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University of Ghana http://ugspace.ug.edu.gh UNIVERSITY OF GHANA COLLEGE OF BASIC AND APPLIED SCIENCES ESTIMATION OF ENTRANCE SURFACE DOSE AND IMAGE QUALITY ASSESSMENT OF ADULT PATIENTS UNDERGOING COMPUTED RADIOGRAPHY EXAMINATIONS BY DANIEL ACKOM (10507037) THIS THESIS IS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF MPHIL MEDICAL PHYSICS DEGREE. DEPARTMENT OF MEDICAL PHYSICS, SCHOOL OF NUCLEAR AND ALLIED SCIENCES JULY, 2016 University of Ghana http://ugspace.ug.edu.gh DECLARATION This thesis is the result of research work undertaken by Daniel Ackom in the Department of Medical Physics, University of Ghana, under the supervision of Prof. Cyril Schandorf, Dr Stephen Inkoom and Mr. Edem Kwabla Sosu. I hereby declare that this thesis is the result of my own original research and that no part of it has been presented for another degree in this University or elsewhere. Duly other works and/or researches done by other researchers cited in this work have been acknowledged under references. …………………………….. …………………………….. DANIEL ACKOM PROF. CYRIL SCHANDORF (STUDENT) (PRINCIPAL SUPERVISOR) Date ……………………….. Date.……………………….. …………………………….. ……………………………... DR STEPHEN INKOOM MR EDEM KWABLA SOSU (CO-SUPERVISOR) (CO-SUPERVISOR) Date ……………………….. Date………………………… ii University of Ghana http://ugspace.ug.edu.gh ABSTRACT The entrance surface dose (ESD) of radiographic examinations of adult patients undergoing computed radiography (CR) examinations have been estimated and image quality using ImageJ software version 1.48. In all 619 adult patients were involved in the study comprising 243 males and 376 females. The mean ESDs calculated were in the range of 0.29±0.0041 mGy to 6.08±0.55 mGy for chest PA, lumbar spine AP/LAT, cervical spine AP/LAT, skull AP/LAT and abdomen AP. All the results were lower compared with published results and diagnostic reference levels set by the IAEA and UK show except chest PA examination which the value estimated at UGH was by a factor of 2.56, 2.20 and 1.65 higher than the mean ESDs values by Inkoom et al, the IAEA and Public Health of UK respectively. An increment by factor of 3.07, 2.87 and 2.15 were found in the estimated mean ESDs for chest PA examination at KBTH for the published results respectively. Image quality of the radiographic images was assessed in terms of CNR and SNR. The CNR for all examinations were between 3.54±3.27 and 20.63±8.65 for all examinations. A maximum difference 17.09 in CNR was found between KBTH and UGH. The results obtained for SNR for both hospitals showed that 92.86% of all the images assessed were at least 1.22 higher than the Rose Model of the threshold value of ≥5. The study therefore has shown that the estimated mean ESD of the hospitals were within the recommended references values and the images were of good quality. Hence, there is the potential to reduce the dose to patients while keeping images of diagnostic quality. iii University of Ghana http://ugspace.ug.edu.gh DEDICATION To my uncles Mr George Affran-Annan and Mr John Annan whose hands have been a guide on my shoulders throughout my education. To my Mother, Martha Annan, no parent could have done anymore. To my siblings Bernard and Gloria Ackom, thank you for always being there for me. iv University of Ghana http://ugspace.ug.edu.gh ACKNOWLEDGEMENTS I wish to express my heartfelt gratitude to the persons whose guidance, enthusiastic encouragement, sympathetic comments during the planning and development of this study. I am indebted to my supervisors, Prof Cyril Schandorf, Dr Stephen Inkoom and Mr Edem Kwabla Sosu who have been a constant source of words of encouragement and motivation, valuable and constructive suggestions and guidance in the preparation of this thesis. This work would not have been successful without the help of Mr Prince Gyekye and Mr Emmanuel Akrobeto of National Regulatory Authority. To the Authorities and radiography staff at radiology departments of University of Ghana and Korle-Bu Teaching hospitals, thank you all for your selfless support during gathering of data for this study. My dearest thanks to the Annan and Ackom family especially Mr John Annan, Mr George Affran-Annan and his family, my mother, Martha Annan and Madam Joyce Hutton Mills for their continual support and encouragement. My colleagues of 2016 medical physics class, may God bless you all for your good company throughout our studies and I will never forget all the chats and beautiful moments we had together. Special thanks to Madam Ethel Jobson-Mitchual and her family for their inspiration and support throughout this work. Finally, I would like to thank my dearest Sheila Ofosua Gyekye for her subservient help, patience, support, care, hard work and for being there for me. v University of Ghana http://ugspace.ug.edu.gh TABLE OF CONTENTS DECLARATION .........................................................................................................ii ABSTRACT ............................................................................................................... iii DEDICATION ............................................................................................................ iv ACKNOWLEDGEMENTS ......................................................................................... v TABLE OF CONTENTS ............................................................................................ vi LIST OF TABLES ....................................................................................................... x LIST OF FIGURES .................................................................................................... xi LIST OF PLATES ................................................................................................... xiii LIST OF ABBREVIATIONS ................................................................................... xiv CHAPTER ONE .......................................................................................................... 1 INTRODUCTION ....................................................................................................... 1 1.1. Statement of Problem ........................................................................................ 2 1.3. Relevance and Justification of the Study .......................................................... 4 1.5. Structure of the Thesis ...................................................................................... 5 CHAPTER TWO ......................................................................................................... 6 LITERATURE REVIEW............................................................................................. 6 2.1. Advancement in Radiography Systems ............................................................ 6 2.2. Entrance Surface Dose ...................................................................................... 9 2.2.1. Methods of Estimating ESD ...................................................................... 9 2.2.1.1. ESD Estimation by the use of Semiconductor Dosimeters ................... 10 vi University of Ghana http://ugspace.ug.edu.gh 2.2.1.2. Measurement of ESD with TLD ........................................................... 13 2.2.1.3. Measurement of ESD with Ionization Chamber ................................... 16 2.2.1.4. Measurement of ESD with Dose Area Product Meter .......................... 18 2.3. Radiation Dose and Image Quality ................................................................. 19 2.4. Optimization.................................................................................................... 20 2.5. Image Quality Assessment with ImageJ Software in Medical Diagnostics .... 21 CHAPTER THREE.................................................................................................... 27 MATERIALS AND METHOD ................................................................................. 27 3.1. Study Area ....................................................................................................... 27 3.2 CR Systems used .............................................................................................. 27 3.3. The Ionization Chamber .................................................................................. 29 3.4. Quality Control (QC) on the CR systems ....................................................... 29 3.4.1. Tube Voltage Accuracy............................................................................ 30 3.4.2. Exposure Timer Accuracy........................................................................ 31 3.4.3 kVp, Timer and Exposure Reproducibility ............................................... 31 3.4.4 X-ray Beam Quality Test (Half Value Layer Determination) .................. 32 3.4.5 Current-Time Linearity ............................................................................. 33 3.4.6. Collimation and Beam Alignment ........................................................... 33 3.4.7. Tube Output Measurement ....................................................................... 34 3.4. Patient Data Collection ............................................................................... 34 3.5. Exposure Factors and Techniques ................................................................... 35 vii University of Ghana http://ugspace.ug.edu.gh 3.6. ESD Estimation ............................................................................................... 36 3.7. Image Quality Assessment .............................................................................. 37 CHAPTER FOUR ...................................................................................................... 38 RESULTS AND DISCUSSION ................................................................................ 38 4.1. Quality Control (QC) on CR Systems ............................................................. 38 4.2. X-Ray Output Measurement ........................................................................... 40 4.3. Summary of Examinations .............................................................................. 43 4.4. ESD Estimation ............................................................................................... 44 4.5. Comparison of ESDs with other Published Results ........................................ 48 4.6. Image Quality Assessment .............................................................................. 50 4.6.1. Contrast-to-Noise Ratio ........................................................................... 51 4.6.2. Signal-To-Noise Ratio ............................................................................. 54 4.7. Optimization of Patient Protection .................................................................. 57 CHAPTER FIVE ........................................................................................................ 63 CONCLUSION AND RECOMMENDATION ......................................................... 63 5.1. Conclusion ...................................................................................................... 63 5.2 Recommendations ............................................................................................ 64 REFERENCES........................................................................................................... 66 APPENDIX A ............................................................................................................ 72 APPENDIX B ............................................................................................................ 74 QC RESULTS AT KBTH.......................................................................................... 74 viii University of Ghana http://ugspace.ug.edu.gh APPENDIX C ............................................................................................................ 77 QC RESULTS AT UGH ............................................................................................ 77 APPENDIX D ............................................................................................................ 79 ix University of Ghana http://ugspace.ug.edu.gh LIST OF TABLES Table 2. 1: Timetable of developments in digital radiography .................................... 8 Table 4.1: Summary of QC results on CR at KBTH ................................................. 39 Table 4.2: Summary of QC results on CR at UGH .................................................... 40 Table 4.3: Estimated mean ESD at KBTH................................................................. 47 Table 4. 4: Estimated mean ESD at UGH .................................................................. 47 Table 4.5: Mean CNR calculated from images obtained for KBTH and UGH ......... 51 Table 4.6: Calculated SNR for the KBTH and UGH ................................................. 55 Table 4. 7: Optimized level of ESD and CNR ........................................................... 62 Table A1: Summary of individual patient exposures at KBTH ................................. 72 Table A2: Summary of individual patient exposures at UGH ................................... 73 Table B.1: kVp accuracy results ................................................................................ 74 Table B. 2: kVp, Exposure and Timer reproducibility............................................... 74 Table B. 3: mAs Linearity.......................................................................................... 75 Table B. 4: Timer Accuracy ....................................................................................... 75 Table B. 5: Tube Output measurement ...................................................................... 76 Table C. 1: kVp accuracy and Reproducibility results .............................................. 77 Table C. 2: Timer Accuracy and Reproducibility ...................................................... 77 Table C. 3: Tube Output measurement ...................................................................... 78 Table D.1: Datasheet used to collect exposure parameters at the hospitals ............... 79 x University of Ghana http://ugspace.ug.edu.gh LIST OF FIGURES Figure 2.1: Diode dosimeter circuit configuration (b) Complete diode dosimeter .... 11 Figure 2.2: (a) Structure of MOSFET (b) Complete MOSFET dosimeter .............. 11 Figure 2.3: Setup for entrance diode calibration (b) Setup for exit diode calibration in total body irradiation (TBI) condition (Allahverdi et al., 2008). ............................... 13 Figure 2. 4: TLDs in different forms (Radiation Products Design, 2014). ................ 14 Figure 2. 5: ROIs drawn to measure CNR (b) ROIs for SNR measurement ............. 23 Figure 2. 6: Main window of the ImageJ plugin developed for performing the physical analysis and image quality checks. .............................................................. 25 Figure 2. 7: Schematic diagram of ROIs chosen. Source: (Song et al., 2004) ........... 26 Figure 3.1: Schematic diagram for different QC tests ............................................... 30 Figure 3.2: Schematic diagram of setup for measurement. ........................................ 35 Figure 4.1: Output curve of CR system at KBTH ...................................................... 41 Figure 4. 2: Output curve of CR system at UGH ....................................................... 42 Figure 4.3: Chart showing the percentage of each examination at KBTH ................ 43 Figure 4. 4: Chart showing the percentage of each examination at UGH.................. 44 Figure 4. 5: Chart comparing the mean ESDs from the two hospitals ....................... 46 Figure 4.6: Comparison of ESD at KBTH with published results and DRLs from international organizations. ........................................................................................ 50 Figure 4. 7: Comparative bar chart of mean CNR obtained from KBTH and UGH . 52 Figure 4.8: Comparative bar chart of SNR obtained from the two hospitals ............. 56 Figure 4. 9: CNR as a function of ESD (mGy) for Chest PA .................................... 57 Figure 4. 10: CNR as a function of ESD (mGy) for Lumbar spine AP ..................... 58 Figure 4. 11: CNR as a function of ESD (mGy) for Lumbar spine LAT .................. 58 Figure 4. 12: CNR as a function of ESD (mGy) for cervical spine AP ..................... 59 xi University of Ghana http://ugspace.ug.edu.gh Figure 4. 13: CNR as a function of ESD (mGy) for cervical spine LAT .................. 59 Figure 4. 14: CNR as a function of ESD (mGy) for Skull AP ................................... 60 Figure 4.15: CNR as a function of ESD (mGy) for Skull LAT ................................. 60 Figure 4. 16: CNR as a function of ESD (mGy) for Abdomen AP ........................... 61 xii University of Ghana http://ugspace.ug.edu.gh LIST OF PLATES Plate 3.1: CR system at the University of Ghana Hospital. ....................................... 28 Plate 3.2: Fujifilm IP cassette type CC ...................................................................... 28 Plate 3. 4: (a) Beam alignment tool (b) Collimator tool ............................................ 34 xiii University of Ghana http://ugspace.ug.edu.gh LIST OF ABBREVIATIONS ALARA As Low As Reasonably Achievable AP Anterior-Posterior BSF Backscatter Factor CCD Charge-Couple Detectors/Devices CNR Contrast-to-Noise Ratio CR Computed Radiography DAP Dose Area Product DDR Direct Digital Radiography DQE Detector Quantum Efficiency DRL Diagnostic Reference Level ESAK Entrance Surface Air Kerma ESD Entrance Surface Dose FFD Focus-to-Film Distance FSD Focus-to-Surface Distance HIS Hospital Information System HVL Half Value Layer IAEA International Atomic Energy Agency ICRP International Commission on Radiation Protection IPEM Institute of Physics and Engineers in Medicine KAP Kerma Area Product KBTH Korle-Bu Teaching Hospital kVp Kilovoltage Peak LAT Lateral mAs Milliampere seconds xiv University of Ghana http://ugspace.ug.edu.gh MOSFET Metal Oxide Semiconductor Field Effect Transistor MTF Modulation Transfer Function NPS Noise Power Spectrum PA Posterior-Anterior PACS Picture Archiving Communication Systems QA Quality Assurance QC Quality Control RMI Radiation Measuring Instrument ROI Region of Interest SNR Signal-to-Noise Ratio SSD Source-to-Surface Distance TBI Total Body Irradiation TLD Thermoluminescence Dosimeter UGH University of Ghana Hospital WHO World Health Organization xv University of Ghana http://ugspace.ug.edu.gh CHAPTER ONE INTRODUCTION 1.0. Background X-ray was discovered in 1895 by a German experimental physicist, Wilhelm Roentgen while working on emissions from electric current vacuum. He observed a glow from barium platinocyanide (BaPt(CN)4) coated screen kept across the room whenever the current was passed between the two electrodes in a charged cathode ray tube (Bansal, 2006). The first medical applications of x-rays was reported few months after the discovery where a two dimensional projection image of patients anatomy was created on a photographic plate using the specific attenuation properties of x-rays within tissues (Jin Mo Goo et al., 2000). Since the discovery of x-rays, diagnostic imaging has evolved and advanced to an extent that it has become an indispensable component of patient diagnosis, management and in certain cases, treatment. The type of x-ray systems used in medicine are conventional (computed or direct digital radiography), dental, fluoroscopy, orthovoltage, mammography, megavoltage and computed tomography. Conventional film-intensifying screen radiography has been used for decades and has served the medical profession well. With the advent of computers, digital imaging has now become the standard of human imaging (Mattoon and Smith, 2004). Direct digital and CR by the use of flat panel detectors and reusable phosphor screens, offer convenient and reliable way to replace the film. In CR, a photostimulable phosphor plate is used for detection of x-rays instead of the conventional film screen. The exposed plate is then scanned with helium neon laser and the emitted light is captured by photomultiplier tube and converted to analogue electrical system, which is then digitised. For direct digital radiography (DDR), a 1 University of Ghana http://ugspace.ug.edu.gh semiconductor based sensor directly converts x-ray energy into electrical signals, hence eliminating the middle step of latent image and image plate reader. Each of these type of radiography system can either be manual or with automatic exposure control system. Computed and direct digital radiography as at 2011, formed only 4% of the conventional x-ray machines in Ghana to help improve the delivery of quality healthcare system (Inkoom et al., 2011). They have the advantage of being low cost, non-invasive, familiarity with medical professionals, relative harmlessness and fast imaging times. However, there are reports showing that low doses of ionizing radiation exposure encountered in diagnostic examinations may induce malicious conditions (Kothan and Tungjai, 2011). Use of x-ray facilities and equipment has increased rapidly in medical practices. Diagnostic radiology has an enormous share of public dose from man-made sources. With the advent of new digital imaging systems in radiography departments, it is therefore important to focus on keeping the dose as low as reasonably practicable whilst producing an image of diagnostic quality for digital radiography. The lack of consistent feedback to technologists concerning the use of optimal acquisition techniques is also a major problem with the use of computed and direct digital radiography. This problem, along with the much larger dynamic range of digital systems, has led to a gradual increase in patient radiation dose (Williams et al., 2007). 1.1. Statement of Problem Image quality is one of the most important aspects of diagnostic radiology. The concept of image quality has been undergoing a transformation with the widespread use of digital-projection radiography. An ideal image is an image with high contrast, high spatial resolution and low noise level (Vladimirov, 2010). However, these factors 2 University of Ghana http://ugspace.ug.edu.gh are interconnected and depend on each other for every image. Since the introduction of CR in Ghana in the year 2012, no attempt has been made to conduct the assessment of the ESD to patient and associated image quality to trigger the need for optimization of protection of patients. The current study seek to bridge this knowledge gap and propose procedures for management of patient dose in CR and DR in Ghana. To fulfil the Medical Exposure Directive (MED, 97/43, Euratom) requirement of a good image quality at a radiation dose that is as low as reasonably achievable, optimization of medical imaging systems is therefore deemed necessary (Smans et al., 2010). CR systems are much more tolerant to inappropriate techniques with a good or even perfect image. This is due to the high latitude of digital detectors and phosphor plates which make possible systematic over-exposure or unnecessary high doses. Also, for CR systems with automatic exposure control (AEC), until now, there is no certain approach with regards to how to keep constant image quality at different tube potentials in order to calibrate the AEC (Vladimirov, 2010). 1.2. Objectives To address the above problem, this work seeks to: Accurately estimate the ESD of the patients (adults) undergoing selected CR examinations [chest (PA), abdomen (AP), lumbar spine (AP and LAT), cervical spine (AP and LAT) and Skull (AP and LAT)] Compare ESDs to the diagnostic reference levels established by international organizations. Determine the potential for optimization of protection of patients for the selected examinations under the study. 3 University of Ghana http://ugspace.ug.edu.gh Make appropriate recommendations from the findings 1.3. Relevance and Justification of the Study This study would be helpful in the optimization of patient protection and setting standards of good practice in the selected hospitals. All radiographic examinations are performed purposely to provide reliable diagnostic information allowing rapid and suitable treatment of the patient. These examinations must be performed with greater care to ensure sufficient image quality while exposing the patient to the lowest dose possible. According to literature, radiation dose to patients in diagnostic x-ray examinations can be best estimated in terms of ESD per radiograph (Suliman and Habbani (2007). To this point, the ESD is defined as the absorbed dose in air, including the contribution from backscatter, assessed at a point on the entrance surface of a specified object (IAEA: TRS-457, 2007). Literature also suggests that ESD is most appropriate for simple x-ray projections since it meets the three (3) basic conditions set out by the International Atomic Energy Agency (IAEA) which state that ESD is simple to measure; permits direct measurement on patient during examination; and is a representative of dose received by patients (Hamza et al., 2014). 1.4. Scope and limitation This study was carried on the CR systems at the University of Ghana and Korle-Bu Teaching hospitals in the Greater Accra Region. It focussed on some selected CR examinations including chest (PA), lumbar spine (AP and LAT), cervical spine (AP and LAT), skull (AP and LAT) and abdomen (AP). 4 University of Ghana http://ugspace.ug.edu.gh 1.5. Structure of the Thesis The thesis is organized as follows: Chapter one presents the general background to the thesis. It also presents the problem statement with set objectives to solve the problem identified. The relevance, justification and scope of the study are also dealt with in chapter one. Chapter two presents literature review of previous work done in relation to this work. The step-by-step procedure followed to collect data for the study is presented in chapter three. Chapter four deals with analysis of the data obtained. It further presents the discussion of the various results obtained. Finally, Chapter five summarizes the findings of the study, draws conclusions and gives recommendations for further study and applications of the findings of this work. 5 University of Ghana http://ugspace.ug.edu.gh CHAPTER TWO LITERATURE REVIEW This chapter reviews various literature in relation to this work in terms of ESD and image quality of adult patients undergoing CR examinations. The chapter discusses the various ways of determining the ESD and objective image quality assessment using ImageJ. 2.1. Advancement in Radiography Systems The discovery of x-rays by Wilhelm Roentgen a century ago was the first step to enable the visualization of the human internal anatomical structures. This led to a revolution in the field of medicine in particular with the use of x-rays for diagnosis and treatment of different kinds of diseases and thus helping to improve healthcare delivery in the world (IAEA: TRS-457, 2007). In the last 50 years diagnostic imaging has grown from a basic application to a more advance level with the introduction of different modalities. It is very clear that medical imaging has become established as having an important role in patient management, and especially radiologic diagnosis (Doi, 2006). Radiological facilities are found in even the smallest hospital and emergency units involved in health care. A hospital without radiography is unbelievable (Bansal, 2006). In the 1950s, most diagnostic images were acquired by the use of screen–film systems and a high-voltage x-ray generator for conventional projection x-ray imaging (Doi, 2006). Most radiographs were obtained by manual processing of films in darkrooms, but with the passage of time, the latter is being taken over by the use of automated film processing systems (Doi, 2006). Hence, during the 6 University of Ghana http://ugspace.ug.edu.gh past two decades, CR has replaced the screen-film radiography in many radiological facilities (Körner et al., 2007). Over the years, many significant modifications have been made in the techniques and the equipment of radiography. In 1977 a group of researchers first described the experimental digital subtraction and was introduced into clinical use as the first digital imaging system in 1980 (Ovitt et al, 1980). X-ray images for general radiography were first recorded digitally with cassette-based storage-phosphor plates and were also introduced in 1980 (Moore, 1980). In 1990, the first digital radiography (DR) system evolved and this was a charge-couple devices (CCD) slot-scan system and subsequently in 1994 investigations of the selenium drum digital radiograph system were published (Neitzel et al, 1994). The first flat-panel detector digital radiography systems based on amorphous silicon as reported by Antonuk et al, (1995) and amorphous selenium reported by Zhao and Rowlands, (1995) were introduced in 1995. The work of Kandarakis et al, (1997) suggests that gadolinium-oxide sulphide scintillators were introduced in 1997 and have been used for portable flat-panel detectors since 2001 (Puig, 2003). The latest development in digital radiography is the introduction of the dynamic flat-panel detectors for digital fluoroscopy and angiography (Colbeth et al, 2001). Table 2.1 summarises the various developments made in digital radiography systems over the years. 7 University of Ghana http://ugspace.ug.edu.gh Table 2. 1: Timetable of developments in digital radiography (Körner et al., 2007) Year Development 1980 CR, storage phosphors 1987 Amorphous selenium-based image plates 1990 Charge-coupled device (CCD) slot-scan direct radiography (DR) 1994 Selenium drum digital radiography (DR) 1995 Amorphous silicon-cesium iodide (scintillator) flat-panel detector 1995 Selenium-based flat-panel detector 1997 Digital subtraction angiography 1997 Gadolinium-based (scintillator) flat-panel detector 2001 Gadolinium-based (scintillator) portable flat-panel detector Dynamic flat-panel detector fluoroscopy-digital subtraction 2001 Angiography 2006 Digital Tomosynthesis 2009 Wireless DR (flat-panel detector) Today, manufacturers provide a variety of digital imaging solutions based on various detector and readout technologies (Körner et al., 2007). Of the many importance derived from the use of digital radiography systems, the implementation of a fully digital picture archiving and communication system (PACS) in which images are stored digitally and are retrieved anytime cannot be ignored (Körner et al., 2007). With the advent of PACS, image distribution in hospitals can now be achieved electronically by means of web-based technology without loss of image fidelity. Körner et al., (2007) further suggest that other advantages of digital radiography include higher patient throughput, increased dose efficiency, and the greater dynamic range of digital 8 University of Ghana http://ugspace.ug.edu.gh detectors with possible reduction of radiation exposure to patients. The dynamic range of an x-ray imaging system is defined as the ratio of the largest and smallest input xray intensities that can be imaged (Williams et al., 2007). For all systems, the smallest useful intensity is determined by the intrinsic system noise. The signal must be large enough to exceed this noise, combined with the x-ray quantum noise. As we gradually move towards the future of fully digital radiography systems, it therefore behooves radiologists and other radiography technologists to be abreast with the technical principles, image quality criteria, radiation exposure and other issues related to the various digital radiography systems that are being produced (Körner et al., 2007). 2.2. Entrance Surface Dose ESD is a quantity which depends on the x-ray penetrating power and parameters used during the x-ray examination. It is evaluated because the dose to patient is greater at the skin surface compared to the internal organ, indicating that radiation effect on the skin is more significant (Fahmi, Aisyahton, and Zaky, 2013). The International Atomic Energy Agency defines ESD as absorbed dose in air, including the contribution from backscatter which is assessed at a point on the entrance surface of a specified object (e.g. patient or phantom) (IAEA Human Health Series No. 24, 2007). Research have shown that to estimate the amount of radiation in dose a patient receives during x-ray examinations, the most commonly used quantity is the ESD (Fahmi et al., 2013). 2.2.1. Methods of Estimating ESD Radiation doses resulting from diagnostic radiological examinations are small and are usually below the level of thresholds needed to cause deterministic effects. Exceptions 9 University of Ghana http://ugspace.ug.edu.gh are found for interventional procedures in radiology and cardiology that may involve high doses to the patient’s skin (IAEA TRS-457, 2007). The greatest source of artificial ionization radiation exposure to the public or population is mainly from diagnostic radiology and therefore, doses delivered in diagnostic radiological procedures should be accurately determined in order to maintain a reasonable balance between image quality and patient dose. There are various methods of estimating the ESD of patients undergoing radiological examinations. In diagnostic radiology, ESD can be estimated mostly by the use of common dosimeters for such as the ionization chambers, TLD and semiconductor detectors. 2.2.1.1. ESD Estimation by the use of Semiconductor Dosimeters Dosimeters other than ionization chambers exist and these have been used for diagnostic measurements (IAEA:TRS-457, 2007). Silicon and Germanium detectors which are semiconductor detectors have been used mainly for energy spectrometry and have replaced scintillators in this application where highest energy resolution is required. Some characteristics of semiconductor detectors also make them suitable to be used as dosimeters for measuring dose or dose rate as a substitute for an ionization chamber (Attix, 1986). In semiconductors with gap widths of 1 eV to 2 eV the number of electron-hole pairs created by radiation in the vicinity of a junction between two different materials is detected directly by electrical means. Hence, semiconductor detectors work like solid state ionisation chambers. Examples of the semiconductor dosimeters are the diode dosimeter circuit and complete diode dosimeter shown in Figure 2.1 (a) and (b) respectively and also the metal oxide semiconductor field effect transistor (MOSFET) dosimeter as shown in Figure 2.2 with the structure in (a) and the complete MOSFET dosimeter in (b). 10 University of Ghana http://ugspace.ug.edu.gh Figure 2.1: Diode dosimeter circuit configuration (b) Complete diode dosimeter (Radiation Products Design, 2014) Figure 2.2: (a) Structure of MOSFET (b) Complete MOSFET dosimeter (Cygler and Scalchi, 2009) With semiconductor detector, Owoade, Sambo, & Tijani, (2015) assessed the entrance surface air kerma (ESAK) in patients undergoing chest x-ray from conventional diagnostic radiology in Ogun State, Nigeria. The semiconductor device was used to measure the output of the x-ray system and by applying equation 2.1 below, the entrance surface air kerma for each patient was calculated. Tube Output mAs BSF FDD ESAK 2 FSD 11 2 2.1 University of Ghana http://ugspace.ug.edu.gh where mAs is the current-time product, BSF is the backscatter factor, FDD is the focus-to-detector distance and FSD is the focus-to-surface distance. The results showed significant differences in the ESAK of the various x-ray systems used. The results compared well to similar studies from other countries except in chest examination where the ESAK was higher than those in literature (Owoade et al., 2015). Herath et al., (2015) obtained x-ray tube output measurement in assessing ESD for postero-anterior erect chest x-ray examination of adult patients in a selected teaching hospital in Sri Lanka. According to the authors, the ESD was then calculated using equation 2.1 with FFD of 180 cm. The results from the study were below the diagnostic reference level set by the IAEA. A study of diode calibration for dose determination in total body was done by Allahverdi et al., (2008). Allahverdi et al., (2008) used four p-type diodes in connection to a MULTIDOSE electrometer as a dosimeter for the measurement. The diodes were calibrated with water phantom of dimension 30 30 32 cm3 . The setup for the study is shown in Figure 2.3. The calibration factor was determined by these authors using the following equation: Fcal absorbed dose ( D ) (measured with chamber ) diode reading ( M ) (measured with diode) 2.2 where Fcal is the calibration factor. Allahverdi et al., (2008) found by measurement that the effect of angle incident on diode response was significant and should have been taken into account. Variation in thickness correction factor was found to be 0.7%. 12 University of Ghana http://ugspace.ug.edu.gh Figure 2.3: Setup for entrance diode calibration (b) Setup for exit diode calibration in total body irradiation (TBI) condition (Allahverdi et al., 2008). Semiconductors have the advantage of being small, give instantaneous response to irradiation, produce large signals from modest amount of radiation and are not influenced by external factors like atmospheric pressure (IAEA TRS-457, 2007). However, dose measurement with semiconductor detectors has some drawbacks which include silicon having low atomic number (Z) of 14. This makes semiconductor detectors to have large energy dependence by comparison with an ionisation chamber (Maryanski et al, 1993). The detectors exhibit radiation damage and therefore, require recalibration as the radiation sensitivity decreases with increasing radiation. Also, due to their construction they show directional dependence, which can have significant effects in some cases (Fong et al, 2001). 2.2.1.2. Measurement of ESD with TLD Thermoluminescence is thermally activated phosphorescence; the most remarkable and the most widely known of a number of different ionizing radiation induced thermally activated phenomena. TLDs are a crystalline material that upon exposure to ionising radiation are able to absorb or retain the radiation energy and store it at the 13 University of Ghana http://ugspace.ug.edu.gh crystal lattice, known as electron traps (Khan, 1994). The use of TLDs in patient dosimetry has been carefully considered in the publication of the European Commission (Zoetelief et al., 2000). According to these authors, the availability of TLDs in various forms (e.g. powder, chips, rods, ribbons) and made of various materials. The availability of TL dosimeters in a variety of physical forms as shown in Figure 2.4 below makes them especially suitable for skin dose measurements. Figure 2. 4: TLDs in different forms (Radiation Products Design, 2014). Shahbazi-Gahrouei, (2006) who measured ESD for routine x-ray examinations in Chaharmahal and Bakhriari hospitals placed three TLD chips on the skin of each patient and the doses were averaged for each radiography and mean of ESD of all patients were calculated. According to this author, there was no significant difference among the values of ESD obtained by the three detectors. However the ESDs were higher that the diagnostic reference levels set by international organizations. The work of Freitas & Yoshimura, (2009) on diagnostic reference levels for most of frequent radiological examinations carried out in Brazil employed TLDs. According to the authors, a pair of packed TLDs were attached directly to the patient’s skin at a point close to the centre of the incident x-ray beam. The ESDs were calculated for only the examination which showed relevant clinical information on the radiograph. The 14 University of Ghana http://ugspace.ug.edu.gh results of the study compared to previous published data in Brazil showed a significant difference even though different methods were used. A study of radiation exposure to critical organs in the panoramic dental examination using TLDs was conducted on different panoramic systems by Taghi et al, (2012). In this study, the absorbed dose to organs and sensitive tissues in the head and neck region during panoramic radiography were determined by placing TLDs packed in sachets on the thyroid gland, left and right parotid glands, occipital region and the eye lids. The results according to these authors show that there were differences between patient doses examined by different panoramic systems. Mortazavi et al., (2004) also measured ESD on the thyroid gland in orthopantomography by employing TLDs. The ESD was measured by placing three TLD chips each on the thyroid of forty (40) patients undergoing panoramic radiography examination. The doses received by each patients were averaged and the mean ESD of all patients were then calculated. However, the results obtained according these authors were inconsistent with the only reported data in literature where the same method was employed. (Herath et al., 2015). In an assessment of dose in thyroid and salivary glands in dental radiology using TLD a group of researchers put a set five TLDs in the thyroid gland of an Alderson-Rando phantom and irradiated it with 10 exposures using a set of exposure parameters provided by the equipment for all techniques. The TLD response ( RTLDs ), in air kerma for a 137 Cs source, was obtained by using the following formula: RTLDs LavgTLDs BGavg FN (mGy) 2.3 Where LavgTLDs is the average TLD readings, BGavg is the background radiation and FN is the batch’s angular coefficient (Horizonte et al., 2011). The main advantages of TLDs are tissue equivalent composition, smaller in size, flexible in shape and are associated with good spatial resolution. They give a good 15 University of Ghana http://ugspace.ug.edu.gh measurement accuracy when correctly used and are invisible in the x-ray image. TLDs have also proven to be useful as personal dosimeters for radiation workers (Toivonen et al., 2003). However, TLDs have a number of undesirable features which should be considered before they are used. Toivonen et al., (2003) suggest that to measure the dose received at a whole part of an exposed body, large numbers of TLDs are required due to their small in size. Dose assessment can only be made retrospectively with TLD. Dose assessment with TLD has also been shown to be laborious, capital intensive and potentially intrusive when large patients are involved in the study (Mcparland, 1998). Also, the main problem with TLDs is that there is typically a 0.1 𝑚𝐺𝑦 minimum absorbed dose to produce a reasonably accurate resultThis value is above most of the paediatric entrance surface air kerma values and thus makes the use of TLD in paediatric dosimetry difficult (IAEA Human Health Series No. 24, 2013). 2.2.1.3. Measurement of ESD with Ionization Chamber The ionization chamber is a device used for the detection and measurement of the ionizing radiation. It does not measure radiation dose directly, instead it measures the energy transferred to a small volume of air (kerma or exposure) by ionizing radiation. It is the most widely used type of dosimeter for precise measurements of ionizing radiation (Attix, 1986). Ionization chamber is used to measure kerma area product (KAP) of an x-ray beam which is then converted to ESD. This procedure is referred to as the indirect measurement of ESD. Incidence air kerma can be obtained with suitable ionization chamber in the absence of phantom (IAEA TRS-457, 2007). Kothan and Tungjai, (2011) estimated x-radiation output using mathematical model. The authors further compared the calculated x-ray output with the measured output using ionization chamber. The radiation output was measured at a source-to-chamber 16 University of Ghana http://ugspace.ug.edu.gh distance of 100 cm. The results according to the authors shows a small difference between the calculated and measured radiation outputs. Dose area product (DAP) could be measured directly through the use of ionization chamber at the surface of xray tube collimator which can be converted into ESD using the equation ESDDAP with CF DAP BSF CFDAP Area( FSD) 2.4 where FSD is the focus-to-skin distance, BSF is the backscatter factor and CFDAP is the correction factor for the dose area product meter (Tamboul et al, 2014). The authors opined that the results obtained by this method do not exceed the reference values published from previous studies. The work of Wanai et al., (2011) who estimated the ESD level of common diagnostic x-ray examinations at Songklanagarind hospital made use of the ionization chamber to measure the entrance surface air kerma of the x-ray equipment. The ESD was then estimated by the multiplication of the entrance surface air kerma by an appropriate backscatter factor for each examination. Here, the authors also suggests that most of the results obtained were in good agreement of the diagnostic reference levels set by the IAEA except the skull lateral which showed higher ESD compared to the corresponding reference level by the IAEA. A feasibility study on the reduction of the ESD to neonates by the use of a new digital mobile x-ray was done by Utsunomiya et al., (2013). The authors measured the scattered radiation around the incubator with an ionization chamber survey meter. The doses measured around the incubator were below measurable limits according to these authors. The advantage of ionization chambers used in diagnostic radiology (unsealed type) according to the IAEA is that no corrections for possible changes in air density are required (IAEA TRS-457, 2007). 17 University of Ghana http://ugspace.ug.edu.gh 2.2.1.4. Measurement of ESD with Dose Area Product Meter According to Kothan and Tungjai, (2011), there are various mathematical models which can also be used to estimate the ESD but to employ the mathematical model, the output of the x ray machine needs to be measured. This is because, the mathematical model is a function of the output of the x-ray machine. The output of the x ray machine can also be determine from a mathematical model (Kothan and Tungjai, 2011). The output is directly proportional to the peak tube voltage, 𝑘𝑉𝑝 , the tube current, 𝑚𝐴𝑠, exposure time 𝑠 and inversely proportional to the square of the distance (𝑑2 ) from the source. This is given as Output (mR ) K kV pn mA s 1 d2 2.5 where, 𝑛 and 𝑘 are constant values but 𝐾 is the slope of the graph between 𝑚𝑅/𝑚𝐴𝑠 and 𝑘𝑉𝑝 2. The output is also dependent on the type of x-ray machine; single phase, three phase or three phase and high frequency (Kothan & Tungjai, 2011). Hence, the current pulse factor which also affects the radiation output of each type of x ray machine should also be included in the mathematical model. The results obtained by these authors agreed well with the measured results from the ionization chamber. Fahmi et al, (2013) determined the ESD in chest radiography of anthropomorphic phantom using two different indirect methods; the dose area product (DAP) with correction factor ESDDAP with CF and the manual calculation method (ESDc). The authors compared their results to the Standard Diagnostic Reference Level (DRL) introduced by The European Union Council Directive 97/43/EURATOM and the adoption of detector Correction Factor (CF) during ESD measurement. The researchers used DAP meter to measure the dose per area and manually converted 18 University of Ghana http://ugspace.ug.edu.gh DAP to the ESD value with an appropriate backscatter factor using the following equations: ESDDAP with CF DAP BSF CFDAP Area( FSD) 2.6 ESDDAP with CF DAP BSF Area( FSD) 2.7 FSD Area( FSD) Area( FFD) FFD 2 2.8 Equation 2.6, and 2.8 were developed by Meade et al, 2003 and equation 2.7 was derived by Livingstone et al, 2006 respectively. Fahmi et al, (2013) used an equation derived by Osibote et al (equation 2.5) to calculate the ESD 2 kVp ESDC Output (OP) mAs BSF 80 2.9 The results of the authors showed that the ESDDAP with CF was found to be slightly different compared to the Standard but not to a significant. 2.3. Radiation Dose and Image Quality The optimization of x-ray imaging parameters remains a continuous challenge in radiology since digital imaging provides new possibilities because of the wide dynamic range of digital detectors and the digital image processing possibilities. Aichinger et al., (2012) suggests that the knowledge of the relationship that links image quality and radiation dose is a prerequisite to any optimisation of medical diagnostic radiology. This is due to the fact that the ALARA principle requires that the dose received by the patient undergoing radiological examination should be kept 19 University of Ghana http://ugspace.ug.edu.gh “as low as reasonably achievable”. Also, European directive on usage of medical exposures 97/43/Euratom (MED) requires that medical exposures have to be justified and carried out in an optimized manner (Commission of the European Communities (CEC), 1997). It further emphasizes that European Union member states shall adopt appropriate criteria for radiological equipment in order to indicate when an action is necessary, as well as, if appropriate, decommissioning the medical equipment (Commission of European Communities (CEC), 2010). According to Aichinger et al., (2012), the quality of the various components of the imaging chain (focal spot, imaging geometry, image receptor, video camera and amplifier, image-processing software, image display) has an overall effect on the image signal obtained at the viewing station. The authors further opine that these given facts must be taken into account when considering optimisation of image quality and exposure. This is because image quality and radiation exposure cannot be investigated independently of one another. Imaging procedures, including post processing, have to be optimised to reach the necessary quality with lowest dose. 2.4. Optimization New optimisation strategies have to be defined for digital radiography. It is important that optimisation includes post processing which depends on different detectors and exposure/dose parameters. Optimisation, a strategy of reducing dose to the patient whilst still producing an image of diagnostic quality, is imperative in radiography (Tugwell et al., 2014) and is therefore recommended by both the International Commission on Radiological Protection (ICRP Publication 73, 1996) and the European Medical Exposure Directive (CEC, 1997). This principle is important for all 20 University of Ghana http://ugspace.ug.edu.gh examinations that involve ionizing radiation, however it is especially important for high dose examinations. Furthermore, the physical characteristics of the x-ray tube assembly and the radiographic device that are used in various radiological examinations must be adapted to the medical requirements in such a way that the image receptor generally can produce an optimum x-ray image with respect to the representation of the object details which are needed for diagnosis at the lowest possible level of exposure (Aichinger et al., 2012). These medical requirements can be derived from national or international recommendations and guidelines. Aichinger et al, (2012) further suggests that it is practical to optimize image quality first with respect to medical indication since the whole-image information can be obtained from the radiation image which exists in front of the imaging chain. This is based on the assumption that theoretically, an ideal image receptor is being used and then taken into account the real image receptor being employed. Image quality in medical imaging systems can be described and quantified by three characteristics; contrast, noise and sharpness. Contrast is mostly associated with screen film techniques whilst the derived quantity SNR is mostly associated with digital imaging technique as an important image quality parameter. Pascoal et al., (2006) states that the routine assessment and control of image quality, both technical and clinical, is a fundamental task associated with good practice. According to these authors, in addition to subjective visual methods, currently, there are also available automated methods that can be used to assess technical image quality associated with diagnostic imaging systems. 2.5. Image Quality Assessment with ImageJ Software in Medical Diagnostics Image quality assessment still remains a challenge in the field of image processing. It is still not satisfyingly solved and new approaches are still appearing. According to 21 University of Ghana http://ugspace.ug.edu.gh the IAEA (IAEA: TRS-457, 2007), optimization processes which involves balancing radiation dose and image quality in radiology does not always lead to reduction in radiation dose and it is therefore important to emphasize that image quality must always be sufficient to meet clinical requirements. It is essential to maintain the appropriate level of image quality required for clinical confidence. Image quality assessment plays an essential role in the various image processing applications. It seeks to quantify a visual quality or, anatomically, an amount of distortion or degradation in a given picture. These distortions (artifacts) are inevitably part of any digital image processing chain from acquisition, processing and transmission of images (Kud, 2012). A great deal of effort has been made to research which seeks to develop various image quality metrics that correlate very well with the perceived quality measurement but only limited success has been achieved (Wang, Bovik, and Lu, 2002). The work of Mohammadi, Ebrahimi-moghadam, & Shirani, (2014) suggests that the importance of efficient and reliable image quality evaluation has increased due to the increasing demand for image-based applications. This according to these authors is due to the fact assessing image quality is of fundamental importance for numerous image processing applications where the goal of image quality assessment methods is to automatically evaluate the quality of images in agreement with human quality judgements. Generally image quality can be evaluated mostly by using two approaches; objective and subjective methods (Kud, 2012). Subjective image quality assessment is by visual inspection of the 2-D images by human observer. Objective image quality assessment uses mathematical models to predict the quality of an image accurately and automatically. ImageJ is an image analysis programme created by the National Institute of Health, USA, that performs image quality assessment objectively (Ferreira & Rasband, 2012). The authors opined that the 22 University of Ghana http://ugspace.ug.edu.gh programme can display, edit, analyse, process, save and print 8-bit, 16-bit and 32-bit images. The programme calculates the area and pixel value statistics of user-defined selections. Image processing functions such as contrast manipulation, sharpening, smoothing, edge detection and median filtering can be done with ImageJ software. ImageJ can also calculate signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) by choosing one or more specific regions of interest (ROI) in the image. The program uses one ROI for calculating the SNR and two ROIs to measure the CNR. Ahmed et al., (2014) made a comparison of sonogram affirmed iterative reconstruction and filtered back projection on image quality and dose reduction in paediatric head computed tomography: a phantom study. The authors employed ImageJ to analyse the images by determining the CNR and SNR of the images acquired. Two regions of interest and three regions of interest were drawn on bone and soft tissue as shown in Figure 2.5 (a and b) below to calculate CNR and SNR respectively. Figure 2. 5: ROIs drawn to measure CNR (b) ROIs for SNR measurement (Ahmed et al., 2014) The CNR and SNR was then calculated by using the following equation: CNR Mean signal of bone ( ROI 1) Mean signal of tissue ( ROI 2) S tan dard deviatio of ROI 2 23 2.10 University of Ghana http://ugspace.ug.edu.gh SNR Mean signal value within ROI S tan dard deviation within ROI 2.11 The results according to the authors showed that CNR remained constant within minimal change in standard deviation. It also shows that Sinogram-Affirmed Iterative Reconstruction enhances SNR while it reduces noise with possible reduction of dose of 68% (Ahmed et al., 2014). In the work of Reis et al., (2014) on optimisation of paediatrics CR for full spine curvature measurements using phantom: a pilot study, the authors determined the CNR by using the ImageJ software. They also used two ROIs and by employing equation 2.10 above, the CNR was calculated. The results obtained showed a highest CNR at 66 kVp (C. Reis et al., 2014). Donini et al., (2014), developed a free software for performing physical analysis of systems for digital radiography and mammography. The software as shown in Figure 2.6 was used as a plugin for ImageJ software. According to Donini et al., (2014), the plugin is able to assist users in calculating various physical parameters such as the response curve also known as the signal transfer property, modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE). This plugin can further assist users to compute some image quality checks like defective pixel analysis, uniformity, dark analysis and lag. 24 University of Ghana http://ugspace.ug.edu.gh Figure 2. 6: Main window of the ImageJ plugin developed for performing the physical analysis and image quality checks. Donini et al., (2014) validated the software by comparing MTF and NPS curves on a common set of images with those obtained with other similar programmes. The results show a very good agreement (Donini et al., 2014). Song et al., (2004) report on automated region detection based on the contrast-to-nose ratio (CNR) in the nearinfrared tomography. The authors employed ImageJ software to determine the CNR by drawing regions of interest (ROI) on the image acquired as shown in Figure 2.7. The CNR was then calculated by employing the following formula: CNR ROI background 2 2 wROI ROI wbackground background 2.12 where ROI is the mean of the signal values in the target; background is the mean value over the variable background, ROI and background are the standard deviations of the 25 University of Ghana http://ugspace.ug.edu.gh target and of the background areas, respectively. Also the noise weights wROI and wbackground are given by wROI AreaROI AreaROI Areabackground and wROI 2.13 AreaROI AreaROI Areabackground 2.14 Figure 2. 7: Schematic diagram of ROIs chosen. Source: (Song et al., 2004) Song et al., (2004) used the CNR as a basis for determining the detectability of objects within reconstructed images from diffuse near-infrared tomography. The results show that there was a maximal value of CNR near the location of an object within the image and also the size of the true region could be estimated from the CNR (Song et al., 2004). The work of Sun et al., (2012) on optimization of chest radiographic imaging parameters: a comparison of image quality and entrance skin dose for digital chest radiography systems also employed ImageJ software to estimate the noise levels. This was done by placing ROIs of the same size and shape at different areas on the background of the images acquired from the different systems. Significant differences of image noise were found in the digital chest radiography from the different systems. 26 University of Ghana http://ugspace.ug.edu.gh CHAPTER THREE MATERIALS AND METHOD This chapter presents the procedures that were followed to undertake this research. It involves the description of the study area, the CR systems used, the type of ionization chamber used and the types of test or examination that were performed using the set of equipment available. 3.1. Study Area This research was conducted in two major hospitals in Ghana within Accra in the Greater Accra Region. These hospitals were the University of Ghana Hospital and Korle-Bu Teaching Hospital (Room 5). These hospitals were chosen because of the availability of CR systems since most of the hospitals in Ghana are still using the conventional screen-film x-ray systems which employs dark rooms to process their films. 3.2 CR Systems used The CR system available at the University of Ghana Hospital was manufactured by Philips Medical Systems, Japan in the year 2002. The system was installed at the hospital in 2004. The tube head of the CR at this hospital has a model number 989000085271 with maximum kV of 150 kV, 300 mAs, 2.5 mmAl filtration at 75 kV. Image of this CR system is presented in Plate 3.1. This CR serves the whole student population of the university community and the general public. On an average, this CR serves forty (40) patients per day but during the university clinical examination 27 University of Ghana http://ugspace.ug.edu.gh for first year students, the number increases as high as one hundred and ninety (190) patients per day. At the Korle-Bu Teaching Hospital Room 5, the CR system being employed there was manufactured by Shimadzu Corporation, Japan in the year 2012. The tube model is 53224558, 1.5 mmAl filtration at 70 kV and maximum kV of 150 kV. Averagely, this CR serves forty (40) patients per day. Plate 3.1: CR system at the University of Ghana Hospital. All these systems employ Fujifilm cassette reader and IP standard cassettes type CC shown in plate. 3.2 of dimensions 35×43cm, 35×35cm, 24×30m and 18×24cm Plate 3.2: Fujifilm IP cassette type CC 28 University of Ghana http://ugspace.ug.edu.gh 3.3. The Ionization Chamber The solid state ionization chamber employed in this study for the quality control and the output measurements of the two CR systems was manufactured by Piranha RTI (Piranha 657) with serial number CB2-11020219, dose range of 4 µGy/h - 273Gy/h or 0.4 mR/h – 31 kR/h, active detector area of 10×10 mm and was last calibrated on 5th April, 2011. 3.4. Quality Control (QC) on the CR systems Performance of each CR system was assessed through quality control. According to Outif, (2004), the purpose of QC on x-ray system is to detect any change in the performance of x-ray system, which may lead to an unacceptable image quality and/or high dose to patient and staff. The setup for the QC is presented in Figure 3.1. The following parameters were assessed during the quality control process: Tube voltage accuracy Timer accuracy Voltage reproducibility Exposure reproducibility Half value layer Current-time linearity Collimation 29 University of Ghana http://ugspace.ug.edu.gh Figure 3.1: Schematic diagram for different QC tests 3.4.1. Tube Voltage Accuracy The beam was collimated to the sensitive part (10×10 mm) of the ionization chamber. To assess the kVp accuracy, the test was performed with variable kVp and constant mAs. The set kVp ( kVpset ) on the console and the measured kVp ( kVpmeas. ) were recorded. The percentage differences between kVpset and its corresponding kVpmeas. were calculated using the equation % Diff (kVp ) kVpset kVpmeas. 100 kVpmeas. The results for this test for different x-ray systems. 30 3.1 University of Ghana http://ugspace.ug.edu.gh 3.4.2. Exposure Timer Accuracy An accurate exposure timer is critical for proper exposure during examinations to deliver a reasonable patient dose. Any significant variation from the desired exposure will compromise the balance between image quality and patient dose as well as staff radiation dose. The test was done with the same setup in Figure 3.1. Several exposures with constant kVp and mA and with variable exposure time that cover the range of possible exposure time of the x-ray system were performed. The set exposure time, t set and measured exposure time, tmeas. were recorded and the percentage difference was calculated using the formula: % Diff ( Exposure time, ms ) tset tmeas . 100 tmeas. 3.2 3.4.3 kVp, Timer and Exposure Reproducibility The kVp, timer and exposure reproducibility tests also known as the coefficient of variation (CV) was performed with constant kVp and constant mAs with the same setup presented in Figure 3.1. Five sets of exposures were taken and in each case the measured kVp, exposure (mGy) and exposure time (ms) were recorded. The coefficient of variation for each parameter (kVp, exposure and exposure time) was calculated using the following equation CV 3.3 where is the estimated standard deviation of the five sets of measured parameter (kVp, exposure and exposure time) and is their corresponding mean for each parameter. 31 University of Ghana http://ugspace.ug.edu.gh 3.4.4 X-ray Beam Quality Test (Half Value Layer Determination) The quality of an x-ray beam is the ability of the beam to penetrate an object and this is called the penetrating power. Hard x-ray beam is an x-ray beam with high penetrability and x-ray beam with low penetrability is referred to as soft x-ray beam. The quality of x-ray beam is expressed in terms of the half value layer (HVL). The HVL is the thickness of a material required to reduce the intensity of an x-ray beam to half of its original value. The test was performed with the same setup shown in Figure 3.1. One exposure was taken and the HVL was automatically given by the solid state ionization chamber (Piranha 657) used. However this can be done manually by using radiation measuring instrument (RMI) or meter. The meter is placed at a 100 cm source-to-surface distance (SSD). The x-ray beam is collimated to the sensitive part of the meter. With a set kVp and mAs combination, an exposure is taken three times. Another set of exposure is taken by introducing aluminium attenuator 1 mm thick into the primary x-ray beam on the collimated area of the meter. A varying thickness of aluminium are added until the exposure falls to below 50% of the initial un-attenuated value of the beam intensity. A graph of exposure/current-time ratio as a function of aluminium thickness is plotted. From the graph, the thickness of Al required to reduce the intensity of the x-ray beam to its initial value is the half value layer (HVL) of the radiation beam. The HVL can also be determined mathematically by using the following equation: HVL ln 2 3.4 where is the linear attenuation coefficient of the absorbing material. The results of this test for the various CR systems employed. 32 University of Ghana http://ugspace.ug.edu.gh 3.4.5 Current-Time Linearity The same setup shown in Figure 3.1 was used to perform this test. Several exposures with constant kVp and variable mAs were taken. The output for each CR system was recorded. Each output measured was divided by the corresponding mAs. The mAs linearity variance (LV) was calculated using the following equation: Output / mAs Output / mAs max min LV 2 Output / mAs avg 3.5 where Output / mAs max , Output / mAs min and Output / mAs avg are respectively the maximum, minimum and average values of the tube output per mAs recorded. 3.4.6. Collimation and Beam Alignment The collimator and beam alignment test tools shown in plates 3.4 (a and b) were used to assess the alignment of the x-ray field and how perpendicular the x-ray beam to the image receptor. The collimator tool was placed at 100 cm SSD on the x-ray table under the x-ray tube. The beam alignment tool was placed at the centre of the collimator tool. The collimator shutters were adjusted in such a way that the edges of the light field coincide with the rectangular outline on the collimator tool. That is, when an image of the test tool is generated, the image of the two steel balls overlaps only when the beam is accurately perpendicular to the image receptor. If an x-ray field falls within the image of the rectangular frame, there is a good alignment. If an edge of the x-ray field falls on the first spot, 1.0 cm , on either side of the line, it shows that the edges of the x-ray field are misaligned by a percentage of 1%. 33 University of Ghana http://ugspace.ug.edu.gh Plate 3. 3: (a) Beam alignment tool (b) Collimator tool 3.4.7. Tube Output Measurement This test was also performed using the setup presented in Figure 3.1. Several exposures were taken with variable tube voltage (40 kVp to 150 kVp with interval of 10 kVp) and variable mAs (50 mAs to 2 mAs). A ratio of exposure/current-time product was calculated for each kVp set. A graph of exposure/current-time product (mGy/mAs) as a function set tube voltage (kVp) was plotted. From this graph, the output (mGy/mAs) of each kVp used for each type of examination was calculated . 3.4. Patient Data Collection To collect various information needed to estimate the ESD of each type of radiographic examination considered, a data sheet was designed. The data sheet captures information such as area of examination (e.g. Chest), patient age, sex, patient thickness, applied tube voltage (kVp), current-time product (mAs) and focus-to-film distance (FFD) for each type of examination. The data sheet was used to record patient data and technique factors for each examination done. Patient thickness was measured using a pair of callipers. Focus-to-film distance was measured by employing tape 34 University of Ghana http://ugspace.ug.edu.gh measure (5 m long). The setup for each examination is presented in the schematic diagram in Figure 3.2 Figure 3.2: Schematic diagram of setup for measurement. 3.5. Exposure Factors and Techniques Exposure parameters and patient data were collected for a total of 619 patients with 243 males representing 39.3% and 376 females representing 60.7% for the two hospitals. The minimum and maximum age of the patients that formed part of this study were 18 and 82 years. This age group is considered adult due to the fact that it is within the adult age category by the International Commission on Radiological Protection (ICRP publication 101a, 2006). Summary of results of ESDs estimated, exposure parameters and patient information for individual patient exposures are 35 University of Ghana http://ugspace.ug.edu.gh presented in Tables A1 and A2 in Appendix A. For a good radiographic technique for the projections chosen , the European guidelines on quality criteria for diagnostic radiographic images requires 125 kVp, 180 cm FFD for chest; 70 – 90 kVp, 115 cm FFD for lumbar spine AP; 80 – 100 kVp, 115 cm FFD for lumbar spine LAT; 70 -85 kVp, 115 cm FFD for skull AP/PA and LAT; 70 kVp, 115 cm for cervical spine AP/PA and LAT; and 70 -100 kVp, 115cm for abdomen (European Commission, 1996). 3.6. ESD Estimation The ESD received by each patient undergoing each type of examination considered in this study was calculated by using the following equation (IAEA: TRS-457, 2007) : ESD ESAK BSF 3.6 where ESAK is the entrance surface air kerma given by 100 ESAK Output (mGym A s ) mAs FSD 1 1 1 2 3.7 FSD is the focus-to-skin distance which is obtained by subtracting the patient thickness from the focus-to-film distance. That is FSD FFD t p where 3.8 t p is the patient thickness. The output measurement of each patient examination is obtained from the output/current-time ratio (mGy/mAs) versus tube voltage (kVp) graph. A backscatter factor of 1.35 was used. This is because according to the European commission, (1996), backscatter factors vary between 1.3 and 1.4 for the x-ray qualities used for various projections included in the quality criteria except for mammography. Therefore, a single average value of 1.35 can be used in most cases without appreciable error. The results were then compared to the diagnostic reference 36 University of Ghana http://ugspace.ug.edu.gh levels (DRLs) set by international organisations like the International Atomic Energy Agency and Public Health of United Kingdom. 3.7. Image Quality Assessment A sample of ten images were acquired for each type of examination from each CR system employed. Image quality was assessed using ImageJ software (version 1.48). This is an image processing software developed by National Institutes of Health. It works by using objective physical characteristics of the imaging system such as spatial resolution, contrast and noise. The images were assessed by measuring the CNR and the modulation transfer function (MTF) which determines the spatial resolution of the imaging system. To calculate CNR, two ROIs were marked on the images using a bespoke software (ImageJ). ROI1 was applied mid-way of the vertebral body (maximum density) and ROI2 in the lung region with a homogenous density (minimum density). The formula in equation 3.9 was used to the estimate CNR: CNR Mean signal of bone ( ROI 1) Mean signal of tissue ( ROI 2) S tan dard deviatio of ROI 2 3.9 The results obtained were then compared to the kVp at which each examination was performed and the corresponding ESD. The SNR was determined by drawing two ROIs; one on area of higher density and the other on the background of the image. The SNR was then calculated using equation 3.10: SNR Mean signal value within ROI S tan dard deviation within ROI 37 3.10 University of Ghana http://ugspace.ug.edu.gh CHAPTER FOUR RESULTS AND DISCUSSION This chapter presents results of the study and discussions of the findings arising from the analysis of the results. The results and discussions cover; quality control, x-ray beam output, patient ESDs, image quality (CNR and SNR) measurements. 4.1. Quality Control (QC) on CR Systems To obtain a good balance between the dose received by patients undergoing radiographic examinations and the corresponding image quality, the assessment of physical operations of the x-ray systems is very essential. Results obtained from assessing the CR systems performance are presented in Tables 4.1 and 4.2 for KBTH and UGH respectively. The applied voltage across x-ray tube during examination has effect on beam quality, image quality and the corresponding patient dose. The accuracy and reproducibility of the tube voltage selected on the control console is therefore important for a proper exposure technique selection. Table 4.1 and 4.2 show that the tube voltage accuracy and reproducibility of the CR systems were within the acceptable limit (≤ ± 6.0 % and ≤ 0.05) set by the Institute of Physics and Engineers in Medicine. The exposure timer accuracy was also within the acceptable limit of ≤ ±10% set by the Institute of Physics and Engineers in Medicine (IPEM) (IPEM Report 91, 2005). This shows that the x-ray tube generators were capable of terminating exposure after the preselected time interval has elapsed. The kVp, timer and exposure reproducibility for both CR systems were also below the limit of ≤ 0.05 set by IPEM (IPEM Report 91, 2005). Therefore, the results show that the performance of CR systems were consistent. 38 University of Ghana http://ugspace.ug.edu.gh Table 4.1: Summary of QC results on CR at KBTH Acceptable Parameter Deviation of Deviation by CR system (IPEM) Remarks Tube Voltage Accuracy -1.900% ≤ ± 6.000 % Pass Timer Accuracy 4.490% ≤ ±10.000% Pass 0.001 ≤ 0.050 Pass 0.011 ≤ 0.050 Pass 0.002 ≤ 0.050 Pass Tube Voltage Reproducibility at 80kVp (Coefficient of Variation) Timer Reproducibility at 80kVp (Coefficient of Variation) Exposure Reproducibility at 80kVp (Coefficient of Variation) Half Value Layer at 70 kVp Current-time Linearity 3.230 mm Al 0.026 ≥ 2.100 mm Al Pass ≤ 0.100 Pass The half value layers of the CR systems as observed in Tables 4.1 and 4.2 were respectively 3.23 mmAl and 5.05 mmAl at KBTH and UGHs. Comparing these values to the acceptable limit of ≥ 2.10 mmAl shows that the x-ray beams were of good quality. Hence, much of the x-rays with lower energies which do not form part of image but contributes to patient dose were eliminated. The two hospitals considered did not have a quality assurance (QA) and QC programmes in place. This shows that with proper QA/QC programmes in place, there is the potential to achieve dose optimisation and patient protection with good or even perfect image quality. 39 University of Ghana http://ugspace.ug.edu.gh Table 4.2: Summary of QC results on CR at UGH Acceptable Deviation of Parameter deviation by Remarks CR system (IPEM) Tube Voltage Accuracy -1.150% ≤ ± 6.000 % Pass Timer Accuracy -5.250% ≤ ±10.000% Pass 0.006% ≤ 0.050 Pass 0.014 ≤ 0.050 Pass 0.032 ≤ 0.050 Pass Tube Voltage Reproducibility at 80kVp (Coefficient of Variation) Timer Reproducibility at 80kVp (Coefficient of Variation) Exposure Reproducibility at 80kVp (Coefficient of Variation) Half Value Layer at 81 kVp 5.050 Current-time Linearity 0.051 ≥ 2.100 mm Al Pass ≤ 0.100 Pass 4.2. X-Ray Output Measurement Figures 4.1 and 4.2 show the relationship between the x-ray output (mGy/mAs) and the applied tube voltages (kVp) at a constant tube current-time product (mAs) of the CR systems used in this study. R2 values of 0.9856 and 0.9971 observed from the output curves of Figures 4.1 and 4.2 show that there exist a strong correlation between the parameters measured and thus show a good fit. The nature of the graph is also in good agreement with the graph obtained from output versus kilovoltage obtained from the work of Tamboul et al., (2014). 40 University of Ghana http://ugspace.ug.edu.gh 0.12 Output (mGy/mAs) = 2E-06kVp2.2066 R² = 0.9856 0.1 Tube Output (mGy/mAs) 0.08 0.06 0.04 0.02 0 0 20 40 60 80 100 Tube Voltage (kVp) Figure 4.1: Output curve of CR system at KBTH 41 120 140 160 University of Ghana http://ugspace.ug.edu.gh 0.06 Output (mGy/mAs) = 2E-05kVp1.7118 R² = 0.9971 0.05 Output (mGy/mAs) 0.04 0.03 0.02 0.01 0 0 20 40 60 Set voltage (kVp) Figure 4. 2: Output curve of CR system at UGH 42 80 100 120 University of Ghana http://ugspace.ug.edu.gh 4.3. Summary of Examinations Ionizing radiation is used daily in hospitals and clinics to perform diagnostic imaging procedures. The largest source of exposure to the Ghanaian population is due to artificial sources with more than 95% resulting from diagnostic x-ray examinations (Inkoom et al., 2012). This is due to the common use of x-rays in conventional, computed and direct digital radiography systems. Figure 4.3 and 4.4 present the summary of the number of each examination. It was observed from Figures 4.3 and 4.4 that 34 % of all radiographic examinations conducted in these hospital were chest examinations. This confirms the fact that medical chest x-ray examination is the most common radiological examination performed in Ghana today. Abdomen, 10% Skull LAT, 6% Chest PA, 34% Skull AP, 6% Cervial spine LAT, 9% Cervial spine AP, 8% Lumber spine LAT, 14% Lumber spine AP, 13% Figure 4.3: Chart showing the percentage of each examination at KBTH Chest x-ray examination forms 46.4% of all x-ray examinations in Ghana (Schandorf and Tettey, 1998) and 40 % of all x-ray examinations conducted worldwide (WHO, 43 University of Ghana http://ugspace.ug.edu.gh 2016). It is also evident from the Figures 4.2 and 4.3 that skull x-ray is the least requested examination which forms on 6 % of all the examinations considered for each of the hospitals. This is due to the fact that skull examination is primarily conducted in these hospitals on accident patients to determine whether there is a fracture in the skull and then also most head (skull) scans are done using computed tomography. Abdomen, 7% Skull LAT, 6% Skull AP, 6% Chest PA, 33% Cervial spine LAT, 12% Cervial spine AP, 12% Lumber spine AP, 12% Lumber spine LAT, 12% Figure 4. 4: Chart showing the percentage of each examination at UGH 4.4. ESD Estimation Figure 4.5 compares the distribution of the mean ESDs obtained from the hospitals. Tables 4.3 and 4.4 in addition to mean ESD, present 1st quartile, 3rd quartile, maximum and minimum ESDs and standard deviation values of means of the ESDs for each anatomical projection considered with their standard deviations. The highest mean 44 University of Ghana http://ugspace.ug.edu.gh ESD of 3.01 mGy and 6.08 mGy were estimated for lateral projection of lumbar spine for KBTH and UGH respectively. There was a difference of 3.07 mGy for lumbar LAT projection between the two hospitals which shows that the mean ESD recorded at UGH was higher than that of KBTH by a factor of 2.02. Lumbar spine AP projection recorded the second highest mean ESD for UGH which was by a factor of 3.57 higher the corresponding mean ESD recorded for KBTH. The third highest mean ESD was recorded at UGH for abdomen also by a factor of 1.70 was higher than that recorded at KBTH. However, the mean ESDs recorded for skull AP projection for both hospitals were almost the same with a difference of 0.04 mGy. A lower mean ESD of 0.29 mGy was recorded for cervical spine AP and LAT projections at KBTH. The values of mean ESD were lower by factors of 1.76 and 2.07 as compared to that recorded at UGH for both AP and LAT projections respectively. For chest examination, there was a difference of 0.1 mGy between the mean ESDs recorded at KBTH and UGH. This shows that the mean ESD recorded at KBTH was by a factor of 1.30 higher compared to that of UGH of 0.33±0.01 mGy. Comparing the 1st quartile values for chest examinations in the two hospitals shows that the dose recorded at KBTH was by a factor of 1.19 higher than the dose recorded at KBTH. The 3rd quartile values were as the same as the mean values were the dose recorded at UGH was lower than the dose recorded at KBTH by a factor of 1.30. 45 University of Ghana http://ugspace.ug.edu.gh 6.08 Mean ESD (mGy) 3.5 3.01 2.62 1.7 1.74 1.54 0.98 0.33 Chest PA 0.53 0.43 Lumber spine AP 0.29 Lumber spine LAT 0.72 0.6 0.51 Cervical spine AP Cervical spine LAT UGH KBTH 0.29 Skull AP Skull LAT Abdomen Figure 4. 5: Chart comparing the mean ESDs from the two hospitals 46 University of Ghana http://ugspace.ug.edu.gh Table 4.3: Estimated mean ESD at KBTH Area of Examination ESD (mGy) 1st quartile Mean 3rd quartile Max Min StdDev Chest PA 0.38 0.43 0.43 0.71 0.12 0.1022 Lumbar spine AP 0.80 0.98 1.05 1.57 0.63 0.2594 Lumbar spine LAT 2.91 3.01 3.05 4.19 2.46 0.2447 Cervical spine AP 0.29 0.29 0.30 0.30 0.29 0.0042 Cervical spine LAT 0.29 0.29 0.30 0.30 0.29 0.0041 Skull AP 1.77 1.74 1.83 1.87 1.19 0.1945 Skull LAT 0.52 0.53 0.54 0.54 0.52 0.0090 Abdomen 1.32 1.54 1.39 3.49 1.27 0.6107 *Max is maximum, Min is minimum and StdDev is standard deviation Table 4. 4: Estimated mean ESD at UGH Area of Examination ESD (mGy) 1st quartile Mean 3rd quartile Max Min StdDev Chest PA 0.32 0.33 0.33 0.36 0.30 0.01 Lumbar spine AP 3.39 3.50 3.65 3.99 3.03 0.23 Lumbar spine LAT 5.81 6.08 6.32 7.12 4.50 0.55 Cervical spine AP 0.49 0.51 0.53 0.59 0.44 0.03 Cervical spine LAT 0.56 0.60 0.63 0.71 0.51 0.05 Skull AP 1.67 1.70 1.73 1.79 1.65 0.04 Skull LAT 0.72 0.72 0.73 0.74 0.71 0.01 Abdomen 2.50 2.62 2.70 2.96 2.45 0.15 47 University of Ghana http://ugspace.ug.edu.gh Table 4.3 also shows that a patient received a maximum dose as 0.71 mGy during chest examination at KBTH as compared to 0.36 mGy of UGH for the same type of examination. In general, Figure 4.4 further shows that all the mean ESDs recorded at UGH were higher than their corresponding mean ESDs recorded at KBTH except for chest PA and skull AP projections. The differences in the doses may be attributed to the different exposure factors and techniques being employed by the radiographers at the hospitals. The difference in the doses may also be due to variations in the number of examinations per day and the thickness of the patients being examined. 4.5. Comparison of ESDs with other Published Results The mean ESDs obtained from this study have been compared to other published data and diagnostic reference levels (DRLs) set by international organizations (Public health of UK and IAEA). Figure 4.6 shows a bar chart for graphical comparison of the two hospitals. The Figure shows that the highest means ESD was recorded for the lateral projection of lumbar spine examination for both hospitals but they were however lower than their corresponding mean ESDs from literature. At UGH, the mean ESD was lower than that of Inkoom et al., (2012) by a factor of 1.61, 1.64 and 2.47 for UK (Public Health England, 2016) and IAEA (Rehani, 2001) respectively. For the same lumbar LAT projection, KBTH recorded a lower value by a factor of 3.24, 3.32 and 4.98 for Inkoom et al., UK and IAEA respectively. Figure 4.6 further shows that all the mean ESDs for the various examinations were lower than their corresponding published data and DRLs except for chest examinations in which the ESDs (0.43 mGy for KBTH and 0.33 mGy for UGH) obtained from this study for both hospitals were higher than their corresponding published data and reference levels by UK and IAEA. There was a 48 University of Ghana http://ugspace.ug.edu.gh difference of 0.19 mGy, 0.18 mGy and 0.13 mGy between the estimated mean ESD for chest examination at UGH and Inkoom et al., (2012) and the DRLs set by UK (Public Health England, 2016) and IAEA (Rehani, 2001) respectively. The difference shows that the mean ESD recorded at UGH was 135.71%, 117.86 % and 63.40 % higher than the ESDs obtained by Inkoom et al., and the DRLs set by the Public health of UK and the IAEA respectively. A percentage increment of 207.14%, 188.92 % and 116.68 % were found in the estimated mean ESD for chest examination at KBTH for Inkoom et al., (2012), UK and IAEA respectively. This differences was due to the selection of exposure parameters and technique factors including the coning of the xray beam. Other reason for the large variation in the mean ESD for chest may be due to patient size, suboptimal usage of the equipment or equipment malfunctioning generally because of the scarceness of regular quality control and radiation protection programme. These differences suggest that much can be done to reduce the ESDs by adequate changes of physical parameters without loss of image quality. Therefore, the radiology departments of the hospitals should undertake a review of their radiographic practice in order to bring their doses to optimum levels. 49 University of Ghana http://ugspace.ug.edu.gh 18.00 16.00 14.00 Mean ESD (mGy) 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Chest PA UGH Lumber spine AP Lumber spine LAT KBTH Cervical spine AP Inkoom et al., 2012 Cervical spine LAT UK (2016) Skull AP Skull LAT Abdomen IAEA (2001) Figure 4.6: Comparison of ESD at KBTH with published results and DRLs from international organizations. 4.6. Image Quality Assessment Image quality is one of the most important aspect of radiology. The aim of radiology is to acquire images which contain adequate information for clinical purpose with minimum radiation dose to the patient. For optimization to be achieved, image quality assessment must be made to balance the patient dose. To assess the image quality of 50 University of Ghana http://ugspace.ug.edu.gh radiographic examinations, CNR and SNR have been calculated using values generated from ImageJ software version 1.48. 4.6.1. Contrast-to-Noise Ratio CNR is a measure for assessing the ability of an imaging procedure to generate clinically useful image contrast. CNR was calculated for ten (10) radiographs for each type of anatomical projection considered. The mean and standard deviation values of the calculated CNR have been presented in Table 4.5. Figure 4.7 presents the graphical representation of the mean CNR calculated for both hospitals. Table 4.5: Mean CNR calculated from images obtained for KBTH and UGH Mean CNR Area of Examination KBTH UGH kVp KBTH UGH Chest 12.8 + 5.77 6.15 +3.14 62.0 - 68.0 125 Cervical spine AP 13.64 + 4.11 10.89 + 3.47 60.0 - 62.0 66.0 - 70.0 Cervical spine LAT 19.18 + 6.78 12.89 + 4.56 60.0 - 62.0 66.0 - 70.0 Lumbar spine AP 11.48 + 8.01 8.97 + 3.6 64.0 - 74.0 77.0 - 85.0 Lumbar spine LAT 3.79 + 11.97 4.81 + 3.51 80.0 - 95.0 96.0 - 98.0 Skull AP 7.43 + 3.02 9.89 + 3.87 73.0 - 75.0 75.0 - 78.0 Skull LAT 20.63 + 8.65 3.54 + 3.27 62.0 - 64.0 70.0 - 74.0 51 University of Ghana http://ugspace.ug.edu.gh 25.00 20.00 CNR 15.00 10.00 5.00 0.00 Chest Cervical spine AP Cervical spine LAT Lumber spine AP KBTH Lumber spine LAT Skull AP Skull LAT UGH Figure 4. 7: Comparative bar chart of mean CNR obtained from KBTH and UGH The results in Table 4.5 and Figure 4.7 show that the highest CNR of 20.63 + 8.65 was obtained for skull LAT projection at KBTH where the image was acquired at a kVp range of 62 to 64 kVp as against 3.54 + 3.27 where the examination was done at a kVp range of 70 to 74 kVp for the same projection at UGH. The CNR recorded for skull LAT projection was also the lowest among all the projections considered. A difference of 17.09 between CNR for skull LAT projection accounted for the CNR of KBTH 52 University of Ghana http://ugspace.ug.edu.gh being higher than that recorded at UGH by a factor of 5.83. The second highest CNR was recorded for cervical LAT projection at KBTH where the CNR recorded was by a factor of 1.48 higher than that recorded at UGH. A similar higher CNR was recorded at KBTH over their corresponding values obtained at UGH for chest PA, cervical AP, and lumbar AP. In all the three examinations, the kVp used to acquire images at KBTH were lower than their corresponding kVp at UGH as shown in Table 4.6. However, UGH recorded a higher CNR of 13.32 + 9.37 as compared with 4.63 + 0.48 recorded at KBTH for skull AP projection even though the range of kVp used for this examination was higher at UGH than kVp used at KBTH for the same examination. The CNR recorded for skull AP examination at KBTH was by a factor of 2.88 lower than the CNR recorded at UGH. The same trend of higher CNR was recorded for lumbar spine lateral projection at UGH as compared to CNR recorded at KBTH. Here also, the kVp range used to acquire the image at UGH were higher than that used at KBTH according to table 4.6. Generally, Figure 4.7 and table 4.6 show that the CNR obtained at KBTH for all the examinations were higher than the CNR recorded at UGH except for skull AP and lumbar spine LAT projection where higher CNR was recorded at UGH as against KBTH. The variation in CNR between the two hospitals is mainly due to the difference in kVp used for the various examination. A similar result was obtained by Hess and Neitzel, (2012) who determined the CNR for paediatric extremities using kVp range of 40 kVp to 60 kVp. According to the authors, the highest CNR was obtained at 40 kVp. Also, the use of higher kVp is a well-known strategy to reduce dose on paediatrics, but at the same time decreases the CNR (Reis et al., 2014). The results of this study supports the assertion that the use higher kVp generally decreases the CNR. This is because with lower kVp, there is less scatter radiation reaching the detector with increasing mAs. Contrast is the most significant 53 University of Ghana http://ugspace.ug.edu.gh factor influencing the choice of tube potential for imaging different parts of the human body with different attenuations. The CNR obtained for objects containing several hundred pixels like radiograph has the potential to provide a useful parameter for comparing imaging performance for x-ray beams with different beam qualities. The relationship between the CNR measurements and tube potential in general reflects the variation in the number of details that can be detected (Doyle, 2008). 4.6.2. Signal-To-Noise Ratio One of the most common indicator of image quality is the SNR. The ability to detect an object (and hence resolve it from its neighbour) is related to the signal to noise ratio (SNR) of the object. The common way to quantify the level of noise in an image is to estimate the SNR. SNR has been calculated for ten images which were used to determine the CNR. Table 4.6 and Figure 4.8 present the results obtained. The results presented in Table 4.6 and Figure 4.8 show that the highest SNR of 22.35± 0.86 was recorded for chest PA examination at KBTH as against 9.10 ± 0.81 SNR of the same anatomical projection at UGH. This shows that SNR recorded at KBTH was 146% more than the SNR recorded at UGH. The second SNR was also recorded at KBTH which was by a factor of 1.07 higher than the corresponding SNR recorded at UGH for cervical spine AP anatomical projection. A similar trend of KBTH having higher SNR than the SNR recorded at UGH was found for lumbar spine AP/LAT projections where the SNRs recorded at UGH were by factors of 1.43 and 1.11 lower than the SNR recorded at KBTH for lumbar AP/LAT projections respectively. 54 University of Ghana http://ugspace.ug.edu.gh Table 4.6: Calculated SNR for the KBTH and UGH Mean SNR Area of Examination KBTH UGH Chest 22.35 ± 0.86 9.10 ± 0.81 Cervical spine AP 17.99 ± 7.67 16.81 ± 7.1 Cervical spine LAT 12.92 ± 3.93 17.58 ± 10.75 Lumbar spine AP 15.48 ± 1.27 10.85 ± 8.32 Lumbar spine LAT 13.59 ± 8.00 12.28 ± 3.46 Skull AP 4.63 ± 0.48 13.32 ± 9.37 Skull LAT 6.10 ± 3.60 14.17 ± 9.99 However, higher SNRs were recorded at UGH over KBTH for cervical LAT and skull AP/LAT projections which accounted for 36.07%, 187.69% and 132.30% increase in SNR for cervical LAT, skull AP/LAT respectively at UGH over the SNR at KBTH. To be able to detect objects in a medical image, the threshold SNR is 5 (Bath, 2010). This threshold was developed by Rose Albert who was interested to find the threshold value of SNR of an object to be visible by human observer and this is known as the Rose Model. The assumption of the Rose Model is that the factor that limits the detection of an object in a radiographic image is the radiation dose used to produce the image (IAEA: STI/PUB/1564, 2014). Comparing the results obtained to the Rose Model show that 92.86 % of the images assessed had SNR greater than 5 for both hospitals except the SNR recorded for skull AP projection at KBTH which had a value lower than 5. This shows that clinicians would be able to extract useful information from these images and there is the potential of reducing the dose patient received undergoing the examinations considered. 55 University of Ghana http://ugspace.ug.edu.gh 25.00 20.00 SNR 15.00 10.00 5.00 0.00 Chest Cervical spine AP Cervical spine LAT Lumber spine AP KBTH Lumber spine LAT Skull AP Skull LAT UGH Figure 4.8: Comparative bar chart of SNR obtained from the two hospitals The differences in the SNR is due to the kVp and mAs factors used for the examination. This is because the selection of exposure parameters such as the increase of kVp and mAs result in more signal reaching the detector that should reduce the noise in the image and improve the SNR. 56 University of Ghana http://ugspace.ug.edu.gh 4.7. Optimization of Patient Protection In order to ensure dose and image quality optimisation for the various examinations, the CNR was plotted as a function of the ESD for all examinations as shown in Figures 4.9 to 4.16. Optimisation technique employed in a radiology departments usually consists of selections of tube potential, filtration and method of scatter removal and there is a need to find an image quality parameter which can be used in clinical imaging tasks to compare and evaluate different options. CNR has been examined as such a parameter in this study. For a radiograph to be acceptable for diagnosis, it is dependent on its ability to display the correct anatomical part being imaged with optimum levels of CNR. 14 12 10 CNR 8 6 4 2 0 0 0.1 0.2 0.3 0.4 ESD (mGy) Figure 4. 9: CNR as a function of ESD (mGy) for Chest PA 57 0.5 0.6 University of Ghana http://ugspace.ug.edu.gh 18 16 14 12 CNR 10 8 6 4 2 0 0.6 0.8 1 1.2 1.4 1.6 1.8 ESD (mGy) Figure 4. 10: CNR as a function of ESD (mGy) for Lumbar spine AP 7 6 5 CNR 4 3 2 1 0 2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 ESD (mGy) Figure 4. 11: CNR as a function of ESD (mGy) for Lumbar spine LAT 58 4.3 University of Ghana http://ugspace.ug.edu.gh 20 18 16 CNR 14 12 10 8 6 4 0.284 0.286 0.288 0.29 0.292 0.294 0.296 0.298 0.3 ESD (mGy) Figure 4. 12: CNR as a function of ESD (mGy) for cervical spine AP 20 19 18 CNR 17 16 15 14 13 12 0.2 0.22 0.24 0.26 0.28 0.3 ESD (mGy) Figure 4. 13: CNR as a function of ESD (mGy) for cervical spine LAT 59 0.32 University of Ghana http://ugspace.ug.edu.gh 16 15 14 13 CNR 12 11 10 9 8 7 6 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 ESD (mGy) Figure 4. 14: CNR as a function of ESD (mGy) for Skull AP 9 8.5 8 7.5 CNR 7 6.5 6 5.5 5 4.5 4 0.35 0.36 0.37 0.38 0.39 ESD (mGy) Figure 4.15: CNR as a function of ESD (mGy) for Skull LAT 60 0.4 0.41 University of Ghana http://ugspace.ug.edu.gh 12 11 10 CNR 9 8 7 6 5 4 1 1.5 2 2.5 3 3.5 4 ESD (mGy) Figure 4. 16: CNR as a function of ESD (mGy) for Abdomen AP In general, the relationship between the CNR measurements and tube potential reflects the variation in the detectability of objects in the image. From figures 4.9 to 4.16, the level of CNR as a measure of image quality as well as low dose to patient has been summarized in the Table 4.7. When the values of CNR and ESD presented in Table 4.7 are implemented in the clinical setting, the ESD for chest PA examination can be reduced by 26.9 % at UGH and 65.4% 61 University of Ghana http://ugspace.ug.edu.gh Table 4. 7: Optimized level of ESD and CNR Type of Examination CNR ESD (mGy) Chest PA 11.60 0.26 Lumbar spine AP 6.40 0.84 Lumbar spine LAT 3.60 3.26 Cervical spine AP 17.60 0.30 Cervical spine LAT 19.00 0.24 Skull AP 15.60 0.92 Skull LAT 4.60 0.38 Abdomen AP 10.65 0.26 62 University of Ghana http://ugspace.ug.edu.gh CHAPTER FIVE CONCLUSION AND RECOMMENDATION This chapter provides the conclusions drawn from this study and recommendations on the estimation of ESD and image quality assessment of adult patients undergoing CR examination in Ghana. 5.1. Conclusion The mean ESD estimated for all the examinations considered under this study was in the range of 0.290±0.004 mGy to 6.08±0.55 mGy which was due to the different exposure parameters and technique factors used for various examinations in the two hospitals considered. The exposure parameters used for the examinations considered were generally higher at UGH as compared to the exposure parameters used at KBTH. Mean ESD of 6.08±0.55 mGy was obtained for lumbar LAT anatomical projection at UGH was the highest among all the ESDs estimated. However, a lower mean ESD of 0.290±0.004 mGy was recorded for cervical AP/LAT projection at KBTH. The mean ESD obtained were at least lower compared to other published results and DRL set by international organization except the mean ESD for chest examination which was higher than the other published results. The CNR calculated was in the range of 3.53±3.27 to 20.63±8.65. The results showed that the CNR obtained for KBTH were generally higher than the CNR obtained at UGH. The differences were mainly due to the use of different kVp for the examinations. It was also found that lower kVp increases CNR which in turn decreases 63 University of Ghana http://ugspace.ug.edu.gh patient dose. The lowest CNR was recorded for skull LAT projection at UGH whilst the highest CNR was also recorded for the same projection at KBTH. The SNR was calculated as a means of quantifying the level of noise in the images acquired from the CR systems used. There was a higher SNR of 22.35 ± 0.86 for chest PA examination at KBTH and a lower SNR of 4.63 ± 0.48 which also occurred at KBTH for skull AP projection. The results obtained was compared to the Rose model where the limit of SNR was set at 5 . All the results obtained were greater than 5 except skull AP examination at KBTH. The results therefore showed that the CR systems produce images that contain useful clinical information. The values of the CNR and SNR can be used as the baseline for future quality control monitoring and research. The plot of CNR against the ESDs for all the examinations showed that there is a potential to reduce doses to patients while keeping images of diagnostic quality. 5.2 Recommendations Hospital Authorities There should be a comprehensive QA/QC programme in place and this should include training of staff in effective use of equipment and approved imaging protocols. Continuous checks and maintenance culture of the CR systems must be established. There should be standardized imaging protocols established to be used uniformly by radiographic staff in order to avoid the risk of unnecessary exposure to patients and staff. The findings of this work should be used to develop institutional level optimization of protection of patients consistent with clinically acceptable image quality. 64 University of Ghana http://ugspace.ug.edu.gh Further Work Further research should be conducted to extend work to optimization of patient dose while keeping images of diagnostic quality acceptable to radiologist and referring clinicians. Future studies should be extended to other hospital that uses CR systems for examination and this should include patient dose to paediatric patients who are more sensitive to radiations than adults. Regulatory Authority. The regulatory authority should provide applicable regulations and guidance document on radiation protection and safe use of CR in Ghana. 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IAEA-CN-96-46P, 92–93. http://doi.org/10.1063/1.3047411 71 University of Ghana http://ugspace.ug.edu.gh APPENDIX A Table A1: Summary of individual patient exposures at Korle-Bu Teaching Hospital Area of Examination Patient Males Females kVp mAs FFD FSD ESD range range (cm) (cm) (mGy) Thickness (cm) Age (yrs) Chest PA 17.2 - 23.8 18.0 - 81.0 33 72 62.0 - 68.0 12.5 - 18 180 156.2 - 162.8 0.12 - 0.71 Cervical spine AP 10.9 - 13.0 19.0 - 81.0 8 18 60.0 - 62.0 10.0 - 12.0 100 87.0 - 89.1 0.29 - 0.30 Cervical spine LAT 10.9 - 13.4 19.0 - 81.0 9 19 60.0 - 62.0 10.0 - 12.0 100 87.0 - 89.1 0.29 - 0.30 Lumbar spine AP 16.6 - 24.4 19.0 - 80.0 13 26 64.0 - 74.0 16.0 - 25.0 100 75.6 - 83.4 0.63 - 1.57 Lumbar spine LAT 20.5 - 26.5 21.0 - 80.0 17 26 80.0 - 95.0 36.0 - 40.0 100 73.2 - 79.5 2.46 - 4.19 Skull AP 18.0 - 20.4 19.0 - 63.0 7 12 72.0 - 75.0 22.0 - 32.0 100 79.6 - 82.0 1.19 - 1.87 Skull LAT 16.0 - 18.0 19.0 - 63.0 7 12 62.0 - 64.0 14.0 -16.0 100 82.0 - 84.0 0.52 - 0.54 Abdomen 17.1 - 23.7 19.0 - 72.0 9 22 73.0 - 83.0 25.0 - 45.0 100 76.3 - 82.9 1.27 - 3.49 72 University of Ghana http://ugspace.ug.edu.gh Table A2: Summary of individual patient exposures at UGH Area of Examination Patient kVp mAs FFD FSD ESD Thickness (cm) Age (yrs) Males Females range range (cm) (cm) (mGy) Chest PA 11.0 - 27.0 18.0 - 81.0 49 53 125 2.5 180 153.0 - 169.0 0.30 - 0.36 Cervical spine AP 10.0 - 20.0 21.0 - 82.0 14 23 66.0 - 70.0 10.0 - 12.0 100 80.0 - 90.0 0.44 - 0.59 Cervical spine LAT 17.0 - 27.0 21.0 - 82.0 14 23 66.0 - 70.0 10.0 - 12.0 100 73.0 - 83.0 0.51 - 0.71 Lumbar spine AP 18.0 - 24.3 23.0 - 81.0 19 18 77.0 - 85.0 40.0 - 50.0 100 75.7 - 82.0 3.03 - 3.99 Lumbar spine LAT 21.0 - 31.5 23.0 - 81.0 19 18 96.0 - 98.0 40.0 - 50.0 100 68.5 - 79.0 4.50 - 7.12 Skull AP 18.0 - 21.5 19.0 - 73.0 8 11 75.0 - 78.0 32.0 - 34.0 100 88.5 - 92.0 1.65 - 1.79 Skull LAT 16.6 -18.0 19.0 - 73.0 8 11 70.0 - 74.0 16.0 - 18.0 100 92.0 - 93.4 0.71 - 0.74 Abdomen 17.5 - 24.4 24.0 - 74.0 9 12 77.0 - 85.0 40.0 - 50.0 100 85.6 -92.5 2.45 - 2.96 73 University of Ghana http://ugspace.ug.edu.gh APPENDIX B QC RESULTS AT KBTH Table B.1: kVp accuracy results Set voltage Recorded kVp Exposure Exposure time (kV) voltage (kV) difference (mGy) (ms) 50 49.0407 -1.9186 0.0871 19.5880 60 59.2908 -1.1819 0.1474 19.5740 70 69.0489 -1.3587 0.2138 19.5926 80 78.7921 -1.5099 0.2871 19.5837 90 88.5723 -1.5864 0.3694 19.5833 100 98.5291 -1.4709 0.4598 20.0665 110 108.8694 -1.0278 0.5648 24.6019 120 119.3909 -0.5076 0.6720 25.1037 Table B. 2: kVp, Exposure and Timer reproducibility Tube voltage Exposure Exposure time (kV) (mGy) (ms) 78.7248 0.2856 20.0665 78.8805 0.2843 19.5829 78.8092 0.2855 20.0763 78.7437 0.2855 20.0763 78.8939 0.2843 20.0665 74 University of Ghana http://ugspace.ug.edu.gh Table B. 3: mAs Linearity Set mAs Focal spot Exposure (mGy) mGy/mAs 0.50 Small 0.0069 0.0137 1.25 Small 0.0273 0.0219 2.20 Small 0.0553 0.0251 5.00 Small 0.1365 0.0273 11.00 Small 0.3159 0.0287 22.00 Small 0.6581 0.0299 40.00 Small 1.2289 0.0307 63.00 Large 1.9444 0.0309 80.00 Large 2.5308 0.0316 100.00 Large 3.2504 0.0325 Table B. 4: Timer Accuracy Set time (ms) Measured time (ms) % Deviation 125 124.9780 -0.0176 80 79.8100 -0.2375 63 63.2450 0.3889 40 39.6570 -0.8575 32 31.5910 -1.2781 25 24.5650 -1.7400 20 20.0940 0.4700 16 15.5680 -2.7000 12 12.5390 4.4917 6.3 6.0480 -4.0000 75 University of Ghana http://ugspace.ug.edu.gh Table B. 5: Tube Output measurement Set voltage Exposure (kV) Set mAs (mGy) Tube output (mGy/mAs) 40 50.0 0.2272 0.0045 50 40.0 0.3781 0.0095 60 32.0 0.4913 0.0154 70 20.0 0.4417 0.0221 90 16.0 0.5927 0.0370 109 10.0 0.5541 0.0554 117 8.0 0.5081 0.0635 125 6.3 0.4478 0.0711 141 4.0 0.2994 0.0748 150 2.0 0.1954 0.0977 76 University of Ghana http://ugspace.ug.edu.gh APPENDIX C QC RESULTS AT UGH Table C. 1: kVp accuracy and Reproducibility results Set kVp Set mAs Recorded voltage kVp difference (kV) (mAs) (kV) (%) 60 32.0 64.45 7.4 60 26.0 64.18 7.0 70 32.0 73.69 5.3 70 16.0 73.56 5.1 70 8.0 73.29 4.7 81 32.0 88.06 8.7 81 16.0 87.85 8.5 81 8.0 87.08 7.5 Table C. 2: Timer Accuracy and Reproducibility Set time (ms) 49.2 Exposure time (ms) 47.87 Time difference (%) -2.7 24.9 23.15 -7.0 49.2 48.84 -0.7 24.9 23.65 -5.0 12.9 11.58 -10.2 51.8 50.39 -2.7 25.9 24.68 -4.7 12.9 12.08 -6.3 77 University of Ghana http://ugspace.ug.edu.gh Table C. 3: Tube Output measurement kvp mAs Exposure (mGy) Tube Output (mGy/mAs) 60 8 0.2395 0.0183 60 16 0.2634 0.0201 60 25 0.2713 0.0207 70 8 0.3305 0.0252 70 16 0.3664 0.0280 70 25 0.3739 0.0285 80 8 0.4502 0.0343 80 16 0.4909 0.0375 78 University of Ghana http://ugspace.ug.edu.gh APPENDIX D Table D.1: Datasheet used to collect exposure parameters at the hospitals Area of examination Chest AP/ Patient ID 1. 2. 3. 4. 5. 6. 7. 8. 9. 10 Lumbar spine AP/ Patient ID 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Patient Age (yrs) Sex (M/F) Thickness (cm) (V) Tube Current mAs Age Sex Thickness kVp mAs 79 kVp FSD ESAK FFD (cm) mGy (cm) FSD ESAK FFD