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The African Review of Physics (2016) 11:0011 67 Evaluation of Image Quality of Head Computed Tomography at the University of Maiduguri Teaching Hospital, Nigeria K. L. Ibrahim1,*, A. Ahidjo 2, I. C. Nwobi3 and S. M. Abdullahi1 Department of Physics, University of Maiduguri, Maiduguri, Nigeria 2 University of Maiduguri Teaching Hospital, Maiduguri, Borno State, Nigeria 3 Department of Medical Radiography, University of Maiduguri, Nigeria 1 Assessment of image quality of head computed tomography (CT) at University of Maiduguri Teaching Hospital (UMTH) is crucial for diagnosis and treatment of patients with head disease or injury. Therefore, it is an essential practice to evaluate the image quality in order to determine if they meet the established standard criteria for clinical diagnostic task. This paper determined the factors that affect image quality of head CT at UMTH by subjective assessment including image noise and artifacts, it also try to establish a baseline on image quality criteria for the locality. A total of 78 head CT images were retrospectively selected using Philips Brilliance 16-slice 3rd generation CT scanner. Image quality criteria for brain general CT were evaluated based on recommendations by the European guidelines on quality criteria for CT. Image noise and the overall image quality of head CT were graded on a 5-point scale, while artifacts on head CT images were graded on a 3-point scale. The results obtained show that brain CT examinations has a high mean score value of 84% with no significance difference when compared with a published work. Assessment of image noise shows that 87% of head CT images meet acceptable standard for clinical diagnostic task, while that of overall image quality shows that nearly 81% meet acceptable standard for clinical diagnostic task. Artifacts assessment shows that 47% of head CT images obtained have artifacts; about 6% of the CT images were affected while 35% of the CT images were not affected for the clinical diagnostic task. In general, it has been shown that head CT images obtained at UMTH Maiduguri, Nigeria achieved optimum performance of diagnostic quality. 1. Introduction The advent of computed tomography (CT) has revolutionized diagnostic medicine as it provides a non-invasive way to allow visualization of internal structure within the human body. CT is a technique that obtains cross-sectional images of an object using multiple x-ray measurements taking at different angle around the object [1]. Due to its accuracy, higher sensitivity, and wider availability, CT has become the diagnostic modality of choice for head trauma and other head diseases. For clarity, the human head comprises of the brain and skull; where the skull is the bony skeleton of the head and the most complex osseous structure in the body shielding the brain [2], the brain on the other hand is an organ of soft nervous tissue that controls thought, memory, emotion, touch, motor skills, vision, respirations, temperature, hunger and every process that regulates our body. The human brain contains over 100 billion neurons [3], it is protected and supported by the bones of the skull, cranial ________________ * [email protected] meninges, cerebrospinal fluid, and blood brain barrier. Indications for CT of the head are numerous, which include acute head trauma, suspected intracranial hemorrhage, calcification, vascular occlusive disease, shunt malfunction, mental status change, headache, cranial nerves dysfunction, seizures etc. Head CT plays a critical role in the evaluation of intracranial abnormalities (such as trauma, stroke, and hemorrhage) [4]. CT is a proven useful tool in the diagnosis of the bony skull abnormalities; it provides accurate visualization and objective information of the bony structures and cranial structures [5,6]. Due to its high sensitivity, CT allows rapid assessment of the extent and type of brain pathology, which ensures patients who require urgent surgical intervention receive such care at the earliest opportunity [7]. However, the clinical quality that makes head CT useful for medical diagnostic task lies in its ability to produce quality images with superior resolutions, uniformity, low noise, and absence of artifacts. At University of Maiduguri Teaching Hospital (UMTH), a lot of head CT images are being produced but the factors that affect the image The African Review of Physics (2016) 11:0011 quality were not assessed. The consequence of this is that there may be production of poor images, which may lead to an inappropriate and missed diagnosis of the patient or increase in radiation dose to the patient as a result of repeat. There are many tasks which involve evaluation of image quality, these different tasks are best performed by different assessment methods and the outcome is often referred to as technical (or physical) image quality or clinical image quality depending on the method used [8]. Methods to assess image quality often focus on the technical (or physical) aspects of the image according to international commission on radiation units and measurements [9], which are more straightforward and objective. This is a limitation for many hospitals in developing countries including UMTH because they lack the phantoms and quality control kits needed to assess the physical image quality. To address these challenges, we focused on the clinical aspect of image quality by subjective assessment using visual grading methods of diagnostic quality. This assessment method is expensive, time consuming and there is inconsistency in observer perception. However, it is considered the most important evaluation of image quality according to the International Atomic Energy Agency (IAEA) since it occurs in the clinical setting using images of clinical patients. In addition, the benefit of visual grading is not limited to specific diagnosis. 2. Materials and Methods Patients All head CT examinations in this study were retrospective. Approval for this study was obtained from our local research ethic committee of the Radiology Department, University of Maiduguri Teaching hospital (UMTH). Inclusion criteria were patients (male and female) up to age 1 and above with indications for head CT. A total of 78 head CT images were randomly selected at the hospital’s CT scan machine from January 2012 to October 2013. CT Imaging Technique The instrument used for collecting data is Philips Brilliance 16-Slice 3rd Generation CT scanner (axial/helical) located at the Radiology Department, UMTH Maiduguri, Nigeria. The CT scanner is made up of three primary systems, including the gantry (with table or couch), the computer, and the operating console. During the CT scan, the patient position is supine, head first into the gantry with the head in the head-holder whenever possible. The table height is centered 68 such that the external auditory meatus (EAM) is at the center of the gantry. To reduce or avoid ocular lens exposure, the gantry is tilted to an angle only for axial CT scan. Helical CT scan does not require gantry tilt because of the fast scan time. Contrast is administered to patients if indicated by the radiologist. Table 1. Head CT protocols. Scan type Axial Helical Rotation time(s) 0.5 1.5 Tube voltage (kV) 120 120 Tube current (mAs) 120-400 120-350/slice FOV(mm) 250 250 Gantry tilt ±30; 0.5 increment - Slice thickness(mm) 6 5 Imaging protocols for head CT (Table 1) were in conformity with the requirements of the American Association of Physicist in Medicine [10] and the European Guidelines on head CT examinations. Criteria for Image Quality Evaluation All head CT images were viewed and assessed on the Philips Brilliance work station on a DICOM monitor. The criteria for image quality evaluation were subjective using visual grading methods of diagnostic quality. Images were assessed with the help of an experienced radiologist and a radiographer. Fig. 1 shows a typical head CT image of patients (a) normal and (b) abnormal. The criteria for grading image quality of anatomical structures were recommended by the European guidelines on image quality criteria for computed tomography [11]. The African Review of Physics (2016) 11:0011 (a) 69 Image criteria scoring were based on the perception of YES for fulfilling a criterion or NO for not fulfilling a criterion. Percentage criteria fulfillment was recorded and the result of image criteria of the scanner in this locality A was compared with a reference scanner B using the method of Calzado et al. [12]. Subjective evaluation of image noise and overall image quality were both graded on a 5point scale; 1= unacceptable 2= below average 3= average 4= above average 5= excellent. A score greater than or equal to 3 is considered to meet the acceptable standard for the clinical diagnostic task. All artifacts were graded on a 3-point scale; 1= not present 2= present and affecting diagnosis 3= present and not affecting diagnosis. The grading method for this study was adopted from Namasivayam et al. [13] and Udayasankar et al. [14] Statistical Analysis Simple descriptive statistical tools like mean and percentages were used to analyze the data with the aid of Microsoft Excel 2007. Two-sample t-test was used in our study to compare percentages. 3. Results (b) Fig.1: Head CT images (a) normal (b) abnormal The criteria for brain general CT examinations were listed as follows: Visualization: (i) Whole cerebrum (ii) Whole cerebellum (iii) Whole skull base Critical Reproduction: (iv) Visually sharp reproduction of the border between white and grey matter (v) Visually sharp reproduction of the basal ganglia (vi) Visually sharp reproduction of the ventricular system (vii) Visually sharp reproduction of the cerebrospinal fluid space around the mesescephalon (viii) Visually sharp reproduction of the cerebrospinal fluid space over the brain Image Criteria of Brain CT Examination The results on image quality criteria evaluation for brain general CT examination at UMTH (A) and that of reference scanner (B) [12] are summarized in Table 2. The figures in Table 2 represent the percentage of examinations for which the criteria were fulfilled. The African Review of Physics (2016) 11:0011 70 Table 2: Image criteria scoring for brain general CT examination Criteria % criteria fulfillment (A) % criteria fulfillment (B) (i) visualization of whole cerebrum 92 90 (ii) visualization of whole cerebellum 75 100 (iii) visualization of whole skull base 95 100 (iv) visually sharp reproduction of the border between 80 30 (v) visually sharp reproduction of the basal ganglia 68 20 (vi) visually sharp reproduction of the ventricular 96 100 78 90 white and grey matter system (vii) visually sharp reproduction of the cerebrospinal fluid space around the mesescephalon (viii) visually sharp reproduction of the cerebrospinal 90 89 fluid space over the brain Mean image quality score (%) It can be seen that individual criterion in the index study were fulfilled to a larger extent with percentage fulfillment of each greater than 50% and a high mean score value of 84% was obtained. Subjective Assessment of Image Noise The results of subjective image noise are summarized in Table 3 From Table 3, subjective assessment of image noise as adopted shows that scores 3, 4, and 5 have a total of 68(87.18%), which meets the acceptable standard for clinical diagnostic task. Test scores 1 and 2 with total of 10(12.82%) did not meet the acceptable standard. 84 78 Table 3: Subjective assessment of image noise of head CT Scores Frequency Percentage 1 7 8.97 2 3 3.85 3 26 33.33 4 20 25.64 5 22 28.21 Total 78 100 (%) The African Review of Physics (2016) 11:0011 Assessment of Overall Image Quality The results of overall image quality by subjective assessment are summarized in Table 4. Table 4: Assessment of overall image quality of head CT Scores Frequency Percentage (%) 1 8 10.26 2 7 8.97 3 24 30.77 4 16 20.51 5 23 29.49 Total 78 100 From the results in Table 4, it can be shown that scores 3, 4, and 5 have a total of 63(80.77%), which meets the acceptable standard for clinical diagnostic task. Scores 1 and 2 with a total of 15(19.23%) did not meet the acceptable standard. Artifacts Assessment The results of assessed image artifacts are summarized in Table 5. Table 5: Assessment of artifacts in head CT images Scores Frequency Percentage (%) 1 46 58.97 2 5 6.41 3 27 34.62 Total 78 100 Artifact result shows that score 1 has 46 (58.97%), which represents images without any form of artifacts. Score 2 has 5 (6.41%), which represent images with artifacts degrading image quality for the diagnostic task. Test score 3 has 27 (34.62%) representing images with artifacts without interfering diagnosis. 4. Discussion Assessment of image quality is crucial to assure that the CT scanner has achieved optimum performance of diagnostic quality. Methods to 71 access the image quality often focus on the physical and technical aspect of the image according to International Commission on Radiation Units [9], which is straight forward and objective. However, objective assessment of the physical image quality could not be conducted because there are no phantoms and quality control kits at the time of the study and has limited our study only to subjective assessment using visual grading based on human perceptions. From the results obtained, the image quality evaluation for brain general CT examination in our study was compared with that of Calzado et al,[12] and the result has not shown any significant difference t(169) = 0.991, (P =0.32). When comparing individual criterion, it can be shown that all image criteria in this locality has no percentage score of less than 50%, unlike criteria (iv) and (v) in the published work with percentage score of 30% and 20%, respectively. This is a very good indication that image quality criteria in our locality achieved optimum performance of diagnostic quality. The subjective assessment of image noise and overall image quality were both graded on a 5point scale and artifacts were graded on a 3-point scale as adopted in [13,14]. The Image noise has total percentage of 87.18% of scores greater than or equal to 3 representing an acceptable level for clinical diagnostic task, only 12.82% were below acceptable level. In this case, acceptable level of image noise means that the grainy appearance on head CT images is moderate, low, or even absent to allow visualization of low and high contrast objects as well as any disease or abnormality present on the images. When the grainy appearance is high, that is, when the noise level is high, it is obvious that low contrast and high contrast objects are not discernible as well as any disease or abnormal structure that may be present on the images. Thus, these images are considered unacceptable and discarded for clinical diagnostic task. The overall image quality has a total of 80.77% of scores greater than or equal to 3 that represents the acceptable level for clinical diagnostic and 19.23% below the acceptable level. The overall image quality evaluation tends to find a balance of all the image quality factors affecting anatomical structures, including image noise and artifacts. Both the assessment of image noise and overall image quality has modal score of 3, which is comparable to that of Namisavayam et al. [13] for neck CT. Although the result of their study is that all CT images were acceptable for the diagnostic task at standard protocol, some of the images in our The African Review of Physics (2016) 11:0011 study were below acceptable limit at standard protocol. Artifacts were found to be present in 32 (41.03%) out of the total 78 head CT images assessed. The result shows that 5 (6.41%) of head CT images with artifacts affected the image quality for clinical diagnostic task while 27 (34.62%) of head CT images with artifacts did not affect the image quality for the clinical diagnostic task. Udayasankar et al, [14] graded only streak artifacts using the same adopted grading method. Our study on the other hand graded stair-step artifact, streak artifact, and motion artifact. These were the artifacts found in head CT in our study. Stair step artifacts were present in 26 (81.25%) out of the 32 images with artifacts, and these were mostly viewed around the edges of sagittal and coronal reformatted images and have reduced the image quality in these formats. Smaller collimation and overlapping reconstruction were used to reduce these artifacts, only one image was degraded and not useful for the diagnostic task. There were 5 (15.63%) images with streak artifact, three of these images were badly affected and discarded as a result of beam hardening and scatter due to metal and bone. Only 1 image was presented with motion artifact, which is rarely common, which was the case of a 1year old baby. The image quality was highly degraded and not useful for clinical diagnostic task. Under clinical conditions, the subjective image quality of head CT scans and the capability of CT to assess different features are influenced by a number of variables, including device, FOV, voxel size, tube voltage and current, and other technical factors [15]. This might be responsible for the poor images produced. Clinical assessment of different features might also be affected by the observer performance and viewing conditions, as well as by hardware and software specifications and the presence of artifacts. In general, the result indicate very good image quality performance from head CT images obtained at the hospital’s CT scan machine with about 90% head CT protocols meeting the standard for good imaging. 5. 72 when compared to that of a published work. Each individual criterion in this study fulfilled a percentage score greater than 50% and the mean image quality score was rated high (84%). Factors affecting image quality like image noise and artifacts were also evaluated using adopted method and the results indicate very good image quality performance from CT images obtained at the hospital’s CT scan machine. The major shortcoming is that there is dearth of information on image quality data in our locality and underscores the importance of this study as base line for further investigation. Acknowledgements The authors acknowledge the Management and Staff of Radiology Department, University of Maiduguri Teaching Hospital, Nigeria who permitted the study to be conducted at the CT scan unit. References [1] [2] [3] [4] [5] [6] [7] [8] [9] Conclusion A generally accepted principle is that image quality is most meaningfully defined and measured in relationship with the intended task of the image. Clinical performance by subjective evaluation was used in this study. It has been shown that head CT images obtained from UMTH Maiduguri, Nigeria reveal generally a good image quality performance [10] [11] Y. Zhang, Ph.D. dissertation, University of Rochester, Rochester (2009). S. Standing, Gray’s Anatomy (Elsevier Inc., London, 2008) p.454. M. Frederic, T. Michael and T. Robert, Human Anatomy (Benjamin-Cummings Publishing Co., US) Chapter 15. D. A. Perron, How to Read a Head CT in Emergency Medicine, Chapter 69, Ed. James G. Adams, Saunders (an imprint of Elsevier Inc., 2008). B. Kotrikova, R. Krempien, K. Freier and J. Muhling, Eur. Radiol. 17, 1968 (2007). T. K. Pilgram et al., Radiology 173, 675 (1989). J. P. Coles, Br. J. Anaesth. 99(1), 49 (2007). M. Tapiovaara, Relationships Between Physical Measurements and User Evaluation of Image Quality in Medical Radiology – A Review. (STUK-A219. Helsinki, 2006) p.7. International Commision on Radiation Units, “Medical imaging- the assessment of image quality”, ICRU Report 54. 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