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Adult patient doses in interventional neuroradiology Nikolaos A. Gkanatsios, Walter Huda, and Keith R. Peters Department of Radiology, University of Florida, P.O. Box 100374, Gainesville, Florida 32610 a) and Department of Radiology, SUNY Upstate Medical University, 750 E. Adams Street, Syracuse, New York 13210 共Received 9 April 2001; accepted for publication 5 February 2002; published 11 April 2002兲 We investigated radiation doses to 149 adult patients who underwent interventional neuroradiologic procedures, consisting of 132 patients who had diagnostic imaging examinations and 17 patients who had therapeutic procedures. The interventional procedures were carried out on a biplane system capable of performing fluoroscopy and digital subtraction angiography 共DSA兲. The x-ray imaging system was interfaced to a patient dosimetry system, which computed surface 共skin兲 doses based on the selected radiographic technique factors in each of the radiographic and fluoroscopic imaging modes. For each patient, an assessment was made of the maximum surface dose received during the procedure, which predicts the possibility of inducing deterministic effects. Knowledge of the surface doses, beam quality and x-ray cross sectional area permitted the computation of the total energy imparted to each patient. Energy imparted values were converted to effective dose, which provides an estimate of the stochastic radiation risk to the patient. The median surface dose for the frontal plane during diagnostic imaging examinations was 1.3 Gy, with a maximum surface dose of 5.1 Gy. The median surface dose for the frontal plane during therapeutic procedures was 2.8 Gy with a maximum surface dose of 5.0 Gy. Ratios of the lateral to frontal median surface doses were 0.47 for diagnostic examinations and 0.68 for interventional procedures. The median energy imparted was 1.8 J during fluoroscopy, and 4.3 J during radiography, showing that on average, 66% of the patient exposure comes from radiographic imaging 共DSA兲. For diagnostic examinations, the median patient effective dose was 33 mSv, with a maximum of 152 mSv. For therapeutic procedures, the median patient effective dose was 74 mSv, with a maximum of 156 mSv. In interventional neuroradiology, surface doses could induce deterministic effects, and the corresponding effective doses are noticeably higher than those normally encountered in diagnostic radiology. © 2002 American Association of Physicists in Medicine. 关DOI: 10.1118/1.1470206兴 Key words: dosimetry, surface dose, energy imparted, effective dose, neuroradiology INTRODUCTION Interventional neuroradiology studies the vasculature and potential pathologies of the brain by means of catherization performed with the transfemoral artery technique.1,2 Most interventional neuroradiologic procedures are considered to be of a diagnostic nature where the objective is to identify a neurologic problem. An increasing proportion of interventional neuroradiologic procedures may be categorized as therapeutic, where the procedure is undertaken to treat a neurologic condition. Examples of therapeutic procedures include brain aneurysm obliteration using detachable coils, arteriovenous malformation embolization using liquid acrylics, and arteriovenous fistula closure with detachable balloons.3 Imaging in interventional neuroradiology is normally accomplished by use of fluoroscopy and digital subtraction angiography 共DSA兲. In general, neuroradiologic procedures require good image quality, long fluoroscopic times and a significant number of angiographic images to visualize and evaluate any vascular pathology.4,5 Interventional neuroradiologic procedures have the potential to result in high patient doses to the head region.6,7 The surface 共skin兲 dose is an important dosimetric parameter, as it can be used to predict the possibility of inducing determin717 Med. Phys. 29 „5…, May 2002 istic radiation injuries, such as skin erythema or epilation.8,9 Surface doses may be calculated from measured x-ray tube output data, or may be measured using radiation detectors such as thermoluminescent dosimeters. Another important dose parameter is the effective dose to the patient. The effective dose takes into account the individual dose and radiosensitivity of all the irradiated organs and tissues.10 The magnitude of the effective dose is taken to be a measure of the stochastic risks to patients undergoing the specified radiologic examination.10,11 The effective dose is generally the most important dosimetric parameter when patient surface doses do not exceed the threshold dose dictating the induction of deterministic effects. In this study, values of surface dose, energy imparted and effective dose were determined for 149 adult patients undergoing interventional neuroradiologic procedures. Patient doses were quantified to investigate how patient surface doses compare with the threshold for the induction of deterministic effects, and how the corresponding effective doses compare with the effective doses of other types of radiologic examinations. Our analysis included an investigation regarding the importance of fluoroscopy relative to radiographic image acquisitions, as well as the relative importance of the frontal versus the lateral imaging projection plane. In addi- 0094-2405Õ2002Õ29„5…Õ717Õ7Õ$19.00 © 2002 Am. Assoc. Phys. Med. 717 718 Gkanatsios, Huda, and Peters: Interventional neuroradiology tion, we investigated how patient doses during diagnostic neuroradiologic procedures compared to those encountered during therapeutic interventional procedures. 718 TABLE I. Values of water equivalent thickness and x-ray beam area of exposure. Body region Head PA Head LAT Chest/Neck PA Abdomen/Pelvis PA METHOD Interventional neuroradiologic imaging The x-ray imaging system used in this study was a biplane, Toshiba 共Toshiba America Medical Systems, Tustin, CA兲 KXO-80 x-ray generator and the Toshiba DFP2000A/A3 digital fluorography system configured to perform interventional neuroradiologic procedures. Radiographic acquisitions were performed using digital subtraction angiography 共DSA兲 where a mask frame obtained prior to the administration of iodinated contrast was subtracted digitally from subsequent images of the vasculature containing iodinated contrast. For radiographic acquisitions, the frame rate ranged between 1 and 3 frames per second, with the total number of frames for a given run ranging between 10 and 50 frames. Biplane fluoroscopy of the head region was used for target position verification. In diagnostic procedures, the acquisition of radiographic images was done in biplane mode almost exclusively. In therapeutic procedures, both biplane and single plane imaging, either frontal or lateral were used during different stages of the evaluation of the embolization progress. A total of 149 consecutive adult patients were considered in this study, including 132 patients who underwent diagnostic examinations, and 17 patients who underwent therapeutic procedures. The transfemoral artery technique was used to guide the catheter to the vertebral or carotid arteries. A limited amount of fluoroscopy was performed over the abdomen–pelvis and thoracic regions of each patient. Based on a study of 10 patients, about 30 seconds of frontal plane fluoroscopy were spent on the abdomen–pelvis region with an additional two minutes at the upper thoracic, lower neck region. The amount of fluoroscopy performed over the abdomen–pelvis and thoracic regions was found to be independent of the patient and the type of the interventional neuroradiologic procedure.12 Computation of surface doses to the head region excluded fluoroscopy performed on the abdomen–pelvis and thoracic region of the patient. Fluoroscopy of the body region, however, was explicitly included when computing patient effective doses. a Water thickness 共cm兲 Exposure areaa (cm2 ) 22.2 17.8 15.0 20.4 279 335 175 175 At entrance to the patient. Surface doses from fluoroscopy and radiography were combined to obtain the total surface dose to each patient for the frontal and lateral planes, respectively. The on-line patient exposure meter computes patient skin exposures by using the x-ray tube radiation output at the selected technique factors 共kV and mA兲 as measured during calibration of the system. The dosimetry system also utilizes additional information about the patient location relative to the x-ray tube as well as measured exposure times.5,12 All skin exposures included the contribution of backscatter radiation. An exposure of 2.58⫻10⫺4 C/kg 共1R兲 was taken to correspond to 9.3 mGy absorbed dose for muscle tissue in converting entrance skin exposure values into patient surface doses.13 Energy imparted and effective dose The energy imparted, , to a patient undergoing any radiologic x-ray examination can be estimated by modeling the patient as a slab of water with thickness z using the expression ⫽ 共 z 兲 ⫻ESE⫻A, J 共1兲 where is the energy imparted per entrance exposure-area product, ESE is the exposure measured free-in-air at the beam entrance plane of the patient, and A is the area of exposure also measured at the entrance plane.14,15 The pa- Surface dose A patient dosimetry system 共PEMNET兲 共Clinical Microsystems Inc., Arlington, VA兲 was operational on each of the two x-ray-imaging planes. The PEMNET unit is a microprocessor-based system running its own on-board software, which is passively hardwired to the x-ray generator where it samples a multitude of signals 共i.e., kVp, mA, C-arm and table location, etc.兲.5 These input signals permit the computation of surface doses to patients, assuming that the same skin area is continually exposed to the x-ray beam. Medical Physics, Vol. 29, No. 5, May 2002 FIG. 1. Histogram distribution of surface doses from use of fluoroscopy for 149 patients undergoing interventional neuroradiologic procedures. Black bars correspond to the frontal imaging plane and gray bars correspond to the lateral plane. Each histogram bin on the x axis corresponds to 0.15 Gy. 719 Gkanatsios, Huda, and Peters: Interventional neuroradiology FIG. 2. Histogram distribution of surface doses from use of radiography 共DSA兲 for 149 patients undergoing interventional neuroradiologic procedures. Black bars correspond to the frontal imaging plane and gray bars correspond to the lateral plane. Each histogram bin on the x axis corresponds to 0.30 Gy. rameter depends on the patient water equivalent thickness, z, the x-ray tube voltage and x-ray beam half-value layer.14,15 The free-in-air entrance exposure was obtained from the patient exposure data recorded by the dosimetry system for each plane, corrected for backscattered radiation. The area of exposure was estimated using the geometry and the image receptor size for an average patient. Table 1 shows the water equivalent thickness and x-ray beam exposure areas for the principal body regions and projections that were used to compute energy imparted. For any given projection, patient effective doses can be estimated from a knowledge of the energy imparted and x-ray beam quality.16 In this paper, values of energy imparted, , were converted into corresponding values of patient effective dose, E, by using E/ conversion factors that accounted for the body region being irradiated, the specific projection and the x-ray beam quality. The average effective dose per unit energy imparted for the head projections was approximately 5 mSv J⫺1 . Corresponding effective dose per unit energy imparted values were approximately 15 mSv J⫺1 719 FIG. 3. Histogram distribution of the total surface doses for 149 patients undergoing interventional neuroradiologic procedures. Black bars correspond to the frontal plane and gray bars correspond to the lateral plane. Each histogram bin on the x axis corresponds to 0.30 Gy. for the chest and approximately 14 mSv J⫺1 for the abdomen. RESULTS Entrance surface doses Figure 1 shows the histogram distribution of surface doses to the head region during fluoroscopy for all 149 patients included in this study. The median values of the fluoroscopic surface doses were 0.45 Gy and 0.12 Gy for the frontal and lateral projection planes, respectively. Maximum surface doses were 2.6 Gy for the frontal plane and 2.7 Gy for lateral plane. Figure 2 shows the histogram distribution of the surface doses to the head region received from radiography. The median values of the radiographic surface doses were 0.83 Gy and 0.50 Gy for the frontal and lateral planes, respectively. Maximum radiographic surface doses of 4.8 Gy and 3.8 Gy were recorded in the frontal and lateral planes, respectively. Figure 3 shows the histogram distributions of the combined surface doses to the patients’ head region from use of TABLE II. Surface doses in Gy for diagnostic angiographic and therapeutic embolization interventional neuroradiologic procedures. Diagnostic procedures Therapeutic procedures Projection Median dose Maximum dose Median dose Maximum dose Fluoroscopy Frontal Lateral 0.43 0.11 2.6 1.6 0.74 0.77 2.3 2.7 Radiography Frontal Lateral 0.73 0.49 3.4 3.8 1.8 1.3 4.8 2.6 Fluoroscopy⫹Radiography Frontal Lateral 1.3 0.61 5.1 4.1 2.8 1.9 5.0 5.2 Imaging procedure Medical Physics, Vol. 29, No. 5, May 2002 720 Gkanatsios, Huda, and Peters: Interventional neuroradiology 720 TABLE III. Energy imparted (J) statistics for interventional neuroradiologic procedures. Imaging mode Fluoroscopy Radiography Fluoroscopy⫹Radiography 25th Median 75th Maximum percentile value percentile value 1.2 2.4 3.2 1.8 4.3 6.7 3.2 6.5 9.0 13 21 27 fluoroscopy and radiography during an interventional neuroradiologic procedure. The medians of the combined surface dose were 1.4 Gy and 0.70 Gy for the frontal and lateral plane, respectively. The maximum surface dose received by a patient was of the order of 5 Gy for both imaging planes. Most of the surface dose occurs as a result of radiography, which accounts for 65% of the combined surface dose in the frontal plane and 81% of the surface dose in the lateral plane. Table II provides a breakdown of the measured surface doses between diagnostic and therapeutic interventional neuroradiologic procedures. Ratios of the lateral to frontal median surface doses were 0.47 for diagnostic examinations and 0.68 for interventional procedures. In general, diagnostic procedures resulted in surface doses that were a factor of 2 to 3 times smaller than surface doses accounted for therapeutic procedures. Maximum surface doses for diagnostic examinations and therapeutic procedures, however, were similar. Energy imparted and effective doses Table III shows data for energy imparted resulted from use of fluoroscopy and radiography. The median value of energy imparted for a complete neuroradiologic procedure was computed to be 6.7 J, with 66% on average due to radiographic acquisitions. An average of 70% of the energy imparted resulted from use of the frontal projection plane. Figure 4 shows the histogram distribution of the effective FIG. 5. Histogram distribution of the total effective dose for 149 patients undergoing interventional neuroradiologic procedures. Each histogram bin on the x axis corresponds to 10 mSv. dose to the patients from use of fluoroscopy and radiography. The median values of effective dose were 10 mSv and 23 mSv for fluoroscopy and radiography, respectively. The maximum computed values of effective dose were 76 mSv for fluoroscopy and 114 mSv for radiography. Figure 5 shows the histogram distribution of the effective dose to patients from complete interventional neuroradiologic procedures. Computed effective doses for interventional neuroradiologic procedures had a median value of 36 mSv and a maximum of 156 mSv. Table IV provides a breakdown for the computed effective doses for diagnostic and therapeutic interventional procedures. The maximum effective doses computed for diagnostic and therapeutic procedures were similar. The median effective doses, however, were two to three times larger for therapeutic procedures in comparison to diagnostic examinations. DISCUSSION The accuracy and precision of measuring surface doses with the patient dosimetry system used in this study was estimated to be within 5% of the true surface dose by simulating a full diagnostic neuroradiologic examination.5,12 This level of accuracy is satisfactory for the purpose of determining surface doses to patients undergoing interventional neuroradiologic procedures. Surface doses depicted in Figs. 1–3 and Table II were based on the assumption that the same skin TABLE IV. Patient effective doses in mSv for diagnostic angiographic and therapeutic embolization interventional neuroradiologic procedures. Diagnostic procedures FIG. 4. Histogram distribution of the effective dose for 149 patients undergoing interventional neuroradiologic procedures. Black bars correspond to effective doses from fluoroscopy and gray bars correspond to effective doses from radiography. Each histogram bin on the x axis corresponds to 10 mSv. Medical Physics, Vol. 29, No. 5, May 2002 Imaging mode Fluoroscopy Radiography Fluoroscopy⫹Radiography Therapeutic procedures Median Maximum Median Maximum dose dose dose dose 9.8 19 33 75 76 152 22 41 74 76 114 156 721 Gkanatsios, Huda, and Peters: Interventional neuroradiology area was continually exposed to the x-ray beam. During interventional neuroradiologic procedures, a 20° to 30° rotation of the x-ray source in the sagittal plane of the patient may be used when acquiring radiographic images. Although the central axis of the x-ray beam changes position on the surface of the head with rotation of the x-ray source, there may be parts of the skin surface, which are continually exposed to the x-ray beam. Surface dose data presented in this study should be considered as an upper limit of the patient surface dose since they assume that there was a skin region that was always in the direct x-ray beam. There were no cases of epilation or skin erythema in this series of 149 adult patients who underwent interventional neuroradiologic procedures. The data shown in Fig. 3 共total surface dose兲, however, indicate that up to 30% of the patients in this study exceeded the nominal threshold skin dose for the induction of deterministic effects 共2 Gy兲. Observable effects such as total epilation are more likely to occur at doses in excess of 6 Gy.6 At lower doses, signs of erythema would be fleeting and faint, which would make detection difficult. Additional factors which could affect the threshold radiation doses for the induction of deterministic effects include the anatomic location and size of the irradiated region, tissue vascularity and oxygenation, as well as patient age, genetic background and hormonal status.9,17,18 In addition, in neuroradiologic examinations radiation doses are delivered over an extended time period, which may be as long as several hours. This is likely to reduce the effectiveness of the radiation due to antagonistic cellular repair processes.19 The median surface dose reported in this study may be compared to the data available in the scientific literature. Bergeron et al.20 measured a range of maximum surface doses between 0.13 and 1.3 Gy in eight patients, with a mean value of 0.62 Gy. Norbash et al.21 studied 12 typical interventional neuroradiologic procedures and measured a range of 0.31–2.7 Gy to the skin surface of the head with a mean value of 1.5 Gy, and showed that rotation of the x-ray tube was effective in reducing patient surface doses. O’Dea et al.22 reported the mean and maximum surface doses from 94 therapeutic embolization procedures to be 2.1 Gy and 8.8 Gy, respectively. The same study reported a mean of 0.94 Gy and a maximum of 6.0 Gy for 160 cerebral angiographic procedures. Detailed comparison with these studies is problematic due to differences in clinical cases and settings, imaging protocols and radiographic equipment. Nevertheless, surface doses in interventional neuroradiologic examinations are high enough to cause concern about induction of deterministic effects. Furthermore, patient doses appear higher than in the past, which is likely a result of use of more complicated and sophisticated protocols in therapeutic neuroradiology.3 For most patients undergoing interventional neuroradiologic procedures, their maximum surface dose was below the threshold dose for induction of deterministic effects. In these cases, surface dose has major limitations as an indicator of radiation risk to the patient since surface dose does not take into account either the x-ray beam quality 共i.e., penetrating power兲 or the size of the x-ray beam incident on the Medical Physics, Vol. 29, No. 5, May 2002 721 patient. An alternative dose parameter to surface dose is energy imparted, or integral dose, which is straightforward to measure or calculate.23,24 Energy imparted is a measure of the total energy deposited in a volume 共i.e., head, chest, abdomen, etc.兲 from x-ray exposures and is generally taken to be proportional to patient stochastic risk for a given type of radiographic examination. The principal limitation of energy imparted as a dosimetric parameter is the fact that it does not account for radiosensitivity of the organs and tissues that are being irradiated. The ease with which energy imparted can be converted into a corresponding value of patient effective dose, however, makes the latter ideal for quantifying patient stochastic risks. The median effective dose of 36 mSv reported in this study may be compared with those in the scientific literature. Feygelman et al.25 studied ten cases and reported values ranging from 1.6 –14 mSv with a mean of 6.2 mSv. Bergeron et al.20 reported an average of 1.8 mSv with a range of 0.44 – 3.4 mSv for a limited number of eight patients undergoing similar procedures. McParland26 reported a median of 7.0 mSv with a range of 2.1–20 mSv when he computed effective doses to patients undergoing cerebral angiography. A wider range was reported by Berthelson and Cederblad4 who computed effective doses between 3.5 mSv and 25 mSv. As in the case of surface doses, the most likely reason for higher effective doses recorded in this study is the use of a biplane imaging system, and more extensive and complicated procedures. Different imaging equipment, setup and imaging protocols can play a major role to how different effective doses may be found among institutions. The fact that this study was done at an academic institution that trains new neurointerventional radiologists may also account for some of the differences in the effective doses recorded between this work and others. Most of these examinations were performed by three radiologists while training radiology residents and some of the examinations were carried out by two fellows. There were no significant differences between radiologists and fellows that will indicate any obvious trends with years of experience. However, there were some differences between the radiologists themselves that could indicate personal preferences in practice. Effective doses for interventional neuroradiologic procedures may be compared to effective doses for other common radiologic examinations that also use ionizing radiation. Effective doses for chest x rays are of the order of 0.05 mSv, for skull radiographic examinations of the order of 0.2 mSv, for abdominal radiographic examinations between 0.5 and 1.5 mSv, and for excretory urogram examinations between 2.5 and 5.0 mSv. Patients undergoing barium enemas receive doses between 3.0 and 7.0 mSv, head CT scans measure effective dose between 1 and 2 mSv, and body CT scans about 5 mSv. A routine coronary angiographic procedure measures between 3.0 and 6.0 mSv. The average nuclear medicine procedure has an effective dose of 5 mSv. Interventional neuroradiologic effective doses are therefore markedly higher than those normally encountered in diagnostic radiology. Patient doses in interventional neuroradiology can also be compared with natural background in the United States 共3 722 Gkanatsios, Huda, and Peters: Interventional neuroradiology mSv/year兲, as well as regulatory dose limits for radiation workers 共50 mSv/year兲 and for members of the public 共1 mSv/year兲.27–33 The data in Tables II and IV demonstrate that radiation doses associated with therapeutic procedures are markedly higher than those associated with diagnostic procedures. Median surface doses encountered in therapeutic procedures were approximately a factor of 3 higher than those encountered in diagnostic procedures, and effective doses in therapeutic procedures were a factor of 2 higher when compared to those in diagnostic procedures. Diagnostic neuroradiologic procedures usually require the practitioner to catherize the vessels to the brain only up to the level of the middle neck and use a limited number of DSA acquisitions. Therapeutic procedures require the catheters to be migrated to the specific site of abnormal vasculature within the brain, which increases the complexity and procedure duration. The diagnostic complexity of therapeutic procedures requires far more DSA acquisitions increasing the total dose to the patient quickly. Therapeutic procedures also utilize much smaller catheters 共0.59 mm diameter for therapeutic versus 1.65 mm diameter for diagnostic procedures兲, which may require use of magnification radiography and will result in elevated surface doses.34 As shown in Fig. 4, as the dose to the patient increases implying a more complex procedure, most of the dose comes as a result of radiographic 共DSA兲 exposures and less of it is fluoroscopy related. Quantitative radiation risk may be obtained for a given value of effective dose, although the risk will be modified by factors such as age and sex of the exposed individual. A risk coefficient of 5⫻10⫺5 cancers and genetic abnormalities per mSv of radiation dose was derived from the ICRP10 attempt to estimate absolute stochastic risks from whole-body irradiation. The radiation risk associated with the median effective dose of 36 mSv is therefore of the order of 0.2%. The risk from a neuroradiologic interventional procedure can be compared to other every day risks. An effective dose of 36 mSv is comparable to the risk of dying from lung cancer when smoking ⬃600 packs of cigarettes, or the risk of dying in an automobile when driving a distance of ⬃30 000 miles. 19 These risk factors, however, need to be treated with great caution. Since patients undergoing interventional neuroradiologic procedure are generally older, the radiation risk could be significantly lower than the one obtained using the nominal ICRP risk coefficients.35 Of greater significance is the uncertainty associated with the extrapolation of radiation risks from high doses to those normally encountered in diagnostic radiology.36,37 It is important to note that the nominal radiation risk is expected to be very small in comparison to the benefit to the individual undergoing the interventional procedure, given that the latter is performed to decrease the chance of future or further neurologic damage, to improve the quality of life of the patient or literally to save lives. As such, the risk benefit ratio to the exposed individual is likely to be small. The primary goal of the interventional neuroradiologist is to ensure that the image quality associated with the fluoroscopic and radiographic exposures are adequate for the imMedical Physics, Vol. 29, No. 5, May 2002 722 aging task at hand. Diagnostic imaging information should not compromise optimal patient care. Secondary objectives should be to try to prevent the incidence of deterministic effects of radiation and to minimize the resultant patient stochastic risk. These secondary objectives require that surface doses do not exceed the threshold doses for the induction of deterministic effects, and the elimination of all unnecessary patient exposures. Application of these radiation protection principles will ensure that patient risks are minimized without compromising the undoubted clinical benefits to the patient that can be achieved from interventional neuroradiology. 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