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The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education and professional practice of medical physics. The AAPM has more than 8,000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines: •Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. •Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline (MPPG) represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiation requires specific training, skills, and techniques as described in each document. As the review of the previous version of AAPM Professional Policy (PP)‐17 (Scope of Practice) progressed, the writing group focused on one of the main goals: to have this document accepted by regulatory and accrediting bodies. After much discussion, it was decided that this goal would be better served through a MPPG. To further advance this goal, the text was updated to reflect the rationale and processes by which the activities in the scope of practice were identified and categorized. Lastly, the AAPM Professional Council believes that this document has benefitted from public comment which is part of the MPPG process but not the AAPM Professional Policy approval process. The following terms are used in the AAPM's MPPGs: Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline.Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.

Purpose: A recent work has demonstrated the feasibility of estimating the dose to individual organs from multidetector CT exams using patient-specific, scanner-independent CTDIvol-to-organ-dose conversion coefficients. However, the previous study only investigated organ dose to a single patient model from a full-body helical CT scan. The purpose of this work was to extend the validity of this dose estimation technique to patients of any size undergoing a common clinical exam. This was done by determining the influence of patient size on organ dose conversion coefficients generated for typical abdominal CT exams. Methods: Monte Carlo simulations of abdominal exams were performed using models of 64-slice MDCT scanners from each of the four major manufacturers to obtain dose to radiosensitive organs for eight patient models of varying size, age, and gender. The scanner-specific organ doses were normalized by corresponding CTDIvol values and averaged across scanners to obtain scanner-independent CTDIvol-to-organ-dose conversion coefficients for each patient model. In order to obtain a metric for patient size, the outer perimeter of each patient was measured at the central slice of the abdominal scan region. Then, the relationship between CTDIvol-to-organ-dose conversion coefficients and patient perimeter was investigated for organs that were directly irradiated by the abdominal scan. These included organs that were either completely (“fully irradiated”) or partly (“partially irradiated”) contained within the abdominal exam region. Finally, dose to organs that were not at all contained within the scan region (“nonirradiated”) were compared to the doses delivered to fully irradiated organs. Results: CTDIvol-to-organ-dose conversion coefficients for fully irradiated abdominal organs had a strong exponential correlation with patient perimeter. Conversely, partially irradiated organs did not have a strong dependence on patient perimeter. In almost all cases, the doses delivered to nonirradiated organs were less than 5%, on average across patient models, of the mean dose of the fully irradiated organs. Conclusions: This work demonstrates the feasibility of calculating patient-specific, scanner-independent CTDIvol-to-organ-dose conversion coefficients for fully irradiated organs in patients undergoing typical abdominal CT exams. A method to calculate patient-specific, scanner-specific, and exam-specific organ dose estimates that requires only knowledge of the CTDIvol for the scan protocol and the patient’s perimeter is thus possible. This method will have to be extended in future studies to include organs that are partially irradiated. Finally, it was shown that, in most cases, the doses to nonirradiated organs were small compared to the dose to fully irradiated organs.

Purpose: Monte Carlo radiation transport techniques have made it possible to accurately estimate the radiation dose to radiosensitive organs in patient models from scans performed with modern multidetector row computed tomography (MDCT) scanners. However, there is considerable variation in organ doses across scanners, even when similar acquisition conditions are used. The purpose of this study was to investigate the feasibility of a technique to estimate organ doses that would be scanner independent. This was accomplished by assessing the ability of CTDIvol measurements to account for differences in MDCT scanners that lead to organ dose differences. Methods: Monte Carlo simulations of 64-slice MDCT scanners from each of the four major manufacturers were performed. An adult female patient model from the GSF family of voxelized phantoms was used in which all ICRP Publication 103 radiosensitive organs were identified. A 120 kVp, full-body helical scan with a pitch of 1 was simulated for each scanner using similar scan protocols across scanners. From each simulated scan, the radiation dose to each organ was obtained on a per mA s basis (mGy∕mA s). In addition, CTDIvol values were obtained from each scanner for the selected scan parameters. Then, to demonstrate the feasibility of generating organ dose estimates from scanner-independent coefficients, the simulated organ dose values resulting from each scanner were normalized by the CTDIvol value for those acquisition conditions. Results: CTDIvol values across scanners showed considerable variation as the coefficient of variation (CoV) across scanners was 34.1%. The simulated patient scans also demonstrated considerable differences in organ dose values, which varied by up to a factor of approximately 2 between some of the scanners. The CoV across scanners for the simulated organ doses ranged from 26.7% (for the adrenals) to 37.7% (for the thyroid), with a mean CoV of 31.5% across all organs. However, when organ doses are normalized by CTDIvol values, the differences across scanners become very small. For the CTDIvol, normalized dose values the CoVs across scanners for different organs ranged from a minimum of 2.4% (for skin tissue) to a maximum of 8.5% (for the adrenals) with a mean of 5.2%. Conclusions: This work has revealed that there is considerable variation among modern MDCT scanners in both CTDIvol and organ dose values. Because these variations are similar, CTDIvol can be used as a normalization factor with excellent results. This demonstrates the feasibility of establishing scanner-independent organ dose estimates by using CTDIvol to account for the differences between scanners.

The purpose of this study was to present a method for generating x-ray source models for performing Monte Carlo (MC) radiation dosimetry simulations of multidetector row CT (MDCT) scanners. These so-called "equivalent" source models consist of an energy spectrum and filtration description that are generated based wholly on the measured values and can be used in place of proprietary manufacturer's data for scanner-specific MDCT MC simulations. Required measurements include the half value layers (HVL1 and HVL2) and the bowtie profile (exposure values across the fan beam) for the MDCT scanner of interest. Using these measured values, a method was described (a) to numerically construct a spectrum with the calculated HVLs approximately equal to those measured (equivalent spectrum) and then (b) to determine a filtration scheme (equivalent filter) that attenuates the equivalent spectrum in a similar fashion as the actual filtration attenuates the actual x-ray beam, as measured by the bowtie profile measurements. Using this method, two types of equivalent source models were generated: One using a spectrum based on both HVL1 and HVL2 measurements and its corresponding filtration scheme and the second consisting of a spectrum based only on the measured HVL1 and its corresponding filtration scheme. Finally, a third type of source model was built based on the spectrum and filtration data provided by the scanner's manufacturer. MC simulations using each of these three source model types were evaluated by comparing the accuracy of multiple CT dose index (CTDI) simulations to measured CTDI values for 64-slice scanners from the four major MDCT manufacturers. Comprehensive evaluations were carried out for each scanner using each kVp and bowtie filter combination available. CTDI experiments were performed for both head (16 cm in diameter) and body (32 cm in diameter) CTDI phantoms using both central and peripheral measurement positions. Both equivalent source model types result in simulations with an average root mean square (RMS) error between the measured and simulated values of approximately 5% across all scanner and bowtie filter combinations, all kVps, both phantom sizes, and both measurement positions, while data provided from the manufacturers gave an average RMS error of approximately 12% pooled across all conditions. While there was no statistically significant difference between the two types of equivalent source models, both of these model types were shown to be statistically significantly different from the source model based on manufacturer's data. These results demonstrate that an equivalent source model based only on measured values can be used in place of manufacturer's data for Monte Carlo simulations for MDCT dosimetry.

The larger coverage afforded by wider z -axis beams in multidetector CT (MDCT) creates larger cone angles and greater beam divergence, which results in substantial surface dose variation for helical and contiguous axial scans. This study evaluates the variation of absorbed radiation dose in both cylindrical and anthropomorphic phantoms when performing helical or contiguous axial scans. The approach used here was to perform Monte Carlo simulations of a 64 slice MDCT. Simulations were performed with different radiation profiles (simulated beam widths) for a given collimation setting (nominal beam width) and for different pitch values and tube start angles. The magnitude of variation at the surface was evaluated under four different conditions: (a) a homogeneous CTDI phantom with different combinations of pitch and simulated beam widths, (b) a heterogeneous anthropomorphic phantom with one measured beam collimation and various pitch values, (c) a homogeneous CTDI phantom with fixed beam collimation and pitch, but with different tube start angles, and (d) pitch values that should minimize variations of surface dose—evaluated for both homogeneous and heterogeneous phantoms. For the CTDI phantom simulations, peripheral dose patterns showed variation with percent ripple as high as 65% when pitch is 1.5 and simulated beam width is equal to the nominal collimation. For the anterior surface dose on an anthropomorphic phantom, the percent ripple was as high as 40% when the pitch is 1.5 and simulated beam width is equal to the measured beam width. Low pitch values were shown to cause beam overlaps which created new peaks. Different x-ray tube start angles create shifts of the peripheral dose profiles. The start angle simulations showed that for a given table position, the surface dose could vary dramatically with minimum values that were 40% of the peak when all conditions are held constant except for the start angle. The last group of simulations showed that an “ideal” pitch value can be determined which reduces surface dose variations, but this pitch value must take into account the measured beam width. These results reveal the complexity of estimating surface dose and demonstrate a range of dose variability at surface positions for both homogeneous cylindrical and heterogeneous anthropomorphic phantoms. These findings have potential implications for small-sized dosimeter measurements in phantoms, such as with TLDs or small Farmer chambers.

Tube current modulation was designed to reduce radiation dose in CT imaging while maintaining overall image quality. This study aims to develop a method for evaluating the effects of tube current modulation (TCM) on organ dose in CT exams of actual patient anatomy. This method was validated by simulating a TCM and a fixed tube current chest CT exam on 30 voxelized patient models and estimating the radiation dose to each patient’s glandular breast tissue. This new method for estimating organ dose was compared with other conventional estimates of dose reduction. Thirty detailed voxelized models of patient anatomy were created based on image data from female patients who had previously undergone clinically indicated CT scans including the chest area. As an indicator of patient size, the perimeter of the patient was measured on the image containing at least one nipple using a semi-automated technique. The breasts were contoured on each image set by a radiologist and glandular tissue was semi-automatically segmented from this region. Previously validated Monte Carlo models of two multidetector CT scanners were used, taking into account details about the source spectra, filtration, collimation and geometry of the scanner. TCM data were obtained from each patient’s clinical scan and factored into the model to simulate the effects of TCM. For each patient model, two exams were simulated: a fixed tube current chest CT and a tube current modulated chest CT. X-ray photons were transported through the anatomy of the voxelized patient models, and radiation dose was tallied in the glandular breast tissue. The resulting doses from the tube current modulated simulations were compared to the results obtained from simulations performed using a fixed mA value. The average radiation dose to the glandular breast tissue from a fixed tube current scan across all patient models was 19 mGy. The average reduction in breast dose using the tube current modulated scan was 17%. Results were size dependent with smaller patients getting better dose reduction (up to 64% reduction) and larger patients getting a smaller reduction, and in some cases the dose actually increased when using tube current modulation (up to 41% increase). The results indicate that radiation dose to glandular breast tissue generally decreases with the use of tube current modulated CT acquisition, but that patient size (and in some cases patient positioning) may affect dose reduction.

The purpose of this work is to examine the effects of patient size on radiation dose from CT scans. To perform these investigations, we used Monte Carlo simulation methods with detailed models of both patients and multidetector computed tomography (MDCT) scanners. A family of three-dimensional, voxelized patient models previously developed and validated by the GSF was implemented as input files using the Monte Carlo code MCNPX. These patient models represent a range of patient sizes and ages (8 weeks to 48 years) and have all radiosensitive organs previously identified and segmented, allowing the estimation of dose to any individual organ and calculation of patient effective dose. To estimate radiation dose, every voxel in each patient model was assigned both a specific organ index number and an elemental composition and mass density. Simulated CT scans of each voxelized patient model were performed using a previously developed MDCT source model that includes scanner specific spectra, including bowtie filter, scanner geometry and helical source path. The scan simulations in this work include a whole-body scan protocol and a thoracic CT scan protocol, each performed with fixed tube current. The whole-body scan simulation yielded a predictable decrease in effective dose as a function of increasing patient weight. Results from analysis of individual organs demonstrated similar trends, but with some individual variations. A comparison with a conventional dose estimation method using the ImPACT spreadsheet yielded an effective dose of 0.14 mSv mAs(-1) for the whole-body scan. This result is lower than the simulations on the voxelized model designated 'Irene' (0.15 mSv mAs(-1)) and higher than the models 'Donna' and 'Golem' (0.12 mSv mAs(-1)). For the thoracic scan protocol, the ImPACT spreadsheet estimates an effective dose of 0.037 mSv mAs(-1), which falls between the calculated values for Irene (0.042 mSv mAs(-1)) and Donna (0.031 mSv mAs(-1)) and is higher relative to Golem (0.025 mSv mAs(-1)). This work demonstrates the ability to estimate both individual organ and effective doses from any arbitrary CT scan protocol on individual patient-based models and to provide estimates of the effect of patient size on these dose metrics.

To determine retrospectively the frequency of two artifact patterns that mimic pathologic lesions on computed tomographic (CT) head images acquired in the axial scanning mode with two different multi-detector row CT systems at the same institution.The institutional review board approved this Health Insurance Portability and Accountability Act-compliant study and waived informed consent. The study involved two groups of consecutive patients, a group of 22 (nine men, 13 women; mean age, 56 years; age range, 27-85 years) examined with one multi-detector row CT system with four detector rows, and another group of 13 (seven men, six women; mean age, 69 years; age range, 53-81 years) examined with a different four-detector row CT system. Examinations in each group took place in a 4-week period. CT images were retrospectively evaluated by a neuroradiologist and a physicist for presence, appearance, location (within the image set and on individual images), and size of artifacts. Elimination of artifacts was verified by scanning a water phantom after scanner service and repair.A pseudolesion, or artifact, was identified in scans of four of 22 patients examined with the first scanner and eight of 13 patients examined with the second scanner. The artifact on images obtained on the first scanner, an approximately 2-cm-diameter faintly hyperattenuating and nonenhancing area with hypoattenuating collar, was found at gantry isocenter on every fourth image. A different pattern was found on images obtained on the second scanner: a 1.1-cm-diameter circular area of hypoattenuation with a faintly attenuating rim, that mimicked a cyst. This artifact was observed also at the CT scanner gantry isocenter on every fourth image. Artifacts disappeared after recalibration (first scanner) or collimator cleaning (second scanner).CT scanning in the axial mode can produce a regularly repeating artifact when data from one detector row of a multi-detector row CT scanner are compromised. Because of the risk of misinterpreting such patterns, routine assessment of each detector element is recommended for multi-detector row CT scanners that are routinely used in the axial scanning mode.

The purpose of this work was to extend the verification of Monte Carlo based methods for estimating radiation dose in computed tomography (CT) exams beyond a single CT scanner to a multidetector CT (MDCT) scanner, and from cylindrical CTDI phantom measurements to both cylindrical and physical anthropomorphic phantoms. Both cylindrical and physical anthropomorphic phantoms were scanned on an MDCT under the specified conditions. A pencil ionization chamber was used to record exposure for the cylindrical phantom, while MOSFET (metal oxide semiconductor field effect transistor) detectors were used to record exposure at the surface of the anthropomorphic phantom. Reference measurements were made in air at isocentre using the pencil ionization chamber under the specified conditions. Detailed Monte Carlo models were developed for the MDCT scanner to describe the x-ray source (spectra, bowtie filter, etc) and geometry factors (distance from focal spot to isocentre, source movement due to axial or helical scanning, etc). Models for the cylindrical (CTDI) phantoms were available from the previous work. For the anthropomorphic phantom, CT image data were used to create a detailed voxelized model of the phantom's geometry. Anthropomorphic phantom material compositions were provided by the manufacturer. A simulation of the physical scan was performed using the mathematical models of the scanner, phantom and specified scan parameters. Tallies were recorded at specific voxel locations corresponding to the MOSFET physical measurements. Simulations of air scans were performed to obtain normalization factors to convert results to absolute dose values. For the CTDI body (32 cm) phantom, measurements and simulation results agreed to within 3.5% across all conditions. For the anthropomorphic phantom, measured surface dose values from a contiguous axial scan showed significant variation and ranged from 8 mGy/100 mAs to 16 mGy/100 mAs. Results from helical scans of overlapping pitch (0.9375) and extended pitch (1.375) were also obtained. Comparisons between the MOSFET measurements and the absolute dose value derived from the Monte Carlo simulations demonstrate agreement in terms of absolute dose values as well as the spatially varying characteristics. This work demonstrates the ability to extend models from a single detector scanner using cylindrical phantoms to an MDCT scanner using both cylindrical and anthropomorphic phantoms. Future work will be extended to voxelized patient models of different sizes and to other MDCT scanners.

The purpose of this study was to focus attention on the technique factors commonly used in survey CT scans (e.g., scout, topogram, or pilot scans) to measure the radiation exposure from typical survey CT scans, to compare their exposure to that of typical chest radiographs, and to explore methods for radiation exposure reduction.The default survey CT scans on 21 CT scanners, representing three different vendors and 11 different models, were investigated. Exposure measurements were obtained with an ion chamber at isocenter and adjusted to be consistent with standard chest radiographic exposure measurement methods (single posterior-anterior projection). These entrance exposures were compared with those of typical chest radiographs, for which the mean for average-sized adults is 16 mR (4.1 x 10(-6) C/kg).The entrance exposures of the default survey CT scans ranged from 3.2 to 74.7 mR (0.8 to 19.3 x 10(-6) C/kg), which is equivalent to approximately 0.2 to 4.7 chest radiographs. By changing the default scan parameters from 120 kVp to 80 kVp and the tube position from 0 degrees (tube above table) to 180 degrees (tube below table), the entrance exposure for the survey CT scan was reduced to less than that of one chest radiograph for all CT scanners.For institutions at which the interpreting radiologists do not rely heavily on the appearance of the survey CT image, we recommend adjusting the technique parameters (kilovoltage and X-ray tube position) to decrease radiation exposure, especially for vulnerable patient populations such as children and young women.

OBJECTIVE: The purpose of our study was to accurately estimate the radiation dose to skin and the eye lens from clinical CT brain perfusion studies, investigate how well scanner output (expressed as volume CT dose index [CTDI(vol)]) matches these estimated doses, and investigate the efficacy of eye lens dose reduction techniques. MATERIALS AND METHODS: Peak skin dose and eye lens dose were estimated using Monte Carlo simulation methods on a voxelized patient model and 64-MDCT scanners from four major manufacturers. A range of clinical protocols was evaluated. CTDI(vol) for each scanner was obtained from the scanner console. Dose reduction to the eye lens was evaluated for various gantry tilt angles as well as scan locations. RESULTS: Peak skin dose and eye lens dose ranged from 81 mGy to 348 mGy, depending on the scanner and protocol used. Peak skin dose and eye lens dose were observed to be 66-79% and 59-63%, respectivelAmy, of the CTDI(vol) values reported by the scanners. The eye lens dose was significantly reduced when the eye lenses were not directly irradiated. CONCLUSION: CTDI(vol) should not be interpreted as patient dose; this study has shown it to overestimate dose to the skin or eye lens. These results may be used to provide more accurate estimates of actual dose to ensure that protocols are operated safely below thresholds. Tilting the gantry or moving the scanning region further away from the eyes are effective for reducing lens dose in clinical practice. These actions should be considered when they are consistent with the clinical task and patient anatomy.

A recent work has demonstrated the feasibility of estimating the dose to individual organs from multidetector CT exams using patient-specific, scanner-independent CTDIvol-to-organ-dose conversion coefficients. However, the previous study only investigated organ dose to a single patient model from a full-body helical CT scan. The purpose of this work was to extend the validity of this dose estimation technique to patients of any size undergoing a common clinical exam. This was done by determining the influence of patient size on organ dose conversion coefficients generated for typical abdominal CT exams.Monte Carlo simulations of abdominal exams were performed using models of 64-slice MDCT scanners from each of the four major manufacturers to obtain dose to radiosensitive organs for eight patient models of varying size, age, and gender. The scanner-specific organ doses were normalized by corresponding CTDIvol values and averaged across scanners to obtain scanner-independent CTDIvol-to-organ-dose conversion coefficients for each patient model. In order to obtain a metric for patient size, the outer perimeter of each patient was measured at the central slice of the abdominal scan region. Then, the relationship between CTDIvol-to-organ-dose conversion coefficients and patient perimeter was investigated for organs that were directly irradiated by the abdominal scan. These included organs that were either completely ("fully irradiated") or partly ("partially irradiated") contained within the abdominal exam region. Finally, dose to organs that were not at all contained within the scan region ("nonirradiated") were compared to the doses delivered to fully irradiated organs.CTDIvol-to-organ-dose conversion coefficients for fully irradiated abdominal organs had a strong exponential correlation with patient perimeter. Conversely, partially irradiated organs did not have a strong dependence on patient perimeter. In almost all cases, the doses delivered to nonirradiated organs were less than 5%, on average across patient models, of the mean dose of the fully irradiated organs.This work demonstrates the feasibility of calculating patient-specific, scanner-independent CTDIvol-to-organ-dose conversion coefficients for fully irradiated organs in patients undergoing typical abdominal CT exams. A method to calculate patient-specific, scanner-specific, and exam-specific organ dose estimates that requires only knowledge of the CTDIvol for the scan protocol and the patient's perimeter is thus possible. This method will have to be extended in future studies to include organs that are partially irradiated. Finally, it was shown that, in most cases, the doses to nonirradiated organs were small compared to the dose to fully irradiated organs.

Lymphoscintigraphy is a nuclear medicine procedure that is used to detect sentinel lymph nodes (SLNs). This project sought to investigate fusion of planar scintigrams with CT topograms as a means of improving the anatomic reference for the SLN localization. Heretofore, the most common lymphoscintigraphy localization method has been backlighting with a 57Co sheet source. Currently, the most precise method of localization through hybrid SPECT/CT increases the patient absorbed dose by a factor of 34 to 585 (depending on the specific CT technique factors) over the conventional 57Co backlighting. The new approach described herein also uses a SPECT/CT scanner, which provides mechanically aligned planar scintigram and CT topogram data sets, but only increases the dose by a factor of two over that from 57Co backlighting. Planar nuclear medicine image fusion with CT topograms has been proven feasible and offers a clinically suitable compromise between improved anatomic details and minimally increased radiation dose.

Children are exposed to ionizing radiation during pre- and post-operative evaluation for craniofacial surgery.The primary purpose of the study was to decrease effective radiation dose while preserving the diagnostic quality of the study.In this prospective study 49 children were positioned during craniofacial CT (CFCT) imaging with their neck fully extended into an exaggerated sniff position, parallel to the CT gantry, to eliminate the majority of the cervical spine and the thyroid gland from radiation exposure. Image-quality and effective radiation dose comparisons were made retrospectively in age-matched controls (n = 49).When compared to CT scans reviewed retrospectively, the prospective examinations showed a statistically significant decrease in z-axis length by 16% (P0.0001) and delivered a reduced effective radiation dose by 18% (P0.0001). The subjective diagnostic quality of the exams performed in the prospective arm was maintained despite a slight decrease in the quality of the brain windows. There was statistically significant improvement in the quality of the bone windows and three-dimensional reconstructed images.Altering the position of the head by extending the neck during pediatric craniofacial CT imaging statistically reduces the effective radiation dose while maintaining the diagnostic quality of the images.

The aims of this study were to estimate the dose to radiosensitive organs (glandular breast and lung) in patients of various sizes undergoing routine chest CT examinations with and without tube current modulation; to quantify the effect of tube current modulation on organ dose; and to investigate the relation between patient size and organ dose to breast and lung resulting from chest CT examinations.Thirty voxelized models generated from images of patients were extended to include lung contours and were used to represent a cohort of women of various sizes. Monte Carlo simulation-based virtual MDCT scanners had been used in a previous study to estimate breast dose from simulations of a fixed-tube-current and a tube current-modulated chest CT examinations of each patient model. In this study, lung doses were estimated for each simulated examination, and the percentage organ dose reduction attributed to tube current modulation was correlated with patient size for both glandular breast and lung tissues.The average radiation dose to lung tissue from a chest CT scan obtained with fixed tube current was 23 mGy. The use of tube current modulation reduced the lung dose an average of 16%. Reductions in organ dose (up to 56% for lung) due to tube current modulation were more substantial among smaller patients than larger. For some larger patients, use of tube current modulation for chest CT resulted in an increase in organ dose to the lung as high as 33%. For chest CT, lung dose and breast dose estimates had similar correlations with patient size. On average the two organs receive approximately the same dose effects from tube current modulation.The dose to radiosensitive organs during fixed-tube-current and tube current-modulated chest CT can be estimated on the basis of patient size. Organ dose generally decreases with the use of tube current-modulated acquisition, but patient size can directly affect the dose reduction achieved.

The purpose of this study was to present a method for generating x-ray source models for performing Monte Carlo (MC) radiation dosimetry simulations of multidetector row CT (MDCT) scanners. These so-called “equivalent” source models consist of an energy spectrum and filtration description that are generated based wholly on the measured values and can be used in place of proprietary manufacturer’s data for scanner-specific MDCT MC simulations. Required measurements include the half value layers (HVL1 and HVL2) and the bowtie profile (exposure values across the fan beam) for the MDCT scanner of interest. Using these measured values, a method was described (a) to numerically construct a spectrum with the calculated HVLs approximately equal to those measured (equivalent spectrum) and then (b) to determine a filtration scheme (equivalent filter) that attenuates the equivalent spectrum in a similar fashion as the actual filtration attenuates the actual x-ray beam, as measured by the bowtie profile measurements. Using this method, two types of equivalent source models were generated: One using a spectrum based on both HVL1 and HVL2 measurements and its corresponding filtration scheme and the second consisting of a spectrum based only on the measured HVL1 and its corresponding filtration scheme. Finally, a third type of source model was built based on the spectrum and filtration data provided by the scanner’s manufacturer. MC simulations using each of these three source model types were evaluated by comparing the accuracy of multiple CT dose index (CTDI) simulations to measured CTDI values for 64-slice scanners from the four major MDCT manufacturers. Comprehensive evaluations were carried out for each scanner using each kVp and bowtie filter combination available. CTDI experiments were performed for both head (16 cm in diameter) and body (32 cm in diameter) CTDI phantoms using both central and peripheral measurement positions. Both equivalent source model types result in simulations with an average root mean square (RMS) error between the measured and simulated values of approximately 5% across all scanner and bowtie filter combinations, all kVps, both phantom sizes, and both measurement positions, while data provided from the manufacturers gave an average RMS error of approximately 12% pooled across all conditions. While there was no statistically significant difference between the two types of equivalent source models, both of these model types were shown to be statistically significantly different from the source model based on manufacturer’s data. These results demonstrate that an equivalent source model based only on measured values can be used in place of manufacturer’s data for Monte Carlo simulations for MDCT dosimetry.

To use Monte Carlo simulations of a current-technology multidetector computed tomographic (CT) scanner to investigate fetal radiation dose resulting from an abdominal and pelvic examination for a range of actual patient anatomies that include variation in gestational age and maternal size.Institutional review board approval was obtained for this HIPAA-compliant retrospective study. Twenty-four models of maternal and fetal anatomy were created from image data from pregnant patients who had previously undergone clinically indicated CT examination. Gestational age ranged from less than 5 weeks to 36 weeks. Simulated helical scans of the abdominal and pelvic region were performed, and a normalized dose (in milligrays per 100 mAs) was calculated for each fetus. Stepwise multiple linear regression was performed to analyze the correlation of dose with gestational age and anatomic measurements of maternal size and fetal location. Results were compared with several existing fetal dose estimation methods.Normalized fetal dose estimates from the Monte Carlo simulations ranged from 7.3 to 14.3 mGy/100 mAs, with an average of 10.8 mGy/100 mAs. Previous methods yielded values of 10-14 mGy/100 mAs. The correlation between gestational age and fetal dose was not significant (P = .543). Normalized fetal dose decreased linearly with increasing patient perimeter (R(2) = 0.681, P.001), and a two-factor model with patient perimeter and fetal depth demonstrated a strong correlation with fetal dose (R(2) = 0.799, P.002).A method for the estimation of fetal dose from models of actual patient anatomy that represented a range of gestational age and patient size was developed. Fetal dose correlated with maternal perimeter and varied more than previously recognized. This correlation improves when maternal size and fetal depth are combined.

Single photon emission computed tomography/computed tomography (SPECT/CT) delivers in a single imaging modality the functional-metabolic information from the SPECT image, combined with the detailed anatomical information from a diagnostic quality CT scanner.In this review, we provide the details for the acquisition, processing, and display of the SPECT, as well as the CT, and the fused SPECT/CT images, with one of the newest devices that combines a dual-headed gamma camera with a multislice CT scanner. Also, we go over the performance characteristics, including the planning and installation requirements for this type of scanners. In addition, we describe what are the current and feasible near-future applications of this new and exciting hybrid imaging modality.The ability to combine an optimized state-of-the-art SPECT image, with resolutions down to 5 mm, with a diagnostic quality CT image-using slices as thin as 1.25 mm-provides a diagnostic advantage that potentially can deliver a more convenient and faster diagnosis, with clinical implications in a significant percentage of patients. This imaging technique has been investigated in a wide range of studies for the oncologic patient, including but not limited to bone scintigraphy, (111)In-pentetreotide scintigraphy, lymphoscintigraphy, (67)Ga and labeled leukocyte infection imaging, (131)I-metaiodobenzylguanidine, parathyroid scintigraphy, (131)I diagnostic scintigraphy, and (111)In ProstaScint, and for planning of radionuclide therapy.Therefore, this evolving and exciting imaging modality will continue to grow and define its place as an integral part of the evaluation of the cancer patient.

Rationale and Objectives: The impact of varying image acquisition parameters on the precision of measurements using quantitative computed tomography is currently based on studies performed before the advent of helical image acquisition and multidetector-row scanners. The aim of this study was to evaluate helical multidetector-row quantitative computed tomography to determine the factors contributing to the overall precision of measurements on quantitative computed tomography conducted using current vintage computed tomographic (CT) scanners. Materials and Methods: The effects of CT protocol parameters (x-ray tube voltage and current, pitch, gantry rotation speed, detector configuration, table height, and reconstruction algorithm) and short-term scanner variation were examined on two commercially available quantitative CT (QCT) systems (ie, a combination of reference phantoms and analysis software) using seven multidetector-row CT scanners (available from a single vendor) operated in helical mode. Combined with simulated patient repositioning using three ex vivo spine specimens, precision (coefficient of variation) estimates were made on the basis of three scenarios: "best case," "routine case," and "worst case.". Results: The overall best-case QCT precision was 1.4, provided that no changes were permitted to the bone mineral density (BMD) scan protocol. Routine-case examination (with a BMD reference phantom in place) that permitted some variation in the x-ray tube current and table speed produced a precision of 1.8. Without any constraints on the clinical QCT examinations, the worst-case precision was estimated at 3.6. Conclusions: Although small in appearance, these errors are for single time points and may increase substantially when monitoring changes through QCT measurements over several time points. This calls for increased caution and attention to detail whenever using helical multidetector-row quantitative computed tomography for the assessment of BMD change. © 2009 AUR.

Pregnant women with shortness of breath are increasingly referred for CT Angiography to rule out Pulmonary Embolism (PE). While this exam is typically focused on the lungs, extending scan boundaries and overscan can add to the irradiated volume and have implications on fetal dose. The purpose of this work was to estimate radiation dose to the fetus when various levels of overscan were encountered. Two voxelized models of pregnant patients derived from actual patient anatomy were created based on image data. The models represent an early (< 7 weeks) and late term pregnancy (36 weeks). A previously validated Monte Carlo model of an MDCT scanner was used that takes into account physical details of the scanner. Simulated helical scans used 120 kVp, 4x5 mm beam collimation, pitch 1, and varying beam-off locations (edge of the irradiated volume) were used to represent different protocols plus overscan. Normalized dose (mGy/100mAs) was calculated for each fetus. For the early term and the late term pregnancy models, fetal dose estimates for a standard thoracic PE exam were estimated to be 0.05 and 0.3 mGy/100mAs, respectively, increasing to 9 mGy/100mAs when the beam-off location was extended to encompass the fetus. When performing PE exams to rule out PE in pregnant patients, the beam-off location may have a large effect on fetal dose, especially for late term pregnancies. Careful consideration of ending location of the x-ray beam - and not the end of image data - could result in significant reduction in radiation dose to the fetus.

Multidetector CT (MDCT) systems offer larger coverage and wider z-axis beams, resulting in larger cone angles. One impact on radiation dose is that while radiation profiles at isocenter are constant when contiguous axial scans are performed, the increased beam divergence from the larger cone angle results in significant surface dose variation. The purpose of this work was to measure the magnitude of this effect. Contiguous axial scans were acquired using an MDCT for two sizes of cylindrical phantoms and an anthropomorphic phantom. Film dosimetry and/or radiation detector measurements were performed on the surface of each phantom. Detailed mathematical models were developed for the MDCT scanner and all phantoms. Monte Carlo simulations of contiguous axial scans were performed for each phantom model. From cylindrical phantoms, film dosimetry at the surface showed differences between peak and valley that reached 50%. From the anthropomorphic phantom, measured values ranged from 7.9 to 16.2 mGy at the phantom surface. Monte Carlo simulations demonstrated these variations in both cylindrical and anthropomorphic phantoms. The magnitude of variation was also related to object size. Even when contiguous axial scans are performed on MDCT, surface radiation profiles show considerable variation. This variation will increase as MDCT cone angles increase and when non-contiguous scans (e.g. pitch > 1) are acquired. The variation is also a function of object size. While average surface doses may remain constant, peak doses may increase, which may be significant for radiation sensitive organs at or near the surface (e.g. breast, thyroid).

Purpose: Implementing Quantitative Computed Tomography (QCT) on Multi‐Slice Computed Tomography (MSCT) scanners requires investigating the effects of axial vs. helical scan modes and protocol parameter variations on quantitative data. While previous work in this area focused on single‐slice axial techniques, technological developments in Computed Tomography(CT) justify more complex assessment. Method and Materials: All scans were obtained using two phantoms designed for bone mineral (BM) densitometry: a reference phantom (three different density cores) and a QA torso phantom. Both phantoms have known properties and are required for long‐term quantitative BM density assessment. The scan acquisition parameters that were varied included kV, mA, rotation speed, pitch, image thickness, detector configuration, reconstruction algorithm, table height, and tube temperature. To assess long‐term scanner drift, the QA Torso phantom was scanned multiple times over three months on each of seven MSCT scanners (five GE Lightspeed‐ 16s, one GE Lightspeed Qx/i, and one GE Lightspeed‐Plus). The daily variability of the individual MSCT scanners and scanner‐to‐scanner variability was determined by coefficient of variation (mean/variance) from the QA Torso phantom data sets over time. All data were collected and analyzed in Hounsfield Units (HU) to provide insight about variations upstream of the actual BM density analysis through commercial software.Results: This study found no significant difference (p > 0.05) in mean HU between phantom images obtained using axial and helical scan mode, or when varying most of the other scan acquisition parameters. However, varying kV and reconstruction algorithm did result in significant (p

Purpose: The results of a long-term, comprehensive CT quality control (QC) program were analyzed to investigate differences in failure rates based on QC test, scanner utilization pattern, and number of channels, as well as explore issues regarding testing frequency. Methods: CT QC data were collected over a 4-yr period for 26 CT scanners representing two different vendors and using three different QC programs culminating in over 100 scanner-years of QC data. QC tests analyzed included water tests [mean CT number, standard deviation, and uniformity], linearity tests [air, water, and acrylic], and artifact analysis [water phantom and large phantom]. The data were organized based on scanner use, number of channels, scanner modality, and QC test. Logistic regression model analysis with generalized estimating equation method was used to estimate failure rates for each group. Results: A significant difference between failure rates with respect to QC test was found (p-value = 0.02). Large phantom artifacts, standard deviation of water, and water phantom artifacts had the three highest failure rates. No significant difference was found between failure rates organized by scanner use, scanner modality, or number of channels. Conclusions: Standard deviation of water is the most important quantitative value to collect as part of a daily QC program. Uniformity and linearity tests have relatively low failure rates and, therefore, may not require daily verification. While its failure rates were moderate, daily artifact analysis is suggested due to its potentially high impact on clinical image quality. Weekly or monthly large phantom artifact analysis is encouraged for those sites possessing an appropriate phantom.

Purpose: While several ionization chambers and solid state detectors are commercially available for CT, their energy dependency has yet to be determined. This work evaluates the energy response of a 0.6‐cc ionization chamber and solid state dosimeter against an ionization chamber specifically calibrated at several diagnostic energy levels (reference chamber) as a function of HVL for in‐air and in‐CTDI phantom measurements. Methods: For two CTscanner models of known difference in beam spectra, HVL's for all available kVp's were obtained with measurements (in‐air) and from Monte Carlo simulations (in head and body CTDI phantom). The reference ionization chamber was calibrated by the UWADCL using reference beams with HVL's in the CT range. Calibration factors for the reference chamber at each measured/simulated HVL were calculated based on polynomial fit of the UWADCL‐beam factors. For each scanner, exposure conditions were established at each kVp by varying mAs to obtain similar results in the reference chamber. Both test dosimeters were then scanned at these conditions and the exposure (or dose) relative to the reference chamber was obtained; no specific energy corrections were applied for either test device. The same procedure was performed with the head and body CTDI phantoms for both CTscanners. Results: HVL's from the scanners ranged from 3.49 to 9.7mm Al in‐air and from 3.1 to 8.8mm Al in‐phantom. Doses measured by the test chamber matched those of the reference chamber across scanners and kVp's to within 3.5% in‐air, 4.9% in head phantom and 11.8% in body phantom. Differences in the solid state device were larger: as high as 16%, 21%, and 37% for in‐air, head, and body phantom, respectively. Conclusions: While ionization chambers have some energy dependence in the diagnostic range, solid state detectors exhibit a more pronounced energy dependence and correction factors should be applied based on HVL.

Purpose: Reduced kVp scanning is being recommended for pediatric patients as well as when contrast studies are being performed. The purpose of this work is to quantitatively compare the organ doses resulting from scans at different kVP and determine the ratio of mAs at each kVp that results in equivalent organ dose. Methods: Using Monte Carlo simulations, two fixed tube current chest exams with 120kVp and 80kVp were simulated for 18 voxelized adult female patient models. Each patient had lung and glandular breast tissues contoured, so that doses to these tissues could be estimated. The resulting organ doses for two kVps were compared for a given mAs, which allowed a comparison to identify the ratio of mAs values needed to achieve the same organ dose for 120kVp and 80kVp. For a further comparison, these values were compared to CTDIw values obtained at both kVps in both 16cm and 32cm CTDI phantoms. Results: For lung dose, the ratio of dose from 120kVp to 80kVp was 3.96 (standard deviation of 0.17) for a given mAs across all 18 patient models; similarly, the glandular breast dose ratio was 4.15 (standard deviation of 0.12). For the CTDIw phantom values, the ratio of dose values from 120kVp to 80kVp were 3.37 and 3.78 for 16 cm and 32 cm phantoms, respectively. Conclusions: Monte Carlo simulation results indicate that lowering tube voltage from 120 to 80 results in almost 4 fold radiation dose reduction for a given mAs. Stated another way, the mAs at 80kVp could be increased by up to a factor of four compared to mAs used at 120kVp and still receive a similar organ dose in chest scans. This is an even higher ratio than predicted by CTDI phantom results. Future studies include pediatric models with contoured lungs and breast.

The Calibration Laboratory Accreditation Subcommittee (CLA) of the AAPM oversees the accreditation of the ADCLs (Accredited DosimetryCalibration Laboratories) who provide calibration services to clinical users in North America and around the world. The AAPM accreditation is similar to other accreditation programs that use the international standard for calibration and testing laboratories (ISO17025) but with additional focus on technical competencies through a strict set of AAPM‐developed Criteria. The purpose of this accreditation is to provide users with the highest level of confidence in equipment calibrations which form the basis for radiation treatments in the clinic. The CLA is currently working towards obtaining recognition by the National Cooperation for Laboratory Accreditation (NACLA) that the accreditation program satisfies the ISO standard 17011. This standard covers accrediting bodies and one of its requirements is that AAPM develop a pool of trained assessors. The main of aim, therefore, of this presentation is to give potential assessors (who could be members of the AAPM outside of the CLA) an introduction to the accreditation process with particular focus on on‐site assessments. Accreditation by the CLA requires a site visit every four years in which every aspect of the laboratories performance (as related to calibration services) is assessed. Quality manuals and procedures are reviewed and sample calibrations witnessed to verify that the laboratories are continuing to provide the high quality of service demanded by the AAPM Criteria. Although this lecture is aimed specifically at a small sub‐group of AAPM members there is much in the discussion of quality systems and the audit process that would of value to a wider audience. The details may be different but the approach set out here (e.g. development and review of Quality Systems, gathering of objective evidence) can be applied to a variety of QA applications in clinics and elsewhere. Since it is not possible to cover the whole accreditation process in one lecture those interested in becoming assessors for the CLA should contact the lead author prior to the meeting for additional material to review. Educational Objectives: 1. Understand the role of the CLA in the accreditation of the ADCLs. 2. Understand the requirements of the AAPM Criteria for Accreditation. 3. Understand the steps in the accreditation process. 4. Understand how an accreditation audit should be carried out.

Purpose: To develop a filmless technique for the measurement of CT beam width for QA. Method and Materials:Images of pre‐patient collimated CT beams were acquired using Fuji computed radiography(CR) cassettes and direct‐exposure film (DF). CR cassettes were exposed using 80kV, 10mAs to avoid signal saturation. The DF was exposed at 80kV, 30mAs to achieve a proper optical density. The DF image was digitized then linearized. To achieve a proper signal range for CR, various plate‐reading protocols were evaluated. An IDL program was developed to automatically handle both the digitized film and a variety of CRimage sizes. “Unprocessed” logarithmic CR data were linearized, which is critical to validate the FWHM. The program executes in an interactive fashion, whereby the user selects the beam to analyze. The program determines the background signal for subtraction then samples multiple cross‐beam profiles to form an averaged profile. The profile is smoothed and the FWHM is determined. Results: Under proper conditions of image acquisition, results show both DF and CR are appropriate techniques for beam width measurements. The CR technique demonstrated a high level of reproducibility with a coefficient of variation below 0.5%. Also, the CR results were within 0.3mm of specifications. Using various sizes of CR cassettes had a minimal effect on the results. However, if the properly exposed image is saturated due to an inappropriate CR plate reading protocol, the inaccuracy is substantial (>1mm). The technique worked well for annual tests of six GEscanners. Over a wide range of beam width configurations, all measurements were within 0.9mm of specifications. Conclusion: A technique for determination of CT beam width in a filmless environment was developed and tested. The results obtained using CR yield accurate measurements of CT beam profile width. The technique is effective, practical, and has proven to be robust.

Purpose: It is the expectation of CT simulations to get the best CTimages with free breathing scans. CTimaging protocol settings needed to achieve optimal image quality have been identified for every scanner in a Radiation Oncology Department having various makes and models of CT simulators. Method and Materials: A Catphan® phantom was used to evaluate the image quality of 5 different makes/models. Various pitch, detector configuration, and gantry speed parameters were studied, while maintaining constant kVp, effective mAs, head display field of view (DFOV), reconstruction filter, and image thickness settings. Three aspects of image quality were evaluated: 1) helical artifact presence, 2) high contrast resolution, and 3) low contrast resolution. Results: For all CTs, more artifacts appear when the channel width is equal to the image thickness. Less helical artifacts are present if the channel width is half or less of the image thickness. At these settings, however, artifacts are still present in varying degrees depending on pitch. For a 16‐channel CT of one manufacturer, the artifact‐free pitch setting is 1.4 or less, but for their 16‐channel large bore CT, it is 0.9 or less. For another manufacturer's 16‐channel CT, the channel width has to be 1/4 of the image thickness with a pitch of 0.6 to obtain artifact‐free images. The optimal settings for all scanners are different. Only slight differences in high and low contrast resolution are observed, even at varying gantry rotation speeds; these differences seem insignificant at body DFOV (clinical setting). Conclusions: In order to obtain optimal CTimage settings, a study should be conducted for each CT model, for there is a vast difference in optimal settings among them. Faster rotation gantry speed should be used as possible, for reduced motion artifact is expected for patient free‐breathing scans.

Purpose: To estimate the absorbed dose for a range of settings of CT topograms and scans to determine the best technique for node localization in breast lymphoscintigraphy while minimizing the dose to the patient. Method and Materials: This study was performed with an anthropomorphic thorax phantom on a Symbia‐T6® SPECT/CT to produce mechanically registered datasets [Siemens Medical Solutions USA, Hoffman Estates IL]. The data included a SPECT, anterior and lateral topograms, a CT, and static anterior and lateral emission scintigrams. CT topograms and scans were acquired at 80, 110, and 130 kVp with varied mAs settings to compare the quality of the images and dose estimates for lower dose settings. Scintigram and SPECT scans were fused to each topogram and CT, respectively, to allow for qualitative comparison of localization. Exposure measurements for topograms were acquired using an ion chamber at isocenter, and entrance skin doses were estimated for the CT topograms. A CTDI body phantom was used to acquire data for estimation of effective dose for the CT scans.Results: The skin dose for a CT topogram ranges from 0.089 mGy to 0.235 mGy, and the effective dose for the CT scan ranges from 0.308 mSv (lowest dose settings) to 5.265 mSv (clinical scan setting). Qualitative comparison showed the 80 kVp, 20 mA topogram and 110 kVp, 10 mAs CT to be sufficient for localization. The skin dose is 0.089 uSv and the effective dose is 0.392 uSv for these techniques, respectively. Conclusion: The absorbed dose to the patient can be substantially reduced for CT topograms and CTs that are to be used for localization. These do not have to be of diagnostic quality in order to provide enough anatomical information to localize the sentinel nodes in breast lymphoscintigraphy. Conflict of Interest: Research sponsored by Siemens Medical Solutions USA.

Purpose: Technical advances in CT have led to an increase in the number of coronary CTangiography (CTA) exams being performed. These exams often use low pitch values to accomplish retrospective cardiac gating, which results in increased radiation dose. This study aims to estimate the dose to breast and lungtissue from a multidetector coronary CTA exam using voxelized models derived from actual patient anatomy spanning a range of sizes. Method and Materials: Nineteen voxelized models of female patient anatomy were created based on image data from clinical CT exams. Breast tissue was contoured by a radiologist, then glandular and lungtissue were automatically segmented. Previously validated Monte Carlo models of 16 and 64 slice scanners were used taking into account source spectra and path, filtration, collimation, and geometry. Simulated CTA exams used manufacturer‐recommended acquisition parameters and accounted for overscan. Organdose was calculated for the glandular breast and lungtissues.Results: Glandular breast dose estimates for the nineteen patient models ranged from 7 to 73 mGy, with an average of 47 mGy. Lungdose estimates ranged from 22 to 63 mGy, with an average of 46 mGy. Dose to both organs was significantly greater for smaller patients. Dose on the 16 slice scanner was higher than the 64 by approximately 14% for breast and 10% for lung, a difference not attributable to pitch or mA. Conclusion: Radiation dose to the breast and lung in CTA imaging was estimated using models of actual patient anatomy,scanner specifics, and acquisition protocols. Organdose was significantly larger for smaller patients, suggesting the need for patient size‐dependent tube current settings for CTA exams.

Purpose: To use detailed Monte Carlo simulations to investigate fetal dose from a multidetector CT (MDCT) using voxelized models created from actual patient images including early and late term pregnancies. Method and Materials: Detailed voxelized models of anatomy were created based on image data from a cohort of pregnant patients who had previously undergone abdomen/pelvis CT scans. The gestational ages ranged from less than 5 weeks to 33.7 weeks. Three regions, corresponding to fetus, gestational sac, and uterus, were contoured on each image series by a radiologist. A MDCT model was created using details about the source spectra, filtration, collimation, and geometry. To simulate an actual scan, a helical source path was defined and particles were transported through the anatomy of the voxelized patient models; radiation dose was tallied in voxels belonging to the three regions of interest. The simulated abdomen/pelvis scan used a helical scan of 120kVp, pitch 1, and 4 × 5 mm total nominal beam collimation. Dose on a per mAs basis was separately calculated for the fetus when the fetus was distinguishable from the gestational sac and uterus in the original image. These doses were then compared to two generally accepted fetus dose estimations: the Felmlee et al method and the ImPact estimation of dose to a uterus (for fetuses < 8 weeks). Results: The radiation dose to the fetus in the models with gestational ages of

Purpose: To investigate the feasibility of using air scan to determine quantitatively whether CT ring artifacts are clinically significant. Method and Materials: Clinical contrast medium Optiray 320 was used to make CT artifact tools, which were attached to CTdetectors at various locations. Preliminary scans of standard uniform phantoms were acquired on a 16‐slice CT scanner with and without artifact tools. Images of an anthropomorphic phantom (ATOM, CIRS, Inc.) were acquired with the chest pulmonary embolus protocol to determine whether the rings are clinically significant. Air scans were acquired with the same artifact tools in place using a clinical as well as a QC protocol. Standard deviation values of ROI placed in images of uniform phantoms and air scans were recorded. The air scan data associated with the rings that are least visible in the anthropomorphic phantoms were used to investigate whether a threshold exists which indicates if rings are clinically significant. Results: Sensitivity of this method varies slightly with image thickness, mAs, scan mode, and ROI size. Evaluation of QC air scans showed good separation of the standard deviation values between “blank” air scans and those with artifacts for different ring diameters. Different threshold values were found based on size of ROI used to measure standard deviation, and it is necessary to use at least two different ROI sizes for either small or large ring artifacts. Conclusion: While air scans are overly sensitive for the visual detection of ring artifacts in CTimages, they may be useful in establishing quantitative criteria for determining whether the rings present are clinically significant. With stack mode viewing, a quick and simple evaluation may be feasible for multidetector row CT scanners of large number of data channels for those institutions without the means to develop automated off‐line analysis.

The Americal College of Radiology introduced an accreditation program for Computed Tomography(CT) in 2002. The CT accreditation application requires the participation of a qualified medical physicist (with specific credentials) in order to submit appropriate phantom images and radiationdose measurements for CT scanners. Examples of acceptable CT accreditation phantom images will be presented, along with examples of the more commonly made mistakes. A brief review of the dosimetry procedures to follow, and pitfalls to avoid, will also be covered. Special attention will be directed toward the accreditation process for multi‐slice CT scanners.