Radiation issues in computed tomography screening

Heart-centered positioning and tailored beam-shaping filtration for reduced radiation dose in coronary artery calcium imaging: A Multi-Ethnic Study of Atherosclerosis (MESA) Study

PURPOSE

Cardiac computed tomography has a clear clinical role in the evaluation of coronary artery disease and assessment of coronary artery calcium (CAC) but the use of ionizing radiation limits the clinical use. Beam-shaping "bow-tie" filters determine the radiation dose and the effective scan field-of-view diameter (SFOV) by delivering higher X-ray fluence to a region centered at the isocenter. A method for positioning the heart near the isocenter could enable reduced SFOV imaging and reduce dose in cardiac scans. However, a predictive approach to center the heart, the extent to which heart centering can reduce the SFOV, and the associated dose reductions have not been assessed. The purpose of this study is to build a heart-centered patient positioning model, to test whether it reduces the SFOV required for accurate CAC scoring, and to quantify the associated reduction in radiation dose.

METHODS

The location of 38,184 calcium lesions (3151 studies) in the Multi-Ethnic Study of Atherosclerosis was utilized to build a predictive heart-centered positioning model and compare the impact of SFOV on CAC scoring accuracy in heart-centered and conventional body-centered scanning. Then, the positioning model was applied retrospectively to an independent, contemporary cohort of 118 individuals (81 with CAC > 0) at our institution to validate the model's ability to maintain CAC accuracy while reducing the SFOV. In these patients, the reduction in dose associated with a reduced SFOV beam-shaping filter was quantified.

RESULTS

Heart centering reduced the SFOV diameter 25.7% relative to body centering while maintaining high CAC scoring accuracy (0.82% risk reclassification rate). In our validation cohort, imaging at this reduced SFOV with heart-centered positioning and tailored beam-shaping filtration led to a 26.9% median dose reduction (25-75th percentile: 21.6%-29.8%) without any calcium risk reclassification.

CONCLUSIONS

Heart-centered patient positioning enables a significant radiation dose reduction while maintaining CAC accuracy.

Accuracy of emphysema quantification performed with reduced numbers of CT sections

OBJECTIVE

The purpose of this study was to assess the accuracy of emphysema measurements obtained from systematic samples of evenly spaced CT images compared with measurements obtained from the entire scan.

MATERIALS AND METHODS

Evenly spaced transverse sections from the CT studies of 136 heavy smokers who underwent screening for lung cancer in the National Lung Screening Trial and of 112 subjects who underwent imaging before lung volume reduction surgery were sampled retrospectively. The samples were acquired both by selection of specific numbers of evenly spaced images and by selection of images at specific distance intervals. The percentage of lung pixels with attenuation below specific thresholds was used as an emphysema index. The image sample error was determined as the difference in emphysema index between the image samples and the entire scan.

RESULTS

The largest absolute image sample errors in the National Lung Screening Trial cohort with image sample sizes of five, 10, and 20 were 2.2, 0.8, and 0.5 index percentage points, respectively, at 1-mm section thickness (-960 HU threshold), and 2.6, 1.1, and 0.5 index percentage points at 5-mm section thickness (-930 HU threshold). The largest errors in the lung volume reduction surgery cohort for image sample sizes of five and 10 were 5.6 and 2.3 index percentage points at 8- to 10-mm section thickness (-900 HU threshold). Image sample errors were equivalent for the two sampling methods.

CONCLUSION

Systematic sampling resulted in very small errors in emphysema quantification and may be useful for decreasing radiation exposure in clinical research studies of emphysema.

The development, validation and application of a multi-detector CT (MDCT) scanner model for assessing organ doses to the pregnant patient and the fetus using Monte Carlo simulations

The latest multiple-detector technologies have further increased the popularity of x-ray CT as a diagnostic imaging modality. There is a continuing need to assess the potential radiation risk associated with such rapidly evolving multi-detector CT (MDCT) modalities and scanning protocols. This need can be met by the use of CT source models that are integrated with patient computational phantoms for organ dose calculations. Based on this purpose, this work developed and validated an MDCT scanner using the Monte Carlo method, and meanwhile the pregnant patient phantoms were integrated into the MDCT scanner model for assessment of the dose to the fetus as well as doses to the organs or tissues of the pregnant patient phantom. A Monte Carlo code, MCNPX, was used to simulate the x-ray source including the energy spectrum, filter and scan trajectory. Detailed CT scanner components were specified using an iterative trial-and-error procedure for a GE LightSpeed CT scanner. The scanner model was validated by comparing simulated results against measured CTDI values and dose profiles reported in the literature. The source movement along the helical trajectory was simulated using the pitch of 0.9375 and 1.375, respectively. The validated scanner model was then integrated with phantoms of a pregnant patient in three different gestational periods to calculate organ doses. It was found that the dose to the fetus of the 3 month pregnant patient phantom was 0.13 mGy/100 mAs and 0.57 mGy/100 mAs from the chest and kidney scan, respectively. For the chest scan of the 6 month patient phantom and the 9 month patient phantom, the fetal doses were 0.21 mGy/100 mAs and 0.26 mGy/100 mAs, respectively. The paper also discusses how these fetal dose values can be used to evaluate imaging procedures and to assess risk using recommendations of the report from AAPM Task Group 36. This work demonstrates the ability of modeling and validating an MDCT scanner by the Monte Carlo method, as well as assessing fetal and organ doses by combining the MDCT scanner model and the pregnant patient phantom.

Description and implementation of a quality control program in an imaging-based clinical trial

RATIONALE AND OBJECTIVES

The American College of Radiology Imaging Network is participating in the National Lung Screening Trial, a large, multicenter, randomized controlled trial, comparing multidetector helical computed tomography (MDCT) versus chest radiography (CXR) in screening for lung cancer. Because the threshold for detection of disease is an inherent function of image quality, and consistent image quality is necessary to track changes in suspicious findings, our purpose was to develop an image quality control (QC) program across all clinical sites for both modalities.

MATERIALS AND METHODS

The primary goals of the QC program include standardization of imaging protocols, certification of imaging equipment, and ongoing, periodic evaluation of the equipment calibration and image quality. Minimum standards for equipment and standardized cross-platform acquisition protocols are achieved via radiologist and physicist attestation forms and web-distributed technique charts, respectively. Imaging equipment performance standards are implemented through an initial machine certification process that includes equipment calibration. Ongoing assessment of equipment performance and calibration, as well as adherence to established imaging protocols. is accomplished via periodic submission of calibration records and phantom images. Participant-specific image acquisition parameters are entered into a web-based centralized database and variations from established protocols are automatically flagged for review. Participant radiation dose can be estimated from the image acquisition parameters applied to the imaging equipment calibration measurements. A radiologist visual review committee also evaluates participant images for diagnostic quality. Data are collected from 23 independent centers, representing 14 models of MDCT scanners from four manufacturers, and CXR systems that include film-screen, computed radiography, and direct digital radiography systems.

RESULTS

Widespread imaging protocol variation in extant clinical practice-as well as variability in equipment technology, image acquisition parameters, manufacturer terminology, and user interface-have required careful standardization as a prerequisite to trial participation and ongoing image QC. Acceptable ranges for image acquisition parameters have been refined to accommodate continuously evolving equipment platforms and the scope of participant size and body habitus.

CONCLUSION

Standardization of imaging protocols is a critical component of image-based clinical trials, predicated on ongoing dialogue between sites and a centralized review committee.

CT Radiation Dose and Image Quality

Image quality is proportional to radiation dose. Improvements in image quality come at a cost of increased radiation dose. CT scanners are more robust today than even 5 years ago. X-ray tubes are capable of producing high levels of almost continuous radiation for rapid CT volume acquisitions and angiographic studies. All parameters of modern CT scanners provide rapid subsecond, large-volume CT acquisitions. How high we go needs to be tempered by how high we should go regarding radiation dose from CT examinations. Radiologists, referring physicians, medical physicists, CT technologists, CT equipment manufacturers, and regulators need to evaluate the appropriateness of radiation dose for different CT studies.

Coronary calcium measurements: effect of CT scanner type and calcium measure on rescan reproducibility--MESA study

PURPOSE

To evaluate the effect of scanner type and calcium measure on the reproducibility of calcium measurements.

MATERIALS AND METHODS

This investigation was approved by the institutional review boards of each study site and by the Institutional Review Board of the Los Angeles Biomedical Research Institute. Informed consent for scanning and participation was obtained from all participants. The study was Health Insurance Portability and Accountability Act compliant. The Multi-Ethnic Study of Atherosclerosis (MESA) is a multicenter observational study of 6814 participants undergoing demographic, risk factor, and subclinical disease evaluations. Coronary artery calcium was measured by using duplicate CT scans. Three study centers used electron-beam computed tomography (CT), and three used multi-detector row CT. Coronary artery calcium was detected in 3355 participants. Three calcium measurement methods-Agatston score, calcium volume, and interpolated volume score-were evaluated. Mean absolute differences between calcium measures on scans 1 and 2, excluding cases for which both scans had a measure of zero, was modeled by using linear regression to compare reproducibility between scanner types. A repeated measures analysis of variance test was used to compare reproducibility across calcium measures, with mean percentage absolute difference as the outcome measure. Rescan reproducibility in relation to misregistrations, noise, and motion artifacts was also examined. Variables were log transformed to create a more normal distribution.

RESULTS

Concordance for presence of calcium between duplicate scans was high and similar for both electron-beam and multi-detector row CT (96%, kappa = 0.92). Mean absolute difference between calcium scores for the two scans was 15.8 for electron-beam and 16.9 for multi-detector row CT scanners (P = .06). Mean relative differences were 20.1 for Agatston score, 18.3 for calcium volume, and 18.3 for interpolated volume score (P < .01). Reproducibility was lower for scans with versus those without image misregistrations or motion artifacts (P < .01 for both).

CONCLUSION

Electron-beam and multi-detector row CT scanners have equivalent reproducibility for measuring coronary artery calcium. Calcium volumes and interpolated volume scores are slightly more reproducible than Agatston scores. Reproducibility is lower for scans with misregistrations or motion artifacts.

Calcified coronary artery plaque measurement with cardiac CT in population-based studies: standardized protocol of Multi-Ethnic Study of Atherosclerosis (MESA) and Coronary Artery Risk Development in Young Adults (CARDIA) study

Calcified coronary artery plaque, measured at cardiac computed tomography (CT), is a predictor of cardiovascular disease and may play an increasing role in cardiovascular disease risk assessment. The Multi-Ethnic Study of Atherosclerosis (MESA) and the Coronary Artery Risk Development in Young Adults (CARDIA) study of the National Heart, Lung, and Blood Institute are population-based studies in which calcified coronary artery plaque was measured with electron-beam and multi-detector row CT and a standardized protocol in 6814 (MESA) and 3044 (CARDIA study) participants. The studies were approved by the appropriate institutional review board from the study site or agency, and written informed consent was obtained from each participant. Participation in the CT examination was high, image quality was good, and agreement for the presence of calcified plaque was high (kappa = 0.92, MESA; kappa = 0.77, CARDIA study). Extremely high agreement was observed between and within CT image analysts for the presence (kappa > 0.90, all) and amount (intraclass correlation coefficients, >0.99) of calcified plaque. Measurement of calcified coronary artery plaque with cardiac CT is well accepted by participants and can be implemented with consistently high-quality results with a standardized protocol and trained personnel. If predictive value of calcified coronary artery plaque for cardiovascular events proves sufficient to justify screening a segment of the population, then a standardized cardiac CT protocol is feasible and will provide reproducible results for health care providers and the public.

Asbestos-related Pleuropulmonary Diseases: Evaluation with Low-Dose Four–Detector Row Spiral CT

PURPOSE

To evaluate the depiction of lung and pleural asbestos-related lesions with low-dose four-detector row spiral computed tomography (CT).

MATERIALS AND METHODS

Eighty-three male workers with a mean duration of occupational exposure to asbestos of 18 years underwent CT as part of a medicolegal investigation. CT examination included low-dose multi-detector row spiral CT of the entire thorax, with reconstruction of contiguous 5-mm-thick images, and thin-section CT, which served as the reference standard for the detection of pleural and parenchymal asbestos-related abnormalities. Two main groups of abnormalities were identified: (a) pleural plaques and diffuse pleural thickening and (b) thickened interstitial short lines, curvilinear subpleural lines, ground-glass opacity with or without bronchiectasis, and honeycombing. The frequencies of the depiction of these abnormalities on the low-dose multi-detector row images and the thin-section images were compared by using the McNemar test.

RESULTS

No significant differences were observed between the low-dose and thin-section CT images in the depiction of either (a) parietal pleural fibrosis consisting of pleural plaques (identified in 67 [81%] vs 65 [78%] workers, P =.157), which appeared mainly as thick, calcified pleural linear structures; or (b) features of parenchymal fibrosis, which consisted of various combinations of intralobular and septal lines (identified in 12 [14%] vs 13 [16%] workers, P =.564), subpleural curvilinear lines (identified in 10 [12%] vs eight [10%] workers, P =.157), and ground-glass opacity with (identified in six [7%] vs six [7%] workers) or without (identified in five [6%] vs three [4%] workers, P =.317) traction bronchiectasis. A honeycombing pattern was depicted on only the thin-section CT images (P <.001). Emphysema (identified in 26 [31%] vs 14 [17%] workers at low-dose and thin-section CT, respectively; P <.001) and noncalcified nodules (identified in 18 [22%] workers vs one [1%] worker, P <.001) were depicted significantly more frequently on the low-dose images than on the thin-section images.

CONCLUSION

Low-dose multi-detector row spiral CT accurately depicts asbestos-related disease.

[Imaging pulmonary embolism]

The diagnostic performance of computed tomography images of pulmonary embolism is directly related to the acquisition parameters. Any physician evaluating these scans must have proper knowledge of the acquisition, injection, reconstruction, and radiation parameters. Cardiac gating and morphological and functional image processing should be understood since they are now routine techniques particularly important for preoperative assessment of chronic thromboembolism. Elementary knowledge of the imaging techniques reduces the risk of diagnostic limitations. Understanding these techniques does not require any particularly advanced knowledge of physics, data processing or technology, but is necessary to chose the appropriate technical facilities and equipment adapted for diagnostic purposes. While specific training is not a prerequisite, interpretation of an angioscan of the pulmonary vessels does require precise knowledge of the pulmonary anatomy in addition to the technical knowledge mentioned above. Proper analysis may reach the 4th and 5th generation vessels. Different analysis methods have been developed which take into account the technical parameters and avoid the need for serial images. Each slice can then be analyzed within an acquisition Volume. Differential diagnosis is also very technique-dependent, minimally operator-dependent but highly machine-dependent. Differential diagnosis becomes less and less a problem with advancing equipment. Sufficient knowledge of the physiological and pathogenic basis is relatively easy to retain.