Di Zhang, Maryam Khatonabadi, Hyun Kim, Matilda Jude, Edward Zaragoza, Margaret Lee, Maitraya Patel, Cheryce Poon, Michael Douek, Denise Andrews-Tang, Laura Doepke, Shawn McNitt-Gray, Chris Cagnon, John DeMarco, Michael McNitt-Gray
KEYWORDS: Diagnostics, Image quality, Computed tomography, Liver, Monte Carlo methods, Scanners, Data modeling, Medical imaging, Radiation effects, Inflammation
Purpose: While several studies have investigated the tradeoffs between radiation dose and image quality (noise) in CT
imaging, the purpose of this study was to take this analysis a step further by investigating the tradeoffs between patient
radiation dose (including organ dose) and diagnostic accuracy in diagnosis of appendicitis using CT. Methods: This
study was IRB approved and utilized data from 20 patients who underwent clinical CT exams for indications of
appendicitis. Medical record review established true diagnosis of appendicitis, with 10 positives and 10 negatives. A
validated software tool used raw projection data from each scan to create simulated images at lower dose levels (70%,
50%, 30%, 20% of original). An observer study was performed with 6 radiologists reviewing each case at each dose
level in random order over several sessions. Readers assessed image quality and provided confidence in their diagnosis
of appendicitis, each on a 5 point scale. Liver doses at each case and each dose level were estimated using Monte Carlo
simulation based methods. Results: Overall diagnostic accuracy varies across dose levels: 92%, 93%, 91%, 90% and
90% across the 100%, 70%, 50%, 30% and 20% dose levels respectively. And it is 93%, 95%, 88%, 90% and 90%
across the 13.5-22mGy, 9.6-13.5mGy, 6.4-9.6mGy, 4-6.4mGy, and 2-4mGy liver dose ranges respectively. Only 4 out of
600 observations were rated "unacceptable" for image quality. Conclusion: The results from this pilot study indicate that
the diagnostic accuracy does not change dramatically even at significantly reduced radiation dose.
Recently published AAPM Task Group 204 developed conversion coefficients that use scanner reported CTDIvol to
estimate dose to the center of patient undergoing fixed tube current body exam. However, most performed CT exams use
TCM to reduce dose to patients. Therefore, the purpose of this study was to investigate the correlation between organ
dose and a variety of patient size metrics in adult chest CT scans that use tube current modulation (TCM).
Monte Carlo simulations were performed for 32 voxelized models with contoured lungs and glandular breasts tissue,
consisting of females and males. These simulations made use of patient's actual TCM data to estimate organ dose. Using
image data, different size metrics were calculated, these measurements were all performed on one slice, at the level of
patient's nipple. Estimated doses were normalized by scanner-reported CTDIvol and plotted versus different metrics.
CTDIvol values were plotted versus different metrics to look at scanner's output versus size.
The metrics performed similarly in terms of correlating with organ dose. Looking at each gender separately, for male
models normalized lung dose showed a better linear correlation (r2=0.91) with effective diameter, while female models
showed higher correlation (r2=0.59) with the anterior-posterior measurement. There was essentially no correlation
observed between size and CTDIvol-normalized breast dose. However, a linear relationship was observed between
absolute breast dose and size. Dose to lungs and breasts were consistently higher in females with similar size as males
which could be due to shape and composition differences between genders in the thoracic region.
The purpose of this study was to investigate the accuracy of Monte Carlo simulated organ doses using cylindrical ROIs
within the organs of patient models as an alternative method to full organ segmentations. Full segmentation and
placement of circular ROIs at the approximate volumetric centroid of liver, kidneys and spleen were performed for 20
patient models. For liver and spleen, ROIs with 2cm diameter were placed on 5 consecutive slices; for the kidneys 1cm
ROIs were used. Voxelized models were generated and both fixed and modulated tube current simulations were
performed and organ doses for each method (full segmentation and ROIs) were recorded. For the fixed tube current
simulations, doses simulated using circular ROIs differed from those simulated using full segmentations: for liver, these
differences ranged from -5.6% to 10.8% with a Root Mean Square (RMS) difference of 5.9%. For spleen these
differences ranged from -9.5% to 5.7% with an RMS of 5.17%; and for kidney the differences ranged from -12.9% to
14.4% for left kidney with an RMS of 6.8%, and from -12.3% to 12.8% for right kidney with an RMS of 6.6%. Full
body segmentations need expertise and are time consuming. Instead using circular ROIs to approximate the full
segmentation would simplify this task and make dose calculations for a larger set of models feasible. It was shown that
dose calculations using ROIs are comparable to those using full segmentations. For the fixed current simulations the
maximum RMS value was 6.8% and for the TCM it was 6.9%.
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