Methods of CT Dose Estimation in Whole-Body ¹⁸F-FDG PET/CT

Abstract

We evaluated the effective dose (ED) of the CT component of wholebody PET/CT using software dedicated to CT dose estimation and from dose-length product (DLP) values to establish practical methods of ED estimation. Methods: Eighty adult patients who underwent 18 F-FDG whole-body PET/CT were divided into groups A and B, each consisting of 20 men and 20 women. In group A, ED of the CT component was calculated using CT-Expo for 6 anatomic regions separately, and whole-body ED was obtained by summing the regional EDs (CT-Expo method). DLP was calculated for each of the 6 regions and multiplied by a corresponding conversion factor described in International Commission on Radiological Protection publication 102 to obtain the ED for each region (regional DLP method). Whole-body ED was also calculated as the product of a whole-body DLP value provided by the scanner automatically and a conversion factor (simple DLP method). Moreover, the ED/DLP values were calculated using whole-body ED estimated by the CT-Expo method and the scannerderived DLP, to optimize the conversion factor. In group B, the optimized conversion factor was applied for the estimation of ED by the simple DLP method. Results: In group A, the regional DLP method allowed an accurate estimation of mean whole-body ED as a result of counterbalance of mild overestimation in men and mild underestimation in women, regarding the CT-Expo method as a standard. The simple DLP method using a conversion factor for the trunk (0.015 mSv/mGy/cm) caused overestimation. On the basis of the ED/DLP values in group A, a modified conversion factor of 0.013 mSv/mGy/cm and sex-specific conversion factors of 0.012 and 0.014 mSv/mGy/cm for men and women, respectively, were determined. In group B, the use of the modified conversion factor improved accuracy, and the use of sex-specific conversion factors eliminated sex-dependent residual errors. Conclusion: ED of the CT component of whole-body PET/CT can be assessed by multiplying the scannerderived DLP by a conversion factor optimized for whole-body PET/CT. PET with 18 F-FDG has been accepted as a valuable tool in oncology practice. CT images are commonly acquired together with PET images in a single imaging session with an integrated PET/CT scanner (1) and are used for diagnosis on CT images themselves, localization of lesions delineated by PET, and attenuation correction of PET images. The problem of CT acquisition additional to PET is an increase in radiation exposure. The effective dose (ED) derived from the CT component varies widely from 5 to 25 mSv (2-8) and often exceeds the ED from 18 F-FDG injection. Although a large amount of radiation exposure is required to acquire highquality CT images for diagnostic purposes, lesion localization and attenuation correction can be achieved on CT images of lower quality. Dose reduction with preserving clinical utilities should be pursued in each facility considering the purpose of CT and using dose reduction technologies (9,10).Estimation of ED is a prerequisite for optimization and mo...

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