Uncertainties in effective dose estimation for CT transmission scan in total body PET-CT imaging with Auto mA3D tube current modulation

Background: Positron Emission Tomography-Computed Tomography (PET-CT) is a useful hybrid imaging modality in the diagnosis of various malignancies. This modality imposes almost 20 mSv radiation dose to the patient. The purpose of the present study was to evaluate the uncertainties in calculated CT effective dose in TUBE CURRENT MODULATION-activated scans by Impact-Dose software.  Materials and Methods: Sixty total body DICOM (30 male and 30 female) whole body PET-CT images were selected. Volume CT Dose Index (CTDIvol) was recorded for each of the procedures. The image was divided into 5 regions of head & neck, chest, abdomen, pelvis and lower limbs according to special anatomical markers. Effective doses for total body and separate organs were calculated by means of Impact-Dose software once with global CTDIvol and once with a summation of doses calculated by 5 Regional CTDIvol and related scan ranges. Results: The difference among effective doses for some organs and total body were considerable. The mean and standard deviation (SD) of the coefficient of variations (CV%) for total body, breast, gonads, liver, lung, red bone marrow (RBM), thyroid, kidneys, and uterus were 12.56, 11.61, 9.44, 8.1, 11.31, 5.93, 8.61, 6.03 and 12.49, respectively. Uncertainties were higher for smaller patients by 19 noise indexes while these changes were higher for bigger patients and 22 noise indexes. Conclusion: The tube current variation depends on the acquisition and patient parameters. For measuring and reporting the total body and organs’ effective doses in order to estimate the risks of CT’s radiation for total body PET-CT procedures, the tube current variations must be considered.