Objective: Radiation exposure in CT is modulated by automatic exposure control (AEC) mainly based on scout images. We evaluated CT radiation dose in whole-body PET/CT in relation to scout imaging direction and arm position, and investigated the behavior of AEC.
Methods: Eighty adult patients who underwent whole-body 18F-FDG PET/CT were divided into groups A, B, C, and D. The posteroanterior scout image alone (PA scout) was used for AEC-based dose modulation in groups A and B, while the posteroanterior and lateral scout images (PA + Lat scout) were used in groups C and D. Patients in groups A and C were imaged with their arms beside the head, while those in groups B and D were imaged with their arms at the sides of the trunk. Dose-length product provided by the scanner was recorded. The tube current value, a determinant of radiation dose, for each slice was plotted against slice location to produce a tube current modulation curve. The scan range was divided into seven anatomical regions, and regional tube current was defined as average tube current for each region. Effective dose was calculated for each region and then summed together.
Results: Regional tube current was higher in the body trunk and proximal thigh using the PA scout than using the PA + Lat scout, resulting in higher dose-length product and effective dose using the PA scout. A marked dose increase was shown in the shoulder especially using the PA scout. Spike-like high current at the top of the head was often observed in tube current modulation curves using the PA scout but not using the PA + Lat scout. Raising the arms increased tube current in the head and neck and decreased it in the chest and abdomen. Although dose-length product did not differ significantly depending on arm position, raising the arms decreased effective dose significantly.
Conclusions: AEC-based CT dose modulation in whole-body PET/CT is affected by scout imaging direction and arm position, which should be considered to determine an optimal imaging protocol for whole-body PET/CT.
Keywords: Arm position; PET/CT; Radiation dose; Scout imaging.