Fundamental investigation of thin slices in computed tomography, 1

Based on ''Performance Evaluation Standards of Computed Tomography (CT) Scanners'', slice thickness and its related exposure doses were investigated using a CT performance phantom. Slice thickness was 4.2 mm in CT scanning with a 4-mm-thick slice and 10.2 mm in CT scanning with a 10-mm-thick slice, which were almost the same as actural thickness. However, slice thickness was 2.7 mm in CT scanning with a 2-mm-thick slice, which was increased by approximately 35 %. Exposure doses were 1.8 rad with a 2-mm-thick slice, 2.5 rad with a 4-mm-thick slice, and 4.6 rad with a 10-mm-thick slice. When the exposure doses obtained with 2-mm- and 4-mm-thick slices were calculated according to the dose with a 10-mm-thick slice, the doses were increased by 35 % in a 4-mm-thick slice and a two-fold increase was seen in a 2-mm-thick slice. When ordinary conditions of scanning, such as 120 kV, 35 mA, and 8 sec (280 mAS), changed to 120 kV, 50 or 75 mA, and 8 sec (400 or 600 mAS) in CT scanning with a 2-mm-thick slice, exposure doses were 2.8 or 4.3 times higher than the doses obtained by ordinary conditions. It is therefore necessary to consider local exposure doses when performing CT scanning with thin slices. (Namekawa, K.)