A comparison of fixed tube current (FTC) and automatic tube current modulation (ATCM) CT methods for abdominal scanning : implications on radiation dose and image quality

Abstract

PURPOSE: There has been a huge increase in the use of abdominal CT scanning in recent years. This has contributed to an increase in radiation dose administered to patients. Abdominal CT scans generally require higher exposure factors when compared to other anatomical regions. This drives a need for urgent optimisation of the radiation dose and image quality for abdominal CT examinations. The aim of this thesis is to evaluate Fixed Tube Current (FTC) and Automatic Tube Current Modulation (ATCM) on image quality and radiation dose during abdominal CT examinations across a range of scanning parameters. MATERIALS AND METHODS: Using a Toshiba Aquilion 16 CT scanner (120 kVp, 0.5 seconds tube rotation), an adult ATOM dosimetry and abdominal anthropomorphic phantom were exposed to a series of FTC and ATCM CT protocols with variations in tube current as follows: FTC - 100, 200, 250, 300 and 400mA; ATCM - low dose+, low dose, standard, quality and high quality. The pitch factors evaluated included were 0.688, 0.938 & 1.438 and the detector configurations included were 0.5×16 mm, 1.0×16 mm and 2.0×16 mm. Radiation doses for nine abdominal organs were directly measured using the Metal Oxide Semiconductor Field Effect Transistors (MOSFET). Effective dose (ED) was measured and estimation comprised of three methods: mathematical modelling with k-factors and dose length product DLP, direct with MOSFET and indirectly with Monte Carlo simulation (ImPACT). Effective risk (ER) was estimated using MOSFET data and Brenner’s equations / BEIR VII 2006 report. The raw data for ATCM radiation dose was corrected using an equivalence equation. The ATCM corrected and uncorrected data were compared against FTC. Image quality was assessed using SNR (five abdominal organs) and a relative visual grading analysis (VGA) method (five different axial images). Image quality evaluation was performed by the researcher after testing agreement between against five different observers. RESULTS: There were no significant differences in the mean radiation doses between FTC and corrected ATCM across a range of acquisition protocols (P>0.05). This was with the exception of the 300mA/quality protocols, and for a fast pitch factor with 0.5×16mm detector configurations. These had significantly lower doses for FTC (P<0.05). These differences were up to 13% for the mean abdominal organ doses, effective doses and the effective risk. In addition, for all acquisition parameters, the mean radiation dose was significantly higher (P<0.05; 17%-23%) for uncorrected ATCM when compared to FTC. In terms of image quality, there were no differences in SNR values between FTC and ATCM for the majority of acquisition protocols, excepting the higher mean SNR value (P<0.05) for the FTC at 100mA/low dose + and 200 mA/ low dose (pancreas, left and right kidneys). Conversely, the mean SNR values were significantly higher (P<0.05) for the ATCM scans for 300mA/quality and fast pitch factor (1.438) (liver, spleen and pancreas) than FTC. Finally, relative VGA scores for both FTC and ATCM demonstrated no significant difference, except for ‘quality’ ATCM scans (image # 1, image # 2) and a fast pitch factor (1.438) for image #2 and #3. CONCLUSION: FTC and corrected ATCM were generally similar in terms of radiation dose and image quality except for some acquisition parameters; 300mA/quality tube current and fast (1.483) pitch factor FTC was lower than the corrected ATCM. However, the uncorrected ATCM produced higher radiation dose when compared with FTC techniques. In addition, FTC and ATCM generally produced similar SNR, again with the exception of some protocols. The SNR was higher for FTC than ATCM at lower tube current (pancreas, left and right kidneys), at 300mA/quality and fast pitch factor (1.438) SNR values for ATCM higher than FTC (liver and spleen). However, the ATCM technique is able to produce higher mean relative VGA scores for upper and middle abdominal organs. Further investigation of image quality and radiation dose difference between FTC and ATCM is required