Purpose: To investigate the effect of maximum tube current threshold for dose modulation at MDCT on the reduction rate of radiation dose and variability of image noise.
Materials and methods: A chest CT phantom was scanned with a 64-detector row CT scanner using an automatic exposure control (AEC) technique. The technique consisted of X-Y-Z modulation and Z modulation. We set the desired image noise at 10 HU. Maximum tube current threshold was set at 200 and 300 mA and minimum tube current threshold was fixed at 10 mA. For comparison, we also performed scanning with a fixed tube current at 300 mA. Parameters for helical scanning were: tube voltage, 120 kV; rotation time, 0.5 sec/rot; collimation, 64x0.5 mm; beam pitch, 0.825; reconstructed section thickness, 5.0 mm. After scanning, we recorded the actual mA value of each image. Image noise was also measured with and without the dose modulation technique.
Results: Using X-Y-Z modulation, when maximum tube current threshold was 300 and 200 mA, the reduction rates of radiation dose with the dose modulation technique were 29.9% and 45.0%, respectively, compared with the fixed tube current technique. Actual mean image noise was 9.6 and 11.4 HU corresponding to 300 and 200 mA. The standard deviation of image noise at 200 mA (2.47 HU) was larger than that at 300 mA (1.96 HU).
Conclusion: A lower maximum tube current setting (200 mA) could achieve larger dose reduction. However, it induced larger variability of the level of image noise in the AEC technique at MDCT.