Optimization of the Exposure Parameters Used in Multislice Helical CT Scanning

Objective To determine the optimal exposure parameters used in multislice helical CT scanning by analyzing the effect of different exposure parameters and slice thickness on the image quality and CT dose index (CTDI).Materials and Methods Marconi Mx8000 scanner and its QA phantoms were used in this study. Exposure voltage of 90kV, 120kV and 140kV, with all the other parameters being the same, were used in three study groups respectively. Each group selected different mAs values, or adopted different slice thickness, to make several exposure parameter combinations. QA phantoms were scanned with each of the above parameter combinations. The image quality was evaluated based on the measurement of the standard deviation of the CT number of water within a selected ROI and both the high and the low contrast resolution.Results When kV was fixed, the image noise was decreasing with the increasing of mAs, while both the high and the low contrast resolution together with the CTDI was increased, too. When mAs was fixed, the same results occurred with the increase of kV. No obvious difference in CTDI value was found between 4mm×5mm, 2mm×8mm and 2mm×10mm thickness scanning modes. According to the image quality standard, the optimal exposure parameter combinations were: 300mAs or 375mAs in 90kV group; 150mAs, 200mAs, 250mAs or 300mAs in 120kV group; 100mAs, 150mAs, 200mAs, 250mAs or 300mAs in 140kV group.Conclusion When kV value is fixed, a linear relation exists between the image quality and both mAs and CDTI. When mAs is fixed, the image quality shows a linear relation only with kV, not with CTDI.