High resolution CT of the lungs: Proper mA s settings for clinical use

Fifty-five patients undergoing routine chest CT were examined with four additional high resolution computed tomography (HRCT) slices to determine the proper milliampere-second settings for clinical HRCT of the lungs. The following technical factors were used: 1-mm collimation, 2-s scan time, 120 kVp and different milliampere settings (160 mA, 100 mA, 60 mA, 30 mA). On 47 out of 54 patients all the four HRCT cuts were at exactly the same level. These images were independently reviewed and assessed by three radiologists. The body mass index was calculated in order to determine the effect of the size of the patient on the milliamperage setting needed. The standard deviation of CT values of 1 mm and 10 mm slice was measured in a Plexiglas phantom with different mA settings. The slice thickness was measured with thermoluminescence dosimeter. In the phantom study the change of milliamperage from 160 mA to 30 mA at 120 kVp and 2 s increased standard deviation in CT units from about 4.6 to 8.9 in 1 mm slice and from about 2.9 to 4.3 in 10 mm slice. The Computed Tomography Dose Index (CTDI) values of both 1 mm and 10 mm slices at 160 mA-30 mA were approximately from 9 to 2 cGy (rad). In the patient study 1 mm HRCT scans with 120 kVp/100 mA/2 s showed the same diagnostic information as 120 kVp/160 mA/2 s scans for all patients regardless of size. If only the normal weight patients are considered, all the details were also well seen with 120 kVp/60 mA/2 s settings. In our study contrary to earlier reports 30 mA/2s was inadequate for diagnosis. We conclude that at 120 kVp medium-dose settings 60-100 mA/2 s or 120-200 mA s are in clinical practice appropriate for HRCT studies of the lungs in most patients.