Technique factors and image quality as functions of patient weight at abdominal CT

PURPOSE: To investigate how changes in kilovolt peak and milliampere second settings, and patient weight affect transmitted x-ray energy fluence and t he image contrast-to-noise ratio (CNR) at abdominal computed tomography (CT ). MATERIALS AND METHODS: Cylinders of water were used as patient models, and x-ray spectra, including x-ray tube potentials of 80-140 kVp, were investig ated. The mean photon energy and energy fluence transmitted through water c ylinders with varying diameters and the image contrast for fat, muscle, bon e, and iodine relative to water were determined. The effect of changing the x-ray tube potential on CNR also was investigated. RESULTS: At a constant kVp, increasing patient weight from 10 kg to 120 kg reduced the transmitted energy fluence by two orders of magnitude. Changing the x-ray tube potential from 80 kVp to 140 kVp increased the mean photon energy from approximately 52 keV to approximately 72 keV and thus reduced t he image contrast relative to water by 12% for muscle, 21% for fat, 39% for bone, and 50% for iodine (approximate reduction values). Increasing the x- ray tube potential from 80 kVp to 140 kVp increased the CNR by a factor of 2.6 for muscle and by a factor of 1.4 for iodine. CONCLUSION: With changes in patient weight at abdominal CT, x-ray tube pote ntials must be varied to maintain a constant detector energy fluence. Incre asing the x-ray tube potential generally improves CNR.