Optimization of kilovoltage and tube current-exposure time product based on abdominal circumference: An oval phantom study for pediatric abdominal CT

OBJECTIVE. This CT study evaluates image noise and radiation dose using a modified CT dose index phantom to approximate pediatric abdominal shape. Contrast-to-noise ratio (CNR) and radiation dose were measured. MATERIALS AND METHODS. The oval shape was simulated by fixing 1000-mL saline bags aside cylindric phantoms with variable circumferences. The doses at the center and peripheral holes in the phantom were recorded. Measurements were obtained at 50-400 mAs and 80-140 kVp. Diluted iodine contrast agent filled the center hole, and distilled water filled the peripheral holes. CNR was defined as the difference in CT number between diluted iodine and water divided by the standard deviation (SD) of CT number of water. RESULTS. Dose increased linearly with increases in tube current-exposure time product and by a power function (∝kVp ⁿ, where n = 2.64-3.09) for increases in kilovoltage. A range of scanning parameters was established for each circumference from which technique optimization curves were created to determine the best tube current-time product and kilovoltage pairs when noise was less than 20 HU and dose was less than 2.5 cGy. CNR increased by 40% as kilovoltage was reduced from 140 to 80 kVp. A dose reduction of 70% was observed for 140 versus 80 kVp for the same CNR. CONCLUSION. Because pediatric patients of the same age and weight come in all shapes and sizes, abdominal circumference is a useful clinical parameter on which to base CT scan techniques controlling radiation output - namely kilovoltage and tube current-time product. Low-kilovoltage techniques for patients with small circumference show better iodine CNR.