Scout view X-ray attenuation versus weight-based selection of reduced peak tube voltage in cardiac CT angiography.

OBJECTIVES: The study evaluated the relationship between cardiac computed tomography (CT) scout view x-ray attenuation and CT image noise compared with weight or body mass index (BMI).

BACKGROUND: Decreasing peak tube voltage from 120 to 100 kVp on the basis of body size reduces radiation exposure. Methods to better predict CT image noise may lead to more effective selection of reduced tube voltage in cardiac CT.

METHODS: Image quality was graded subjectively (1 [excellent] to 4 [nondiagnostic]) and objectively (SD of the aortic attenuation value) in cardiac CT angiograms (N = 106) acquired at either 100 or 120 kVp. X-ray attenuation characteristics on the scout view (120 kVp, 30 mA) were measured within a 3-cm region of interest across the chest in the frontal x-ray. Receiver-operating characteristic curve analysis was performed comparing scout view attenuation versus weight and BMI in predicting CT image noise and quality.

RESULTS: CT image noise correlated with both BMI (r = 0.40; p < 0.001) and the scout view attenuation value (r = 0.52; p < 0.001). In linear regression models with controlling for BMI (or weight) and tube potential, scout view attenuation was the best predictor of the CT image noise (p < 0.001), and increased model fit statistic from 0.23 to 0.41 (p model <0.001). At 120 kVp, scout view attenuation predicted CT image noise <30 Hounsfield units (HU) more accurately than BMI (area under the curve: 0.89 vs. 0.77). For CT images acquired at 120 kVp, those with a scout view attenuation <-120 HU had significantly lower noise and higher signal-to-noise ratios, with similar mean aortic attenuation values. A majority (89.3%) of "low-noise" CT images at 120 kVp had scout view attenuation values of <-120 HU.

CONCLUSIONS: Scout view attenuation predicts cardiac CT image noise better than weight or BMI and could enable broader application of reduced x-ray tube voltage as a radiation sparing technique.