Optimization of Hybrid Iterative Reconstruction Level in Pediatric Body CT

Abstract

The objective of our study was to attempt to optimize the level of hybrid iterative reconstruction (HIR) in pediatric body CT. One hundred consecutive chest or abdominal CT examinations were selected. For each examination, six series were obtained: one filtered back projection (FBP) and five HIR series (iDose(4)) levels 2-6. Two pediatric radiologists, blinded to noise measurements, independently chose the optimal HIR level and then rated series quality. We measured CT number (mean in Hounsfield units) and noise (SD in Hounsfield units) changes by placing regions of interest in the liver, muscles, subcutaneous fat, and aorta. A mixed-model analysis-of-variance test was used to analyze correlation of noise reduction with the optimal HIR level compared with baseline FBP noise. One hundred CT examinations were performed of 88 patients (52 females and 36 males) with a mean age of 8.5 years (range, 19 days-18 years); 12 patients had both chest and abdominal CT studies. Radiologists agreed to within one level of HIR in 92 of 100 studies. The mean quality rating was significantly higher for HIR than FBP (3.6 vs 3.3, respectively; p < 0.01). HIR caused minimal (0-0.2%) change in CT numbers. Noise reduction varied among structures and patients. Liver noise reduction positively correlated with baseline noise when the optimal HIR level was used (p < 0.01). HIR levels were significantly correlated with body weight and effective diameter of the upper abdomen (p < 0.01). HIR, such as iDose(4), improves the quality of body CT scans of pediatric patients by decreasing noise; HIR level 3 or 4 is optimal for most studies. The optimal HIR level was less effective in reducing liver noise in children with lower baseline noise.