Optimizing photon counting CT enterography: determining the optimal virtual monoenergy for bowel imaging

Objective To determine the optimal virtual monoenergy for viewing the bowel at photon-counting CT enterography using quantitative assessment of mural attenuation, contrast-to-noise ratio, signal-to-noise ratio and noise. Methods This study was institutional review board approved and Health Insurance Portability and Accountability Act compliant. Consecutive adults (>= 18 years) who underwent photon-counting CT enterography from 5/1/2022-5/31/2022 with available Spectral Postprocessing (SPP) images for retrospective virtual monoenergy creation were identified. Nine virtual monoenergetic series (40-120 keV, 10 keV increments) were created. Two region-of-interest measurements were placed in the stomach wall, jejunum wall, ileum wall, and each psoas muscle by two radiologists on 0.6 mm images in PACS. Region-of-interests were copied to other virtual monoenergies to ensure identical placement and size. Attenuation (HU) and noise (HU standard deviation) were recorded from each region-of-interest. Signal-to-noise ratio and contrast-to-noise ratio were computed for stomach, jejunum, ileum, and all bowel combined. Pairwise comparisons for attenuation, noise, signal-to-noise ratio and contrast-to-noise ratio for each virtual monoenergy were performed with ANOVA. A p <.05 indicated statistical significance. Results 50 patients (32 female; mean[SD] age: 57 years) were included. Attenuation and noise for all bowel regions were highest at 40 keV with statistically significant pairwise comparisons from 40 to 70 keV (all p <.05), but similar for 70-120 keV (all p >.05). Signal-to-noise ratio was similar from 40 to 70 keV (all p >.05) for all bowel regions. Contrast-to-noise ratio decreased with increasing keV. Contrast-to-noise ratio was similar for all bowel at 40 keV and 50 keV (p =.06), for stomach from 40 to 70 keV (all p >.05), for jejunum from 40 to 50 keV (p =.21), and for ileum from 40 to 60 keV (all p >.05). Conclusion 50 keV virtual monoenergetic images from photon-counting CT enterography optimizes contrast-to-noise ratio while mitigating noise and should routinely be utilized for bowel assessment at photon-counting CT enterography. As most photon-counting CT users primarily interpret virtual monoenergetic images in clinical practice, knowledge of the optimal virtual monoenergy can inform protocol development.