Dual-energy CT-based virtual non-contrast imaging for stopping power ratio estimation

Iodinated contrast-enhancing agents improve the visualization of relevant organ structures for proton therapy treatment planning when employed in computed tomography (CT). However, the presence of iodine can result in inaccuracies in dose calculations owing to erroneous conversions from CT numbers into stopping power ratios (SPRs).

This study proposes an algorithm for generating virtual non-contrast (VNC) SPR images from dual-energy CT (DECT) scan data. The algorithm builds upon our previously established method, which facilitates the calculation of the VNC relative electron density in human tissue from post-contrast DECT data. The VNC SPR was calculated using the Bethe equation based on an empirical relationship between the relative electron density and mean excitation energy of human tissues.

The feasibility of the proposed VNC algorithm was assessed through analytical DECT image simulations on a digital phantom comprising various tissue types and iodine surrogates. The phantom was designed with inserts representing standard human tissues and iodine-enhanced soft-tissue materials. We evaluated the accuracy of VNC SPRs compared with that of non-contrast base substances to determine the performance of the proposed algorithm.

The proposed VNC algorithm successfully eliminated the distinct contrast between each iodinated material and the base material of the phantom housing in both the VNC SPR and relative electron density images. The relative deviation of the VNC SPR values from the non-contrast values remained within ±0.9% for all phantom materials, yielding a root-mean-square error (RMSE) of 0.41%.

In this DECT simulation study, converting the VNC relative electron density into the mean excitation energy enabled the creation of a VNC SPR image with high accuracy.