Deep-learning-based direct synthesis of low-energy virtual monoenergetic images with multi-energy CT

Purpose: We developed a deep learning method to reduce noise and beam-hardening artifact in virtual monoenergetic image (VMI) at low x-ray energy levels. Approach: An encoder-decoder type convolutional neural network was implemented with customized inception modules and in-house-designed training loss (denoted as Incept-net), to directly estimate VMI from multi-energy CT images. Images of an abdomen-sized water phantom with varying insert materials were acquired from a research photon-counting-detector CT. The Incept-net was trained with image patches ( $64\times 64\text{pixels}$ ) extracted from the phantom data, as well as synthesized, random-shaped numerical insert materials. The whole CT images ( $512\times 512\text{pixels}$ ) with the remaining real insert materials that were unseen in network training were used for testing. Seven contrast-enhanced abdominal CT exams were used for preliminary evaluation of Incept-net generalizability over anatomical background. Mean absolute percentage error (MAPE) was used to evaluate CT number accuracy. Results: Compared to commercial VMI software, Incept-net largely suppressed beam-hardening artifact and reduced noise (53%) in phantom study. Incept-net presented comparable CT number accuracy at higher-density ( $P$ -value [0.0625, 0.999]) and improved it at lower-density inserts ( $P\text{-}\text{value}=0.0313$ ) with overall MAPE: Incept-net [2.9%, 4.6%]; commercial-VMI [6.7%, 10.9%]. In patient images, Incept-net suppressed beam-hardening artifact and reduced noise (up to 50%, $P\text{-}\text{value}=0.0156$ ). Conclusion: In this preliminary study, Incept-net presented the potential to improve low-energy VMI quality.