PFCM: Poisson Flow Consistency Models for Low-Dose CT Image Denoising.

Abstract

X-ray computed tomography (CT) is widely used for medical diagnosis and treatment planning; however, concerns about ionizing radiation exposure drive efforts to optimize image quality at lower doses. This study introduces Poisson Flow Consistency Models (PFCM), a novel family of deep generative models that combines the robustness of PFGM++ with the efficient single-step sampling of consistency models. PFCM are derived by generalizing consistency distillation to PFGM++ through a change-of-variables and an updated noise distribution. As a distilled version of PFGM++, PFCM inherit the ability to trade off robustness for rigidity via the hyperparameter $\text {D} \in \text {(}{0},\infty \text {)}$ . A fact that we exploit to adapt this novel generative model for the task of low-dose CT image denoising, via a "task-specific" sampler that "hijacks" the generative process by replacing an intermediate state with the low-dose CT image. While this "hijacking" introduces a severe mismatch-the noise characteristics of low-dose CT images are different from that of intermediate states in the Poisson flow process-we show that the inherent robustness of PFCM at small D effectively mitigates this issue. The resulting sampler achieves excellent performance in terms of LPIPS, SSIM, and PSNR on the Mayo low-dose CT dataset. By contrast, an analogous sampler based on standard consistency models is found to be significantly less robust under the same conditions, highlighting the importance of a tunable D afforded by our novel framework. To highlight generalizability, we show effective denoising of clinical images from a prototype photon-counting system reconstructed using a sharper kernel and at a range of energy levels.