Increasing spatial resolution in photon counting CT by exploiting the non-linear partial volume effect

Photon counting CT (PCCT) uses energy-resolving detectors to count and assign individual incident x-ray photons to preset energy bins, offering improved image quality, energy discrimination capability, and dose efficiency. Existing PCCT studies typically explore the spatial resolution and spectral performance improvement of PCCT separately. In this work, we leverage spectral information to improve spatial resolution through the non-linear partial volume (NLPV) effect and demonstrate its utility in PCCT images with simulated phantom studies. A fictitious “third material” was defined to represent the magnitude of the sub-pixel iodine transition sharpness, or the NLPV effect. After a projection-domain “three-material decomposition” of iodine/water/NLPV, the iodine and NLPV components were used to characterize the sub-pixel iodine profile and generate two sub-pixel values to replace the original pixel value at detector pixels with large iodine transition. With the improved sub-pixel iodine profile characterization, we generated the NLPV-enhanced iodine projections and reconstructed the iodine images. Phantoms pertinent to knee anatomy were simulated, and pig stifle experiments will be performed in the future. We show that with the improved NLPV algorithm, the image-domain spatial resolution improved by ~62% in the noiseless case and ~40% in the noisy case.