Ultra-High-Resolution Imaging of Rectal Cancer with Photon-Counting Detector Computed Tomography: An Exploratory Pilot Study on the Impact of Slice Thickness and Monoenergetic Reconstruction Level on Image Quality

Background/Objectives: To evaluate, in an exploratory pilot study, the influence of slice thickness and monoenergetic (ME) level on quantitative and qualitative image quality of ultra-high-resolution (UHR) photon-counting detector CT (PCD-CT) in rectal cancer (RC). Methods: This prospective pilot study enrolled 21 patients with suspected RC undergoing pre-treatment PCD-CT between July 2023 and February 2025. All underwent a QuantumPlus portal venous phase, whose 3 mm/60 keV reconstruction served as the standard reference, plus a separate pelvic UHR acquisition. Sixteen UHR reconstructions were generated (0.6–3 mm; 45–70 keV). Signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and noise were analyzed using linear mixed-effects models. Three readers independently rated overall image quality, perceived noise, mesorectal fascia (MRF) sharpness and lesion conspicuity relative to the reference; ratings were analyzed using mixed model ordinal regression. The primary analysis was the within-UHR comparison of reconstruction settings; the comparison with the reference, which differed in contrast phase and acquisition parameters, was descriptive. Results: Nineteen patients (mean age 61.7 ± 10.7 years, 73.7% male) were included. CNR showed a significant slice thickness–ME level interaction (p < 0.001), ranging from 4.85 ± 2.43 (3 mm/45 keV) to 0.86 ± 1.04 (0.6 mm/70 keV). SNR increased and noise decreased with thicker slices, while both decreased with higher ME levels (all p < 0.001). ME level influenced all four qualitative parameters (all p < 0.001); slice thickness significantly affected overall quality and perceived noise (p < 0.001) but not MRF sharpness or lesion conspicuity (p ≥ 0.115). Overall quality was rated favorably in 82.9% at 70 keV versus 45.6% at 45 keV. Lesion conspicuity reached 71.9–86.0% favorable ratings, whereas perceived noise was consistently rated worse than the reference, and UHR SNR remained below the reference in all settings. In a descriptive pooled summary, 3 mm/60 keV (68.0%) and 2 mm/70 keV (66.2%) were rated favorably most often, with overlapping confidence intervals. Conclusions: Slice thickness and ME level markedly influence UHR PCD-CT image quality in RC. One candidate working reconstruction for future validation is 3 mm at 60 keV, with 2 mm at 70 keV as a plausible alternative. Because the reference differed in acquisition and contrast phase and no diagnostic endpoint was assessed, these results do not establish a diagnostic benefit of UHR PCD-CT.