Optimization and Initial Diagnostic Performance of Photon-Counting Detector CT for Rectal Cancer T Staging

Background The potential of photon-counting detector (PCD) CT for rectal cancer (RC) staging remains, to the knowledge of the authors, unexplored. Purpose To determine the optimal virtual monoenergetic images (VMIs) and quantum iterative reconstruction (QIR) strength in PCD CT for RC T staging and to compare the diagnostic performance with that of MRI. Materials and Methods In this prospective study, participants with RC underwent preoperative contrast-enhanced abdominal PCD CT and rectal MRI between September and November 2024. Portal venous phase PCD CT images including VMIs at 40-70 keV in 10-keV increments and polychromatic images (referred to as T3D by the manufacturer) were reconstructed with five QIR levels (off and levels 1-4), and image quality was evaluated. Two junior radiologists and a senior radiologist independently performed T staging evaluations from T3D, optimal PCD CT images, and MRI scans. Diagnostic performance was compared across modalities using DeLong and McNemar tests. Results Seventy-two participants (mean age, 61 years ± 11.8 [SD]; 40 men) were included for image analysis; 51 underwent radical surgery for T staging analysis. VMIs at 40 keV with QIR level 4 (QIR-4) showed the best signal-to-noise ratio (12.1 ± 2.8), contrast-to-noise ratio (6.2 ± 2.6), and maximal Likert scores (median score, 5; IQR, 5-5) for tumor conspicuity, tumor margin clarity, and overall image quality versus other reconstructions (all <i>P</i> < .001). Compared with T3D, optimal PCD CT improved T3-T4 diagnostic performance for the two junior radiologists (area under the receiver operating characteristic curve [AUC], 0.80 vs 0.64 [<i>P</i> = .03] and 0.82 vs 0.64 [<i>P</i> = .002], respectively) but not for the senior radiologist (AUC, 0.86 vs 0.80; <i>P</i> = .13). Optimal PCD CT demonstrated comparable performance to MRI in overall T staging (accuracy, 65%-76% vs 67%-80%) and T3-T4 staging (AUC, 0.80-0.86 vs 0.78-0.88) across all readers (all <i>P</i> > .05). Conclusion In participants with RC, portal venous abdominal PCD CT at 40 keV and with QIR-4 yielded the optimal image quality for RC T staging, showed enhanced T3-T4 diagnostic performance among less-experienced radiologists, and achieved performance that was comparable to that of MRI. © RSNA, 2025 <i>Supplemental material is available for this article.</i>