Optimizing contrast timing in photon-counting detector CT for the assessment of peripheral arthritis.

Objective: Photon-counting detector CT (PCD-CT) allows for iodine mapping of inflamed tissues in peripheral immune-mediated arthritis, supporting diagnosis and disease activity assessment. This study aims to identify the optimal timing for image acquisition after intravenous iodinated contrast administration to maximize enhancement and contrast with surrounding tissues.

Methods: High-resolution PCD-CT scans of bilateral wrist-hand regions were obtained from 26 patients with peripheral arthritis, both native and post-contrast (1 ml/kg intravenous iodinated contrast at 2.5 ml/sec flow) at 120-, 180-, and 240-s delay phases. Iodine maps were constructed from spectral data. Phases were compared based on densities and iodine concentrations measured in synovial, tenosynovial, and periungual tissues, with muscle, fat, and vessels as controls. We used descriptive statistics and mixed-effects linear regression inferential models for the comparisons. Synovitis and tenosynovitis were verified by ultrasound measurements.

Results: No significant differences (p > 0.05) were found in iodine concentration or density across the 120-, 180-, and 240-s post-contrast phases in inflamed synovial, tenosynovial, and periungual soft tissues. Inflamed tissues showed significant and consistent differences in iodine concentration from muscle and fat (p < 0.0001) across all phases, while the greatest differentiation from vessels was in the 120-s phase. The effective dose was identical across all post-contrast phases (0.028 ± 0.0035 mSv).

Conclusion: Iodine uptake in inflamed tissues was identical across all three post-contrast phases. However, the 120-s phase offered the highest contrast between inflammation and surrounding vascular structures while minimizing scan time, supporting its use for standardized follow-up imaging.