Added Value of Photon-counting CT for the Detection of Calcium Pyrophosphate Deposition Disease in the Wrist

Background

While polarized light microscopy of synovial fluid is the gold standard for detecting calcium pyrophosphate deposition (CPPD) disease, non-invasive detection of subtle wrist involvement remains challenging. Dual energy CT (DECT) is currently used for the detection of gout, however, its value in the detection of CPPD is controversial. Photon-counting CT (PCCT) offers ultra-high spatial resolution and intrinsic spectral capabilities, potentially enabling superior material characterization compared with DECT. We evaluated the diagnostic performance and added value of spectral PCCT for detecting CPP deposits in the wrist.

Methods

In this retrospective study, consecutive, non-contrast wrist PCCT examinations acquired between January-July 2026 were evaluated. All examinations in patients aged ≥ 18 years, regardless of clinical indication, were included. Two readers independently assessed 17 anatomical wrist regions per patient in two steps: (1) for the presence of calcification on standard CT (sCT) alone, and (2) sCT combined with spectral color-coded reconstructions for either CPPD or gout with a 2-week interval between the two assessments. Disagreements were resolved by consensus. Diagnostic performance (sensitivity, specificity, PPV, NPV, accuracy) was evaluated per wrist site and patient level. Inter-reader agreement was assessed using Cohen’s kappa and intraclass correlation coefficients.

Results

At the site level across 17 anatomical regions, spectral PCCT demonstrated high specificity, ranging from 85.6% to 100% (e.g., STT: 100%, TFC: 97.7%, SL: 97.1%). At the overall patient level, PCCT showed moderate accuracy (66.9%) and specificity (69.7%), with low sensitivity (52.2%) and PPV (25.0%), but high NPV (88.3%). Overall regional agreement was moderate (ICC = 0.446, p < 0.001).

Conclusion

While spectral PCCT demonstrated high site-specific specificity, its overall sensitivity and patient-level accuracy remained limited. Current spectral PCCT technology and post-processing tools are not yet sufficiently robust to support definitive clinical diagnosis of CPPD in the wrist.