To assess the potential of spectral photon-counting (PC) radiography (SPCR) for the detection and characterization of monosodium urate (MSU) and calcium hydroxyapatite (HA) crystals, based on effective atomic number (Z _eff ) values derived from specific X-ray attenuation characteristics at different energy levels.
Suspensions of either pure agar, synthetic MSU (200 mg/ml) or HA (100 and 150 mg/ml) crystals in agar were sealed in industry-standard polystyrene vials and supported on a 2.5-mm-thick plastic table. Samples were scanned using a vendor microfocus X-ray tube and a spectral PC detector prototype with four energy thresholds per acquisition (15, 25, 30, and 35 keV). Material decomposition calibration was performed using polymethyl methacrylate (PMMA) and polyvinylchloride (PVC) slabs. Using a custom post-processing software based on polynomial material decomposition, Z _eff of the respective samples were computed. All samples were additionally scanned using dual-energy CT (DECT, 80 kV and tin-filtered 150 kV) and analyzed with a proprietary post-processing algorithm for gout.
MSU crystal suspension attenuated significantly less than both HA samples. MSU and HA suspensions differed significantly in Z _eff (mean ± SD: 7.74 ± 0.28 vs. 9.43 ± 0.41, p < .001). Z _eff values from SPCR were comparable to DECT-based reference values (p = 0.16) and were independent of the radiation dose level (0.18 - 18 mAs, p = 1).
This in vitro feasibility study demonstrates the potential of SPCR for discriminating MSU from HA crystal suspensions based on Z _eff differences. Further studies have to corroborate these initial findings ex vivo and in vivo, and to compare the diagnostic performance of SPCR with DECT in imaging of crystal-associated arthropathies.