Purpose: We aimed to determine the in-vitro diagnostic performance of
multi-energy spectral photon-counting CT (SPCCT) in crystal-related
arthropathies.
Methods: Four crystal types (monosodium urate, MSU; calcium
pyrophosphate, CPP; octacalcium phosphate, OCP; and calcium
hydroxyapatite, CHA) were synthesized and blended with agar at the
following concentrations: 240, 88, 46, and 72 mg/mL, respectively.
Crystal suspensions were scanned on a pre-clinical SPCCT system at 80
kVp using the following four energy thresholds: 20, 30, 40, and 50 keV.
Differences in linear attenuation coefficients between the various
crystal suspensions were compared using the receiver operating
characteristic (ROC) paradigm. Areas under the ROC curves (AUC),
sensitivities, specificities, and diagnostic accuracies were calculated.
Crystal differentiation was considered successful if AUC>0.95.
Results: For each paired comparison of crystal suspensions, AUCs were
significantly higher in the first energy range (20-30 keV). In the first
energy range, MSU was confidently differentiated from CPP (sensitivity,
0.978; specificity, 0.990; accuracy, 0.984) and CHA (sensitivity, 0.957;
specificity, 0.970; accuracy, 0.964), while only moderately
distinguished from OCP (sensitivity, 0.663; specificity, 0.714;
accuracy, 0.688). CPP was confidently differentiated from OCP
(sensitivity, 0.950; specificity, 0.954; accuracy, 0.952), while only
moderately from CHA (sensitivity, 0.564; specificity, 0.885; accuracy,
0.727). OCP was accurately differentiated from CHA (sensitivity, 0.898;
specificity, 0.917; accuracy, 0.907).
Conclusions: Multi-energy SPCCT can accurately differentiate MSU from
CPP and CHA, CPP from OCP, and OCP from CHA in vitro. The distinction
between MSU and OCP, and CPP and CHA is more challenging.