Photon-counting detector spectral calibration enabling iodine quantification for spectral CT

Photon-counting detector (PCD) computed tomography (CT) can provide grayscale and spectral images within the same acquisition. To achieve accurate material quantification, PCD-CT requires a pixel-per-pixel calibration. In this work, a novel step-wedge phantom made of various iodine concentrations and water thicknesses was used for such calibration. The phantom allowed for water/iodine material decomposition (MD) and so for a direct quantification of iodine concentration, relevant for CT angiography protocols. The general methodology refines a simulated forward model from measurements of known material thicknesses. However, this technique requires an accurate PCD response model which is difficult to simulate. Here we present a case where this exhaustive methodology led to an inaccurate MD. We tested two alternative models with a reduced number of parameters and a polynomial approach, for spectral accuracy comparison. The reduced parameter methods were compared with real data acquired with a CdZnTe (CZT) PCD with six energy thresholds. The different approaches were evaluated with the calibration measurements taken on the step-wedge phantom and the CT acquisitions of two phantoms: 1) with iodine tubes at different concentrations and 2) an image quality phantom evaluating noise and ring artifacts. The polynomial approach led to the best spectral accuracy but suffered from high noise. The exhaustive model calibration resulted in divergent behavior and inaccurate spectral quantification. Both reduced parameter models achieved iodine quantification accuracy below 0.5 mg/ml and but only the model with two incident energies per bin presented the least noise and ring artifacts. For this last method the impact of the number of energy bins (2-6) was also evaluated. The configuration with 4, 5 or 6 bins provided superior MD, with accurate iodine quantification and reduced ring artifacts.