Spectral photon counting computed tomography (SPCCT)

The SPCCT (Philips Research, Haïfa, Israel) is a modified clinical CT that allows axial and helical scans over 360 degrees with a gantry rotation time of 1 s. SPCCT images were acquired at 100 mA tube current and 120 kVp tube voltage using a conventional X-ray tube emitting a spectrum of photons ranging from 30 to 120 keV. The detection system comprises 28 tiles of cadmium-zinc-telluride (CZT) sensors of 2 mm thickness and a pixel pitch of 500 μm × 500 μm, flip-chip bonded to Philips’ proprietary ChromAIX2 ASICs providing an in-plane field-of-view of 169 mm and a z-coverage at 2.5 mm in isocenter⁴⁰. The scanner system is equipped with energy resolving PCDs allowing up to 5 consecutive energy bins at the detector level, which leads to a complete analysis of the transmitted spectrum through the animal. The photon counts are measured in one-sided bins by the scanner and then processed by subtraction into two-sided bins^20,40,41. Spectral energy resolution (FWHM) is about 8% at 60 keV as shown previously⁴⁰. The detailed detector response function is taken into account in the decomposition process. Iodine and gadolinium can be separated from a material decomposition process based on a maximum likelihood method.²⁴ Of note, only the gadolinium separation is based on its K-edge energy, while the iodine separation is based on a 2 components model (water and iodine). Therefore, 2 energy bins were set at 50.2 keV (30–51, 51–64) to coincide with the K-edge energy of gadolinium, the extra energy bins were set to better managing the photon count rate (64–72, 72–85, 85–120). Further technical details are provided in previous studies^33,42.

Conventional images and contrast material maps (i.e. iodine and gadolinium images) derived from SPCCT acquisitions were reconstructed on a voxel grid of 0.25 × 0.25 × 0.25 mm. A Gaussian filter of 2 pixels was applied on material decomposition iodine and gadolinium K-edge images for noise reduction, and contrast material overlay images were created using FIJI software (ImageJ)³⁹.