A Gantry-Mounted Photon-Counting Detector Computed Tomography Prototype for Image Guided Proton Therapy

Currently, on-board kilovoltage imaging systems on proton therapy machines typically employ scintillator-based energy-integrating detectors (EIDs) to acquire planar images or cone beam computed tomography (CBCT) scans. However, EID-based CBCT is limited by suboptimal image quality and poor quantification accuracy, falling short of the requirements for online adaptive proton therapy and other advanced imaging tasks for proton therapy. This study aims to experimentally demonstrate the feasibility of on-board photon-counting detector (PCD) CT imaging by integrating a PCD with a proton therapy gantry.

A cadmium telluride (CdTe)-based PCD, featuring a pixel size of 100μm and 2 energy channels, was mounted onto the surface of the existing flat-panel EIDs of an IBA Proteus ONE proton therapy system. The PCD-CT scans followed the same acquisition protocol as the clinical EID-CBCT for head imaging. Geometric distortions were estimated and corrected during image reconstruction. To assess nonspectral image quality, standardized phantoms with known material compositions were scanned under matched x-ray exposure and beam width conditions. The resulting PCD-CT images were compared with EID-CBCT in terms of modulation transfer function (MTF) and contrast-to-noise ratio (CNR), defined based on 15 different material inserts from 2 physical phantoms. In addition, spectral PCD-CT data were used to estimate the electron density ratio of each material relative to water. The estimated values were then compared with reference values.

Compared with the EID, the PCD exhibits significantly higher detective quantum efficiency, especially at high spatial frequencies. When the MTFs of PCD-CT and EID-CBCT were matched by adjusting the reconstruction kernel, PCD-CT consistently achieved higher CNR across all material inserts tested. For electron density ratios estimated from spectral PCD-CT, the mean absolute percent error was 1.8%.

The feasibility of gantry-mounted PCD tomographic imaging for proton therapy has been demonstrated for the first time. The PCD-CT prototype provides superior nonspectral image quality compared with conventional EID-CBCT. Additionally, its spectral imaging capability enables accurate estimation of electron density ratios.