Image quality and artefact reduction for tantalum clips in ocular proton therapy using photon-counting CT

Objective Tantalum fiducials (“clips”) used in intraocular proton therapy generate strong artefacts in conventional computed tomography, limiting the visibility of tumours and anatomical structures. This study investigated the potential of photon-counting computed tomography (PCCT) for reducing tantalum-related artefacts. Materials and methods Two phantom models containing tantalum clips were examined using a photon-counting CT system and two conventional CT scanners with energy-integrating detectors (EICT). Reconstruction parameters, including tube voltage, image resolution, reconstruction kernel, virtual monoenergetic imaging (VMI), and iterative metal artefact reduction (iMAR), were systematically optimised. Image quality was evaluated using objective metrics assessing homogeneity, noise, contrast, structural similarity, and artefact volume, complemented by a blinded subjective observer study. Correlations between objective and subjective results were analysed using Spearman’s rank correlation. Results Objectively, PCCT showed no significant advantage over the modern EICT scanner but significantly outperformed the older-generation EICT therapy simulator. In contrast, blinded observers consistently preferred PCCT images. iMAR was the dominant factor influencing image quality. VMI alone did not yield relevant improvements. High-energy VMI (110 keV) combined with iMAR achieved the best subjective ratings. Higher spatial resolution due to smaller voxel sizes showed no clinically relevant benefit for artefact reduction. Soft reconstruction kernels provided the best balance between artefact suppression and image noise. Objective rankings correlated strongly with subjective assessments. Conclusion PCCT combined with iMAR improves subjective image quality in intraocular proton therapy and reduces the perceived severity of tantalum artefacts. Nevertheless, tantalum remains a major challenge in image-guided proton therapy, requiring further optimisation of imaging and reconstruction strategies. Key Points Question Photon-counting CT may improve visualisation of tantalum clips and reduce metal artefacts in ocular proton therapy planning compared with conventional CT systems. Findings Photon-counting CT achieved the highest subjective image quality ratings, while iterative metal artefact reduction remained the dominant factor influencing artefact suppression. Relevance statement Photon-counting CT combined with iterative metal artefact reduction can improve visualisation of tantalum clips and surrounding anatomy in ocular proton therapy planning and may support more reliable tumour localisation despite limited objective differences compared with state-of-the-art conventional CT.