Weighted Directional Total Nuclear Variation for Joint Yttrium-90 PET/SPECT Reconstruction with CTAC-derived Guidance

TL;DR

This paper introduces a novel joint reconstruction method, w-dTNV, that leverages CT-guided directional regularization to improve Yttrium-90 PET/SPECT imaging accuracy for personalized dosimetry.

Contribution

The proposed w-dTNV regularizer effectively exploits physical coupling and anatomical guidance to enhance lesion recovery and contrast in joint PET/SPECT reconstructions.

Findings

w-dTNV outperforms existing methods in phantom recovery coefficients.

w-dTNV yields higher tumor-to-background ratios in patient data.

The method improves stability and accuracy of post-SIRT Yttrium-90 activity estimates.

Abstract

Quantitative post-treatment activity imaging is essential for personalised dosimetry after Yttrium-90 selective internal radiation therapy (SIRT). Yttrium-90 PET offers high spatial resolution but is extremely low-count, whereas bremsstrahlung SPECT has higher count statistics but is degraded by blur, scatter, and septal penetration. Since both modalities image the same microsphere distribution, joint synergistic reconstruction can exploit their physical coupling, with CT attenuation correction (CTAC) providing additional anatomical guidance. We propose weighted directional total nuclear variation (w-dTNV), a joint variational regulariser for coupled PET/SPECT reconstruction with CTAC-guided anisotropy. w-dTNV penalises the nuclear norm of a dual-modality Jacobian, promoting co-located, geometry-consistent edges without forcing intensity correlation. Directionality is derived from the CTAC attenuation map $\mu$ and applied to PET/SPECT gradients, allowing efficient per-voxel spectral computations. PET/SPECT scale disparity is mitigated using data-driven modality normalisation from preliminary reconstructions. We evaluate w-dTNV on a NEMA IEC phantom with 20 bootstrapped PET noise realisations and on 9 post-SIRT patients with 45 lesions, against dTV, w-TNV, and sequential hybrid kernel expectation maximisation (SHKEM). In the phantom, w-dTNV improves recovery coefficients over dTV and w-TNV, and improves recovery over SHKEM for the smallest spheres. In patients, w-dTNV gives higher tumour-to-background ratios than SHKEM at comparable background activity. These results suggest that CTAC-guided synergistic variational coupling improves lesion recovery and clinical lesion contrast, offering a practical route towards more stable post-SIRT Yttrium-90 activity estimates for personalised dosimetry.