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Fast GPU-based Monte Carlo code for SPECT/CT reconstructions generates improved Lu-177 images

Journal article
Authors Tobias Rydén
Jakob Heydorn Lagerlöf
Jens Hemmingsson
Ida Marin
Johanna Svensson
Magnus Båth
P. Gjertsson
Peter Bernhardt
Published in Ejnmmi Physics
Volume 5
Issue 1
ISSN 2197-7364
Publication year 2018
Published at Institute of Clinical Sciences, Department of Radiation Physics
Institute of Clinical Sciences, Department of Oncology
Language en
Keywords Monte Carlo, GPU, SPECT, OSEM, receptor radionuclide therapy, geometric transfer-function, neuroendocrine tumors, detector response, photon transport, simulation, lu-177-dotatate, collimator, camera, toxicity, Radiology, Nuclear Medicine & Medical Imaging
Subject categories Radiological physics


Background: Full Monte Carlo (MC)-based SPECT reconstructions have a strong potential for correcting for image degrading factors, but the reconstruction times are long. The objective of this study was to develop a highly parallel Monte Carlo code for fast, ordered subset expectation maximum (OSEM) reconstructions of SPECT/CT images. The MC code was written in the Compute Unified Device Architecture language for a computer with four graphics processing units (GPUs) (GeForce GTX Titan X, Nvidia, USA). This enabled simulations of parallel photon emissions from the voxels matrix (1283 or 2563). Each computed tomography (CT) number was converted to attenuation coefficients for photo absorption, coherent scattering, and incoherent scattering. For photon scattering, the deflection angle was determined by the differential scattering cross sections. An angular response function was developed and used to model the accepted angles for photon interaction with the crystal, and a detector scattering kernel was used for modeling the photon scattering in the detector. Predefined energy and spatial resolution kernels for the crystal were used. The MC code was implemented in the OSEM reconstruction of clinical and phantom Lu-177 SPECT/CT images. The Jaszczak image quality phantom was used to evaluate the performance of the MC reconstruction in comparison with attenuated corrected (AC) OSEM reconstructions and attenuated corrected OSEM reconstructions with resolution recovery corrections (RRC). Result: The performance of the MC code was 3200 million photons/s. The required number of photons emitted per voxel to obtain a sufficiently low noise level in the simulated image was 200 for a 1283 voxel matrix. With this number of emitted photons/ voxel, the MC-based OSEM reconstruction with ten subsets was performed within 20 s/iteration. The images converged after around six iterations. Therefore, the reconstruction time was around 3 min. The activity recovery for the spheres in the Jaszczak phantom was clearly improved with MC-based OSEM reconstruction, e.g., the activity recovery was 88% for the largest sphere, while it was 66% for AC-OSEM and 79% for RRC-OSEM. Conclusion: The GPU-based MC code generated an MC-based SPECT/CT reconstruction within a few minutes, and reconstructed patient images of Lu-177-DOTATATE treatments revealed clearly improved resolution and contrast.

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