Abstract
BackgroundMonte Carlo simulations provide accurate models of nuclear medicine imaging systems as they can properly account for the full physics of photon transport. The accuracy of the model included in the maximum-likelihood–expectation-maximization (ML-EM) reconstruction limits the overall accuracy of the reconstruction results. In this paper, we present a Monte Carlo-based ML-EM reconstruction method for pinhole single-photon emission computed tomography (SPECT) that has been incorporated into the SIMIND Monte Carlo program. The Monte Carlo-based model, which accounts for all of the physical and geometrical characteristics of the camera system, is used in the forward-projection step of the reconstruction, while a simpler model based on ray-tracing is used for back-projection. The aim of this work was to investigate the quantitative accuracy of this combination of forward- and back-projectors in the clinical pinhole camera GE Discovery NM 530c.ResultsThe total activity was estimated in 99mTc-filled spheres with volumes between 0.5 and 16 mL. The total sphere activity was generally overestimated but remained within 10% of the reference activity defined by the phantom preparation. The recovered activity converged towards the reference activity as the number of iterations increased. Furthermore, the recovery of the activity concentrations within the physical boundaries of the spheres increased with increasing sphere volume. Additionally, the Monte Carlo-based reconstruction enabled recovery of the true activity concentration in the myocardium of a cardiac phantom mounted in a torso phantom regardless of whether the torso was empty or water-filled. A qualitative comparison to data reconstructed using the clinical reconstruction algorithm showed that the two methods performed similarly, although the images reconstructed using the clinical software were more uniform due to the incorporation of noise regularization and post-filtration in that reconstruction technique.ConclusionsWe developed a Monte Carlo-based reconstruction method for pinhole SPECT and evaluated it using phantom measurements. The combination of a Monte Carlo-based forward-projector and a simplified analytical ray-tracing back-projector produced quantitative images of acceptable image quality. No explicit calibration is necessary in this method since the forward-projector model maintains a relationship between the number of counts and activity.
Highlights
Monte Carlo simulations provide accurate models of nuclear medicine imaging systems as they can properly account for the full physics of photon transport
An attractive, but seldom clinically used, aspect of single-photon emission computed tomography (SPECT) is that it can be utilized for activity quantification in vivo, with applications in, e.g. radionuclide therapy dosimetry [1, 2] and myocardial perfusion studies [3]
Our aims were to take advantage of the accurate forward-projection modelling provided by a full Monte Carlo (MC) simulation with the SIMIND program and to combine it with an approximate back-projector, thereby creating an integrated program for MC-based pinhole SPECT reconstruction
Summary
Monte Carlo simulations provide accurate models of nuclear medicine imaging systems as they can properly account for the full physics of photon transport. We present a Monte Carlo-based ML-EM reconstruction method for pinhole single-photon emission computed tomography (SPECT) that has been incorporated into the SIMIND Monte Carlo program. The accuracy of SPECT-based activity quantification depends on the accuracy with which radiation transport in the patient and the detector are modelled in the tomographic reconstruction algorithm. Typical photon energies in SPECT imaging range from 50 to 400 keV Within this energy range, there is a considerable probability that an emitted photon will interact within the patient before reaching the detector, which is difficult to model accurately. For imaging using radionuclides that emit photons of multiple energies, high-energy photons can be down-scattered into lower energy windows All of these phenomena must be considered to achieve reliable activity quantification. The absolute quantitative accuracy of gamma camera SPECT quantification has typically been reported to be within 20% for a range of radionuclides [2, 4]
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