Fully 3D Monte Carlo reconstruction in SPECT: proof of concept and is it worthwhile?

In Single Photon Emission Computed Tomography (SPECT) with parallel hole collimation, image reconstruction is usually performed as a set of 2D analytical or iterative reconstructions. This approach ignores the 3D nature of scatter and detector response function that affect the detected signal. To deal with the 3D nature of the image formation process, iterative reconstruction can be used by considering a 3D projector modeling the 3D spread of photons. This has already been studied using approximate analytical models for the 3D projector. In this paper, we investigate the value of using accurate Monte Carlo simulations to determine the 3D projector including all physical effects affecting the imaging process (attenuation, scatter, camera point spread function) used in a fully 3D Monte Carlo (F3DMC) reconstruction approach. Given the 3D projector, the reconstruction problem is solved using the maximum likelihood expectation maximization (MLEM) algorithm. To validate the concept, 2 small datasets were simulated and 4 reconstruction strategies were compared: filtered backprojection, MLEM without attenuation correction, MLEM including all corrections with approximate analytical models (MLEMC) and F3DMC. Results suggest that F3DMC multiplies imaging sensitivity by about 103, increases signal-to-noise ratio by 25 to 70% compared to MLEMC and improves spatial resolution. The practical feasibility of the approach on real data sets is discussed. Index Terms - Image reconstruction, Monte Carlo simulations, SPECT. (authors)