Unified reconstruction for ECT by Inverse Monte Carlo

Tomographic reconstruction for SPECT with simultaneous compensation for attenuation, scatter, and distance dependent collimator resolution is provided by an Inverse Monte Carlo (IMOC) reconstruction algorithm. Monte Carlo techniques are used to solve the photon transport equation for SPECT acquisition from an arbitrary source distribution within the body contour, forming a matrix of projection probabilities. This matrix, multiplied by the unknown source distribution vector, is set equal to the acquired projection vector. The resulting large, non-sparse system of equations is solved for the source distribution using an iterative Maximum Likelihood EM estimator. Reconstruction of experimentally acquired projections from phantoms shows quantitative compensation for scatter and attenuation. Comparison with filtered backprojection (FBP) reconstruction shows an improvement in resolution recovery and contrast for the IMOC algorithm. For a cold defect in an active cylinder, the contrast was improved from -0.38 +- 0.06 for FBP, to -0.78 +- 0.04 for IMOC. Preliminary reconstruction of a clinical HIPDM human brain study shows improved contrast. The IMOC algorithm is well suited for reconstruction of projection data from fan beam or special acquisition geometries provided a Monte Carlo model of the required acquisition technique is employed. Developed here for SPECT, the technique is valid for PET as well as for transmission CT. These preliminary results indicate that IMOC will contribute to improvement in quantitative ECT reconstruction