A model based, anatomy dependent method for ultra-fast creation of primary SPECT projections

Monte Carlo (MC) is the most common method for simulating virtual SPECT projections. It is useful for optimizing procedures, evaluating correction algorithms and more recently image reconstruction as a forward projector in iterative algorithms; however, the main drawback of MC is its long run time. We introduced a model based method considering the effect of body attenuation and imaging system response for fast creation of noise free SPECT projections. Collimator detector response (CDR) was modeled by layer by layer blurring of activity phantom using suitable Gaussian functions. Using the attenuation phantom, in each angle, attenuation factor (AF) was calculated for each voxel. This calculated AF is the weight for the emission voxel and states the detection probability of photons that are emitted from that voxel. Finally weighted ray sum of the blurred phantom was driven to create a projection. For the next projection, our phantom was rotated and the procedure was repeated until all projections were acquired. Root Mean Square error (RMS) between all 60 modelled projection and real MC simulated projections was decreased from 0.58 ± 0.15 using simple Radon to 0.19 ± 0.03 using our suggested model. This value was 0.56 ± 0.16 using blurred Radon without attenuation modelling, and 0.21 ± 0.03 using attenuated Radon without CDR modelling. Our suggested model that considers the effect of both attenuation and CDR simultaneously results in more accurate analytical projections compared with conventional Radon model. Creation of 60 primary SPECT projections in less than one minute may make this method as a proper alternative for MC simulation. This model can be used as a forward projector during iterative image reconstruction for correction of CDR and attenuation that is necessary for quantitative SPECT.