Projection data de-truncation in direct asymmetric-fan-beam SPECT reconstruction

The authors have recently developed a new sampling geometry-asymmetric fan (AsF) beam-which completes sampling of the imaging field of view (FOV), and thereby avoids the problems caused by projection truncation in transmission imaging with SPECT systems. The AsF geometry for a particular angular view covers only a half of the FOV. The problem in the raw projection data is that a sharp cut-off that causes spurious oscillations within the central region of a reconstructed image. But, since the 360° acquisition provides complete sampling, each individual projection can be completed by the proper use of the opposing views data. Several methods were investigated to prepare the data for filtering and backprojection. The first approach was the simplest one, using just the mirror reflection of original data; `fooling´ the Fourier transform. The second one was the use of the single opposing projection, the one separated by the 180°, without any processing. The last one, yielding the best results, provides missing data by resorting and interpolation. It uses four projection bins, two from each of the two neighboring angular views, that do not exactly oppose the original projection