A processing technique for the truncated projections of asymmetric-fan-beam transmission imaging

The authors have developed a new imaging geometry-the asymmetric-fan-beam (AsF)-to expand the imaging field of view (FOV) for transmission imaging on current SPECT systems. The AsF geometry samples slightly more than one-half of the FOV in each projection and yields half-truncated projection data. Although each projection profile is not complete, the combined acquired data set meets the minimum sampling requirement because the other half of the FOV is sampled in the opposite projections after a full 360° rotation of the detector system. To take advantage of the simple convolution-backprojection algorithm for reconstruction, the key is in the handling of the projection profile for convolution. The authors have investigated such a technique to process the truncated projection profile for reconstruction without the side effects caused by the truncation. This technique entails filling in the truncated portion of each profile with interpolated data derived from other projections. After convolution, only the corresponding half of the original projection profile is backprojected in reconstruction. This is done to minimize propagation of the errors of interpolation in the reconstruction. Reconstructed images from phantoms and human subjects demonstrate that this processing technique yields good quality transmission images