Parallel-beam tilted-head analytic SPECT reconstruction

Parallel-beam Tilted-Head SPECT (TH-SPECT) was previously implemented on a SPECT system for its potential to image breast lesions and nearby axilla of seated, upright women. All TH-SPECT reconstructions will contain artifacts since the tilted orbit does not satisfy the Orlov sampling criteria. However, it is not clear which reconstruction method, if any, is better suited for TH-SPECT data. Here a geometric derivation of the ramp filter for tilted parallel-beam geometries is presented. A new filter is then heuristically suggested, specifically designed to boost frequencies near the unmeasured null space present in all TH-SPECT geometries. This filter, H(nu(11),tau(11)), was combined with the ramp filter and then implemented and compared with an iterative Ordered Subsets Expectation Maximization (OSEM) algorithm and a ramp-only FBP algorithm, using TH-SPECT data. Contrast values as well as axial elongation present in all TH-SPECT reconstructions were characterized using a mini-Defrise disk phantom placed inside a fillable breast phantom and imaged from 0-15 degree head tilt. At small tilt angles about the mini-Defrise disk phantom, contrasts were generally equal between H(nu(11),tau(11)) -filtered FBP and OSEM reconstructions compared at similar noise values, and poorer contrasts were obtained for ramp-only FBP results. In conclusion, measured results indicate OSEM reconstruction may offer better shape and uniform activity distribution of the breast compared to FBP methods. H(nu(11),tau(11))-filtered TH-SPECT FBP, however, performed more comparably to OSEM than ramp-only FBP methods, and other similar filtering techniques may provide even greater improvements.