Generation of planar images from lung ventilation/perfusion SPECT

Objective To develop a method of producing lung ventilation and perfusion (V/Q) planar images using forward projection of reconstructed single-photon emission computed tomography (SPECT) images through approximate attenuation (mu) maps generated from the lung emission scans alone, as transmission-based mu maps may not be routinely available. Methods Synthetic mu maps are derived from Tc-99m photopeak and "scatter" windows for the attenuation correction of the SPECT images. The attenuation-corrected SPECT images are forward projected at appropriate angles to give the equivalent of planar images. This method allows high-count planar images, as well as the SPECT images, to be produced from a single SPECT acquisition. In addition, isolated "single lung" views of lateral and medial projections without "shine-through" from the contra-lateral lung, which have not been available previously, can be formed. Results Comparison of reprojected images produced from CT-derived or synthetic mu maps displayed similar detail and radiopharmaceutical distribution. In a blinded comparison of "true" planar images with those from reprojecting the SPECT data using the synthetic mu maps, no difference in mismatched defect detection was found, and hence it was confirmed that the reprojected planar images could replace true planar images with no loss in planar diagnostic sensitivity. Conclusions The reprojected planar images provide high-count, high-quality images, which are comparable with conventional 2D images.