Problems created in attenuation corrected SPECT images by artifacts in the attenuation maps

Attenuation of photons is a well recognized problem in SPECT, leading to artifacts and inaccuracies in reconstructed images. Most of the correction methods available in clinics use attenuation maps obtained with different transmission scanning systems. However, when maps are inaccurate this procedure may introduce artifacts to the final images which are difficult to identify, but which could inadvertently alter diagnosis and study outcome. The objective of this study was to investigate the issues of quality of the attenuation map and the effects that map artifacts may have on the attenuation corrected emission images. The investigated topics were related to the problem of low transmission counts, the cross-talk contributions from emission isotope, truncation of the transmission data and the methods of map reconstruction, segmentation and scaling. Examples of patient studies displaying particular problems guided our investigation and simulations were used in the analysis. Attenuation maps and final emission images were visually checked for artifacts and for presence of perfusion defects. Quantitative evaluation of map uniformity and defect visibility and size variations was performed.