Use of asymmetric fan‐beam transmission computed tomography for attenuation correction of cardiac SPECT imaging

Photon attenuation is often the largest source of infidelity in cardiac single photon emission computed tomography (SPECT). This investigation evaluates the feasibility of using an asymmetric fan‐beam transmission CT (AsF–TCT), acquired immediately before or after the emission computed tomogram (ECT), to correct for photon attenuation in cardiac SPECT. AsF–TCTs are reconstructed to generate patient‐specific attenuation maps. The maps are used with an iterative maximum‐likelihood algorithm to reconstruct attenuation‐corrected (AC) SPECT images. AsF attenuation correction significantly improves the accuracy of images of a cardiac‐thorax phantom modeling a large patient with a normal myocardial perfusion study. Similar improvements in image uniformity are obtained for patient studies. Using a high‐activity line source and rapid camera rotation, a suitable attenuation map is obtained with a four minute transmission acquisition; shorter acquisitions could be used for SPECT systems with higher count‐rate performance. Misregistration between the ECT and TCT can create artifacts in the reconstructed AC images, with lateral patient shifts as small as 2–3 cm causing severe artifacts. Axial, anterior‐posterior, and rotational misregistrations require larger shifts before artifacts become significant. Finally, applying AC improves assessments of both defect size and severity. Without AC, cardiac SPECT can significantly over‐estimate the size of perfusion defects, particularly in the septum and inferior wall of the left ventricle. AC improves estimates of lesion severity and results in better contrast between adjacent normal and defect regions. This investigation demonstrates that AsF–TCT is a viable technique for attenuation correction for cardiac SPECT, and may improve the diagnostic accuracy of this modality.