Effects of misalignment between transmission and emission scans on attenuation-corrected cardiac SPECT

Misalignment between transmission and emission scans in attenuation- corrected (AC) cardiac SPECT can introduce errors of measured activity. The severity of these errors, however, has not yet been fully elucidated. Methods: We performed 2 phantom measurement as well as a study of patients with low likelihood of coronary artery disease. Transmission and emission scans were acquired using a triple-head SPECT system with a collimated ²⁴¹Am line source and an offset fanbeam collimator. The left ventricular myocardium was divided into five segments, and the mean regional activity was calculated for each segment using a semiquantitative polar map approach. Misalignment between transmission and emission data was created by shifting the emission data along the x, y or z axis. Results: in the heart phantom, a shift between the transmission and emission data produced a decrease or increase in relative regional activity in each segment resulting in heterogeneous activity distribution. A 7-mm (1-pixel) shift produced up to 15% change in relative regional activity, suggesting that even a small misalignment between transmission and emission data can produce serious errors in measured activity. In the clinical data, the effects of misalignment were less significant than those observed in the phantom data but were still measurable and visually identifiable. Conclusion: The results indicate that a small misalignment between the transmission and emission data can produce serious errors in measured activity, and thus, geometrical precision is essential and accurate diagnosis of AC SPECT images.