Factors affecting non-uniform attenuation compensation for SPECT myocardial perfusion imaging

The purpose of this study was to evaluate factors affecting the quality of the measured attenuation map and nonuniform attenuation compensation (AC) for SPECT myocardial perfusion imaging. Acquiring this map with low noise and accurate attenuation coefficients is important for simultaneous transmission-emission (STE) systems using fan beam geometry. The authors have investigated several factors affecting the measured attenuation map and AC using phantom and patient data. Myocardial uniformity decreased as extent of truncation increased. Finer sampling (128×128) in acquisition and reconstruction provided more accurate myocardial wall thickness and better contrast compared to 64×64 matrix. Downscatter correction improved accuracy of attenuation coefficients and myocardial uniformity. Reliable myocardial uniformity required scan durations of at least 14 minutes for myocardial activity of 350 μCi. Over a broad range of ML-EM iterations (20 to 50) in transmission (TCT) reconstruction measured attenuation coefficients for water were similar. Scatter correction improved normal myocardial wall contrast and moderate or severe defect contrast. In conclusion, methods to achieve accurate and low noise AC in myocardial imaging should use the best affordable sampling and scan duration, adequate activity of Gd-153 TCT source, proper positioning of patients and scatter correction