A comparison of 180°and 360°acquisition for attenuation-compensated thallium-201 SPECT images

This study compared attenuation compensated, myocardial SPECT images reconstructed from 180°and 360°data to determine if either data acquisition method might yield improved image quality. Specifically, this study analyzed how the use of either 180°or 360°data affects: (1) the relative count density distribution, (b) defect contrast and (c) level of statistical noise in the left ventricular (LV) wall in the reconstructed SPECT images. Methods: Using the three-dimensional MCAT phantom simulating ²⁰¹Tl uptake in the upper torso and the SIMSET Monte Carlo code, noise- free projection datasets for both 180°(45°LPO to 45°RAO) and 360°acquisition were generated with the effects of nonuniform attenuation, collimator-detector response and scatter. In addition, low-noise experimental phantom data were acquired over 180°and 360°. Assuming the same total acquisition time, four sets of noisy projection data were simulated from scaled noise-free, simulated data for the following acquisitions: (a) 180°and (b) 360°data acquired on a 90°dual-detector system and (c) 180°and (d) 360°data acquired on a 120°triple-detector system. For each of the four acquisition schemes, 400 realizations of noisy projection data were generated, and the normalized s.d. in the reconstructed images was calculated for five ROIs in the LV wall. Images were reconstructed with nonuniform attenuation compensation using ML-EM algorithm for 25, 50 and 75 iterations. Results: Both the simulated noise-free and experimental low-noise images reconstructed from 180°and 360°data showed nearly identical count densities and defect contrasts in the LV wall. For the 90°dual-detector system, 180°images showed less noise, while for the 120°triple-detector system, 360°showed less noise; however, these differences in noise level were extremely small after a smoothing filter was applied. The 180°images acquired with the 90°dual-detector system showed the same noise level as the 360°images acquired with the 120°triple-detector system, so neither system geometry had an advantage with respect to reduced noise in the SPECT images. Conclusion: When nonuniform attenuation compensation is included in the reconstruction, the count density in the LV wall is nearly identical for 180°and 360°SPECT images, and the 90°dual-detector and 120°triple- detector SPECT systems produced similar SPECT images for the same total acquisition time.