Correction of the respiratory motion of the heart by tracking of the center of mass of thresholded projections: a simulation study using the dynamic MCAT phantom

During normal breathing, heart motion is about 15 mm along the body axis in humans. We propose a method to track and to correct this motion after a list-mode acquisition which involves the recording of a signal proportional to respiratory volume. We use the dynamic MCAT (DMCAT) chest phantom to simulate 24 temporal frames regularly spaced during the respiratory cycle, for 60 projection angles over 360°. A 15-mm respiratory translation motion is simulated for the heart, liver and spleen. Thresholding of projections is used to reduce the influence of static activity on calculation of the axial center-of-mass (aCOM). Variation in the impact of attenuation as a function of projections and noise in the low-count projections rebinned from list-mode acquisitions is seen to limit ones ability to track respiratory motion using the aCOM. By including the recording of a signal proportional to the relative respiratory volume with the list-mode acquisition counts from different respiratory cycles can be combined to produce projections with common respiratory volumes. We have determined that the aCOMs determined from summing these common-volume based projections over the anterior to left-anterior oblique projection angles can be used to track respiratory motion as a function of the volume signal. Using this information on the variation of the aCOM as a function of the volume signal, the entire list-mode acquisition can then be rebinned into a projection set which is corrected for respiratory motion. After motion tracking, the mean absolute difference between the true motion curve and the aCOM curve is 0.10 cm for noisy studies. After correction no heart motion is visible on a cine display of projections. The polar map of myocardial MIBI uptake after motion correction is closer to that obtained when no respiratory motion is present than without correction.