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 body axis in humans. In this work, we propose a method to track and to correct this motion after a list-mode respiratory-gated SPECT acquisition of a cardiac MIBI study. We use the dynamic MCAT (DMCAT) chest phantom to simulate 24 temporal frames regularly spaced during the respiratory cycle, for 60 projection angles. A 15 mm respiratory translation motion is simulated for the heart, the liver and the spleen. Since in thresholded anterior projections the activity is essentially stemming from these organs, the motion of the axial center of mass (aCOM) in these projections is expected to faithfully reflect heart motion. The mean aCOM position is determined using the 24 frames of thresholded anterior projections. Using the mean aCOM curve, the sixty projections of all frames are axially shifted up or down, in order to match an arbitrary common position for their aCOM. After motion tracking, the mean absolute difference between the true motion curve and the aCOM curve is less than 0.05 cm for noisy studies respectively. After correction no heart motion is visible on a cine display of projections. A polar map of myocardial MIBI uptake after motion correction is closer to that obtained when no respiratory motion is present than without correction