The effect of contraction and twist on myocardial PET and SPECT image resolution: a mathematical phantom study

Motion of the heart degrades cardiac PET and SPECT resolution. A mathematical phantom was constructed to model contractile and non-uniform twisting motions of the left ventricular myocardium as the heart beats. The phantom was used to estimate the effect of motion on cardiac ECT image resolution and on quantitation in regions of interest (ROIs). Cardiac motion with contraction and twist was simulated at 16 time gates. The movement of mid-myocardial points was tracked, from which motion blur was computed. Contractile motion was greater near the base and twisting motion was greater near the apex. The blur due to both motions was equivalent to 7-9 mm FWHM Gaussian smoothing. Clinical PET resolution of 7 mm was degraded to 10-11 mm FWHM and clinical SPECT resolution of 14 mm to 16-17 mm FNHM. Weighting coefficients were used to characterize how activity in short and long axis ROIs was related to activity in source model ROIs. The effect of contraction was dependent on whether short or long axis slices were used for ROIs and on the spatial location of the ROIs. The weights for short axis ROIs showed the effect of twist in blurring activity to adjacent angular sectors. This effect was not seen in long axis ROIs where the angular range encompassed by the constant thickness slices was greater. A mathematical heart phantom that models contractile and twisting motions is valuable for investigating the effect of these motions on cardiac ECT studies.