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

Motion of the heart blurs cardiac PET and SPECT images. A mathematical phantom was constructed that models the contractile and non-uniform twisting motions of the left ventricular myocardium as the heart beats. The phantom was used to estimate the effect of normal heart motion on ECT image resolution and quantitation in regions of interest (ROIs). The movement of mid-myocardial points was tracked, from which motion blurring was computed. Clinical PET resolution of 7 mm was degraded to 10-12 mm FWHM in the direction of motion and clinical SPECT resolution of 14 mm to 15-16 mm FWHM. Contractile motion was greater near the base and twisting motion was greater near the apex. The blur due to both motions was equivalent to 7-10 mm FWHM Gaussian smoothing in the direction of motion. Activity weight coefficients characterized how activity in source model ROIs contributed to activity in short and long axis ROIs. The effect of contraction was dependent on whether the ROIs were from short or long axis slices and on the ROI location. The activity weights for short axis slices showed the effect of twist in blurring activity to adjacent angular sectors. This effect was not seen for constant thickness long axis slices because the angular range of the slices was greater, particularly near the apex. The contrast of perfusion defects in short axis ROIs was decreased by cardiac twist. A mathematical heart phantom that models contractile and twisting motions is useful for investigating the effect of these motions on cardiac ECT studies.