A mathematical model of motion of the heart for use in generating source and attenuation maps for simulating emission imaging

This manuscript documents the alteration of the heart model of the three-di mensional (3D) mathematical cardiac torso (MCAT) phantom to represent cardi ac motion. The objective of the inclusion of motion was to develop a digita l simulation of the heart such that the impact of cardiac motion on single- photon emission computed tomography (SPECT) imaging could be assessed and m ethods of quantitating cardiac function could be investigated. The motion o f the gated 3D MCAT's (gMCAT) heart is modeled using 128 separate and evenl y spaced time samples from a blood volume curve approximating an average he art cycle. Sets of adjacent time samples can he grouped together to represe nt a single time interval within the heart cycle. Maximum and minimum chamb er volumes were selected to be similar to those of a normal healthy person while the total heart volume stayed constant during the cardiac cycle. Myoc ardial mass was conserved during the cardiac cycle and the bases of the ven tricles were modeled as moving towards the static apex. The orientation of the 3D MCAT heart was changed during contraction to rotate back and forth a round the long axis through the center of the left ventricle (LV) using the end systolic time interval as the time point at which to reverse direction . Simple respiratory motion was also introduced by changing the orientation of the long axis of the heart to represent its variation with respiration. Heart models for 24 such orientations spanning the range of motion during the respiratory cycle were averaged together for each time sample to repres ent the blurring of the heart during the acquisition of multiple cardiac cy cles. Finally, an option to model apical thinning of the myocardium was inc luded. As an illustration of the application of the gMCAT phantom, the gate d heart model was evaluated by measuring myocardial wall thickening. A line ar relationship was obtained between maximum myocardial counts and myocardi al thickness, similar to published results. Similar results were obtained f or full width at half maximum (FWHM) measurements. With the presence of api cal thinning, an apparent increase in counts in the apical region compared to the other heart walls in the absence of attenuation compensation turns i nto an apparent decrease in counts with attenuation compensation. The apica l decrease was more prominent in end systole (ES) than end diastole (ED) du e to the change in the partial volume effect. These observations agree with clinical trends. It is concluded that the gMCAT phantom can be used to stu dy the influence of various physical parameters on radionuclide perfusion i maging. (C) 1999 American Association of Physicists in Medicine. [S0094-240 5(99)02911-9].