Measurement of the biomechanics of 3-D cardiac function with gated nuclear medicine studies

A new model to quantify functional and structural parameters of the myocardium from gated SPECT or PET data was developed. The model consisted of a set of nodes that move with the deformation of the left ventricle. Initially node positions were determined automatically using the prolate-spheroidal transform of an image of the left ventricle in a single gate of the cardiac cycle. The nodes were connected creating a set of tetrahedral volume elements that filled the space of the left ventricular muscle. Next, optimum node positions in each gate and node intensities were determined by least squares fitting the model intensities at each node for each gate to the measured data. The optimization of the node placements was driven by the gated nuclear medicine images with the optimization procedure regularized by constraining: (1) local cardiac muscle incompressibility, (2) fiber extension uniformity over the entire myocardium, and (3) smoothness of the node trajectories over time and space. We used microPET ¹⁸FDG-rat data to test our method by determining optimum positions for 774 nodes (3040 elements) in eight gates. Values of node intensities unchanged between the gates were also determined. In addition to blood pool volumes, the quantitative parameters describing cardiac biomechanics of myocardium volumes, regional fiber stretch values, and regional wall thickening values were also measured.