Four-dimensional superquadric-based cardiac-thorax phantom for Monte Carlo simulation of radiological imaging systems [SPECT application]

A four-dimensional (x, y, z, t) composite superquadric-based object model of the human upper torso for Monte Carlo simulation of radiological imaging systems has being developed. The phantom includes heart, lungs, liver, gall bladder, spinal column, ribs, sternum, and breasts and models the real temporal geometric conditions of a beating heart for frame rates up to 64 per cardiac cycle. Phantom objects are described by boolean combinations of superquadric ellipsoids and toroids. Moving spherical coordinate systems are chosen to model ventricular wall movement whereby points of the ventricle walls are assumed to move towards a moving center-of-gravity point. Due to the non-static coordinate system, the atrial/ventricular valve plane of the mathematical heart phantom moves up and down along the left ventricular long axis resulting in reciprocal emptying and filling of atria and ventricles. Compared to the base movement, epicardial apex as well as the superior atria area are almost fixed in space. Since geometric parameters of the objects are directly applied on intersection calculations of the photon ray with object boundaries during Monte Carlo simulation, no phantom discretization artifacts are involved.