The quantification of planar scintigraphy using three dimensional modelling of organ shadows

A method to define biodistribution from planar scintigraphic data is presented. Sampling regions or regions of interest (ROI's) are first defined as the projection contour of organs. From anatomical knowledge the three dimensional volume of the organ whose shadow has been traced is reconstructed. When the relevant organ volumes including the whole body or relevant whole body part have been defined, the count rate density in each pixel is assumed to represent the sum of the product of the projecting organ partial volumes times the tracer concentrations in the underlying organs. The solution (the definition of organ concentrations) is found by a matrix inversion. Validation is obtained in two ways. The first method is illustrative: an image is reconstructed from the defined volumes modulated by the computed concentrations. This image, if the method is correct, should be a crisp equivalent of the original data and closely similar to it following blurring with a convolution kernel equal to the imaging system's point spread function. The second is analytical: from reconstructed S.P.E.C.T. data in which voxel values are directly proportional to organ concentrations, a planar image is reconstructed. Organ average voxel values are computed in the S.P.E.C.T. image with volumetric ROI's and in the reprojected planar image using the method described here. The methods should yield the same concentration data but for a scaling factor. The method has the advantage that it yields volumetric and concentration data which are directly applicable to MIRD type analysis. In effect, the standard man assumptions in MIRD analysis have been incorporated in the calculation of the concentrations.