Imaging of Volume Objects by Shadow Casting

A novel technique for imaging the spatial distribution of radio isotopes is described that has application in diagnostic Nuclear Medicine. The penetrating nature of gamma and X-ray radiation renders conventional imaging techniques useless, and scanning methods or shadow casting techniques employing restrictive apertures comprised of an opaque material are required. The new technique consists of using coded apertures that enclose or partially enclose the object to cast shadow patterns on a recording media. For cylindrical and spherical shadowing configurations, the shadow pattern is shown to specify the three-dimensional Fourier Transform of the source on concentric cylinders or spheres. By appropriate aperture design, this sampling contains sufficient information to permit true volume reconstruction. This is unlike previous shadowing techniques using planar geometries, where the encoding of depth by the scale of the aperture pattern placed a fundamental restriction on the relation between depth and lateral resolution. The new imaging configuration is such that the physical constraints of the aperture construction, that prohibited the planar system from achieving high spatial resolution, are largely removed. In deriving the new imaging concept, this paper addresses the fundamental dimensionality-reduction question of how three-dimensional source information is encoded onto two dimensions, with detailed results presented for the cylindrical and spherical geometries. In addition to identifying the factors that affect the fidelity of the new system, results are presented regarding the trade-off between resolution and noise behavior.