Effects of attenuation in SPECT

The general goal of any SPECT imaging procedure is to obtain cross-sectional image(s) of the intensity distribution of a given radionuclide in a given organ. These images are usually reconstructed from a complete set of projection images, each acquired at a different angle, by using a standard reconstruction algorithm, such as filtered backprojection. It is straightforward to show that an exact analytic reconstruction can be obtained (in the limit of infinitely fine sampling and an infinite number of counts) only if the number of counts measured for each projection ray is directly proportional to the line integral of the activity distribution along the ray. Unfortunately, there are several physical effects which violate this requirement for an exact, analytic reconstruction. The effects to be considered in this presentation are (1) attenuation of emitted photons, and (2) scattered photons which are detected