Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) are functional imaging techniques that find application in diagnosis of ischemic heart disease, mapping of local brain metabolism, detection of tumors and areas of infection, drug development.
3-D images of spatial distribution of a pharmaceutical in PET and SPECT are reconstructed indirectly by observing the radiation emitted by a radioisotope that is bound to the drug.
Iterative algorithms based on statistical inference, such as Maximum Likelihood Expectation Maximization (MLEM), have proven to be effective for tomographic reconstruction.
Correlation of drug distribution with the underlying tissue morphology provides the potential to use information from an anatomical image of the same subject to improve the reconstruction and to decrease the uncertainty associated with the estimation of nuclear activity distribution. These techniques ultimately would allow for reduction of the radiopharmaceutical dose required for given resolution. Furthermore new possibilities are emerging from combined imaging techniques: PET/CT, SPECT/CT, PET/MR and SPECT/MR.