Reconstruction Methods

As discussed earlier, SPECT combines conventional nuclear medicine image techniques and methods for image reconstruction from projections. Aside from radiopharmaceuticals and instrumentation, image reconstruction methods are another important engineering and technological aspect of the SPECT imaging technique.

In x-ray CT, accurate transaxial images can be obtained through the use of standard algorithms for image reconstruction from projections. The results are images of attenuation coefficient distribution of various organs within the patient’s body. In SPECT, the goal of image reconstruction is to determine the distribution of administered radiopharmaceutical in the patient. However, the presence of photon attenuation affects the measured projection data. If conventional reconstruction algorithms are used without proper compensation for the attenuation effects, inaccurate reconstructed images will be obtained. Effects of scatter and the finite collimator–detector response impose additional difficulties on image reconstruction in SPECT.

In order to achieve quantitatively accurate images, special reconstruction methods are required for SPECT. Quantitatively accurate image reconstruction methods for SPECT consist of two major components. They are the standard algorithms for image reconstruction from projections and methods that compensate for the image-degrading effects described earlier. Often, image reconstruction algorithms are inseparable from the compensation methods, resulting in a new breed of reconstruction method not found in other tomographic medical imaging modalities. The following ssssssections will present the reconstruction problem and a brief review of conventional algorithms for image reconstruction from projections. Then quantitative SPECT reconstruction methods that include additional compensation methods will be described.