ADVANCED IMAGING METHODS

Single photon emission computed tomography (SPECT) and positron emission tomography (PET) are both advanced imaging techniques using radioactive isotopes to generate tomographic images of subject organ function or blood flow. Both are expensive technologies. SPECT imaging is available in only one or two veterinary schools, and PET scanning is currently available only in larger human medical center.
The isotopes used for SPECT imaging are the same as those used for standard planar scintigraphy. SPECT imaging differs from planar scintigraphy in that with SPECT the camera head rotates around the patient. This allows the collection of images of the same anatomic location from multiple different positions. The data recorded from the different camera positions or individual detectors is recorded on a computer then reconstructed into selected image slices using back projection algorithms developed originally for CT imaging. SPECT images have the advantage of allowing separation of overlapping structures, producing serial images of the “slices” of an organ and allowing three-dimensional reconstruction of serial images.

PET scanning is performed using a ring detector system. The technique depends on simultaneous or “coincidental’" detection of a pair of 5 11 keV photons emanating at 180o from a positron/electron annihilation event somewhere in the subject. Since the point of annihilation must have occurred somewhere along a straight line between the two activated detectors, no collimation of the detectors is required. Filtered back-projection techniques are again used to generate a computed image. The major advantage of PET is that many of the isotopes used such as carbon- 11, nitrogen- 13, oxygen- 15, and fluorine-18 are components of natural metabolic processes, thus PET scanning can provide detailed insight into previously inaccessible functions, particularly in the brain. Currently the high cost of the equipment and the high cost of producing the radionuclides used with PET have limited the utilization of this imaging modality in veterinary medicine.