SPECT and PET Instrumentation

Abstract

Abstract Imaging instrumentation for nuclear cardiology is designed for imaging radionuclides that emit gamma rays (single photons) and radionuclides that emit positrons. SPECT uses mechanical devices, called collimators, to determine the direction of travel of detected photons. The most common collimators are parallel-hole designs that eliminate all photons that are not perpendicular to the detector face. Photons that pass through the collimator encounter a scintillation crystal; scintillation produces a flash of light with brightness proportional to the photon’s energy. This flash is detected by an array of devices called photomultiplier tubes (PMTs). Determining the location and brightness of the scintillation allows formation of a planar projection. Acquiring planar projections at several positions around the patient provides the information necessary to reconstruct tomographic images. Advanced SPECT system use unique designs to improve sensitivity. PET does not image positrons directly but uses a ring of scintillation detectors to detect the annihilation photon pairs that are emitted in opposite directions when a positron (antimatter) and an electron (matter) meet, and their mass is converted to energy. By only accepting photon pairs that occur in coincidence, the line between the two detectors gives the direction of travel of the photons (electrical collimation). Improved PET designs utilize the difference in arrival time of the photon pair to estimate the location of the annihilation and lengthen the axial FOV (along the table) to improve sensitivity. Some improved SPECT and PET designs have replaced PMTs with solid-state detectors.