SPECT (Single-Photon Emission Computed Tomography)

During the last three decades, there has been much excitement in the development of diagnostic radiology. The development is fueled by inventions and advances made in a number of exciting new medical imaging modalities, including ultrasound (US), x-ray CT (computed tomography), PET (positron emission tomography), SPECT (single-photon emission computed tomography), and MRI (magnetic resonance imaging). These new imaging modalities have revolutionized the practice of diagnostic radiology, resulting in substantial improvement in patient care.

Single-photon emission computed tomography (SPECT) is a medical imaging modality that combines conventional nuclear medicine (NM) imaging techniques and CT methods. Different from x-ray CT, SPECT uses radioactive-labeled pharmaceuticals, that is, radiopharmaceuticals, that distribute in different internal tissues or organs instead of an external x-ray source. The spatial and uptake distributions of the radiopharmaceuticals depend on the biokinetic properties of the pharmaceuticals and the normal or abnormal state of the patient. The gamma photons emitted from the radioactive source are detected by radiation detectors similar to those used in conventional nuclear medicine. The CT method requires projection (or planar) image data to be acquired from different views around the patient. These projection data are subsequently reconstructed using image reconstruction methods that generate cross-sectional images of the internally distributed radiopharmaceuticals. The SPECT images provide much improved contrast and detailed information about the radiopharmaceutical distribution as compared with the planar images obtained from conventional nuclear medicine methods.

As an emission computed tomographic (ECT) method, SPECT differs from PET in the types of radionuclides used. PET uses radionuclides such as C-11, N-13, O-15, and F-18 that emit positrons with subsequent emission of two coincident 511 keV annihilation photons. These radionuclides allow studies of biophysiologic functions that cannot be obtained from other means. However, they have very short half-lives, often requiring an on-site cyclotron for their production. Also, detection of the annihilation photons requires expensive imaging systems. SPECT uses standard radionuclides normally found in nuclear medicine clinics and which emit individual gamma-ray photons with energies that are much lower than 511 keV. Typical examples are the 140-keV photons from Tc-99m and the ∼70-keV photons from TI-201. Subsequently, the costs of SPECT instrumentation and of performing SPECT are substantially less than PET.

Furthermore, substantial advances have been made in the development of new radiopharmaceuticals, instrumentation, and image processing and reconstruction methods for SPECT. The results are much improved quality and quantitative accuracy of SPECT images. These advances, combined with the relatively lower costs, have propelled SPECT to become an increasingly more important diagnostic tool in nuclear medicine clinics