PET and SPECT are medical imaging techniques that involve introducing a synthetic radionuclide into a molecule of biological interest, such as a ligand, peptide, or antibody, and administering it to either an animal or a patient. The uptake of the radiotracer is then measured over time, allowing researchers to obtain valuable information about cellular and molecular processes. Although both PET and SPECT rely on similar principles to produce images, their instrumentation, radiochemistry, and experimental applications differ due to inherent differences in the physics of photon emission. Refer to the schematic figures below for a detailed illustration of each principle.
PRINCIPLE OF PET
PET scans involve the use of a radiotracer that accumulates in the tissue under investigation. The radionuclide present in the radiotracer decays, emitting a positron that travels a short distance before colliding with an electron. This collision produces two gamma rays (photons) that are emitted in opposite directions with an energy of 511 keV. The PET camera detects these photons and simultaneously locates their origin within a specific time frame, using opposing detectors that correspond to multiple rings of scintillation crystals. By collecting a statistically significant number of these radioactive events, mathematical algorithms generate a three-dimensional image that displays the distribution of the positron-emitting molecules throughout the body.
PRINCIPLE OF SPECT
In SPECT imaging, the radiopharmaceutical is introduced into the animal’s body and emits gamma-ray photons, which pass through the animal and are detected by a set of collimated radiation detectors, usually based on NaI(TI) scintillation detectors. Unlike PET imaging, which relies on positron-emitting radionuclides, SPECT utilizes gamma-emitting radionuclides that are easier to produce and can be used with a wider range of biomolecules. To create a 3D image of the radiopharmaceutical distribution in the animal’s body, projection data is acquired from different views by rotating the detectors around the animal or, in preclinical studies, by moving the animal bed. The acquired projections are then reconstructed using mathematical algorithms.
IN VIVO IMAGING MODALITIES
Molecular imaging modalities are a group of medical imaging techniques that allow for the visualization and quantification of biological processes at the molecular and cellular levels. These techniques utilize specific molecular probes that are designed to target and bind to specific biomolecules in the body, such as receptors, enzymes, and transporters. The probes are typically labeled with a radioactive isotope or fluorescent dye, which allows them to be detected and imaged in vivo.
There are several molecular imaging modalities, including positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), and optical imaging. Each of these modalities has its own strengths and weaknesses, and the choice of modality will depend on the specific biological process being studied and the clinical application.
PET and SPECT are nuclear medicine imaging techniques that rely on the detection of radiation emitted by a radiotracer. MRI, on the other hand, uses strong magnetic fields and radio waves to generate images. Optical imaging uses light to generate images, and can be used to visualize biological processes at the cellular level.
Molecular imaging modalities have a wide range of applications in both clinical and preclinical settings. They are used to study a variety of diseases, including cancer, cardiovascular disease, neurological disorders, and infectious diseases. Molecular imaging can provide valuable information about disease progression and response to therapy, and can also be used for drug development and clinical trials.