Positron emission tomography (PET) is a nuclear imaging tool for molecular and functional imaging of biological processes. While functional imaging is used to monitor parameters such as perfusion and metabolic rate, molecular imaging is done to study and measure cellular events like gene expression and receptor binding.
The miniaturized version of PET called the micro-PET is used in small animal imaging. The development of micro-PET imaging has opened up new possibilities for non-invasive and repetitive imaging of small animals in preclinical studies.
With the development of new probes and reporter genes, the applications of micro-PET in research studies focusing on enzyme activity, protein-protein interactions, metabolism, and gene expression has been enhanced. Also, the results of small animal PET imaging is extrapolatable and can be easily translated to the clinic. Micro-PET can reduce the number of animals needed for experiments by allowing non-invasive and serial studies.
Principles of micro-PET
PET imaging involves detection of photons generated by the sample tissue as a result of a process called positron decay. The sample is injected with radio-labeled biomolecules. With decay of the radioisotopes, the sample emits positrons that destroy electrons present in the sample, thus producing high-energy gamma rays. PET systems have detectors that pick up these gamma rays and the data collected can be reconstructed to produce high-resolution images of the sample.
Applications of micro-PET
- Molecular and functional micro-PET imaging is highly useful in neurology, oncology, and cardiology
- Clinical uses of micro-PET include estimating enzyme reactions, interactions between ligand and receptor, cell proliferation, and cellular metabolism.
- Advanced applications of micro-PET such as in Alzheimer’s disease pathophysiology or therapeutics are still at an early stage. The technique is currently being used to enhance in vivo Alzheimer’s disease diagnosis, monitoring propagation of the disease, and advancing clinical trials of the disease.
- The technique is being used for accelerating radiopharmaceuticals development
Strengths of micro-PET
- Micro-PET offers excellent depth of imaging as the source of radiation is introduced into the sample
- It provides good, in vivo measurements of metabolic pathways and target tissues deep inside the body
- Micro-PET’s acquisition time is very fast
- The technique offers precise quantitative analysis of radiolabeled biomolecules
- It enables small animals to act as their own controls, thus minimizing the number of animals required for study
- The technique is very sensitive to different biological tissues as they intake the radioisotopes at different rates.
Weaknesses of PET imaging
- Volumetric differences between tissues in small animals and humans pose huge challenges in PET imaging.
- Half-lives of radioisotopes used in this technique are very short and hence cyclotrons may need to be present with the experimental apparatus for constant generation of these isotopes
- Use of radiation can be harmful to small animals
- Radiation also alters the size of the tumor in cancer research studies and thus additional control groups may be required
- Spatial resolution offered by micro-PET is not very good
- It often needs to be combined with other tools such as micro-MRI or CT to achieve a well rounded study involving both anatomical and molecular imaging. This increases the cost as well as the need for specialized facilities.
Reviewed by Catherine Shaffer, M.Sc.
- MicroPET imaging and transgenic models: a blueprint for Alzheimer's disease clinical research, https://www.ncbi.nlm.nih.gov/pubmed/25151336
- Micro-PET imaging and small animal models of disease, https://www.ncbi.nlm.nih.gov/pubmed/12900267
- Preclinical Imaging https://en.wikipedia.org/wiki/Preclinical_imaging
- Positron Emission Tomography: applications in drug discovery and drug development, https://www.ncbi.nlm.nih.gov/pubmed/16181131
- Small Animal PET Imaging, http://ilarjournal.oxfordjournals.org/content/49/1/54.full