Introduction to PET/MRI

Background: PET and MRI
Positron Emission Tomography (PET) is an imaging technique that provides metabolic/physiological/functional/pharmacological biodistribution information inside the body. With this information, one can diagnose if there is a cancerous tissue in the body and further investigate the glucose metabolism, for example. Patients undergo PET scans to monitor the tumors (in brain, breast, lungs, lymphs, abdomen, reproductive organs etc.) and is a good diagnostic tool providing high sensitivity information.

Generally, a positron emitting radionucleide/radioisotope/radiotracer is administered to the patient. For each positron that annilate with an electron in the body, two gamma photons (511 keV each) are emitted in opposite direction (near 180-degrees). The PET detectors capture these photons within a certain time window and the data is processed to obtain a 2D/3D reconstructed image. Tumor regions can be identified in the body if these radionucliedes bind to the cancerous tissue compared to a healthy ones. This is a simplistic explanation of the basic principle of PET physics. At BNL, the main research involves the design and development of novel PET systems, mainly for small animal imaging applications.

Magnetic Resonance Imaging (MRI) is another non-invasive imaging tool that provides structural/morphological/anatomical information. Even X-ray Computed Tomography (CT) does the same. Infact, PET/CT systems are currently used extensively in clinical setting, as opposed to stand-alone imaging systems. But MRI provides better soft-tissue contrast compared to CT. Different tissue contrast can be highlighted using a variety of MR imaging sequences. Furthermore, there is no additional radiation dose to the patient in MRI, unlike CT.

PET pros: High sensitivity (pico molar range); Good physiological detail.
PET cons: Poor spatial resolution and limited anatomical detail.

MRI pros: Better soft-tissue contrast compared to CT ; Good spatial resolution (few microns) and no additional radiation dose.
MRI cons: Limited or no functional detail.

So by combining PET and MRI systems, the demerits of both PET and MRI systems are negated and they mutually complement each other making them powerful tools to extract accurate diagnostic information. There are many reports and publications on PET/MRI research conducted by various research groups around the world.