PET/MRI Technology

PET/MRI is a new imaging modality, where the camera (or detector) used to image the PET has been engineered to fit and function within the bore of the MRI.  This allows for the simultaneous acquisition of both PET and MRI data while the patient is in the scanner.

PET (or positron emission tomography) is an imaging modality that is used to detect the presence of a specific type of radiation (ie positrons).  We take advantage of this, by labeling small molecules with positron emitters.  The most common approach is to label glucose (sugar) with F18 (radioactive fluorine); this is termed FDG (fluorodeoxyglucose) to describe the injected radiotracer.  Since the late 1990s, FDG PET/CT has been the cornerstone of oncologic imaging.

MRI (or magnetic resonance imaging) is a noninvasive tool for imaging that does not require the use of ionizing radiation.  MRI uses unique properties of magnetic fields and resonance to image water protons in the body.  This technique has developed into a powerful tool for discriminating soft tissues within the body, and is now commonly used for numerous applications.

In order to make PET/MRI feasible, significant technological hurdles had to be over come.  The major issue is with the PET detector itself.  In PET/CT the crystals detect photons by releasing electrons, which results in a cascade within a photomultiplier tube.  This technology does not function within a PET/MRI as the electrons would be displaced due to the strong magnetic field.  Therefore new detectors had to be engineered that can function with high fidelity within a clinical 3.0T magnet.  This was solved by using what is termed silicon photomultipliers. 

Numerous other engineering hurdles had to be developed as well, including solving attenuation correction issues, preventing heating of the detectors, workflow considerations and minimizing interplay between MR imaging and PET acquisition.

The PET/MRI installed at UCSF is a GE Healthcare Signa 3.0T time-of-flight PET/MRI.  It is the first clinical time-of-flight PET/MRI installed in the United States, and has unparalleled imaging capabilities.  It has a 60 cm bore, identical to the majority of 3.0T magnetics available.  The PET detectors are only 5 cm wide and narrow the bore of the original magnet from 70 cm to 60 cm.   The PET detector uses a 25 mm deep lutetium base scintillator with a 25 cm z-axis field of view for the PET detector.  The detectors have a sensitivity of 21 cps/kBq.  Every MR sequence that is available on our clinical 3.0T magnets is available on the PET/MRI, meaning that there is no loss in MRI imaging when simultaneous acquisitions are performed.