Technologist Viewpoint on PET/MR

Abstract

PET and MR are a good match for each other in some cases, but I do not think PET/MR will fully replace PET/CT. PET is well known for its use in metabolic and molecular imaging but has poor spatial resolution and lacks anatomic information. MR provides high resolution and good soft-tissue contrast but has poor sensitivity to molecular events. PET/MR causes less radiation exposure than PET/CT and allows for simultaneous acquisition of PET and MR data, eliminating the need to reposition the patient during the scan. The PET/MR system differs from a stand-alone PET or MR scanner in that the PET detectors are in the center of the bore of the MR scanner. Thus, the bore size is smaller than in most PET/CT systems but is about the same size as in a stand-alone MR scanner. In a PET/MR system, the PET component uses a solid-state detector—avalanche photodiodes—because photomultiplier tubes interfere with the magnetic field. There is a filter between the PET system and the MR system that does not cause any degradation in the MR images. In MR imaging, the body part being scanned must be in the center of the bore. Data must be simultaneously acquired in the same position in both PET and MR. In a multibed PET/MR study, the table will slowly move to allow the body part being scanned to remain in the center of the bore. PET/MR scans generally take longer than PET/CT scans. In PET/MR, it takes longer to position the patient on the table because coils have to be placed on top of the patient for the MR portion of the examination, whereas in PET/CT no extra equipment is needed. Patient comfort is key to a successful scan. MR imaging systems use coils to collect signal from the scanner to create images. The PET/MR system has 4 body coils that are placed on the anterior portion of the body. There is also a head and neck coil, as well as a spine coil on which the patient lies to receive signal from the posterior portion of the body. Flex coils are provided for imaging the extremities. Because MR imaging is sensitive to motion, it is important that patients remain still during the examination. Breathing artifacts can be significant in the chest and abdomen; therefore, some sequences are acquired while patients are holding their breath. Some patients may be claustrophobic because of the small bore size. The coils placed on top of the patient can sometimes exacerbate claustrophobia. Other factors that can limit a patient’s ability to undergo PET/MR are the presence of metal such as shrapnel or a bullet near the eyes or spine and the presence of a pacemaker, defibrillator, or aneurysm clips.