The emergence of PET/MR imaging has been solidly underway for a few years and the combination of the two modalities represents significant improvements to PET alone, namely MR’s excellent soft-tissue contrast and ability to capture not only physiologic but also biochemical and metabolic data. A review in the March issue of The Journal of Nuclear Medicine presents an updated perspective on how PET/MR is moving forward.
Since the first integrated scanner was developed in 2007, researchers have come a long way in discovering its strengths and weaknesses, according to Ciprian Catana, MD, PhD, of Massachusetts General Hospital and Harvard Medical School in Boston, and colleagues.
“Given the complementary nature of each modality’s strengths and weaknesses, integrating PET and MR imaging offers the opportunity to gain in a single examination many of the positive attributes of both and mitigate some of their limitations,” wrote the authors. “The wealth of information provided by MR enables PET/MR to go far beyond simple anatomic registration of PET molecular imaging, and the simultaneous acquisition of PET and MR data opens opportunities impossible to realize using sequentially acquired data.”
One of the major weaknesses of the combination is MR’s challenge when differentiating very dense bone from areas that consist of air. Both appear as signal void in imaging. Catana et al presented atlas-based attenuation correction and ultra-short echo time-sequences as significant improvements.
PET has inherent challenges when it comes to motion artifacts. MR provides a means of discerning high-temporal resolution motion, which can be applied to PET imaging for effective motion correction.
The patient populations that stand to gain the most benefit from PET/MR are those that are sensitive to radiation dose, especially pediatric patients and patients requiring multiple imaging studies.
Imaging of the head and neck cancers, pelvic cancers and non-malignant growths are especially well imaged due to MR’s soft tissue sensitivity, according to the authors. About 20 percent of patients at Massachusetts General Hospital with head and neck cancers require both 18F-FDG PET and MR imaging for effective evaluation. MR also may improve specificity in breast cancer imaging.
However, there are cases when a combination of PET/CT and MR perform better than PET/MR. The authors found that sensitivity of MR in pelvic imaging was approximately 80 percent or less for primary cancers that involve metastases to bone and lymph nodes outside the pelvis.
Despite any challenges with bone when imaging soft tissues, bone cancer patients with multiple myeloma were shown to benefit from a whole-body MR scan as opposed to skeletal imaging alone.
PET/MR also provided an advantage for patients with liver cancer due to its ability to improve 18F-FDG PET’s relatively deficient imaging of lesions smaller than 10mm.
Treatment monitoring could be a unique area for PET/MR expansion, as well as for use in novel radiotherapies. This is especially the case for therapies that target specific tumor processes due to MR’s ability to assess tumor vascularity. Cardiac and stem cell therapy applications also represent another possible application of PET/MR.
A future PET/MR application may be neuropsychiatric disease. Neurotransmittor balance and the impact on disorders such as schizophrenia could improve patient management. Finally, PET/MR may be used to improve Alzheimer’s imaging due to MR’s imaging of aggregation and neuronal degeneration.