A new study suggests positron emission tomography brain scans could reveal hidden inflammation in patients with multiple sclerosis who are being treated with highly-effective treatments. The new technique could lead to more advanced MS treatments.

The study started when researchers at Brigham Multiple Sclerosis Center and the Ann Romney Center noticed patients who were being treated with the most effective MS treatments available were experiencing worsening symptoms. The team has worked for the past eight years on developing an approach of imaging cells called microglia. Microglia are immune cells in the brain that are thought to have a role in MS disease progression but cannot be seen by a routine MRI. The team developed a technique called F18 PBR 06 PET imaging. It involves the injection of a tracer, or dye, which binds to the microglia cells. According to the researchers, increased microglial activity means more atrophy of gray matter in the brain.

In their paper, the authors describe the term “smoldering” inflammation. Just as a smoldering fire can burn slowly without smoke or flame, smoldering inflammation may linger in patients with MS, driving disease progression and symptoms, even when it cannot be assessed on MRI.

The newly published study involved performing PET scans on 22 people with MS and eight healthy controls. Researchers measured the glial activity load on the PET scans, a new measure where lab members looked at the level of smoldering inflammation from microglia in MS patients. They compared those scans to patients’ disability and fatigue levels and not only found that PET scans could show hidden inflammation caused by microglia, but the damage to patients’ brains correlated with the disability and fatigue levels they were experiencing. The researchers were also able to better classify patients with MS between high-efficacy and low-efficacy treatments. Those being treated with low-efficacy treatments had more abnormalities on their PET scans, suggesting more microglial cell activation. Those using high-efficacy treatments had a lower degree of PET abnormality than those on no or low efficacy treatments, but still had an abnormal increase of microglial activation compared to healthy people, suggesting that while high-efficacy treatments helped to reduce neuroinflammation, there was residual inflammation despite treatment, which could account for future worsening and progression independent of relapse activity in these MS patients.

One limitation to the study is the initial group was small. The authors note that PET scans can also be expensive and expose patients to some level of radiation, whereas MRIs do not. The authors said that radiation could potentially be reduced because of the long half-life and the requirement for a lower administered dose of the F18 PBR06 tracer being used. The tracer also produces better imaging characteristics compared to previously used tracers with shorter half-lives.

The researchers said despite the limitations, the study shines an important light on the power of PET scanning, specifically for the purposes of finding microglial activation. They said before the technique can be used routinely in a clinical setting, it must be validated on a larger sample size. Other longer half-life PET tracers have been approved by the FDA for clinical use, for example, amyloid PET tracers for studying Alzheimer’s disease. If approved, F-18 PBR06 could also be used as a tool to personalize and predict a patient’s treatment course in MS and other brain diseases. However, the authors note that even prior to approval, F-18 PBR06 can be used to help advance drug development and perform multicentric clinical trials.

The findings were published in Clinical Nuclear Medicine.