Macromolecular Proton Fraction Mapping and MRI Reveal That Loss of Myelin Denotes Severity of Multiple Sclerosis

Researchers reported that the study could have important applications in clinical trials and treatment monitoring.

The study’s findings were published online September 12, 2014, in the journal Radiology. Loss of myelin, the fatty protective sheath surrounding nerve fibers, is a characteristic of MS, an inflammatory disorder of the central nervous system that can lead to a variety of serious neurologic symptoms and disability. MS is typically considered a disease of the brain’s signal-conducting white matter, where myelin is most abundant, but myelin is also present in smaller amounts in gray matter, the brain’s data processing hub that is comprised mostly of nerve cell bodies.

Although the myelin content in gray matter is small, it is still very important to proper function, as it enables protection of thin nerve fibers connecting neighboring areas of the brain cortex, according to Vasily L. Yarnykh, PhD, associate professor in the department of radiology at University of Washington (Seattle, USA). “The fact that MS patients lose myelin not only in white but also in gray matter has been proven by earlier postmortem pathological studies,” he said. “However, the clinical significance of the myelin loss, or demyelination, in gray matter has not been established because of the absence of appropriate imaging methods.”

To determine more about the correlation between MS and demyelination in both white and gray matter, Dr. Yarnykh and colleagues used an advanced MRI technique that provides data on the content of biologic macromolecules—molecules present in tissues and composed of a large number of atoms, such as proteins and carbohydrates. The new modality, known as macromolecular proton fraction (MPF) mapping, has been hampered in the past because of the length of time required for data collection, but improvements now allow much faster generation of whole-brain maps that reveal the macromolecular content in tissues.

“The method utilizes a standard MRI scanner and doesn’t require any special hardware—only some software modifications,” Dr. Yarnykh said. “MPF mapping allows quantitative assessment of microscopic demyelination in brain tissues that look normal on clinical images, and is the only existing method able to evaluate the myelin content in gray matter.”

The researchers examined 30 MS patients, including 18 with relapsing-remitting MS (RRMS), the most common type of MS initially diagnosed, and 12 with the more advanced type of disease known as secondary progressive MS (SPMS). Fourteen healthy control participants were also included in the study. Each participant underwent MRI on a 3-Tesla scanner, and the researchers reconstructed three-dimensional (3D) whole-brain MPF maps to look at normal-appearing white matter, gray matter, and MS lesions. The researchers further compared their imaging technique findings with clinical tests characterizing neurologic dysfunction in MS patients.

The findings showed that MPF was significantly lower in both white and gray matter in RRMS patients compared with healthy control subjects, and was also substantially decreased in both normal-appearing brain tissues and lesions of SPMS patients compared to RRMS patients with the largest relative decrease in gray matter. MPF in brain tissues of MS patients significantly correlated with clinical disability and the strongest associations were found for gray matter.

“The major finding of the study is that the loss of myelin in gray matter caused by MS in its relative amount is comparable to or even larger than that in white matter,” said Dr. Yarnykh. “Furthermore, gray matter demyelination is much more advanced in patients with secondary-progressive MS, and it is very strongly related to patients’ disability. As such, we believe that information about gray matter myelin damage in MS is of primary clinical relevance.”

The improved technique has potentially critical applications for MS treatments targeted to protect and restore myelin. “First, this method may provide an objective measure of the disease progression and treatment success in clinical trials,” Dr. Yarnykh said. “And second, assessment of both gray and white matter damage with this method may become an individual patient management tool in the future.”

Dr. Yarnykh and colleagues are currently conducting further research on the new technology with the support of the US Multiple Sclerosis Society (New York, NY, USA) and the National Institutes of Health (Bethesda, MD, USA). “This study was done on the participants at a single point in time,” he said. “Now we want to compare MS patients with control participants to see how myelin content will evolve over time. We further plan to extend our method to the spinal cord imaging and test whether the combined assessment of demyelination in the brain and spinal cord could better explain disability progression as compared to brain demyelination alone.”

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