Understanding spinal cord injury using advanced MRI scans
By: Sarah Morris, University of British Columbia
Every year about half a million people suffer a spinal cord injury. Most of these injuries occur due to trauma like car accidents or falls. The damage is often complex, and every injury is different, and so it can be very difficult for doctors to predict whether an individual patient can regain function. This is distressing for patients, who must wait through their recovery to find out if they have a permanent disability. MRI scans, which give detailed maps of where the healthy and damaged tissue is, might help doctors make better predictions about which patients can recover.
Your spinal cord is about the width of your pinky finger: the butterfly-shaped core is made of a type of tissue called grey matter and the rest is white matter. White matter is composed of nerve fibers which carry signals from the brain to the body and back again. The fibres conduct signals very quickly with the help of myelin, a protective layer of fatty tissue that surrounds each nerve. Myelin is gradually lost after a spinal cord injury as the tissue breaks down, disrupting the communication between the body and brain and contributing to disability.
Measuring myelin loss would tell us a lot about spinal cord injury. It could help doctors to predict what degree of recovery is possible for individual patients, allowing them to plan physiotherapy and help patients get back to their lives as quickly as possible. It would also help us track if new treatments are working.
When a patient is admitted to the hospital with a spinal cord injury, they will be immediately scheduled for an MRI. The MRI scanner uses a strong magnet and radio-waves to take images of the spinal cord which the doctor can use to assess the injury. On a standard MRI, spinal cord injuries are bright around the injury site. This brightness is definitely tissue damage, but it could be caused by many things: both loss of myelin and, for example, more water in the tissue due to inflammation, appear bright on a standard MRI scan.
So to measure myelin loss, we need a scan which is more specific. MRI researchers have developed two main methods for measuring myelin. The first detects how much water is trapped inside myelin. It is called myelin water imaging and has been used extensively to investigate brain development in childhood, and a variety of brain and spinal cord diseases including multiple sclerosis and Alzheimer’s.
The second technique involves transferring MRI signal from the fat molecules in myelin to water and is called magnetisation transfer. Magnetisation transfer scans can be acquired faster than myelin water imaging, but it is less specific, because fats which are not myelin can contribute to the signal. In 2015 inhomogeneous magnetisation transfer, a new magnetisation transfer technique with better specificity to myelin, was proposed. Inhomogeneous magnetisation transfer shows real promise as a fast myelin imaging technique, but before we can use it widely, we need to validate that it works. One way to check is to compare MRI images with a gold-standard map of where the myelin is.
For this research, we were lucky to have access to the International Spinal Cord Injury Biobank, which houses spinal cords donated by people who died shortly after sustaining a spinal cord injury. We scanned these cords with inhomogenenous magnetisation transfer MRI and then cut the cords into very thin slices which we stained with a special dye which attaches to myelin. When we compared the MRI and the stained tissue, we found that the areas of high myelin measured by the MRI scan and the bright areas of the myelin staining map matched very well. From our results, we are confident that inhomogeneous magnetisation transfer can accurately measure myelin.
This is a great first step, but there’s still a way to go before doctors can confidently use MRI images to predict how a spinal cord injury will heal over the long term. Our next step will be scanning spinal cord injury patients with both inhomogeneous magnetisation transfer and myelin water imaging to see how well they measure myelin loss after spinal cord injury in live humans. This is hard to do, as the spinal cord is small and moves around as the patient breathes. We will optimise our scanning techniques to get the highest quality images possible. Then we will compare the myelin maps with information about each patient’s recovery, to begin to build a model of how myelin loss affects disability.
Ultimately, we believe advanced myelin MRI could be an extremely valuable tool to see inside the spinal cord and measure myelin loss, helping doctors make more accurate predictions during the scary and uncertain recovery period and thereby improving outcomes for people living with spinal cord injury.