Bright spots & black holes: 2 of 2
Gadolinium enhancement" sees active inflammation
Gadolinium is a contrast agent. Infused into the bloodstream a few minutes before an MRI, it lights up areas where immune-system cells are leaking out of tiny blood vessels and into the central nervous system. This is a hallmark of active inflammation, a key event in the MS attack on myelin. (Normally, immune cells are kept away from brain tissues by the blood-brain barrier, a layer of specialized cells inside capillary walls.)
Studies show that a gadolinium-enhancing lesion lights up for three to six weeks and then fades. Thus these scans are indicators of active disease during the preceding 45 days or so.
Unfortunately, fading doesn't always mean a lesion has fixed itself and disappeared. It may remain on a "T2 weighted" MRI scan.
T2 weighted scans give a good one-year look back
The MRI computer program can produce pictures based on two different proton relaxation times, called T1 and T2 weighted scans.
The T2 scans reflect the level of disease activity in the preceding year. The lesions these scans reveal continue to show up as high-water bright spots after the gadolinium enhancements have gone away. They are referred to as plaques. Occasionally lesions repair themselves and disappear forever. Others re-inflame later on, sometimes again and again. Over time, scientists have learned, repeated inflammation changes their nature altogether.
T1 weighted scans reveal tissue loss
Dark areas on the T1 weighted scans point to loss of myelin and loss of the nerve fibers, or axons. These areas are named for what they look like: "black holes". The darker the spot, the more extensive the tissue loss. They develop where a lesion has been re-inflamed time and again.
As one clinician put it, "Black holes are where a plaque used to be. The area is now totally inactive."
Black holes do correlate with the clinical picture. Individuals with black holes have permanent disabilities.
"MTR" MRI zeroes in on tissue loss
MTR, or magnetization transfer ratio, delivers an extra pulse of energy to the protons that are bound to myelin and then measures how much of that energy is transferred to the free water nearby. These calculations allow researchers to estimate more precisely how much tissue damage has occurred in specific lesions.
"MRS" MRI sees more than water
Magnetic resonance spectroscopy, or MRS, brings MRI full circle because the technology was originally developed not to make images of human beings but to scan laboratory samples for the presence of specific chemicals. MRS can measure the amount of important marker chemicals in nerve cells and axons, including, for example, a key byproduct of nerve-cell breakdown, providing yet another way to calculate how much permanent damage has occurred.
Tractography
The potential of MRI for revealing bodily processes in action within a living human being has not peaked. Among the newer techniques is one called "tractography", or "diffusion weighted imaging" and "diffusion tensor imaging". Focusing on white matter (the myelin-dense areas of the brain), this technology can outline the individual fiber tracts of nerve cells.
Tesla magnet
MRI machines with much bigger magnets are also being developed. The garden-variety MRI device has a 1.5 Tesla magnet, roughly as powerful as the magnets used in junkyards to pick up automobiles. An experimental 8 Tesla magnet at the University of Ohio has produced extraordinarily detailed images--and is so powerful it unhooked the bra of a visitor standing at a respectful distance outside. MRI will continue to provide scientists with insights on MS.
How often should I have an MRI?
Most of the MRI applications described here are for research studies only. They are done to answer questions about MS, not to guide the care and treatment of an individual. Frequent, or "serial", MRIs produce useful research information only if head position, scan power, and other factors are replicated exactly.
Some specialists advise patients to ask for an MRI every two years, An MRI helps clarify those hidden exacerbations. We can see if a disease-modifying therapy is doing its job. And for those who are not on therapy, we can track changes, which may help persuade the person to opt for therapy after all.
Other specialist feel that an MRI offers little help on how to treat most of his patients once they have been diagnosed. The data on adjusting treatment based on MRI changes alone just aren't conclusive yet. Many change medications in response to side effects or other clinical factors.
In short, like so much else in MS, the answer varies from person to person and from circumstance to circumstance.