JACK SIMON, MD, PhD: There are several ways we use T2-weighted imaging. One way is to look at the burden disease, which is the total amount of disease the patient has with that technique, and it gives you an idea of how -- sometimes how long the disease has been going on and how aggressive it's been. But we also can now count the number of T2 lesions, which seems to be a fairly convenient, simple approach to watching the disease over time. We can come up with a quantitative measure of change over time.

There is in fact a poor correlation between the T2 burden of disease and disability, for example, as measured by the EDSS. So we really can't use it as a measure of how the patient is going to look. But the T2 measures are good for following the course of disease, looking at how active it's been over a period of time, how much disease burden the patient's had up until that point in time.

There are reasons for the poor correlation between T2 burden of disease, for example, and disability, which we are just beginning to understand. For example, a single lesion in a functionally exquisite portion of the central nervous system, such as the optic nerve, causes fairly important signs and symptoms. But the same lesion in average white matter may have no clinical significance at the time.

In addition, we know that there's hidden pathology -- pathology of what is called normal appearing white matter, which we don't pick up by the standard T2 burden measures.

BRIAN APATOFF, MD, PhD: MRI, particularly for patients that have elected not to go on treatment is a very important way of really documenting how stable the disease is. So even though a patient is not having any actual relapses in the course of a year or two, they might be increasing their MRI burden of disease at a rate of 5-6% a year.

Similarly, there can be progression of brain atrophy measures on patients that are not on therapy. So these are essential to document, relatively stable disease activity for patients -- particularly who are not on therapy, or for trying to assess treatment failures to document what the pattern of accumulating lesions might be.