of interest). Chemical shift information is often used as an additional refinement to further clarify pictures produced through standard MRI.
In the body, ascertaining the presence of certain substances at a particular site can often provide important indicators of disease or abnormality. Inositol aspartate seems to be present in all neurons and can serve as a marker for mapping the nervous system using MRS. Choline and creatine are associated with much new growth and with tumor production, and can therefore provide useful targets in spectroscopic studies. N-acetyl aspartate acid (NAA) is another useful marker for neurons and demonstrates an intriguing use for MRS in tracking the course of disease. "It is possible to create maps of [the brain using] NAA," said Graeme Bydder, by first generating "a normal one as a baseline" and then a subsequent MRS map "to observe neuronal fallout, for example, in Pick's disease. This allows us to detect the absence of neurons even where the MR imaging is normal." Since NAA serves as a marker for neurons, MRS studies of newborns over time can map the growth of the brain, and possibly detect abnormalities that would show up in no other tests.
Bydder's latest research interest involves imaging the movement in vivo of anisotropically weighted water, what he described as "diffusion-weighted imaging," a very novel approach that Bydder nonetheless relates to the field: "Ideas in magnetic resonance have come from a lot of different sources, and this is another one that has actually been borrowed from spectroscopy." Bydder credited Michael Mosley from the University of California, San Francisco, with the important work necessary to turn the idea into a clinical application. The motion of the water molecules due to diffusion causes signal loss that allows MRI to differentiate particular tissues. The anisotropic quality means that water protons diffuse at different rates from different directions. When this diffusion factor is added to the other MRI parameters, certain biochemical phenomena in the body may be revealed that would not show up in a straightforward search for differences in proton density or in T1 or T2 times. Tissues can in this way be distinguished by their "diffusibility," a further marker to discriminate fine detail.
Bydder called the use of this new technique in brain studies "an interesting area. It brings together the molecular motion of water, the properties of myelinated nerve fibers, the gross anatomy of the brain in terms of being able to distinguish the directions of white-matter fibers, and the diffusion of water" as influenced by the presence of disease. It is MRI that unites all of these factors, he concluded, which could prove to be a revolutionary diagnostic advance. Bydder cited improved diagnosis of multiple sclerosis as another application.