What is Diffusion Tensor MRI (DT MRI)?

DT MRI can be peformed on a conventional MRI machine. It is used to study white matter tracts.

“What the mind does not know, the eye cannot see”–old Bangladeshi proverb

“You can’t trust your eyes if your imagination is out of focus”–Mark Twain.

We really need something visual in central pain and hopefully DT plus PET scans will do the trick. Without a seeable dimension, central pain will remain unreal to the world even as it is, unfortunately, the most potent reality to those who suffer from it. Until then, it is a kind of dark energy, known only to theoretical scientists. Somehow the public must see people on fire. Then, a cure would be at hand.

It would be very helpful in central pain to understand which tracts connect, and how, in instances of central pain. Once pain reaches the cord, it travels in insulated tracts, or white matter. It is of great interest to know how behavior in tracts alters and which tracts connect, or fail to connect, in Central Pain. A “fiber” in DT MRI is not really an actual neuron, but rather a grouping of them.

Diffusion tensor MRI is possible due to the development of software which can detect the direction of the diffusion of water. The parameters most looked for are mean diffusivity and fractional anisotrophy (breakup). Images corresponding to white matter can be derived. There was a need to study connectivity of tracts in widely separated areas. This led to tractography.

Recently. Brun and others have exploited this to develop DT tractography. Brun used path shape descriptions, path shape comparison, spectral clustering, and what are called spectral embeddings to do tractography.

Another method determines path shapes using variations on the Hausdorff distance, which is the greatest distance which separates pairs of points on a path. A clustering could be derived to make an image. Remember again that the diffusion of water underlies signal.

O’Donnell and Westin have expanded clustering techniques to “simultaneously cluster multiple subjects”. This method groups clusters on the basis of shape and location. Tractogrophy is drawn until functional anisotrophy (roughly, statistial unlikelihood of water diffusing in that fashion in random matter) of the direction of maximal diffusion fell below 0.1 or when the fiber rapidly changes direction.

Spectral clustering groups data based on its similarity to other data. These clusters are stored in a matrix. Shape and position are the basis of similarity and hence, direction of diffusion, or direction of tract.

Do not worry if you do not get all this. Neither do most radiologists. Still, it is something that should be used extensively in cases of central pain to determine which tracts have gone wrong. We anxiously await such research. It will allow study of the various central pain if the MRI is done during eliciting of the pain, ie. causing the evoked level of pain. It should take us a step further than PET scans alone. See O’Donnell, Amercian Journal of Neuroradiology, May 2006, 1032 ff.

DT MRI is presently used to follow multiple sclerosis progression but is NOT used to monitor central pain. It should be so utilized.