Diffusion sequence and demyelination

MR Images of 28-Year-Old Woman with History of Multiple Sclerosis and Recent Hypoesthesia of Left Leg

Figures 1-3. Axial (1) T2-weighted fast spin-echo image (5,000/128) obtained with two signals averaged, 6-mm section thickness, echo train of 23, and 230 × 512 matrix, (2) fast FLAIR (fluid-attenuated inversion recovery) image (6,000/105/2,200 [inversion time, msec]) obtained with two signals averaged, 6-mm section thickness, and 182 × 256 matrix, and (3) T1-weighted contrast-enhanced spin-echo image (540/14) obtained with two signals averaged, 6-mm section thickness, and 230 × 512 matrix. Active plaque with contrast enhancement is seen. Two additional, small plaques are also recognized on the FLAIR image.







Figures 4, 5. (4) Axial diffusion-weighted (x sensitizing direction) multishot echo-planar image (800/123) obtained with one signal acquisition and 6-mm section thickness and (5) corresponding ADC map.

Figures 6, 7. (6) Axial diffusion-weighted (z sensitizing direction) multishot echo-planar image and (7) corresponding ADC map. The ADC values of active plaque are reduced; however, depending on the orientation of the corticospinal fibers, the conspicuity may be lower with respect to conventional sequences. Additionaly, the small plaques are not recognized.


Myelination
Diffusion-weighted imaging is exquisitely sensitive to the direction of the axons in white matter. The measured diffusion coefficients appear to be larger when measured along the fibers (in the range of 1.0 × 10-3 mm2/sec) than across the fibers (in the range of 0.6 × 10-3 mm2/sec). Diffusion-weighted imaging will probably be a valuable technique in identification of myelination disorders (1), although the exact value of this technique has not yet been established (2-4).
References

1. Wimberger DW, Roberts TP, Barkovich AJ, Prayer LM, Moseley ME, Kucharczyk J. Identification of "premyelination" by diffusion-weighted MRI. J Comput Assist Tomogr 1995;19:28-33.

2. Doran M, Hajnal JV, Van Bruggen N, King MD, Young IR, Bydder GM. Normal and abnormal white matter tracts shown by MR imaging using directional diffusion weighted sequences. J Comput Assist Tomogr 1990;14:865-873.

3. Sakuma H, Nomura Y, Takeda K, et al. Adult and neonatal human brain: diffusional anisotropy and myelination with diffusion-weighted MR imaging. Radiology 1991; 180:229-233.

4. Douek P, Turner R, Pekar J, Patronas N, Le Bihan D. MR color mapping of myelin fiber orientation. J Comput Assist Tomogr 1991; 15:923-929.