Dirty-appearing white matter in multiple sclerosis: volumetric MR imaging and magnetization transfer ratio histogram analysis

Abstract

Background and purpose: In contrast to "normal-appearing" white matter (NAWM) in patients with multiple sclerosis (MS), there are subtle, abnormal and diffuse signal intensity changes often seen on T2-weighted MR images, which we have referred to as "dirty-appearing" white matter (DAWM). These areas of DAWM have slightly higher signal intensity than that of NAWM, but lower than that of lesion plaques. Our study was designed to determine the volumetric and magnetization transfer ratio (MTR) features of DAWM in patients with MS.

Methods: Dual-echo fast spin-echo MR imaging and magnetization transfer imaging were performed in 22 patients with relapsing-remitting MS. Slightly hyperintense DAWM areas were manually outlined on the basis of T2-weighted imaging findings. The volume and MTR of DAWM were calculated and compared with the volume and MTR of NAWM and T2 lesion plaques.

Results: The average volume of DAWM (18.3 mL) was greater than the average volume of T2 lesion plaques (11.0 mL, P =.04), and the mean MTR in DAWM (38.7%) differed significantly (P <.0001) from that in NAWM (40.7%) and plaques (33.3%). There was a modest negative correlation between either mean MTR (r = -0.60; P =.003) of DAWM or peak height (r = -0.50; P =.02) of DAWM with T2 lesion load. Neither DAWM volume nor total T2 abnormality (DAWM + plaques) volume correlates with the Expanded Disability Status Scale.

Conclusion: The results of this study indicate that MTR is able to differentiate DAWM from lesion plaques and NAWM and that DAWM might be a different pathologic process of the disease. The notion and quantification of these subtle imaging findings of DAWM areas may improve our understanding of certain stages of disease progression and disease burden in patients with relapsing-remitting MS.

Fig 1.

The PD-weighted scan shows DAWM areas, which are typically located in the periventricular and centrum semiovale regions of white matter in MS patients. A, DAWM with a single plaque. B, DAWM with several plaques. C, pure DAWM without plaques visible. D, outlined DAWM masks from C.

Fig 2.

The average MTR histograms were generated from NAWM, DAWM, and lesion plaques. Note that the histogram of DAWM was located between the histograms of NAWM and lesion plaques.