Evidence of Long-T2 Fraction and Higher Myelin Water Fraction in the Corticospinal Tract

Introduction The corticospinal tract (CST) can be identified on heavily T2-weighted MR images as areas of bright intensity within the posterior internal capsules [1]. The CST is known to have thicker myelin sheaths than other brain structures [1,2] and this difference in myelin morphology may give rise to a difference in the water T2 behaviour in the CST. The T2 decay curve from central nervous system tissue can be separated into several exponential components and in normal brain tissue the T2 decay curve has three main components, which arise from three distinguishable water environments. There is a shorter component (T2 ~ 20ms), which has been designated as signal from the water trapped within the myelin layers; an intermediate component (T2 ~ 80ms) where the signal arises from intraand extracellular water, and a very long component (T2 > 2s) from cerebrospinal fluid (CSF) [3,4]. These three components can be quantified separately as fractions of the total signal in the area of interest. In addition, a long T2 component (200ms < T2 < 800ms) has been observed in the white matter of subjects with PKU and MS, as well as in the CST of normal controls [2,5]. The relationship between myelin content and the amount of long T2 signal has not been studied in healthy white matter. The purpose of this study is to examine the fraction of long T2 component and the fraction of myelin water (MWF) in the CST and to compare the results with a nearby structure, the anterior internal capsule (AIC).