What causes the hyperintense T2-weighting and increased short T2 signal in the corticospinal tract?

Abstract The corticospinal tract (CST) appears hyperintense on both T 2 -weighted images and myelin water maps. Here, an extended multiecho T 2 relaxation sequence with echoes out to 1120 ms was used to characterize the longer T 2 times present in the CST. The T 2 distribution from the CST was compared to other white matter structures in 14 healthy subjects. The intra-/extracellular T 2 peak of the CST was broadened relative to other white matter structures and often split into two distinct peaks. In the CST, it appeared that the intracellular and extracellular water environments had unique T 2 times, causing the intracellular water peak to be pushed down into the myelin water T 2 regime and the extracellular peak to be pushed up to longer T 2 times. The conventional myelin water T 2 limits of 5-40 ms resulted in an artificial increase in myelin water fraction (MWF), causing the CST to be bright on myelin water images. When the upper limit for MWF was decreased to 25 ms, the CST regions exhibited MWF values similar to those found for adjacent anterior and posterior regions. The CST has unique magnetic resonance characteristics, which should be taken into consideration when being examined, especially when compared to pathological tissue.