MR of postnatal brain development

Myelination of the human brain has been studied in vivo by means of magnetic resonance (MR) imaging (Barkovich et al., 1988; Bird et al., 1989; Martin et al., 1991; Nakagawa et al., 1998; Staudt et al., 1993; van der Knaap and Valk, 1990). The fundamental work in this domain was done by Barkovich and colleagues (1988), later complemented by others (Bird et al., 1989; Huppi et al., 1998; Koenig et al., 1990; Martin et al., 1991; Nakagawa et al., 1998; Staudt et al., 1993; Takeda et al., 1997; van der Knaap and Valk, 1990). To better understand the following description, a brief review of MR terminology may be helpful. Most MR images used to assess myelination are based on the concept of T1 or T2 weighting (table 3.1), which reflect differences in tissue water. Tl-weighted images are typically more "anatomic," whereas T2-weighted images typically show subtle abnormalities reflected in changes in water, or edema. More importantly, images can be Tl- or T2-weighted to optimize tissue characteristics.

Tl-weighted images look like "cut brain." Cerebrospinal fluid spaces, such as the ventricles and sulci, are dark on Tl-weighted images. Fatty tissues are bright. Myelin, containing phospholipids, is also bright relative to other intracranial structures. Thus, by the process of myelination, brain areas that are myelinated appear bright or hyperintense relative to other areas on the image. Moreover, areas of the brain with very tightly packed fiber bundles, such as the corpus callosum, extrude any free water from their myelin fibers and thus appear even brighter on the image.

T2-weighted images are essentially inverted in their signal characteristics. Hence, myelinated brain, with an abundance of phospholipid, appears hypointense.

Table 3.1 MR milestones of CNS myelination

Anatomic Region

Age at Which Myelination Appears

Tl-Weighted Images

Tl-Weighted Images

Inferior cerebellar peduncle

Present at birth

Present at birth

Middle cerebellar peduncle

1 month

1-2 months

Superior cerebellar peduncle

Present at birth

Present at birth

Posterior cerebellar peduncle

Was this article helpful?

0 0

Post a comment