medscape/viewarticle/811594_6 Magnetization Transfer Imaging MT is a quantitative MRI technique based on interactions and exchanges between mobile protons in a free water pool and those bound to macromolecules. By using MR sequences with and without an off-resonance saturation pulse, MT allows calculation of an index, the magnetization transfer ratio (MTR). Decreases in the MTR indicate that protons bound to the brain tissue matrix have a diminished capacity to exchange magnetization with the surrounding free water. Thus, this index provides an estimate of the extent of tissue structure disruption and affords a potential window into the macromolecular environment that is not directly visible using cMRI techniques [Horsfield et al. 2003]. In MS, MTR can be used to quantify the integrity of myelinated white matter in large areas of the brain [Filippi and Agosta, 2007]. Changes in the MTR of cerebral white matter are highly weighted by changes in myelin content because of the overwhelming contribution of myelin to the macromolecules involved in the magnetization transfer phenomenon [Schmierer et al. 2004]. Decreased MTRs have been reported in acute gadolinium-enhanced lesions and chronic MS lesions, with the most prominent changes found in T1-hypointense lesions [Filippi and Agosta, 2007]. The MTR decrease in acute lesions, consistent with demyelination, can be followed by a variable recovery over the subsequent months that probably reflects remyelination (Figure 6). Partial or complete recovery of MTR values is more likely to occur when the initial decrease is only modest, whereas a high initial MTR decline predicts whether the lesion will evolve into a T1-hypointense lesion (chronic black hole) [Dousset et al. 1992; Deloire-Grassin et al. 2000]; hence, the degree of MTR change has been proposed as a marker of overall lesion severity. At least in some lesions dramatic changes in normal appearing white matter areas can be seen months before the formation of new T2 lesions [Filippi et al, 1998; Fazekas et al. 2002; Pike et al. 2000]. This observation further supports the notion that in a subgroup of MS lesions, primary myelin damage precedes the inflammatory mediated blood–brain barrier disruption. Chen and colleagues recently developed a voxelwise analysis method to monitor longitudinal MTR changes in individual newly formed MS lesions [Chen et al. 2008]. This method could be of value to assess the neuroprotective (slowing degeneration of neural tissue) or reparative (restoring tissue integrity and function) effects of new treatments in MS. Some lesion regions were seen to exhibit significant increases in MTR, consistent with remyelination, that were ongoing for approximately 7 months after enhancement and then stabilized. The same study showed that decreases in MTR consistent with demyelination were ongoing for approximately 33 months after enhancement. These observations of continuing demyelination and remyelination for months and years after lesion formation indicate that the window of opportunity for a therapeutic intervention may be longer than is usually assumed.
Posted on: Sun, 20 Oct 2013 07:25:28 +0000
Trending Topics
Recently Viewed Topics
© 2015