Magnetic resonance imaging (MRI) contrast can be highly sensitive to the presence of myelin; however, quantitative MRI approaches to measure myelin content remain a challenge. With ultra-short echo time (UTE) MRI, it may be possible to directly image the rapidly-relaxing non-aqueous protons in the phospholipid bilayers that comprise myelin. UTE-MRI brings with it technical challenges, and the MRI characteristics of the ultra-short T2 signal from myelin is not yet well understood. Several studies have presented UTE-MRI of brain thought to reflect myelin, but questions remain and further work is needed to validate this approach.
Attendees will learn:
The NMR signals from macromolecular protons, such as those in the phospholipid bilayers that comprise much of the non-aqueous portion of myelin, are generally expected to have extremely broad line widths. That is, their observed signals decay rapidly (one might say that they have an ultra-short T2) and are not directly visible with conventional MRI. It is possible to indirectly detect these protons through their effect on the surrounding water signal via magnetization transfer (MT), and various measures of MT contrast have been shown to correlate with myelin content in brain and nerve. However, some studies have shown that it may be possible to directly image these macromolecular protons using ultra-short echo time (UTE) MRI.
UTE-MRI refers to a category of MRI methods that aim to capture 'ultra-short T2' signals by reducing the time between signal excitation and acquisition to ≤ 100 µs. Typically this is achieved using a non-selective hard-pulse excitation followed by a high-bandwidth center-out radial acquisition. In this form, UTE MRI can be somewhat slow and suffer from moderate SNR and resolution. Also, a raw UTE image of the brain includes signal from water and (perhaps) from the macromolecular protons and therefore is not particularly informative. In order to provide specific information about myelin, its ultra-short T2 signal needs to be distinguished from the background water signal. To some extent this can be done by subtracting a conventional MRI from a UTE-MRI, or by suppressing the water signal prior to the UTE acquisition. Recent studies have used such approaches and found evidence of a myelin sensitive signal, but is it myelin specific and is it really the direct signal of macromolecular protons?
This presentation will will consider this possibility by reviewing relevant experimental studies and considering the technical challenges of UTE-MRI.