Ultrashort echo time 1H-MRI for direct imaging of white matter ultrashort T2* components at 7 Tesla
Myelin is a multi-lamellar membrane that surrounds the axon of most nerve cells, forming an electrically insulating layer, which increases the speed of action potential transmission by a factor of up to 300. Thus, myelin is of utmost importance for complex neuronal functions. In demyelinating diseases such as multiple sclerosis, the myelin sheath of the neurons is damaged. For the evaluation of disease progression and for therapy monitoring (e.g. remyelination), non-invasive mapping of the myelin content is desirable.
In magnetic resonance imaging (MRI), myelin tissue has ultrashort transverse relaxation times (T2* << 1 ms). With conventional clinical techniques, the signal is acquired after a few milliseconds so that myelin is not directly detectible. Only indirect, mostly non-quantitative estimates of the myelin content are currently available.
An approach to directly image those white matter ultrashort T2* components is based on ultrashort echo time (UTE) pulse sequences. These enable the detection of fast decaying MRI signals that originate from non-water protons in the myelin sheaths. However, UTE imaging techniques are currently not standard on clinical MR systems. For clinical research, a fast 3D UTE technique that enables imaging of the whole brain with an isotropic spatial resolution  – similar to a 3D MP-RAGE pulse sequence – would be highly valuable. At 3 Tesla, inversion recovery (IR) enhanced UTE pulse sequences for imaging the white matter ultrashort T2* components in the human brain have been proposed . A study with multiple sclerosis patients indicated that the IR UTE sequence is able to identify the presence of demyelination .
This project is focused on bringing this promising technique to the field strength of 7 Tesla and on evaluating its potentials and limitations. Different methods to suppress signals from long T2* components will be evaluated and artifact reduction techniques applied (Figure 1). This shall enable non-invasive mapping of the white matter ultrashort T2* components at 7 Tesla for clinical application (Figure 2).
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