Ultra-kurze Echozeiten 1H-MRT bei 7 Tesla

Radiologie

Direktor:
Prof. Dr. med. Michael Uder

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 [1] – 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 [2]. A study with multiple sclerosis patients indicated that the IR UTE sequence is able to identify the presence of demyelination [3].

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).

Contact:

Max Müller
PhD student
max.muelleratuk-erlangen.de

References:

[1] Nagel AM, Laun FB, Weber MA, Matthies C, Semmler W, Schad LR.
Sodium MRI using a density‐adapted 3D radial acquisition technique.
Magn Reson Med. 2009 Dec;62(6):1565-73.

[2] Sheth V, Shao H, Chen J, Vandenberg S, Corey-Bloom J, Bydder GM, Du J.
Magnetic resonance imaging of myelin using ultrashort Echo time (UTE) pulse sequences: Phantom, specimen, volunteer and multiple sclerosis patient studies.
Neuroimage 2016;136:37-44.

[3] Jang H, Ma YJ, Chang EY, Fazeli S, Lee RR, Lombardi AF, Bydder GM, Corey-Bloom J, Du J.
Inversion Recovery Ultrashort TE MR Imaging of Myelin is Significantly Correlated with Disability in Patients with Multiple Sclerosis.
AJNR Am J Neuroradiol 2021; 42(5): 868-874.

Figure 1: UTE sequences and ultra-high field applications are susceptible to different kinds of image artifacts. Hardware imperfections, such as deviations in the k-space gradient trajectory need to be corrected. The use of measured k-space data points for the image reconstruction leads to markedly improved image quality. Furthermore, blurred/folded-in lipid signals from the scalp area are also causing artifacts in the IR UTE difference contrast. These are reduced by carefully adjusting the IR pulse parameters, such that lipid signals are minimized during the readouts.
Figure 2: High resolution (1 mm)3 IR UTE acquisition of a healthy volunteer. Acquisition time was 4 min. The ultrashort echo and the second echo as well as the calculated IR UTE difference and fraction contrasts are shown. A good suppression of long T2* components as well as a sufficient artifact reduction can be observed. The white matter ultrashort T2* components are thus selectively highlighted by the IR UTE technique.
 
Ansprechpartner
Prof. Dr. rer. nat. Armin Nagel
Telefon: 09131 85-25900
DECT: 09131 85-45618
E-Mail: armin.nagel@uk-erlangen.de
Visitenkarte
Prof. Dr. rer. nat. Frederik B. Laun
Telefon: 09131 85-26268
DECT: 09131 85-45622
E-Mail: frederik.laun@uk-erlangen.de
Visitenkarte