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Ultra High Field - Taking Advantage of Higher Sensitivity

Applications at Ultra High Field (UHF) directly benefit from the high sensitivity. The gained SNR performance can, for example, be invested into higher resolutions and/or shorter scan times, or can be taken advantage of in X-nuclei imaging.

Ultra-high Resolution MRI

To avoid partial volume effects and thus improve data quality and data analysis, highest resolution is desired. However, if the signal in the individual voxels of the investigated subject is not large enough, the resulting low SNR prohibits analysis of the images. The greater SNR obtainable with Ultra High Field (UHF) instruments, therefore can be directly carried over into higher resolution. This enables researchers to push the resolution in the direction of “in vivo MRI histology” as well as to benefit from the increased data quality in a range of disease models [1,2].

In addition to anatomical imaging, many MRI methods benefit from the sensitivity increase. For example, in BOLD fMRI more refined stimulation paradigms can be defined, as the increased SNR places less demands on the strength of the stimulations. Furthermore, with increasing resolution, fMRI accuracy becomes less and less limited by voxel size but rather by how particularly minutely (both spatially and temporally) the blood flow to the point of neuronal activity is regulated [3]. Furthermore, for high resolution fMRI the reduced partial volume effect promises to lead to further improvement in SNR [4]. High resolution fMRI with small voxel sizes will additionally benefit from UHF since it operates in the thermal noise-dominated regime and thus, in this case, a significant sensitivity gain compared to lower magnetic fields is expected [4].

Ultra high resolution, high contrast T2 weighted in vivo mouse brain data acquired at 15.2 Tesla with the MRI CryoProbe

Ultra high resolution, high contrast T2 weighted in vivo mouse brain data acquired at 15.2 Tesla with an MRI CryoProbe. Method: RARE, Resolution: (29 x 29) µm2, Slice Thickness: 203 µm, Slices: 12, Scan time: 26 min.

References:

[1] Petiet A, Aigrot M-S, Stankoff B. Gray and White Matter Demyelination and Remyelination Detected with Multimodal Quantitative MRI Analysis at 11.7T in a Chronic Mouse Model of Multiple Sclerosis. Frontiers in Neuroscience. 2016;10:491. doi:10.3389/fnins.2016.00491.
www.ncbi.nlm.nih.gov/pmc/articles/PMC5081351/

[2] Ong HH, Webb CD, Gruen ML, Hasty AH, Gore JC, Welch EB. Fat-water MRI of a diet-induced obesity mouse model at 15.2T. Journal of Medical Imaging. 2016;3(2):026002. doi:10.1117/1.JMI.3.2.026002.
www.ncbi.nlm.nih.gov/pmc/articles/PMC4877437/

[3] Polimeni, J. and Uludağ, K., Neuroimaging with ultra-high field MRI: Present and future. NeuroImage, 2018; 168: 1-6. doi: 10.1016/j.neuroimage.2018.01.072
www.ncbi.nlm.nih.gov/pubmed/29410013

[4] Uludağ, K., and Blinder P., Linking brain vascular physiology to hemodynamic response in ultra-high field MRI. Neuroimage, 2018; 168 279-295. doi: 10.1016/j.neuroimage.2017.02.063.
www.ncbi.nlm.nih.gov/pubmed/28254456

Higher Throughput

Limited available time of the MRI system, large numbers of needed animals, or instable models, often make shorter measurement times necessary.
Longer measurements times are often needed to achieve sufficient SNR when using lower field strengths. Using UHF, measurement times can be significantly shortened, since, for example, an increase in sensitivity by a factor of two allows to acquire images with the same resolution and similar quality in a quarter of the time [5]. Therefore, the number of data averages can be reduced, and the time saved can be invested into additional subjects or further studies.

Exemplary high resolution, fast T2 weighted in vivo mouse brain data acquired at 15.2 Tesla with the MRI CryoProbe in 1 minute

Exemplary high resolution, fast T2 weighted in vivo mouse brain data acquired at 15.2 Tesla with an MRI CryoProbe in 1 minute. Method: RARE, Resolution: (47 x 49) µm², Slice Thickness: 400 µm, Slices: 12, Scan time: 1 min.

A Boost for X-Nuclei Imaging

An additional benefit of the SNR gain at UHF is that imaging of X-nuclei with low gyromagnetic ratios, quadruple moments, and low abundances can be significantly improved or even made feasible for the first time [5,6,7,8].
This opens up a multitude of different research applications, such as sodium (²³Na) imaging. Sodium MRI is currently used for a wide range of applications. On clinical systems, for example, sodium concentration measurements are used to study tissue viability [9]. Due to the high sensitivity, the use of UHF greatly facilitates sodium imaging [7]. Preclinical UHF focuses, among others, on using sodium concentrations and distributions as metrics to help cellular engineers improve human mesenchymal stem cell (hMSC) conditioning for treatment of ischemic stroke [10]. Furthermore, UHF MRI can potentially lead to a breakthrough for oxygen (17O) imaging which allows direct access to the cellular oxygen metabolism. Cellular oxygen metabolism is altered in several diseases such as Alzheimer’s and Parkinson’s as well as in cancer. Thus, 17O MRI has the potential to visualize local pathology changes in the brain, underling the importance of this imaging approach [8].
Another application of X-nuclei imaging of metabolism is found with deuterium imaging, which can be used to map glucose metabolism. In addition to the ability to map glucose metabolism as opposed to glucose uptake, deuterium metabolic imaging (DMI), has the additional advantage over positron emission tomography (PET), of using non-radioactive substrates [11]. The increased sensitivity of deuterium at UHF, makes DMI a viable alternative to PET.

References:

[5] Nowogrodzki A. The world’s strongest MRI machines are pushing human imaging to new limits. Nature 563, 24-26 (2018), doi: 10.1038/d41586-018-07182-7
www.ncbi.nlm.nih.gov/pubmed/30382222

[6] Öz G, Tkáč I, Uğurbil K. Animal models and high field imaging and spectroscopy. Dialogues in Clinical Neuroscience. 2013;15(3):263-278.
www.ncbi.nlm.nih.gov/pmc/articles/PMC3811099/

[7] Deutsches Krebsforschungszentrum: www.dkfz.de/en/medphysrad/projectgroups/t7_x-nuclei/t7_x-nuclei_Na_MRI

[8] Deutsches Krebsforschungszentrum:
www.dkfz.de/en/medphysrad/projectgroups/t7_x-nuclei/t7_x-nuclei_O_MRK

[9] Thulborn KR, Lu A, Atkinson IC, Damen F, Villano J. Quantitative Sodium MR Imaging and Sodium Bioscales for the Management of Brain Tumors. Neuroimaging clinics of North America. 2009;19(4):615-624. doi:10.1016/j.nic.2009.09.001.
www.ncbi.nlm.nih.gov/pmc/articles/PMC3718497/

[10] Using High Fields to Combat Ischemic Stroke with Cell Therapy
Bruker: www.bruker.com/events/webinars/using-high-fields-to-combat-ischemic-stroke-with-cell-therapy.html

[11] De Feyter H, Behar K, Corbin Z, Fulbright R, Brown P, McIntyre S, Nixon T, Rothman D, de Graaf R. Deuterium metabolic imaging (DMI) for MRI-based 3D mapping of metabolism in vivo. Sci. Adv. 2018; 4. doi: 10.1126/sciadv.aat7314
https://advances.sciencemag.org/content/4/8/eaat7314