Ultra High Field Magnetic Resonance

Group Members:
Rolf Pohmann
Jonas Bause
Morteza Bakhtiary
Matthias Valverde
Former group members:


Ultra High Field Magnetic Resonance

Human venogram acquired at 9.4 T (left) and image of the rat brain from 16.4 T.
The High-Field MR and Methodology group focuses mainly on the development of MR techniques, with emphasis on MR imaging and spectroscopy at ultra high field. In addition, the characteristics and advantages of ultra high magnetic fields are investigated.
With a 9.4 T MR scanner for humans and a 16.4 T instrument for small animals, our institute has a unique instrumentation that makes it a pioneer in the investigation of ultrahigh field imaging. Thus, the main goals of our group were to develop hardware and imaging techniques to make it possible to fully take advantage of the high magnetic field. In addition, the special properties of MR imaging at those field strengths are investigated and applications for which especially high gains are expected are implemented and characterized.

High magnetic field strengths have a high potential for magnetic resonance imaging and spectroscopy: The signal-to-noise ratio increases, the longitudinal relaxation time - and thus the potential to store additional information -  grows, the sensitivity to detect small local changes in the magnetic properties of tissue gets stronger and the spectral dispersion and thus the information contained in an NMR spectrum increases. Simultaneously, however, the physical properties of the MR signal change drastically as the field gets higher, requiring novel approaches in both hardware and measuring techniques.
We aim to develop the technology that make it possible to fully take advantage of the expected gain at ultra high field:
  • Development of hardware (rf-coils and preamplifiers) necessary to enable ultra high field MR as well as techniques for homogenization of the excitation field,
  • Evaluation of the potential advantages and limitations imposed by using high magnetic fields,
  • Development of optimized measuring techniques with the aim of taking full advantage of the improved imaging characteristics,
  • Support of other groups within the department by developing and optimizing imaging techniques adapted to their applications.

The main projects currently are:

  • Hardware Development
    The short wavelengths of the signals transmitted and recorded at ultra high field MR requires the development of novel coil designs and of techniques for homogenization of the transmit field. With the design of separate multi-channel arrays for transmit and receive, we are able to use sophisticated parallel transmit technology for optimized excitation profiles and to acquire the data in a highly sensitive way due to the use of multiple receive surface coils. Sophisticated algorithms for B1-shimming were implemented and, for further optimization of the transmit field, techniques for multi-channel excitation are being developed. As alternative to the complex multi-channel transmit methodology, novel patch antenna designs were developed that are able to obtain acceptable transmit homogeneity with reduced effort for setup and adjustments. In spite of the small bore sizes in both scanners, that do not support traveling waves, these antennae were shown to be capable of exciting large regions over the brain and, in combination with multi-channel receive arrays, of acquiring images with high spatial resolution and high SNR.

  • Anatomical Imaging
    The high field strengths used in our scanners open the potential of increased SNR with different or even novel contrast mechanisms. Investigations at 16.4 T have shown considerably improved image quality in images with contrast based on the relaxation times as well as on image phase and magnetization transfer [2]. In spite of the high T1 values of around 2250 ms in gray matter at that field, the SNR gain still increases almost proportional to the field strength. With T2* and image phase, excellent contrast can be obtained with very high spatial resolutions in a way that is exclusive to high field. In human imaging at 9.4 T, inplane resolutions down to 175 μm were possible with phase imaging and intra-cortical structures became visible that cannot be observed with other imaging techniques. A high amount of anatomical information was contained in T2*-images, which can be used to detect large strains of nerve fibers or regions with high iron content. With susceptibility-weighted imaging (SWI) it was possible to visualize venous structures at very fine detail with SNR increases of much more than linear with magnetic field strength when compared to similar data acquired at 3 T (Fig. 2). In addition, the highly enhanced effect of magnetization transfer can serve as an additional means for contrast enhancement. Due to the high SNR, isotropic spatial resolutions below 50 μm can easily be obtained in ex vivo measurements, while in vivo experiments in the abdomen of living mice reached a voxel size of 66 × 66 × 100 μm3 within ten minutes in spite of the rapid motion due to breathing.

  • Functional Imaging
    In functional imaging, isotropic spatial resolutions of 1 mm for humans and 150 μm in rats were reached with high signal variations due to brain activation. Using a special EPI sequence with correction of image distortions due to field inhomogeneities, it was possible to accurately overlay phase information, which distinctly shows the positions of veins, and functional activation maps with the aim of comparing the main BOLD-signal contributions with the venous architecture. For fMRI based on gradient-echo EPI, it could be shown that the major signal contributions arise from the draining veins as expected from theoretical predictions. It will have to be shown whether the expectation of improved weighting of the small vasculature in comparison to larger veins for spin-echo based imaging realizes the promises for improved spatial specificity at high field strengths.

  • Spectroscopy
    In addition to the high sensitivity, in vivo MR spectroscopy profits from the increased spectral dispersion, causing the expectation of an especially high gain at ultra high field. Thus, proton spectroscopy in the rat brain at 16.4 T was implemented and optimized acquisition and quantification techniques were developed. A localized STEAM sequence with short echo times and improved water suppression was established and a novel, improved algorithm to take the signals of macromolecules into account was developed. With these techniques, the high sensitivity and spectral dispersion made it possible to quantify up to 20 metabolites, including glycine and acetate (Fig. 3). Additional studies verified the high precision and accuracy of these measurements even in short experiments or from difficult deep brain regions, like the medulla oblongata or the cerebellum [3]. Applications in different rat strains or brain regions showed the capability of detecting even small variations in the concentration of single metabolites [4].

  • 17O imaging
    Imaging of 17O constitutes an especially challenging field due to its low natural abundance and NMR sensitivity and its limited availability. The increased sensitivity at 16.4 T made it possible to measure for the first time spatially resolved T1 and T2*-maps of the rat head in vivo based on the natural abundance signals. The main interest of 17O imaging, however, is the direct measurement of the local oxygen consumption in the brain, which can be quantified by observing the evolution of the 17O-signal during and after ventilation of the animal with 17O2-gas. Using this technique, we were for the first time able to detect functional activation by direct and quantitative imaging of the oxygen consumption (Fig. 4). This makes it possible to accurately localize those brain regions where stimulation causes increased energy demand and can, due to its quantitative fashion, give valuable additional and complimentary information to investigations based on the BOLD-effect or perfusion measurements.

  • Perfusion measurements:
    Quantifying the perfusion in the brain or other organs without the need to use contrast agent is challenging due to the low amplitude of the signal obtained with Arterial Spin Labeling techniques. We concentrate on increasing the sensitivity of ASL measurements by developing and applying highly sensitive techniques like continuous ASL or a specially developed technique for quantifying low perfusion, as in muscle or in white matter brain tissue. In addition, using ultra-high field ASL will yield improved perfusion images due to the high intrinsic sensitivity at high field and the increased longitudinal relaxation time.

Last updated: Monday, 19.01.2015