Magnetic Resonance Imaging at ultra high fields requires novel techniques, both concerning hardware and imaging sequences. At field strengths of 9.4 T (human) and 16.4 T (rodents), I investigate the capabilities of ultra high field MRI. Specifically, our aim is to

• quantify the MR parameters at ultra high field strength,
• find solutions to some of the major challenges involved in ultra high field MRI, like B1-homogeneity or safety,
• develop techniques that take full advantage of the potential of the high field strength and assess the gain compared to conventional MR instruments
• help in appying those techniques to medical or biological applications

In addition, I assist other groups by developing or optimizing specialized techniques for their applications.

For a comprehensive overview over our projects, please see our project group web page.

High sensitivity perfusion imaging with Arterial Spin Labeling

Arterial Spin Labeling is a promising technique for quantitative perfusion imaging without the need for contrast agents. Since ASL generally suffers from a low signal-to-noise ratio, we concentrate on implementing and developing techniques with high sensitivity. To reach this goal, we follow several different approaches:

• In contrast to the most commonly used techniques of pulsed arterial spin labeling, continuous arterial spin labeling has a considerably increased sensitivity, but also a higher complexity. We have implemented a number of different variants of continuous ASL, which are now used in neuroscientific and biological applications (publication).

  

  Perfusion images acquired with different CASL and a PASL technique.

• In contrast to the cortex in the brain, which has a very high perfusion, the white matter or, even more, the skeletal muscle are only poorly perfused, making highly quantitative measurement difficult. For making it possible to obtain quantitative values of the perfusion from those tissues, a highly sensitive technique based on single-voxel spectroscopy was developed. This FAIRPRESS techniques has been successfully used to quantify the perfusion in the white matter as well as in muscels in the legs of rats and humans.
• In addition to the gain in intrinisc signal-to-noise ration at ultra-high field, the longer longitudinal relaxation time will help to further improve the accuracy of perfusion images. Using the most sensitive CASL sequences is, however, hampered by the high SAR with is necessary to label the inflowing blood. Using a dual-coil CASL technique, that applies the labelling field with separate, local coils at the neck of the volunteer, it will be possible to reduce SAR far enough to be able to take full advantage of the improved signal at higher field.

Measurement of MR imaging parameters at ultra high field

When increasing the field strength, MR relaxation times, magnetization transfer effects, and susceptibility-induced field variations will change. We have measured and quantified those parameters at field strengths up to 16.4 T and analysed the effects of those changes on SNR and contrast of the resulting images (publication).

The relaxation times T₁, T₂ and T₂^* were measured with high accuracy and spatial resolution using inversion recovery, single spin echo and gradient echo sequences, respectively, with varying delays. The MTR was measured by off-resonance irradiation at different frequencies and power levels and the resulting values were used to determine the parameters of a two-pool model [2] to allow for comparison to other field strengths. Local frequency variations were deduced from phase changes in gradient echo images with varying echo time.

With values between 1834 ms (white matter ) and 2376 ms (hippocampus), T₁ was significantly increased compared to lower fields, while T₂ and T₂^* are relatively low (corpus callosum: 20 ms / 13.6 ms, cortex: 24 ms / 21 ms). The MTR varies between 51% and 61% and thus is considerably stronger than at lower field. Image phase shows distinct differences between different anatomical structures and can be a valuable contrast mechanism at high field. In all parameter maps, all major anatomical structures were clearly visible.

Comparisons to publications at lower field show an increase in contrast-to-noise ratio with field strength for all contrast mechanisms. Especially phase and T₂^* imaging have great potential for use in neuroscientific and preclinical applications.

Flip angle mapping

Highly accurate, fast, and simple mapping of the excitation field is a crucial requirement for ultra high field MR imaging. In an extensive study, we are comparing the accuracy and precision of the most popular flip angle mapping techiques, both theoretically and in experiments under different settings. The results will be used to further improve the performance of these sequences to enable highly accurate shaping of the transmit field using B1-shimming or Transmit SENSE.

For information about the other projects in our group, please see  our project group web page.