What is the Use of Ultra-High Field?

Magnetic Resonance Imaging at ultra high field strengths can help to increase the signal amplitude and thus to improve spatial or temporal resolution. But how much? And how does that affect the contrast?

Corrected SNR distributions over three slices of the same volunteer at field strengths 3 T, 7 T and 9.4 T. The relative SNR differences between the fields at the bottom show the far above linear increase of SNR with field strength.

With a 9.4 T scanner for humans and a 14.1 T small animal scanner, our group is on the forefront of ultrahigh field MRI. Accordingly, a major research focus has been the investigation of the specific properties of MR imaging at such high fields and the characterization of advantages and problems involved.
In a comparison between 9.4 T human scanning and the more conventional field strengths of 3 T and 7 T, we investigated the signal-to-noise ratios (SNR) obtained in the same subjects, using for each field the respective high-end multi-channel receive head coils. After correction for differences in relaxation times or excitation profiles, we found a strongly superlinearily increasing SNR with field strength, with a  relation of SNR ~ B01.65 describing the SNR behavior with field. While the physical accuracy of this value is limited by the differences in the used receive coil arrays, it shows that the measurement sensitivity, and accordingly the spatial or temporal resolution that can be reached, increases rapidly with field strength.

Parameter maps in a rat brain at 16.4 T. In addition to the still useful T1 and T2-contrast, T2*, magnetization transfer ratio (MTR) and phase provide strongly improved contrast mechanisms compared to lower magnetic fields.

In a previous study, we investigated the effect of the field strength on the most prominent contrast mechanism, namely the relaxation times T1, T2 and T2*, as well as magnetization transfer and phase in the rat brain at 16.4 T. We found a still increasing contrast-to-noise ratio for T1 and T2-weighted measurements, while T2*, MT and phase imaging open completely new contrast options at ultra high fields.

1.
Pohmann, R.; Speck, O.; Scheffler, K.:
Signal-to-noise ratio and MR tissue parameters in human brain imaging at 3, 7, and 9.4 tesla using current receive coil arrays.
Magnetic Resonance in Medicine 75 (2), pp. 801 - 809 (2016)
2.
Pohmann, R.; Shajan, G.; Balla, D.:
Contrast at high field: Relaxation times, magnetization transfer and phase in the rat brain at 16.4 T.
Magnetic Resonance in Medicine 66 (6), pp. 1572 - 1581 (2011)
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