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Peter Grünberg Institute / Institute of Complex Systems

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Low-field NMR and MRI

We perform nuclear magnetic resonance experiments at extremely low magnetic fields, using our nitrogen-cooled superconducting quantum interference device (SQUID) as a detector. A pulsed magnetic field Bp of a few Millitesla is used to polarize the sample. After switching off this polarization field, the free induction decay is observed in the detection field BM which is of the order of the earth’s magnetic field.

Low field NMR

Low field NMR

Compared to conventional nuclear magnetic resonance (NMR) at fields of a few tesla flux density, low field (LF) NMR at a few microtesla or less requires much less experimental effort and allows to study different relaxation processes. For a fixed relative homogeneity, the NMR line width scales linearly with the measurement field and makes the NMR lines very narrow at low fields, coming close to the lifetime-limited natural widths.

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J coupling spectroscopy

J coupling spectroscopy

Inside a magnetically shielded room, the measurement field can be reduced even further down to the Nanotesla range. Chemical shift information is absent in such low fields, and so is the homonuclear J-coupling between the degenerate states of identical nuclei. This opens the window for heteronuclear “pure J spectroscopy”, i.e., the investigation of electron-mediated scalar coupling between different nuclei.

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Magnetic resonance imaging

Magnetic resonance imaging

Using our tuned high-Tc rf SQUID system, two-dimensional LF MRI measurements were demonstrated using filtered back-projection reconstruction. A slice of pepper was imaged with a spatial resolution of 0.2 mm. Work is ongoing towards a tuned HTS SQUID system for biological LF MRI measurements.

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