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NMR goes optical

07 Sep 2006

A novel laser-based technique for NMR could lead to real-time two-dimensional imaging of samples, with a resolution limited only by light diffraction.

Physicists in the US have invented a new form of nuclear magnetic resonance (NMR) that could significantly improve the resolution and sensitivity of the technique.

The method, developed by Michael Romalis and co-workers at Princeton University, involves shining a laser on a sample and measuring how it rotates the plane of polarization of the beam. The new technique could lead to real-time two-dimensional imaging of samples, with a resolution limited only by light diffraction.

Most NMR experiments involve placing a sample in a magnetic field, which encourages the spin of the nuclei to point in the same direction as the field. The frequency with which the spins wobble or "precess" about this direction provide useful information about the local molecular environment. But because the magnetic moment is too small to be detected individually, the spins are deliberately driven out of alignment by applying a radio-frequency pulse to a metal coil. As they return to equilibrium, the nuclei create a bulk magnetization that induces an oscillating electric current in the coil.

The new technique developed by Romalis and colleagues is completely different. Rather than measuring the frequency shifts of signals in an NMR spectrum, it involves shining a plane-polarized visible laser beam on a sample and measuring how the nuclear spins rotate the plane of polarziation of the beam. Romalis has been able to demonstrate the technique, which is known as nuclear-spin optical rotation (NSOR), for both liquid xenon and water.

There could be several advantages to the new technique. In particular, it works with small, tightly focused laser beams which could allow samples to be studied at micrometre resolution in real time. Obtaining even 100-micrometre resolution in Magnetic Resonance Imaging (MRI), in contrast, is difficult. The technique is also particularly suitable for heavy nuclei, which usually have poor spectra in traditional NMR. It could even be used to create three-dimensional maps of tissues because near-infrared light can penetrate into such material.

One problem with the technique is that it is currently not as sensitive as conventional NMR. It also only works for transparent samples.

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