Copyright Nature 2006

Nuclear magnetic resonance (NMR) has revolutionized medical diagnostic imaging. Over the last 60 years, many Nobel Prizes have been awarded for its development and application. In all that time the basic technique used to measure NMR signals has changed very little: detecting currents induced in coils surrounding the sample by the net magnetic field. The major limitation of such an induction-based approach is spatial resolution but, thanks to new research, optical NMR detection could alleviate this problem.

It is known that intense, circularly polarized laser light can shift NMR frequencies — the inverse Faraday effect — but it is now believed that these shifts are too small to detect. Savukov and colleagues at Princeton University have taken an opposite tack: they pass linearly polarized light through a sample and measure the subsequent polarization rotation. This rotation is caused by the magnetic field (the Faraday effect), which is induced by nuclear magnetization. With this new nuclear-spin optical rotation (NSOR) technique, the spatial resolution attainable could potentially only be restricted by the diffraction limit of the laser light. From their experiments using 532-nm laser light, Savukov and co-workers interpolate a polarization rotation of 15 microradians for Xe and 0.4 microradians for H for every centimetre travelled through fully polarized atoms (at a concentration of 1 mole per litre). Improvements to the signal-to-noise ratio — which at present is not as high as that offered by conventional NMR techniques — could mean that the NMR of the future is able to image on a far smaller scale than today.