Improving the power of MRI

Two papers in this week's issue of Science report on efforts to improve the resolution of nuclear magnetic resonance spectroscopy, which may pave the way for a new, nanoscale imaging technique.

Magnetic Resonance Imaging (MRI) is a commonly-used technique in the medical sciences, because it can be used to see inside opaque objects - such as a person - non-invasively and without using ionising radiation like X-rays.

A related technique, Nuclear Magnetic Resonance (NMR) spectroscopy, is used by scientists to investigate the properties of molecules like proteins.

Both approaches rely on positive charges (protons) in the nuclei of materials being excited by bursts of radio waves.

These radio pulses alter the way the charges 'spin', causing the charges to emit their own, brief radio signals which are, in turn, detected and used to build up a picture of the molecular structure of the object - or tissue - under scrutiny.

In a hospital MRI scanner, patients lie inside a powerful magnetic field, which aligns the spins of all of the protons in the hydrogen atoms in the body. When radio signals are briefly applied, the proton spins are momentarily knocked off kilter. As they snap back into alignment with the field, the scanner picks up the signals they emit to produce a three-dimensional image.

But the signals are extremely weak, meaning that they readily merge into the background magnetic "noise", limiting the resolution of medical magnetic resonance imaging. So far, the only way to improve the resolution has been to keep everything extremely cold (just a few miliKelvin), which is a little impractical.

Now, step forward "nano-nuclear magnetic imaging", the brain child of two independent research groups, one led by John Mamin at IBM's Research Division in California and the other by Thomas Staudacher at the Max Planck Institute in Stuttgart. Both are confident that this new approach can image individual molecules measuring just a few nanometres in length, and at room temperature.

It uses a structure called a diamond-based magnetometer. This comprises a thin layer of diamond (carbon atoms) in which some of the carbons have been replaced by nitrogen atoms - this introduces into the diamond lattice "vacancies", which acts like extra electrons, making the surface very sensitive to the presence of even very tiny electromagnetic fields.

This makes it perfect for sensing the weak signals generated by the changing spins of individual nuclei, pushing the potential resolution down to a level at which it may be possible to use MRI to see inside individual living cells.