Spin improves medical imaging

Researchers at York University have developed a technique for changing the spin properties of atoms to make MRI and NMR scanners much more sensitive.

NMR (nuclear magnetic resonance) is a method of obtaining structural information in chemistry, and MRI (magnetic resonance imaging) is used for clinical imaging inside the body. The more the spin properties of the atomic nuclei of the molecules being examined can be encouraged to line up in one direction, the higher the signal-to-noise ratio and the clearer the image.

Prof Gary Green, director of neuroimaging at York Neuroimaging Centre, worked on the project with Prof Simon Duckett from the university’s chemistry department. Green explained that when molecules are placed in a strong magnetic field, as happens during MRI and NMR, atoms will try and line up either with or against the field, according to their spin direction.

‘The concept of nuclear resonance involves putting radio frequency fields in and getting signals back out, detecting the difference in spin directions,’ said Green. ‘This shows, for example, what the molecule is, what the chemical group is and where the molecule is in a structure, including in the body.

‘However, there’s usually an almost equal proportion of what we call spin up and spin down. For some time, people have been trying to overcome that sensitivity limitation by influencing the spin properties of molecules to make them all spin in one direction. This is called hyper-polarisation.’

Several well-established techniques exist for doing this. These include optical pumping, methods to transfer spin from electrons to nuclei — dynamic nuclear polarisation — and transferring spin order from chemicals with particular spin structures into other molecules to cause polarisation. But each method has drawbacks. Optical pumping is not very efficient and dynamic polar nuclearisation is very slow; it takes hours to polarise even very small amounts of compound, and these have to be cooled down to 20º Kelvin. Until now the chemical methods have required a special type of hydrogenation, the incorporation of a form of hydrogen called parahydrogen into a molecule.

‘Parahydrogen has a highly-ordered antisymmetric spin structure, where one nucleus spins up and one spins down,’ said Green. ‘If you react molecules with parahydrogen, you can get that highly ordered spin structure into that new molecule, and you can see where those hydrogens go in MRI or NMR.

‘The key advantage of using parahydrogen is all the chemical reactions can be done at body or room temperature, instead of extremely cold temperatures earlier techniques required.’

Green explained that as well as dynamic nuclear polarisation and parahydrogen hydrogenation, there are electronic methods to improve the signal-to-noise ratio in NMR. However, these require a cryoprobe costing thousands of pounds and only give a 30-fold increase in signal-to-noise ratio, whereas the hydrogenation method can give 30,000 times ratios.

The breakthrough made at York is the discovery that the hydrogenation reaction is not necessary. The researchers have used a catalyst to transfer the spin order from parahydrogen to other molecules.

The method will be fully in the public domain when a patent is granted next month. Green said: ‘In any molecule you can move spin around — it’s all to do with energy gaps and energy differences. This can be done using radio frequency pulses or the natural evolution of those spins in particulate physical environments — polarisation transfer.

‘We’ve designed a specific catalyst that makes this possible almost instantaneously at normal temperatures. Quite a large range of molecules can be spin polarised this way, and you don’t have to change the chemical structure.’

This method can increase signal-to-noise ratio by a claimed several hundred fold, and it has a range of potential applications including quantum computing, magnetic sensors and in spintronic magnetic storage.

The researchers began work on the project a year ago, with proof of concept sponsorship from the White Rose University Consortium.

York has signed an agreement with international magnetic resonance technology specialist Bruker BioSpin, which is building a prototype device to allow standard NMR machines to detect the increased signal. It is due early next year.

The scientists are now working on a range of other catalysts that will be able to induce hyperpolarisation in a much larger range of chemicals for different detection technique.

Berenice Baker