Sensor detects variation in magnetic field strength

A high-precision sensing technique based on the principle of nuclear magnetic resonance (NMR) has been developed that can detect the smallest variations in magnetic field strength in a water droplet. The new sensor, which includes a sensitive digital radio receiver to allow background noise to be reduced, was produced by scientists from ETH Zurich and the University of Zurich and was able to measure small changes in strong magnetic fields with unprecedented precision.

The team, whose work was reported in Nature Communications [Gross et al. Nat. Commun. (2016) DOI: 10.1038/ncomms13702], magnetized a water droplet inside a magnetic resonance imaging (MRI) scanner and then measured the magnetic variations – shown to be up to a trillion times smaller than the seven tesla field strength of the MRI scanner used in the experiment – within the droplet. To date, it has only been possible to measure such small variations in weak magnetic fields, with highly sensitive measurement detecting variations of around a trillionth of the field strength. However, this new approach offered a similarly sensitive method for strong fields of over one tesla, such as those used in medical imaging.

For NMR, radio waves are used to excite atomic nuclei in a magnetic field, resulting in the nuclei emitting their own weak radio waves, which are then measured with a radio antenna. Their frequency identifies the strength of the magnetic field. To eliminate the detrimental influence of the radio antenna on measurements, they cast the droplet and antenna in a prepared polymer where its magnetic susceptibility matched exactly that of the copper antenna.

“Ultimately, we hope that our sensor will be able to provide information on heart disease – and do so non-invasively and in real time”Klaas Prüssmann

To test the sensor, they positioned it in front of the chest of a volunteer inside an MRI scanner to detect periodic changes in the magnetic field, which pulsated in time with the volunteer’s heartbeat. The measurement curve is similar to an electrocardiogram, but measures a mechanical process rather than electrical conduction. As researcher Klaas Prüssmann said, “Ultimately, we hope that our sensor will be able to provide information on heart disease – and do so non-invasively and in real time”.

As well as sensitivity to patient physiology, the sensor could provide new options in MRI and NMR equipment for recording field errors for retrospective correction, real-time field correction, as well as hardware characterization and monitoring. The technique could also be used in developing new contrast agents for MRI, as the image contrast is mainly dependent on the speed of a magnetized nuclear spin reverting to its equilibrium state, known as process relaxation. The direct measurement of the nuclear spin components could lead to breakthroughs in NMR spectroscopy, with potential applications in biological and chemical research.