The highly sensitive magnetic field sensor. (Photograph: ETH Zurich / Peter Rüegg)
A team of scientists from the Institute for Biomedical Engineering, which is operated together by ETH Zurich and the University of Zurich, have been successful in measuring minute changes in strong magnetic fields with exceptional precision.
During the experimental phase, the scientists were able to magnetize a droplet of water inside a magnetic resonance imaging (MRI) scanner, a device that is used for medical imaging. The team was able to identify even the tiniest modifications of the magnetic field strength within the water droplet. Compared to the seven tesla field strength of the MRI scanner used in the experiment, these modifications were up to a trillion times smaller.
Until now, it was possible only to measure such small variations in weak magnetic fields.
Klaas Prüssmann, Professor of Bioimaging, ETH Zurich and the University of Zurich
The Earth is an example of a weak magnetic field, where the field strength is only a few dozen microtesla. For these kinds of fields, highly sensitive measurement techniques are already able to identify changes of approximately a trillionth of the field strength, says Prüssmann.
“Now, we have a similarly sensitive method for strong fields of more than one tesla, such as those used, inter alia, in medical imaging.”
Newly Developed Sensor
The scientists based the sensing method on the principle of nuclear magnetic resonance, which also acts as the foundation for magnetic resonance imaging and the spectroscopic techniques that biologists apply to reveal the 3D structure of molecules.
A new high-precision sensor, part of which is a highly sensitive digital radio receiver had to be constructed in order to measure the changes.
“This allowed us to reduce background noise to an extremely low level during the measurements,” says Simon Gross.
Gross took up this topic for his doctoral thesis in Prüssmann’s group and is lead author of the paper published in the Nature Communications journal.
Eliminating Antenna Interference
For nuclear magnetic resonance, radio waves are used to stimulate atomic nuclei in a magnetic field. This causes the nuclei to release weak radio waves of their own, which are then calculated using a radio antenna; their precise frequency signifies the strength of the magnetic field.
As the scientists highlight, building the sensor in such a way that the radio antenna did not warp the measurements was a challenge. The team has to position it in the close proximity to the water droplet, but as it is composed of copper it becomes magnetized in the strong magnetic field, causing a variation in the magnetic field within the droplet.
The scientists formulated a trick: they placed the droplet and antenna in a specially created polymer; its magnetizability (magnetic susceptibility) precisely matched that of the copper antenna. This way, the team was able to remove the unfavorable impact of the antenna on the water sample.
Broad Applications Expected
This measurement technique for very small variations in magnetic fields enables the scientists to now explore the causes of such modifications. They anticipate that their method will find application in several areas of science, partly in the field of medicine. However, most of these applications are still in their basic level.
In an MRI scanner, the molecules in body tissue receive minimal magnetization – in particular, the water molecules that are also present in blood. The new sensor is so sensitive that we can use it to measure mechanical processes in the body; for example, the contraction of the heart with the heartbeat.
Simon Gross, Doctoral Student
The scientists performed an experiment where they placed their sensor in front of the chest of a volunteer inside an MRI scanner. They were able to identify periodic variations in the magnetic field, which pulsated in time with the heartbeat. The measurement curve is suggestive of an electrocardiogram (ECG), however unlike the latter measures a mechanical process (the contraction of the heart) instead of an electrical conduction.
“We are in the process of analysing and refining our magnetometer measurement technique in collaboration with cardiologists and signal processing experts,” says Prüssmann. “Ultimately, we hope that our sensor will be able to provide information on heart disease – and do so non-invasively and in real time.”
Development of Better Contrast Agents
The new measurement method could also be used in the progress of new contrast agents for magnetic resonance imaging: in MRI, the image contrast depends principally on how rapidly a magnetized nuclear spin reverts to its equilibrium state. Experts refer to this process as relaxation. Contrast agents manipulate the relaxation attributes of nuclear spins even at low concentrations and are used to emphasize specific structures in the body.
In strong magnetic fields, sensitivity problems had formerly limited researchers to measurement of only two of the three spatial nuclear spin components and their relaxation. They had to rely upon an indirect measurement of relaxation in the vital third dimension. The new high-precision measurement method enables the direct measurement of all three dimensions of nuclear spin in strong magnetic fields for the first time.
Direct measurement of all three nuclear spin parts also facilitates future developments in nuclear magnetic resonance (NMR) spectroscopy for applications in chemical and biological research.