Scientists develop first diamond quantum sensor-based cardiomagnetometer

Abnormal current propagation in the heart is the root cause of most heart diseases. Now, researchers at the Tokyo Institute of Technology and the University of Tokyo have successfully developed a new cardiomagnetography technique that can image these currents with millimeter resolution. The technique is based on diamond quantum sensors that can sense the heart's magnetic field at room temperature (currently commonly used cardiomagnetometers are superconducting quantum interferometers, which require refrigeration) at high resolution, and the new technique has been demonstrated in experiments with mice; the technique could help deepen our understanding of several cardiac arrhythmias.

"The new quantum sensor is capable of imaging cardiac currents with millimeter-level resolution. With the help of nitrogen vacancies in diamond, the new method can help scholars study cardiovascular diseases better and in greater detail."

The research results were published on August 23 in the journal Communications-Physics under the title "Millimeter-scale cardiac magnetography in open-chested live rats" [1].

01Indirect measurement of cardiac currents and high-resolution perception of cardiomagnetism

Cardiac problems, such as tachycardia (tachycardia) and fibrillation (fibrillation), are mainly due to defects in the way electric currents propagate through the heart. Unfortunately, it is difficult for physicians to study these defects because measuring these currents requires highly invasive procedures and exposure to x-ray radiation.

Fortunately, there are other options. For example, cardiac magnetography (MCG) is a promising alternative method for indirectly measuring cardiac currents. This technique involves sensing small changes in the magnetic field near the heart caused by cardiac currents, which can be done in a completely contactless manner; for this purpose, various types of quantum sensors have been developed that are suitable for this purpose. However, their spatial resolution is limited to the centimeter level: insufficient to detect cardiac currents propagating at the millimeter level. Moreover, each of these sensors has a significant part of its practical limitations, such as size and operating temperature.

In a recent study, scientists led by Associate Professor Takayuki Iwasaki of Tokyo Tech (Japan) have now developed a novel device that can perform MCG at higher resolutions [2]. Their approach is based on a diamond quantum sensor composed of nitrogen vacancies (NV) that act as special magnetic "centers" sensitive to the weak magnetic field generated by the heart current.

02NV-MCG system: Towards millimeter-scale imaging

But how can the state of these "nitrogen vacancies" be observed to extract information about cardiac currents?

As it turns out, the sensors are also fluorescent - meaning they can easily absorb light at a specific frequency and then re-emit it at a different frequency. Most importantly, the intensity of the light re-emitted at the nitrogen vacancy varies depending on the strength and direction of the external magnetic field.

The team therefore created an MCG setup that uses a 532-nm (green) laser to excite the diamond sensor and a photodiode to capture the re-emitted photons (light particles); they also developed mathematical models to accurately map these captured photons to the corresponding magnetic fields and, in turn, to the cardiac currents responsible for these photons.

Solid-state quantum sensor-based cardiomagnetics. a) Schematic of the rat cardiomagnetics (MCG) device. The heart of a living rat remains approximately one millimeter below a diamond chip containing a central collection of nitrogen vacancies (NV). The rat is automatically scanned along the XY axis for magnetic field mapping and manually scanned along the Z axis for height adjustment. Electrocardiogram (ECG) signals are monitored simultaneously with the MCG through an ECG analyzer, with the NV center excited by a green laser. b) Energy level map of the NV center. c) Magnified view of the heart and diamond. d) Magnetometer principle. e) Magnetic field sensitivity of the rat's heart signal band ~200 Hz.

Optical images of the rat heart. b, c) Measured magnetic field maps at the moment of the R-wave peak with an accuracy better than 2 mm; superimposed gray solid lines are shown from magnetic resonance imaging (MRI). d) Rat heart contours. e, f) Magnetic field maps fitted to a multi-current dipole model.

The system has unprecedented, 5.1 mm-level spatial resolution to create detailed two-dimensional maps of cardiac currents measured in laboratory rat hearts. In addition, unlike other MCG sensors that require low temperatures, the diamond sensor can operate at room temperature. This allows the researchers to place their sensors very close to the heart tissue, thereby amplifying the measured signal. "Our non-contact sensor combined with our current model will allow for more accurate observation of heart defects using small mammalian models." Dr. Iwasaki emphasized [3].

03Future, expansion to more bio-current scientific research

Overall, this experiment demonstrated MCG and associated current estimation in live mammals using a solid-state quantum sensor to bring NV colored hearts close to a signal source under ambient operating conditions. The reported implementation of millimeter-scale MCG technology is an important step in the development of tools for the study of various cardiac diseases.

Through incremental technical improvements in magnetic field sensitivity, solid-state quantum sensors will add tremendous value to medicine, health care: further expansion from MCG to the study of various other bio-current-driven phenomena is expected. dr. Iwasaki commented, "Our technology will make it possible to study the origin and progression of various cardiac arrhythmias, as well as other bio-current-driven phenomena possible."

Reference link：

[1]https://www.nature.com/articles/s42005-022-00978-0

[2]https://www.nationaltribune.com.au/measuring-currents-in-heart-at-millimeter-resolution-with-diamond-quantum-sensor/

[3]https://phys.org/news/2022-08-currents-heart-millimeter-resolution-diamond.html