“TDK is working on developing a wide variety of sensors as one of its “Attracting Tomorrow” technologies. High-sensitivity magnetic sensors that can measure weak biomagnetic fields are also one of them. It is expected to bring solutions to intractable diseases such as fetal heart disease in the abdomen that cannot be detected by current electrocardiographs, and ischemic heart disease that is difficult to detect early.
TDK is working on developing a wide variety of sensors as one of its “Attracting Tomorrow” technologies. High-sensitivity magnetic sensors that can measure weak biomagnetic fields are also one of them. It is expected to bring solutions to intractable diseases such as fetal heart disease in the abdomen that cannot be detected by current electrocardiographs, and ischemic heart disease that is difficult to detect early.
TDK has accumulated spintronics technology in the HDD head manufacturing process, and has developed MR (Magnetoresistive Effect) device technology through this technology. By applying this technology, a small, high-sensitivity biomagnetic sensor has been developed that is capable of sensing weak biomagnetic fields that were previously only measurable with SQUID fluxmeters. In addition, through joint research with Tokyo Medical and Dental University Graduate School, TDK developed a biomagnetic field measurement system using a multi-channel sensor array, and successfully realized the world’s first measurement of cardiac magnetic field by MR magnetic sensor, and realized the visualization of cardiac magnetic field distribution ( image). Unlike SQUID fluxmeters that require a liquid helium cooling device (Dewar), which are expensive and bulky, systems using MR magnetic sensors can measure with high sensitivity at ambient temperature (uncooled), and are also lightweight. , excellent operability and mobility, so it can be used not only for medical diagnostic purposes such as magnetocardiography, but also in fields such as health care and sports science.
Features of TDK MR Biomagnetic Sensors
・ Small, high-sensitivity biomagnetic sensor using MR devices developed by spintronics technology
・ Magnetic resolution reaches tens of pT (10-11T) comparable to the SQUID field
・Can measure biomagnetic fields such as cardiac magnetic field and muscle magnetic field in a non-invasive state at room temperature (non-cooling)
・ Visualization of cardiac magnetic field distribution through a multi-channel sensor array
・The system cost is about 1/10 of the SQUID fluxmeter
・ No cooling system required, excellent operability and mobility
・ Measurement can be performed in a relatively simple magnetically shielded room
・ Joint research with Tokyo Medical and Dental University Graduate School
・Development of sensors capable of measuring brain magnetic fields with a magnetic resolution of several pT
Heart disease is as famous as cancer (malignant growth) and cerebrovascular disease (stroke), and it is the leading cause of death in the world. Currently, an electrocardiograph (ECG: electro-cardiograph) is widely used as an examination device for examining cardiac activity.
The electrical activity of the heart originates in a tissue located in the right atrium called the sinoatrial node, which is the innate pacemaker. The electrical signal from the sinoatrial node is first transmitted to the entire atrium, and then, through a tissue called the atrioventricular node, it is divided into left and right branches and transmitted to the entire ventricle, thereby causing the heart to repeatedly beat rhythmically. This is called the electrical conduction system of the heart. Through the transmission of this electrical excitation of the heart, a potential difference will appear between various parts of the body surface. The electrocardiograph detects the potential difference by pasting multiple electrodes on the limbs, chest and other parts, and amplifies it to Display the potential difference in the form of a waveform, etc. Display and record.
Figure 1 shows the electrical conduction system of the heart, the basic flow trajectories of the heart’s active current, and a typical ECG waveform pattern. The P wave represents atrial contraction, the QRS wave represents the waveform accompanying ventricular contraction, and the T wave and U wave represent the gradual weakening of ventricular excitation.
Figure 1. Cardiac electrical conduction system and ECG waveform
The electrocardiograph cannot grasp the activity of the heart in a spatial form, and can only be roughly estimated from the ECG waveform. However, if the detailed activity of the heart muscle can be observed, the diagnostic accuracy can be greatly improved. The magnetocardiograph (MCG:magneto-cardiograph) has become the solution to this problem. According to the “right-hand spiral rule” of electromagnetism, when a current flows, a magnetic field is generated around it, so by measuring the magnetic field generated around the heart, it is possible to infer the flow and location of the current. Another advantage of the magnetograph is that there is no need to stick electrodes on the body surface, and non-invasive measurements can be performed while wearing clothes. However, the cardiac magnetic field is an extremely weak biological magnetic field, so a highly sensitive magnetic sensor is indispensable for a magnetograph.
Types of Magnetic Sensors and Biomagnetic Fields
The first electrocardiogram was produced in 1903, measured by a device designed by Dutch physiologist Wilhelm Eintofen. And because the magnetic field of the heart is very weak, only 1/1 millionth of the earth’s magnetic field, it was not until 1963 in the second half of the 20th century that it was successfully measured for the first time. A set of magnetic flux detection coils wound 2 million times was used for the measurement. After that, in order to prevent interference due to factors such as the geomagnetic field, the measurement was carried out in a special magnetically shielded room. However, even so, the measurement was only limited to confirming that the magnetic field was generated from the heart, and the accuracy did not reach the standard that can help the diagnosis of heart disease. .
A major breakthrough in biomagnetic field measurement was the SQUID fluxmeter developed around 1970. SQUID is the English abbreviation of “superconducting quantum interference device”, which is a magnetic sensor with a Josephson junction in the part of the coil using a superconductor. The Josephson junction device was originally developed as an arithmetic device to increase the processing speed of computers, but because of its extremely high sensitivity to magnetism, it is used in high-sensitivity fluxmeters. The SQUID fluxmeter can not only measure the cardiac magnetic field generated by the cardiac activity, but also the muscle magnetic field and the brain magnetic field.
However, to enable the SQUID, the superconducting coil needs to be cooled by liquid helium, and a special magnetic shielding chamber for shielding magnetic noise needs to be prepared, so the system is large and expensive. For this reason, biomagnetic field measurement devices using SQUID fluxmeters are currently only introduced in rare cases such as research. For example, there are only about 40 units in Japan, and most of them are used to measure magnetoencephalography, and only 2 units are used to measure magnetocardiography (2016).
Several papers have pointed out that the analysis of the electrocardiogram against the magnetocardiogram can effectively diagnose various types of heart disease. However, in order to develop and popularize a practical magnetocardiograph, a magnetic sensor with extremely high sensitivity that can replace the SQUID fluxmeter is required. To this end, TDK has successfully developed biomagnetic sensors using advanced MR (magnetoresistive) devices. At the beginning of development, the magnetic resolution was around several hundred pT, but with the improvement of technical capabilities, it has reached several tens of pT in the SQUID field in 2017, and research is currently being carried out with the goal of measuring several pT of brain magnetic fields.
Figure 4 summarizes the various magnetic sensor sensitivities (approximate measurement range) and biomagnetic field strengths that have been used in practice.The unit of magnetic field strength is[Wb/m]the unit of magnetic flux density is[Wb/m2], the two can be related to each other by the magnetic permeability, but the magnetic permeability of the biological tissue as a non-magnetic body is the same as that of the air, which is almost 1, and the magnetic field strength is indicated by the magnetic flux density in the figure. 1Wb/m2=1T (Tesla). 1T is 104G (Gaussian) in the cgs unit system.
Figure 2 Various types of magnetic sensors and their approximate measurement ranges
HDD magnetic head technology and MR devices
The MR devices used in TDK’s biomagnetic sensors are briefly described below.
It has been known for a long time that when an external magnetic field is applied to a substance, the resistance changes slightly, a phenomenon known as the magnetoresistive effect (MR effect). It is collectively referred to as the “electromagnetic effect” together with the Hall effect, which is a kind of physical effect. When the electrons or holes carrying charges move in the magnetic field, the Lorentz force will act, thereby distorting the moving direction. MR sensors utilize the magnetoresistance effect of semiconductors and ferromagnetic materials, and are now widely used as magnetic sensors for detecting magnetic data of automatic ticket gates, magnetic ink patterns on banknotes, and the like.
In addition to this conventional magnetoresistance effect, there are also cases in which a magnetoresistance effect with an unusually large resistance change rate is exhibited in a structure such as a ferromagnetic multilayer film. This phenomenon is the giant magnetoresistance effect discovered by Peter Greenberg and Albert Fehr in 1987. It cannot be explained by electromagnetic effects, but rather a spintronic phenomenon related to the spin of electrons.
Since then, the giant magnetoresistance effect has been used as an HDD reading device, and in the second half of the 1990s, the recording density of HDDs has been improved by leaps and bounds. TDK quickly mastered advanced spintronics technology, and successively developed HDD magnetic heads such as GMR magnetic heads and TMR magnetic heads, contributing to the large-capacity HDD (Figure 3).
Figure 3 TDK’s development of magnetic heads and changes in HDD high recording density
This report mainly introduces biomagnetic sensors using spintronics-type MR magnetic sensors, which incorporate the thin-film technology cultivated by TDK in the manufacture of HDD magnetic heads. In addition, the sensitivity limit of the conventional spintronics MR magnetic sensor has remained at the 10nT (10-8T) level. In order to realize the pT Order magnetic field measurement that cannot be realized in the MR period, corresponding technologies are required to greatly improve the SN ratio of the sensor.
TDK has successfully realized a biomagnetic sensor with extremely high magnetic resolution of tens of pT (10-11T) by completely eliminating the environmental noise such as the geomagnetic field, which is 1 million times the biological magnetic field, and the noise emitted by the MR device and circuit itself. , its magnetic resolution has reached about 1000 times that of previous products. This magnetic resolution is comparable to the field of SQUID fluxmeters and can be used to measure biological magnetic fields such as cardiac magnetic fields.
Principle of spintronic MR magnetic sensor
The spintronic MR device is a sandwich structure in which a ferromagnetic thin film is sandwiched by a non-magnetic thin film. One side of the ferromagnetic film is a pinned layer (pinned layer) whose magnetization direction is fixed by pinning, while the other side is a free layer, and the magnetization direction of the ferromagnetic film changes following the direction of the external magnetic field. Since the device resistance changes in proportion to the relative angle of the magnetization directions of the pinned layer and the free layer, the magnetic field strength can be known from the magnitude of the current.
Fig.4 Basic structure and sensor principle of MR device
Unlike fluxgate sensors or MI (Magnetic Impedance) sensors, MR magnetic sensors do not require complex oscillating currents because they only need to supply DC power to obtain signals.
Although MR devices have excellent temperature characteristics, their resistance values vary slightly with temperature. In order to control this temperature drift to a minimum, in the MR magnetic sensor, a plurality of devices are formed on the substrate, and differential temperature compensation is performed through a punch bridge structure. A typical Wheatstone bridge circuit composed of 4 devices is shown in Figure 5. Arrows indicate the magnetization direction of the pinned layer.
Figure 5 Example of bridge structure for temperature compensation (Wheatstone bridge)
World’s first visualization of cardiac magnetic field with TDK biomagnetic sensor
TDK’s MR magnetic sensor unit combines multiple MR devices in a bridge structure and has a built-in low-noise circuit. TDK developed a sensor array in which the sensor cells are arranged in a lattice pattern, and conducted joint research with the Tokyo Medical and Dental University Graduate School. The world’s first (2016) cardiac magnetic field was measured and visualized using an MR magnetic sensor (image). ).
At the same time, TDK succeeded in obtaining clearer images by realizing multi-channelization of up to 64ch. Figure 6 shows TDK’s MR magnetic sensor unit and 64ch (channel) sensor array.
Figure 6 MR magnetic sensor unit (upper left) and 64ch MR magnetic sensor array
An example of cardiac magnetic field distribution measurement and visualization using TDK’s 64ch MR magnetic sensor is shown in Figure 7. After overlaying the chest X-ray, the blue waveform maps the electrocardiogram (ECG), the green waveform maps the magnetocardiogram (MCG), and the photo maps the magnetic field distribution of the heart. The black dots are the sensor channels, and the magnetocardiogram waveform is the magnetic field intensity time waveform obtained through the sensor channels indicated by the yellow dots (①②). The reason why the waveform peaks are in opposite directions is because the magnetic field lines are in different directions.
The closed white curves in the photo, similar to the isobars on the weather map, are shown as isomagnets corresponding to the R-waves of the electrocardiogram combined with the same magnetic field strength around the heart at the time of measurement. The red and blue parts indicate different directions of the magnetic field lines. The red part represents the outflow direction of the magnetic field lines, and the blue part represents the inflow direction of the magnetic field lines. The R wave of the electrocardiogram shows the ventricular contraction process. According to the distribution of the magnetic field, the orientation of the magnetic field lines, and the right-handed spiral rule, the cardiac activity current at this time can be estimated to flow in the direction of the green arrow.
Figure 7 Example of measuring and visualizing cardiac magnetic field distribution with a 64ch MR magnetic sensor array
Advantages of the biomagnetic measurement system developed by TDK
TDK’s biomagnetic measurement system uses MR devices. Compared with high-cost large-scale SQUID fluxmeters that require a liquid helium cooling device (Dewar), the system cost is reduced to about 1/10, and it is ), it is excellent in operability and mobility, and has various advantages in research and clinical applications. At the same time, the magnetic shielding room is relatively simple. Due to the high sensitivity of previous SQUIDs, they are easily affected by external disturbance magnetic fields, so they require a strict magnetic shielding environment. Compared with SQUIDs, MR sensors have a wider dynamic range and can still work even in a simple magnetically shielded environment. TDK has demonstrated through practice that the cardiac magnetic field distribution can be measured within a portable, compact magnetic shield that TDK has never seen before. At the same time, it is difficult to replace the sensor configuration in the cooling dewar in the SQUID fluxmeter, and other devices need to be used separately for different object parts. instrument. However, TDK sensors are uncooled and do not require a Dewar, so the arrangement and density of the sensors can be freely changed according to the target site.
The application of magnetocardiography in clinical diagnosis is still in its infancy. If the magnetocardiograph that can be used at room temperature can be practically used and popularized, it is expected to bring great innovation to the diagnosis of heart disease and other diseases.
For example, all images obtained by X-ray film, X-ray-CT, MRI, etc. are still images. Although it can play a role in the diagnosis of morphological damage such as fractures, it cannot be functionally diagnosed. Therefore, an electrocardiograph is used in the diagnosis of heart disease, and the magnetic field measured by the magnetocardiograph has a strength and direction. vector. The transmission path of the active current can be estimated by calculating the source based on the distribution of the magnetic field around the heart, that is, by “reverse solution”. At the same time, by comparing with the electrocardiogram, useful information for diagnosing diseases can also be obtained from the magnetic field waveform.
Of particular interest is the diagnosis of ischemic heart disease. Ischemic heart disease is a condition in which there is a temporary shortage of blood to the heart muscle, which can lead to angina or myocardial infarction if the condition worsens. Although early detection is difficult to achieve with an electrocardiogram, successful detection may be possible through temporal mapping of the magnetic field distribution using a magnetocardiograph.
At the same time, some newborns suffer from congenital heart disease, but if the disease can be detected in the fetal stage, early response to abnormality can be achieved, or smooth medical treatment can be achieved after delivery.
The fetus in the womb is wrapped in a substance called fetus. Fetal fat has extremely high electrical insulating properties, shielding almost all current from the heart. For this reason, measuring the fetal ECG is difficult. In the past, only ultrasonic diagnostic equipment (ECHO) could be used, but only its shape can be understood in ECHO examination. However, since the magnetic lines of force penetrate without being affected by the tire grease, it is possible to measure the magnetocardiogram. In addition, the magnetic field strength decays rapidly according to the distance from the source. Therefore, the fetal cardiac magnetic field can be measured independently without being affected by the maternal cardiac magnetic field. This is also the unique advantage of magnetocardiograph.
Fig. 8 Measurement of fetal cardiac magnetic field by magnetocardiograph using MR magnetic sensor (schematic diagram)
In addition, it is non-invasive and can easily measure weak biomagnetic fields, so it is not only for medical use, but also is expected to be used in fields such as wearable health care equipment or sports science in the future.
Shown below is a comparison of the advantages and disadvantages of biomagnetic measurement systems using SQUIDs versus systems using MR magnetic sensors.
Table 1 Comparison of systems using SQUIDs and systems using MR magnetic sensors
The SQUID fluxmeter used for measuring biological magnetic fields, etc., has an expensive and bulky system itself, and also requires regular replenishment of liquid helium for cooling, so the high operating cost has become its fundamental disadvantage. For this reason, low-cost and easy-to-use systems that can replace SQUID fluxmeters are now being sought worldwide.
To meet this demand, TDK has developed a biomagnetic sensor using a small, high-sensitivity MR magnetic sensor by utilizing the advanced thin-film technology and spintronics technology accumulated in the manufacture of HDD heads. Its magnetic resolution is tens of pT (10-11T), which is about 1000 times that of previous MR magnetic sensors, and has reached the level of SQUID fluxmeters. In addition, through joint research with Tokyo Medical and Dental University Graduate School, TDK developed a biomagnetic field measurement system using a multi-channel MR magnetic sensor array, and successfully realized the world’s first normal temperature measurement of the cardiac magnetic field, and realized the visualization of the cardiac magnetic field distribution.
Currently, TDK is also advancing the development of a biomagnetic field measurement system with a resolution of several pT. By further digging into the fields that can only be measured by the SQUID fluxmeter in the past, it is expected to be used for the diagnosis of atrial diseases such as atrial fibrillation and brain diseases (epilepsy, ALS, etc.).
In addition, a resolution level of several pT will enable the measurement of the brain magnetic field, which is weaker than the cardiac magnetic field. In the biomagnetic field, the study of magnetoencephalography is more active than that of magnetocardiography. Compared with the electroencephalograph that measures the potential difference, the magnetoencephalograph is characterized by not being affected by the skull, so that clear information can be obtained. However, in order to identify the abnormal part of the epilepsy patient’s brain that emits a unique brain wave waveform, to clarify the cause of the intractable disease ALS (Amyotrophic Lateral Sclerosis), to identify the alpha wave emitted from the brain during meditation, etc., The required resolution needs to be below 0.5pT. Although this is a very difficult technical field in the field of MR magnetic sensors, TDK is making continuous efforts to help carry out the most advanced biomagnetic field research by relying on the unique characteristics and advantages of MR magnetic sensors.
TDK’s MR magnetic sensors have various applications in addition to biomagnetic field measurement. Due to its normal temperature, non-invasive measurement characteristics, it can also be used in the field of non-destructive inspection, such as magnetic testing (MT) for detecting subtle defects that cannot be found by visual inspection.
At the same time, by utilizing the unique compactness of MR devices, the convenience of mobile devices such as smartphones can be improved, and it can also be used in wearable VR (Virtual Reality) devices, health care devices, in-vivo inspection devices, and artificial organs.
In the future, please pay attention to the biomagnetic measurement system that realizes extremely high-sensitivity biomagnetic field measurement in a simple and low-cost manner, and TDK’s magnetic sensor technology developed using advanced spintronics technology.
TDK is working on developing various applications of magnetic sensor technology including MR magnetic sensors.