TDK Biomagnetic Sensor: Effective Alternative to SQUID (Superconducting Quantum Interference Device)
Capable of Detecting Extremely Weak Biomagnetic Signals at Room Temperature
The World’s First Ambient Temperature Sensor to Visualize the Heart’s Magnetic Field Distribution

CONTENTS

• 1
  Background
• 2
  Types of Magnetic Sensors and Biomagnetic Fields
• 3
  HDD Head Technology and MR Elements
• 4
  Principle of the Spintronics Type MR Magnetic Sensor
• 5
  Visualization (Imaging) of the Magnetic Fields of the Heart for the First Time in the World Using Biomagnetic Sensors from TDK
• 6
  Merits of the biomagnetic measurement system developed by TDK
• 7
  Summary
• 8
  For Inquiries and Document Requests

Background

Heart disease, along with cancer (malignant growth) and cerebrovascular disease (cerebral hemorrhage), is the leading cause of death worldwide.
The electrocardiograph is widely used as a diagnostic device to determine the status of cardiac activity.

The source of electrical activity of the heart is a bundle of cells known as the sinus node, which is a natural pacemaker located in the right atrium. The electrical signals produced by the sinus node are first conveyed to the entire atrium, and then pass through the tissue known as the atrioventricular node, after which they branch to the right and left, and are conveyed to the entire ventricle, causing the rhythmic repetitive beating of the heart. This is known as the impulse conduction system. This conduction of the electrical excitation of the heart appears as the potential difference between different parts on the surface of the body. The electrocardiograph detects and amplifies this potential difference using multiple electrodes attached to the extremities as well as the chest, and displays and records it as waveforms.

Figure 1 shows the cardiac impulse conduction system, general flow of cardiac activity current, and the pattern diagram of a typical electrocardiogram waveform. P-wave and QRS-wave are waveforms which accompany the contraction of the atrium and ventricles respectively, while T-wave and U-wave are waveforms of the processes by which the excitation of ventricles are cooling down.

Figure 1 Cardiac impulse conduction system and electrocardiogram waveform

It is not possible to spatially understand the working of the heart using an electrocardiograph, and it can only be roughly estimated from the waveforms in an electrocardiogram. If we were able to observe the detailed working of the cardiac muscles, it would result in a remarkable improvement in the accuracy of diagnosis. The magnetocardiograph (MCG) is a solution to these problems. According to the “right-hand screw rule” in electromagnetism, when current flows, it produces magnetic fields around it. Hence, by measuring the magnetic fields produced around the heart, it is possible to estimate the flow of current, and the parts to which it flows. Another advantage of the magnetocardiograph is that it can carry out measurement completely non-invasively while the subject is fully clothed, and does not require electrodes to be attached to the surface of the body. But, cardiac magnetic fields are extremely weak biomagnetic fields, and hence the magnetocardiograph needs to be equipped with highly sensitive magnetic sensors.

Types of Magnetic Sensors and Biomagnetic Fields

The first electrocardiogram was measured in 1903 using a device invented by W. Einthoven, a Dutch physicist. In comparison with this, cardiac magnetic fields are extremely weak at about 1 millionth or less of geomagnetism, and hence they were fist measured only in the latter half of the 20th century, in the year 1963. The first measurement used a pair of magnetic flux detection coils wound as many as 2 million times. After this, measurement was carried out in a special magnetically shielded room, in order to prevent disturbance from geomagnetism, but even then, it was barely enough to determine that magnetic fields were being produced by the heart, and the accuracy was not enough to be useful in diagnosis of heart disease.
The SQUID flux meter developed around 1970 has brought about major advances in the measurement of biomagnetic fields. SQUID is the acronym for “superconducting quantum interference device”, and it is a magnetic sensor in which a Josephson junction is created in a part of the coil using a superconductor. The Josephson junction element was originally developed as a logic element to increase the computer processing speed, but since it is extremely sensitive to magnetism, it was used as a highly sensitive flux meter. Using this SQUID flux meter, it has become possible to measure not only the magnetic fields which accompany cardiac activity, but also to measure brain magnetic fields.

However, to operate SQUID, it is necessary to cool the superconductive coil using liquid helium, and to set up a special magnetically shielded room to block magnetic noise, as a result of which the system becomes bulky and expensive. This is the reason why biomagnetic field measurement devices using a SQUID flux meter are used to a very limited extent, mainly for research purposes. For example, there are only about 40 devices in Japan, and most of them are used for magnetoencephalograms and only 2 devices are for magnetocardiograms (in 2016).
Several papers have pointed out that analyzing an electrocardiogram by comparing it with a magnetocardiogram can be useful in the diagnosis of various heart diseases. However, in practical terms, an extremely highly sensitive magnetic sensor that can replace the SQUID flux meter is required for development and spread of the magnetocardiograph. TDK has developed new biomagnetic sensors using advanced MR (magnetoresistive) elements. When they were initially developed, the magnetic resolution was around a few hundreds of pT (pico teslas), but with improvements in technology, as of 2017, a figure of a few tens of pT was achieved, which is in the SQUID domain. Currently research is continuing with the goal of reaching a just a several pT, at which point it would even be possible to measure the brain’s magnetic fields.

The sensitivity (approximate measurement range) and biomagnetic field intensity of various magnetic sensors that have been actually developed has been summarized in Figure 4. The unit of magnetic field intensity is [Wb/m], and the unit of magnetic flux density is [Wb/m²]. Both are related through magnetic permeability, but the magnetic permeability of biological tissue, which is non-magnetic material, is approximately 1, which is the same as air, and hence, in this figure, the intensity of the magnetic field is represented by the magnetic flux density. 1Wb/m² = 1T (Tesla). 1T is equal to 10⁴G (Gauss) in the cgs unit system.

Figure 2 Approximate range of measurement of various magnetic sensors

HDD Head Technology and MR Elements

We shall briefly describe the MR elements used in the biomagnetic sensors from TDK.
We have known for a long time that when an external magnetic field is applied on a substance, the electrical resistance changes slightly, and this is known as the magnetoresistive effect (MR effect). This type of physical effect, along with the Hall effect, is collectively known as the “galvanomagnetic effect”. When electrons and electron holes carrying electric charge move inside a magnetic field, the Lorentz force acts, causing the direction of movement to bend. MR sensors use the magnetoresistive effect of semiconductors and ferromagnetic materials, and are being widely used as magnetic sensors to detect magnetic data on tickets at automated ticket gates, magnetic ink patterns on banknotes, etc.

Other than the conventional type of magnetoresistive effect, a multi-layered film made of ferromagnetic materials demonstrates a magnetoresistive effect with an abnormally large rate of change of resistance. In 1987, P. Grunberg and A. Fert discovered this phenomenon known as the giant magnetoresistive effect. It cannot be explained by the galvanomagnetic effect, and it is a spintronics phenomenon that is related to the spin of electrons.
The giant magnetoresistive effect is used as the reading element in an HDD, and since the latter half of the 1990s, it has resulted in rapid growth of recording density in HDDs. TDK has quickly adopted advanced spintronics technology, and has developed HDD heads such as the GMR head and TMR head in quick succession, and we have been contributing to the capacity enlargement of HDDs. (Figure 3).

Figure 3 Phases of magnetic head development by TDK and increasing recording density of HDDs

This article presents biomagnetic sensors, which use MR magnetic sensors of the spintronics type, developed by application of thin-film technology, cultivated through the manufacture of HDD heads. So far, even the spintronics type MR magnetic sensors had a limit on sensitivity equal to 10nT (10^-8T), and to enable measurement of magnetic field gradients on the order of a few pT, which is considered impossible with MR elements, a technology that largely increases signal-to-noise ratio as a sensor is required..

By eliminating environmental noises including one of earth magnetism, which is a million times as intense as noise of biomagnetic fields, and noises of MR elements and circuits themselves, TDK has been able to develop biomagnetic sensors with a very high magnetic resolution of a few tens of pT (10^-11T), which is about 1000 times higher than before. This is close to the domain of the SQUID flux meter, and it also enables measurement of biomagnetic fields such as cardiac magnetic fields.

Principle of the Spintronics Type MR Magnetic Sensor

The spintronics type MR element has a sandwich structure made of a thin-film of non-magnetic material sandwiched between thin ferromagnetic films. One of the ferromagnetic films is a pin layer (fixed layer) in which the direction of magnetization is fixed by pinning, and the other ferromagnetic film is a free layer in which the direction of magnetization follows the direction of the external magnetic field. Since the electrical resistance of the element varies proportionately with the relative angle between the direction of magnetization of the pin layer and free layer, the intensity of the magnetic field can be measured from the magnitude of the current.

Figure 4 Basic structure and sensor principle of the MR element

Unlike fluxgate sensors and MI (Magneto-Impedance) sensors, MR magnetic sensors receive signals just by applying DC power source, and do not require complex oscillation circuits.

An MR element also has excellent temperature characteristics, and the resistance value changes only slightly due to changes in temperature. In order to minimize this temperature drift, multiple elements are formed on the circuit board of the MR magnetic sensor, and a bridge configuration is used for differential temperature compensation. An ordinary Wheatstone bridge circuit in which 4 elements have been combined is shown in Figure 5. The arrows indicate the direction of magnetization of the pin layer.

Figure 5 Example of the bridge configuration for temperature compensation (Wheatstone bridge)

Visualization (Imaging) of the Magnetic Fields of the Heart for the First Time in the World Using Biomagnetic Sensors from TDK

The MR magnetic sensor unit combines multiple MR elements in a bridge configuration, and has a built-in low-noise circuit. TDK has developed a sensor array in which these sensor units are arranged in the form of a lattice, and through collaborative research with the graduate school of the Tokyo Medical and Dental University, we have become the first organization in the world to successfully carry out measurement and visualization (imaging) of the cardiac magnetic fields using MR magnetic sensors (in 2016).
TDK has also been successful in obtaining clearer images by increasing the number of channels up to 64. Figure 6 shows an MR magnetic sensor unit from TDK and a 64ch (channel) sensor array.

Figure 6 Image showing the MR magnetic sensor unit (upper left) and a 64ch MR magnetic sensor array

Figure 7 shows an example of measurement and visualization of cardiac magnetic field distribution using TDK's 64ch MR magnetic sensor arrays. The blue waveform is an electrocardiogram (ECG), the green waveform is a magnetocardiogram (MCG), and the picture is the magnetic field distribution of the heart superimposed on the X-ray image of the chest. The black dots are sensor channels and the waveform of the magnetocardiogram is the time waveform of magnetic field intensity obtained from sensor channels indicated by yellow dots (①and②). The peak of one of the waveforms faces the opposite direction of the other one because of the difference in direction of the lines of magnetic force.
The white curved lines in the photo, which are similar to the isobar lines of a weather map, connect points with magnetic field intensity equal to around the heart at the time of measurement supporting R waves of the electrocardiogram. The red area and blue area represent the differences in the direction of the lines of magnetic force. The red area shows the direction in which the line of magnetic force is ballooning. The blue area shows the direction in which the line of magnetic force is drawing inwards. R waves of the electrocardiogram accompany the contraction of the ventricles. Judging from the magnetic field distribution, direction of the line of magnetic force, and the right hand screw rule, it can be estimated that the action potential of the heart at this time is flowing in the direction shown by the green arrow.

Figure 7 Example of measurement and visualization of cardiac magnetic field distribution using a 64ch MR magnetic sensor array

Merits of the Magnetocardiograph Used to Replace the SQUID

The biomagnetic measurement system from TDK, which uses MR elements, has a system cost that is 1/10th of the SQUI flux meter, which is expensive, bulky, and requires a liquid helium cooling device (Dewar). In addition, it is capable of measurement at room temperature (without cooling), and also has excellent operability and portability, which are various advantages from the research aspect as well as the clinical aspect.
And, a relatively simple shielded room is adequate for measurement. Because the earlier SQUID had high sensitivity, it was easily affected by disturbance magnetic fields, and therefore required strong magnetic shielding, but MR sensors have a wider dynamic range than SQUID, and hence they work even with simple magnetic shielding. TDK is demonstrating, through proof-of-concept, that measurement of the cardiac magnetic field distribution is actually possible inside our unprecedentedly innovative portable compact magnetic shield.
In a SQUID flux meter, it was difficult to change the arrangement of sensors inside the Dewar for cooling, and a separate device was necessary for each targeted part, such as a magnetoencephalography for brain magnetic fields and a magnetocardiograph for cardiac magnetic fields. On the other hand, TDK's sensors require neither cooling nor a Dewar, the arrangement and density of sensors can be changed freely depending on target parts.

The application of magnetocardiograms for clinical diagnosis is still in the initial phase, and once magnetocardiographs which can be used at room temperature are implemented and become freely available, they may usher in major innovations in the diagnosis of heart diseases as well as other diseases.
For example, the images obtained as X-ray images, X-ray-CT, and MRI, are static morphological images. These are useful for diagnosis of morphological disorders such as fractures, but cannot be used for functional diagnosis. Therefore, while the electrocardiograph is used for diagnosis of heart disease, the magnetic field measured using the magnetocardiograph is a vector having magnitude and direction. By obtaining the source from the magnetic field distribution around the heart, in other words, “Solving the inverse problem”, it can also estimate the transmission route of the activity current. And, by comparing it to the electrocardiogram, it is also possible to obtain useful information from the magnetic field waveform leading to diagnosis of diseases.
It is especially expected to help in diagnosis of ischemic heart disease. Ischemic heart disease is a disease in which there is a temporary deficiency of blood flowing to the cardiac muscles, and if it becomes serious, it could lead to angina pectoris or myocardial infarction. Early discovery is difficult using an electrocardiogram, but detection may be possible by temporal mapping of the magnetic field distribution using a magnetocardiograph.

And, for newborns with congenital heart disease, discovery of the disease during the fetal stage can smooth early actions in emergency situations and the treatment process after birth.

A fetus in a uterus is covered with a material called vernix. Vernix has high electrical insulation and shuts off almost all the current from the heart. Therefore, taking an electrocardiogram of a fetus is quite difficult and there was no way other than using an ultrasonograph (echo), which can only show shapes. On the other hand, the lines of magnetic force can pass through without being affected by vernix, which enables magnetocardiograms. Also, the magnetic field intensity weakens rapidly in inverse proportion to the square of the distance from the source, and hence it is possible to measure the cardiac magnetic fields of the fetus only without being affected by the cardiac magnetic fields of the mother. This is also another merit which is characteristic to the magnetocardiograph.

Figure 8 Magnetocardiogram measurement of a fetus with a magnetocardiograph using a MR magnetic sensor (image)

Further, since it is capable of measuring weak magnetic fields easily and non-invasively, it is expected to be used not only in the medical field, but also in the fields of wearable health care devices and sports science.

A comparison between the strengths and weaknesses of a system for measurement of biomagnetism using SQUID, and a system using an MR magnetic sensor, is shown below.

Summary

SQUID flux meters have been used measurements of biomagnetic fields, but the system itself is expensive and bulky and has basic flaws such as high running cost including the cost of periodically supplied liquid helium for cooling. As a result, it is expected to have a system which is low-cost and easy-to-use and can substitute SQUID flux meters worldwide.
To meet such a need, TDK developed biomagnetic sensors using compact and highly sensitive MR magnetic sensors by development of applications using advanced thin-film technology and spintronics technology cultivated through the manufacture of HDD heads. Their magnetic resolution is of the order of a few tens of pT (10^-11T), which is about 1000 times that of the existing MR magnetic sensors, and it is approaching the domain of the SQUID flux meter. And, through collaborative research with the graduate school of the Tokyo Medical and Dental University, TDK has developed a biomagnetic measurement system using a multi-channel MR magnetic sensor array, and we are the first organization in the world to successfully carry out measurement of the cardiacmagnetic fields at room temperature, and visualization (imaging) of the cardiac magnetic field distribution.

TDK is also working on the development of a biomagnetic field measurement system with a resolution of a few pT. By coming closer to the domain which could only be measured using a SQUID flux meter up to now, it is expected to be useful in the diagnosis of atrial diseases such as atrial fibrillation.
Furthermore, the resolution of a few pT is at a level that enables measuring of brain magnetic fields which are weaker than heart magnetic fields. As for biomagnetic fields, more research is being conducted on magnetoencephalograms than on magnetocardiograms. Compared to a potentiometric electroencephalograph, a magnetoencephalograph is less affected by a skull and is able to obtain sharper information. However, it is expected that a resolution of 0.5 pT or less will be required for identifying the abnormal part that generates brain wave patterns unique to patients with epilepsy, for finding causes of the serious disease ALS (Amyotrophic lateral sclerosis), and for clarifying alpha waves emitted from brains in meditation. Although this is an extremely difficult field of MR magnetic sensors, TDK will work to contribute to advanced research in biomagnetic fields by utilizing the characteristics and advantages of MR magnetic sensors.

MR magnetic sensors from TDK have the potential to be widely used in various applications other than biomagnetic field measurement. Since they can carry out measurement non-invasively and at room temperature, they can potentially be used in the field of non-destructive testing, such as magnetic particle testing (MT), to examine minute flaws that cannot be discovered by visual inspection.
They also achieve the compactness which is characteristic of MR elements, and help in making mobile devices such as smartphones more convenient, and can also be considered for use in applications such as wearable VR (virtual reality) devices, health care devices, and even in internal body examination devices, and artificial organs.
Please watch out for biomagnetic measurement systems which can carry out measurement of extremely highly sensitive biomagnetic fields easily and at low cost, and magnetic sensor technology from TDK using advanced spintronics technology.

TDK has been working on development of MR magnetic sensors and a variety of other applications of magnetic sensor technology.
If you have any projects where the TDK technologies and products presented in this article can be used, please feel free to get in touch with us. TDK is also planning to hold demonstrations where you can observe and also personally experience the working of the biomagnetic measurement system which uses MR magnetic sensors.

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