Biomagnetic Measurements Research Articles

Development of Biomagnetic Measurement System with 39ch SQUIDs Magnetometer for a Three Dimensional Magnetic Measurement

A new 39 channel SQUID magnetometer system with 3D second-order gradiometer has been developed for research use in order to measure the biomagnetic fields. This is the total hardware system consisting of 39 SQUID sensors with detecting coils, readout electronics, two liquid helium dewars for MEG and MCG, a gantry, and a magnetically shielded room.The noise characteristics of frequency domain of three components were measured. The power spectra of the noise field normal component are about 15fT/√Hz at 10Hz and under 10fT/√Hz at white noise. The power spectrums of the noise field tangential component are about 12fT/√HZ at 10Hz and under 10fT/√Hz at white noise. An alpha rhythm occurring from a subject's head was recorded with the amplitude of about 500fT at about 10Hz. We could measure the mixed AEF and SEF overlapping in time using this system. From these results, the system has a sufficient ability to measure weak magnetic fields from the brain.

Biomagnetic 3-dimensional spatial and temporal characterization of electrical activity of human stomach.

Biomagnetic measurements are based on the noninvasive recording of magnetic signals produced by biological sources such as nervous system and muscle. The aim of this study was to obtain multichannel magnetic field recordings from the human gastrointestinal tract and to localize the sources of these signals three-dimensionally. The magnetic field was recorded in eight human healthy subjects using a sensor array with 37 superconducting quantum interference devices (SQUIDs); an electrogastrogram was recorded simultaneously. Biomagnetic source localization was carried out with an iterative nonlinear optimization algorithm using the model of an equivalent current dipole (ECD) and correlated to magnetic resonance imaging (MRI) in four volunteers. Magnetogastrograms and electrogastrograms demonstrated a similar frequency distribution with a peak at 3/min. In all subjects the centers of the calculated dipoles plotted vs time showed a characteristic migration across the stomach area. One volunteer demonstrated tachygastric episodes, during which his magnetic field amplitudes increased fivefold and his dipole migration disappeared. In absence of an attack his recordings changed to normal. This demonstrates multichannel magnetic recordings can be used to localize the sources of the biomagnetic field, which could be useful for the understanding of motility disturbances.

Biopotential amplifier for simultaneous operation with biomagnetic instruments.

A multichannel biopotential amplifier for simultaneous use with biomagnetic measurements in a magnetically shielded room is designed and evaluated. Particular care is taken to make the amplifier electromagnetically compatible with the biomagnetic instruments over the whole frequency spectrum, from DC to RF. The electromagnetically quiet environment allows the use of high electrode impedances; the preamplifier has been designed accordingly. Special care is taken to analyse the coupling mechanisms of mains interference to the amplifier. Over 170 simultaneous electric and magnetic recordings have been performed using the system in a hospital environment.

A new concept for integrated YBa/sub 2/Cu/sub 3/O/sub 7/ magnetometers

We present an integrated magnetometer concept that combines inductance matching by a flux transformer with the low inductance of a multiloop pick-up coil. Using ramp-type Josephson junctions with PrBa/sub 2/Cu/sub 3/O/sub 7/ barriers, a 90 pH dc SQUID together with a five turn flux transformer is coupled to a multiloop pick-up coil. The dependence of the effective area of the magnetometer on the temperature is investigated by successively disconnecting the pick-up loops. The 1/f flux noise of the integrated magnetometer is suppressed by operation in a flux-locked loop mode with bias current modulation. This results in a flux density noise of /spl radic/(S/sub B/)(1 Hz,77 K)=100 fT//spl radic/(Hz). Using this magnetometer, different biomagnetic measurements have been performed.

Plastic-molded LTS DC-SQUIDs for multichannel biomagnetic measurement systems

A plastic molding technique for SQUIDs has been developed to protect the chip from moisture damage. A 1.5-mm-thick layer of molding compound (epoxy resin, elastic resin, and silica powder) was poured over Ketchen-type DC-SQUID that has Nb/AlO/sub x//Nb Josephson junctions. The moisture resistance of these plastic-molded SQUIDs was examined by subjecting them to temperature and humidity storage tests (60/spl deg/C, 90% relative humidity, 1,000 hours), and then measuring their performance characteristics. The results showed that the plastic packaging protected the chip from moisture damage, while not affecting the noise performance of the SQUIDs. The plastic molding therefore improved reliability and ease in handling of such SQUIDs.

Practical aspects for SQUID applications

Classical applications of superconducting quantum interference devices (SQUIDs), like biomagnetic measurements or precision voltage and current measurements have in almost all cases been carried out in a magnetically and electrically well shielded environment. Recently, the demonstration of highly sensitive HTS SQUIDs has increased the interest in SQUIDs for a broader range of applications like geophysical measurements or nondestructive materials evaluation. The operation of SQUIDs in an unshielded environment, however, leads in many cases to a significant reduction in their sensitivity due to numerous sources of interference and excess noise. The paper reviews recent work on aspects related to the operation of SQUIDs in unshielded environments.

Integrated niobium DC SQUID gradiometers for non-destructive evaluation and biomagnetism

We describe the design and fabrication of thin-film Nb gradiometers with integrated DC SQUIDs for use in non-destructive evaluation (NDE) and biomagnetism. Issues of sensitivity, imbalance and field response are considered. Results are presented from eddy-current NDE in an unshielded environment of aluminium plates with sub-surface flaws, and from biomagnetic measurements of spinal and peripheral nerve evoked fields.

The hemodynamics of the umbilical artery in normal and pre-eclamptic pregnancies. A new application of SQUID biomagnetometry.

This study was set to investigate the hemodynamics of the feto-placental circulation in normal and pre-ecclamptic pregnancies using the biomagnetometer SQUID. Thirteen women with pre-eclampsia and 26 healthy women were studied. All were "near term". Biomagnetic signals (waveforms) were recorded from the umbilical arteries. After statistical Fourier analysis, the findings were assigned arbitrarily in terms of spectral amplitudes as high (200-300 fT/ root of Hz), low (80-150 fT/ root of Hz) and borderline (151-199 fT/ root of Hz). In all cases the frequencies considered were distributed in the range 2-7 Hz. Interestingly, the umbilical artery waveforms and the corresponding spectral densities were of high amplitudes in most (88%) normal pregnancies of low amplitudes in most (84.6%) pregnancies complicated with pre-eclampsia. These findings were of statistical significance and were correlated with fetal heart rate (FHR) monitoring, pH and Apgar score at 1 and 5 minutes; high amplitude cases were related with normal FHR monitoring, pH > 7.25 and Apgar score > 7, while low amplitude recordings were connected with abnormal FHR patterns, pH < 7.25 and Apgar score < 7. It is suggested that biomagnetic measurements of the umbilical artery flow, which is an entirely new application of SQUID technology, is a promising procedure in assessing fetal health, especially in high risk pregnancies.

Fetal magnetoencephalographic mapping in normal and pre-eclamptic pregnancies

This study investigates the fetal brain activity in normal and pre-eclamptic pregnancies. Measurements were performed by means of a superconducting quantum interference device (SQUID) in an electrically shielded room of low magnetic noise. The study was prospective. Ten pregnant women with pre-eclampsia and 11 healthy gravidae were included. All were preterm at 28 to 37 weeks' gestation. Biomagnetic signals (waveforms), recorded from the fetal brains in the frequencies 2-7 Hz, were expressed in terms of magnetic power spectral amplitudes: these were low in almost all normal pregnancies, and high in most pregnancies complicated with pre-eclampsia. The pictorial representation of the results in the form of iso-spectral amplitude (ISO-SA) mapping showed two different patterns: (a) iso-contour lines 'organised' in dense concentration zones (pre-eclamptic pattern), (b) iso-contour lines at random distribution without dense concentration zones (normal pattern). Biomagnetic measurements of fetal brain activity is a promising screening procedure for assessing the cerebral function, especially at high risk pregnancies.

Gastrointestinal Transit of a Magnetically Marked Capsule Monitored Using a 37-Channel SQUID-Magnetometer

The gastrointestinal transit of magnetically marked solid pharmaceutical drug dosage forms can be monitored using biomagnetic measurement equipment. The presented study shows that using a 37-channel SQUID-magnetometer the investigation of the gastrointestinal transit of an ingested magnetically marked capsule becomes feasible with a temporal resolution in the order of milliseconds and a spatial resolution in the millimeter range.

Biomagnetic measurements with an integrated YBa2Cu3O7 magnetometer in a hand-held cryostat

With low-noise magnetometers made from high-temperature superconductors, mobile applications of superconducting quantum interference device (SQUID) magnetometry become possible. Due to the high volume heat of evaporation of liquid nitrogen, a SQUID magnetometer can be operated in a small size cryostat for some hours. For the first time biomagnetic measurements are presented using an integrated YBa2Cu3O7 magnetometer mounted in a hand-held cryostat with a content of 100 cm3 of liquid nitrogen. These measurements and the noise properties are compared to those made in a conventional cryostat. The low-noise magnetometer consists of a multiloop pick-up coil coupled inductively to a dc SQUID based on ramp-type Josephson junctions with PrBa2Cu3O7 barriers.

A 35-Series Superconducting Quantum Interference Device Array for High-Dynamic-Range Magnetic Measurements

A 35-Series Superconducting Quantum Interference Device Array has been developed for a magnetometer with a high dynamic range. The coherence of each SQUID was observed in a double-shielded tube made of cryoperm and lead; a maximum voltage swing of 2 mV and flux noise of 0.6 µΦ0/√ Hz were obtained. The nonuniform inductance of the SQUIDs in the array, which causes Φ–V curve modulation, is decreased by using dummy SQUIDs at each end of the array. A gradiometer for biomagnetic measurement was constructed using a 35-SQUID array and a 10×10-mm planar pick-up coil. A magnetic-field sensitivity of 5.4 fT/√ Hz, a maximum slew rate of 1.9 mT/s, and a maximum feedback amplitude of ±2.23 µ T were achieved by using a direct-coupled flux-locked-loop circuit. The dynamic range of the magnetometer input field was ±4.13×108 √ Hz.

Thermal noise in biomagnetic measurements

Studies of weak magnetic fields are generally influenced by magnetic noise emanating from thermal agitation of electric charge (Johnson noise) in electrically conducting materials surrounding the magnetic-field sensor. In this article, the thermal magnetic noise fields generated by slabs with high electric conductivity (copper, aluminum) or high magnetic permeability (mu metal) are studied. The analysis is based both on a previously published phenomenological model and on measurements with an ultrasensitive superconducting magnetometer. Both the spectral densities and spatial correlations of the magnetic field fluctuations are evaluated. The computed correlation coefficients are utilized to develop a practical method for estimating the thermal noise due to thin conducting foils, such as thermal radiation shields in a cryogenic measurement Dewar. Also experiments to reduce the Dewar noise are reported. Finally, estimations are presented for the thermal noise fields arising in the walls of a magnetically shielded room. In practice, thermal magnetic noise, particularly due to the superinsulation in cryogenic Dewars, can be the limiting factor of sensitivity in measurements of weak biomagnetic signals arising in the human heart and brain. The results are useful in the estimation and minimization of the contribution of thermal noise.

High- SQUID magnetometers for biomagnetic measurements

We have designed and fabricated three types of high- SQUID (superconducting quantum interference device) magnetometers based on step-edge Josephson junctions using three different concepts of coupling magnetic flux into the SQUID: (i) a single pickup loop galvanically coupled to the SQUID, (ii) a flux transformer inductively coupled to the SQUID and (iii) a multiloop pickup loop used directly as the SQUID inductance. On a substrate we achieved an effective flux capture area of and for the inductively coupled and multiloop devices, respectively. Due to the low white noise levels of for the inductively coupled magnetometer and for the multiloop device high quality magnetocardiograms were recorded inside a magnetically shielded room without signal averaging.

Suppression of background brain activity influence in localizing epileptic spike sources from biomagnetic measurements.

The origin of inter-ictal epileptic activity can be localized from the magnetoencephalogram (MEG) using the Equivalent Current Dipole (ECD) as a source model. One problem with such localizations is that in many patients, the localization of an epileptic spike source becomes inaccurate because the epileptic spikes are superimposed by pathologic brain rhythmic activities. This paper proposes to use the spatial coherence of the measured magnetic field caused by the measured magnetic field to suppress its influence in the ECD localization. In the method proposed here, the covariance matrix, which expresses the spatial coherence of the rhythmic magnetic field, is first calculated using a data portion where no spikes exist and rhythmic slow waves are evident. Then, ECD localization is performed by minimizing the least-squares cost function modified by the covariance matrix. Experiments using a computer generated ECD field and rhythmic slow waves measured from a patient suffering from complex partial epilepsy prove the basic effectiveness of the method proposed. Localizations of actual spike sources in the same clinical data are performed, and results indicating the effectiveness of the proposed method are obtained.

生体磁気計測用磁気シールドルームの遮蔽性能に関する基礎的検討

生体磁気計測用磁気シールドルームの遮蔽性能に関する基礎的検討

Evaluation of a high-performance magnetically shielded room for biomagnetic measurement

A spherical magnetically shielded room called COSMOS and a method for measuring the shielding factor of well-shielded rooms have been developed. A dc-superconducting quantum interference device is used to detect faint magnetic fields, and the detected signals are averaged to increase the signal-to-noise ratio. The measured magnetic shielding factor for COSMOS is 420000 at 1 Hz.

超電導エレクトロニクス ＳＱＵＩＤ応用 ＳＱＵＩＤ技術の進展と応用分野の展望

Recent developments of SQUID technology are reviewed. First, key elements for the development of high Tc SQUID are briefly shown. Due to these developments, performance of high Tc SQUID has been much improved, and system application is now becoming possible. Second, development of an electronic circuit which makes SQUID robust against environmental noise, is shown. With this circuit, the requirement for magnetic shielding can be reduced. As a result of these developments, constraints in using SQUID, i.e., cooling and shielding, can be relaxed, which is helpful for the extension of the application-field of the SQUID system. Requirements for the SQUID system are briefly mentioned in the application to biomagnetic measurement, nondestructive testing of materials, SQUID microscope and precise measurement. It is emphasized that the requirement for the system is different according to the types of applications. Therefore, it is necessary to develop the system in order to optimize the requirements, including the choice of high Tc or low Tc SQUID. As an example, a nondestructive test of materials using SQUID is shown.

生体磁気計測用SQUID磁束計の開発

生体磁気計測用SQUID磁束計の開発

Biomagnetic measurements using low-noise integrated SQUID magnetometers operating in liquid nitrogen

We have fabricated high-Tc SQUID magnetometers with a pickup coil and a multiturn input coil integrated onto the same substrate. A four-channel system operating in liquid nitrogen has been developed and used to measure biomagnetic signals emanating from both the heart and the brain. The magnetic field sensitivity of the integrated SQUID magnetometer reaches 70 fT/√Hz at 1 kHz and 280 fT/√Hz at 1 Hz. The use of superconductor-normal metal-superconductor (SNS) step-edge junctions with YBa2Cu3O7−δ as the superconductor and Ag–Au alloy as the normal metal allows the measurements to be performed with standard dc bias electronics.