Magnetic Source Analysis Research Articles

Evaluation of Directional Dependence of Sensitivity for Room-Temperature Magnetic Flux Sensors With Wide Sensitivity Region

Recently, the room-temperature (RT) magnetic flux sensors have begun to be applied to biomagnetic measurements. The RT sensors have a large advantage that they can be placed closer to the magnetic sources and obtain larger signals, unlike the superconducting quantum interference device (SQUID) flux sensors. However, when an RT sensor is placed closer to the sources, the dimension of the sensitivity region cannot be negligible, and the directional dependence of sensitivity should be considered to estimate the theoretical magnetic signals for magnetic source analysis. We proposed a method to evaluate the directional dependence of the sensitivity of the RT sensors in response to adjacent magnetic sources using the array of coils arranged along a circular arc. Consequently, it was revealed that a specific magnetoresistance device-based flux sensor with a certain spatial extent had the directional dependence of sensitivity with a bell-shaped profile. We also proposed the multiple sensitivity points model for the bell-shaped profile and estimated the sensitivity distribution over the sensitivity region, which is expected to be effective in improving the accuracy of the magnetic source analysis using an RT sensor array.

Calibration of Room Temperature Magnetic Sensor Array for Biomagnetic Measurement

Biomagnetic field measurement is a promising tool for the investigation of electrical activities in a living body. Room temperature (RT) magnetic sensors with an improved resolution such as magnetoresistance (MR) devices have been recently employed for the detection of weak magnetic fields, which were earlier detected solely using superconducting interference device magnetic sensors. The position, orientation, and sensitivity of each magnetic sensor in a sensor array must be precisely determined for accurate magnetic source analysis. We proposed a calibration method using an array of multiple coils, which is applicable to an RT magnetic sensor array. To demonstrate the validity of the proposed calibration method, we applied it to an MR device-based magnetocardiography (MCG) system equipped with an L-shaped planar sensor array, which was newly developed for the simultaneous observation of both anterior and lateral sides of the body. The deviation of the sensor parameters from the designed values was estimated via calibration. The result of the marker coil localization test indicated that the calibration considerably improved the accuracy of the magnetic source analysis. Finally, we demonstrated a preliminary MCG measurement using the calibrated magnetic sensor array.

Calibration for a Multichannel Magnetic Sensor Array of a Magnetospinography System

A method for calibrating a multichannel superconducting quantum interference device (SQUID) magnetic sensor array of a magnetospinography (MSG) system is proposed. The MSG system detects weak magnetic fields generated by the neural activity of the spinal cord using the array of SQUID sensors. To calibrate 120 SQUID magnetic sensors, we employed an array of precisely machined circular coils to generate fundamental magnetic fields instead of employing Helmholtz coils, which are generally used for the calibration of magnetic sensors. The effective position, orientation, and sensitivity of each SQUID sensor were determined simultaneously by a parametric numerical optimization algorithm. These sensor parameters are essential for improving the accuracy of the magnetic source analysis. Using this method, we could calibrate the magnetic sensors even though they were installed in a non-transparent cryostat. In addition, the shifts from the designed position and orientation of the sensors as well as their variability were evaluated.

Source analysis of cervical spinal cord evoked field detected by vector SQUID sensors

Abstract Chained current element (CCE) analysis is a new approach of magnetic source analysis for the spinal cord evoked field (SCEF) measurements. In the CCE analysis, magnetic sources are described by multiple dipoles arranged along a parameterized curve that corresponds to a modelled spinal cord. Both path of the neural signal propagation and the current distribution along the path are determined at once by searching the parameters that determine the shape and position of the curve. We applied the CCE analysis to the data from an actual SCEF measurement on a rabbit. The possibility of the lesion localization of the spinal cord by the CCE analysis was indicated.

An error in magnetic field of isosceles-triangle coil as a representation of equivalent current dipole

MEG phantoms with saline water, as a practical realization of the Sarvas model, have been used to evaluate the accuracy of magnetic field measurement from MEG systems as well as to confirm the validity of algorithms for magnetic source analysis. But there are several concerns to be addressed before they will be accepted as “the evaluation standard.” The concerns are that mechanical accuracy is hard to achieve, linear current drive is also hard to achieve and that electrode quality degrades easily. On the other hand, “dry phantom” proposed by Ilmoniemi et al., which is an isosceles-triangle coil with an infinitely small base, is free from these concerns and is a promising candidate for the standard. To realize a practical standard with this model, the base should be finite. So, we calculated the magnetic field from a finite-small base isosceles-triangle coil in comparison with the magnetic field of the ideal Sarvas model. Calculations were done for 80-mm legs with 10-mm base triangle and for 80-mm legs with 5-mm base triangle, which results in maximum 2% error and maximum 0.2% error from the ideal Sarvas model, respectively. These results suggest that the isosceles-triangle coil with a 5-mm base is a good selection for the standard phantom.

Auditory cortical response patterns to multiple rhythms of AM sound.

Multiple amplitude modulation (AM) of a tone results in a time series with a complex structure of envelope fluctuations. In this study the cortical responses elicited by such an auditory stimulus were investigated. It was hypothesized that the auditory cortex responds simultaneously with different patterns of activity corresponding to the different rhythm of periodical stimulus fluctuations. The relations of these response patterns to either steady-state responses (SSR) in the 40 Hz range or the transient slow auditory evoked responses were investigated. Auditory evoked magnetic fields were recorded from the left temporal cortex of nine healthy humans in response to 250 Hz tones that were amplitude modulated with the sum of two sinusoids of 38 and 40 Hz. The stimulus was presented continuously to the subject's right ear at the intensity of 70 dB sensation level. In addition, the responses to 250 Hz tone-burst stimuli with duration of 20, 50, 100, and 200 msec, respectively, were recorded. The tone-bursts were presented at an inter-stimulus interval (ISI) of 0.5 sec, which was equivalent to the slowest rhythm of the multiple-AM stimulus. Magnetic source analysis was applied to all elicited waveforms. From all subjects SSR at 38 and 40 Hz were clearly recognizable and were superimposed by a periodical response pattern corresponding to 2 Hz stimulus envelope fluctuations. A positive peak with latency of 70 msec was the mostly pronounced response to tone-burst stimulation at the ISI of 0.5 sec and resembles the low-frequency response pattern to the AM stimulus. The N1-P2 complex showed an almost complete suppression at 0.5 sec ISI. In contrast, it was the most prominent evoked response activity when using a longer ISI of 3 sec. Source locations in overlapping areas were estimated for the SSR in the 40 Hz range, the prominent peak of low-frequency response to the AM stimulus, and the P1 peak at tone-burst stimulation, respectively. These were separated from the N1 source location by about 0.5 cm in anterior and 0.5 cm in medial direction. As a result of multiple AM the stimulus signal envelope fluctuates with frequencies of 2 Hz and around 40 Hz. The auditory cortex responds to this stimulus with different response patterns. Both modulation signals at 38 and 40 Hz were almost linearly represented as steady-state responses. The evoked response patterns to stimulus fluctuations at low-frequency rhythm resemble the slow cortical auditory evoked responses. The 2 Hz stimulus rhythm is perceived as loudness fluctuation, and 40 Hz modulation is perceived as a rough sound. It is assumed that the different cortical representations correspond to both types of perception of the complex AM sound.