Noise Characteristics of Readout Electronics for 64-Channel DROS Magnetocardiography System

김진목,김기담,이용호,유권규,김기웅,권혁찬,Kim J. M.,Kim K. D.,Lee Y. H.,Yu K. K.,Kim K. W.,Kwon H. C.,Sasada Ichiro The Korean Superconductivity Society 2005 Progress in superconductivity Vol.7 No.1

We have developed control electronics to operate flux-locked loop (FLL), and analog signal filters to process FLL outputs for 64-channel Double Relaxation Oscillation SQUID (DROS) magnetocardiography (MCG) system. Control electronics consisting of a preamplifier, an integrator, and a feedback, is compact and low-cost due to larger swing voltage and flux-to-voltage transfer coefficients of DROS than those of dc SQUIDs. Analog signal filter (ASF) serially chained with a high-pass filter having a cut-off frequency of 0.1 Hz, an amplifier having a gain of 100, a low-pass filter of 100 Hz, and a notch filter of 60 Hz makes FLL output suitable for MCG. The noise of a preamplifier in FLL control electronics is $7\;nV/{\surd}\;Hz$ at 1 Hz, $1.5\;nV/{\surd}\;Hz$ at 100 Hz that contributes $6\;fT/{\surd}\;Hz$ at 1 Hz, $1.3\;fT/{\surd}\;Hz$ at 100 Hz in readout electronics, and the noise of ASF electronics is $150\;{\mu}V/{\surd}\;Hz$ equivalent to $0.13\;fT/{\surd}\;Hz$ within the range of $1{\sim}100\;Hz$. When DROSs are connected to readout electronics inside a magnetically shielded room, the noise of 64-channel DROS system is $10\;fT/{\surd}\;Hz$ at 1 Hz, $5\;fT/{\surd}\;Hz$ at 100 Hz on the average, low enough to measure human MCG.

저성능 자기차폐실에서 64채널 DROS 2차 미분계 시스템의 잡음 특성

김진목,이용호,유권규,김기웅,권혁찬,박용기,Kim, J.M.,Lee, Y.H.,Yu, K.K.,Kim, K.,Kwon, H.,Park, Y.K.,Sasada, Ichiro 한국초전도학회 2006 Progress in superconductivity Vol.8 No.1

We have developed a second-order double relaxation oscillation SQUID(DROS) gradiometer with a baseline of 35 mm, and constructed a poorly magnetically-shielded room(MSR) with an aluminum layer and permalloy layers for magnetocardiography(MCG). The 2nd-order DROS gradiometer has a noise level of 20 $fT/{\surd}Hz$ at 1 Hz and 8 $fT/{\surd}Hz$ at 200 Hz inside the heavily-shielded MSR with a shielding factor of $10^3$ at 1 Hz and $10^4-10^5$ at 100 Hz. The poorly-shielded MSR, built of a 12-mm-thick aluminum layer and 4-6 permalloy layers of 0.35 mm thickness, is 2.4mx2.4mx2.4m in size, and has a shielding factor of 40 at 1 Hz, $10^4$ at 100 Hz. Our 64-channel second-order gradiometer MCG system consists of 64 2nd-order DROS gradiometers, flux-locked loop electronics, and analog signal processors. With the 2nd-order DROS gradiometers and flux-locked loop electronics installed inside the poorly-shielded MSR, and with the analog signal processor installed outside it, the noise level was measured to be 20 $fT/{\surd}Hz$ at 1 Hz and 8 $fT/{\surd}Hz$ at 200 Hz on the average even though the MSR door is open. This result leads to a low noise level, low enough to obtain a human MCG at the same level as that measured in the heavily-shielded MSR. However, filters or active shielding is needed fur clear MCG when there is large low-frequency noise from heavy air conditioning or large ac power consumption near the poorly-shielded MSR.

열린 자기차폐실의 심자도 시스템

김진목,이용호,권혁찬,유권규,김기웅,박용기,Kim, J.M.,Lee, Y.H.,Kwon, H.,Yu, K.K.,Kim, K.,Park, Y.K.,Sasada, Ichiro 한국초전도학회 2007 Progress in superconductivity Vol.9 No.1

We have installed a 61-channel magnetocardiography (MCG) system inside a magnetically shielded room (MSR) with a size of $2.4\;m\;{\times}2.4\;m\;{\times}2.4\;m$. The MCG system consists of 1st-order axial gradiometers containing double relaxation oscillation SQUIDs (DROSs) with pick-up coils of a base line of 70 mm. The MSR holds a shielding factor of 50 at 0.1 Hz and 10000 at 100 Hz, when its door in the middle on a front wall is closed. On opening the MSR door, we have obtained the characteristics of the MCG system with a 2.9 Hz noise generated from an air conditioning unit at 13 m distance off the MSR. In an open-door MSR ($140^{\circ}$ opening), a noise at the center channel increases up to $700\;fT/Hz^{l/2}$ at 2.9 Hz and $1.7\;pT/Hz^{1/2}$ at 60 Hz. MCG signals for a healthy human do not show distortion until the door opens to $45^{\circ}$, but show the effect of noise when the door opens further at $90^{\circ}$ and $140^{\circ}$. With the door opens to $45^{\circ}$, MCG measurement can be performed with ease of door operation and without creating claustrophobia for the patient.

뇌자도에서 전체 평균과 이를 이용한 해부학적 위치 추정

권혁찬,김기웅,김진목,이용호,박용기,Kwon H.,Kim K.,Kim J. M.,Lee Y. H.,Park Y. K. 대한의용생체공학회 2004 의공학회지 Vol.25 No.6

In this work, a method is presented to find an anatomical site of a current source crudely in a standard brain using grand average of MEG data. Minimum norm estimation algorithm and truncated singular value decomposition were applied to calculate the distributed sources that can reproduce the measured signals. Grand average over all subjects was obtained from the transformed signals, which would be detected in a standard sensor plane by the obtained distributed current sources. In the simulation study, it was shown that the localized dipole using the grand average is consistent with the mean location of localized dipoles of all subjects within several mm even with large inter-individual differences of sensor positions. This result suggests that the mean location of low level signal source can be estimated as a dipole source in grand average and it was confirmed in the localization of the current source of N100m. when the localized dipole is registered on a standard brain. This result also suggests that the activity region obtained from grand average can be crudely estimated on a standard brain using the source location of the N100m as a reference point. 본 연구에서는 뇌자도 신호의 전체 평균을 구하고 표준뇌를 이용하여 전류원의 해부학적 위치를 대략적으로 추정하는 방법을 제시하였다. Minimum norm estimation 알고리듬과 truncated singular value decomposition을 이용하여 측정된 신호를 재현할 수 있는 전류원 분포를 구하고, 이 전류원에 의해 표준 센서면에서 측정될 것으로 예상되는 신호로 변환한 다음 모든 피실험자에 대한 전체 평균을 구하였다. 모의 실험에서는 서로 다른 위치에서 측정된 피실험자의 뇌자도 신호의 전체 평균에서 얻은 전류원이 각 피실험자의 뇌자도 신호에서 얻은 전류원의 평균값과 수 mm 이내에서 일치하였다. 이는 신호가 미약하더라도 활동 부위의 평균적인 위치를 전체 평균에서 알 수 있음을 보여주며 청각 자극에 대한 N100m 신호의 전류원을 구하여 실험적으로 확인하였다. 또한 이 결과는 전체 평균에서 구한 활동부위의 전류원 정보를 N100m의 전류원을 기준점으로 표준뇌에 대략적으로 표시할 수 있음을 보여준다.

소동물용 뇌자도 측정 시스템 개발

김지은,김인선,강찬석,권혁찬,김진목,이용호,김기웅,Kim, J.E.,Kim, I.S.,Kang, C.S.,Kwon, H.,Kim, J.M.,Lee, Y.H.,Kim, K. 한국초전도학회 2011 Progress in superconductivity Vol.13 No.1

We developed a four-channel first order gradiometer system to measure magnetoencephalogram for mice. We used double relaxation oscillation SQUID (DROS). The diameter of the pickup coil is 4 mm and the distance between the coils is 5 mm. Coil distance was designed to have good spatial resolution for a small mouse brain. We evaluated the current dipole localization confidence region for a mouse brain, using the spherical conductor model. The white noise of the measurement system was about 30 fT/$Hz^{1/2}$/cm when measured in a magnetically shielded room. We measured magnetic signal from a phantom having the same size of a mouse brain, which was filled with 0.9% saline solution. The results suggest that the developed system has a feasibility to study the functions of brain of small animals.

허혈에 의한 다채널 심자도의 ST-T 변화

권혁찬,김기웅,이용호,김진목,임현균,박용기,정남식,고영국,정보영,김진배,조정래,Kwon, H.,Kim, K.,Lee, Y.H.,Kim, J.M.,Lim, H.K.,Park, Y.K.,Chung, N.,Ko, Y.G.,Joung, B.,Kim, J.B.,Cho, J.R. 한국초전도학회 2007 Progress in superconductivity Vol.9 No.1

Myocardial ischemia causes heterogeneity of ventricular repolarization and sometimes produces changes of the ST-T wave in ECG. Therefore, morphological changes of ST-T waveform in ECG have a clinical significance in diagnosing myocardial ischemia. In this study, we investigated the ST-T changes caused by myocardial ischemia in magnetocardiography (MCG). We analyzed MCG patterns of biphasic T, ST segment deviations from baseline, main current angle of $T_{peak}$ and $T_{peak}$ dispersion in 300 CAD patients without ST elevation in ECG, 122 symptomatic patients and 48 normal subjects. MCGs were recorded by multichannel SQUID system in a magnetically shielded room. As results, we found that appearances of the abnormality were strongly correlated with the severity of myocardial ischemia. Also we found that the percentage of the patients showing MCG changes were higher than those in ECG. These results show that morphological changes of ST-T waveform in MCG can be used as a marker of myocardial ischemia.

생체자기계측을 위한 자기차폐실의 설치 및 특성

이용호(Y. H. Lee),권혁찬(H. C. Kwon),김진목(J. M. Kim),임청무(C. M. Lim),이상길(S. K. Lee),박용기(Y. K. Park),박종철(J. C. Park) 한국자기학회 1996 韓國磁氣學會誌 Vol.6 No.4

A magnetically shielded room has been constructed for biomagnetic applications. The room has internal dimensions of 2 m(length)×2 m(width)×2.5 m(height) and it consists of high permeability Mumetal and high conductivity aluminum, utilizing ferromagnetic shielding and eddy current shielding. The dc shielding factor around the center of the room is above 60 ㏈, and the ac shielding factors at 1 and 10 ㎐ are larger than 60 and 80 ㏈, respectively. The internal magnetic field noise at 1 ㎐ is 500 fT/√㎐ and at 10 ㎐ is 100 fT/√㎐, and the field gradient noise at 1 ㎐ is below 7 fT/㎝√㎐. Successful measurements of cardiomagnetic fields using SQUID magnetometer and neuromagnetic fields using SQUID gradiometer have been done.

심자도 측정을 위한 SQUID 센서 기술의 개발 현황

이용호,김진목,유권규,김기웅,권혁찬,Lee, Y.H.,Kim, J.M.,Yu, K.K.,Kim, K.,Kwon, H. 한국초전도학회 2011 Progress in superconductivity Vol.13 No.1

Measurement of magnetic signals generated from electric activity of myocardium provides useful information for the functional diagnosis of heart diseases. Key technical component of the magnetocardiography (MCG) technology is SQUID. To measure MCG signals with high signal-to-noise ratio, sensitive SQUID magnetic field sensors are needed. Present magnetic field sensors based on Nb SQUIDs have field sensitivity good enough to measure most of MCG signals. However, for accurate measurement of fine signal pattern or detection of local atrial fibrillation signals, we may need higher field sensitivity. In addition to field sensitivity, economic aspect of the SQUID system is also important. To simplify the SQUID readout electronics, the output voltage or flux-to-voltage transfer of SQUID should be large enough so that direct measurement of SQUID output can be done using room-temperature preamplifiers. Double relaxation oscillation SQUID (DROS), having about 10 times larger flux-to-voltage transfers than those of DC-SQUIDs, was shown to be a good choice to make the electronics compact. For effective cancellation of external noise inside a thin economic shielded room, first-order axial gradiometer with high balance, simple structure and long-baseline is needed. We developed a technology to make the axial gradiometer compact using direct bonding of superconductive wires between pickup coil and input coil. Conventional insert has mechanical support to hold the gradiometer array, and the dewar neck has equal diameter with the dewar bottom. Boiling of the liquid He can generate mechanical vibrations in the gradiometer array due to mechanical connection structure. Elimination of the mechanical support, and direct mounting of the gradiometer array into the dewar bottom can reduce the dewar neck diameter, resulting in the reduction of liquid He consumption.

Classification of magnetocardiographic maps in coronary artery disease diagnosis

권혁찬,김기웅,김진목,이용호,김태은,임현균,고영국,정남식,Kwon H.,Kim K.,Kim J. M.,Lee Y. H.,Kim T. E.,Lim H. K.,Ko Y. G.,Chung N. The Korean Superconductivity Society 2005 Progress in superconductivity Vol.7 No.1

The diagnostic management of patients with chest pain remains a clinical challenge. Magnetocardiography (MCG) has been proposed as a new non-invasive method for detection of myocardial ischemia. To date, however, MCG technique is not intensively introduced for clinical use. One of the main reasons might be the absence of statistically valid and diagnostically clean criteria, which can determine the presence of certain heart disease. In this work, we suggested a new method to classify the diagnostic value of MCG for the detection of coronary artery disease (CAD) in patients with chest pain. MCG was recorded for three groups (healthy subjects and patients without and with CAD) by means of the 64 channel SQUID gradiometer system installed at a hospital. Using four parameters, which were found to be significantly different between groups, we evaluated a probability, in which parameters can be classified into each group based on the distribution function of the parameter in each group. For all parameters, sum of probabilities was compared between groups to determine the presence of CAD. Our classification method shows that the MCG can be a useful tool to predict the presence of CAD with sensitivity and specificity of higher than $80\%$ each.

흉통환자에서 심자도를 이용한 관상동맥질환의 진단

권혁찬,김기웅,김진목,이용호,김태은,임현균,박용기,고영국,정남식,Kwon, H.,Kim, K.,Kim, J.M.,Lee, Y.H.,Kim, T.E.,Lim, H.K.,Park, Y.K.,Ko, Y.G.,Chung, N. 한국초전도학회 2006 Progress in superconductivity Vol.8 No.1

Magnetocardiography(MCG) has been proposed as a novel and non-invasive diagnostic tool for the detection of cardiac electrical abnormality associated with myocardial ischemia. In our previous study, we have proposed a new classification method of MCG parameters, based on the different populations of the parameters between coronary artery disease(CAD) patients, symptomatic patients and healthy volunteers. We used four parameters, representing the directional changes of the electrical activity in the period of an R-ST-T interval. In patients with chest pain and without ST-segment elevation, who were selected consecutively from all patients admitted to the hospital in 2004, the patients with CAD could be classified with a higher sensitivity than conventional methods, showing that the proposed method can be useful for the diagnosis of CAD with MCG. In this study, we examined the validity of the algorithm with the prior probability distribution in diagnosis of new patients admitted to the hospital in 2005. In the results, presence of CAD could be found with sensitivity and specificity of 81.3% and 71.4%, respectively, in patients with chest pain and non-diagnostic ECG findings.