168IEICE TRANS. ELECTRON., VOL.E88–C, NO.2 FEBRUARY 2005

INVITED PAPER Special Section on Superconducting Electronic Devices and Their Applications

Double Relaxation Oscillation SQUID Systems for BiomagneticMultichannel Measurements

Yong-Ho LEE†a), Hyukchan KWON†, Jin-Mok KIM†, Kiwoong KIM†, In-Seon KIM†,and Yong-Ki PARK†, Nonmembers

SUMMARY Multichannel superconducting quantum interference de-vice (SQUID) systems based on double relaxation oscillation SQUIDs(DROS) were developed for measuring magnetocardiography (MCG) andmagnetoencephalography (MEG) signals. Since DROS provides large flux-to-voltage transfer coefficients, about 10 times larger than the DC SQUIDs,direct readout of the SQUID output was possible using compact room-temperature electronics. Using DROSs, we fabricated two types of mul-tichannel systems; a 37-channel magnetometer system with circular sen-sor distribution for measuring radial components of MEG signals, and twoplanar gradiometer systems of 40-channel and 62-channel measuring tan-gential components of MCG or MEG signals. The magnetometer systemhas external feedback to eliminate magnetic coupling with adjacent chan-nels, and reference vector magnetometers were installed to form softwaregradiometers. The field noise of the magnetometers is around 3 fT/

√Hz at

100 Hz inside a magnetically shielded room. The planar gradiometer sys-tems have integrated first-order gradiometer in thin-film form with a base-line of 40 mm. The magnetic field gradient noise of the planar gradiometersis about 1 fT/cm/

√Hz at 100 Hz. The planar gradiometers were arranged

to measure field components tangential to the body surface, providing effi-cient measurement of especially MCG signals with smaller sensor coveragethan the conventional normal component measurements.key words: SQUID, magnetometer, field noise, magnetoencephalography,magnetocardiography

1. Introduction

Magnetometers based on the superconducting quantum in-terference device (SQUID) are very sensitive low-frequencymagnetic field sensors. As the most widespread applica-tion of SQUIDs, measurements of the magnetic fields fromthe human brain or heart provide useful information for thediagnoses of brain functions or heart diseases [1]. SeveralSQUID systems were developed for measuring biomagneticsignals [2], [3], and the noise levels of low-temperature NbSQUIDs are low enough for MCG and MEG measurements.However, the conventional DC SQUIDs have small flux-to-voltage transfer coefficients, typically about 100 µV/Φ0 (Φ0

is the flux quantum, 2.07×10−15 Wb). Thus the SQUID sen-sitivity is limited by the input noise of the room-temperaturepreamplifier if the SQUID output voltage is measured di-rectly with the room-temperature preamplifier. In order todetect the SQUID output voltage without a degradation ofthe SQUID sensitivity by the preamplifier, a very low-noise

Manuscript received June 8, 2004.Manuscript revised August 3, 2004.†The authors are with Biomagnetism Research Center, Korea

Research Institute of Standards and Science, Doryong 1, Yuseong,Daejeon, 305-340, Korea.

a) E-mail: yhlee@kriss.re.kr

preamplifier or ac flux modulation, matching circuit andphase sensitive detection are usually used. These increasethe complexity of the readout system, and thus, severalimproved SQUID schemes were introduced; DC SQUIDwith additional positive feedback, series array of SQUIDs,double relaxation oscillation SQUID (DROS) [4]. Amongthese, DROS provides large transfer coefficients, more than10 times larger than the standard dc SQUIDs, and largemodulation voltages. Therefore, simple flux-locked loopelectronics could be used for SQUID operation [5]. In or-der to develop magnetocardiography (MCG) and magne-toencephalography (MEG) systems with compact readoutelectronics, we fabricated multichannel DROS magnetome-ters and planar gradiometers. In this paper, we describe thefabrication, operation characteristics and biomagnetic appli-cations of DROS multichannel systems.

2. DROS Magnetometer

2.1 Operation Principle of DROS

The schematic circuit drawing of the DROS is shown inFig. 1(a). The DROS consists of a hysteretic dc SQUID (thesignal SQUID) and a hysteretic junction (the reference junc-tion), shunted by a relaxation circuit of a resistor Rsh andan inductor Lsh. In contrast to the DC SQUID, the DROSuses unshunted, hysteretic tunnel junctions. In the originaldesign of DROS, a reference SQUID was used. But, wereplaced it by the reference junction to remove the possi-bility of flux trapping by the reference SQUID and to re-duce number of wires for SQUID operation [6]. In a certainrange of dc bias current Ib, relaxation oscillations can occurwith an oscillation frequency of roughly Rsh/Lsh. Duringthe oscillations, either the signal SQUID or reference junc-tion participates in the oscillations, and the other stays atthe zero voltage state. Since the frequency bandwidth ofroom-temperature dc preamplifier (typically several MHz)is much narrower than the oscillation frequency of around1 GHz, time-averaged dc voltage can be measured. Sincethe critical current of the signal SQUID modulates betweenIc1,max and Ic1,min by changing the signal flux through it, theconstant reference critical current Ic2 should be in-betweenIc1,max and Ic1,min. If Ic1 > Ic2, the reference junction par-ticipates in the oscillations, and since we measure the out-put voltage across the reference junction, voltage drop ap-pears. On the other hand, if Ic1 < Ic2, the signal SQUID

Copyright c© 2005 The Institute of Electronics, Information and Communication Engineers

LEE et al.: DOUBLE RELAXATION OSCILLATION SQUID169

(a)

(b)

(c)

Fig. 1 Schematic circuit diagram and structure of the DROS. (a)Schematic circuit drawing of the DROS, (b) close-up view of the DROSarea, and (c) overall structure of the magnetometer.

is in the oscillation, and the reference junction stays in thesuperconducting state, resulting in no voltage output. If thesignal flux value is changed that Ic1=Ic2, the two critical cur-rents have equal probability to oscillate, and abrupt changein voltage output occurs with large flux-to-voltage transfercoefficient.

2.2 Design of DROS Magnetometer

The close-up view of the DROS is shown in Fig. 1(b).

The signal SQUID is a Ketchen-type DC SQUID with twosquare holes connected in parallel. By using parallel con-nection, we could reduce the number of input coil turns nec-essary for inductance matching with the pickup coil. Eachsquare hole has an inner dimension of 100 µm × 100 µm.The total SQUID inductance was estimated to be 104 pH,including the parasitic inductance due to slit structure andjunction inductance. The Josephson junctions in the signalSQUID have sizes of 4 µm × 4 µm each, whereas the refer-ence junction has a size of 5 µm × 5 µm. In terms of thearea ratio, the reference critical current corresponds to 78%of the maximum value of the signal critical current, whichis near the mid point of the modulation range of the signalcritical current when modulation parameter βL is 1.

The overall structure of the magnetometer is shown inFig. 1(c). The size of the magnetometer is 16 mm × 10 mm,and the pickup coil has a linewidth of 0.5 mm. The induc-tance of the pickup coil is calculated to be 40 nH. In order toeliminate magnetic crosstalk with adjacent channels, exter-nal feedback scheme was used. The feedback coil is formedon the opposite part of the DROS. The mutual inductancebetween the feedback coil and flux transformer Mf is 3 nH.

The input coil consists of two series-connected coils,10-turns each with a linewidth of 5 µm, integrated on eachSQUID loop and the input coil inductance is calculated tobe 37 nH. The mutual inductance between the input coiland the SQUID is estimated to be 1.76 nH. The field-to-fluxtransfer of the flux transformer circuit was calculated to be0.56 nT/Φ0.

Since the intrinsic noise of the DROS is inversely pro-portional to the square root of the relaxation frequency, therelaxation frequency was designed to be as high as about1 GHz [7]. With the reference critical current of 10 µA,the intrinsic flux noise of the DROS is calculated to be1 µΦ0/

√Hz. The shunt resistance Rsh is 1.5Ω and shunt in-

ductance Lsh is 2 nH, thus the time constant of the relaxationcircuit is 1.3 ns. The shunt inductor consists of two figure-of-eight coils, connected in series and formed symmetricallywith respect to the signal SQUID to eliminate magnetic cou-pling with the signal SQUID as shown in Fig. 1(b).

In order to provide stable operation condition of theDROS, some damping circuits were used. First, since theDROS has hysteretic junctions, a parasitic resonance due tothe junction capacitance and the shunt inductance Lsh canoccur. To damp this LC resonance, a damping resistor Rd

(=50Ω) was used across the signal SQUID and the referencejunction. Second, the signal SQUID washer acts as a groundplane for the input coil, giving rise to a resonance. To dampthis washer resonance, a damping resistor Rw (=5Ω) wasinserted across the signal SQUID loop. Third, a dampingcircuit made of a resistor Rx (=20Ω) and a capacitor Cx

(=0.4 nF) was put in the flux transformer to damp the res-onance in the input coil, and to filter out high-frequencynoises from the pickup coil.

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Fig. 2 Flux-voltage curves of DROS at several bias currents.

2.3 Fabrication of Magnetometers

Since the DROSs are based on hysteretic low-Tc junc-tions, sensors were fabricated using the Nb/AlOx/Nb junc-tion technology. The Nb/AlOx/Nb trilayers were depositedby dc magnetron sputtering, and the junction areas were de-fined by reactive ion etching. The insulator between metallayers is an SiO2 film deposited by plasma-enhanced chem-ical vapor deposition. The resistor is made of reliable Pdthin film. 14 magnetometers were fabricated on 3-inch Siwafer at the same time. We used the 4-level process with5 photomasks. The fabricated magnetometers were gluedonto printed circuit board sensor holders having a diameterof 22 mm, where wiring copper leads were printed with non-magnetic connectors. Each magnetometer holder has a plas-tic protective cap for hermetic sealing and easy-handling,allowing quick attachment or removal from the insert.

2.4 Characteristics of Magnetometers

The magnetometers were cooled by inserting directly into afiberglass liquid helium dewar. Measurements were done in-side a magnetically shielded room (MSR) with two layers ofMu-metal and one layer of Al, which has shielding factorsof about 60 dB at 1 Hz and 90 dB at 100 Hz. The magne-tometers had reference critical currents of 10–20 µA, and themaximum modulation voltages of around 100 µV, which isabout 2 times larger than the DC SQUIDs. The flux-voltagecurves showed almost a step function of the applied fieldas shown in Fig. 2. The maximum flux-to-voltage transferswere around 1 mV/Φ0, which is about 10 times larger thanthe transfer of DC SQUIDs. Using a calibration field coil,the field-to-flux transfer coefficient of the flux transformerwas measured to be 0.54 nT/Φ0, close to the design value of0.56 nT/Φ0.

2.5 Readout Electronics

The single channel electronics for flux-locked loop (FLL)operation and SQUID control consist of a dc bias current,preamplifier, main amplifier, integrator and control circuits,

Fig. 3 Simplified circuit diagram of flux-locked loop circuit.

as shown in Fig. 3. Since the fabricated DROS magne-tometers had large flux-to-voltage transfers, the DROS out-put voltage was measured directly by room-temperature dcpreamplifiers without using matching circuits and ac fluxmodulation.

DC preamplifiers were fabricated using the commonLT1028 op amps (Linear Technology). The input voltagenoise of the preamplifiers was about 1.5 nV/

√Hz at 100 Hz.

With a typical transfer coefficient of 1 mV/Φ0, the pream-plifier contributes an equivalent flux noise of 1.5µΦ0/

√Hz

at 100 Hz. Since the SQUID system noise is typically5 µΦ0/

√Hz at 100 Hz, the contribution of the preamplifier

to the total SQUID system noise is about 10% [8]. Due tothe large modulation voltage of DROSs, the FLL operationwas quite stable against the offset voltage drift of the ampli-fier chain. The operation margin for the offset drift is about±15 µV around the center of modulation voltage. This op-eration margin for the offset voltage is about 3 times largerthan the dc SQUID with additional positive feedback [9].

The control of the SQUID operation could be done ei-ther manually or automatically. In the automatic mode, thesoftware controls the bias current, the voltage offset at theintegrator, and the flux bias. The optimum bias current wasdetermined by the measured white noise. The electronics ofthe magnetometer system consists of 6 boxes for FLL cir-cuits and 3 sub-racks for SQUID controls. Each FLL boxhas 8 modular FLL circuits and was fixed on the ceiling ofthe MSR. The control electronics are within an RF-shieldedAl rack located outside the MSR. The FLL outputs can bepassed selectively through 0.3-Hz high pass filters, 100-Hzlow pass filters and 60-Hz notch filters, and gain-adjustableamplifiers. When the magnetometer was operated inside theMSR without using an additional superconductive shield,the SQUID system noise is about 3 fT/

√Hz at 100 Hz, in-

cluding all the noise contributions, like the residual mag-netic noise of the MSR and the dewar thermal noise.

2.6 37-Channel Magnetometer Insert

The magnetometer system consists of 37 magnetometers tomeasure MEG signals and noise fields at the same time, and11 reference magnetometers to measure noises. Fig. 4 showsthe photograph of the magnetometer system insert. The 37magnetometers were distributed on a semispherical surface

LEE et al.: DOUBLE RELAXATION OSCILLATION SQUID171

Fig. 4 Photograph of 37-channel magnetometer insert.

with a radius of 125 mm and measure the field componentnormal to the head surface. The distance between adjacentmagnetometers is 26 mm, and the 37 channels cover 160 mmin diameter. The reference magnetometers are located be-tween 50 mm and 90 mm above the signal channels. Usingthe 11 reference channels, it is possible to measure 3 vectorfields, Bx, By, and Bz, and 5 first-order gradients, dBx/dx,dBx/dy, dBx/dz, dBy/dy, and dBy/dz. By using these refer-ence signals, first-order or second-order software gradiome-ters can be formed.

The liquid helium dewar has a liquid volume of 29 Land boil-off rate of the liquid helium is about 3.5 L/d withthe insert. The distance between the room temperature andthe liquid helium is 20 mm.

2.7 Application to MEG Measurements

To demonstrate the usefulness of the 37-channel systemfor measuring MEG signals, auditory-evoked response wasmeasured. The measurement consists of DC offset removal,low-pass filter, 60-Hz removal filter, averaging, syntheticsoftware gradiometer, field mapping and source localiza-tion. To apply non-magnetic auditory stimuli, a capaci-tive earphone was used and the auditory signal lines weretwisted in pair. Auditory stimuli of a 1-kHz tone burst, 170-ms duration, and about 70-dB normal hearing level were ap-plied to the right ear of a normal human subject in a ran-dom interval, and fields were measured over the left tem-poral lobe. Sampling rate was 500 Hz. Fig. 5 shows theauditory-evoked signals with the traces superposed together.The signals were processed by applying the modified prin-cipal component elimination method [10], averaging of 100times, and a digital 40-Hz low pass filter. The hardwarehigh-pass, low-pass and 60-Hz removal filters in the SQUIDcontrollers were not used.

Though the signal was obtained without using the soft-ware gradiometer, clear N100m peak, the field componentgenerated at about 100 ms after the stimulus onset, was ob-tained. This peak corresponds to the primary response ofthe auditory cortex to the sound stimulus. The isofield con-tour map of the N100m peak showed a dipolar field pattern,

Fig. 5 Auditory-evoked field. The hatched box on the time axis showsthe stimulus duration.

and source localization using simulated annealing algorithmshowed that the estimated current dipole is located in theprimary auditory cortex.

3. DROS Planar Gradiometer

3.1 Structure of Planar Gradiometer

The overall structure of the planar gradiometer is shown inFig. 6. The basic design and parameters of DROS are verysimilar to those of DROS magnetometer. The inductanceof the signal SQUID is 113 pH, and internal feedback isused with a 1-turn feedback coil formed on the outer partof the SQUID loop. The input coil consists of two series-connected coils, 15-turns each with a linewidth of 5 µm,integrated on each SQUID loop and the input coil induc-tance is calculated to be 87 nH. Assuming the coupling co-efficient with the SQUID to be 0.9, the mutual inductancebetween the input coil and the SQUID is estimated to be2.8 nH. The pickup coil is a first-order gradiometer with twosquare coils connected in series, each with an outer dimen-sion of 12 mm × 12 mm. The linewidth of the pickup coilis 0.5 mm and the baseline is 40 mm. The field gradient-to-flux transfer of the flux transformer circuit is 0.24 nT/cm/Φ0

or 0.95 nT/Φ0. The overall size of the planar gradiometer is12 mm × 52 mm [11]. The planar gradiometers were fabri-cated using the same Nb junction technology used to fabri-cate magnetometers.

3.2 40-Channel Planar Gradiometer System

The readout electronics for the operation of planar gra-diometers are the same as those used for magnetometers.The 40-channel insert consists of 16 rectangular epoxy rods,and each rod has 2∼4 gradiometers in the x- and y-direction.The distance between two parallel gradiometers is 25 mmor 21 mm. The gradiometers were arranged to measurefield components tangential to the body surface; dBx/dzand dBy/dz, where z-axis is normal to the body surface. Atangential gradiometer sensitive to off-diagonal derivativehas a magnetic field peak just above the current dipole when

172IEICE TRANS. ELECTRON., VOL.E88–C, NO.2 FEBRUARY 2005

Fig. 6 Structure of DROS planar gradiometer.

(a) (b)

Fig. 7 Grand average of the field distribution of (a) 400 ms and (b)600 ms components in morphosyntactic violation condition. Bright regionsin (a) and (b) mean the field maxima. The grey scales in (a) and (b) arenormalized by the maximum field of 50 fT and 95 fT, respectively.

the pickup coil surface is arranged parallel to the dipole di-rection [12]. Thus, a less-extensive sensor array is neededto get the essential field distribution. In addition to the 40sensing channels, 4 reference channels were placed at 13 cmdistance from the sensing channels to pickup backgroundnoises and to apply adaptive filtering.

When the planar gradiometers were operated insidethe MSR, the field gradient noises were 2 fT/cm/

√Hz and

1 fT/cm/√

Hz at 1 Hz and 100 Hz, respectively. Multipliedby the baseline, these correspond to the field noises of8 fT/

√Hz and 4 fT/

√Hz at 1 Hz and 100 Hz, respectively.

3.3 Applications to MEG Measurements

Since the 40-channel system has flat-bottom tail, it can bepositioned above the temporal lobe to measure the auditory-evoked responses. When auditory stimuli of a 1-kHz toneburst, 170-ms duration, and about 70-dB normal hearinglevel were applied to the normal subject, we also could ob-tain clear N100m peak. In order to apply the system tothe functional study of the brain, auditory evoked responsesto morpho-syntactic and semantic violations of Korean sen-tences were measured.

Measured field data were transformed to the fields thatwould be detected on a standard sensing plane and averagedacross subjects in different violation conditions. Figs. 7(a)and (b) show the grand average of the tangential field distri-bution of 400 and 600 ms components in morpho-syntacticviolation condition, respectively. Here, the grey scale in (a)and (b) was normalized by the maximum field of 50 and95 fT, respectively, and bright region indicates the maxi-mum. Equivalent current dipoles plotted on a standard brainindicated the inferior frontal region and the superior tem-poral gyrus for 400 ms component, and the middle tempo-

Fig. 8 Arrangement of 62-channel planar gradiometers.

ral gyrus for 600 ms component. Our MEG study localizedthe distinct cortical regions involved in syntactic processes,which may be reflected in Left Anterior Negativity and P600component of event related potentials [13].

3.4 62-Channel Planar Gradiometer System

A 62-channel planar gradiometer insert was fabricated tomeasure MCG signals. The sensor holder is made of epoxyblock where planar gradiometers are attached as shown inFig. 8. The 62-channel insert consists of 21 sensor holdersand each sensor holder has 2∼4 planar gradiometers. Thestandard distance between parallel gradiometers is 35 mm.In addition to the 62 sensing channels, 10 reference chan-nels were installed at a distance of 135 mm from the sensingchannels, and optional software higher-order gradiometerscould be formed to reduce environmental noises [14]. Thesenor coverage area of the 62-channel is 162 mm × 162 mm,which seems to be insufficient compared with the conven-tional MCG system measuring normal-component. How-ever, a simulation study showed that tangential measure-ment can localize current dipoles deeper than normal mea-surement with the same confidence volume [15]. It wasreported that the MCG obtained using this kind of sensordistribution made it easy to estimate visually the electro-physiological behavior of the heart [12]. Fig. 9 comparesthe confidence region diameter in the longitudinal currentdirection between tangential measurement and normal mea-surement. We can see that the normal measurement has anarrower confidence region diameter than tangential mea-surement below 3-cm depth from the chest surface, mean-ing that tangential measurement can localize current dipolesdeeper than the normal measurement with the same confi-dence volume.

The liquid helium dewar has an internal tail diameterof 192 mm and the distance between the liquid helium androom temperature is 20 mm. The liquid capacity is 40 L andthe average boil-off rate is 3.6 L/d in everyday operation, al-lowing the refill once a week. The bed can be moved verti-cally and horizontally to adjust the measurement positions,

LEE et al.: DOUBLE RELAXATION OSCILLATION SQUID173

Fig. 9 Confidence region diameters as a function of depth from the chestsurface for the tangential measurement and normal measurement.

Fig. 10 Software for MCG measurement. (a) Magnetic field map of thenormal component of p-wave, (b) current arrow map of p-wave and (c)superimposed MCG traces. The magnetic fields at the positive pole (+)and the negative pole (−) are about 1.2 pT and −0.6 pT, respectively.

and a nonmagnetic cycle was installed for exercise MCGmeasurements.

MCG data was collected at a sampling rate of typi-cally 1 kHz using a 16-bit, 64-channel A/D card and a per-sonal Windows-based computer. Typical recoding time is 1minute for rest MCG measurements. The signal processingsoftware provides digital filtering, averaging, synthetic gra-diometer formation and baseline correction using the mor-phological filtering [16]. The analysis software providescurrent arrow mapping and the transformed field distribu-tion simulating a sensor system measuring the normal com-ponents of the fields. Based on the current maps and thetransformed field maps, several diagnostic parameters com-patible with those of sensor systems measuring normal com-ponents can be obtained and compared for clinical applica-tions.

Although there are a couple of simple ways to trans-form tangential components to normal components [17], the

method merely gives information of the direction of currentvector corresponding to the field pattern direction. To con-sider more detailed information such as the effect of volumecurrents, we utilized minimum-norm estimate (MNE) of thesource current distribution [18]. Fig. 10 is the dynamic viewprogram which shows the field map (normal component),current arrow map and MCG waveform. The MCG system,together with a magnetically shielded room, was installed ina heart center of a hospital to study myocardiac ischemia.

4. Conclusion

We developed DROS magnetometer and DROS planar gra-diometer systems, and applied to measure biomagnetic mea-surements. Fabricated DROSs provided flux-to-voltagetransfers of typically 1 mV/Φ0, which was large enough thatroom-temperature dc preamplifiers could be used for mea-suring SQUID voltage output. Though the input voltagenoise of the preamplifier was not very low, the contribu-tion of the FLL electronics to the system noise was neg-ligible, about 10% of the system noise. The magnetome-ters have average noise level of 3 fT/

√Hz at 100 Hz, and

the 37-channel magnetometer system was applied to mea-sure auditory evoked responses. The planar gradiometershave noise level of 1 fT/cm/

√Hz at 100 Hz. Two planar gra-

diometer systems, 40-channel and 62-channel systems, werefabricated and applied to measure MEG and MCG signalssuccessfully.

The developed 62-channel MCG system was largeenough to obtain the whole information from the heart ina single measurement. Simulation showed that the tangen-tial measurement can localize current dipoles deeper thanthe normal measurement with the same confidence volume.Thus, by measuring the tangential component, the sensorcoverage can be smaller than the normal measurement, re-ducing the dewar tail diameter, and the boil-off rate of thedewar and maintenance cost.

Acknowledgments

This work was supported in part by the NRL project of theMinistry of Science and Technology, Korea.

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Yong-Ho Lee received the B.S. in Physicsfrom Kyungpook National University in 1984,and M.S. and Ph.D. degrees in Physics from Ko-rea Advanced Institute of Science and Technol-ogy in 1986 and 1989, respectively. Since 1989,he has been working in Korea Research Instituteof Standards and Science to study SQUID sen-sors and biomagnetic measurements.

Hyukchan Kwon received the B.S. andM.S. degrees in Nuclear Engineering fromSeoul National University in 1979 and 1981, re-spectively. Since 1981, he has been working inKorea Research Institute of Standards and Sci-ence to study SQUID sensors and biomagneticmeasurements.

Jin-Mok Kim received the B.S. in ElectricalEngineering from Kyungpook National Univer-sity in 1984. Since 1984, he has been working inKorea Research Institute of Standards and Sci-ence to develop SQUID electronics.

Kiwoong Kim received the B.S., M.S. andPh.D. degrees in Physics from Korea AdvancedInstitute of Science and Technology in 1995,1997 and 2002, respectively. Since 2002, hehas been working in Korea Research Institute ofStandards and Science to study biomagnetic sig-nal processing and analysis.

In-Seon Kim received the B.S. and M.S.degrees in Electrical Engineering from Kyung-pook National University in 1980 and 1982, re-spectively, and Ph.D. degree in Material Sciencefrom Tokyo Institute of Technology in 1993.He joined Korea Research Institute of Standardsand Science in 1984. He is currently develop-ing high-Tc SQUID sensors and systems for bio-magnetism.

Yong-Ki Park received the B.S., M.S. andPh.D. degrees in Material Science from SeoulNational University in 1975, Korea AdvancedInstitute of Science and Technology in 1977and Northwestern University in 1985, respec-tively. Since 1985, he has been working in Ko-rea Research Institute of Standards and Scienceto study SQUID sensors and biomagnetic mea-surements.