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Location of somatosensory-evoked dipoles studied by a SQUID system in a superconducting magnetic shield
International Congress Series 1300 (2007) 387 – 390
www.ics-elsevier.com
Location of somatosensory-evoked dipoles studied by a SQUID system in a superconducting magnetic shield Hiroshi Ohta a,⁎, Toshiaki Matsui a , Yoshinori Uchikawa b a
National Institute of Information and Communications Technology, Tokyo, 184-8795 Japan b Tokyo Denki University, Hatoyama, Saitama, 350-0394 Japan
Abstract. A whole-head SQUID system in a superconducting magnetic shield was used to study somatosensory response experimentally. The low-noise data obtained by the 64-channel SQUID system in the superconducting magnetic shield allowed a reliable application of the MUSIC (multiple signal classification) estimation of dipoles. © 2007 Published by Elsevier B.V. Keywords: Superconducting magnetic shield; SQUID; SEF; N20; MUSIC; Multiple signal classification
1. Introduction This is a report on neuromagnetic SQUID measurement in a superconducting magnetic shield as shown in Fig. 1. The superconducting magnetic shield gives us a very low-noise SQUID system as shown in Fig. 2 The low-noise data help us estimate current dipoles by MUSIC (multiple signal classification approach). 2. Noise spectra of a SQUID system in a superconducting magnetic shield Fig. 2 shows that the SQUID has a very low noise in the superconducting magnetic shield especially below 1 Hz. This is important because most ambient magnetic noises are
⁎ Corresponding author. Nukuikita-machi 4-2-1, Koganei, Tokyo, 184-8795, Japan. Tel.: +81 42 327 7934; fax: +81 42 327 6669. E-mail address: [email protected] (H. Ohta). 0531-5131/ © 2007 Published by Elsevier B.V. doi:10.1016/j.ics.2006.12.086
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H. Ohta et al. / International Congress Series 1300 (2007) 387–390
Fig. 1. A whole-head SQUID system in a superconducting magnetic shield.
largest below 1 Hz. The best sensitivity of the SQUID system of SNS junctions is 0.9 fT/ Hz− 1 / 2 in the Johnson noise limit in a superconducting magnetic shield. 3. Neuromagnetic SQUID measurement The median nerve in the right wrist was stimulated by current pulses. Somatosensoryevoked magnetic field was measured by the 64-channel whole-head SQUID magnetometer. The first order gradiometers have a diameter of 18 mm and a length of 30 mm. Some information about the SQUID is given in [1–4]. Fig. 3 shows the data of the 64-channel SQUID system in the superconducting magnetic shield. The figure shows 64 traces of each channel versus the common time axis. The arrow indicates the N20 response at 20 ms after somatosensory stimulation. The signal of N20 is very weak because the stimulation by current pulses was very mild. We estimated the
Fig. 2. Noise spectra of a SQUID in the superconducting magnetic shield and in a magnetically-shielded room of Permalloy. Compare the noise spectrum (a) in the superconducting magnetic shield and the noise spectrum (c) in the magnetically-shielded room. The SQUID system in the superconducting magnetic shield is more than 100 times more sensitive than those in a magnetically shielded room of Permalloy below 1 Hz. The noise spectrum (b) was obtained when both the SQUID sensor and the pick-up coil were tightly wrapped by a superconducting lead (Pb) foil. Therefore the noise spectrum (b) indicates the intrinsic noise of the SQUID sensor. The data of (d) is cited from [5] to indicate the intrinsic noise of their SQUID sensors.
H. Ohta et al. / International Congress Series 1300 (2007) 387–390
389
Fig. 3. Data of the 64-channel SQUID system in the superconducting magnetic shield. The figure shows 64 traces of each channel versus the common time axis. The arrow indicates the N20 at 20 ms after somatosensory stimulation. Two hundred events are averaged.
evoked dipole in the subject's brain by an extension of a radar theory — MUSIC (multiple signal classification approach). 4. Low-noise data support MUSIC (multiple signal classification) Music is not very new. M. Hamalainen et al. [6] pointed out in 1993 the importance of the early MUSIC paper by J. C. Mosher et al. published in 1992 [7]. Nevertheless the MUSIC approach was not very popular until recently. The reason for this could be that there are few studies which confirm advantage of MUSIC over traditional estimation of dipoles.
Fig. 4. Comparison between MUSIC and the traditional least-squares fit of dipoles for N20. Agreement of the locations of the estimated dipoles is excellent between MUSIC and the traditional least-squares fit.
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H. Ohta et al. / International Congress Series 1300 (2007) 387–390
The figure in the left-hand side of Fig. 4 shows a result of the MUSIC estimation of dipoles while the figure in the right-hand side of Fig. 4 shows a result of the traditional iterative least-squares estimation of dipoles. Fig. 4 indicates the following: 1) The N20 component of the somatosensory-evoked magnetic field data recorded with the SQUID system in the superconducting magnetic shield has a large enough Signal-to-Noise ratio for MUSIC estimation of dipoles because of the superconducting magnetic shield. 2) The dipole estimated by MUSIC locates close to the location of the dipole calculated by the traditional least-squares fitting of dipoles. MUSIC estimates the signal subspace of large singular values and the noise subspace of small singular values from the correlation matrix of observed magnetic field practically. The orthogonality of gain vectors to the noise subspace determines location of dipoles. Therefore MUSIC gives wrong estimation of dipoles if observed data contain large noise. On the other hand the traditional least-squares approach can avoid using the noise subspace. Generally, however, the results of the iterative least-squares fitting of dipoles depend on the initial guess of the parameters. Furthermore, the traditional least-squares approach may use the results of MUSIC as initial conditions. Poor Signal-to-Noise ratios of used data have so far masked advantage of MUSIC over the traditional least-squares estimation. The low-noise data support promising MUSIC estimation of dipoles encouragingly. 5. Conclusion Sensitivities of the SQUID system of SNS junctions in a superconducting magnetic shield are more than 100 times better than sensitivities of the same SQUID's in a magnetically shielded room of Permalloy. We performed a standard experiment of somatosensory-evoked magnetic field of human brains in the superconducting magnetic shield. Estimated locations of the evoked current dipole for N20 by an extension of a radar theory named MUSIC(multiple signal classification approach) is confirmed to be close to the locations of the current estimated by a traditional least-squares fitting. Our results suggest that low-noise data obtained with the SQUID system in a superconducting magnetic shield is well suited for a reliable application of the MUSIC approach. References [1] H. Ohta, et al., Neuromagnetic SQUID measurement in a superconducting magnetic shield, IEEE Transactions on Applied Superconductivity 9 (1999) 407. [2] H. Ohta, et al., 64-channel whole-head SQUID system in a superconducting magnetic shield, Superconductor Science and Technology 12 (12) (1999) 763. [3] H. Ohta, T. Matsui, Whole-head SQUID system in a superconducting magnetic shield, Physica C: Superconductivity 341–348 (2000) 2713. [4] H. Ohta, et al., Neuromagnetic SQUID measurements in a helmet-type superconducting magnetic shield of BSCCO, IEEE Transactions on Applied Superconductivity 3 (1) (1993) 1953. [5] D. Drung, S. Bechstein, K.P. Franke, Th. Schurig, Improved direct-coupled dc SQUID read-out electronics with automatic bias voltage tuning, IEEE Transactions on Applied Superconductivity 11 (March 2001) 880. [6] M. Hamalainen, et al., Magnetoencephalography, Reviews of Modern Physics 65 (2) (April 1993) 413. [7] J.C. Mosher, P.S. Lewis, R.M. Leahy, Multiple dipole modeling and localization from spatio-temporal MEG data, IEEE Transactions on Biomedical Engineering 39 (6) (1992) 541.
www.ics-elsevier.com
Location of somatosensory-evoked dipoles studied by a SQUID system in a superconducting magnetic shield Hiroshi Ohta a,⁎, Toshiaki Matsui a , Yoshinori Uchikawa b a
National Institute of Information and Communications Technology, Tokyo, 184-8795 Japan b Tokyo Denki University, Hatoyama, Saitama, 350-0394 Japan
Abstract. A whole-head SQUID system in a superconducting magnetic shield was used to study somatosensory response experimentally. The low-noise data obtained by the 64-channel SQUID system in the superconducting magnetic shield allowed a reliable application of the MUSIC (multiple signal classification) estimation of dipoles. © 2007 Published by Elsevier B.V. Keywords: Superconducting magnetic shield; SQUID; SEF; N20; MUSIC; Multiple signal classification
1. Introduction This is a report on neuromagnetic SQUID measurement in a superconducting magnetic shield as shown in Fig. 1. The superconducting magnetic shield gives us a very low-noise SQUID system as shown in Fig. 2 The low-noise data help us estimate current dipoles by MUSIC (multiple signal classification approach). 2. Noise spectra of a SQUID system in a superconducting magnetic shield Fig. 2 shows that the SQUID has a very low noise in the superconducting magnetic shield especially below 1 Hz. This is important because most ambient magnetic noises are
⁎ Corresponding author. Nukuikita-machi 4-2-1, Koganei, Tokyo, 184-8795, Japan. Tel.: +81 42 327 7934; fax: +81 42 327 6669. E-mail address: [email protected] (H. Ohta). 0531-5131/ © 2007 Published by Elsevier B.V. doi:10.1016/j.ics.2006.12.086
388
H. Ohta et al. / International Congress Series 1300 (2007) 387–390
Fig. 1. A whole-head SQUID system in a superconducting magnetic shield.
largest below 1 Hz. The best sensitivity of the SQUID system of SNS junctions is 0.9 fT/ Hz− 1 / 2 in the Johnson noise limit in a superconducting magnetic shield. 3. Neuromagnetic SQUID measurement The median nerve in the right wrist was stimulated by current pulses. Somatosensoryevoked magnetic field was measured by the 64-channel whole-head SQUID magnetometer. The first order gradiometers have a diameter of 18 mm and a length of 30 mm. Some information about the SQUID is given in [1–4]. Fig. 3 shows the data of the 64-channel SQUID system in the superconducting magnetic shield. The figure shows 64 traces of each channel versus the common time axis. The arrow indicates the N20 response at 20 ms after somatosensory stimulation. The signal of N20 is very weak because the stimulation by current pulses was very mild. We estimated the
Fig. 2. Noise spectra of a SQUID in the superconducting magnetic shield and in a magnetically-shielded room of Permalloy. Compare the noise spectrum (a) in the superconducting magnetic shield and the noise spectrum (c) in the magnetically-shielded room. The SQUID system in the superconducting magnetic shield is more than 100 times more sensitive than those in a magnetically shielded room of Permalloy below 1 Hz. The noise spectrum (b) was obtained when both the SQUID sensor and the pick-up coil were tightly wrapped by a superconducting lead (Pb) foil. Therefore the noise spectrum (b) indicates the intrinsic noise of the SQUID sensor. The data of (d) is cited from [5] to indicate the intrinsic noise of their SQUID sensors.
H. Ohta et al. / International Congress Series 1300 (2007) 387–390
389
Fig. 3. Data of the 64-channel SQUID system in the superconducting magnetic shield. The figure shows 64 traces of each channel versus the common time axis. The arrow indicates the N20 at 20 ms after somatosensory stimulation. Two hundred events are averaged.
evoked dipole in the subject's brain by an extension of a radar theory — MUSIC (multiple signal classification approach). 4. Low-noise data support MUSIC (multiple signal classification) Music is not very new. M. Hamalainen et al. [6] pointed out in 1993 the importance of the early MUSIC paper by J. C. Mosher et al. published in 1992 [7]. Nevertheless the MUSIC approach was not very popular until recently. The reason for this could be that there are few studies which confirm advantage of MUSIC over traditional estimation of dipoles.
Fig. 4. Comparison between MUSIC and the traditional least-squares fit of dipoles for N20. Agreement of the locations of the estimated dipoles is excellent between MUSIC and the traditional least-squares fit.
390
H. Ohta et al. / International Congress Series 1300 (2007) 387–390
The figure in the left-hand side of Fig. 4 shows a result of the MUSIC estimation of dipoles while the figure in the right-hand side of Fig. 4 shows a result of the traditional iterative least-squares estimation of dipoles. Fig. 4 indicates the following: 1) The N20 component of the somatosensory-evoked magnetic field data recorded with the SQUID system in the superconducting magnetic shield has a large enough Signal-to-Noise ratio for MUSIC estimation of dipoles because of the superconducting magnetic shield. 2) The dipole estimated by MUSIC locates close to the location of the dipole calculated by the traditional least-squares fitting of dipoles. MUSIC estimates the signal subspace of large singular values and the noise subspace of small singular values from the correlation matrix of observed magnetic field practically. The orthogonality of gain vectors to the noise subspace determines location of dipoles. Therefore MUSIC gives wrong estimation of dipoles if observed data contain large noise. On the other hand the traditional least-squares approach can avoid using the noise subspace. Generally, however, the results of the iterative least-squares fitting of dipoles depend on the initial guess of the parameters. Furthermore, the traditional least-squares approach may use the results of MUSIC as initial conditions. Poor Signal-to-Noise ratios of used data have so far masked advantage of MUSIC over the traditional least-squares estimation. The low-noise data support promising MUSIC estimation of dipoles encouragingly. 5. Conclusion Sensitivities of the SQUID system of SNS junctions in a superconducting magnetic shield are more than 100 times better than sensitivities of the same SQUID's in a magnetically shielded room of Permalloy. We performed a standard experiment of somatosensory-evoked magnetic field of human brains in the superconducting magnetic shield. Estimated locations of the evoked current dipole for N20 by an extension of a radar theory named MUSIC(multiple signal classification approach) is confirmed to be close to the locations of the current estimated by a traditional least-squares fitting. Our results suggest that low-noise data obtained with the SQUID system in a superconducting magnetic shield is well suited for a reliable application of the MUSIC approach. References [1] H. Ohta, et al., Neuromagnetic SQUID measurement in a superconducting magnetic shield, IEEE Transactions on Applied Superconductivity 9 (1999) 407. [2] H. Ohta, et al., 64-channel whole-head SQUID system in a superconducting magnetic shield, Superconductor Science and Technology 12 (12) (1999) 763. [3] H. Ohta, T. Matsui, Whole-head SQUID system in a superconducting magnetic shield, Physica C: Superconductivity 341–348 (2000) 2713. [4] H. Ohta, et al., Neuromagnetic SQUID measurements in a helmet-type superconducting magnetic shield of BSCCO, IEEE Transactions on Applied Superconductivity 3 (1) (1993) 1953. [5] D. Drung, S. Bechstein, K.P. Franke, Th. Schurig, Improved direct-coupled dc SQUID read-out electronics with automatic bias voltage tuning, IEEE Transactions on Applied Superconductivity 11 (March 2001) 880. [6] M. Hamalainen, et al., Magnetoencephalography, Reviews of Modern Physics 65 (2) (April 1993) 413. [7] J.C. Mosher, P.S. Lewis, R.M. Leahy, Multiple dipole modeling and localization from spatio-temporal MEG data, IEEE Transactions on Biomedical Engineering 39 (6) (1992) 541.