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Whole-head SQUID system in a superconducting magnetic shield
ELSEVIER
Physica C 341-348 (2000) 2713-2714
www.elsevier.nl/locate/physc
Whole-head SQUID system in a superconducting magnetic shield H. Ohta a* and T. Matsui b aCommunications Research Laboratory, Ministry of Posts and Telecommunications, Nukuikita-machi 21-2-1, Koganei-shi 184-0015, Tokyo, Japan bCommunications Research Laboratory, Ministry of Posts and Telecommunications A whole-head 64-channel SQUID magnetometer of SNS (Superconductor/ Normal metal / Superconductor) junctions has been constructed to be operated in a superconducting magnetic shield of Bi(Pb)SrCaCuOx demonstrating the sensitivity of 5 femto Tesla in our laboratory only 10 meter far from both an elevator and power transformer banks. Signal-to-noise ratio of data in neuromagnetic measurements is exellent because a SQUID of SNS junctions has a smaller telegraph noise than that of SIS tunnel junctions and because shielding factors of the superconducting magnetic shield does not reduce even at as low frequencies as 0.05 Hz unlike that of Permalloy shield room. A movie is made to compare the evoked current dipole in the MEG data with the MRI images.
1. I N T R O D U C T I O N The whole-head SQUID system in a superconducting magnetic shield has yielded excellent SN ratio of data in neuromagnetic measurements according to our movies about topographies of magnetic field above human brains. We describe characteristics of the superconducting magnetic shield, a 64-channel whole-head SQUID of SNS junctions, and neuromagnetic SQUID measurements.
spray-coated by high-temperature plasma in the atmosphere.
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2. S U P E R C O N D U C T I N G SHIELD
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MAGNETIC
Magnetic noises from traffic including automobiles and far-away tram cars have peaks at so low frequencies as less than one Hertz. The shielding factor of superconducting shield does not reduce even at these low freqencies while that of Permalloy does. We first made a helmet-size superconducting shield of bulk BSCCO measuring somatosesary evoked magnetic field from human brains successfully. In order to make a body-size magnetic shield, we switched from heavy bulk superconductors to thick-film superconductors on a nickel cylinder. The inside wall of a nickel cylinder is covered with thick film of BSCCO which is *The work was supported by the Institute of Physical and Chemical Research in part.
Figure 1. Whole-head SQUID system in a superconducting magnetic shield.
The high-temperature plasma spray-coating guarantees the best adhesion between the superconducting thick film and nickel. The nickel cylinder coated with l m m thick BSCCO film is cooled by helium gas from a closed-cycle helium refrigerator. The 2-meter tall cryostat for the superconducting shield works as a part of the refrigerator.
0921-4534/00/$ - see front matter <~ 2000 Elsevier Science B.V. All rights reserved. P[I S0921-4534(00)01406-4
2714
H. Ohta, T Matsui/Physica C 341-348 (2000) 2713-2714
The cryostat is tilted in any angle because we use a closed-cycle refrigerator instead of liquid nitrogen above whose liquid level the temperature of the superconductor would go up. We used high-Tc-phase BixPb2-xSr2Ca2Cu3Oy instead of low-Tc-phase Bi2Sr2Ca1Cu2Ox to backup lack of power of the refrigerator connected with the cryostat by the 10-meter flexible stainless tubes which cut mechanical vibrations. It takes about one day to cool the cylinder down to lower than liquid nitrogen temperature. The magnetic shield can reduce a magnetic field to around - 80 dB or 10 -4 even at as low frequency as 0.05 Hz. We insert both the FRP dewar and the SQUID cryostat into the horizontally-tilted magnetic shield horizontally without using any lifting machine on the ceiling [1]. 3. N E U R O M A G N E T I C REMENT
Top ZO) nm
•
V
Figure 2. One shot from our movie about topographies of magnetic field above a human brain during stimulation of the median nerve in the right wrist.
SQUID MEASU-
Sematosensorily evoked magnetic field was measured by a whole-head SQUID magnetometer of SNS junctions. A typical junction sensitivity s is 5 [fr/vfH-z]. The bandwidth A f is wider than several hundred Herz. We average about one hundred events (N). Therefore the system sensitivity S is around S = s v f ' ~ / N ~ 5fT. The neuromagnetic measurements have been done in the superconducting magnetic shield. A median nerve in the right wrist was stimulated by current pulses. The data show that the N20 peaks of 50 fT have a good Signal-to-Noise ratio. The movie in Fig. 2 and Fig. 3 confirmed the evoked current dipole in the brain rotates at a frequency of about I0 Hz. The current dipole is located in the central sulcus of the MRI image in Fig. 3. 4. C O N C L U S I O N A superconducting magnetic shield of highTc superconductor Bi(Pb)SrCaCuOx has been constructed whose diameter is 65 cm and length is 160 cm, respectively. The 64channel SQUID of SNS junctions combined with the body-size superconducting magnetic shield has demonstrated the sensitivity of 5 fT in our laboratory only 10 meter far from
Figure 3. The evoked current dipole in the brain is located in the central sulcus in the MR/image.
both an elevator and power transformer banks.
REFERENCES . H. Ohta, M. Aono, T. Matsui, Y. Uchikawa, K. Kobayashi, K. Tanabe, S. Takeuchi, K. Narasaki, S. Tsunematsu, Y. Koyabu, Y. Kamekawa, K. Nakayama, T. Shimizu, K. Koike, K. Hoshino, H. Kotaka, E. Sudoh, H. Takahara, Y. Yoshida, K. Shinada, M. Takahata, Y. Yamada and K. Kamijo, IEEE Trans. on Applied Superconductivity 9 (1999) 4073.
Physica C 341-348 (2000) 2713-2714
www.elsevier.nl/locate/physc
Whole-head SQUID system in a superconducting magnetic shield H. Ohta a* and T. Matsui b aCommunications Research Laboratory, Ministry of Posts and Telecommunications, Nukuikita-machi 21-2-1, Koganei-shi 184-0015, Tokyo, Japan bCommunications Research Laboratory, Ministry of Posts and Telecommunications A whole-head 64-channel SQUID magnetometer of SNS (Superconductor/ Normal metal / Superconductor) junctions has been constructed to be operated in a superconducting magnetic shield of Bi(Pb)SrCaCuOx demonstrating the sensitivity of 5 femto Tesla in our laboratory only 10 meter far from both an elevator and power transformer banks. Signal-to-noise ratio of data in neuromagnetic measurements is exellent because a SQUID of SNS junctions has a smaller telegraph noise than that of SIS tunnel junctions and because shielding factors of the superconducting magnetic shield does not reduce even at as low frequencies as 0.05 Hz unlike that of Permalloy shield room. A movie is made to compare the evoked current dipole in the MEG data with the MRI images.
1. I N T R O D U C T I O N The whole-head SQUID system in a superconducting magnetic shield has yielded excellent SN ratio of data in neuromagnetic measurements according to our movies about topographies of magnetic field above human brains. We describe characteristics of the superconducting magnetic shield, a 64-channel whole-head SQUID of SNS junctions, and neuromagnetic SQUID measurements.
spray-coated by high-temperature plasma in the atmosphere.
bclow 77K ¢ by a closcd-¢ycl
N
2. S U P E R C O N D U C T I N G SHIELD
/
II H i m
/
~
~ , ~
MAGNETIC
Magnetic noises from traffic including automobiles and far-away tram cars have peaks at so low frequencies as less than one Hertz. The shielding factor of superconducting shield does not reduce even at these low freqencies while that of Permalloy does. We first made a helmet-size superconducting shield of bulk BSCCO measuring somatosesary evoked magnetic field from human brains successfully. In order to make a body-size magnetic shield, we switched from heavy bulk superconductors to thick-film superconductors on a nickel cylinder. The inside wall of a nickel cylinder is covered with thick film of BSCCO which is *The work was supported by the Institute of Physical and Chemical Research in part.
Figure 1. Whole-head SQUID system in a superconducting magnetic shield.
The high-temperature plasma spray-coating guarantees the best adhesion between the superconducting thick film and nickel. The nickel cylinder coated with l m m thick BSCCO film is cooled by helium gas from a closed-cycle helium refrigerator. The 2-meter tall cryostat for the superconducting shield works as a part of the refrigerator.
0921-4534/00/$ - see front matter <~ 2000 Elsevier Science B.V. All rights reserved. P[I S0921-4534(00)01406-4
2714
H. Ohta, T Matsui/Physica C 341-348 (2000) 2713-2714
The cryostat is tilted in any angle because we use a closed-cycle refrigerator instead of liquid nitrogen above whose liquid level the temperature of the superconductor would go up. We used high-Tc-phase BixPb2-xSr2Ca2Cu3Oy instead of low-Tc-phase Bi2Sr2Ca1Cu2Ox to backup lack of power of the refrigerator connected with the cryostat by the 10-meter flexible stainless tubes which cut mechanical vibrations. It takes about one day to cool the cylinder down to lower than liquid nitrogen temperature. The magnetic shield can reduce a magnetic field to around - 80 dB or 10 -4 even at as low frequency as 0.05 Hz. We insert both the FRP dewar and the SQUID cryostat into the horizontally-tilted magnetic shield horizontally without using any lifting machine on the ceiling [1]. 3. N E U R O M A G N E T I C REMENT
Top ZO) nm
•
V
Figure 2. One shot from our movie about topographies of magnetic field above a human brain during stimulation of the median nerve in the right wrist.
SQUID MEASU-
Sematosensorily evoked magnetic field was measured by a whole-head SQUID magnetometer of SNS junctions. A typical junction sensitivity s is 5 [fr/vfH-z]. The bandwidth A f is wider than several hundred Herz. We average about one hundred events (N). Therefore the system sensitivity S is around S = s v f ' ~ / N ~ 5fT. The neuromagnetic measurements have been done in the superconducting magnetic shield. A median nerve in the right wrist was stimulated by current pulses. The data show that the N20 peaks of 50 fT have a good Signal-to-Noise ratio. The movie in Fig. 2 and Fig. 3 confirmed the evoked current dipole in the brain rotates at a frequency of about I0 Hz. The current dipole is located in the central sulcus of the MRI image in Fig. 3. 4. C O N C L U S I O N A superconducting magnetic shield of highTc superconductor Bi(Pb)SrCaCuOx has been constructed whose diameter is 65 cm and length is 160 cm, respectively. The 64channel SQUID of SNS junctions combined with the body-size superconducting magnetic shield has demonstrated the sensitivity of 5 fT in our laboratory only 10 meter far from
Figure 3. The evoked current dipole in the brain is located in the central sulcus in the MR/image.
both an elevator and power transformer banks.
REFERENCES . H. Ohta, M. Aono, T. Matsui, Y. Uchikawa, K. Kobayashi, K. Tanabe, S. Takeuchi, K. Narasaki, S. Tsunematsu, Y. Koyabu, Y. Kamekawa, K. Nakayama, T. Shimizu, K. Koike, K. Hoshino, H. Kotaka, E. Sudoh, H. Takahara, Y. Yoshida, K. Shinada, M. Takahata, Y. Yamada and K. Kamijo, IEEE Trans. on Applied Superconductivity 9 (1999) 4073.