The detection of MEG signals from deep sources by a SQUID system for spinal cord evoked field
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1
Kanazawa Institute of Technology, Japan
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2
Tokyo Metropolitan University, Department of System Design, Japan
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3
Tokyo Medical and Dental University, Section of Orthopedic and Spinal Surgery, Japan
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4
CNRS and Universite Paris Descartes, Laboratorie de Psychologie de la Perception, France
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5
Kanazawa Institute of Technology, Applied Electronics Laboratory, Japan
The MEG signals from deep sources such as the brainstem are quite weak because of the intensity decay by distance. To detect them, we must average a huge number of responses to improve the S/N ratio. The measurement inevitably requires a long time. We applied a biomagnetometer system developed recently for the spinal cord evoked magnetic field (SCEF) measurement to the detection of the magnetic field generated by deep sources in the brain. The measurement system is equipped with an array of 40 SQUID vector gradiometers with a long baseline length of 68 mm. Its cryostat has a cool-to-warm separation of less than 7 mm. The white noise level of the SQUID sensor is typically less than 3 fT/rtHz. These features were originally aimed at detecting the SCEF from deep in the cervix, but they are also an advantage for targeting the brain deep sources. Brainstem response measurements were performed on two subjects without hearing problems. The MEG signals synchronized with repetitive auditory stimulation of click sounds were recorded. The sounds were generated by Eartone 3A transducer and guided to the subjects' left ear through a 1.75-m plastic tube. The repetition rate was 9.52 Hz and the sound intensity was 70 or 75 dBSL. The observation area was positioned over the right lateral-occipital area of each subject. All signals were digitally recorded with an analog band-pass filter of 100 - 5000 Hz. After digitization, 8000 responses were averaged to improve the S/N ratio. A magnetic field component around 5 ms after the actual stimulus onset was clearly observed from both subjects. Its distribution was widely spread over the observation area of 90 mm x 140 mm and its peak amplitude was about 5 - 7 fT. Based on the peak latency, this component corresponds to the wave V of an auditory brainstem response (ABR). After noise reduction using synthesized spatial filters, earlier ABR waves were also observed.
Conference:
Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010.
Presentation Type:
Poster Presentation
Topic:
Instrumentation and Multi-modal Integrations: MEG, Low-field MRI,EEG, fMRI,TMS,NIRS
Citation:
Adachi
Y,
Fujii
S,
Kawabata
S,
De Cheveigne
A and
Kado
H
(2010). The detection of MEG signals from deep sources by a SQUID system for spinal cord evoked field.
Front. Neurosci.
Conference Abstract:
Biomag 2010 - 17th International Conference on Biomagnetism .
doi: 10.3389/conf.fnins.2010.06.00422
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Received:
09 Apr 2010;
Published Online:
09 Apr 2010.
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Correspondence:
Yoshiaki Adachi, Kanazawa Institute of Technology, Kanazawa, Japan, adachi@ael.kanazawa-it.ac.jp