Source: http://aoot.osa.org/boe/abstract.cfm?uri=boe-3-5-981
Timestamp: 2019-04-23 14:34:44+00:00

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We report on the measurement of somatosensory-evoked and spontaneous magnetoencephalography (MEG) signals with a chip-scale atomic magnetometer (CSAM) based on optical spectroscopy of alkali atoms. The uncooled, fiber-coupled CSAM has a sensitive volume of 0.77 mm3 inside a sensor head of volume 1 cm3 and enabled convenient handling, similar to an electroencephalography (EEG) electrode. When positioned over O1 of a healthy human subject, α-oscillations were observed in the component of the magnetic field perpendicular to the scalp surface. Furthermore, by stimulation at the right wrist of the subject, somatosensory-evoked fields were measured with the sensors placed over C3. Higher noise levels of the CSAM were partly compensated by higher signal amplitudes due to the shorter distance between CSAM and scalp.
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Fig. 1 (Top) Vision of a flexible fiber-coupled magnetometer system. (Middle) Schematic of the microfabricated sensor head. (Bottom) Photograph of the microfabricated sensor head.
Fig. 2 Sensitivity of the chip-scale magnetic sensor measured inside the magnetically-shielded room BMSR-2. (Inset) Bode plot for a typical CSAM determined by use of a coil driven by a signal generator. The 300 µs time constant was used in the MEG recordings to achieve an effective bandwidth of up to 150 Hz.
Fig. 3 Sketch of the measurement positions on the head used to detect magnetoencephalographic signals. Spontaneous activity around 10 Hz linked to closing and opening of the eyes was measured with the sensor positioned above O1 (international 10-20 system for electrode positioning), whereas signals related to an electrical stimulation at the wrist were obtained over position C3.
Fig. 4 Time-frequency analysis of the CSAM signal (left) and a SQUID signal (right) obtained during a repeated sequence of 20 s of eyes open followed by 20 s of eyes closed. The eyes-closed sections start at 20 s and 60 s, lasting for 20 s as indicated, and the increase in α–power in the 10 Hz band is immediately visible both in the CSAM and the SQUID signal. Measurement position was O1, as sketched in Fig. 3 (International 10-20 system).
Fig. 5 Averaged SEF for three different subjects with the CSAM and SQUID data taken sequentially over position C3 as indicated in Fig. 3. Left side: CSAM result; Right side: SQUID result. The stimulus artifact at 0 ms is visible in the curves, as are the N20m and later responses, which can vary in timing. The field strength is much smaller in the SQUID curves due to the much larger distance between source and sensor, which is estimated to be 2.5 cm for the CSAMs and 6 cm for the SQUIDs.
Fig. 6 Sketch of the geometry of brain current dipole source Q parallel to the surface of a horizontally layered conductor and the two sensors used in this study. Both sensors measure Bz, which is zero directly above Q, and therefore the sensors are assumed to be offset horizontally.
(1) B z = μ 0 4π Q → ×( r → − r → Q )⋅ e → z | r → − r → Q | 3 .

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