Recording brain activities in unshielded Earth's field with optically pumped atomic magnetometers

Abstract

Understanding the relationship between brain activity and specific mental function is important for medical diagnosis of brain symptoms, such as epilepsy. Magnetoencephalography (MEG), which uses an array of high-sensitivity magnetometers to record magnetic field signals generated from neural currents occurring naturally in the brain, is a noninvasive method for locating the brain activities. The MEG is normally performed in a magnetically shielded room. Here, we introduce an unshielded MEG system based on optically pumped atomic magnetometers. We build an atomic magnetic gradiometer, together with feedback methods, to reduce the environment magnetic field noise. We successfully observe the alpha rhythm signals related to closed eyes and clear auditory evoked field signals in unshielded Earth's field. Combined with improvements in the miniaturization of the atomic magnetometer, our method is promising to realize a practical wearable and movable unshielded MEG system and bring new insights into medical diagnosis of brain symptoms.

Fig. 1. Unshielded MEG system using OPMs.

(A) The overall arrangement of the practical unshielded MEG system. The OPMs are placed at the center of the two sets of three orthogonal coils. The diameter of the coils for the outer layer is 3.6 m, and for the inner layer is 1.5 m. (B and C) Detailed arrangements for detecting the alpha rhythm signal (B) and AEF signal (C). (D) Amplitude responses of the two OPMs. The orange dashed line shows the −3-dB level relative to the signal amplitude at 1 Hz. arb. unit, arbitrary units. (E) Phase responses of the two OPMs. (F) The green circle shows the ability of the unshielded MEG system to reduce the common-mode magnetic field noise under different frequencies and is a combined effect from the atomic gradiometer (orange triangle) and field stabilization (blue square). (Photo credit: Wei Xiao, Rui Zhang, and Hong Guo, Peking University, Beijing, China.)

Fig. 2. Spontaneous alpha rhythm signal.

(A) The green region shows the measured spectral density of the atomic gradiometer with no person present and indicates a noise floor of 20 fT/Hz^1/2 for frequencies between 5 and 30 Hz. When the person is present and asked to keep his eyes open, the measured result is shown in the yellow region. When the person closes his eyes, a clear alpha rhythm signal around 10 Hz is observed, see the blue region. (B) The time domain signal acquired when the person is asked to repeat closing and opening his eyes every 30 s. (C) The spectrogram for the acquired 300-s data shown in (B).

Fig. 3. AEF signal.

(A) The measured AEF signals in the time domain for three individuals. The auditory stimulus has a frequency of 440 Hz and is repeated for 480 times. The purple dashed arrow shows the start time of the stimulus. The AEF signal occurs ~100 ms after the stimulus. (B) The map of the 480 measured results for person no.2.