Portable Magnetometry for Detection of Biomagnetism in Ambient Environments

We present a method of optical magnetometry with parts-per-billion resolution that is able to detect biomagnetic signals generated from the human brain and heart in Earth’s ambient environment. Our magnetically silent sensors measure the total magnetic field by detecting the free-precession frequency in a highly spin-polarized alkali-metal vapor. A first-order gradiometer is formed from two magnetometers that are separated by a 3-cm baseline. Our gradiometer operates from a laptop consuming 5 W over a USB port, enabled by state-of-the-art microfabricated alkali-vapor cells, advanced thermal insulation, custom electronics, and compact lasers within the sensor head. The gradiometer has a sensitivity of 16 ${\mathrm{fT}/\mathrm{cm}/\mathrm{Hz}}^{1/2}$ outdoors, which we use to detect neuronal electrical currents and magnetic cardiography signals. Recording of neuronal magnetic fields is one of a few available methods for noninvasive functional brain imaging that usually requires extensive magnetic shielding and other infrastructure. This work demonstrates the possibility of a dense array of portable biomagnetic sensors that are deployable in a variety of natural environments.

Figure 1

Magnetic gradiometer operation. (a) Two ${}^{87}\mathrm{Rb}$ vapor cells separated by a 3-cm baseline are optically pumped with a laser to near-unity polarization. A quantum nondemolition measurement of the ${}^{87}\mathrm{Rb}$ polarization along the probe axis occurs with a weak detuned laser undergoing optical rotation. ${}^{87}\mathrm{Rb}$ undergoes free precession about a total field $B$, leading to a decaying sine-wave signal for each magnetometer cell. The total field $B$ and the quantization axis are determined by the local Earth’s field $B_E$. (b) A custom frequency counter extracts ${}^{87}\mathrm{Rb}$ free-precession frequencies for each cell by detecting optical-rotation zero crossings in a single-shot detection period of 2.3 ms. The counter calculates a total field for each cell, $B_1$ and $B_2$, by dividing by the ${}^{87}\mathrm{Rb}$ gyromagnetic ratio $\gamma /2\pi \approx 7\mathrm{kHz}/\mu \mathrm{T}$. (c) The single-shot pump and probe measurement is repeated at a rate of 300 Hz. The two magnetometers (mag.) measure total fields $B_1$ and $B_2$, which are streamed to a laptop and subtracted to obtain the first-order total-field gradient $dB/dx$. (d) We take spectral densities of the total-field magnetometers and gradiometer time-domain data to demonstrate the unshielded noise floor and common-mode rejection of the all-optical gradiometer in Earth’s ambient environment. The red line is a fit to the gradiometer noise floor, giving 15.7 $\mathrm{fT}/\mathrm{cm}\sqrt{\mathrm{Hz}}$.

Figure 2

Auditory evoked fields detected unshielded in Earth’s field with a portable first-order gradiometer. (a) The vacuum-packaged ${}^{87}\mathrm{Rb}$ vapor cells are optically pumped and probed by diode lasers integrated into the gradiometer head. Optical rotation of the probe laser is detected with a photodiode amplifier assembly (PDA). The gradiometer is controlled by compact electronics, including a custom counter that streams the total fields $B_1$ and $B_2$ and the gradient $dB/dx$ to the laptop. The two ${}^{87}\mathrm{Rb}$ vapor cells are placed near a subject’s auditory cortex to measure the total-field gradient $dB/dx$ with a 3-cm baseline. The gradiometer is centered approximately over the location of the auditory current dipole, so the radial component of the auditory magnetic field contributes with opposite sign to the two channels. (b) A picture of the in-the-field MEG recording apparatus with a subject. (c) For a given subject, data are recorded for roughly 20 min while time-randomized 1-kHz auditory stimuli are applied to the left ear of the subject. A filtered average of 462 epochs is shown for the $dB/dx$ gradient data taken above a subject’s right ear. The bands indicate the standard error of the mean. The auditory evoked fields observed include the prominent N100m peak along with an indication of the P40m and P150m responses.

Figure 3

Field-deployable magnetocardiography. (a) A picture of a next-generation gradiometer that is able to detect human heartbeats in magnetically noisy environments. (b) As the chest of a subject is presented to the sensor, a real-time filter (0.1–50-Hz band-pass, 60- and 120 Hz notches) is applied to the total field gradient $dB/dx$ to demonstrate the heartbeat measurements. (c) The result of averaging data for 10 s, triggered on the $R$ peak.