A compact, high performance atomic magnetometer for biomedical applications

Abstract

We present a highly sensitive room-temperature atomic magnetometer (AM), designed for use in biomedical applications. The magnetometer sensor head is only 2 × 2 × 5 cm3 and is constructed using readily available, low-cost optical components. The magnetic field resolution of the AM is <10 fT Hz–1/2, which is comparable to cryogenically cooled superconducting quantum interference device (SQUID) magnetometers. We present side-by-side comparisons between our AM and a SQUID magnetometer, and show that equally high quality magnetoencephalography and magnetocardiography recordings can be obtained using our AM.

Figure 1

(a) Photograph of the prototype AM adjacent to optical tweezers. The external dimensions are 1.5×1.5×3 cm³. (b) AM enclosed in an outer protective jacket (2×2×5 cm³) with three-axis coils wrapped on the outside. (c) Magnetic field sensitivity of the AM prototypes. The AM1 and AM2 traces are the magnetic noise power spectral density (PSD) of the two magnetometer channels measured within the MSR at the time of AER studies, and the gradient trace is the PSD of the difference of the outputs from the two magnetometers; i.e., synthetic gradiometer. The gradiometer arrangement cancels the low frequency environmental noise inside the MSR and also removes a large interference at 7 Hz from an air conditioning fan located above the MSR. A digital notch filter was used to suppress interference at 60 Hz. The roll-off in the noise spectrum seen at frequencies above 50 Hz is due to the filter settings on the lock-in amplifier.

Figure 2

(a) Photograph of two AMs positioned over the chest of a subject for recording the MCG. (b) Close-up photograph of two AMs positioned over the parietal cortex for MEG-AER recordings. AM 2 was used to record the AER, while AM 1 was used as reference sensor for background noise cancellation.

Figure 3

(left) Signal-averaged MCG waveforms from subject #1, obtained using the SQUID and AM. The peak-to-peak amplitude of the signal is about 75 pT. The insets show the raw recordings, except for application of a 60 Hz notch filter. (right) 8–12 Hz MEG recording showing blocking of the alpha rhythm, obtained by instructing the subject to alternately open and close his eyes every ten seconds.

Figure 4

Averaged MCG waveforms from nine subjects, recorded with a SQUID (red) and AM (blue). The x-axis in each of the plots is time (0.7 s full-scale). The y-axis is magnetic field, arbitrarily scaled to facilitate comparison.

Figure 5

Averaged AER recordings made using a SQUID and AM. The left graph for each of the four subjects is the AER recorded using a 7 channel vector SQUID system. (right) Comparison of AERs measured using the AM (blue) and a SQUID channel (red) with similar morphology. The x-axis shows time in seconds and the y-axis shows magnetic field in fT. The SQUID and AM recordings were vertically offset to facilitate comparison.