An integrated full-head OPM-MEG system based on 128 zero-field sensors

Abstract

Compact optically-pumped magnetometers (OPMs) are now commercially available with noise floors reaching 10 fT/Hz^1/2. However, to be used effectively for magnetoencephalography (MEG), dense arrays of these sensors are required to operate as an integrated turn-key system. In this study, we present the HEDscan, a 128-sensor OPM MEG system by FieldLine Medical, and evaluate its sensor performance with regard to bandwidth, linearity, and crosstalk. We report results from cross-validation studies with conventional cryogenic MEG, the Magnes 3,600 WH Biomagnetometer by 4-D Neuroimaging. Our results show high signal amplitudes captured by the OPM-MEG system during a standard auditory paradigm, where short tones at 1000 Hz were presented to the left ear of six healthy adult volunteers. We validate these findings through an event-related beamformer analysis, which is in line with existing literature results.

Keywords: HEDscan; OPM; cross-validation; magnetoencephalography; optically-pumped magnetometer; system integration.

Figure 1

(A) Photograph of the integrated controls system for 128 sensors. (B) Drawing of the OPM sensor schematic. (C) Photograph of an OPM sensor head.

Figure 2

(A) Photograph of the setup inside the MSR. The same patient bed was used for both OPM MEG and cryoMEG recordings. The active coil panels can be seen on the side. The dewar of the 4-D Neuroimage cryogenic MEG system can be seen in the background. (B) Photograph of the adjustable, self-localizing helmet.

Figure 3

(A) Open-loop and (B) Closed-loop show the sensor response to a magnetic field at 1 Hz as a function of field amplitude. The black squares are measured field amplitudes and the red line represents 100% linearity. (C) Open-loop (black) and closed-loop (blue) sensor linearity shown as a fractional deviation from unity as a function of field amplitude.

Figure 4

(A) Normalized response of the OPM sensors operating in closed loop as a function of frequency shown for 8 sensors. The red line indicates the 3 dB point. (B) Normalized response of one OPM sensor operating in open loop as a function of frequency (red) and corresponding phase (blue).

Figure 5

(A) Noise floor in the magnetically shielded room. (B) Noise floor after signal-space projection (SSP) with 4 projections.

Figure 6

Auditory-evoked responses from the same subject measured with the HEDscan OPM MEG system [(A): 148 averages, 100 channels] and 4-D cryogenic MEG system [(B): 148 averages, 246 channels]. The colored pattern in the top left corners show the distribution of channels on the head, showing a dipolar pattern with blue signals having a positive, and red signals having a negative amplitude of the M100 signal.

Figure 7

(A) Magnetic field distribution measured with the HEDscan system in six different subjects 123 ms – 135 ms post trigger to the left ear of the subject. Blue represents inward going fields and red represents outward going fields. (B) Event-related Beamformer results for subject 6 measured with the HEDscan system in response to sounds delivered to the left ear. It can be seen that the activity starts in the contralateral right auditory cortex (131 ms, top row), is followed by bilateral activity (144 ms, middle row), and finally activity appears in the ipsilateral left auditory cortex (148 ms, bottom row).