Scalp sensor for simultaneous acoustic emission detection and electroencephalography during transcranial ultrasound

Abstract

Focused ultrasound is now capable of noninvasively penetrating the intact human skull and delivering energy to specific areas of the brain with millimeter accuracy. The ultrasound energy is supplied in high-intensities to create brain lesions or at low-intensities to produce reversible physiological interventions. Conducting acoustic emission detection (AED) and electroencephalography (EEG) during transcranial focused ultrasound may lead to several new brain treatment and research applications. This study investigates the feasibility of using a novel scalp senor for acquiring concurrent AED and EEG during clinical transcranial ultrasound. A piezoelectric disk is embedded in a plastic cup EEG electrode to form the sensor. The sensor is coupled to the head via an adhesive/conductive gel-dot. Components of the sensor prototype are tested for AED and EEG signal quality in a bench top investigation with a functional ex vivo skull phantom.

Fig. 1:

Schematic of the scalp sensor for AED and EEG data acquisition. Acoustic and electric fields can be obtained simultaneously though the same interface.

Fig. 2:

Picture of scalp sensor with the A) piezoelectric disk (backside showing) embedded in the B) scalp EEG electrode (electric insulator layer and Ag-AgCL layer removed).

Fig. 3:

Experimental apparatus for testing the scalp senor with phantom AED and EEG signals sources.

Fig. 4:

Picture of the AED/EEG scalp sensors placed on the A) phantom. B) Sonicating transducer (272 kHz) placed below the phantom. C) The rubber tube within the location of the focused ultrasound transducer beam focus and the location of the electric field antennae outside the beam focus.

Fig. 5:

Hydrophone beam plot (1 mm resolution) of the FUS transducer used to excite the water in the rubber hose of the phantom. Scanning performed to create this image was conducted without the skull and in a water tank and at a lower acoustic power than used in AED experiments. The approximate size and location of the rubber hose exterior boundaries in relation to the beam focus is depicted with yellow dotted lines.

Fig. 6:

Locations of sensors and electrodes during experiments and equvelnt locations where they would be placed on a human scalp.

Fig. 7:

AED results from sensor S1 during cavitation experiments. A) Time domain of acoustic emissions of the talc water (red) and degassed water (blue) flowing through the rubber tube during sonication. B) Frequency domain of the time series in (A). C) Magnified image of (B) showing the sub-harmonic peak. The example with cavitation is shown in red where; a large increase in sub-harmonics is evident during sonication in the talc water. Each data point is averaged after conducting the experiment 3 times.

Fig. 8:

EEG DAQ for simultaneous comparison of conventional scalp EEG electrodes vs. combined AED/EEG sensors. S6 and S8 are conventional scalp electrodes and S5 and S7 are the custom sensors. A) Amplitude variation test. B) Frequency variation test. No averaging was used. The FUS sonication timestamp is shown above the EEG signals.

Fig. 9.

Continuous 272 kHz wave sonication during simultaneous acquisitions of AED and EEG. (Left: AED; right: EEG). The sonication timestamp is indicated above the EEG.

Fig. 10:

Pulsed wave sonication during simultaneous acquisitions of AED and EEG with Pulse Repetition Frequency (PRF) and 500 Hz and a pulse duration of 1 ms. (Left: AED; right: EEG). The sonication timestamp is indicated above the EEG.