An integrated and highly sensitive ultrafast acoustoelectric imaging system for biomedical applications

Abstract

Direct imaging of the electrical activation of the heart is crucial to better understand and diagnose diseases linked to arrhythmias. This work presents an ultrafast acoustoelectric imaging (UAI) system for direct and non-invasive ultrafast mapping of propagating current densities using the acoustoelectric effect. Acoustoelectric imaging is based on the acoustoelectric effect, the modulation of the medium's electrical impedance by a propagating ultrasonic wave. UAI triggers this effect with plane wave emissions to image current densities. An ultrasound research platform was fitted with electrodes connected to high common-mode rejection ratio amplifiers and sampled by up to 128 independent channels. The sequences developed allow for both real-time display of acoustoelectric maps and long ultrafast acquisition with fast off-line processing. The system was evaluated by injecting controlled currents into a saline pool via copper wire electrodes. Sensitivity to low current and low acoustic pressure were measured independently. Contrast and spatial resolution were measured for varying numbers of plane waves and compared to line per line acoustoelectric imaging with focused beams at equivalent peak pressure. Temporal resolution was assessed by measuring time-varying current densities associated with sinusoidal currents. Complex intensity distributions were also imaged in 3D. Electrical current densities were detected for injected currents as low as 0.56 mA. UAI outperformed conventional focused acoustoelectric imaging in terms of contrast and spatial resolution when using 3 and 13 plane waves or more, respectively. Neighboring sinusoidal currents with opposed phases were accurately imaged and separated. Time-varying currents were mapped and their frequency accurately measured for imaging frame rates up to 500 Hz. Finally, a 3D image of a complex intensity distribution was obtained. The results demonstrated the high sensitivity of the UAI system proposed. The plane wave based approach provides a highly flexible trade-off between frame rate, resolution and contrast. In conclusion, the UAI system shows promise for non-invasive, direct and accurate real-time imaging of electrical activation in vivo.

Mapping Biological Current Densities With Ultrafast Acoustoelectric Imaging: Application to the Beating Rat Heart

“…The corresponding frequency of the UAI signal is indicated at the top right. demonstrated previously in phantoms [15], a higher number of plane waves provided improved lateral resolution.…”

“…The ultrasound probe was connected to the first 128 channels and used for the emission of plane ultrasound waves and the recording of the echoes necessary to produce B-mode images. The second connector was fitted with a break-out board which received the UAI signal from a pair of recording electrodes via a custom-made high-pass instrumentation amplifier with high common-mode rejection ratio, as previously described [15]. A custom-made low-pass instrumentation amplifier was also used to record the signal from the same pair of electrodes for ECG measurements.…”

“…We have recently proposed the use of the acoustoelectric effect at high frame rates using ultrasound plane waves at high frequency (5MHz) in a technique called Ultrafast Acoustoelectric Imaging (UAI) [15]. We demonstrated that the technique can produce 2D images of the measured UAI signal with high spatial and temporal resolution.…”

Selective Mapping of Deep Brain Stimulation Lead Currents Using Acoustoelectric Imaging

“…As an US beam is pulsed and swept in a conductive medium, a recording electrode detects the high frequency AE interaction signal, which is proportional to the local pressure and current. Feasibility of AEI for mapping current densities has been demonstrated in a variety of preparations, including time-varying dipoles (Olafsson et al, 2008, Wang et al, 2011, Wang et al, 2016, Berthon et al, 2017) and imaging of the cardiac depolarization wave in the live rabbit heart (Olafsson et al, 2009, Qin et al, 2015). The primary goals for this study are to 1) assess the performance (spatial resolution, sensitivity, and accuracy) of AEI for detecting and resolving current densities near a DBS device using stimulation parameters resembling those used clinically, and 2) demonstrate feasibility and benchmark performance of AEI through a human skullcap.…”

In vivo acoustoelectric imaging for high-resolution visualization of cardiac electric spatiotemporal dynamics

“…t US and t phys indicate the US propagation time (in microseconds) and physiologic time (in milliseconds), respectively. Detailed derivations of the general AE equation have been previously described [12][13][14].…”