To address this limitation, researchers at Institut Langevin are developing an ultrafast acoustoelectric imaging (UAI) system for non-invasive imaging of cardiac electrical activation. The team has now built a compact UAI scanner that provides accurate 2D images of current distributions in real-time. (Phys. Med. Biol. 61 5808).

"There has been some exciting work in the literature over the last few years regarding use of the acoustoelectric effect to map electrical currents with conventional focused ultrasound beams," said first author Beatrice Berthon. "We wanted to see whether the concepts inherent to ultrafast ultrasound imaging could benefit acoustoelectric imaging, by reducing peak pressures while increasing sensitivity and achieving sufficient frame rates to track electrical activation in the heart."

Rapid tracking

The acoustoelectric effect is one in which a propagating acoustic wave creates localized modulation of the medium's electrical impedance. The local intersection of the acoustic wave with a current density is detected as a variation in voltage, measured by a pair of remote electrodes. UAI triggers this effect using ultrasound plane waves to image current densities.

The use of plane waves to insonify the medium enables each emission to map the entire field-of-view. This enables high frame rates of up to 10000 Hz, making UAI ideal for tracking transient and fast events such as cardiac activation, which typically occurs within 100 ms. Plane waves can also be emitted at different angles and the resulting electrode measurements coherently compounded to produce a high-quality image.

The UAI scanner is based on a 256-channel ultrasound research platform fitted with a 128-element linear ultrasound array on one 128-element connector, and a break-out board on the second. Channels 1 to 128 are used to emit acoustic plane waves, while the UAI signal (measured by copper wire electrodes) is differentially amplified and sampled via a single channel of the break-out board. Using one platform to both emit the acoustic wave and receive and process the UAI data creates a compact and portable system.

Berthon and colleagues characterized the scanner by measuring the UAI signal in a saline phantom, with DC current injected via a pair of copper wire electrodes. The peak pressure did not exceed 3 MPa, 36% lower than the FDA recommendations for limiting tissue damage. They employed a novel holographic image formation algorithm to reconstruct 2D UAI images fast enough for real-time image display, an important feature for clinical applications.

Without averaging, the scanner could measure a signal for currents as low as 0.56 mA, with a mean signal-to-noise ratio (SNR) of 6.0. The authors note that this value is close to physiologically relevant current densities. When varying the ultrasound probe voltage, signal was observed down to 0.3 MPa with a mean SNR of 5.2. This shows the scanner's promise for in vivo imaging without damaging tissues.

Resolution measurements of UAI data for 3 to 33 plane waves emitted at different angles showed that resolution improved with increasing number of plane waves, reaching an equivalent value to a focused wave (at equivalent peak pressure) for 13 plane waves.

The SNR also increased with the number of plane waves, reaching a similar value to focused waves when three plane waves were coherently compounded. With 33 plane waves, the SNR was 27 dB higher than for focused waves at equivalent peak pressure.

Clinical potential

To demonstrate the scanner's ability to map time-varying currents in real time, the researchers injected a 3 Hz sinusoidal current through a pair of electrodes in the saline pool. The UAI signal matched the sinusoidal form of the current. Next, they injected a 10 Hz sinusoidal current via two neighbouring electrode pairs with 180° phase difference between the two pairs. The 10 Hz sinusoidal waveform was observed in the UAI signal, as well as the 180° phase shift.

Biological current distributions follow complex geometries as they propagate in the heart. With this in mind, the researchers used the UAI scanner to image a complex current density distribution generated via four pairs of electrodes positioned around the recording pair. Using 3 MPa with 33 angles, the UAI image clearly showed three groups of close electrode pairs, with intensities related to the distance between electrodes and to the ultrasound probe. The current distribution between the most distant electrodes was also seen, with a lower intensity.

"We recently used UAI in vivo and obtained some promising results," Berthon told medicalphysicsweb. "We are currently conducting additional studies to validate these results and hope to present them in the near future. We are also working to further improve the sensitivity of our system by developing an improved electrode arrangement and by using 2D plane waves, which will also enable 3D imaging."

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