Electric current mapping using the acousto-electric effect

Conventional methods for mapping cardiac current fields lack either spatial resolution (e.g. ECG) or are time consuming (e.g., intra-cardiac catheter electrode mapping). We present a method based on the acousto-electric effect (AEE) with potential for rapid mapping of current fields in the heart with high spatial resolution. The AEE is a pressure-induced conductivity modulation, in which focused ultrasound can be used as a spatially localized pressure source. When an ultrasound beam is focused between a pair of recording electrodes in a homogeneous conductive medium, an induced voltage will be produced due to the pressure-modulated conductivity and local current density. The amplitude of the voltage change should be proportional to fluctuations in current density, such as those generated during the cardiac cycle, in the region of focused ultrasound. Preliminary experiments demonstrate the feasibility of this method. A 540 kHz ultrasound transducer is focused between two tin electrodes lying parallel to the beam axis. These electrodes inject current into a 0.9% saline solution. A pair of insulated stainless steel electrodes exposed at the tip is used to record voltage. To simulate a cardiac current, a low frequency current waveform is injected into the sample such that the peak current density (8 mA/cm ²) approximates cardiac currents. The transducer is pulsed at different delays after waveform initiation. Delays are chosen such that the low frequency waveform is adequately sampled. Using this approach an emulated ECG waveform has been successfully reconstructed from the ultrasound modulated voltage traces.