Cardiac Activation Mapping using Ultrasound Current Source Density Imaging.

Abstract

Intracardiac ablation procedures to correct drug-resistant arrhythmias require accurate maps of cardiac activation. Conventional methods are time-consuming and have poor spatial resolution (5- 10 mm). The goal of this dissertation was to develop a new method, Ultrasound Current Source Density Imaging (UCSDI), to map biological currents. UCSDI is based on the acousto-electric (AE) effect, a modulation of the electric resistivity by acoustic pressure. If a current passes through the focal region of an ultrasound transducer, a voltage modulated at the ultrasonic frequency can be measured with a pair of electrodes located distal to the focal zone. By sweeping the focal zone, UCSDI can map a distributed current field. UCSDI has several potential advantages as a technique for mapping cardiac activation currents: high spatial resolution determined by the typically sub-mm focal characteristics of the ultrasound beam, short mapping time using electronically steered ultrasonic beams, and automatic registration with B-mode ultrasound images without sophisticated mathematical algorithms. UCSDI was first validated by mapping an artificially generated 2D current distribution. It was compared to sequential electrode mapping, computer simulation as well as to an inverse algorithm. In this study it was possible to use UCSDI to locate monopolar current sources to within 1-mm of their true locations without making any prior assumptions about the source geometry. UCSDI was then used to detect and map biological currents in an isolated rabbit heart. Both UCSDI and normal low frequency electrocardiograms (ECG) were measured simultaneously by tungsten electrodes embedded in the left ventricle. The motion of the heart was significantly reduced by perfusing it with an excitation contraction de-coupler. Measured UCSDI maps showed temporal and spatial patterns consistent with a spreading activation wave and timing consistent with normal ECG signals. UCSDI was then combined with ultrasonic strain imaging in a new method for electromechanical imaging. This combined method was used to make localized measurements of electromechanical delay. This method could be useful in cardiac resynchronization therapy for placing pacemaker leads.