Ultrasonic Guidance for Radiofrequency Ablation

Abstract

Similar to transthoracic echocardiography, intracardiac echocardiography images cardiac structures with ultrasound waves. These sonic waves are emitted from an ultrasound crystal mounted onto the distal aspect of an intravascular catheter. The initial cardiac application was to evaluate intracoronary atherosclerotic lesions. Since tissue resolution varies inversely with the wavelength, and depth of penetration varies directly with wavelength, ideal imaging of the coronary endothelial surface requires high frequencies on the order of 30 mHz. For catheter ablation procedures, these ultrasound catheters were occasionally used to verify catheter tip-myocardial tissue contact.¹ Modification of the intracoronary ultrasound catheters to deliver lower frequencies (9 mHz) provided greater depth of penetration and not only allowed verification of tissue contact, but also facilitated identification of specific cardiac structures important in the ablation of arrhythmias and ablation location.^2–5 There are two commercially available ultrsound catheters. The first is a rotary driven transducer that images tangentially through a fluid filled catheter providing a 360° view around the catheter shaft that is angled at 15° consistent with the angle of the transducer (Figure 1). The second is a phased array system that mounts several transducers in an array on the shaft of a catheter, thus the image observed is pie shaped (Figure 2).