CLINICAL CARDIOLOGY: PHYSICIAN UPDATE Use of Intracardiac Echocardiography to Guide Ablation of Atrial Fibrillation

Intracardiac Echocardiography Ultrasound technology has progressed rapidly over the past several years, leading to significant miniaturization of transducer design. The original catheter-based transducers, known as intravascular ultrasound (IVUS) catheters, are high frequency (20 to 40 MHz) and are used to image the walls of coronary arteries and other small vessels. They contain a single ultrasound crystal that rapidly rotates in the end of the catheter, offering a circumferential, 2-dimensional image. Although this high-frequency transducer provides excellent near-field spatial resolution, its acoustic depth is limited to several millimeters, thus making it suitable for intracoronary imaging. Transducers with lower frequencies (5 to 12 MHz) have been developed to apply catheter-based echocardiography to intracardiac imaging, which requires greater ultrasound penetration than intravascular imaging. Two types of ultrasound catheters are currently in use. The first is a mechanical echo catheter, which is similar to IVUS catheters, with a rotating ultrasound transducer driven by a motor unit at the opposite end of a braided drive shaft, which results in a 360-degree view perpendicular to the catheter. The second type is a fixed or phased array echo catheter that uses electronically controlled multiple transducers affixed to one side of the catheter shaft, which results in a wedge-shaped image sector similar to that of a transesophageal echo probe. Both types of catheters provide high resolution, real-time images of anatomic structures and of other intracardiac devices and catheters. Echo catheters are currently 6 to 10 French, and are typically introduced through a sheath in a femoral vein. Phased array catheters offer a large depth of field and the possibility of Doppler imaging, whereas mechanical catheters offer better near-field resolution.