First In Vivo Use of a Capacitive Micromachined Ultrasound Transducer Array–Based Imaging and Ablation Catheter

Electrophysiologic interventions have had increasing popularity as treatment options for arrhythmias, particularly atrial fibrillation.1 Atrial fibrillation is the most common arrhythmia, with an incidence of 2 to 3 per 1000 people aged 55 to 64 years annually,2 and is associated with considerable morbidity and mortality.3,4 There are at least 3 techniques available to interventionalists that can accurately locate intracardiac regions of interest, with fluoroscopy being the most established technique. However, the exposure to ionizing radiation, as well as the inability of fluoroscopy to clearly delineate subsurface intracardiac regions and confirm catheter contact with the endocardium, has made echocardiography and electroanatomic mapping (EAM) the most attractive as adjunct modalities.5–9 Intracardiac echocardiography (ICE), in particular, has found multiple uses in the electrophysiology laboratory, providing real-time display of intracardiac structures.6–8 The use of ultrasound image guidance for interventional radiofrequency ablation (RFA) procedures in the heart has begun to play a more important role in the minimization of fluoroscopic radiation exposure to the patient while imaging important intracardiac features, eg, the membranous fossa of the interatrial septum for puncture access to the left atrium, the pulmonary veins, and Doppler blood velocities associated with pulmonary vein stenosis.10 Important considerations in the design of ICE catheters are the (1) imaging and handling performance features, (2), compatibility with current interventional practice, and (3) cost in both procedural time and purchasing expense. Overall, a catheter that can be itself guided into place with EAM, perform high-quality image guidance of RFA therapy, and improve procedural throughput at a lower cost is a worthwhile goal. As a general guide, Table 1 compares the common RFA catheter with an “ideal” catheter and with the initial features of a proposed progenitor, a microlinear capacitive micromachined ultrasound transducer (ML-CMUT) catheter. Table 1 Performance Parameter Objectives for Catheters Used in Radiofrequency Ablation Procedures The miniaturization of ICE catheters has revolutionized interventional procedures by integrating ultrasound transducers onto flexible, low-profile catheters, which allow imaging in the restricted spaces of vascular and cardiac structures.11–13 Two distinctly different ICE catheter prototypes, descriptively referred as ML catheters because of their small ultrasonic array designs, have been developed within a program to build a series of miniaturized high-frequency forward-looking transducer arrays.14 Piezoceramic arrays, namely those made with lead zirconate titanate (PZT), are considered standard design types for most ultrasound applications. Currently in their third generation of development, our early ML devices are called ML-PZT array catheters. The second array transducer type is the CMUT, now in its second generation of development15; these CMUT arrays are used in the assembly of the ML-CMUT array catheter (Figure 1). By comparison to PZT, the CMUT as an acoustic transducer is relatively new as an ultrasound transceiver. This silicon-based array, however, has several design aspects that may yield a considerable advantage over the PZT array type, especially at small sizes. The small-element CMUT array in combination with a local buffer-preamplifier offers excellent transmit and receive sensitivity with a wide bandwidth, allows special element shaping, does not require acoustic matching layers, and may be considerably less expensive in large-scale manufacturing. Figure 1 Prefinished distal tip (a) of the 9F microlinear capacitive micromachined ultrasound transducer (CMUT) intracardiac imaging catheter with a metal radiofrequency ablation tip electrode, and the 24-element CMUT array (b) with silicon die dimensions of 1.9 ... The principal objective of this work is to describe the first in vivo use of a forward-looking ML-CMUT array catheter in a true 9F profile, the catheter compatibility with EAM guidance, and the use of a specially integrated RFA tip capable of both monitoring and delivery of intracardiac ablation. This work represents the first in vivo ablation and direct simultaneous collection of tissue echo data for the thermal strain temperature at the exact ablation site. In addition, we describe the initial use of the ML-CMUT catheter in exploratory cardiac imaging from the perspective of the epicardial surface from within the pericardium.