First in vivo use of a capacitive micromachined ultrasound transducer array-based imaging and ablation catheter

Abstract

Objectives: The primary objective was to test in vivo for the first time the general operation of a new multifunctional intracardiac echocardiography (ICE) catheter constructed with a microlinear capacitive micromachined ultrasound transducer (ML-CMUT) imaging array. Secondarily, we examined the compatibility of this catheter with electroanatomic mapping (EAM) guidance and also as a radiofrequency ablation (RFA) catheter. Preliminary thermal strain imaging (TSI)-derived temperature data were obtained from within the endocardium simultaneously during RFA to show the feasibility of direct ablation guidance procedures.

Methods: The new 9F forward-looking ICE catheter was constructed with 3 complementary technologies: a CMUT imaging array with a custom electronic array buffer, catheter surface electrodes for EAM guidance, and a special ablation tip, that permits simultaneous TSI and RFA. In vivo imaging studies of 5 anesthetized porcine models with 5 CMUT catheters were performed.

Results: The ML-CMUT ICE catheter provided high-resolution real-time wideband 2-dimensional (2D) images at greater than 8 MHz and is capable of both RFA and EAM guidance. Although the 24-element array aperture dimension is only 1.5 mm, the imaging depth of penetration is greater than 30 mm. The specially designed ultrasound-compatible metalized plastic tip allowed simultaneous imaging during ablation and direct acquisition of TSI data for tissue ablation temperatures. Postprocessing analysis showed a first-order correlation between TSI and temperature, permitting early development temperature-time relationships at specific myocardial ablation sites.

Conclusions: Multifunctional forward-looking ML-CMUT ICE catheters, with simultaneous intracardiac guidance, ultrasound imaging, and RFA, may offer a new means to improve interventional ablation procedures.

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 × 1.4 mm. The integrated front-end electronics are bonded underneath the array. The CMUT subelement membranes are shown in c, which constitute the functional element widths depicted in d.

Figure 2

Graphic depiction (a) of a microlinear capacitive micromachined ultrasound transducer (CMUT) catheter with the special ultrasound transparent ablation tip, which contacts the endocardial wall for radiofrequency ablation (RFA) and simultaneous thermal strain echo collection. The tip thermocouple and steering assembly are omitted for clarity. Actual images show the catheter without the special tip (b) and after the tip attachment procedure (c). ASIC indicates application-specific integrated circuit; and Pt-Ir, platinum-iridium.

Figure 3

Qualitative image quality comparison of the microelectromechanical system–based second-generation microlinear capacitive micromachined ultrasound transducer (ML-CMUT) prototype (left) and the third-generation piezoceramic microlinear lead zirconate titanate transducer (right), with system image sizes adjusted to correct for scale. Both were used in these examples to help guide the ablation tip of a separate radiofrequency ablation (RFA) catheter into position on the surface of the endocardium. The ML-CMUT image (left) shows the RFA catheter surrounded by a collection of echogenic bubbles near the tip (arrows) during an ablation procedure.

Figure 4

Imaging the right atrium (RA) and appendage from the oblique sinus within the pericardial sac while investigating catheter steering and the potential for an epicardial ultrasound examination with the microlinear capacitive micromachined ultrasound transducer catheter. Ao indicates aorta; IVC, inferior vena cava; PA, pulmonary artery; RAA, right atrial appendage; and SVC, superior vena cava.

Figure 5

Microlinear capacitive micromachined ultrasound transducer (ML-CMUT; ML 18) catheter imaging the placement of a radiofrequency ablation (RFA) catheter near the inferior right atrial isthmus. The imaging catheter has been advanced from the superior vena cava, whereas the RFA catheter was introduced femorally. The wide-bandwidth ML-CMUT here can resolve the very fine wire braid pattern (arrow) within the RFA catheter shaft in the range (at ≈5 mm) of the best focus for this small tip-mounted array.

Figure 6

The microlinear capacitive micromachined ultrasound transducer (ML-CMUT) ablation sites recorded in the porcine model are indicated as numbered red dots. The NavX reconstruction of the partially mapped right heart is shown in two anterior views. The electroanatomic mapping catheter was advanced from the inferior vena cava (IVC) and used to define the right atrial (RA) and right ventricular (RV) chambers before tracking ML-CMUT ablation locations. Ablations were performed to test the handling of the ML-CMUT in the RA, through the tricuspid annulus (TA) to the RV, and additionally in the coronary sinus (CS) and great cardiac vein.

Figure 7

The microlinear capacitive micromachined ultrasound transducer ablation sites (A indicates region of single ablations; and B, multiple ablations) located in the porcine right ventricular free wall are shown explanted subsequent to radiofrequency ablation. These sites showing erythematous endocardium corresponded well to the NavX electroanatomic mapping–recorded ablation site record.

Figure 8

Comparison of the in vivo time course of ultrasound-based thermal strain (TS) between the microlinear lead zirconate titanate (ML-PZT) and microlinear capacitive micromachined ultrasound transducer (ML-CMUT) catheters for ablations under identical conditions performed on the right ventricular endocardial wall. The arrow shows the strong nonlinear TS response, which becomes more pronounced as the tissue heating continues. Further in vivo characterization of this response is an important focus of future studies. RFA indicates radiofrequency ablation; and UCRA, ultrasound-compatible radiofrequency ablation.

Figure 9

Thermal strain imaging based on in vivo echoes from a porcine endocardium undergoing simultaneous ablation. The far left panel is the unprocessed B-mode sector image in gray scale gradients on a decibel scale. The middle panel shows the ultrasound-based “strain” image of the tissue approximately 12 seconds after the start of ablation; the round dark region at a 2-mm depth is likely a small blood vessel. The color bar describes the (unitless) thermal strain, which can be converted to temperature units (as suggested in Figure 8) pending verification of the scaling factor relationship, which is to be determined. A schematic of the ultrasound-compatible radiofrequency ablation tip and approximate ultrasound image sector is shown at the far right.