Aaron Dentinger

Forward-looking volumetric intracardiac imaging using a fully integrated CMUT ring array

Atrial fibrillation is the most common type of cardiac arrhythmia that now affects over 2.2 million adults in the United States alone. Currently fluoroscopy is the most common method for guiding interventional electrophysiological procedures. We are developing a 9-F forward-looking intracardiac ultrasound catheter for real-time volumetric imaging. We designed and fabricated a 64-element 10-MHz CMUT ring array with through-wafer via interconnects. We also designed custom front-end electronics to be closely integrated with the CMUT array at the tip of the catheter for improved SNR. This integrated circuit (IC) is composed of preamplifiers and protection circuitry, and can directly interface a standard imaging system. This multi-channel IC is capable of passing up to ±50-V bipolar pulses. An 8-channel front-end IC was fabricated based on this circuit topology. Additionally, a flexible PCB was designed for the integration of ring array with front-end electronics. We have acquired a PC-based real-time imaging platform and demonstrated real-time imaging with the ring array. We have also shown volume images using off-line full synthetic aperture image reconstruction method. The presented experimental results demonstrate the performance of our forward-looking volumetric intracardiac imaging approach. We are currently working on the final catheter integration and further development of our real-time imaging methods.

Forward-looking intracardiac ultrasound imaging using a 1-D CMUT array integrated with custom front-end electronics

Minimally invasive catheter-based electrophysiological (EP) interventions are becoming a standard procedure in diagnosis and treatment of cardiac arrhythmias. As a result of technological advances that enable small feature sizes and a high level of integration, nonfluoroscopic intracardiac echocardiography (ICE) imaging catheters are attracting increasing attention. ICE catheters improve EP procedural guidance while reducing the undesirable use of fluoroscopy, which is currently the common catheter guidance method. Phased-array ICE catheters have been in use for several years now, although only for side-looking imaging. We are developing a forwardlooking ICE catheter for improved visualization. In this effort, we fabricate a 24-element, fine-pitch 1-D array of capacitive micromachined ultrasonic transducers (CMUT), with a total footprint of 1.73 mm x 1.27 mm. We also design a custom integrated circuit (IC) composed of 24 identical blocks of transmit/ receive circuitry, measuring 2.1 mm x 2.1 mm. The transmit circuitry is capable of delivering 25-V unipolar pulses, and the receive circuitry includes a transimpedance preamplifier followed by an output buffer. The CMUT array and the custom IC are designed to be mounted at the tip of a 10-Fr catheter for high-frame-rate forward-looking intracardiac imaging. Through-wafer vias incorporated in the CMUT array provide access to individual array elements from the back side of the array. We successfully flip-chip bond a CMUT array to the custom IC with 100% yield. We coat the device with a layer of polydimethylsiloxane (PDMS) to electrically isolate the device for imaging in water and tissue. The pulse-echo in water from a total plane reflector has a center frequency of 9.2 MHz with a 96% fractional bandwidth. Finally, we demonstrate the imaging capability of the integrated device on commercial phantoms and on a beating ex vivo rabbit heart (Langendorff model) using a commercial ultrasound imaging system.

Forward-looking intracardiac imaging catheters using fully integrated CMUT arrays

Atrial fibrillation, the most common type of cardiac arrhythmia, now affects more than 2.2 million adults in the US alone. Currently, electrophysiological interventions are performed under fluoroscopy guidance, which besides its harmful ionizing radiation does not provide adequate soft-tissue resolution. Intracardiac echocardiography (ICE) provides realtime anatomical information that has proven valuable in reducing the fluoroscopy time and enhancing procedural success. We developed two types of forward-looking ICE catheters using capacitive micromachined ultrasonic transducer (CMUT) technology: MicroLinear (ML) and ring catheters. The ML catheter enables real-time forward-looking 2-D imaging using a 24-element 1-D CMUT phased-array that is designed for a center frequency of 10 MHz. The ring catheter uses a 64-element ring CMUT array that is also designed for a center frequency of 10 MHz. However, this ring-shaped 2-D array enables real-time forward-looking volumetric imaging. In addition, this catheter provides a continuous central lumen that enables convenient delivery of other devices such as RF ablation catheter, EP diagnostic catheter, biopsy devices, etc. Both catheters are equipped with custom front-end ICs that are integrated with the CMUT arrays at the tip of the catheters. The integration of the ICs with the CMUT arrays was accomplished using custom flexible PCBs. We also developed several image reconstruction schemes for the ring catheter on a PC-based imaging platform from VeraSonics. We performed a variety of bench-top characterizations to validate the functionality and performance of our fully integrated CMUT arrays. Using both catheters, we demonstrated in vivo images of the heart in a porcine animal model. We have successfully prototyped the first CMUT-based ICE catheters and proven the capabilities of the CMUT technology for implementing high-frequency miniature transducer arrays with integrated electronics.

The feasibility of using thermal strain imaging to regulate energy delivery during intracardiac radio-frequency ablation

A method is introduced to monitor cardiac ablative therapy by examining slope changes in the thermal strain curve caused by speed of sound variations with temperature. The sound speed of water-bearing tissue such as cardiac muscle increases with temperature. However, at temperatures above about 50°C, there is no further increase in the sound speed and the temperature coefficient may become slightly negative. For ablation therapy, an irreversible injury to tissue and a complete heart block occurs in the range of 48 to 50°C for a short period in accordance with the well-known Arrhenius equation. Using these two properties, we propose a potential tool to detect the moment when tissue damage occurs by using the reduced slope in the thermal strain curve as a function of heating time. We have illustrated the feasibility of this method initially using porcine myocardium in vitro. The method was further demonstrated in vivo, using a specially equipped ablation tip and an 11-MHz microlinear intracardiac echocardiography (ICE) array mounted on the tip of a catheter. The thermal strain curves showed a plateau, strongly suggesting that the temperature reached at least 50°C.

Experimental Studies With a 9F Forward-Looking Intracardiac Imaging and Ablation Catheter

Objective. The purpose of this study was to develop a high‐resolution, near‐field‐optimized 14‐MHz, 24‐element broad‐bandwidth forward‐looking array for integration on a steerable 9F electrophysiology (EP) catheter. Methods. Several generations of prototype imaging catheters with bidirectional steering, termed microlinear (ML), were built and tested as integrated catheter designs with EP sensing electrodes near the tip. The wide‐bandwidth ultrasound array was mounted on the very tip, equipped with an aperture of only 1.2 by 1.58 mm. The array pulse echo performance was fully simulated, and its construction offered shielding from ablation noise. Both ex vivo and in vivo imaging with a porcine animal model were performed. Results. The array pulse echo performance was concordant with Krimholtz‐Leedom‐Matthaei model simulation. Three generations of prototype devices were tested in the right atrium and ventricle in 4 acute pig studies for the following characteristics: (1) image quality, (2) anatomic identification, (3) visualization of other catheter devices, and (4) for a mechanism for stabilization when imaging ablation. The ML catheter is capable of both low‐artifact ablation imaging on a standard clinical imaging system and high–frame rate myocardial wall strain rate imaging for detecting changes in cardiac mechanics associated with ablation. Conclusions. The imaging resolution performance of this very small array device, together with its penetration beyond 2 cm, is excellent considering its very small array aperture. The forward‐looking intracardiac catheter has been adapted to work easily on an existing commercial imaging platform with very minor software modifications.

Array signal processing for local arterial pulse wave velocity measurement using ultrasound

A new signal processing approach to estimation of local arterial pulse wave velocity (PWV) in superficial arterial segments using long-axis ultrasound measurements is proposed. The method is designed to be resistant to estimation bias due to pulse wave reflections. It is evaluated using a laboratory test tank, and it appears to estimate local PWV with less bias than previously accepted methods, and with similar estimation variance to those methods

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

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.

The acoustic lens design and in vivo use of a multifunctional catheter combining intracardiac ultrasound imaging and electrophysiology sensing

A multifunctional 9F intracardiac imaging and electrophysiology mapping catheter was developed and tested to help guide diagnostic and therapeutic intracardiac electrophysiology (EP) procedures. The catheter tip includes a 7.25-MHz, 64-element, side-looking phased array for high resolution sector scanning. Multiple electrophysiology mapping sensors were mounted as ring electrodes near the array for electrocardiographic synchronization of ultrasound images. The catheter array elevation beam performance in particular was investigated. An acoustic lens for the distal tip array designed with a round cross section can produce an acceptable elevation beam shape; however, the velocity of sound in the lens material should be approximately 155 m/s slower than in tissue for the best beam shape and wide bandwidth performance. To help establish the catheters unique ability for integration with electrophysiology interventional procedures, it was used in vivo in a porcine animal model, and demonstrated both useful intracardiac echocardiographic visualization and simultaneous 3- D positional information using integrated electroanatomical mapping techniques. The catheter also performed well in high frame rate imaging, color flow imaging, and strain rate imaging of atrial and ventricular structures.

Miniaturized ultrasound imaging probes enabled by CMUT arrays with integrated frontend electronic circuits

Capacitive micromachined ultrasonic transducer (CMUT) arrays are conveniently integrated with frontend integrated circuits either monolithically or in a hybrid multichip form. This integration helps with reducing the number of active data processing channels for 2D arrays. This approach also preserves the signal integrity for arrays with small elements. Therefore CMUT arrays integrated with electronic circuits are most suitable to implement miniaturized probes required for many intravascular, intracardiac, and endoscopic applications. This paper presents examples of miniaturized CMUT probes utilizing 1D, 2D, and ring arrays with integrated electronics.

10C-6 Fully Integrated CMUT-Based Forward-Looking Intracardiac Imaging for Electrophysiology

Minimally invasive percutaneous electrophysiological mapping of the heart chambers is becoming a standard procedure to diagnose and treat cardiac arrhythmias. Due to advances in technology that enable small feature sizes and a high level of integration, non-fluoroscopic intracardiac imaging is attracting more attention to better guide electrophysiologal (EP) interventions. In this effort, we are developing a forward-looking intracardiac ultrasound imaging catheter, which is also equipped with several EP electrode sensor bands and a metal RF ablation tip enclosure. A 24-element fine-pitch (63 mum) 1-D array, based on capacitive micromachined ultrasonic transducer (CMUT) technology, has been fabricated for high-frame-rate imaging. Through-wafer vias are incorporated in the device to connect the signal and ground electrodes to the flip-chip bond pads on the backside of the array. The total footprint of the array measures 1.73 mm x 1.27 mm. Also a custom-designed integrated circuit (IC) has been fabricated to be closely integrated with the CMUT array for improved SNR. This IC comprises some of the important front- end electronics of an ultrasound imaging system. It measures 2 mm x 2 mm and is composed of 24 individual transmit/receive blocks. The transmit circuitry is capable of delivering 25 -V unipolar pulses. The receive circuitry includes a transimpedance preamplifier followed by a line driver buffer. A CMUT array was flip-chip bonded directly on to the IC for initial testing. All of the 24 elements of the array and the IC are functional. Array uniformity was tested by measuring the resonant frequency in air. A standard deviation of 0.37 percent was measured around the mean value of 17.9 MHz. The same array operates at 9.2 MHz in immersion with a 104 percent fractional bandwidth. Imaging performance of the described front-end was tested on a commercial phantom and also in ex- vivo environment on an isolated perfused rabbit heart (Langendorfl). The final goal is to integrate the CMUT array and the front-end electronics at the tip of a 10 -F catheter. A flexible printed circuit board (PCB) has been designed and the first sub-assembly is ready for cable attachment and final catheter integration.