(Invited) CMUT Based Microscale Integrated Ultrasound Systems for Minimally Invasive Intracardiac and Intravascular Imaging

Although catheter based intravascular ultrasound (IVUS) imaging of arterial cross sections and intracardiac echography (ICE) imaging of the heart in two dimensions have proven to be very useful in many interventional procedures used in the diagnosis and treatment of coronary and structural heart diseases, extension of these techniques to three-dimensional (3-D) volumetric imaging will have a dramatic impact.  Truly volumetric images in front of an IVUS catheter will enable accurate evaluation and safer crossing of chronic total occlusions (CTOs) in coronary and peripheral arteries. Three dimensional ICE imaging in the heart can improve the outcome of challenging procedures such as trans-catheter valve replacements by providing the clinicians exceptional capability for real-time spatial mapping. These imaging devices would be enabled by miniature ultrasound systems that can be placed at the tip of mechanically flexible catheters.

We have been developing technologies for integration of capacitive micromachined ultrasonic transducer (CMUT) arrays and custom designed CMOS front end electronics on the same silicon chip for ultimately miniaturized ultrasound systems for 3-D IVUS and ICE imaging. To implement these CMUT-on-CMOS systems, we post-process CMOS wafers to fabricate CMUTs using a low temperature process [1]. The CMOS electronics are designed specifically for low noise operation with CMUTs and use smart power management to reduce the power consumption, and time division multiplexing to reduce the cable count in the catheter to about 10 for an imaging array with over 100 elements [2]. With this approach, a 300um thick, 1-2mm diameter donut shaped silicon contains most of the required front end functionality, resulting in very flexible 3-7F catheters for 3-D IVUS and ICE imaging applications.

Figure 1 shows a CMUT-on-CMOS chip with 1.4-mm-diameter dual-ring CMUT array on a front-end IC implemented in 0.35-µm CMOS process after silicon donut shaping using deep reactive ion etching (left) and after initial flex tape electrical connections (right). The dual-ring array has 56 transmit elements and 48 receive elements on two separate concentric annular rings. The IC incorporates a 25-V pulser for each transmitter element and a low-noise transimpedance amplifier (TIA) for each receiver, along with digital control. The final shape of the silicon chip is a 1.5-mm-diameter donut with a 430-µm center hole for a guide wire.  The overall front-end system requires only 13 external connections and provides 4 parallel RF outputs while consuming an average power of 20-mW. The frequency of operation is around 20-MHz, suitable for forward looking volumetric IVUS imaging of CTOs. This device has been tested on wire phantoms and ex vivo chicken heart samples to demonstrate its capability to collect 3-D ultrasound imaging data at 60 fr/s rate and dynamic range comparable to commercial IVUS systems [3]. In addition to providing details of this type of CMUT-on-CMOS systems, we  will discuss the use of this approach for implementing MRI compatible intracardiac imaging catheters as well as integration of an IVUS imaging system on a 0.014” diameter guidewire.

1. J. Zahorian, M. Hoffman, T. Xu, G. Gurun, S. Satir, M. Karaman, and F.L. Degertekin “Monolithic CMUT on CMOS Integration for Intravascular Ultrasound Applications,” IEEE Trans. on UFFC, vol. 58, pp. 2659-2667, 2011.
2. G. Gurun, P. Hasler, and F.L. Degertekin, “Frontend Receiver Electronics for High Frequency Monolithic CMUT-on-CMOS Imaging Arrays,” IEEE Trans. on UFFC, vol. 58, 1658-1668, 2011.
3. G. Gurun, C. Tekes, J. Zahorian, T. Xu, S. Satir, M. Karaman and F.L. Degertekin, “Single-Chip CMUT-on-CMOS front-end System for Real Time Volumetric IVUS and ICE Imaging,”  IEEE Trans. on UFFC, to appear in February 2014.