Capacitive micromachined ultrasonic transducers for medical imaging and therapy

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) have been subject to extensive research for the last two decades. Although they were initially developed for air-coupled applications, today their main application space is medical imaging and therapy. This paper first presents a brief description of CMUTs, their basic structure, and operating principles. Our progression of developing several generations of fabrication processes is discussed with an emphasis on the advantages and disadvantages of each process. Monolithic and hybrid approaches for integrating CMUTs with supporting integrated circuits are surveyed. Several prototype transducer arrays with integrated frontend electronic circuits we developed and their use for 2-D and 3-D, anatomical and functional imaging, and ablative therapies are described. The presented results prove the CMUT as a MEMS technology for many medical diagnostic and therapeutic applications.

Figures

Fig. 1

Schematic cross-sections for different CMUT structures. (a) CMUT fabricated using sacrificial release process. (b) CMUT fabricated using simple wafer bonding process. (c) CMUT fabricated using LOCOS process. (d) CMUT fabricated using the thick-buried-oxide process. (e) CMUT with added mass on its plate. (f) CMUT with the compliant post structure.

Fig. 2

Multichip hybrid integration. (a) left panel top: A 32×32 2-D CMUT array flip-chip bonded on the top side of a rigid interposer. Left panel bottom: Four ICs flip-chip bonded on the bottom side of the interposer. Right panel: The cross-sectional view of the multichip assembly. (b) Eight ICs and a 64-element CMUT ring array flip-chip bonded on flexible printed circuit board. Each leg of the flexible PCB is folded for placement in the catheter tip.

Fig. 3

A 132-element 1-D CMUT array fabricated using the simple wafer bonding process.

Fig. 4

(a) A 16×16 2-D CMUT array flip-chip bonded on the first-generation custom frontend IC. (b) 3-D rendered image of a wire phantom obtained in synthetic aperture mode with a 5-MHz array (The phantom is schematically shown in the inset).

Fig. 5

(a) A 16×16 2-D CMUT array flip-chip bonded on the second-generation custom frontend IC with transmit beamforming capability. (b) 3-D rendered image of a wire phantom with a 2.5-MHz array.

Fig. 6

(a) Left panel: A 12-F forward-looking ring catheter. Right panel: A 64-element CMUT ring array. (b) The 3-D rendered image of a metal spring and three different cross-sectional images captured in real time.

Fig. 7

(a) Left panel: The 9-F forward-looking microlinear catheter. Right panel: A 24-element CMUT microlinear array. (b) A 2D image obtained with the CMUT microlinear array in-vivo in a pig heart.

Fig. 8

Photoacoustic images of the chicken breast phantom, reconstructed using the data from a 64 × 64 aperture. (a) Schematic of tubes embedded in chicken breast phantom. (b) 3-D rendered pulse-echo image (grayscale). (c) 3-D rendered photoacoustic image (red).