High-Resolution 2D Imaging Using High-Frequency 1D CMUT Arrays
High-frequency arrays (> 20 MHz) are required for real-time applications with demanding resolution specifications. It has been challenging to manufacture piezoelectric transducer arrays with consistent pitch and kerf at the dimensions necessary for high-frequency operation. We have demonstrated high-frequency, (45 MHz) single-element CMUTs in the past [1] and, more recently, high-frequency CMUT arrays fabricated by using surface micromachining and wafer-bonding techniques [2, 3].
We have also used the high-frequency 1D arrays for high-resolution imaging [4]. For imaging experiments, we used three different designs (Fig. 1). Design 1 operates in immersion at 20 MHz with 85% fractional bandwidth, Design 2 at 26 MHz, 52% fractional bandwidth, Design 3 at 41 MHz, 32% fractional bandwidth. Designs 2 and 3 have narrower bandwidths than usual for CMUTs because their designs have low fill factor. The fill factor can be easily improved to yield arrays operating at high frequencies with fractional bandwidth of over 80%. A target made of wires positioned as shown in Fig. 2 was imaged to characterize imaging performance. B-scans were made of both a nylon target, which has an acoustic impedance mismatch to oil of 2:1, and a glass fiber target with a mismatch of 10:1. The nylon wires were 76 µm in diameter, and the glass fibers, 150 µm. The B-scans are shown in Fig. 2. Finally, B-scans were made of a rabbit eye. Fig. 3 shows the cross section of a rabbit eye and the relative size and location of the transducer aperture. Figs.3.b and 3.c show the B-scans of the rabbit eye using the Design 1 transducer operated in conventional (10 MHz) and collapse modes (20 MHz), respectively.
FIGURE 1. High-frequency 1D CMUT arrays.
FIGURE 2. High-resolution images of a wire target. (a) Wire positions in the imaging phantom. (b) Image from 20 MHz transducer array, glass fiber target, 40 dB. (c) Image from 26 MHz transducer array, nylon target, 40 dB. (d) Image from 40 MHz transducer array, 40 dB.
FIGURE 3. Experimental images of a rabbit eye. (a) Anatomy of the rabbit eye. (b) Rabbit eye imaged by 10 MHz CMUT array, 76 dB (c) Rabbit eye imaged by 20 MHz CMUT array, 70 dB.
References
[1] Ergun AS, Huang Y, Cheng CH, Oralkan Ö, Johnson J, Jagannathan H, Demirci U, Yaralıoglu GG, Karaman M, and Khuri-Yakub BT, “Broadband capacitive micromachined ultrasonic transducers ranging from 10 kHz to 60 MHz for imaging applications and more,” Proceedings of IEEE International Ultrasonics Symposium, pp. 1039–1043, 2002.
[2] Oralkan Ö, Hansen ST, Bayram B, Yaralioglu GG, Ergun AS, and Khuri-Yakub BT, “ High-frequency CMUT arrays for high-resolution medical imaging,” in Proceedings of IEEE Ultrasonic Symposium, 2004, pp. 399-402.
[3] Yeh D, Oralkan Ö, Ergun AS, Zhuang X, Wygant IO, Cheng CH, Huang Y, Yaralioglu GG, and Khuri-Yakub BT, “High-frequency CMUT arrays for high-resolution medical imaging,” Proc. SPIE Medical Imaging Conference, pp. 87-98, 2005.
[4] Yeh D, Oralkan Ö, Wygant IO, Ergun AS, Wong JH, and Khuri-Yakub BT, “ High-Resolution Imaging with High-Frequency 1-D Linear CMUT Arrays,” presented at the 2005 IEEE International Ultrasonics Symposium, Rotterdam, The Netherlands, Sept. 18 - 21, 2005
Acknowledgements
This project was funded by National Institutes of Health under grant HL67647.
