3D Photoacoustic Imaging Using CMUT Arrays

Photoacoustic imaging has typically relied on a single mechanically scanned focused piezoelectric transducer for detection of the laser-generated ultrasound. Using a CMUT array in place of a mechanically scanned element has a number of advantages. 3D images can be acquired in one shot using large, two-dimensional arrays which can be reliably fabricated using CMUT technology. To demonstrate the potential of CMUTs for photoacoustic imaging we performed a series of experiments.

A diagram illustrating the experimental setup is shown in Fig. 1.a. For these experiments, the phantom to be imaged is suspended in an oil tank of size 5 cm x 5 cm x 3 cm. Vegetable oil is used between the array and phantom to couple ultrasound. Vegetable oil is also used because it is nonconducting and thus the array and electronics do not need to be insulated. The phantom consists of three 0.86-mm inner diameter (1.27-mm outer diameter) polyethylene tubes passing through a 2 cm x 2 cm x 3 cm block of tissue mimicking material (ATS Laboratories, Bridgeport, CT). The center tube is filled with India-ink to provide optical contrast for the photoacoustic imaging. The CMUT array is located at the bottom of the tank. The phantom is illuminated from the side of the tank by a Q-switched Nd:YAG laser. Ideally the laser should uniformly illuminate the material being imaged. Thus the laser beam is de-focused to a 1/e2 diameter of approximately 6 mm. A ground glass diffuser in front of the tank further diffuses the laser light. The laser used has a 1.064 µm wavelength and 12-ns FWHM pulse duration. The energy of each laser pulse is 2.3 mJ. The laser was fired at a rate of 10 Hz. A photograph of the phantom and tank is shown in Fig. 1.b. CMUT technology allows the fabrication of large two-dimensional arrays. The advantages of larger arrays include the ability to image larger targets with an improved signal to noise ratio. Larger arrays also result in improved lateral resolution due to a larger aperture size. To simulate this effect, array tiling was performed. In our experiment the CMUT array was placed on an X-Y translational stage. After one data set was acquired, the array was translated 4 mm (length of the array) along the x-direction and another data set was acquired. Further data sets were obtained by also tranlating 4 mm along the y-direction. In all, 9 data-sets were acquired. The array tiling results in an image that is equivalent to an image taken with an array of size 48 x 48 elements. This experiment demonstrates that large area arrays are necessary for clinical applications.

Example pulse-echo and photoacoustic data acquisitions are shown in Fig. 1.c. The signal from the ink-filled tube can be identified in both figures. The individual element acquisitions are bandpass filtered and then used for image reconstruction. As is evident, the ink-filled tube gives a very strong signal in the photoacoustic A-scan. Using the recorded signals we reconstructed both the photoacoustic and pulse-echo images using a standard delay and sum reconstruction algorithm. A 3D image of the photoacoustic image overlayed on the pulse-echo image is shown in Fig. 2. The photoacoustic image is shown in red color while the pulse-echo image is rendered in grayscale. The images from the 48x48 array shown in Fig. 2.b clearly illustrate the advantages of larger arrays.

These results demonstrate some of the advantages of CMUT technology for photoacoustic imaging. A transducer array such as the one used in this work has clear acquisition time advantages over a mechanically scanned system. By increasing the laser repetition rate, real-time images could be obtained with the system described above. Image resolution could be improved by using a larger aperture size as shown by tiling the array. The ability to obtain simultaneous pulse-echo and photoacoustic images enable to get both anatomical and functional images at once. Furthermore, the use of 2D arrays enable imaging a volume in a single shot. Details of the work described can be found in [1-3].

 

 

 

FIGURE 1. Experimental photoacoustic imaging setup diagram (b) Photograph of the experimental setup.
(c) Pulse-echo signal (top), Photoacoustic signal (bottom).

 

 

 

FIGURE 2. Superimposed structural and functional images. Traditional pulse-echo images are shown in grayscale. Photoacoustic images are shown in red. (a) Received by the 16x16 aperture. (b) Received by the 48x48 aperture.

 

 

 

References

 

[1] I. O. Wygant, X. Zhuang, P. S. Kuo, D. T. Yeh, Ö. Oralkan, and B. T. Khuri-Yakub. “Photoacoustic imaging using a two-dimensional CMUT array,” in Proc. IEEE Ultrason. Symp., 2005, pp. 1921-1924.

[2] S. Vaithilingam, I. O. Wygant, P. S. Kuo, X. Zhuang, Ö. Oralkan, P. D. Olcott, and B. T. Khuri-Yakub, “Capacitive micromachined ultrasonic transducers (CMUTs) for photoacoustic imaging,” in Proc. SPIE Photons Plus Ultrasound: Imaging and Sensing 2005: The Sixth Conference on Biomedical Thermoacoustics, Optoacoustics, and Acousto-optics, vol. 6086, pp. 1-11, 2006. [Invited]

[3] S. Vaithilingam, I. O. Wygant, S. Sifferman, X. Zhuang, Y. Furukawa, Ö. Oralkan, S. Keren, S. S. Gambhir and B. T. Khuri-Yakub, “Tomographic photoacoustic imaging using capacitive micromachined ultrasonic transducer (CMUT) technology,” presented at the IEEE Ultrason. Symp., Vancouver, BC, Canada, 2006.

 

Acknowledgements

 

This project was funded by National Institutes of Health under grants CA99059 and by Canon, Inc.