Conventional 2D Imaging Using 1D CMUT Arrays

We have built 1D linear CMUT arrays of equivalent size, channel count, and operating frequency to state-of-the-art medical arrays employing piezoelectric transducers. These arrays contain 128 individually-addressable elements, each 6 mm high by 200 µm wide (Fig. 1). The element pitch is 250 µm, ideal for the chosen 3 MHz operating frequency. A lossy lens was not used to focus in the elevation direction and reduce cross-talk.

To test the imaging performance of 1-D CMUT arrays, we built a resolution phantom roughly mimicking the attenuation properties of soft tissue with embedded wire targets. Using a custom designed PC-based experimental system, we acquired a complete set of 128 x 128 RF A-scans from all transmit-receive element combinations (Fig. 2). Wire target echoes exhibited an 80% fractional bandwidth around 3 MHz including the effect of attenuation in the medium. B-scans were reconstructed off-line over a 90 degree sector to a depth of 210 mm using RF beamforming and synthetic phased array approaches, as illustrated in Fig. 3. The 6-dB lateral and axial resolutions at 135 mm depth were 0.0144 radians and 0.3 mm, respectively (Fig. 4). The electronic noise floor was more than 50 dB below the maximum mainlobe magnitude. We also performed preliminary investigations on the effects of crosstalk on image quality. In the near field, some artifacts extended to a depth of 2 cm. A tail was also observed in the PSF in the axial direction, indicating some crosstalk. The tail amplitude relative to the mainlobe was less than -20 dB, as shown Fig.C.8.b. The details of this work can be found in [1]. Recently, clinical images have also been obtained on commercial scanners using 1D CMUT arrays [2].

 

 

FIGURE 1. 1D CMUT array.

 

 

 

FIGURE 2. A 16x16 2D array integrated with frontend electronic circuits.

 

 

 

FIGURE 3. Experimental and simulated mages of a resolution test phantom.

 

 

 

FIGURE 4. Axial and lateral point spread functions.

 

 

References

 

[1] Oralkan Ö, Ergun S, Johnson JA, Karaman M, Demirci U, Kaviani K, Lee TH, and Khuri-Yakub BT, “Capacitive Micromachined Ultrasonic Transducers: Next Generation Arrays for Acoustic Imaging?”, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 49, pp. 1596-1610, Nov. 2002.

[2] Mills DM, Smith LS, “Real-time in vivo imaging with capacitive micromachined ultrasound transducer (CMUT) linear arrays,” Proceedings of the 2003 IEEE Ultrasonics Symposium, pp. 568-571, 2003.