Top orthogonal to bottom electrode (TOBE) capacitive micromachined ultrasound transducers (CMUTs) are a new transducer architecture that permits large 2-D arrays to be addressed using row-column addressing. Here, we demonstrate the feasibility of 3-D photoacoustic imaging using N laser pulses and N receive channels. We used a synthetic aperture approach to simulate a large 2 X 2 cm array using a smaller die. A hair phantom in an oil immersion medium was excited by a laser, and the received signal was dynamically focused to obtain high-resolution images. We found the TOBE CMUT to have a center frequency of 3.7 MHz with a bandwidth of 3.9 MHz. Lateral and axial resolutions were 866 ¿m and 296 μm, respectively.

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Synthetic transmit aperture (STA) ultrasound imaging offers near-ideal reconstruction across an entire field of view. This performance comes at the cost of SNR compared with scanning using only dynamic receive focusing. SNR may be enhanced by using spatial encoding using a Hadamard sequence. An encoding based on a Hadamard sequence has two main drawbacks: the array must be capable of transmitting a pulse and an inverted pulse at the same time, and the inverted transmission must be symmetrical with respect to the non-inverted transmission. These are often not the case in practice, and thus Hadamard encoding may require twice as many transmission events and special consideration of the inverted waveform. As an alternative, we propose the use of S-sequences, which are similar to Hadamard sequences, but use half the elements and do not require an inverted pulse. This encoding is implemented on a commercial ultrasound system and compared with STA imaging using single-element emissions and Hadamard encoding in terms of SNR and resolution using a point target. We find that the two encodings perform very similarly despite the increased transmit power and doubling of transmit events in our implementation of Hadamard imaging. Both encodings give up to 19 dB signal improvement over single-element STA imaging, while maintaining resolution. Finally, we show sample in vivo human carotid images with all three methods which illustrate the suitability of S-sequence-encoded STA imaging for a clinical setting.