One-dimensional optoacoustic receive array employing chirped excitation and GPU-based beamforming

Optical techniques are a promising technology to realize high frequency ultrasound arrays. High sensitivity and broad bandwidth have been demonstrated with thin film etalon sensors. We have previously demonstrated a 256-element etalon array with true parallel detection. A disadvantage of this approach is lower signal-to-noise ratio (SNR) due to the use of a linear CCD array. We are also exploring a serial approach where the optical probe beam is rapidly scanned to produce a 500-element etalon array. Higher SNR is achieved with photodiode detection and high optical probe power. The optoacoustic sensor is a thin film etalon consisting of a parylene layer with gold coatings on a glass substrate. A fiber coupled 785 nm diode laser is focused on the etalon, where the beam is rapidly scanned over a 5 mm line at a 50 Hz rate. The reflected beam is coupled into a multimode fiber and sent to an AC-coupled photodetector. The 25 MHz source transducer is driven with a 4 μs duration chirped waveform at a 50 kHz repetition rate. A 500 MHz 8-bit oscilloscope records the signals from all 500 elements before transfer to a workstation. Pulse compression, beamforming, and image display are performed with a graphics processing unit (GPU). The object is a wire target placed approximately 3 mm below the etalon surface. The chirped excitation provides an SNR increase of over 18 dB with respect to traditional impulse excitation. After pulse compression, the array data clearly shows the scattered wavefront from the wire target. The wire target is clearly visible, where the -6 dB width is 95 um. Our current system uses the GPU to perform off-line beamforming, where real-time execution is currently under development. An advantage of the serial approach over parallel detection is higher SNR, since the entire probe laser power is used to acquire the signal from an individual element. We are currently improving the system to provide B-mode images at video frame rates. We believe these r- sults suggest the potential of optoacoustic arrays for video-rate ultrasound biomicroscopy.