One-dimensional optoacoustic receive array employing parallel detection and video-rate acquisition

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. Etalon arrays usually involve synthetic aperture techniques, where a single probe laser beam or photodiode is scanned over the sensing region. True parallel detection suitable for video-rate B-mode imaging remains a challenge. We demonstrate a 256-element one-dimensional receive array employing true parallel detection. The optoacoustic sensor is a thin film etalon consisting of a photoresist layer with gold coatings on a glass substrate. A fiber coupled 785 run diode laser is brought to a line focus (3.6 mm length) on the etalon. The line focus is imaged with a one-to-one lens relay onto a 512 element line scan CCD camera with a 14 × 14 μm pixel size. The etalon array is tested with a 25 MHz f/2 transducer. Both the transducer and probe laser are modulated with a frequency sweep from 10 MHz to 40 MHz within a 14 ms time window. The CCD camera acquires 1200 lines with an exposure time of 10 μs per line. The frequency domain data are processed off-line, where each frame involves background subtraction, spectral apodization, zero padding, and an inverse Fourier transform. The reconstructed time-domain wavefield data for the entire array spans 16 μs. Both the measured pulse duration (60 ns) and main lobe width (154 μm) agree well with the expected values from a 25 MHz f/2 transducer. Although the camera has 512 pixels, only 256 were illuminated with the line focus. A swept frequency acquisition is easily adapted for higher frequency (i.e. 75 MHz) signals. Each B-mode data frame was acquired in less than 15 ms, which approaches the speed necessary for video-rate acquisition. We believe these results suggest the potential of optoacoustic arrays for video-rate ultrasound biomicroscopy.