Backward-mode multiwavelength photoacoustic scanner using a planar Fabry-Perot polymer film ultrasound sensor for high-resolution three-dimensional imaging of biological tissues

Department of Medical Physics and Bioengineering, University College London, Gower Street, London WC1E 6BT, UK.
Applied Optics (Impact Factor: 1.78). 03/2008; 47(4):561-77. DOI: 10.1364/AO.47.000561
Source: PubMed


A multiwavelength backward-mode planar photoacoustic scanner for 3D imaging of soft tissues to depths of several millimeters with a spatial resolution in the tens to hundreds of micrometers range is described. The system comprises a tunable optical parametric oscillator laser system that provides nanosecond laser pulses between 600 and 1200 nm for generating the photoacoustic signals and an optical ultrasound mapping system based upon a Fabry-Perot polymer film sensor for detecting them. The system enables photoacoustic signals to be mapped in 2D over a 50 mm diameter aperture in steps of 10 microm with an optically defined element size of 64 microm. Two sensors were used, one with a 22 microm thick polymer film spacer and the other with a 38 mum thick spacer providing -3 dB acoustic bandwidths of 39 and 22 MHz, respectively. The measured noise equivalent pressure of the 38 microm sensor was 0.21 kPa over a 20 MHz measurement bandwidth. The instrument line-spread function (LSF) was measured as a function of position and the minimum lateral and vertical LSFs found to be 38 and 15 microm, respectively. To demonstrate the ability of the system to provide high-resolution 3D images, a range of absorbing objects were imaged. Among these was a blood vessel phantom that comprised a network of blood filled tubes of diameters ranging from 62 to 300 microm immersed in an optically scattering liquid. In addition, to demonstrate the applicability of the system to spectroscopic imaging, a phantom comprising tubes filled with dyes of different spectral characteristics was imaged at a range of wavelengths. It is considered that this type of instrument may provide a practicable alternative to piezoelectric-based photoacoustic systems for high-resolution structural and functional imaging of the skin microvasculature and other superficial structures.

    • "Optical detection of ultrasound addresses many of the challenges associated with transducers. Examples include optical interferometers (Speirs and Bishop 2013), fiber Bragg detectors (Rosenthal et al 2011), micro-ring resonators (Ling et al 2011), and Fabry–Pérot sensors (Zhang et al 2008). These detectors have the benefit of broadband sensitivity and a point-like detection spot, therefore, spatial resolution is high compared to piezoelectric transducers. "
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    • "Illumination of the coating through the fibre core was performed with pulsed laser light (2 ns pulse duration, 1064 nm, 35 µJ pulse energy, SPOT-10- 500-1064, Elforlight, UK), and through the photoacoustic effect a broad-band acoustic field was transmitted. This field propagated through the acoustic lens and was subsequently detected optically using a custom planar Fabry-Pérot etalon with a thickness of 10 µm exhibiting a broadband sensitivity of 0 − 110 MHz [10]. Using a custom 3D-printed holder, the source fibre was centered with respect to the acoustic lens, with its distal end located at a distance of 4.5 mm from the acoustic lens; the etalon was located on the other side of the lens at a distance of 7 mm from the lens centre with its surface perpendicular to the axial direction. "
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    • "Different spacer thicknesses have been employed, chosen to optimise the sensitivity and bandwidth for the given application. For example, as part of the photoacoustic scanner described in [9] "

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