A broadband all-optical plane-wave ultrasound imaging system for high-resolution 3D imaging of biological tissues is presented. The system is based on the planar Fabry-Perot (FP) scanner for ultrasound detection and the photoacoustic generation of ultrasound in a Carbon-Nanotube-Polydimethylsiloxane (CNT-PDMS) composite film. The FP sensor head was coated with the CNT-PDMS film to act as an ultrasound transmitting layer for pulse-echo imaging. Exciting the CNT-PDMS coating with nanosecond laser pulses generated monopolar plane-wave ultrasound pulses with MPa-range peak pressures, and a-6dB bandwidth of 22 MHz, that were transmitted into the target. The resulting scattered acoustic field was detected across a 15 mm × 15 mm scan area with a step size of 100 and an optically defined element size of 64. The-3dB bandwidth of the sensor was 30 MHz. A 3D image of the scatterer distribution was then recovered using a k-space reconstruction algorithm. To obtain a measure of spatial resolution, the instrument line-spread function (LSF) was measured as a function of position. At the centre of the scan area the depth dependent lateral LSF ranged from 46 to 65 for depths between 1 and 12 mm. The vertical LSF was independent of position and measured to be 44 over the entire field of view. To demonstrate the ability of the system to provide high-resolution 3D images, phantoms with well-defined scattering structures of arbitrary geometry were imaged. To demonstrate its suitability for imaging biological tissues, phantoms with similar impedance mismatches, sound speed and scattering properties to those present in tissue, and ex-vivo tissue samples were imaged. Compared to conventional piezoelectric based ultrasound scanners this approach offers the potential for improved image quality and higher resolution for superficial tissue imaging. Since the FP scanner is capable of high-resolution 3D photoacoustic imaging of in-vivo biological tissues, the system could ultimately be developed into an instrument for dual-mode all-optical ultrasound and photoacoustic imaging.

A full-wave model for nonlinear ultrasound propagation through a heterogeneous and absorbing medium in an axisymmetric coordinate system is developed. The model equations are solved using a nonstandard or k-space pseudospectral time domain method. Spatial gradients in the axial direction are calculated using the Fourier collocation spectral method, and spatial gradients in the radial direction are calculated using discrete trigonometric transforms. Time integration is performed using a k-space corrected finite difference scheme. This scheme is exact for plane waves propagating linearly in the axial direction in a homogeneous and lossless medium and significantly reduces numerical dispersion in the more general case. The implementation of the model is described, and performance benchmarks are given for a range of grid sizes. The model is validated by comparison with several analytical solutions. This includes one-dimensional absorption and nonlinearity, the pressure field generated by plane-piston and bowl transducers, and the scattering of a plane wave by a sphere. The general utility of the model is then demonstrated by simulating nonlinear transcranial ultrasound using a simplified head model.

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