King's College London

Photoacoustic endoscopy (PAE) can provide 3D functional, molecular and structural
information of tissue deep inside the human body, and thus could be well suited for guiding minimally invasive procedures such as tumour biopsy and fetal surgery. One of the major challenges in the development of miniature PAE probes, in particular, forward-viewing PAE probes, is the integration of a sensitive and broadband ultrasound sensor with the light delivery and scanning system into a small footprint. In this work, we developed a forward-viewing PAE probe enabling optical-resolution microscopy imaging based on a transparent ultrasound sensor coated on the distal end of a multimode optical fibre. The transparent sensor comprised a transparent polyvinylidene fluoride (PVDF) thin film coated with indium tin oxide (ITO) electrodes with a diameter of 2 mm. Excitation laser light was focused and raster-scanned across the facet of the probe tip through the multimode fibre and the PVDF-ITO thin film via wavefront shaping. The sensor had an optical transmission rate of 55–72% in the wavelength range of 400 to 800 nm, a centre frequency of 17.5 MHz and a -10 dB bandwidth of 25 MHz. Singular value decomposition was used to remove a prominent trigger-induced noise, which enabled imaging close to the probe tip with an optically defined lateral resolution of 2 um. The performance of the imaging probe was demonstrated by obtaining high-fidelity photoacoustic microscopy images of carbon fibres. With further optimisation of the sensitivity, the probe promises to guide minimally invasive procedures by providing in situ, in vivo characterisation of tissue.