###### Abstract  
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 \(\upmu\)m. 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. photoacoustic imaging; photoacoustic endoscopy; endomicroscopy; ultrasound; transparent piezoelectric transducer; wavefront shaping +
Footnote †: journal: Medical Imaging  
clinical disciplines such as oncology [4], dermatology [5; 6; 7] and cardiology [8]. Recent developments of photoacoustic endoscopy (PAE) have significantly extended the landscape of the clinical applicability of PAI to examining internal tissues [9; 10]. Catheter-based side-viewing PAE has been the main research focus for imaging gastrointestinal tract tumours [11; 12] and atherosclerotic plaques [13; 14]. In contrast, forward-viewing probes are more suitable for guiding minimally invasive procedures such as needle biopsy, fetal surgery, and nerve blocks [15; 16]. Optical fibres such as coherent bundles and multimode fibres are commonly used for delivery of the excitation light [17; 18; 19; 20; 21; 22; 23; 24]. However, as conventional ultrasound detection usually relies on opaque piezoelectric transducers, a major challenge in the development of a miniature forward-viewing PAE probe has been the integration of the laser delivery module with the ultrasound detection module within a small space at the endoscopy probe tip. As a result, early studies employed transmission-mode implementations with the transducer placed between the light delivery fibre and tissue objects [17; 18; 20], which is not practical for clinical endoscopy applications. Optical interferometry-based ultrasound sensors, owing to good optical transparency and broad acoustic bandwidths, have played an important role in the development of forward-viewing PAE probes. In 2018, [PERSON] et al. developed an all-optical forward-viewing PAE probes based on the photoacoustic computed tomography (PACT) principle [15], in which a dichromatic Fabry-Perot sensor was integrated at the distal tip of a rigid fibre bundle. Excitation laser (590 nm) was transmitted through the bundle and a continue wave laser (1500-1630 nm) was focused into and scanned across individual fibre cores to interrogate the sensor for ultrasound detection. High-fidelity 3D images of microvascular anatomies of animal and human tissues were obtained after tomographic image reconstructions. Recently, the same group further developed a flexible forward-viewing PAE by integrating a fibre bundle with a Fabry-Perot sensor using a miniature telecentric relay system [16; 17].