## 4 Discussion  
We developed an optical-resolution forward-viewing photoacoustic endomicroscopy probe based on an optically transparent ultrasound sensor using a ITO-coated PVDF thin film. Although transparent ultrasound sensors have been used in PAI in recent years, this work is the first study, to the best of our knowledge, that a transparent piezoelectric sensor is used in PAE. The outer diameter of the endomicroscopy probe is 2 mm defined by the diameter of the transparent sensor, which can be further reduced towards the diameter of a multimode fibre. With future optimisation, such a forward-viewing probe could be well suited for the guidance of minimally invasive procedures such as fetal surgery and tumour biopsy.  
Although optical ultrasound sensors possess several distinct advantages such as large bandwidths, and high sensitivity with miniature footprints, the transparent PVDF-ITO sensor benefits from a simple manufacturing process, a high stability, and a low cost. The optical transparency of the PVDF-ITO sensor also facilitates the integration of PAM with other optical imaging modalities such as confocal, fluorescence, two-photon, and Raman imaging to provide more comprehensive information of tissue. Compared to LiNbO3, PVDF transducers benefit from lower acoustic impedance mismatch with tissue, higher frequency bandwidths and greater flexibility.  
It is significant to be able to set the imaging plane close to the fibre tip as a large numerical aperture can thus be retained for high spatial resolution. In this work, the closest distance to the PVDF thin film that the imaging sample can be placed was 200 \(\upmu\)m due to the  
Figure 5: Examples of photoacoustic microscopy (PAM) images of carbon fibres acquired with the developed imaging probe. Images on the left column are detected ultrasound signals before and after the singular value decomposition (SVD) denoising at different imaging planes with the distance from the probe tip of 200 \(\upmu\)m (**a**), 320 \(\upmu\)m (**e**) and 620 \(\upmu\)m (**i**). (**b,f,j**) are optical microscopy images of carbon fibre networks at different imaging planes. The right two columns are PAM images before and after SVD denoising at 200 \(\upmu\)m (**c,d**), 320 \(\upmu\)m (**g,h**) and 620 \(\upmu\)m (**k,l**). Scale bars represent 10 \(\upmu\)m.  
conductive epoxy and electrical wire attached on the PVDF surface. The SVD filter was able to eliminate trigger-induced electrical noises which overwhelmed photoacoustic signals generated from objects close to the sensor (200-620 nm), which substantially improved the spatial resolution of the imaging probe from 3.7 to 2.0 \(\upmu\)m as indicated by the sizes of the optical foci at different imaging planes (Figure 5). The PVDF had a thickness of \(\sim\)50 \(\upmu\)m and a \(-\)10 dB bandwidth of 25 MHz. A thinner PVDF film will provide a larger bandwidth and thus a higher axial resolution for 3D imaging.  
High-resolution OR-PAM imaging has been demonstrated on phantoms comprising carbon fibres. In future works, the performance, such as the imaging depth of the imaging probe, will need to be tested on an in vivo animal model towards clinical translation. A pre-amplifier could be developed to improve the signal-to-noise ratio, which has been demonstrated effective in a benchtop PAM configuration [3]. In addition, better electrical grounding and shielding of the sensor could also be developed to reduce environmental noise. Another challenge for future clinical translation is related to fibre bending. Since the light transport through multimode fibres needs to be characterised before imaging, significant geometry changes degrade light focusing performance and thus the imaging quality. Potential solutions include employing a bending-insensitive fibre or computationally correcting the bending-induced distortions as demonstrated in previous studies [44; 45].