Fabrication of end-coated fibre-optic ultrasound transmitters

Obliquely emitting fibre-optic optical ultrasound (OpUS) probes show strong potential for guiding minimally invasive procedures, but the imprecise coating methods used to date deposit excess coating along the side of the fibre, which has been suspected to cause imaging artefacts. In this study, we aimed to confirm the source of these artefacts and to introduce an alternative coating method. Optical fibres cleaved at 40° were coated either by conventional dip-coating (DC) or by a new dip-transfer (DT) method, where the curvature of coating droplets allows for accurate alignment of angle-cleaved end-faces. To demonstrate the advantage of DT over DC deposition, fibre-optic ultrasound sources were fabricated using both methods and thoroughly characterised using planar and cylindrical acoustic field scans. Furthermore, a source fabricated with each method was assembled into a fibre-optic OpUS imaging probe and used to image a controlled phantom. DT transmitters generated significantly higher ultrasound pressures than DC transmitters (1.64 ± 0.14 MPa vs 0.53 ± 0.22 MPa, p < 0.05), with comparable mean bandwidths (24.7 ± 1.6 MHz vs 17.3 ± 6.9 MHz) with significantly lower variability. Cylindrical field scans revealed that DC transmitters exhibited strong secondary acoustic lobes (20% of primary beam amplitude), whereas sources fabricated with the DT method did not generate detectable spurious emissions. Imaging trials confirmed that coating of the fibre sides is the primary source of artefacts in obliquely emitting OpUS probes. The DT method effectively eliminates these artefacts while simultaneously enhancing acoustic performance. The higher acoustic performance and the absence of spurious emissions and their associated image artefacts will further advance the quality of OpUS image guidance of minimally invasive interventions. Furthermore, this study reinforces the need for specialised coating methods for obliquely-viewing OpUS probes and highlights the importance the coating coverage in determining image quality.