Lewis Thompson, India; (2025) Multi-Modality Fibre-Optic Imaging for Minimally Invasive Applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

All-optical ultrasound (OpUS) is an imaging paradigm which uses light to both generate and receive ultrasound and has progressed from benchtop to in vivo studies in recent years, demonstrating promise for minimally invasive surgical applications. This technique has advantages in terms of ease of miniaturisation through the use of optical fibres, high sensitivity, broad ultrasound bandwidth and immunity to electromagnetic interference. Additionally, the use of optics allows for easy modification to allow a lateral viewing configuration, which enables the operator to have a view of the specific vessel wall, such as coronary arteries. Another promising feature of OpUS is the potential to integrate other optical modalities, such as photoacoustic imaging or laser ablation via additional optical fibres or wavelength-selective coatings. Recently there has been interest in using miniaturised hybrid probes for guiding minimally invasive procedures. One promising hybrid imaging methodology is the combination of ultrasound imaging and fluorescence sensing. With ultrasound imaging, an image is formed by the combination of the received reflections of ultrasound waves, thereby providing structural contrast. With fluorescence sensing, excitation light is delivered to the imaging target where it is selectively absorbed by molecules, known as fluorophores, which emit light via fluorescence, thereby providing molecular contrast. These two complementary modalities have been realised previously for imaging of the gastrointestinal tract and for assessment of cancer progression. This thesis focuses on the integration of complementary optical modalities into a miniaturised lateral Optical Ultrasound (OpUS) probe. Ultrasound transmitters were fabricated using a novel composite coating to assess the feasibility of creating a wavelength-selective coating with the appropriate transmission windows for complementary fluorescence sensing. These transmitters were capable of generating ultrasound pressures in excess of 1 MPa with corresponding −6 dB bandwidths > 20 MHz. After modification to generate a lateral field-of-view, these transmitters were integrated into a clinically-compatible catheter and housing. A novel rapid acquisition imaging methodology was demonstrated, enabling imaging resolutions as low as 45 μm and 120 μm in the axial and lateral extent respectively whilst achieving subsecond acquisition periods. To demonstrate the potential of the device for clinical imaging, an ex vivo swine oesophagus was imaged using the working channel of a mock endoscope for device delivery. Further, fluorescence sensing was incorporated to present a fibre-optic dual-modality side-viewing device for rapid pullback imaging. This work demonstrates the promise of all-optical hybrid imaging probes to provide rapid diagnostics and guidance alongside conventional endoscopy.

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