Endoscopic ultrasonography (EUS) is an important clinical tool for the assessment of tumours in the abdominal cavity and guiding minimally invasive surgical procedures. However, one of the drawbacks of EUS is the lack of soft tissue contrast which can compromise its ability to visualise the microvasculature and thus delineate tu mour margins. Photoacoustic endoscopy (PAE) could potentially overcome this drawback as it can visualise the microvasculature with high contrast due to the strong optical absorption of haemoglobin. Previously, we have demonstrated an all-optical laboratory demonstrator PAE probe using standard bulk optical components. We now present a miniaturized flexible PAE probe with a 6 mm outer diameter and improved acoustic sensitivity capable of high resolution volumetric imaging in the forward-viewing configuration.
The probe comprises a 1.5 mm diameter flexible coherent fibre bundle consisting of 18,000 fibre-optic cores with a transparent 80MHz Fabry-Pérot ultrasound sensor at its distal end. The FP sensor thus acts as a high density 2D array composed of 18,000 individually addressable ultrasound detectors, each with a bandwidth of 80MHz and an element size and centre-to-centre spacing of 40 µm and 44 µm, respectively. The pulsed excitation light is coupled through all of the fibre-optic cores from the proximal end, and the generated photoacoustic waves are detected in backward mode by sequentially interrogating the FP sensor through individual fibre optic cores. This configuration allows for very fine sampling of the photoacoustic waves and thus high lateral spatial resolution and image fidelity. This new approach to photoacoustic endoscopy offers significant advantages over previous piezoelectric based distal-end scanning probes. These include a forward viewing capability and a wider detection bandwidth and finer spatial sampling than achievable with piezoelectric receivers, a high degree of miniaturisation, no moving parts at the distal end and relatively simple and inexpensive fabrication.
We have characterized the PAE probe in terms of its field of view, noise-equivalent pressure (NEP), and lateral and axial PSF. The lateral field of view is 5 mm and the NEP is 500 Pa over 20MHz bandwidth. The lateral spatial resolution is 50 µm at a depth of 1 mm decreasing to 150µm at a depth of 3 mm. The axial resolution is 33 µm over this depth range. The probe have also been evaluated using a variety of tissue phantoms and ex vivo tissues and shown to provide excellent high resolution 3D images of the vasculature. It is anticipated that this novel forward-viewing probe will provide new opportunities for the photoacoustic assessment of tumours in the liver and other abdominal organs, cancer in the GI tract and guiding minimally invasive procedures in abdominal surgery and foetal medicine.