Simultaneous photoacoustic imaging and white light video endoscopy for guiding laparoscopic surgery (Conference Presentation)

Abstract

White light endoscopy is widely used both as a diagnostic tool for the assessment of abdominal cancers and to help guide their surgical excision using minimally invasive procedures. However, the information it provides is limited to visual inspection of the tissue surface. Endoscopic ultrasonography provides depth-resolved morphological images but exhibits poor label-free microvascular contrast thus limiting its ability to identify and delineate deep seated tumours. These drawbacks can potentially be addressed by using a laparoscopic probe that provides co-registered photoacoustic (PA) and white light endoscopy images. With the help of PA contrast, the probe can visualise depth-resolved microvasculature and thus offers the prospect of more sensitive detection of tumours based on abnormal vascular anatomy and function. However, it is challenging to implement such a probe using conventional piezoelectric transducers. Their opaque nature makes it difficult to achieve forward-viewing capability in a small footprint as required for laparoscopic use as well as incorporate videoscopy. Furthermore, achieving sufficiently widebandwidth (tens of MHz) and λ/2 spatial sampling as required for high resolution endoscopic PA imaging present further challenges. To address these challenges, we present a rigid miniature forward-viewing endoscope that is based on a transparent optical ultrasound sensor which offers a wideband response up to 50 MHz with sub-100 µm spatial sampling. The probe is designed for laparoscopic use. It is 260 mm long and 9 mm in outer diameter to permit insertion via a standard 12 mm abdominal trochar and comprises a lens relay system with a high-finesse FP ultrasound sensor at its distal end. The sensor is designed to operate in the 1500 – 1600 nm spectral range with high transmission in the visible to near-infrared region (550 – 1200 nm). The latter not only enables delivery of near-infrared pulsed excitation light through the sensor to acquire PA images but also transmission of visible CW light for simultaneous acquisition of wide-field video images at the probe tip. A MEMS scanning mirror located at the proximal end of the probe scans the FP sensor via the optical relay with 8 focused beams from a CW tunable laser source (1550 nm centre wavelength) to map the generated photoacoustic waves. High-resolution 3D tomographic images are reconstructed using a time reversal algorithm and fused with the white light video images. The probe has 8 mm lateral field-of-view and the NEP is 200 Pa over 20 MHz bandwidth. The lateral spatial resolution is 52 µm at a depth of 1 mm decreasing to 110µm at a depth of 7 mm. The axial resolution is 29 µm over this depth range. To demonstrate potential clinical applicability, the probe was evaluated in an in vivo sheep study and shown to provide excellent high resolution 3D images of vascular structures in the liver, kidney and placentomes. This novel forward-viewing PAE probe could provide new opportunities for the photoacoustic assessment of tumours in the liver, cancer in the GI tract and guiding minimally invasive procedures in abdominal surgery and foetal medicine.