Endoscopic probe for ultrasound-assisted photodynamic therapy of deep-lying tissue

Endoscopic photodynamic therapy (PDT) has attracted much attention as a minimally invasive, minimally toxic treatment. PDT can selectively destroy cancer cells by reactive oxygen generated through chemical reaction of photosensitizers that absorb incident light with a certain wavelength. However, the efficacy of PDT is limited due to shallow light penetration mainly caused by Rayleigh scattering in biological tissue. We previously demonstrated that the air bubbles temporarily induced by ultrasound (US) act as a Mie scattering medium, thus increasing light penetration, called US-assisted light penetration increase. For endoscopic applications of the proposed method, the endoscopic probe for US-assisted PDT should be developed; it should be as small as possible while being able to deliver laser for treatment and provide high acoustic energy for air bubble generation in the laser pathway. Satisfying those requirements is very challenging. In this paper, we report a recently developed endoscopic probe for US-assisted PDT that meets the requirements. The probe consists of two 3 MHz US transducers placed side by side and tilted at an angle of 25^◦ and an optical fiber for laser delivery. The probe size was measured at 60.5 mm × 14 mm × 14 mm suitable for endoscopic applications. In the experiment, it was shown that the probe could increase the laser intensity by 28%, compared to the laser only illumination. Even this increase is higher than the previous measurement using a commercial ring-shaped 1.1 MHz HIFU transducer with outer and inner diameters of 64 mm and 22.6 mm.