Radiation profile measurement methods for optical light diffusers: comparison of imaging and non-imaging methods

Abstract

Interstitial Photodymanic Therapy (iPDT) selectively treats malignant brain cancer. A photoactive drug, which accumulates specifically in tumour cells, is delivered to the patient. By irradiation of the targeted brain region with low power laser light, radical oxygen is generated, leading to apoptosis or necrosis. For the illumination, glass fibers are connected to a laser source and inserted through boreholes into the human skull. At the distal fiber end, light is decoupled from the fiber and spread into the surrounding tissue. Various approaches target the decoupling process from the fiber core to the surrounding tissue including the use of polymer diffusers, surface roughening or internal glass modification using ultrafast laser sources. While the manufacturing process is a widely debated topic, the measurement of the radiation profile of the fibers is rarely discussed. Therefore, this study targets the introduction of a novel method, using a plain camera sensor, which was brought in close contact to the fibre diffuser surface. Thereby, an angle-resolved profile should be accessible. The gained profiles were afterwards compared to the state-of-the-art method, a camera setup recording an image of the diffuser surface through an optical lens, as well as to an integrating sphere, collecting all emitted rays along the diffuser length. The results showed that the main characteristics are similar in between all three methods, however its development strongly differs. This could be explained by the limitations of the camera setup, which is limited by the acceptance angle of the used camera lens. Rays emitted under a flat angle from the fibre surface miss the lens aperture and won’t be detected. For the camera sensor and the integrating sphere setup, those rays are detected. The results show a dependency of the radiation profile on the selected method. The novel camera sensor method allows an angle-resolved measurement detecting almost the full radiation profile. Furthermore, a simulation model can be easily established, which could be used to predetermine the radiation profile of manufactured diffusers.