Status and opportunities for future use of terahertz radiation for clinical applications

An overview of terahertz (THz) development is presented in view of possible medical applications, including details of the current technologies which could be effectively used. Although no clinical THz technologies are currently in use, its principal applicability has been already demonstrated in skin cancer detection and treatment, dental caries detection and pharmaceutical screening. Fundamental limitations of THz studies are highlighted which have to be overcome before clinical applications can be realized.

Investigation on cavitation bubble dynamics induced by clinically available Ho:YAG lasers

Endoscopic laser lithotripsy is the preferred technique for minimally invasive destruction of ureteral and kidney stones, and is mostly performed by pulsed holmium:yttrium-aluminum-garnet (Ho:YAG) laser irradiation. The absorbed laser energy heats the water creating a vapor bubble which collapses after the laser pulse, thus producing a shock wave. Part of the laser energy strikes the stone through the vapor bubble and induces thermomechanical material removal. Aim of the present study was to visualize the behavior and the dynamics of the cavitation bubble using a specially developed ultra-short-time illumination system and then to determine important characteristics related to clinically used laser and application parameters for a more detailed investigation in the future.In accordance with Toepler’s Schlieren technique, in the ultra-short-time-illumination set-up the cavitation bubble which had been induced by Ho:YAG laser irradiation at the fiber end, was illuminated by two Q-switched lasers and the process was imaged in high contrast on a video camera. Cavitation bubbles were induced using different pulse energies (500 mJ/pulse and 2000 mJ/pulse) and fiber core diameters (230 μm and 600 μm) and the bubble dynamics were recorded at different times relative to the Ho:YAG laser pulse. The time-dependent development of the bubble formation was determined from the recordings by measuring the bubble diameter in horizontal and vertical directions, together with the volume and localization of the center of the bubble collapse.The results show that the bubble dynamics can be visualized and studied with both high contrast and high temporal resolution. The bubble volume increases with pulse energy and with fiber diameter. The bubble shape is almost round when a larger fiber core diameter is used, and elliptical when using a fiber of smaller core diameter. Moreover, the center of the resulting bubble is slightly further away from the fiber end and the center of the bubble collapse for a smaller fiber core diameter.The experimental set-up developed gives a better understanding of the bubble dynamics. The experiments indicate that the distance between fiber tip and target surface, as well as the laser parameters used have considerable impact on the cavitation bubble dynamics. Both the bubble dynamics and their influence on the stone fragmentation process require further investigation.

Internal structuring of silica glass fibers: Requirements for scattered light applicators for the usability in medicine

Background:Diffuser fibers have been used for some time in the fields of laser-induced thermotherapy and photodynamic therapy. For their applicability the breaking strength, the thermostability and a homogeneous radiation profile are of great importance. Flexible applicators offer special benefits because they introduce a totally new range of application possibilities.Objective:The aim of the presented investigations was to develop a totally new flexible diffuser fiber generation which can be produced cheaper and without the use of any further materials. For this purpose it was proposed to induce scattering micro dots directly into silica fibers by generating a local change of the refractive index in the core of the optical fiber. The resulting diffuser was expected to create a homogeneous radiation profile containing at least 80% of the light coupled into the optical fiber, i.e. less than 20% prograde (forward) emission.Materials and methods:On the basis of former research results, scattering micro dots were induced linearly into the core of an optical silica fiber through a multiple photon process using a femtosecond laser. In addition to the macroscopic optical control by means of a microscope, the form of the radiation profile was examined as well as the non-scattered forward emission which depends on a variety of influencing factors. The processing was optimized according to the observations made. The thermostability of the developed prototypes was assessed by using a thermocamera, and the minimal bending radius was determined. Finally the prototypes were tested and validatedResults:An influence of the processing power, the number and radial position of the scattering micro dots as well as the therapeutic coupled-in wavelength onto the form of the radiation profile and the non-scattered forward emission was determined. Both the form of the radiation profile and the prograde emission were found to be independent of the therapeutic laser power coupled into the fiber. The developed prototype had a nearly homogeneous radiation profile, a forward emission of 12.8±2.1% in average, and a minimum bending radius of 31±6 mm.Conclusion:The non-scattered forward emission of the developed diffusers was within the objective of below 20% and the radiation profile was very nearly homogeneous. In order to improve the reproducibility of the production process, an improved fixation apparatus needs to be developed.

Enhancement of OCT imaging by blood optical clearing in vessels – A feasibility study

The results of a feasibility study of the application of PEG-300 and fructose as two independent optical clearing agents for the reduction of light scattering in biological tissues are presented.An OCT system operating at 1300 nm was used to study optical clearing effects. InThe intradermal injection of fructose in combination with the intravenous injection of PEG-300 led to a rapid optical clearing effect. In the experiments on miceThe experiments on mice have clearly demonstrated that intradermal and intravenous injections of optical clearing agents enhanced light transport through the skin and blood vessels.

Calibration of a three-dimensional photodynamic therapy illumination system and its segmentation assessment for port-wine stains

The uniformity of light dosimetry is an important parameter that affects the efficacy of photodynamic therapy (PDT). Although this uniformity can be improved by a three-dimensional (3D) digital PDT illumination system, it has a low field-of-view (FOV) utilization rate. A checkerboard calibration method using color coding is proposed to calibrate both the projector and camera of the system with a broad common FOV. Experiments reveal that the proposed method increases the utilization rate by up to three times compared with noncolor-coding methods with almost the same accuracy. A fine distinction of phantom lesions in the 3D system can be obtained by clustering, which may be used to optimize the treatment and light-dosimetry evaluation.