The extent of retinal laser coagulations depends on the temperature increase at the fundus and the time of irradiation. Due to light scattering within the eye and variable fundus pigmentation the induced temperature increase and therefore the extent of the coagulations cannot be predicted solely from the laser parameters. We use optoacoustics to monitor the temperature rise in real-time in vivo (rabbit) and ex vivo (porcine eye) and to automatically control the coagulation strength. Continuous wave treatment laser radiation and pulsed probe laser light (1-1100 ns) are coupled into the same
fibre and are imaged onto the retina by a laser slit lamp. The temperature dependent pressure waves are detected by an
ultrasonic transducer embedded in a customary contact lens. Below the coagulation threshold the increase in acoustic amplitude due to thermal tissue expansion is up to 40 %. Best signal to noise ratios > 10 are achieved with probe pulse durations of 1 to 75 ns. Further a time critical algorithm is developed which automatically ceases laser treatment when a certain preset coagulation strength is achieved. Coagulations with similar extent are obtained with this method in vitro and in vivo even when varying the power of the treatment laser by 50 %. These preliminary results are very promising, thus this method might be suitable for an automatic feedback controlled photocoagulation with adjustable coagulation
strength.
A 1.94 μm Laser-Scalpel system (Starmedtec, Starnberg, Germany) emitting at a wavelength at 1.94 μm and a max. laser
power 18 W (cw mode) was used for partial resection of porcine kidney. Partial resection was performed on 9 kidneys.
Six kidneys were extracted directly after resection and pigs were sacrificed. The remaining three pigs were kept alive for
2 weeks post resection in order to investigate the healing process. No complications during the operation and during the 2
weeks survival period were found. Total resection time including haemostasis of the remaining tissue was 10.2±6.5 min,
blood lost was 14.7±29 ml. Haemostatic property of the 1.94 μm Laser-Scalpel was sufficient. Histological evaluation
with H&E stained tissue samples showed medium carbonization and a thermal damage zone of 1mm. In conclusion, the
first experiments show that the 1.94 μm Laser-Scalpel is a very promising device for bloodless and fast kidney resection.
Retinal photocoagulation is an established treatment of different retinal diseases. The treatment relies on a short, local
heating of the tissue which induces a denaturation. The resulting scar formation may for example prevent the further
detachment of the retina. The extent of the coagulation is besides other parameters mostly dependent on the induced
temperature increase. However, until today a temperature based dosimetry for photocoagulation does not exist. The
dosage is rather based on the experience of the treating physicians to achieve visible whitish lesions on the retina. In this
work a technique is presented, which allows an online temperature monitoring during photocoagulation. If an absorbing
material is irradiated with short laser pulses, a thermoelastic expansion of the absorber induces an acoustic wave. Its
amplitude is dependent on the temperature of the absorber. For analyzing the applicability of the optoacoustic
temperature determination for dosimetry, measurements were performed on enucleated porcine eye globes. The pressure
transients are detected by an ultrasonic transducer, which is embedded in an ophthalmologic contact lens. As long as no
strong lesions occur, the determined temperatures are almost proportional to the power of the treatment laser. Using a
spot diameter of 200 μm and different laser powers, the temperature rise at the end of the 400 ms irradiation was found
to be approximately 0.16 °C/mW. The onset of the denaturation was observed around 50°C. The far aim of this project is
an automatic regulation of the treatment laser onto a desired temperature course.
In most retinal laser treatments the therapeutic effect is initiated by a transient temperature increase at and around the
retinal pigment epithelium (RPE). Especially in long exposure time treatments like Transpupillary Thermotherapy (TTT)
choroidal perfusion has a strong influence on the realized temperature at the fundus. The fundus blood circulation and
therefore the heat dissipation is influenced by the intraocular pressure (IOP), which is investigated in the study presented
here. In order to reduce the choroidal perfusion, the IOP is increased by injection of physiological saline solution into the
eye of anaesthetized rabbits. The fundus is irradiated with 3.64 W/cm2 by means of a TTT-laser (λ = 810 nm) for t = 20 s
causing a retinal temperature increase. Realtime temperature determination at the irradiated spot is achieved by a non
invasive optoacoustic technique. Perfusion can be reduced by increasing IOP, which leads to different temperature
increases when irradiating the retina. This should be considered for long time laser treatments.
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