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.
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.
The therapeutic effect of most retinal laser treatments is initiated by a transient temperature increase. Although crucial to the effectiveness of the treatment, the temperature course is not exactly known due to individually different tissue properties. We develop an optoacoustic method to determine the retinal temperature increase in real time during continuous-wave (cw) laser irradiation, and perform temperature calculations to interpret the results exemplary for transpupillary thermotherapy (TTT). Porcine globes ex vivo and rabbit eyes in vivo are irradiated with a diode laser (=810 nm, P3 W, =2 mm) for 60 s. Simultaneously, pulses from a N2-laser pumped dye laser (=500 nm, =3.5 ns, E5 µJ) are applied on the retina. Following its absorption, an ultrasonic pressure wave is emitted, which is detected by a transducer embedded in a contact lens. Using the previously measured temperature-dependent Grüneisen coefficient of chorioretinal tissue, a temperature raise in porcine eyes of 5.8 °C/(W/cm2) after 60 s is observed and confirmed by simultaneous measurements with an inserted thermocouple. In a rabbit, we find 1.4 °C/(W/cm2) with, and 2.2 °C/(W/cm2) without perfusion at the same location. Coagulation of the rabbit's retina occurs at T=21 °C after 40 s. In conclusion, this optoacoustic method seems feasible for an in vivo real-time determination of temperature, opening the possibility for feedback control retinal laser treatments.
In almost all retinal laser treatments the therapeutic effect is initiated by a transient temperature increase. Due to differences in tissue properties and physiology like pigmentation and vascular blood flow an individually different temperature increase might occur with crucial effects on the therapeutic benefit of the treatment. In order to determine the individual retinal temperature increase during cw-laser irradiation in real-time we developed a non-invasive method based on optoacoustics. Simultaneously to the cw-laser irradiation (λ = 810 nm, P < 3 W, t = 60 s) pulses from a dye laser (λ = 500 nm, τ = 3.5 ns, Ε ≈ 5 μJ) are applied concentrically to the cw-laser spot on the eyeground. The absorption of the pulses lead to a consequent heating and thermoelastic expansion of the tissue. This causes the emission of an ultrasonic pressure wave, which amplitude was found to be temperature dependent following in good approximation a 2nd order polynomial. The pressure wave was measured by an ultrasonic transducer embedded in a contact lens placed on the cornea. The experiments were performed in-vivo on rabbits. Simultaneous measurements with a miniaturized thermocouple showed a similar slope with a maximum local deviation of 0.4 °C for a temperature increase of 5.5 °C. On two rabbits measurements pre and post mortem at the same location were performed. The temperature increase after 60 s was found to raise by 12.0 % and 66.7 % post mortem, respectively.
These data were used to calculate the influence of heat convection by blood circulation using a numerical model based on two absorbing layers and assuming a constant perfusion rate for the choriocapillaris and the choroid.
Overall the presented optoacoustic method seems feasible for a non-invasive real-time determination of cw-laser induced retinal temperature increases and might serve as a temperature based dosimetry control during retinal laser treatments.
Tumor thermo treatment such as photodynamic therapy (PDT) or transpupillary thermotherapy (TTT) deal with long term and large laser spot exposures. The induced temperature increase is not exactly known [1]. Under these conditions convective heat transfers due to the blood flow in the choroid and the choriocapillaris must be considered in addition to the usually calculated heat conduction. From an existing analytical model defining a unique convective term for the whole fundus irradiated with Gaussian irradiance distribution lasers [2], we developed a numerical one allowing a precise modelling of convection and calculating heating evolution and temperature profiles of the fundus of the eye. The aim of this study is to present the modelling and several comparisons between experimental results [3] and numerical ones concerning the convective heat transfers inside the fundus of the eye.
KEYWORDS: Temperature metrology, Eye, Ultrasonics, Heat therapy, Transducers, Laser beam diagnostics, Laser therapeutics, In vivo imaging, Signal detection, Lamps
Transpupillary Thermotherapy (TTT) is a retinal laser treatment targeting on occult neovascular membranes. However, the induced temperature increase during irradiation is not exactly known. Model calculations show that differences in pigmentation and vascular perfusion should lead to a different temperature rise under same treatment parameters. In order to evaluate the temperature profiles during TTT, we developed a non-invasive optoacoustic method.
The experiments were carried out on enucleated porcine eyes (ex-vivo) and rabbits (in-vivo). Simultaneously to the irradiation with a TTT-laser (λ=810nm, P≤3W) through a slit lamp, pulses from a pulsed dye laser (λ=500nm, τ=3ns, E=5μJ) were applied onto the irradiated area. This probe irradiation induces an ultrasonic pressure wave, its amplitude depends on the temperature of the absorbing media. We measured this pressure wave by an ultrasonic transducer integrated into a contact lens.
The induced pressure maximum for porcine RPE samples was found to raise by 14.6% between 37°C and 50°C. Using this relation we determined optoacoustically the temperature increase during TTT. A comparison of the acoustic data with simultaneous temperature measurements by a thermo couple positioned in the choroid showed accordance within ±1.2°C. Differences in pigmentation led to a 1.8 fold induced temperature increase between weak and heavy pigmented eyes. First results in the in-vivo animal study showed an average temperature increase of (2.45 ±0.16)°C for an irradiation of 1W/cm2.
In conclusion the developed optoacoustic method seems adequate for real-time temperature determination during retinal laser treatments and might serve as a dosimetry control for TTT.
Endoscopic Electronic-Speckle-Pattern Interferometry (endoscopic ESPI) is a tool for the detection and the measurement of displacements and movements in technical and biological cavities. For an adequate measurement accuracy additional to the geometry of imaging and illumination, information about the shape and curvature of the investigated cavities is required. Investigations to measure the surface topometry by endoscopic ESPI combined with a two-wavelength method have been carried out. The advantage of this arrangement, which uses commercial endoscope imaging systems, is that it is possible to obtain object shape and displacement data with the same endoscopic interferometric measurement system. Therefore, in a first step, the phase difference distribution effected by the change of the illumination wavelength and the object shape is detected by spatial phase shifting and in a second evaluation step, the radial distortions of the endoscope imaging system are corrected and the geometry of endoscopic off-axis illumination is taken into account by an approximation.
The method of endoscopic electronic-speckle-pattern interferometry (ESPI) is a tool for detection and visualization of deformations and movements in technical cavities as well as for non-destructive minimal invasive diagnostics within body cavities. Here, a spacial phase shifting (SPS) method is chosen for additional setup and the investigated specimen in comparison to temporal phase shifting techniques. With a simple arrangement, the SPS technique is adapted to the special conditions of standard endoscopic optical imaging. The characterization and optimization of the interferometer is carried out on technical specimen. To demonstrate possible applications of endoscopic ESPI and spatial phase shifting endoscopic (ESPI) on biological tissues, non-destructive investigations of biological heart valve prostheses are performed. The quality of the obtained phase difference fringes is sufficient for phase unwrapping and for visualization as well as quantitative evaluation. In conclusion, the results of the investigations show that it is possible to detect structural differences in tissues underneath the visible surface by application of SPS endoscopic ESPI even of biological specimen with a wet and low reflecting surface.
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