Selective Laser Trabeculoplasty (SLT) is a treatment option for open-angle glaucoma, however, it lacks an instant evidence for successful irradiation. So far ophthalmologists use the visible appearance of permanent champagne like bubbles as an indicator for appropriate pulse energy. We hypothesize that micro bubbles, which take place far below the appearance of macro bubbles already trigger the therapeutic benefit. Here we present two techniques for real-time detection of the onset of micro bubbles. The trabecular meshwork of freshly enucleated porcine eye globes was irradiated, in contrast to conventional SLT, with a series of 15 pulses with a pulse duration of 1.7 μs and with increasing energy at a repetition rate of 100 Hz per each spot of 200 μm in diameter. Both observation methods, an optoacoustic and an optical, are equally capable of detecting micro bubble nucleation, with sensitivities over 0.83 and specificities over 0.89. We demonstrated an accurate method for detection of micro bubble formation in SLT. In case that the therapeutically demanded pressure reduction is already achieved with these micro bubbles, which needs to be proved clinically, then the methods presented here can be used in a feedback loop controlling the laser irradiation. This will unburden the clinicians from any dosing during SLT.
Photocoagulation is a treatment modality for several retinal diseases. Intra- and inter-individual variations
of the retinal absorption as well as ocular transmission and light scattering makes it impossible to achieve
a uniform effective exposure with one set of laser parameters. To guarantee a uniform damage throughout
the therapy a real-time control is highly requested. Here, an approach to realize a real-time optical feedback
using dynamic speckle analysis in-vivo is presented. A 532 nm continuous wave Nd:YAG laser is used for
coagulation. During coagulation, speckle dynamics are monitored by a coherent object illumination using a
633 nm diode laser and analyzed by a CMOS camera with a frame rate up to 1 kHz. An algorithm is presented
that can discriminate between different categories of retinal pigment epithelial damage ex-vivo in enucleated
porcine eyes and that seems to be robust to noise in-vivo. Tissue changes in rabbits during retinal coagulation
could be observed for different lesion strengths. This algorithm can run on a FPGA and is able to calculate a
feedback value which is correlated to the thermal and coagulation induced tissue motion and thus the achieved
damage.
Photocoagulation is a laser treatment widely used for the therapy of several retinal diseases. Intra- and inter-individual
variations of the ocular transmission, light scattering and the retinal absorption makes it impossible
to achieve a uniform effective exposure and hence a uniform damage throughout the therapy. A real-time
monitoring and control of the induced damage is highly requested. Here, an approach to realize a real time
optical feedback using dynamic speckle analysis is presented. A 532 nm continuous wave Nd:YAG laser is
used for coagulation. During coagulation, speckle dynamics are monitored by a coherent object illumination
using a 633nm HeNe laser and analyzed by a CMOS camera with a frame rate up to 1 kHz. It is obvious that
a control system needs to determine whether the desired damage is achieved to shut down the system in a
fraction of the exposure time. Here we use a fast and simple adaption of the generalized difference algorithm
to analyze the speckle movements. This algorithm runs on a FPGA and is able to calculate a feedback value
which is correlated to the thermal and coagulation induced tissue motion and thus the achieved damage. For
different spot sizes (50-200 μm) and different exposure times (50-500 ms) the algorithm shows the ability to
discriminate between different categories of retinal pigment epithelial damage ex-vivo in enucleated porcine
eyes. Furthermore in-vivo experiments in rabbits show the ability of the system to determine tissue changes in
living tissue during coagulation.
Laser coagulation of the retina is an established treatment for several retinal diseases. The absorbed laser energy and thus the induced thermal damage varies with the transmittance and scattering properties of the anterior eye media and with the pigmentation of the fundus. The temperature plays the most important role in the coagulation process. An established approach to measure a mean retinal temperature rise is optoacoustics, however it provides limited information on the coagulation. Phase sensitive OCT potentially offers a three dimensional temporally resolved temperature distribution but is very sensitive to slightest movements which are clinically hard to avoid. We develop an optical technique able to monitor and quantify thermally and coagulation induced tissue movements (expansions and contractions) and changes in the tissue structure by dynamic laser speckle analysis (LSA) offering a 2D map of the affected area. A frequency doubled Nd:YAG laser (532nm) is used for photocoagulation. Enucleated porcine eyes are used as targets. The spot is 100μm. A Helium Neon laser (HeNe) is used for illumination. The backscattered light of a HeNe is captured with a camera and the speckle pattern is analyzed. A Q-switched Nd:YLF laser is used for simultaneous temperature measurements with the optoacoustic approach. Radial tissue movements in the micrometer regime have been observed. The signals evaluation by optical flow algorithms and generalized differences tuned out to be able to distinguish between regions with and without immediate cell damage. Both approaches have shown a sensitivity of 93% and a specificity above 99% at their optimal threshold.
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