Holger Lubatschowski

Thermal and mechanical damage of corneal tissue after free-running and Q-switched mid-infrared laser ablation

Laser ablation of corneal tissue by mid-infrared laser radiation was studied in dependence of the laser wavelength and the pulse duration. The thermally and mechanically laser induced damage was determined by light microscopy (LM) and scanning electron microscopy (SEM). Fresh porcine eyes were irradiated with three different lasers: Er:YAG ((lambda) equals 2.94 micrometers ), Er:YSGG ((lambda) equals 2.79 micrometers ) and Ho:YAG ((lambda) equals 2.1 micrometers ). The experiments were performed with both, free running (pulse duration (tau) equals 200 microsecond(s) ) and Q-switched pulsed ((tau) equals 100 ns) lasers. The extent of thermally damaged tissue was found to be the same for Er:YAG and Er:YSGG lasers: 8 - 45 micrometers and 2 - 10 micrometers , with the long and the Q-switched pulses, respectively. The Ho:YAG laser induced coagulation zones were 200 - 600 micrometers thick in the free running and 40 - 80 micrometers in the Q-switched mode. The ablation efficiency of Er-lasers was between 4 and 13 times higher than that of the Ho:YAG.

ArF-excimer laser-induced secondary radiation in photoablation of biological tissue

Secondary radiation, emitted during and after the irradiation of corneal, dermal, and dental tissue by an ArF‐excimer laser (193 nm), was qualitatively and quantitatively characterized. Emission of secondary radiation was found in the range of 200–800 nm. The intensity of secondary radiation in the range of 200–315 nm (UVC and UVB) is ∼20% of the total intensity at high laser fluences (>2 J/cm2), and ∼50% at moderate laser fluences (<500 mJ/cm2); 10 μJ/cm2 in the UVC and UVB were measured at the sample surface, at fluences (<1J/cm2) which are of relevance for clinical procedures on soft tissues. In dental tissue processing, very high fluences (>5 J/cm2) are required. As a consequence, laser–induced plasma formation can be observed. Secondary radiation can be used as a visible guide for selective removal of carious altered tissue. The data we have found might be of assistance in estimating potential hazards for future mutagenic studies in the field.

Influence of the ablation plume on the removal process during ArF-excimer laser photoablation

Correction of myopia with the ArF-excimer laser (PRK) sometimes leads to a so called central island formation on the anterior corneal surface. The attenuation of the laser beam by the ablation plume might be one reason for this phenomenon. The attenuation properties of the ablation plume were investigated by a probe beam parallel to the surface of the tissue probe. By varying the laser parameters (fluence, repetition rate, spot size) and the target tissue (cornea, PMMA) the attenuation of the probe beam was measured time and spatial resolved. As a result of this study, a significant influence of the removal process due to scattering and absorption within the ablation plume can be assumed as a function of repetition rate, spot size and air flow on the tissue surface.

Characterization of laser-induced pressure transients by means of piezoelectric PVDF films

For an interpretation of pressure signals detected by piezoelectric PVDF-foils several parameters are of importance. These are for example pressure pulse duration and rise times, active area of the film and specifications concerning the high-frequency technique of the experimental setup. Using an input-resistance of 1 M(Omega) at the storage-oscilloscope leads to reflections of the signal which superimpose each other when reflected a second time at the foil. This might cause an apparent increase of the amplitude. The utilization of a longer cable allows a separation of each reflected pulse, but it causes the problem, that the cables input- impedance approaches its characteristic impedance of 50 (Omega) . This will lead to a time derived signal as well as using 50 (Omega) as input-resistance at the scope. Therefore a direct measurement of short pressure transients requires a very short cable and an input-resistance of 1 M(Omega) . Otherwise the measured signal allows no proportional relation between pressure transient and measured voltage signal. In the latter case a frequency-dependent correction of the signal becomes necessary. This has been developed in this paper by means of Fast Fourier Transform algorithm. After correction in the frequency-domain the signal is transformed back into the time-domain.

Optoacoustic online control for laser cyclophotocoagulation

In this work, we show the capability of laser optoacoustics to localize the position of the ciliary body on enucleated porcine and rabbit eyes. Our findings correspond well with histological sections of the measured area. Different wavelengths for an optoacoustical detection system in combination with laser cyclophotocoagulation have been compared taking grayscale images of the region of interest of rabbit and porcine eyes for various wavelengths in the NIR spectral range. Additionally, the changes in the optical properties of the tissue induced by coagulation with a diode laser were observed. First online measurements of the changes due to coagulation show that the method of laser optoacoustics is suitable for an online therapy control system.

Photoablation of the cornea with a Q-switched Er:YAG laser

In this study the ablation characteristics and the wound healing process of rabbit cornea irradiated with a Q- switched Er:YAG laser was evaluated. The laser, emitting at 2.94 micrometers wavelength, has a pulse width of 100 ns. The spot size on the corneal surface was 1 mm in diameter at a fluence of 750 mJ/cm2. The laser beam was applied by a `flying spot mode, performing refractive ablations of -7 to -8 dpt. As a biological model, the corneas of 9 rabbits were irradiated. The post-treatment follow-up was as long as 39 days. The treated corneas were investigated by light and electron microscopy. The wound healing on rabbit cornea of the Q-switched Er:YAG laser radiation in corneal tissue processing resembles to what is known from ArF- excimer laser application. To shorten the pulse width by means of Q-switching is one major key to the successful application of the Er:YAG laser for PRK.

Erbium laser photoablation of the cornea

In the mid IR region, where water acts as the main chromophore in soft tissue, pulsed erbium lasers emit radiation with sufficient energy for photoablative tissue processing. In this study, the ablation characteristics of these lasers were analyzed in order to evaluate their potential for clinical application in corneal and intraocular surgery. Histopathological examination of thermal side-effects was performed on porcine corneas. Ablation rates were investigated by on-line depth profiling in free-running and in q-switched mode. Structure and intensity of laser induced acoustic transients were analyzed by piezo-electric PVDF-films.

Photoacoustic determination of optical parameters of biological tissue

Tissue optical properties are determined by an interpretation of pressure transients generated by irradiation with pulsed laser light. These pressure signals are detected using piezoelectric transducers. The signals are influenced by numerous processes during generation, propagation and detection. To study each process in particular, specific experiments were performed using biological tissue in vitro as well as different substances with various absorption and scattering properties. Especially acoustical diffraction effects cause a strong distortion of the shape and spatial oscillations in the amplitude of the transients. These phenomenons were investigated experimentally and verified theoretically.

Internal ablative sinostomy using a fiber delivered Q-switched CTE: YAG laser (2.69 microns).

Current trends of laser technology towards low-thermal photoablative pulsed mid-infrared lasers open new, more adequate approaches to experimental surgical procedures which have already been evaluated in the past.Transcorneal laser ablation of the trabecular meshwork (internal sinostomy) in human autopsy eyes was performed with a Q-switched CTE:YAG laser (wavelength: 2.69 μn, pulse width: 1 μs). Beam delivery was achieved with conventional optical quartz fibers (Low-hydroxy-fused-silica: 0,3 ppm, 50 cm length, 200 μn diameter). Light- and scanning-electron-microscopy were used for histological examination and micromorphological analysis.By applying two laser pulses (6 J/cm2) to the functional trabecular meshwork, a round sinostomy with a diameter corresponding to the diameter of the fiber-tip was achieved. It was possible to set several internal sinostomies into the chamber angle opposite to the entering paracentesis of the laser fiber-tip. Collateral thermal tissue alteration reached up to 50 μm, and since fiber-tip contact was maintained during laser application, thermal tissue alteration was also found around the opposite wall of Schlemms canal. At higher energy fluences mechanical (disruptive vaporization) effects were significantly enhanced.It can be concluded, that low-thermal pulsed mid-infrared lasers are adequate instruments to perform transcorneal trabecular ablation (abinterno sinostomy). The laser used in this study (CTE:YAG) bears the advantage that its radiation can easily be delivered in conventional optical quartz fibers.

Optoacoustic imaging for ophthalmology

The feasibility of optoacoustic imaging was investigated for ophthalmologic application in the treatment of glaucoma. Difficulties in the treatment with laser cyclophotocoagulation are mainly due to uncertainties in the localization of the ciliary body. With laser optoacoustics it is possible to localize the position of the ciliary body on enucleated porcine and rabbit eyes. Additionally, the changes in the optical properties of the tissue induced by coagulation with a diode laser were observed.