Ralf Brinkmann

Cw high-power IR laser at 2 μm for minimally invasive surgery

The potential of a new continuous wave Thulium YAG laser is investigated for tissue ablation and cutting focusing on applications in minimally invasive surgery. The laser emits at a wavelength of 2.01μm, which is well suited for tissue ablation due to its high absorption by water. The laser power can be tuned up to 60 W output through a 365 μm core diameter quartz fibre. For the ablation studies, the quartz fibre was placed in contact under various pressures (20 to 90mN) to porcine liver under saline solution in vitro at angles varying between 30° to 60°. The influence of different powers (10 to 60W) and cutting velocities (2 to 10mm/s) on the incision depth and coagulation zones of the tissue were investigated. A maximum incision depth of 3.3 mm was found with a power of 60W, a cutting velocity of 2mm/s and a fibre-tissue angle of 45°. The incisions were surrounded by coagulated tissue between 0.4 and 0.8mm in thickness, sometimes with an inner zone of carbonization of 0.2mm on average. In conclusion, the first experiments show that a cw Thulium laser is very well suited for tissue dissection as required in minimally invasive surgery.

Nucleation and Dynamics of Bubbles Forming Around Laser Heated Microabsorbers

Selective tissue damage on the cellular level can be achieved due to transient microbubbles around laser-heated intracellular pigments. We investigated bubble dynamics and nucleation mechanisms at microabsorbers in suspensions as well as bubble formation in retinal pigment epithelial cells by fast flash photography and time-resolved probe laser transmission.

Temperature dependence of water absorption for wavelengths at 1920 nm and 1940 nm

In this study the absorption coefficient of water at different temperatures was investigated for wavelengths at 1920 nm and 1940 nm respectively. The laser radiation was transmitted via a quartz fibre (O 600 µm core) into a thermal isolated and heated water filled cuvette. At different temperatures the transmittance of water was measured for both wavelengths. According to Beer-Lambert’s law the absorption coefficient α was calculated.

Optoacoustic measurements during us irradiation of the retinal pigment epithelium

The selective microphotocoagulation is a new technique to damage the retinal pigment epithelium (RPE), which is desired for treatment of several retinal diseases. By applying a train of microsecond(s) laser pulses it is possible to selectively destroy these cells and simultaneously spare the adjoining photoreceptor and neural tissue. We applied microsecond laser pulses of a Nd:YLF laser (527 nm), at a repetition rate of 500 Hz to porcine RPE. The light is absorbed in the RPE and by thermoelastic expansion, an optoacoustic (OA) signal will be generated which could be measured by an ultrasonic transducer. With this setup, the baseline temperature increase at the RPE, during irradiation can be determined, since the optoacoustic pressure signal depends on the temperature of the irradiated RPE. We found a linear dependence of the OA amplitude to the RPE sample temperature. At higher irradiance we proved the formation of microbubbles and bubble collapse in the RPE with OA techniques.

Myocardial tissue ablation by single high-energy laser pulses for ELR and TMR

The objective of this study is to compare the ablation sites induced by two different laser and application systems for myocardial laser revascularization. One system used was an 800 W CO2 laser, which is clinically established for transmyocardial laser revascularization (TMR). The second system was a self-designed Holmium laser emitting single high energy pulses for the minimal invasive approach of endocardial laser revascularization (ELR), whereby the laser light is transmitted via optical fiber into the left ventricle to ablate the myocardial channels from the inside. The laser energy was applied to Polyacrylamide (PAA) as transparent tissue phantom and in water as blood phantom. The ablation dynamics were investigated by high speed flash photography recording a picture series of a single event. Reperfused ex- vivo porcine hearts were treated to quantify differences in the thermal-mechanical damage ranges by polarization light microscopy. Ablation dynamics in water revealed oscillatory changes of the axial length of the steam bubbles between 3 mm and 12 mm during the CO2 laser pulse. For the Holmium laser pulse a maximal axial and lateral length of 5 mm was observed. The lateral dimensions of the bubbles were maximal 1 mm with the CO2- and 3.5 mm with the Holmium laser system. In PAA bubbles also collapse during the laser pulse which affects the size of the ablated channels. Using 12 J Holmium laser pulses for ablation of PAA, channel depths around 7 mm were found. Single Holmium laser pulses demonstrate ablations comparable in size and thermal- mechanical collateral damage to those achieved with the standard CO2 laser. The results are very encouraging for single pulse ELR and demonstrate the potential of a catheter based minimal invasive procedure for laser heart reperfusion.

Analysis of cavitation bubble dynamics by optical online monitoring

The formation and the dynamics of cavitation bubbles were investigated when applying pulsed mid-IR laser radiation in water. HeNe laser light reflected at the distal application fiber end was measured in order to probe refractive index changes during ablation. A Cr:Tm:YAG laser, (lambda) equals 2.01 micrometer, was operated in the free running and the Q- switched mode. The pulses were transmitted through a 400 micrometer low-OH quartz fiber into a water filled cuvette. In the Q-switched mode the reflected HeNe-laser power was found to be nearly constant during the cavitation bubble lifetime, whereas in the free running mode, transient changes of the reflected power were measured. Initial bubble wall velocities of 20 m/s in the free running mode and about 330 m/s in the Q- switched mode could be estimated using pulse energies of 110 mJ and 18 mJ, respectively. The lifetime of the cavitation bubble in both cases was measured to be about 300 microseconds. Relative to the reflected power when the fiber tip was in air, it was significantly lower during the lifetime of the bubble. The results indicate that condensation or sublimation of steam takes place on the fiber tip owing to low vapor temperature within the bubble. Rapid cooling due to adiabatic steam expansion and the Joule-Thompson effect at the onset of bubble formation are the most likely explanations for this unexpected result. A simple gas kinetic model predicts temperature gradients in the order of 200 degrees Celsius within the first microseconds. In conclusion, the optical on- line monitoring described is an excellent tool to investigate the kinetics of ablation in any medium in vitro as well as in vivo.

Beam-profile modulation of thulium laser radiation applied with multimode fibers and its effect on the threshold fluence to vaporize water

The threshold fluences at which vaporization is initiated at the tip of a multimode fiber that is submerged in water were investigated when free-running and Q-switched thulium laser pulses (lambda = 2.01 microm) were applied with different pulse energies. We focused on the quantification of temporal and spatial fluence modulations of the beam profile at the tip of a 400-microm fiber. The spatial and the temporal fluence peaks over the average fluence were measured to as high as 1.5 and 4 in the Q-switched mode, respectively, and 2.5 and 40 in the free-running mode, respectively. The fluence peaks significantly influence the vaporization process. An increase in the threshold fluence with increasing pulse energy was found for the Q-switched mode, but there was a decrease for the free-running mode. Pressure transients of the order of 1 kbar and temperatures higher than 200 degrees C were calculated for a 30-mJ Q-switched laser pulse at the onset of vaporization. Collecting all the data allowed us to trace the thermodynamic path of rapid heating and vaporization in a phase diagram of water.

Selective RPE photodestruction: mechanism of cell damage by pulsed-laser irradiance in the ns to μm time regime

The subject of this study was to investigate the threshold radiant exposures for bubble formation at single porcine melanosomes in suspension and for porcine RPE cell damage when using pulse durations in the ns to microsecond(s) time regime. A frequency doubled Nd:YLF laser ((lambda) equals 527 nm) with adjustable pulse duration between 250 ns and 3 microsecond(s) and a Q- switched Nd:YAG laser ((lambda) equals 532 nm, (tau) equals 8 ns) were used for the single pulse irradiation. Fast flash photography was applied to probe vaporization around individual melanosomes while a fluorescence viability assay was used to probe cell vitality. Applying single ns laser pulses to RPE cells, an ED50 threshold radiant exposure of 84 mJ/cm2 was determined, which is close to the vaporization threshold around single melanosomes. When irradiating with pulse durations of 3 microsecond(s) , a threshold of about 223 mJ/cm2 was measured, which is only 40% lower of the vaporization threshold around the single melanosome at that pulse width. This can be explained with heat contribution from adjacent melanosomes, which increases towards longer pulse durations. Calculations are in good agreement with the experimental results when assuming a surface temperature at the melanosome of 140 degrees Celsius and an absorption coefficient of 8000 cm-1 to initiate vaporization. It can be concluded that the origin of RPE cell damage for single pulse irradiation with a duration of 8 ns results from transient microbubbles around the melanosomes, which lead to a transiently increased cell volume and subsequently a rupture of the cell structure. It is also likely that the same effect plays the major role when using pulse durations up to 3 microsecond(s) .

Influence of pulse width and speckle formation on the ablation thresholds in water by means of pulsed mid-IR laser radiation

The evaporation of water by means of fiber guided pulsed mid- infrared laser radiation was investigated. The influence of the pulse duration and speckle formation on the ablation threshold was determined. Optical on-line monitoring of refractive index changes at the distal fiber tip has been used to determine the cavitation threshold. Temporal and spatial speckle pattern at the distal application fiber tip were measured by imaging the fiber tip to a reference plane. An IR- CCD camera and fast IR-photodiodes were used to monitor the intensity modulation. The measured cavitation thresholds show a strong dependence on the pulse energy and the pulse duration. Overheating of water at the evaporation threshold could be calculated for the Q-switched pulse mode. Regarding speckle, spatial peak intensities up to a factor of 1.5 and 2.6 over the average intensity were evaluated for the Q- switched and the free running mode, respectively. Speckle modulation plays an important role in the free running mode to achieve the boiling point of water with the threshold radiant exposure determined. Furthermore, transient intensity fluctuations were measured across the beam profile at the fiber tip.

Toward automated selective retina treatment (SRT): an optical microbubble detection technique

Selective retina therapy (SRT) is an ophthalmological laser technique, targeting the retinal pigment epithelium (RPE) with repetitive microsecond laser pulses, while causing no thermal damage to the neural retina, the photoreceptors as well as the choroid. The RPE cells get damaged mechanically by microbubbles originating, at the intracellular melanosomes. Beneficial effects of SRT on Central Serous Retinopathy (CSR) and Diabetic Macula Edema (DME) have already been shown. Variations in the transmission of the anterior eye media and pigmentation variation of RPE yield in intra- and inter- individual thresholds of the pulse energy required for selective RPE damage. Those selective RPE lesions are not visible. Thus, dosimetry-systems, designed to detect microbubbles as an indicator for RPE cell damage, are demanded elements to facilitate SRT application. Therefore, a technique based on the evaluation of backscattered treatment light has been developed. Data of 127 spots, acquired during 10 clinical treatments of CSR patients, were assigned to a RPE cell damage class, validated by fluorescence angiography (FLA). An algorithm has been designed to match the FLA based information. A sensitivity of 0.9 with a specificity close to 1 is achieved. The data can be processed within microseconds. Thus, the process can be implemented in existing SRT lasers with an automatic pulse wise increasing energy and an automatic irradiation ceasing ability to enable automated treatment close above threshold to prevent adverse effects caused by too high pulse energy. Alternatively, a guidance procedure, informing the treating clinician about the adequacy of the actual settings, is possible.