Dirk Theisen

Influence of optical aberrations on laser-induced plasma formation in water and their consequences for intraocular photodisruption.

The influence of spherical aberrations on laser-induced plasma formation in water by 6-ns Nd:YAG laser pulses was investigated for focusing angles that are used in intraocular microsurgery. Waveform distortions of 5.5λ and 18.5λ between the optical axis and the 1/e2 irradiance values of the laser beam were introduced by replacement of laser achromats in the delivery system by planoconvex lenses. Aberrations of 18.5λ increased the energy threshold for plasma formation by a factor of 8.5 compared with the optimized system. The actual irradiance threshold for optical breakdown was determined from the threshold energy in the optimized system and the spot size measured with a knife-edge technique. For aberrations of 18.5λ the irradiance threshold was 48 times larger than the actual threshold when it was calculated by use of the diffraction-limited spot size but was 35 times smaller when it was calculated by use of the measured spot size. The latter discrepancy is probably due to hot spots in the focal region of the aberrated laser beam. Hence the determination of the optical-breakdown threshold in the presence of aberrations leads to highly erroneous results. In the presence of aberrations the plasmas are as much as 3 times longer and the transmitted energy is 17–20 times higher than without aberrations. Aberrations can thus strongly compromise the precision and the safety of intraocular microsurgery. They can further account for a major part of the differences in the breakdown-threshold and the plasma-transmission values reported in previous investigations.

Laser-induced breakdown in the eye at pulse durations from 80 ns to 100 fs

Nonlinear absorption through laser-induced breakdown (LIB) offers the possibility of localized energy deposition in linearly transparent media and thus of non-invasive surgery inside the eye. The general sequence of events--plasma formation, stress wave emission, cavitation--is always the same, but the detailed characteristics of these processes depend strongly on the laser pulse duration. The various aspects of LIB are reviewed for pulse durations between 80 ns and 100 fs, and it is discussed, how their dependence on pulse duration can be used to control the efficacy of surgical procedures and the amount of collateral effects.

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.

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.

Energy balance of optical breakdown in water

During optical breakdown, the energy delivered to the sample is either transmitted, reflected, scattered, or absorbed. The absorbed energy can be further divided into the energy required to evaporate the focal volume, the energy radiated by the luminescent plasma, and the energy contributing to the mechanical effects such as shock wave emission and cavitation. The partition of the pulse energy between these channels was investigated for 4 selected laser parameters (6 ns pulses of 1 and 10 mJ, 30 ps pulses of 50 (mu) J and 1 mJ, all at 1064 nm). The results indicated that the scattering and reflection by the plasma is small compared to plasma transmission. The plasma absorption can therefore be approximated by A approximately equals (1-T). The ratio of the shock wave energy and cavitation bubble energy was found to be approximately constant (between 1.5:1 and 2:1). For a more comprehensive study of the influence of pulse duration and focusing angle on the energy partition, we therefore restricted our measurements to the plasma transmission and the cavitation bubble energy. The bubble energy was used as an indicator for the total amount of mechanical energy produced. We found that the plasma absorption first decreases strongly with decreasing pulse duration, but increases again for pulses shorter than 3 ps. The conversion of the absorbed energy into mechanical energy is approximately equal to 90% with ns-pulses at large focusing angles. It decreases both with decreasing focusing angle and pulse duration (to less than or equal to 15% for fs-pulses). The disruptive character of plasma-mediated laser surgery is therefore reduced with ultrashort laser pulses.

Myocardial expression of the vascular endothelial growth factor (VEGF) after endocardial laser revascularization (ELR)

Background. Endocardial laser revascularization (ELR) is a new technique to treat patients with severe coronary artery disease (CAD) in a percutaneous approach. The results show a significant improvement of symptoms, but the mechanism of action is still unknown. One main theory is the angiogenesis for which Vascular Endothelial Growth Factor (VEGF) is the keypromotor. We investigated immunohistochemically the VEGF-expression after ELR in porcine hearts over a timeperiod of four weeks. Methods. ELR was performed with a single-pulse Thulium:YAG laser. 15 pigs were treated with ELR and the hearts were harvested at five timeperiods: directly (group I), 3 days (group II), 1 week (group III), 2 weeks (group IV) and 4 weeks (group V) after ELR. Each group consisted of three pigs. Immunohistochemically the VEGF-expression was assessed by staining with a polyclonal antibody against VEGF and cellcounting using an expression index (VEGF-EI) Results. A maximum of VEGF-expression was found three days (group II) after ELR with a VEGF-EI of 97%. At 1 week (group III) the VEGF-EI was similar high with 93%. Along the timecourse the index decreased to 22% at 4 weeks (groupV). Conclusions. Our findings show that ELR leads to an local upregulation of VEGF around the channels. The resulting angiogenesis could be the mechanism for the relief of angina.

Endokardiale Laser Revaskularisation des Myokards mittels 20 J Einzelpuls Holmium Laserstrahlung

Zusammenfassung Die Endokardiale Laser Revaskularisation (ELR) ist eine minimal invasive Technik zur Behandlung von multipler Koronarstenose. Zur Erzeugung der myokardialen Kanale mit nur einem einzelnen Laserpuls wurde ein Holmium-Lasersystem entwickelt, welches eine Pulsenergie bis zu 30 J bei einer Wellenlange von 2,1 μm emittiert. Die Laserpulse werden durch einen optischen Multimode Wellenleiter transmittiert, wodurch der Einsatz uber einen Herzkatheter moglich ist. Die Kanaltiefe im Myokard wurde in vitro an Schweineherzen bei verschiedenen Laserpulsenergien bis zu 20 J und bei Laserpulsdauem von 3 ms und 8 ms gemessen. Histologien wurden unter Polarisations-Mikroskopie zur Visualisierung der thermo-mechanischen Schadensreichweite untersucht. Zur Darstellung der Ablationsdynamik wurden Laserpulse mit einer Energie von 12 J bei Laserpulsdauem von 2,2 ms bis 8 ms in Polyacrylamid (PAA) als transparentes Myokardmodell appliziert und mittels Hochgeschwindigkeitsfotografie (5000 Bilder/s) aufgezeichnet. Bei gleicher Laserpulsenergie sind die Kanaltiefen bei einer Pulsdauer von 3 ms urn 25% langer als bei 8 ms. Die maximale Kanaltiefe von 8 mm wurde bei einer Laserpulsenergie von 16 J und einer Pulsdauer von 3 ms erzielt. Die thermo-mechanische Schadigung urn den Kanalliegt bei 500-700 μm. Die Ablationsdynamik in PAA zeigt bei allen Laserparametem starke laterale Oszillationen der entstehenden Kavitat wahrend des Laserpulses. Zusammenfassend zeigen die Untersuchungen, daβ die ELR mittels einzelner Hochenergie-Laserpulse ein vielversprechendes und einfaches, minimal invasives Verfahren zur Erzeugung myokardialer Kanale darstellt.

Ablation Dynamics of High Energy IR Laser Pulses in Myocardial Revascularization

The laser induced formation of transmural channels in ischemic myocardium traces back to the idea of a direct blood supply from the ventricle to the ischemic areas.