Joerg P. Fischer

Plasma-mediated ablation of brain tissue with picosecond laser pulses

Plasma-mediated ablations of brain tissue have been performed using picosecond laser pulses obtained from a Nd:YLF oscillator/regenerative amplifier system. The laser pulses had a pulse duration of 35 ps at a wavelength of 1.053 µm. The pulse energy varied from 90 µJ to 550 µJ at a repetition rate of 400 Hz. The energy density at the ablation threshold was measured to be 20 J/cm2. Comparisons have been made to 19 ps laser pulses at 1.68 µm and 2.92 µm from an OPG/OPA system and to microsecond pulse trains at 2.94 µm from a free running Er:YAG laser. Light microscopy and scanning electron microscopy were performed to judge the depth and the quality of the ablated cavities. No thermal damage was induced by either of the picosecond laser systems. The Er:YAG laser, on the other hand, showed 20 µm wide lateral damage zones due to the longer pulse durations and the higher pulse energies.

In-vivo measurement of the retinal birefringence with regard to corneal effects using an electro-optical ellipsometer

The phase retardation of light induced by the birefringent parts of the human retina in vivo is measured with an electro-optical ellipsometer using the principle of confocal imaging. A scanning unit allows to examine an area of 25 degrees by 12.5 degrees on the retina with a resolution of 256 by 128 points with both, incident and exit beam transmitting the cornea at a fixed position. Due to this fixed beam position the Muellermatrix of the cornea can be calculated using light which is specularly reflected on blood vessels lying above the nerve fiber layer of the retina. The obtained images show a homogenous radial distribution of the retinal retardation around the fovea without the appearance of the so called Haidinger brushes. The areas with the thickest nerve fiber layers, the arcuate bundles, appear in their typical arc and were measured quantitatively. In addition, an alternative method for the compensation of corneal birefringence is evaluated by focusing the light beam onto the surface of the lens. Hereby, the measured area in the center of the cornea is 3 X 0.75 mm2.

Time-resolved studies of plasma-mediated surface ablation of soft biological tissue with near-infrared picosecond laser pulses

Time-resolved video photographs have been taken in order to investigate plasma induced surface ablation of soft tissue by IR picosecond laser pulses with energies of about 1 mJ. The emission and propagation of shock waves in the irradiated tissue as well as in the surrounding air environment was studied. The pressure amplitudes of the shock transients were determined from the measured shock velocities. A decay of the pressure amplitude below 100 MPa was observed within a distance of about 200 micrometers from the center of laser induced optical breakdown. The dynamics of the ablation crater and the ejection of the ablation fragments was studied on a larger time scale. The maximum expansion of the ablation crater was measured to be about 200 (Mu) m at temporal delays of 4-5 microsecond(s) after the impact of the laser pulse. Furthermore, generation and propagation of a surface deformation wave was observed. Thus, we present a detailed and consistent description of all phenomena occurring during plasma-mediated surface ablation of soft tissue.

Ablation of neural tissue by short-pulsed lasers — a technical report

SummaryThe basis for most laser applications in neurosurgery is the conversion of laser light into heat when the incident laser beam is absorbed by the tissue. Irradiation of neural tissue with laser light therefore leads to its thermal damage. However, due to the diffusion of heat energy into the surrounding tissue, often there is thermal damage to neural tissue outside the area of the target volume. These are the characteristics of thermal laser/tissue interaction. In this paper we discuss how we used three different short-pulsed lasers to achieve non-thermal ablation of neural tissue.Three different short-pulsed lasers were used to generate ultrashort laser pulses in the picosecond to femtosecond range. The interaction of such laser pulses with tissue was predicted to be nonthermal. The short-pulsed lasers were used for the ablation of neural tissue using an in vitro calf brain model. The histopathological examination of the lesions revealed that the neural tissue had been removed very precisely without any sign of thermal damage to the surrounding tissue.

Plasma-mediated ablation using picosecond UV and IR laser pulses and spectroscopical investigations of the plasma spark

In-vitro ablation experiments of calf brain tissue with ultrashort lasers are reported. Excisions have been performed by the mechanism of plasma-mediated ablation with focused picosecond laser pulses. Results achieved with a 35 ps oscillator/regenerative amplifier Nd:YLF laser at 1053 nm and its second and fourth harmonic at 526 and 263 nm, respectively, are presented. Preliminary experiments with 19 ps laser pulses at wavelengths near 3 micrometers generated by optical parametric amplification are reported. Here, during the ablation with low photon energies no plasma was induced. Furthermore, the spectrum of laser-induced plasma sparks on the surface of fresh calf brain tissue was recorded. Atomic line widths were measured and used to determine the electron density of the plasma. With energy densities of about 100 J/cm2 values between 2 (DOT) 1017 and 5 (DOT) 1018 cm-3 were found.

Comparison of tissue ablation by ultrashort laser pulses in the nano-, pico-, and femtosecond range

Plasma mediated ablations of calf brain tissue and corneal tissue have been performed with ultrashort pulses provided by different laser systems: A 30 ps Nd:YLF amplified laser with up to 1.5 mJ of pulse energy and a powerful 180 fs Ti:Sapphire oscillator/regenerative amplifier laser system capable of pulse energies of up to 300 (mu) J. With the femtosecond pulses the threshold in energy density needed for initiating the ablation process was found to be significantly lower than with longer pulse durations. Furthermore, the amount of ablated material is by a factor of two greater with the femtosecond pulses compared to the results obtained with the picosecond pulses. Histological examinations did not show any thermal or structural damage in adjacent tissue with pulses in the pico- and femtosecond range as it could be found after irradiation with longer pulses. The morphology of the excisions is of a very high quality.

Time-Resolved Imaging of the Plasma-Induced Tissue Ablation with Picosecond Laserpulses

In Ophthalmology the application of ultrashort laserpulses is already well established for special intraocular surgeries like posterior capsulotomy or periphere iridotomy (see e. g. [SP85]). Also the application of picosecond pulses for intrastromal ablation in refractive surgery is under investigation. Recently the process of plasma-induced or photodisruptive laser-tissue interaction is also discussed with respect to other medical applications, e. g. lithotripsy, angioplasty and the resection of brain tumours in neurosurgery [Fis+94]. As the pulse duration is much shorter than the thermal diffusion time, tissue ablation without thermal side-effects (carbonisation and coagulation zones) is possible. On the other hand, the expansion of the microplasma leads to the generation of a shock wave, a high pressure transient moving radially outword with an initial velocity exceeding the sound velocity. Histological examinations do not reveal any dammage of adjacent tissue, but with these methods possible effects on a subcellular level cannot be examined.

Photodisruptive ablation of brain tissue using the first, second, and fourth harmonics of a Nd:YLF picosecond laser system

In vitro ablation experiments of calf brain tissue using a picosecond Nd:YLF laser system are reported. The laser lesions were studied with special regard to the ablation rate and to possible injuries of tissue adjacent to the excisions. Histological examinations prove the excellent quality of the lesions, showing no signs of thermal damages or carbonization. Furthermore, the influence of higher photon energies on the ablation threshold was studied.

Measurement of the Z-transfer function in the fovea centralis of the human eye using a new confocal laser scanning device

ABSTRACT A new method for the fast measurement of the Z—transferfunction in the Fovea Centralis of the human eyeusing a confocal laser scanning device is presented. The tested eye is illuminated by a collimated helium—neonlaser beam and therefore focuses the light onto the retina. Reflected light is detected using confocal techniques, i.e. a pinhole in the detection unit assures that only light originating from the focal plane is sampled. Thefocus of the laser beam is scanned in one dimension along the visual axis of the eye through the retina by adding a slide defocus to the incoming beam. The Z—transferfunction can be obtained directly by measuringthe intensity of the reflected light for different focal planes. A fixation target is offered to the eye at infinity tostabilize the accommodation as well as the direct viewing. Different pinholes in the detection unit are testedto investigate signal to noise ratio and depth resolution. Furthermore, the wave aberrations of the tested eyesand the laser scanning system itself are measured using a Hartmann—Shack wavefront sensor. Once the waveaberrations are known the intensity distribution in the plane of the detection pinhole can be calculated usingdiffraction theory. Thus, the intensity which passes through a pinhole of a certain diameter on the optical axisof the system can be obtained and the Z—transferfunction can be simulated. A comparison of the measuredand simulated Z—transferfunctions for different sizes of the detection pinhole has been performed and showssubstantial similarity.