Markolf H. Niemz

Age-related structural abnormalities in the human retina-choroid complex revealed by two-photon excited autofluorescence imaging

The intensive metabolism of photoreceptors is delicately maintained by the retinal pigment epithelium (RPE) and the choroid. Dysfunction of either the RPE or choroid may lead to severe damage to the retina. Two-photon excited autofluorescence (TPEF) from endogenous fluorophores in the human retina provides a novel opportunity to reveal age-related structural abnormalities in the retina-choroid complex prior to apparent pathological manifestations of age-related retinal diseases. In the photoreceptor layer, the regularity of the macular photoreceptor mosaic is preserved during aging. In the RPE, enlarged lipofuscin granules demonstrate significantly blue-shifted autofluorescence, which coincides with the depletion of melanin pigments. Prominent fibrillar structures in elderly Bruchs membrane and choriocapillaries represent choroidal structure and permeability alterations. Requiring neither slicing nor labeling, TPEF imaging is an elegant and highly efficient tool to delineate the thick, fragile, and opaque retina-choroid complex, and may provide clues to the trigger events of age-related macular degeneration.

Ablation of femural bone with femtosecond laser pulses—a feasibility study

Although lasers are nowadays widely accepted as a popular scalpel of minimally invasive surgery (MIS), one of the most common orthopedic surgeries—the replacement of the knee joint—is still performed using an ordinary oscillating saw. Since ultra-short laser pulses are usually considered to be inefficient regardless of their high precision, the newest development of femtosecond laser systems has not yet been clinically applied to any mass ablation situation. However, thin disk Yb:KYW lasers meanwhile provide sufficient output power to ablate bone tissue within a reasonable time frame. Our results mainly focus on ablation rates obtained at different spot distances, repetition rates and pulse energies. It is shown that femtosecond laser pulses at high repetition rates are a promising tool for orthopedic surgery.

Two−photon excited autofluorescence imaging of human retinal pigment epithelial cells

Degeneration of retinal pigment epithelial (RPE) cells severely impairs the visual function of retina photoreceptors. However, little is known about the events that trigger the death of RPE cells at the subcellular level. Two-photon excited autofluorescence (TPEF) imaging of RPE cells proves to be well suited to investigate both the morphological and the spectral characteristics of the human RPE cells. The dominant fluorophores of autofluorescence derive from lipofuscin (LF) granules that accumulate in the cytoplasm of the RPE cells with increasing age. Spectral TPEF imaging reveals the existence of abnormal LF granules with blue shifted autofluorescence in RPE cells of aging patients and brings new insights into the complicated composition of the LF granules. Based on a proposed two-photon laser scanning ophthalmoscope, TPEF imaging of the living retina may be valuable for diagnostic and pathological studies of age related eye diseases.

Ultrashort laser pulses in dentistry: advantages and limitations

Several laser systems are currently under investigation for the purpose of removing hard dental tissues. However, either undesired thermal side effects or the lack of efficiency have already been demonstrated in most cases. In this paper, advantages and limitations of using ultrashort laser pulses with either picosecond or femtosecond durations are discussed. The major advantages associated with these pulse durations is the ability to produce very precise cavities without significant thermal side effects. Even disruptive effects due to shock wave generation seem to be negligible at moderate pulse energies close to the ablation threshold. The quality of these cavities is found to be superior to the quality achievable with other laser systems. Moreover, a spectroscopical analysis of the laser-induced plasma sparks enables an on-line health diagnosis of the irradiated volume. Limitations arise from the development of a suitable delivery system and from the cost of generating ultrashort laser pulses.

Plasma-mediated ablation of biological tissue with picosecond and femtosecond laser pulses

We investigated plasma-mediated surface ablation in corneal tissue using picosecond and femtosecond laser pulses in order to achieve high precision, non-thermal tissue removal with a non-ultraviolet laser source. Experiments utilized three laser systems, a regeneratively amplified Ti:sapphire laser, a synchronously amplified dye laser, and a regeneratively amplified picosecond Nd:YLF laser. Tissue ablation was performed by tightly focusing the laser beam on the tissue surface. Ablation thresholds were determined by monitoring the plasma spark, as well as the tissue surface. Tissue ablations were then analyzed by standard histologic methods and scanning electron microscopy. We observed a decrease in the ablation fluence threshold as the pulse duration is shortened from 200 ps to approximately 140 fs, in agreement with our theoretical predictions. Using identical pulse energies, the femtosecond laser pulses ablated tissue at higher efficiencies than the picosecond laser, with an approximately two-fold improvement in the etch depth curve. Histologic analysis reveal minimal adjacent tissue damage at either pulse duration. Femtosecond laser pulses may offer advantages that make them ideal tools for high precision tissue ablation.

Multifibre Application in Laser-Induced Interstitial Thermotherapy under On-Line MR Control.

Abstract.The application of multiple fibres for the conformal irradiation of tumours by laser-induced interstitial thermotherapy (LITT) has been investigated. A study was performed to evaluate the coagulated zones produced in porcine muscle tissue in vitro. For delivering specified powers into the tissue, a multifibre system was developed which allows the simultaneous use of up to four fibres. A new quantitative method of magnetic resonance imaging (MRI) has been applied for real-time thermometry. It is based on the temperature dependence of the T1 relaxation time and the equilibrium magnetisation. The MR results were compared with the measurements of fibreoptic thermometers. Since the acquisition time of the selected MR sequence takes only 3 s per slice and the calculation of the temperature measurement could be realised within a few seconds, the temperature mapping works closely to real time. The accuracy of the temperature measurements in muscle tissue was 1.5°C. Whereas single-fibre applications induced convex-shaped isotherms, concave structures were generated by a multifibre LITT.

Comparison of three different laser systems for application in dentistry

Three different laser systems have been investigated according to their possible application in dentistry: a free running and a Q-switched microsecond Ho:YAG laser, a free running microsecond Er:YAG laser and picosecond Nd:YLF laser system consisting of an actively mode locked oscillator and a regenerative amplifier. The experiments focused on the question if lasers can support or maybe replace ordinary drilling machines. For this purpose several cavities were generated with the lasers mentioned above. Their depth and quality were judged by light and electron microscopy. The results of the experiments showed that the picosecond Nd:YLF laser system has advantages compared to other lasers regarding their application in dentistry.

Diagnosis of caries by spectral analysis of laser-induced plasma sparks

A picosecond Nd:YLF laser system was used to remove sound and carious enamel by the mechanism of plasma-induced ablation. The plasma spark was optically imaged onto the entrance pupil of a spectrometer. The spectra were scanned between 400 - 700 nm with a typical resolution of 0.2 nm. Calcium in neutral and singly ionized states and the sodium doublet at 589 nm were observed. The second harmonic of the laser wavelength was generated in an external BBO crystal, thereby converting about 10 (mu) J of the pulse energy to radiation at 527 nm. The amplitude of the diffuse reflected second harmonic was used as a reference signal for normalization of the spectra. Several sound and artificial caries regions of different teeth were investigated. The spectra obtained from caries always showed a strong decrease in amplitude of all mineral lines, if compared to sound enamel. These results can be explained by the demineralization process of dental decay. Thus, caries infected teeth are easily distinguished from sound probes, enabling a computer controlled caries removal in the near future. The possible setup of such an automated system is discussed.

Light and Matter

In this and the following chapter, we will discuss basic phenomena occurring when matter is exposed to light. While here we will be concerned with various actions of matter on light, the opposite effect will be discussed in Chap. 3. Matter can act on electromagnetic radiation in manifold ways. In Fig. 2.1, a typical situation is shown, where a light beam is incident on a slice of matter. In principle, three effects exist which may interfere with its undisturbed propagation: reflection and refraction, absorption, scattering.

Medical Applications of Lasers

In this chapter, we will discuss principal applications of lasers in modern medicine. Due to the present boom in developing new laser techniques and due to the limitations given by the dimensions of this book, not all disciplines and procedures can be taken into account. The main intention is thus to focus on the most significant applications and to evoke a basic feeling for using certain techniques. The examples are chosen to emphasize substantial ideas and to assist the reader in grasping some technical solutions. Potential difficulties and complications arising from either method are addressed, as well. However, we should always keep in mind that any kind of laser therapy will not be indicated if alternative methods are available which offer a better rate of success, are less dangerous to the patient, and/or easier to perform.