Volume 29, Issue 11, November 2002
Index of content:
- PH. D. THESES ABSTRACTS
29(2002); http://dx.doi.org/10.1118/1.1518590View Description Hide Description
Understanding the fundamental mechanisms in laser tissue ablation is essential to improve clinical laser applications by reducing collateral damage and laser pulse energy requirements. In this dissertation, skintissue ablation by nanosecond laser pulses has been studied from near infrared to ultraviolet for a clear understanding of the mechanism that can be used to enhance future design of pulsed lasers for dermatology and plastic surgery. Multiple laser and optical configurations have been constructed to generate 9–12 ns laser pulses at 1064, 532, 266, and 213 nm. Through histology and optical transmission measurements of ablation depth as a function of laser pulse energy, the 589 nm spectral line in the secondary radiation from ablatedskin tissue samples was identified as the signature of the occurrence of ablation. Subsequently, this spectral signature has been used to investigate the probabilistic process of the ablation near the threshold at the four wavelengths. In addition, optical breakdown and backscattering in water were investigated along with a nonlinear refraction index measurement using a z-scan technique. Preliminary studies on ablation of a gelatin based tissue phantom are also presented. The current theoretical models describing soft tissue ablation by short laser pulses were critically reviewed. A new plasma-mediated model has subsequently been developed and a laser-induced localized thermal ionization pathway has been investigated. It was found to have significant influence on the initial free electron density during plasma formation due to the combination of strong light absorption and subsequent confined temperature rise in chromophores. Good agreement has been found between the model and experimental results.
Introduction of weighted structural complexity measures for analysis of noisy signals and introduction to nuclear medicine image processing29(2002); http://dx.doi.org/10.1118/1.1518476View Description Hide Description
In this work a sensitive source detection algorithm for nuclear medicine imaging is presented. Special attention is given to weak sources with strength comparable to fluctuations of the underlying noise. The scaling-index-method (SIM) originally developed by Morfill et al. (Max-Planck-Institute for Extraterrestrial Physics) has been extended. The SIM assumes that the local point density in a sphere with radius r around a point in a discrete phase space follows the scaling law For a digitized image the points of this space are represented by the coordinates (x,y) and the gray-level Because the scaling behavior of noise and signal points differs, discrimination between sources and noise is possible. In this work the author extends the SIM by applying weighting functions to the locally calculated scaling indices, which take into account the information available on the noise characteristics. Because of the poor photon density in nuclear medicinediagnostics the image quality is highly influenced by the statistical fluctuations inherent in radioactive decay. Consequently, only those changes in radiopharmaceutical distributions will be detectable for the observer, which are greater than statistical noise. On the other hand the detection of weak sources is one of the most important tasks in nuclear medicinediagnostics. Therefore in several clinical cases, such as total-bone or thyroidal scintigraphy, the applicability of the developed algorithms is studied. This work documents that by using the proposed methods even weak sources hardly detectable for the examining physician are identified with a high significance. Furthermore the SIM allows to discriminate real sources from tomographic artefacts.