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Spectral signature and heterodyne efficiency for different wavelengths in laser Doppler flowmetry
Linköping University, Department of Biomedical Engineering. Linköping University, The Institute of Technology.
Linköping University, Department of Biomedical Engineering. Linköping University, The Institute of Technology.
Linköping University, Department of Biomedical Engineering. Linköping University, The Institute of Technology.
2002 (English)In: Medical and Biological Engineering and Computing, ISSN 0140-0118, E-ISSN 1741-0444, Vol. 40, no 1, 85-89 p.Article in journal (Refereed) Published
Abstract [en]

Laser Doppler perfusion monitoring and imaging technologies generate time traces and two-dimensional flow maps of the microcirculation. With the goal of reaching different tissue depths, these technologies are equipped with lassers operating at different wavelengths λ. The fact that the average scattering angle, at a single scattering event, between a photon and a red blood cell increases with λ is compensated for by a 1/λ effect in the scattering vector, rendering the average frequency shift virtually independent of the choice of wavelength. Monte Carlo simulations showed that the corresponding spectral signature of the Doppler signals for λ=632.8nm and 780nm were close to identical. The theoretical predictions were verified by calculating the centre-of-gravity (COG) frequency of the laser Doppler power spectral density for the two wavelengths from forearm and finger skin, representing a low and high perfusion area, respectively (forearm COG=123 against 121Hz, finger COG=220 against 212 Hz). When the wavelength changes from 632.8nm to 780nm, the heterodyne efficiency of the detector and, thereby, the inherent system amplifcation increase. For tissues with identical microvascular flow conditions, the output signal therfore tends to increase in magnitude when shifting to longer wavelengths.

Place, publisher, year, edition, pages
2002. Vol. 40, no 1, 85-89 p.
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-24544DOI: 10.1007/BF02347700Local ID: 6703OAI: oai:DiVA.org:liu-24544DiVA: diva2:244865
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2017-12-13
In thesis
1. Origin and processing of laser Doppler spectra
Open this publication in new window or tab >>Origin and processing of laser Doppler spectra
2000 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Laser Doppler Flowmetry (LDF) is a technique for studying microvascular blood flow. Laser light is guided to the tissue and the backscattered light, after being Doppler shifted by moving Red Blood Cells (RBCs), is detected using a heterodyne process. In Laser Doppler Perfusion Monitoring, the light is guided to and from the tissue using optical fibers, whilst in Laser Doppler Perfusion Imaging (LDPI), a freely impinging laser beam is used. The Power Spectral Density (PSD) of the photodetector current constitutes the LDF spectrum and can be processed to yield an estimate of the tissue perfusion.

The aim of this thesis was to study the origin and suggest adequate processing of the LDF spectra from both a technical and a physiological perspective.

The orientation and length of the average scattering vector resulting from a RBC/photon interaction, are altered when changing the laser source wavelength. It has been shown theoretically that the change in the orientation and length do not alter the average frequency shift of the scattering event. In vivo measurements on a low and a high perfused area using the wavelengths 632.8 nm and 780 nm respectively, confirm the theoretical findings. The heterodyne efficiency of the detector increases for longer wavelengths, giving higher photodetector signal amplitude.

A method for differentiating high velocity flows, by changing the filtering of the LDF spectra is presented. Emphasis is given to higher frequencies, including information from higher flow velocities.

The scanning mode and the shape of the laser beam. influence the spectral signature in LDPI. In order to maintain a high signal quality, a stepwise mode is to be preferred. The continuous mode induces large spectral components that depend on the scanning speed and the tissue surface roughness. Using a slightly divergent beam minimizes the inlluence of the distance between the detector and the tissue surface.

The physiological perspective includes two randomized and placebo controlled studies of the rela tionship between topical skin analgesia and the perfusion response to different local stimuli. In the first study, it was shown that analgesia using EMLA® cream during local heating, changes the dynamic flow regulation to a persistent and delayed perfusion increase. This was not observed in untreated or placebo treated skin. In the second study, this heating response was positively related to longer treatment times and. hence. to higher intradermal concentrations of the analgesics. By using capillary microscopy. it was shown that analgesic cream treatment for at least one hour reduces the number of physiologically active capillaries by 50%, while LDf perfusion remains unaltered. After local heating, the LDF perfusion increased, in 9/11 subjects by an average of 8.7 times, while the number of capillaries remained decreased. These findings suggest a low capillary influence of the LDF signal in human skin.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2000. 44 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 644
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-29444 (URN)14791 (Local ID)91-7219-805-2 (ISBN)14791 (Archive number)14791 (OAI)
Public defence
2000-09-11, Berzeliussalen, Universitetssjukhuset, Linköping, 09:15 (Swedish)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-02-27

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Nilsson, GertStrömberg, Tomas

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