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Origin and processing of laser Doppler spectra
Linköping University, Department of Biomedical Engineering. Linköping University, The Institute of Technology.
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: urn:nbn:se:liu:diva-29444Local ID: 14791ISBN: 91-7219-805-2 (print)OAI: oai:DiVA.org:liu-29444DiVA: diva2:250259
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
List of papers
1. Critical design parameters in laser Doppler perfusion imaging
Open this publication in new window or tab >>Critical design parameters in laser Doppler perfusion imaging
1996 (English)In: Proc. SPIE 2678: Optical Diagnostics of Living Cells and Biofluids / [ed] Daniel L. Farkas; Robert C. Leif; Alexander V. Priezzhev; Toshimitsu Asakura; Bruce J. Tromberg, SPIE Proceedings Series , 1996, Vol. 2678, 401-408 p.Conference paper, Published paper (Refereed)
Abstract [en]

Laser Doppler Perfusion Imaging (LDPI) is a method for visualization of tissue blood perfusion. A low power laser beam is used to step-wise scan a tissue area of interest and a perfusion estimate based on the backscattered, partially Doppler broadened, light is generated. Although the basic operating principle of LDPI is the same as that of conventional Laser Doppler Perfusion Monitoring (LDPM), significant differences exist between the implementation of the methods which must be taken into account in order to generate high quality perfusion images. The purpose of this study is to investigate the relevance of a number of LDPI design parameters, such as:

(1) The influence of artifact noise when using a continuously moving laser beam instead of a step-wise moving beam to scan the image.

(2) The signal processor output's dependency on the distance between the measurement object and the scanner head when using collimated laser light.

(3) The speed and mode of the scanning.

The results show a substantial rise in the noise level when using a continuously moving beam as opposed to a step-wise. Skin measurements using a collimated laser beam demonstrated an amplification factor dependency on the distance between the skin surface and the scanner head not present when using a divergent laser beam. The scanning speed is limited by the trade-off between the Doppler signal lower cut-off frequency and the image quality.

Place, publisher, year, edition, pages
SPIE Proceedings Series, 1996
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-33211 (URN)10.1117/12.239527 (DOI)19198 (Local ID)9780819420527 (ISBN)19198 (Archive number)19198 (OAI)
Conference
Optical Diagnostics of Living Cells and Biofluids, San Jose, CA | January 27, 1996
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2016-05-04
2. Higher order moment processing of laser Doppler perfusion signals
Open this publication in new window or tab >>Higher order moment processing of laser Doppler perfusion signals
1997 (English)In: Journal of Biomedical Optics, ISSN 1083-3668, E-ISSN 1560-2281, Vol. 2, no 4, 358-363 p.Article in journal (Refereed) Published
Abstract [en]

The laser Doppler technique is used to assess tissue perfusion. Traditionally an integrated, ω-weighted (first-order filter) power spectrum is used to estimate perfusion. In order to be able to obtain selective information about the flow in vessels with different blood cell velocities, higher order filters have been implemented, investigated, and evaluated. Theoretical considerations show that the output of the signal processor will depend on the flow speed, for a given concentration of blood cells, according to Soutνn where v is the average blood cell speed and n is the spectral filter order. An implementation of filters using zero-, first-, second-, and third-order spectral moments was utilized to experimentally verify the theory by using a laser Doppler perfusion imager. Two different flow models were utilized: A Plexiglas model was used to demonstrate the various signatures of the power spectrum for different flow speeds and filter orders, whereas a Delrin model was used to study the relationship between the flow velocity and the output of the signal processor for the different filters. The results show good agreement with theory and also good reproducibility. Recordings made on the skin of the wrist area demonstrated that the flow in small veins can be visualized by the use of higher spectral orders.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-33080 (URN)10.1117/12.281499 (DOI)19050 (Local ID)19050 (Archive number)19050 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13
3. Spectral signature and heterodyne efficiency for different wavelengths in laser Doppler flowmetry
Open this publication in new window or tab >>Spectral signature and heterodyne efficiency for different wavelengths in laser Doppler flowmetry
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.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-24544 (URN)10.1007/BF02347700 (DOI)6703 (Local ID)6703 (Archive number)6703 (OAI)
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2017-12-13
4. Effects on skin blood flow by provocation during local analgesia
Open this publication in new window or tab >>Effects on skin blood flow by provocation during local analgesia
2000 (English)In: Microvascular Research, ISSN 0026-2862, E-ISSN 1095-9319, Vol. 59, no 1, 122-130 p.Article in journal (Refereed) Published
Abstract [en]

Although topical analgesia cream has been used for several years, little is known about its effects on the microcirculation. Previous studies have shown a vasoconstrictive effect after short application times and a vasodilatation after longer application. It has also been shown that vasomotion does not occur in the analgesized skin. The present study was undertaken to investigate the alterations in skin blood perfusion following local cooling, local heating and pin-pricking after the establishment of analgesia. In 11 healthy volunteers, skin analgesia was attained by use of a eutectic mixture of lidocaine and prilocaine (EMLA, Astra Pain Control AB, Sweden) applied to the skin three hours prior to provocation. The changes in skin blood perfusion, after applying three different provocation methods, were studied using the laser Doppler technique. Local cooling and heating to temperatures of +10 and +45°C, respectively, were applied for 9 s by use of a copper probe (Ø12 mm). In the pin-prick provocation method, a combined effect of deflection and penetration of the skin to in total 3 mm was attained. Identical provocation methods were applied to placebo treated and untreated skin areas. After heat provocation, significant differences in the perfusion response between the treatments were seen (P < 0.0001). Skin areas treated with analgesia cream responded with a slow increase in perfusion that persisted beyond the four minute measurement period. Placebo and untreated areas decreased their perfusion over time. After cooling a significant reduction in skin perfusion was seen, irrespective of the treatment. Similarly, after pin-pricking a perfusion increase was seen for all treatments. The findings indicate that topical analgesia influences the myogenic control of the blood flow in those vascular plexa measured by laser Doppler following heat provocation. No differences could be seen in the response to pin-pricking and cooling for the different treatments.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-30586 (URN)10.1006/mvre.1999.2205 (DOI)16177 (Local ID)16177 (Archive number)16177 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13
5. Skin capillary appearance and skin microvascular perfusion due to topical application of analgesia cream
Open this publication in new window or tab >>Skin capillary appearance and skin microvascular perfusion due to topical application of analgesia cream
2000 (English)In: Microvascular Research, ISSN 0026-2862, E-ISSN 1095-9319, Vol. 59, no 1, 14-23 p.Article in journal (Refereed) Published
Abstract [en]

Local topical analgesia changes basal skin perfusion and its regulation. In particular, the response induced by local heating, which in nontreated skin comprises a rapidly increased perfusion followed by a normalization within 30 s, is altered to a delayed and persistent perfusion increase. The response dependency to the analgesia cream application time, that is, the intradermal penetration of the analgesics and in which vascular plexa the response occurs, is not known. The aim of this study was to assess changes in the appearance of superficial skin capillaries and skin microvascular perfusion changes due to different application periods of topical analgesia cream (EMLA). Twelve subjects were treated with EMLA and placebo applied to the volar side of each forearm, respectively. The treatment areas were assigned different application times (20 min, 40 min, 1 h, 2 h, and 3 h). The areas were cleared from the creams and shortly thereafter provoked during 9 s with a probe heated to 45°C. To assess capillary number density and skin perfusion, capillary microscopy, and Laser Doppler perfusion imaging (LDPI), respectively, were used. The number density of physiologically active capillary was significantly decreased with longer application times of EMLA (P < 0.005). The LDPI-signal showed a persistent perfusion increase after provocation associated with increasing application time of the cream. This perfusion pattern was not seen after 20 min of treatment, but was present in 9 of 12 subjects after 3 h of treatment. No significant relationship between changes in the capillary number density and the LDF measurement was found. In conclusion, a longer application time and therefore a higher intradermal concentration and a deeper penetration of the analgesics was associated with a delayed and persistent perfusion increase after local heating. There was a discrepancy between changes in capillary number density and skin perfusion, indicating that the perfusion increase does not occur in the capillaries but in the deeper lying vessels. Hence, the contribution of the capillary perfusion to the LDF-signal is smaller than previously anticipated. Capillary number density and presumably their perfusion were decreased with longer application times.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-30587 (URN)10.1006/mvre.1999.2206 (DOI)16178 (Local ID)16178 (Archive number)16178 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13

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