How deep can you probe with laser-Doppler flowmetry?

Laser-Doppler perfusion measurements have been developed during the past 25 years. The method is based on a Doppler shift of the light scattered back from the examined tissue due to movements of flowing blood cells contained. Today, equipment is commercially available from a Swedish company (Perimed AB) to image the superficial blood perfusion in tissues. It has been possible to develop this unique tool due to a strong research activity within the field at Linköping University. We are from time to time using such equipment in our research, to be able to monitor the superficial blood perfusion before and after laser treatments of tissue.

The aim of the project is to examine various possibilities to modify the probe depth of this technique.

As the instrument works today, it is probing the peripheral blood perfusion in a superficial layer of approximately a few hundred micrometers thickness. In many applications it would be of interest to measure the blood flow also in the slightly deeper lying larger vessels. The intention is that the project should include the following:

• Review the field of laser-Doppler perfusion measurement. To do this properly, you should conduct a literature search. Especially consider the probe depth of the instruments used and their documented clinical applications in this part. This review should be presented in the introduction of your final report. It is probably a good idea to look into what they have published at Department of Biomedical Engineering (IMT, InstitutetförMedicinskTeknik) in Linköping.
• You should measure the optical properties of muscle tissue at the wavelengths of interest (during the laboratory exercises). Optical properties for skin are difficult to measure, and such values can be found in the literature.
• It has been shown that the probed depth could be varied by changing the laser wavelength and/or the signal processing algorithm (by weighting the velocity components of the scatters differently and assuming different velocities at different depths). Study this and support the discussion with modeling arguments. Select a model for light transport that is suited for this. Discuss also any other way you can come up with to increase the probe depth.
• You should calculate the resulting probe depths for a number of light wavelengths of interest, and for a few processing algorithms.
• Discuss how the commercial laser Doppler perfusion equipment could be developed to allow a wider range of applications by changing the above mentioned parameters.

In the project at least these parameters should be considered: The velocity of the blood cells as a function of blood vessel localization below the skin surface, the light wavelength, the optical properties of the tissue.

Suggested key-words: Laser, laser-Doppler measurements, blood perfusion, laser-Doppler perfusion imaging, vessels, skin, tissue optics