Sunday, July 17, 2011

Basics 1: The Doppler-Effect

SLDF measures retinal blood flow. But what are the physical effects behind the scenes? Most know at least that this method uses the Doppler-Effect (ok, its part of the acronym :-) : The light that is reflected or scattered by moving blood cells gets a frequency shift. The effect everyone knows from a car passing by (http://en.wikipedia.org/wiki/Doppler_effect).


Figure 1 shows the interfering high fequency light and the resulting beat at low frequency. This is what the photo diode of the HRF camera records and what is saved in the SER-files: 256*64 recordings, each of ~2 seconds duration. You can see the time measurement either if you load a SER file by the HRF software or load it into SDLF.

Figure 1: Two waves of slightly different wave length (incoming and reflected light) interfering; frequency shift is caused by the moving blood cell


Figure 2: Same scenario, but the cell moving faster resulting in a higher frequency shift


The algorithms in HRF or SLDF analysis software again take the time signals of each pixel recording and determine the different frequency shifts. The distribution and magnitude of the frequency shifts are equivalent to distibution of blood cells moving at different velocities.