PeriScan PIM 3 System Measuring Principle   Scanning Although the scanning motion of the laser beam appears continuous to the eye, each measurement is in reality made up of a number of discrete measurement points. In each point, the laser beam stops for some milliseconds to sample the perfusion in the tissue before moving on to the next site. The maximum number of measurement points is 255x255, which means that more than 65 000 separate tissue sites can be scanned in one image. Generally, it is an advantage to average the perfusion over many points, since each individual point is sensitive to noise and to spatial variations the tissue. Laser Doppler Principle When laser light penetrates the tissue under study it is scattered and partly absorbed. Some of the scattered light returns to the tissure surface, where it is registered by a photo detector inside the instrument. This signal is then processed to extract information about the microcirulatory blood flow. According to the Doppler principle, light particles which hit moving blood cells undergo a change in wavelength/frequency (a Doppler shift), while light particles which encounter static structures return unchanged. The perfusion can be calculated since the magnitude and frequency distribution of the doppler shifted light are directly related to the number and velocity of blood cells but unrelated to their direction of movement. PIM 3 Mechanical Principle The cutaway drawing on the left shows the laser, the digital camera, the photo detector which collects the back scattered light, and the moving mirror used to move the beam from site to site.    Return to previous page

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