Abstract

Laser Doppler Perfusion Imaging (LDPI) creates two-dimensional flow-maps of the tissue perfusion. A laser beam scans the tissue surface step-wise by means of a computer controlled stepper motor system that forms an integral part of a camera-like device positioned above the tissue surface. At each measurement site in the flow-map, the beam is arrested for about 50 msec. During this time period the backscattered light, partly Doppler shifted by the moving blood cells in the superficial microvascular network, is recorded by a photo-detector, positioned next to the stepper motors in the scanner head. At the surface of the photo-detector, frequency-shifted and non-frequency-shifted beams are mixed to form a photocurrent which is composed of a fluctuating portion superimposed on a stationary base level. The frequency content of the fluctuating portion is related to the average speed of the blood cells, while its magnitude is related to their concentration within the scattering volume. The average sampling depth in skin tissue is in the order of 0.2 mm. By using a step-wise scanning procedure, the beam is kept still in relation to the tissue when recording the backscattered Doppler signals and the movement artifact noise generated by a continuously moving beam is avoided. By making the laser beam slightly divergent, the solid angle under which the laser spot is seen on the tissue surface from the photo detector is virtually constant. This implies that the degree of coherent detection can be kept constant thereby making the perfusion signal recorded independent of the distance between the scanner head and the tissue. After signal processing, an output signal is generated that scales linearly with tissue perfusion defined as the product of the blood cell speed and concentration. This signal is stored in the computer memory and the beam is moved to the next measurement site. When all measurement sites (at maximum 4096 points) have been sampled, the stored values are displayed in the form of a colour coded image on a computer monitor. This image can be further analyzed by use of the integrated analyses software LDI-soft or exported to other software packages for further analysis. LDPI has been used with success in a number of applications including axon reflex studies in diabetics, mapping of skin blood flow in association with microdialysis and investigation of the hyperperfusion in psoriatic skin.

© 1996 IEEE