Planar Difference Tomography

Direct measurement and visualization of a three-dimensional shear stress field is of great importance in the Biofluid Mechanics because of the strong correlation between shear stress and pathologic processes of thrombus formation and blood damage. Furthermore, such method has an advantage for the wall shear stress measurement in comparison with all other methods based on the measurement of the velocity field.

The new method is based on liquids, which exhibit the optical effect of streaming birefringence. This feature is associated with the action of shear forces in a flow. An incident light wave is separated in the birefringent fluid into two components, which are linearly polarized, with the planes of polarization being perpendicular to each other, and which propagate at different velocities. This difference is a function of shear stress. These two light components are out of phase when leaving the birefringent fluid and this difference in the optical phase can be visualized and measured. The oldest used technique for visualizing birefringent test volumes is an apparatus called polariscope. A linearly polarized light is directed through the flow, and after having traversed a second polarizer (“analyzer”) acquired with a camera.

Flow visualization in an aneurysm using birefringent liquid (V2O5) and polariscope.

The new method that was patented (patent no.: DE 102006013452 B3; US patent no.: 5 980 568) is based on the idea to locate a source of the polarized light into the flowing birefringent fluid. This is done by adding the reflecting particles into the birefringent fluid and illumination of these particles by a polarized, monochromatic light sheet. This light sheet is acquired by two synchronized cameras from opposite directions. The light intensity in each image is a measure of the optical effect of streaming birefringence on a way of the light from a light source to the camera.

Two images are obtained for each light sheet and a set of parallel sheets are acquired in order to investigate a test volume. Finally the optical effect of the birefringence by the light transmission through the whole test volume is measured. By a combination of two images for one light sheet and one final image with a light transmission through the test volume an optical effect of the birefringence on the way between light sheet and camera are separated. Using a set of parallel light sheet a three dimensional resolution of the effect in a test volume is solved.

Schematical draw of the measurement principle. PF is a polarizer working as polarizer (P) and analyzer (A).

Contact persons

Dr. Eng. Leonid Goubergrits
Dipl.-Ing. Torsten Schneider

Cooperation partner

Prof. Dr.-Ing. C. O. Paschereit Fachgebiet Experimentelle Strömungsmechanik am Institut für Strömungsmechanik und Technische Akustik (ISTA) der TU Berlin

funded by