COMPARISON OF PHANTOM AND COMPUTER-SIMULATED MR-IMAGES OF FLOW IN A CONVERGENT GEOMETRY - IMPLICATIONS FOR IMPROVED 2-DIMENSIONAL MR-ANGIOGRAPHY

The signal loss that occurs in regions of disturbed now significantly decreases the clinical usefulness of MR angiography in the imaging of diseased arteries, This signal loss is most often attributed to turbul ent now; but on a typical MR angiogram, the signal is lost in the nont urbulent upstream region of the stenosis as web as in the turbulent do wnstream region, In the current study we used a now phantom with a for ward-facing step geometry to model the upstream region, The now upstre am of the step was convergent, which created high levels of convective acceleration, This region of the flow held contributes to signal loss at the constriction, leading to overestimation of the area of stenosi s reduction, A computer program was designed to simulate the image art ifacts that would be caused by this geometry in two-dimensional time-o f-flight MR angiography. Simulated images were compared with actual ph antom images and the how artifacts were highly correlated, The compute r simulation was then used to test the effects of different orders of motion compensation and of fewer pixels per diameter, as would be pres ent in MR angiograms of small arteries, The results indicated that the computational simulation of now artifacts upstream of the stenosis pr ovides an important tool in the design of optimal imaging sequences fo r the reduction of signal loss.