SEQUENTIAL 3-DIMENSIONAL TIME-OF-FLIGHT MR-ANGIOGRAPHY OF THE CAROTIDARTERIES - VALUE OF VARIABLE EXCITATION AND POSTPROCESSING IN REDUCING VENETIAN BLIND ARTIFACT

OBJECTIVE. Multiple overlapping three-dimensional (3D) time-of-flight carotid MR angiography potentially combines many of the desirable feat ures of two-dimensional (2D) and single-volume 3D MR angiographic imag ing techniques. Yet the maximum-intensity-projection images from such acquisitions are often degraded by artifact due to nonuniform signal i ntensity of contiguous imaging volumes and inadequate, yet arduous, po stprocessing. The former has been termed venetian blind artifact. To d ate, the severity of the artifact has been minimized by use of very th in slabs with a large percentage of overlap. However, the artifact typ ically is still appreciable, and the required acquisition and postproc essing times are increased. The purpose of this study was to examine t he value of technical modifications of both the multislab acquisition and postprocessing procedures to reduce this artifact on images of hea lthy volunteers. SUBJECTS AND METHODS. Spatially variable RF pulses al ong the direction of flow were applied as excitation pulses in the mul tislab time-of-flight MR angiographic acquisitions to compensate for t he nonuniform blood signal intensity caused by spin saturation. An aut omatic postprocessing technique was used to optimally combine the imag e data in overlapping slices by selecting the higher-intensity pixel o f the tow on a pixel-by-pixel basis. Ratios of the standard deviation of signal intensity to the mean signal intensity were computed as a fu nction of RF profile and postprocessing method along the long axes of the arteries to measure the uniformity of the signal intensity of the blood. The spatially variable and sinc RF pulse acquisitions, combined with automatic and conventional manual postprocessing, were compared. RESULTS. Compared with the since pulse acquisition, the MR angiograms acquired with spatially variable excitation pulses improved the signa l uniformity of the arteries with thicker volumes and less overlap, th ereby reducing the acquisition time by 25% for similar spatial coverag e. When used with the automatic postprocessing technique, the severity of the venetian blind artifact on maximum-intensity-projection images was minimized and the postprocessing time was reduced by roughly a fa ctor of 5. CONCLUSION. The combined use of spatially variable excitati on pulses and an automatic postprocessing technique can improve the un iformity of the signal from blood across the slab and allow thicker sl abs to be acquired with less overlap. Data acquisition and postprocess ing times can be reduced significantly. This work suggests it may be p ossible to easily produce overlapping 3D MR angiograms that should be superior to conventional 2D and 3D studies.