A protocol for the isolation of carotid and vertebral arteries in MR angiography.

MR angiography [1 - 9] is rapidly becoming a technique to evaluate blood flow in the carotid and vertebral arteries. We have been using a time-of-flight pulse sequence [9] to acquire axial images of carotid and vertebral arteries with flow-en­ coded information. These axial images can be reformatted through a computer algorithm to give multiple angle presen­ tations of the collected data in sagittal or coronal planes. We found that many of the reconstructed images were difficult to interpret because of overlap of right and left ver­ tebral and carotid arteries (Fig . 1 ). Presented here is a protocol for optimizing the information obtained in the time-of-flight images. All of the angiograms were obtained on a clinical 1 .5-T system.* Studies of the carotid artery bifurcations were per­ formed with the standard linear head coil , which was used for both excitation and detection. The region of interest (the carotid bifurcation) was located with multiple-view angle , phase-contrast MR angiograms [4]. A series of 128 axial time-of-flight images was then obtained with superior satu­ ration to suppress venous flow. Standard acquisition param­ eters were: 16-cm field of view, 12-cm slab, one excitation, 128 x 256 matrix, and 20° flip angle. The problem of arterial overlap was solved in a straightfor­ ward manner. The 128 axial images were reduced to 64 to 80 consecutive images centered in the excitation slab. The right arteries were isolated by using a circular mask that included only the right carotid and vertebral in the reformatting of the axial images. The positions of the axes of rotation and the circular masks used in the processing are shown in Figure 2. A series of 31 images was produced with views of the vessels at 6° increments through 180°. The analogous pro­ cedure was performed to isolate and image the left arteries. One side benefit of this procedure is that any two consecutive reformatted images of the series can be presented as ste­ reoscopic pairs (Fig. 3). The two sets of images of the right and left arteries are automatically centered by the reformatting process. There