774 Current Concepts of Cerebrovascular Disease and Stroke Magnetic Resonance Angiography Cerebrovascular Applications Paul M. Ruggieri, MD; Thomas J. Masaryk, MD; and Jeffrey S. Ross, MD M agnetic resonance angiography (MRA) can provide significant additional information to complement the routine spin-echo MR ex- amination of the brain. 1 " 13 The idea of a combined study has gained favor because it provides a noninvasive alternative for the complete study of patients with suspected cerebrovascular disease. Because of the rela- tively short time of acquisition, MRA sequences can be easify added to the traditional parenchymal study with- out significantly prolonging the overall examination time (Figure 1). Magnetic resonance is able to create anatomic images through the use of radiofrequency (RF) excitation and refocusing pulses as well as spatial localizing magnetic field gradients. Motion (blood flow) in the presence of the RF pulse sequence creates time-of-flight (TOF) effects on the signal of moving protons, while proton motion during the application and in the direction of the magnetic field gradients produces spin phase phe- nomena. Each of these effects can be manipulated relatively independently to create vascular contrast within a given scan and thus angiographic images. TOF techniques create vascular contrast via the inflow of blood protons into a region of interest previ- ously prepared by an RF excitation, inversion, or satu- ration pulse. The most popular techniques consist of relatively simple gradient echo pulse sequences where the inflow of spins produces high signal within the vessels on the basis of the TOF effects known as "entry slice phenomenon" or "flow-related enhancement." Angiographic images can then be derived from such data sets through the use of computer post-processing techniques which obviate the need for mask acquisitions to create a subtraction angiogram. Since TOF MRA requires only a single data set, the acquisition is less susceptible to problems, such as patient motion and eddy currents, which arise with longer examination times and multiple data sets. 111 TOF data also require less computer memory and post-processing following the acquisition. Although the phase contrast techniques are more sensitive to very slow flow, the TOF tech- niques may be less vulnerable to the phase dispersion (and signal loss) that accompanies complex motion typically seen in regions of arterial flow (i.e., tortuous vessels, stenoses). 12 - 13 All MRA techniques may be implemented in a 2D or 3D mode. Two-dimensional Fourier transform imaging From the Section of Neuroradiology, Cleveland Clinic Founda- tion, Cleveland, Ohio. Reprinted from Current Concepts of Cerebrovascular Disease and Stroke 1991;26:29-36. (2DFT) acquires data in the conventional fashion in which individually acquired image slices are stacked sequentially. With 3D Fourier transform (3DFT) acqui- sitions an entire block or "slab" of anatomic digital data is acquired. This may then be displayed as individual slices reconstructed in any plane or perhaps subjected to a post-processing algorithm which displays only selected tissues (Figure 1). While 2D MRA examinations are superior for the demonstration of slow flow (e.g., ve- nous) lesions, 3DFT sequences can provide thinner image slices with higher spatial resolution, as well as shorter echo times which minimize artifact responsible for the overestimation of (carotid) stenoses. TOF techniques have been most extensively tested in the evaluation of carotid artery atherosclerotic disease and a variety of vascular abnormalities involving the larger vessels of the intracranial circulation. Both areas are particularly amenable to investigation by TOF stud- ies because of the relatively rapid, constant flow in these vessels (particularly the carotid arteries) which results in high vascular contrast, the lack of significant physio- logic motion (e.g., respiration), and the availability of coils which are especially well suited to these applica- tions. With the exception of venous studies, intracranial investigations have been primarily performed with 3D sequences because of the complex flow and the demands for high spatial resolution. 16 - 10 The carotid arteries have been studied with both 2D and 3D sequences. 2 - 4 Extracranial Applications: Carotid Bifurcation MRA techniques will likely have their greatest clini- cal impact in the evaluation of carotid bifurcation atherosclerotic disease. 14 - 15 Traditional MR imaging is particularly sensitive to the parenchymal sequelae of cerebral vascular disease, and many patients are now evaluated by MR for the extent and distribution of ischemic damage. Hence the interest in developing a technique that can be combined with the traditional MR brain examination to evaluate the severity of carotid bifurcation occlusive disease in the same sitting. 14 A number of studies have been conducted which demonstrate the efficacy of 2D and 3D-TOF MRA techniques in the evaluation of the carotid bifurca- tion. 2 - 41617 The sequential 2D-MRA technique has the advantage of high vessel/soft tissue contrast from strong flow-related enhancement which is maintained even in the setting of relatively slow flow (e.g., severe, long segment stenosis). On the other hand, minor motion by the patient during the scan can cause a severe stair-step misregistration artifact. Also, higher gradient strengths are necessary to define thin slices in sequential 2DFT by guest on August 17, 2015 http://stroke.ahajournals.org/ Downloaded from