Current Concepts of Cerebrovascular Disease and Stroke Magnetic Resonance Angiography Cerebrovascular Applications

Magnetic resonance angiography (MRA) can provide significant additional information to complement the routine spin-echo MR examination of the brain." 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 relatively short time of acquisition, MRA sequences can be easify added to the traditional parenchymal study without 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 phenomena. 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 previously prepared by an RF excitation, inversion, or saturation 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. 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 techniques 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).All MRA techniques may be implemented in a 2D or 3D mode. Two-dimensional Fourier transform imaging