Basic Principles of Phase-contrast, Time-of-flight, and Contrast- enhanced MR Angiography

The most widely used MR angiographic imaging techniques can be categorized as phase contrast, time-of-flight, or contrast-enhanced methods. The basic physical principles of each of these techniques are described briefly below. Phase contrast techniques derive contrast between flowing blood and stationary tissues by manipulating the phase of the magnetization, such that the phase of the magnetization from the stationary spins is zero and the phase of the magnetization from the moving spins is non-zero. The phase is a measure of how far the magnetization precess from the time it is tipped into the transverse plane until the time it is detected. The data acquired with phase contrast techniques can be processed to produce phase difference, complex difference, and magnitude images. In phase difference images, the signal is linearly proportional to the velocity of the spins-faster moving spins give rise to a larger signal, and spins moving in one direction are assigned a bright (white) signal, whereas spins moving in the opposite direction are assigned a dark (black) signal. Thus, the vascular anatomy can be assessed, and the speed and direction of the blood flow can be qualitatively determined, by observing the phase