Phase-contrast magnetic resonance angiography

Time of flight (TOF) and phase contrast (PC) magnetic resonance angiography (MRA) are two established magnetic resonance (MR) techniques that were described and developed in the 1980s. Both of these techniques usually do not involve the use of intravascular contrast, but rather exploit the intrinsic contrast provided by flowing blood to produce vascular signal. Bolus infused (gadolinium-enhanced) MR angiography was only introduced in 1993. PC-MRA uses the phase shifts introduced to nuclei with motion in the presence of a magnetic field gradient. A bipolar magnetic field gradient will induce a phase shift to nuclei moving along the gradient dependent on the velocity, as well as acceleration and higher order motion terms. More complex gradient waveforms enable sensitivity to specific motion terms such as velocity or acceleration. By constructing an image in which the intensity is proportional to the phase shift of the nuclei, it is possible to create an angiographic image related to the flow properties of blood (or other liquids such as cerebrospinal fluid). The PC-MRA is a powerful technique and allows for encoding of flow in one or many directions in such a way that the velocity sensitivity can be chosen depending on the vessel of interest. This technique also allows for quantification of flow velocity and flow rate, which is not generally available with other angiographic techniques.