How is phase contrast imaging accomplished, and why is it used?
Moving protons (of blood) in a magnetic field gradient produce a phase shift proportional to their velocity and direction, while stationary protons (of tissue) produce no phase shift. This produces a signal that is proportional to the velocity of blood moving through the slice. Phase contrast cine gradient echo images (i.e., velocity-encoded images) exploit this property to quantify the velocity of blood moving through a stenotic valve in order to estimate the pressure gradient across the valve. To calculate the pressure gradient across a stenosis, the modified Bernoulli equation is used (where pressure gradient [in mm Hg] = 4 × v max 2 and v max is the maximum velocity [in m/sec]). Phase contrast images are also useful to evaluate regurgitant fraction, flow through an intracardiac shunt, flow velocity, and collateral flow.