This patent specification refers to, and hereby incorporates by reference, the items listed at the end and identified by reference (ref) numbers in parenthesis in the discussion below.
Phase contrast imaging is a magnetic resonance imaging (MRI) technique that can be used to visualize moving fluid. It is typically used for MR angiolgraphy that does not require introducing a contrast agent in the subject. Phase contrast imaging relies on the assumption that spins that are moving in the same direction as a magnetic field gradient develop a phase shift that that is proportional to the velocity of the spins. In principle, bipolar gradients (two gradients with equal magnitude but opposite direction) are used to encode the velocity of the spins. Stationary spins undergo no net change in phase after the two gradients are applied, but moving spins experience a different magnitude in phase response to the second gradient compared with the first. The detected net phase shift can be used to calculate the velocity of the spins, or an image obtained with a bipolar gradient can be subtracted from an image of the same slice obtained without using a bipolar gradient to thereby determine velocity from net phase shift and highlight flow. Gradient moment nulling can be used to balance the phase of all gradient pulses to achieve only velocity phase shifts in the echo signal from an effective bipolar pulse.
The most common and available method for hemodynamic MR velocity imaging on commercial clinical scanners is believed to be two-dimensional (2D) phase contrast (PC) imaging using a bipolar gradient pulse (ref 1) to encode the signal phase with velocity in cine gradient echo (GRE) sequences with low flip angle radio frequency (RF) pulses, and cardiac gating (cine-PC) through the cardiac cycle (ref 1, ref 2). Cine-PC 2D imaging measures flow velocity in a single slice plane and scan time is increased proportional to the number of repeated slice planes through arteries or cerebrospinal fluid (CSF) passageways.