Question

A technique in MRA where signal intensity depends on the direction of flow and thus requires gradient application in all three planes for proper signal acquisition:___________

Answer 1

Answer:

A technique in MRA where signal intensity depends on the direction of flow and thus requires gradient application in all three planes for proper signal acquisition is: Phase-contrast (PC-MRA)

Phase-contrast magnetic resonance imaging MRA  is a technique used in moving blood flow, where its speed is encoded in the magnetic resonance signal's phase after the applying bipolar gradient along any axis and the measurement point.

Explanation:

In this technique the bipolar gradient is manipulated varying its magnetic fields to be preset to a maximum expected flow velocity and it´s applied along any axis or axes depending on the direction along which flow is to be measured to get a reversed image of the bipolar gradient and the difference of the two images is calculated. The unaffected phase accrued during the application of the gradient, is 0 for stationary spins.  Since phase-contrast can only acquire flow in one direction at a time, 3 separate image acquisitions in all three directions must be computed to give a complete quantitative measurements of blood flow.  

Although this technique is slow,its strength lays in the possibility of calculating spins moving with a constant velocity of the applied bipolar gradient.  The accrued phase is proportional to both and the 1st moment of the bipolar gradient, thus providing a means to estimate gamma is the Larmor frequency of the imaged spins on moving tissues such as blood, which acquire a different phase since it moves constantly through the gradient, thus also giving its speed of the flow.