In phase contrast MRA, the signal intensity within the vessel is dependent on what factor?

In phase contrast magnetic resonance angiography (MRA), the primary factor influencing the signal intensity within the vessel is the velocity of the flowing blood. Phase contrast imaging exploits the movement of blood to generate a phase shift that is proportional to the velocity of flow. This technique calculates the difference in the phase of the MR signal between stationary tissue and moving blood, which allows for quantification of the blood flow velocity.

When blood moves through the magnetic field, it experiences a change in frequency (due to the Larmor equation), which results in a shift in phase that is directly related to its velocity. As the flow velocity increases, the phase shift becomes more pronounced, leading to increased signal intensity from the vessel in the images produced. Thus, the velocity of the flowing blood is a crucial parameter when determining the signal intensity in phase contrast MRA, as it directly correlates to how effectively the technique can visualize and measure blood flow within the vascular structures.