In SE MRA, what occurs when blood is moving at an intermediate speed?

In time-of-flight (TOF) magnetic resonance angiography (MRA), such as in a Steady-State (SE) technique, the behavior of blood flow significantly impacts the resulting images. When blood is moving at an intermediate speed, there is a dynamic interaction between the blood that is already within the imaging slice and the fresh blood entering the slice.

As blood moves through the magnetic field, those vessels that are at an intermediate speed can lead to an incomplete signal. This happens because some of the moving blood will exit the imaging slice before the RF pulse can fully affect it. Simultaneously, new blood enters the slice from surrounding areas. This process helps maintain the signal because fresh spins from newly entering blood can be excited by the MRI pulse, adding to the overall signal captured.

This concept is crucial for techniques like SE MRA because it allows for the enhancement of blood-to-background contrast, which is vital for visualizing vascular structures effectively. Therefore, the correct choice captures how the continuous flow of blood affects the MRA signal, demonstrating the importance of understanding blood flow dynamics in MRI imaging.