Animation 8.2 (Entry-slice Phenomenon)

This animation supports Figures 8.35-8.38. It demonstrates entry slice phenomenon. It shows flowing spins in the aorta and IVC in four slices orientated perpendicular to the direction of flow. The slices are acquired in the order shown, i.e. the most inferior slice first and ending with the most superior slice. The flow in the aorta is, therefore, in the opposite direction to slice acquisition, whereas the flow in the IVC is in the same direction as slice acquisition.

Fresh spins in both the aorta and the IVC are ones that have traveled from somewhere else in body and "hit" one of the slices for the first time. This is when they receive their first RF excitation pulse. Because the direction of flow is different in the aorta than the IVC, this occurs in a different slice for each vessel. It occurs in the most superior slice for spins in the aorta, and the most inferior slice for spins in the IVC. When spins receive an RF pulse of the first time, they return a much higher signal than when they have received several RF pulses and become saturated.

This is shown in Figures 8.35 and 8.38. In Figure 8.35, high signal intensity is returned from the IVC because this is the most inferior slice in the stack. These spins are fresh, and therefore, return a higher signal than spins in the aorta that have become saturated because they have received several RF pulses as they passed down through the stack of slices from top to bottom.

In Figure 8.38, the effect is reversed. This is the most superior slice in the stack and therefore, the spins in the aorta are fresh and returning a high signal, whereas spins in the IVC are saturated because many have traveled through the stack from the bottom to the top. The other phenomena that is demonstrated in this animation is that as the flow in the IVC is in the same direction as slice excitation, spins become saturated faster than spins in the aorta that have traveled in the opposite direction to slice excitation. Therefore, the IVC returns a lower signal on Figure 8.38, then the aorta returns in Figure 8.35.