Animation 8.3 Time of Flight

This animation refers to Figures 8.39 and 8.40 and shows how slice thickness, velocity of flow, and TE affect the signal that is likely to be returned from flowing spins. It shows three vessels in which there is low, average, and fast flow. The vessels pass through three slices of thin, medium, and thick slice thickness. All spins in all slices and all vessels receive an excitation pulse, but not all of them produce signal. Spins must be present in a slice for both the excitation and rephasing pulses to return signal. As the spins flow down these vessels, they are likely to remain in the thick slice for a longer period of time than in the medium or thin slices. Therefore, thick slices are more likely to produce signal from flowing blood than thinner slices because they are more likely to be present in the slice for both the excitation and rephasing pulses. In addition, slow flowing spins are more likely to remain in all three slices for longer than spins flowing more quickly. Hence, slow flow produces a higher signal than fast flow if the vessels are perpendicular to the slice.

The TE selected will also affect the signal returned because it determines the time interval between the excitation and rephasing pulses. In the first part of this animation, a long TE has been selected. Therefore, a relatively long time elapses between the RF excitation and rephasing pulses. In the fast flowing vessel, most of the spins move out of all three slices during this period and therefore, return little signal. In the slower flowing vessels, however, there are still some spins present in the medium and thick slices to receive both pulses. They will, therefore, produce some signal - the highest signal is likely to come from the slow flowing vessel in the thickest slice. The next part of the animation shows what happens when we select a short TE. A short period of time elapses between the RF excitation and rephasing pulses. In this example, the flow has much less time to exit the slice in all three vessels, and this is particularly marked in the thick slice. Therefore, signal is likely to be produced on all vessels, particularly in medium and thick slices.