Figure 34

Signal losses caused by velocity gradients. This diagram illustrates how signal losses occur in the presence of stenotic or regurgitant flow jets. The two images (bottom left) show signal loss (white arrows) caused by aortic valve regurgitation at two end systole and early diastole. Flow jets consist of a large range of velocities, from very high velocities at the centre of the jet, to relatively low velocities at the edge of the jet (top left). The change in velocity from the centre to the edge of the jet is often referred to as a velocity gradient. Figure33 shows that when bipolar gradients are applied within imaging pulse sequences, velocity related phase shifts occur. Where there is a large range of velocities due to a velocity gradient within a pixel, this results in a large range of phase shifts (right), causing significant dephasing and therefore signal loss within that pixel. Outside the jet there is only a small range of velocities leading to negligible dephasing and no signal loss.