[MR tomography in cardiology. II: Flow phenomena].

Flow phenomena modify the information of MR tomograms and can cause artefacts which degrade the image quality. When ECG triggering and suitable modes of excitation are used, flow effects can give diagnostically relevant information concerning global and regional cardiac function and blood flow in vessels. The signal intensity of blood excited by an SE-mode is usually low. The spin echo increases due to the TR-effect. However, the TE-effect reduces the signal of flowing blood with increasing flow velocity. Phase shifts caused by flow generate line-shaped artefacts in the reconstructed tomograms and decrease the signal intensity of all odd spin echoes. The second order phase errors are compensated by phase recombination occurring in all even echoes. Therefore, in double echo acquisition the second spin echo of flowing blood can be higher than the first echo, especially when the flow velocity is physiologically or pathologically reduced. In multi-slice acquisition, time-of-flight effects can be observed, by which a resonance signal induced in one particular slice is shifted to and recorded in another slice.