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# Table 3 Explanations of terms relating to CMR velocity acquisition, blood flow visualization, flow and wall properties.

Dimension
| Dimension | A dimension refers, in this paper, to one of up to 3 orthogonal (X, Y, Z) spatial dimensions and the temporal dimension, along each of which a series of measurements of velocity components are made. For practical purposes, the spatial dimensions are subdivided into voxels and the time dimension, into a number of phases of an effectively averaged cardiac cycle. |

Directional component | One of up to three orthogonal (X, Y, Z) components of a vector such as velocity, each usually represented by a single number at any given point in space-time. | |

CMR methods
| Phase contrast velocity acquisition | The encoding of velocity in CMR by means of differences of phase between MR signals recovered using two differently velocity encoded acquisition sequences, applied one after the other, usually in quick succession. |

Phase contrast Intravoxel velocity standard deviation | Quantification of the standard deviation of the blood flow velocity distribution within a voxel obtained from the magnitude of phase contrast MRI signals acquired with different first gradient moments. Intravoxel velocity standard deviation can be used to derive turbulence intensity. | |

Fourier velocity encoding | This technique entails a measurement of the spectrum of one or more components of velocity reflecting a 'dimension'. These could represent multiple velocities that might be measured in a single, spatially or temporarily extended flow region in an unstable flow field. | |

Visualization
| Vector plot | A line or arrow representing both the magnitude and direction of velocity at a point, calculated from the magnitudes of X, Y and Z components of velocity, or X and Y if in a single plane. |

Particle trace | The computed behavior of a virtual particle in a flow field. In pulsatile flow several types of particle traces can be computed, as instantaneous streamlines and pathlines | |

Streamline | A line plotted through in a flow field at a given instant in time such that it is aligned with the local velocity vectors at all points along its length. | |

Instantaneous streamline | A streamline plotted at a specific instant in a changing flow field. | |

Pathline | A line through a flow field representing the path traced by a particle or virtual particle released from a given seed point in the flow field. | |

Flow features and properties
| Viscosity | A measure of the resistance of a fluid to internal deformation. |

Vortex | Rotating or swirling motion in a flow field, where streamlines or pathlines tend to curl back on themselves | |

Helical flow | Part of a flow field with rotation around an axis of flow such that streamlines are helical | |

Unstable flow or flow instability | Irregularly fluctuating disturbances of flow consisting of multiple eddies and counter-eddies. Several factors in the heart and great vessels predispose to instability: relatively large luminal diameters, sudden changes of lumen diameter, relatively high flow velocities, velocity changes (particularly deceleration), flow separation and shear within the flow field. | |

Turbulence | A fluid regime characterized by randomly and rapidly fluctuating velocities. | |

Flow separation | The separation of streamlines in a flow field from an adjacent wall. This tends to happen by virtue the forward momentum of flow relative to a curving, irregular or discontinuous boundary, for example where venous flows enter the atria, separate from the tips of valves or where streamlines separate from the inner curvature the distal aortic arch. | |

Recirculation | The recirculation of streamlines or pathlines from a forward stream back into a separation zone, for example beyond each heart valve. | |

Shear | Shear in a flow field is fluid deformation such that adjacent layers move relative one another, for example in the shear layer between a high velocity jet and the adjacent low velocity fluid. In CMR, high shear can results in a range of velocities in a single voxel which can cause local loss of blood signal due to dephasing. | |

Pulse wave velocity | The velocity of propagation of a pulse wave along a vessel, usually an artery, normally several times faster than the velocities of blood flow within the vessel. | |

Wall shear stress | Refers to the stress, meaning force per unit area, parallel to the wall, exerted by shear in the fluid layer immediately adjacent to the wall (fluid-wall shear stress). | |

Wall
| Intramural stress | Intramural stress is a measure of the internal forces acting within a deforming vascular or cardiac wall. The intramural stress dependents on variables such as transmural pressure, wall curvature, wall thickness and any constraints from the outside. The resultant strains depend on the structural properties of the wall. |