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.