Biomedical Engineering Reference
In-Depth Information
Figure 3.5: Velocity-induced phase shifts. Stationary pins do not show any
velocity-induced phase shift or intravoxel dephasing (box A). Constant velocity
flow throughout voxel results in net phase shift (box B). Turbulence and shear
stress within a voxel produce randomly oriented velocity vectors, resulting in a
loss of signal.
sequences at short echo times (see Fig. 3.5). These recent advances are de-
scribed later in this chapter. The velocity distribution inside the voxels is not
the result of an intravascular flow profile, but there are multiple small vessels
inside a single voxel that take course in all directions. In the case of vessels tak-
ing many turns, intravascular velocities and accelerations inside the voxel are
due to spatial arrangement rather than the flow profile. Spin isochromats expe-
rience destructive interference of isochromat vector components. This results
in signal loss and the magnitudes depend upon the cardiac cycle. These signal
losses may be avoided by the use of subtraction of flow-sensitive images from
the flow-insensitive images to demonstrate tissue perfusion and tissue diffusion.
3.1.2.5 Quantification of Flow by Spin-Phase Effect
Intravascular signals of voxel within the vessel are characterized by the mag-
nitude and phase angle of the magnetization vector. Flow-compensated pulse
sequences are sensitive to velocities because the phase angle is a measure of
blood flow velocity or acceleration. The phase angle is adjusted below 360
◦
for
