Biomedical Engineering Reference
In-Depth Information
studies. With higher velocity encoding, pulse is wrapped. Magnitude and speed
images show a drop in signal intensity with increasing velocity.
For quantitative studies, one sets the flow encoding to produce a phase
shift just below 180
◦
for the highest velocities present. The quantitative rela-
tionship between velocity and phase shift reduces the detectability of small
vessels and some aneurysms and reduces the apparent diameter of large
vessels.
3.2.5.3 Phase Dispersion and Flow Compensation
Intravoxel spin-phase dispersion is called intravoxel incoherence or loss of spin-
phase coherence. It imposes a limitation for vascular MRI. This loss of signal
intensity can occur whenever any of the three conditions exists: (1) A wide
spectrum of flow velocities exists within an imaging voxel; (2) higher orders
of motion, such as acceleration and jerk, are not compensated; and (3) local
variations in magnetic field homogeneity are present, such as those produced
by magnetic susceptibility effects. In a long straight vessel with no bifurcation,
blood flow is typically laminar flow. That is, the velocity profile across the vessel
is not constant, but varies across the vessel lumen. The flow at the center of
the lumen of the vessel is faster than that at the vessel wall, where resistance
slows down the blood flow. As a result, the blood velocity is almost zero near the
wall, and increases toward the center of the vessel. The velocity profile becomes
more complicated when the flow is pulsatile and the vessel curves or bifurcates.
In general, shear rate increases near the vessel wall, resulting in greater velocity
variations, intravoxel phase dispersion, and loss of signal intensity. Decreasing
the voxel size is one important strategy for minimizing intravoxel dephasing
in vascular MRI studies. Smaller voxels encompass a smaller range of flow ve-
locities. This reduced size of voxel also reduces SNR in a linear fashion. The
loss of SNR can be offset by the use of long acquisition times. SNR is propor-
tional to the square root of the imaging time. The other alternative is employing
the stronger magnetic fields, as SNR is proportional to magnetic field strength.
Thus, voxel-size reduction will improve nonturbulent flow only such as vascular
structures with well-characterized distribution of velocities within a vessel. It
will not eliminate signal loss due to true turbulence. The reason for this is that
turbulence flow has randomly oriented the velocity vectors. The lower voxel-size
strategy offers similar improvements in the regions with magnetic susceptibility
