Biomedical Engineering Reference
In-Depth Information
changes due to magnetic field gradients. Phase shift induced by flowing blood in
the presence of a flow-encoding gradient is directly proportional to the velocity.
A dispersion of velocities in a vessel, therefore, results in a dispersion of phase
shifts. Consequently, a projection measurement of phase through a vessel with
laminar flow will represent the average velocity provided that the flow-encoding
gradient is not too strong. If the flow becomes complex or turbulent, the dis-
persion of velocity components along the projection may cause an attenuation
of the signal, or even zero signal. Turbulent flow is the flows with different ve-
locities that fluctuate randomly. The difference in velocities across the vessel
changes erratically.
3.2.5.4 Flow Compensation
Spin echoes recover the loss of signal because of magnetic field inhomogeneity
or susceptibility gradients. However, these spin echoes with longer echo times
are less effective in overcoming the phase dispersion due to spins moving at dif-
ferent velocities. Flow compensation is a first-order gradient moment nulling.
It employs the refocusing gradients to re-establish phase coherence. For this,
lobes are added to the read-out and slice-select gradient waveforms. As a result,
the loss of phase coherence due to different velocity distributions is minimized
and velocity-induced phase shifts are canceled. This strategy results in an acqui-
sition at constant velocity. However, high-order motions such as acceleration
and jerks are compensated by the use of waveform complexity. As a result of
additional lobes of gradient waveforms, the echo time and degrade image quality
are increased.
3.2.5.5 First-Order Gradient Moment Nulling
It means that the system applies gradient pulses so that constant velocity spins
and stationary spins have no net phase accumulation at each echo time. For
stationary spins, the signs of the gradients are reversed so that the phase ad-
vance experienced at a given location is compensated by appropriate phase re-
tardation. The first-order gradient moment nulling balances the phase for both
stationary spins and spins moving with constant velocity. This can be accom-
plished with the application of a gradient sequence in which the strength and
duration of the gradient pulses have a 1:2:1 ratio (see Fig. 3.22). Vascular blood
