Biomedical Engineering Reference
In-Depth Information
flow is pulsatile and velocity is not constant between excitation and detection.
However, some phase dispersion will normally occur. In addition, in some
anatomic regions the effects of acceleration become prominent and “acceler-
ation drop out” signal loss becomes apparent in the resulting images. In periph-
eral vascular studies, pulsatile motion and jerk are significant causes of artifacts.
Although acceleration compensation schemes exist, the inevitable trade-off of
increased echo time can make them impractical.
3.2.5.6 Phase Dispersion
When magnetic field gradient is applied to a spin system, the spins within the
voxel accumulate a phase angle in relation to one another. This phase angle
difference is known as “phase dispersion.” To correct for this phase dispersion,
the gradient is typically reversed to rephase the spins. This technique is used
frequently in imaging sequences to refocus stationary spins. These “bipolar”
gradient lobes are of equal strength and duration but have opposite signs (see
Fig. 3.23). Spins that are moving in the direction of the magnetic field gradient are
not refocused and are left with some residual phase. The motion-induced phase
shifts occurring in the presence of magnetic field gradients are arithmetically
defined by position/time derivatives called “moments.” The zeroth moment (
M
0
)
describes the effect of a gradient on the phase of stationary spins. Similarly, the
Figure 3.23:
Gradient reversal.
