Biomedical Engineering Reference
In-Depth Information
first moment (
M
1
) describes its effect on the phase of a spin with constant
velocity. The second moment (
M
2
) describes the gradient's relationship to the
phase of spins experiencing acceleration. The third moment (
M
3
) defines the
effect of jerk on spin phase. Even higher order moments exist, but they are
usually less important.
3.2.5.7 Shorter Echo Times
Shorter echo times (TE) may also reduce the problem of signal loss due to phase
dispersion. Short TE reduces the time for spins to dephase after the RF pulse.
Short TE thereby reduces the signal loss arising from susceptibility gradients,
velocity distributions, and higher orders of motion. For all VMRI techniques,
flow-related phase errors accumulate as a function of TE(
n
+
1), where
n
is
the moment (i.e.,
n
=
1 for velocity and
n
=
2 for acceleration). Phase error is,
proportional to TE(
n
+
1).
The effects of higher order moments become more significant for long echo
delays. This is because the second moment (acceleration) has a cubic depen-
dence on echo time, while the third moment (jerk) has fourth-power depen-
dence. Using the shortest possible TE can therefore minimize signal loss due to
these higher order moments. For example, a VMRI exam obtained with TE
=
3 msec will have approximately one-half the velocity-related phase errors of the
same study performed with TE
=
4 msec.
3.2.5.8 Complex Flow
To minimize the problem of signal loss due to complex flow, several strategies
may be employed. The dispersion of velocities along a projection can be greatly
reduced by obtaining vessel images in thin cross-sections rather than in full
projection. 3D data acquisition overcomes the problem of velocity dispersion
within a voxel. Since the phase contrast technique relies on the phase shift
induced in moving spins, conventional flow compensation techniques cannot be
used on flow-encoding axis. To minimize phase dispersion, the bipolar phase-
encoding gradient is placed symmetrically around the first moment (called PC
flow compensation). However, a slightly shorter echo time can be achieved by
placing this gradient asymmetrically in relation to the first moment. The resulting
