Biomedical Engineering Reference
In-Depth Information
Figure 3.12: Three-dimensional TOF angiogram (left panel) shows cavernous
angioma with visible methemoglobin due to short T1 due to simulated blood
flow. For comparison, SPGR images are shown with high signal intensity center
representing methemoglobin.
result is bright signal intensity in the images, which may simulate flow-related
enhancement.
3.2.2.1.3 Echo Time (TE).
Lower TE reduces motion-induced phase errors.
Partial RF pulses reduce the minimum TE while these RF pulses preserve an
acceptable slab profile. Very low TE may be achieved by removing flow compen-
sation from the gradient waveform. Thus there is a trade-off between minimum
echo time at the cost of flow compensation. This approach is currently used for
clinical imaging.
3.2.2.1.4 Flip Angle.
Flip angle has an effect on intravascular signal intensity
and background suppression. Smaller arteries may be visualized at flip angles
of 15-20
◦
with TR of 40 msec. Stationary tissues exhibit greater saturation at a
larger flip angle. For example, small 3D volumes of 28 slices show intravascular
signal intensity of larger arterial structures at flip angles 20-35
◦
with rapid flow.
Arterial flow begins to saturate at flip angles greater than 40
◦
. It results in reduced
intravascular signal intensity (see Fig. 3.13).
3.2.2.1.5 Flow Compensation.
Flow compensation is critical in 3D TOF
MRA. Motion-induced phase dispersion results in signal void areas. These areas
are frequently identified within the juxtasellar carotid arteries and proximal
middle cerebral arteries. These signal void areas can be minimized by the use
of shortest possible TE with flow compensation applied in the slice-select and
read-out directions. This combined approach reduces the phase dispersion and
