Biomedical Engineering Reference
In-Depth Information
Figure 3.27:
Partition effect.
The phase of the transverse magnetization is made independent of the flow
velocity by the use of velocity compensated gradients. However, higher order
flow terms may cause signal void in the areas of turbulent flow. The use of
short echo times compensates for this. If the stationary tissue is selected as the
volume of interest, it may be saturated using a short TR and a large tip angle
(see Fig. 3.27). During this pulse sequence, fresh unsaturated blood moves into
the imaging slice. This results in good contrast between the unsaturated blood
and the stationary tissue.
The INFLOW technique may be used with 2D multiple single slice or 3D
acquisition with a flow compensated gradient echo sequence. For 2D multiple-
single-slice INFLOW, many thin (2-3 mm) contiguous (or over contiguous) slices
are collected in a plane that is orthogonal to the blood flow. The optimum con-
trast between flowing blood and stationary tissue should be obtained with the
shortest TE, a TR of the order of 40-60 msec, and a large tip angle of 45-90
◦
,
depending on the anatomy being studied and the flow rate of blood. Presatura-
tion of a slab above or below the imaging slice allows selective imaging of the
veins or arteries. The single sided, parallel presaturation slab moves with the
slice position, ensuring good suppression. The slab thickness is adjusted in the
second pass parameters and is typically set to 50 mm. An alternative method for
certain imaging protocols employs a presaturation plane that is perpendicular
to the imaging slice. An example is the use of a sagittal or coronal slice for
imaging the carotid arteries. A perpendicular presaturation slab is necessary to
remove the venous flow.
