Biomedical Engineering Reference
In-Depth Information
3.2.8 Magnetization Transfer
This method is based on suppression of the off-resonant protein-bound water
protons. If magnetization transfer (MT) is combined with TOF MRA, it helps
suppress the background signal at least by 30%. The best-known example is
brain parenchyma where MT-TOF MRA increases conspicuity of small and distal
branches of vessels with slow flow, and aneurysms. MT can be combined with
TONE for further visualization of small vessels.
3.2.9 Flow Eddies
The flow eddies are unique for identification and estimation of stenosis although
these cause overestimation of stenosis by MRA. Flow eddies are mainly con-
tributed by turbulent flow and vertex flow as well as stream separation distal
to stenosis and carotid siphons (vessel turns). This overestimation of stenosis
is the result of accelerated flow through the stenotic area leading to dephasing
and flow void during TE. Common examples of overestimation are estimation
of length of stenosis in the case of poststenosis or mimicking stenosis in the
case of vessel turns.
3.2.10 Bright Blood Imaging
In addition to making the vessels appear black, vascular structures can also be
visualized by making them brighter. Several techniques can be used to enhance
the signal from flowing blood including gradient echo imaging and/or gradient
moment rephasing and/or contrast enhancement. In gradient echo imaging, the
flowing spins are refocused by the rephasing gradient. The patent vessels appear
brighter on the images. This technique can be referred to as “bright blood imag-
ing” and can be further improved by the application of an imaging option known
as “gradient moment rephasing.” It is a first-order velocity compensation tech-
nique to visualize slow moving protons with constant velocity (see Fig. 3.26).
Protons in venous blood or CSF are put into phase with the stationary protons.
So, the intravoxel dephasing is reduced. Gradient moment rephasing compen-
sates the flow by making these vessels containing slow flowing spins appear
brighter. It enhances the signal from blood and CSF.
