(CE
MRA)
Contrast enhanced
MR angiography is based on the T1 values of blood, the surrounding tissue, and paramagnetic
contrast agent.
T1-shortening
contrast agents reduces the T1 value of the blood (approximately to 50 msec, shorter than that of the surrounding tissues) and allow the visualization of blood vessels, as the images are no longer dependent primarily on the inflow effect of the blood.
Contrast enhanced
MRA is performed with a short TR to have low signal (due to the longer T1) from the stationary tissue, short
scan time to facilitate
breath hold imaging, short TE to minimize
T2* effects and a
bolus injection of a sufficient dose of a
gadolinium chelate.
Images of the
region of interest are performed with 3D spoiled
gradient echo pulse
sequences. The enhancement is maximized by timing the
contrast agent injection such that the period of maximum arterial concentration corresponds to the
k-space acquisition. Different techniques are used to ensure optimal
contrast of the arteries e.g.,
bolus timing,
automatic bolus detection,
bolus tracking,
care bolus.
A high
resolution with near
isotropic voxels and minimal pulsatility and misregistration artifacts should be striven for. The
postprocessing with the
maximum intensity projection (
MIP) enables different views of the 3D data set.
Unlike conventional
MRA techniques based on
velocity dependent inflow or
phase shift techniques,
contrast enhanced
MRA exploits the
gadolinium induced T1-shortening effects. CE
MRA reduces or eliminates most of the artifacts of
time of flight angiography or
phase contrast angiography. Advantages are the possibility of in plane imaging of the blood vessels, which allows to examine large parts in a short time and high
resolution scans in one breath hold.
CE
MRA has found a wide acceptance in the clinical routine, caused by the
advantages:
•
3D
MRA can be acquired in any plane, which means that
greater vessel coverage can be obtained at high
resolution with fewer slices (aorta, peripheral vessels);
•
the possibility to perform a time resolved examination
(similarly to conventional
angiography);
•
no use of ionizing radiation; paramagnetic agents have a beneficial
safety.
•
] and 41 definitions [
]
Further Reading:
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