Biomedical Engineering Reference
In-Depth Information
In our experience, two or more images are collected, one of which is velocity
sensitive in a specified direction and the other is velocity compensated image.
For example, a thick slab (100 mm) FLAG sequence in the plane of flow (thick
coronal slab for imaging the abdominal aorta) will yield a projective MR angio-
graphic image. The resultant phase images are phase corrected and subtracted
to yield the projection angiogram. The FLAG sequence can be run with or with-
out cardiac triggering. In the noncardiac triggered version, a “shortest” TR is not
recommended. The FLAG sequence interleaves the velocity compensated and
velocity-sensitive data in consecutive TR periods. If the scan is gated, FLAG se-
quence interleaves the velocity sensitive data such as in consecutive heartbeats.
For better suppression of respiratory motion, the RSE sequence may be used.
RSE interleaves the velocity compensated and velocity-sensitive data in the same
heartbeat. The RSE sequence must be run in the cardiac triggered mode.
3.3.3 Digital Subtraction MRA
Digital subtraction MRA has been compared to digital subtraction angiography
(DSA) as contrast is selectively produced for moving spins during two acqui-
sitions. These moving spins are then subtracted to remove the signal from the
stationary spins, leaving behind an image of the moving spins. An early sub-
traction angiogram may be performed while gating to the cardiac cycle. An
acquisition during systole (fast flow) is generally subtracted from an acquisition
during dystole (slow flow). In this case, the stationary spins were subtracted,
retaining only the moving spins, such as the vasculature, on the resultant image.
This technique is significant as recent techniques were based on same principles.
3.4 Recent Advancement in MRA Techniques
Different newer methods are reported in the literature for MRA from the per-
spective of different applications applied for flow imaging.
3.4.1 Sensitivity Encoding
Sensitivity encoding (SENSE) is used to increase spatial resolution and decrease
venous contamination in peripheral MRA. In this method, single-bolus peripheral
