Biomedical Engineering Reference
In-Depth Information
the system collects and displays the data as a series of thick slices or a single
slab. The slices, or slab, are then projected onto a single plane. The most efficient
way to acquire 2D PC angiogram is to use the shortest possible repetition time
and a large number of excitations, e.g. NEX
=
2-16.
The optimized NEX permits data emerging during an R-to-R interval for each
set of the phase-encoding gradients. Since the data is averaged during the entire
cardiac cycle, the resulting angiogram will be a measure of the average flow.
Averaging also reduces ghosting artifacts. 2D phase contrast images may also
be obtained without the use of flow compensation, to minimize echo time.
3.2.3.1.2 Dynamic-Range Compression.
Compared with the signal com-
ing from all the other spins in the regions being imaged, the MR signal from
moving blood is very small. Since the primary mechanism of stationary-spin
suppression is the subtraction of two excitations drawing most of their MR sig-
nal from nonmoving spin, minor errors in the stationary-spin signal prohibit
good background suppression.
To overcome this problem, a projection-dephasing gradient can be applied
to diminish signal from thick objects. This gradient has little effect on vascular
signal because the vessels are small with respect to the head and neck. For this
reason, the signal from stationary tissues is substantially suppressed, while the
MR signal from vessel is only slightly diminished (see Fig. 3.20). This results
in a reduction in dynamic range (an important imaging enhancement for PC
angiography). Projection dephasing may also modify the appearance of vessels
separated in the direction of projection. Under these circumstances, the region
of overlapped vessels may exhibit enhanced signal intensity, reduced signal
intensity, or no change in intensity.
3.2.3.1.3 Cardiac-Gated 2D Phase Contrast Angiography.
Cardiac
gated 2D phase contrast angiography is based on cine MR acquisition meth-
ods in which TR remains constant, and each step in phase encoding is initiated
by the ECG trigger. The cine gradient echo pulse sequence is modified to include
bipolar gradients for positive and negative flow encoding in a fashion analogous
to multislice/slab 2D phase contrast angiography. The velocity encoding is also
similar to nongated 2D PC MRA. With this approach, up to 32 points in the car-
diac cycle are retrospectively sorted from the scan data. Magnitude and phase
images are then generated for each point in the cardiac cycle. On the phase
