Biomedical Engineering Reference
In-Depth Information
3.3 Acquisition Methods
This section describes the basic theory of MR angiography, mainly the INFLOW,
FLAG, and rapid sequential excitation (RSE) methods.
3.3.1 INFLOW Method—Time of Flight
This method belongs to a class of MR angiographic techniques known as “time-
of-flight.” This technique gives rise to 3D information about the vessels in the
volume of tissue being imaged with high contrast between the stationary tissue
and the flowing blood. The INFLOW method relies on the flow related image en-
hancement caused by the movement of fresh, unsaturated blood into an already
saturated slab of tissue. The INFLOW method has a number of advantages over
other angiographic imaging methods. First, image subtraction is not necessary,
thereby reducing scan time and computing requirements while speeding data
manipulation. Second, high contrast can be obtained virtually independent of
flow velocity. Third, the arteries or veins may be selectively imaged by the use
of presaturation slabs. Finally, the technique does not require the use of self-
shielded gradients. It is less sensitive to motion than the phase contrast methods.
Using the INFLOW technique, angiograms may be obtained in only 10-15 min.
For example, the data can be processed by sending a batch job or processed
interactively with AP500 within 10 min. The choices are available on selecting
INFLOW processing under the ANPROC key. Both batch and interactive pro-
cessing are discussed later in this section.
To achieve the best possible contrast in the final images, the imaging param-
eters must provide for maximum refreshment of blood in the imaging volume.
The threshold minimum velocity (
V
t
) is given by:
V
t
=
d
/
TR
,
(3.16)
where
d
is the slice thickness and TR is the repetition time. For a typical 2D
INFLOW sequence with 2 mm slices and TR
=
50 ms, threshold velocity (
V
t
) will
be 0.04 m/sec. For velocities greater than
V
t
, the signal intensity is essentially
independent of the flow velocity. Typical velocities range from 1 m/sec for the
aorta and 0.8 m/sec in the carotid artery and 0.03 m/sec in small veins.
