Biomedical Engineering Reference
In-Depth Information
3.1.2.2 Inflow Related Artifacts and Their Suppression
High intravascular signal intensity on GRE images shows the vascular ghost
appearance. These ghosts appear due to pulsation artifacts. These pulsation
artifacts affect the image quality, if the images are acquired without electro-
cardiographic (ECG) triggering. These artifacts may be suppressed by the use
of phase encoding steps. For this, the principle frequency is selected as super-
imposed with inspiration rate. It will shift the ghost to the edge of the image.
Motion-insensitive pulse sequence is also used to suppress the artifacts. In gen-
eral, second echo and velocity-insensitive gradients are applied. These methods
compensate for high-order motions and thereby minimize the vascular ghosting.
Hence, these methods are known as flow-compensated techniques. However, for
quantification of vascular features, the spatial-presaturation method is a current
practice. This method uses 90
◦
RF pulse. The 90
◦
RF pulse is applied outside
the imaged volume. This imaged volume represents the stack of slices in any
region. In any selected region, the inflowing spins may produce entry-slice phe-
nomenon. After applying 90
◦
RF pulse, the
z
magnetization of inflowing spins is
completely abolished just before their flow in the imaging volume. This results
in no flow-related enhancement effects and the vessels appear black on GRE
images. The disadvantage of this method is that it cannot completely suppress
signal from stagnant blood imaged with pulse sequences even using the long
repetition times (TR).
In order to overcome this problem, MR-projection angiograms of vessels
have emerged as an alternative method. In this method, many adjacent thin
slices perpendicular to the principal vessel orientation are acquired consecu-
tively. The method is known as 2D Fourier transform gradient recalled echo
(2D-FT GRE). As a result of entry-slice phenomenon, the vessels appear very
bright in these slices, particularly if they have been acquired using flow compen-
sation to suppress the dephasing. These acquired data are three dimensionally
reconstructed using maximal-intensity-projection or surface-rendering methods
to generate vascular angiograms. This method is described later in the section
on techniques. If presaturation pulse is applied in distal to imaged slice, the
signals from veins are suppressed. Similarly, if presaturation pulse is applied in
the proximal direction to the imaged slice, it will suppress arterial signals. This
method was further refined by the use of two image sequences of the veins and
arteries being imaged. It was done by subtracting a flow compensated sequence
