Biomedical Engineering Reference
In-Depth Information
direction of a symmetrical gradient field. This is the case of read-out gradient
when multiple echoes with constant echo spacing are obtained. In a multiecho
sequence obtained in this fashion, echoes occur at all multiple integers of the
echo time (TE) such that even echoes correspond to phase change zero for
stationary and moving spin isochromats independent of their velocity. The in-
travascular signal does not show up and results in no dephasing at even echoes.
So, the rephrasing phenomenon is dramatic, as vessels without signal on the first
echo can have very bright signal on the second echos or even echos. Arteries
do have the pulsatile blood flow and experience the velocity and acceleration
dephasing effects. During diastole, arterial blood is almost stagnant and leads to
high signal intensity. There is complete signal loss during diastole when blood
is moving fast in the veins.
Both slice-transition and spin-phase phenomena are responsible simultane-
ously for the suppression of intravascular signal. In in-plane flow, spin-phase
effects must be dominant, whereas for in-flow perpendicular to the imaging
plane, slice-transition effects must be prominent. In gradient echo imaging, in-
traluminal signal loss occurs due to spin-phase phenomenon. It does not show
high-velocity signal loss. This intraluminal signal loss results from spatial vari-
ation in velocities and accelerations at the site of increased shear stress in the
vascular tree such as arterial bifurcation, arterial proximal parts, pulmonary
arteries, and venous confluences around cardiac valves. However, dephasing
effects and therefore the signal losses on GRE images are pulse sequence de-
pendent.
In general, MR imaging requires the product of the gradient field
G
and the
time to turn-on (
t
p
) to be constant for spatial resolution. So, the dephasing effect
on constant velocity is dependent on
t
p
2
and
t
p
3
, and shorter TE. In other words,
shorter TE and
t
p
will result in less dephasing effect and less signal loss. The
SE and GRE pulse sequences are standardized for using dephasing effect in the
evaluation of valvular diseases.
3.1.2.4 Turbulence-Induced Signal Loss
On GRE images around vascular stenoses, turbulence-induced signal loss further
deteriorates the capacity of this approach in flow quantification in valvular dis-
eases. Spin dephasing and consecutive signal loss make the lesions appear more
prominent. The turbulence-induced effects are corrected by flow-compensated
