Phase contrast MRI uses changes in the phase of tissue magnetization from moving protons to create angiographic images and measure flow velocity. ECG-gated MRI acquires images during systole and diastole, subtracting the former to highlight arteries based on their faster flow during cardiac contraction. Typical velocity encoding values are 20-30 cm/s for veins, 40-60 cm/s for higher arterial flow, and over 60 cm/s to determine flow direction and velocity.
3. Phase Contrast MRA
PC-MRA uses change in the phase of TM of
the blood and form an image
The selective phase shift of the moving p+
is produced
GRE of a given strength is applied to both
stationery as well as moving p+ causing
phase shift in both but at different rates
Initial RF excitation pulse brings all p+ in
phase
Second GRE of same amplitude and
duration but of opposite polarity is applied
5. In stationery p+ reversal of phase shift
occur of exact amount
Canceling the effect of original phase
shift
Resulting into no net phase shift
Hence, flowing p+ changed their position
the phase shift will not be corrected
This shift is directly proportional to the
change in location or distance the p+
travel b/w application of first and second
gradients
The phase shift is used by PC-MRA to
create angiographic image and to
measure flow velocity
7. Velocity encoding gradients are applied
in one or all three directions to acquire
quantitative information
Velocity encoding technique
compensates for projected flow velocity
within the vessel by controlling the
amplitude or strength of the bipolar
gradient
If velocity encoding (VENC) is selected
lower than the velocity within the vessel-
aliasing occurs
Aliasing results in low signal intensity in
the centre of the vessel and
Better delineation of the vessel wall
8.
9. Typical values of the VENC are
◦ 20-30 cm/s for venous flow
◦ 40-60 cm/s for higher velocity with some aliasing
◦ 60-80 cm/s to determine velocity and flow
direction
PC-MRA provides information about flow
direction
If flow is encoded from superior to inferior
Head will appears bright and feet appears
black
PC-MRA are in 2D or 3D acquisition
2D used in routine practice bcoz of
acceptable acquisition time of 1-3 minutes
10. Phase Contrast MRA
Typical Venc
values for different
flow measurement
are listed below:
Aorta: 150 cm/s
MPA: 120 cm/s
Aqueduct: 8-12
cm/s
11. ECG-Gated FSE MRA
In this technique, images of
the vessels are acquired in
systole and in diastole
On diastole image for both
artery and vein are both bright
On systole artery flow will
appear black due to its fast
movement while veins will
appear bright due to it slow
flow
The systole images are
subtracted from diastole
images resulted bright blood
in arteriography
12. Advantages are;
◦ Relative short-time, sensitive to slow flow, acquire coronal
plane
◦ Not possible in patient with arrhythmia
Used in peripheral arteries and aorta
FBI (Toshiba), NATIVE SPACE (s), TRANCE & flow