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.

1. Phase Contrast
ECG-Gated
Rahman
Ud
Din
Lecturer
Medical
Imaging
NWIHS

2. Learning Outcomes
 Phase Contrast MRA
 VENC (Velocity Encoding)
 ECG-Gated FSE MRA

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

4. Phase Contrast MRA

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

6. VENC (Velocity Encoding )

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

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

13. ECG-Gated FSE MRA