magnetic resonance in angiography

1. MAGNETIC RESONANCE
ANGIOGRAPHY
AND ITS ARTIFACTS

2. MR angiography
It is a type of magnetic resonance imaging(MRI) scan that uses a
magnetic field and pulses of radio wave energy to provide pictures
of blood vessels inside the body.
Advantages of MR angiography:
compared with catheter angiography, MRA is less invasive, less
expensive, and faster to perform. For conventional angiography,
catheter is inserted though the patient groin and threaded up into
the artery in the brain.mra does not require this catheter.
As a result, it eliminates related complications such as possible
damge to an artery.
Disadvantages of MR angiography:
it does not depict small vessels or extremely slow blood flow as
well as conventional angiography dose.

3. MR ANGIOGRAPHY IMAGES

4. CONTRAST ENHANCED MR ANGIOGRAPHY
It is a technique involving 3D spoiled gradient-echo(GE)
sequences, with administration of Gd-based contrast. It
can be utilised to assess vascular structures of almost any
part of the body.
It key features are as follows:
T1- weighted spoiled gradient-echo(flip angle 25 degree-
50 degree allows T1- weighting)
central k-space acquisition corresponding to arterial phase
of the study maximizes preferential visualization arteries
use of Gd- based contrast to shorten T1-interval of the
blood which appears bright as a result

5. Black blood imaging
In this techniques blood appears as black as this are
spin-echo based sequences techniques.
However proton in the flowing blood usually do not
receive either 90 degree or 180 degree pulse.
Hence signal is not produced and flowing blood
appear as dark. Slow flow and clots can produced
bright signal because they received both 90 degree
or 180 degree pulse.
Blood flow- black image
Slow flow and clot- bright image

6. BLACK BLOOD IMAGING

7. Bright blood imaging
In this type of imaging blood appear bright most of
this are gradient echo sequences. In this GRE
sequences excitation pulse is slice selected but the
rephrasing which is done by gradient rather than
180 degree pulse is not limited to the slice of
interest and is applied to whole imaging volume
therefore a flowing proton will receive an
excitation pulse is rephrased regardless of its slice
position.
Blood flow- bright image
Slow flow and clot- black image

8. BRIGHT BLOOD IMAGING

9. Non contrast enhanced MR angiography
It is performed in several ways including:
1.Time of flight angiography(TOF- MRA)
2. Phase contrast angiography(PC- MRA)
Generally, these techniques are time- consuming as compared
with contrast enhanced MR angiography.
Non- contrast Contrast image
image

10. Time of flight angiography(TOF)
It is an MRI technique to visualize flow within vessels,
without the need administer contrast. It is based on the
phenomenon of flow- related enhancement of spins
entering into imaging slice.
As a result of being unsaturated, these spins give more
signal that surrounding stationary spins.
TOF can be of three types :
1. 2D time of flight
2. 3D time of flight
3. MOTSA(Multiple overlapping thin slab (acquistion)

11. TIME OF FLIGHT IMAGES

12. 2D time of flight
With 2-D TOF, multiple thin imaging slices are acquired
with a flow compensated gradient- echo sequences. These
images can be combined by using a technique of
reconstruction such as maximum intensity projection(MIP),
to obtain a 3-D image of the vessels analogous to
conventional angiography.
3D time of flight
With 3-D TOF, a volume of images is obtained
simultaneously by phase- encoding in the slice-select
direction. An angiographic appearance can be generated
using MIP, as is done with 2-D TOF.

13. 2D- TOF IMAGE 3D- TOF IMAGE

14. MOTSA(multiple overlapping thin slab acquisition)
Combined advantages of 2D(large cover area) and (3D high
resolution).
Imaging volume is divided. these slabs are combined to single
volume data.

15. MOTSA(MULTIPLE OVERLAPPING THIN SLAB ACQUISITION)
MRA IMAGE

16. Phase contrast imaging(PC-MRA)
It is an MRI technique that can be used to visualize moving fluid. It
is typically used for MR venography as a non IV-contrast requiring
technique.
Spin that are moving in the same direction as a magnetic field
gradient develop a phase shift that is proportional to the velocity of
the spins. This is the basis of phase-contrast angiography.
In the simplest phase- contrast pulse sequences, bipolar gradients
are used to encode the velocity of the spins.
Stationary spins undergo no net change in phase after the two
gradients applied. Moving spins will expierence a different
magnitude of the second gradient compared to the first, because of
its different spatial position. This result in net phase shift.

17. PHASED CONTRAST MRA IMAGES

18. ECG- Gated FSE MRA
In the ECG gated fast spin echo techniques images
of the vessels are acquired during diastole and
systole.
On the diastole image both artery and vein are
bright while on the systole image the artery is dark
due to slow flow .
The systole images are subtracted from diastole
images giving the bright blood arteriography. The
vein and background are subtracted.

19. Advantages of this technique involves:
1. relatively short time
2. sensitivity to slow flow
3. ability to acquire in coronal plane parallel to the vessels.
4.It may not be suitable for patients with
arthythmia(asthamic patient)
5.it is mainly used for peripheral arteries and aorta.
Examples: FBI(fresh blood imaging, Toshiba)
NATIVE SPACE and NATIVE HASTE(SIEMENS)
TRANCE(PHILIPS) and flow prep.

20. SSFP- Based MRA
Balanced SSFP sequences are gradient echo steady state sequences
with very high SNR and motion insensitivity due to balanced
gradients in all three directions.
These sequences are called True FISP(siemens),FIESTA(GE) and
balanced TFE(Philips). The contrast is determined by T2/T1 ration
giving the blood very high signal without dependence or inflow. Veins
other fluid and fat are also bright on this sequences.
Hence some form of subtraction is required arterial spin
labelling(ASL) or inversion recovery pulses are used for this purpose.
Example: SLIP(Spatial labelling inversion pulse, Toshiba)
NATIVE True FISP(Siemens)
IFIR(Inflow inversion recovery, GE)

21. SSFP- BASED MRA IMAGES

22. Contrast Enhanced MRA(CEMRA)
This sequences used for CEMRA is usually a modified T1
weighted spoiled gradient refocused GRE sequences.
Approximate T1 times of blood, muscle and enhanced blood
are 1200ms, 600ms and 100ms
respectively.
A dose of approximately 0.2m mol/kg of gadolinium is
required to make T1 of blood shorter than that of fat and
muscle, so that it will appear brighter than fat.
The most important aspect in CEMRA is timing of peak
arterial enhancement such that it will fill center of the K-
space that is responsible for contrast in the image.

23. The time to fill the center of the K-space is usually the
initial 2-3 seconds of the typical 15-20 seconds run. It
is important to match these 2-3 seconds with peak
arterial enhancement to get good angiogram.
Keyhole imaging can be applied to CEMRA to improve
the temporal resolution.
Examples- TRICKS (GE)-time resolved MRA technique
TWIST(SIEMENS)

24. ANGIOGRAPHY ARTIFACTS
1.METAL ARIFACT
2. BLOOMING ARTIFACT
METAL ARTIFACT
It is intended to reduce the size and intensity of susceptibility
artifacts resulting from magnetic field distortion.
A variety of techniques are used foe reducing metal artifacts
at MRI, both for addressing artifacts due to the presence of
metal in the image plane(in- plane artifacts) and for artifacts
due to metal in an adjacent plane(through- plane artifacts).

25. METAL ARTIFACTS IMAGE

26. BLOOMING ARTIFACTS
Blooming artifacts is a susceptibility artifact encountered on
some MRI sequences in the presence of paramagnetic substance
that affect the local magnetic milieux. Most affected sequences
are T2*, and in many instances they are designed to exploit this
phenomenon to make certain pathologies more conspicuous. As
such although
it is artifact, it is used deliberately to improve detection of
certain small lesion, much as the T1 shortening effect of low
concentration gadolinium are used to detect contrast
enhancement.
The term “blooming” refers to the fact that lesions appear larger
than they actually are.

27. BLOOMING ARTIFACT IMAGE

28. THANK YOU