Introduction to Magnetic Resonance Angiography

1
Introduction to Magnetic Resonance Angiography

• Geoffrey D. Clarke, Ph.D.
• Division of Radiological Sciences
• University of Texas Health Science Center at San
  Antonio

2
Overview

• Flow-Related Artifacts in MRI
• Time-of-Flight MR Angiography
• Contrast-Enhanced MR Angiography
• Phase-Contrast MR Angiography
• Quantitative Flow Imaging

3
Flow Voids Enhancements

• In spin echo imaging vessels appear as signal
  voids
• same volume of blood does not experience both 90o
  and 180o pulses
• In flow effect
• may cause unsaturated blood to appear bright in
  slice that is most proximal to heart
• Saturation effects
• cause diminished signals in blood flowing
  parallel to image plane

4
Vessel Signal Voids
Early multi-slice spin echo images depicted
vessels in the neck as signal voids
5
Multi-slice Spin Echo
MRI Slices
Long TR 90o-180o Fast flow
Flowing Blood
Stationary Tissue
Spins do not get refocused by 180o pulse
Slice 1 Slice 2 Slice 3
6
Field Echoes Bright Blood

• Partial Flip Angle/Field Echo Images
• Short TR, Short TE
• Only one TX RF pulse (?o)
• Blood has Greater Proton Density than Stationary
  Tissues

7
Bright Blood Images
Using gradient (field) echo images with partial
flip angles allowed blood which flowed through
the 2D image plane to be depicted as being
brighter than stationary tissue.
8
Motion Artifacts

• in read-out direction
• data acquired in time short compared to motion
• blurring of edges
• in phase-encode direction
• ghosting presenting as lines smudges
• in slice-select direction
• variable partial volume, difficult to detect

9
The MRI Signal Amplitude Phase
Bo
rf B1
Net Magnetization
Real
Imaginary
Real
Imaginary
10
Dephasing Due to Motion
Gslice
time
180o
BLOOD phase not zero
TISSUE phase equals zero
PHASE
time
Phase Shift Due to Motion in a Gradient Field
-180o
t 0
11
Pulsatile Motion Artifact
Aorta
Artifact
Artifact
Artifact
12
Motion Compensation Gradients
Gslice
time
Phase Shift Due to Motion in a Gradient Field
180o
PHASE
time
-180o
BLOOD phase equals zero
t 0
TISSUE phase equals zero
Only applies for constant flow. More
gradient lobes needed for acceleration.
13
Flow Artifact Correction

• Spatial pre-saturation pulses prior to entry of
  the vessel into the slices
• Surface coil localization
• Shortened pulse sequences
• Cardiac respiratory gating
• Motion Compensation Gradients

14
Magnetic Resonance Angiography (MRA)
15
MRA Properties

• Utilizes artifactual signal changes caused by
  flowing blood to depict vessel lumen
• May include spin preparation to suppress signal
  from stationary tissues or discriminate venous
  from arterial flow
• Does not require exogenous contrast
  administration, but contrast agents may be used
  to enhance MRA for fast imaging

16

• Methods of Magnetic Resonance Angiography
• Signal Amplitude Methods
• 2D Time-of-Flight
• 3D Time-of-Flight
• Signal Phase Methods
• 2D Phase Contrast
• (Velocity Imaging Q-flow)
• 3D Phase Contrast
• (Velocity Imaging Q-flow)

17
Time-of-Flight MRA Method
Bo
M
Imaginary
Real
18
Time of Flight Effect

• T1 of flowing water is effectively shorter than
  the T1 of stationary water
• Two contrast mechanisms are responsible
• T1 saturation of the stationary tissue
• In-flow signal enhancement from moving spins

19
2D Time-of-Flight MRA Conditions

• Field Echo Imaging
• Short TE
• Partial Flip Angle
• generally large
• keeps stationary tissues saturated
• TR and flip angle
• adjusted to minimize stationary tissue
• adjusted to maximize blood

20
2D Time-of-Flight MRA Advantages

• Good stationary tissue to blood
• flow contrast
• Sensitive to flow
• Minimal saturation effects
• Short scan times
• Can be used with low flow rate

21
2D Time-of-Flight MRA Limitations

• Relatively poor SNR
• Poor in-plane flow sensitivity
• Relatively thick slices
• Long echo times (TE)
• Sensitive to short T1 species

22
Improving Contrast in Time-of-Flight MRA
1. Venous Pre-saturation (spatial
suppression) 2. Magnetization Transfer Contrast
(frequency selective irradiation) 3. Fat
Saturation (frequency selective
irradiation) 4. Cardiac Gated MRA 5. Spatial
variation of flip angle
23
Spatial Pre-saturation in Time-of-Flight MRA

• Saturates and dephases spins before they enter
  imaging slice
• Can be used to isolate arteries or veins
• Can be used to identify vessels feeding
• a given territory
• Can be used to establish the direction of flow
  in a particular vessel

24
Magnetization Transfer Contrast
PROTON SPECTRUM
Frequency (Hertz)
0
Free Water
Lipids
Bound Water
0
217 Hz
1500 Hz
Frequency (Hertz)
25
Gradient Echo with MTC Pulse
Off-resonance rf pulse
RF excitation
TX
Digitizer On
Field Echo
RX
Slice Select
Gsl
Rephasing
Spoilers
Read Out
Crushers or Spoilers
Dephasing
Gro
Phase Encode
Gpe
26
MIP 1
Maximum Intensity Projections
MIP 2
OBJECT
27
2D TOF Application
Abdominal Aneurysm
28
3D Time-of-Flight MRA Conditions

• Uses two phase encode gradients and volume
  excitation
• Maximum volume thickness limited by flow velocity
  
• Use minimum TR, adjust flip angle for best
  contrast

29
Three Dimensional Gradient Refocused Echo Imaging
RF pulse (short time)
TX
Field Echo
RX
Slab Select
Secondary Phase Encoding
Digitizer On
Gsl
Crusher
Rephasing
Read Out
Gro
Dephasing
Primary Phase Encoding
Phase Rewinder
Gpe
30
3D Time-of-Flight MRA Advantages

• Higher resolution (thinner slices) available
  allowing for delineation of smoother edges
• Higher signal-to-noise than 2D methods
• Lower slice select gradient amplitudes results in
  fewer phase effect artifacts than 2D method
• Short duration RF pulses can be used to excite
  slab TE can be reduced

31
3D Time-of-Flight MRA Limitations

• Blood signal is easily saturated with slow flow
• Relatively poor background suppression
• Short T1 tissues may be mistaken for vessels

32
3D-TOF ApplicationCerebral Arteries Circle of
WIllis

• TR /TE 40 / 4.7 ms
• 64 partitions, 48 mm slab, 0.75 mm per partition
• Flip angle 25o
• 256 x 256, 18 cm FOV, 0.78 x 1.56 mm pixel
• MTC contrast
• Venous Presaturation

33
Circle of Willis
90o

• Time of Flight MRA

34
Cerebral Venous Angiogram
TOP
Saggital Sinus
Use of arterial presaturation allows
visualization of cerebral venous vessels
FRONT
Straight Sinus
Transverse Sinus
Confluence Of Sinuses
Cerebven.mpeg
35
Multi-Slab 3D TOF MRA

• Hybrid of 2D and 3D methods
• Thin 3D slabs used
• Good inflow enhancement
• Multiples slabs to cover volume of interest
• High resolution
• Short TE
• Relatively time inefficient

36
Gd Contrast Enhanced MRA

• Gd contrast agents decrease T1 and increase CNR
  of blood and soft tissue
• Along with ultra-fast 3D sequences, allow
  coverage of larger VOIs
• Shorter acquisition times allow breath-holding
  for visualization of central and pulmonary
  vasculature

37
MRI Compatible Power Injectors
Programmable Automatic Injection MRI
Compatible Allows rapid arterial injection of
Gd-DTPA
www.medrad.com
38
3D CE-MRA of Aortic Aneurysm

• 44 slices
• 32 sec scan
• TR/TE
• 2.3/1.1 ms
• 1.5 x 1.8 x 1.8 mm pixel

Phase 3 Phase 2
Phase 1
Digital Subtraction X-ray Angiography
Phase 2 Phase 1
Schoenberg SO, et al. JMRI 1999 10347-356
39
Bolus Chase 3D MRA
Station 1 Station 2
Station 3
Earlier venous enhancement noted with fast
injection
Ho VB et al. JMRI 1999 10 376-388
40
Normal Runoff MRA

• Image of tissue surrounding vessel can be
  manually striped off

http//www.uth.tmc.edu/radiology/publish/mra/galle
ry.html
41
Phase-Contrast MRA Method
Bo
?
Imaginary
Real
42
Dephasing Due to Motion
Gslice
time
180o
BLOOD phase not zero
TISSUE phase equals zero
PHASE
time
Phase Shift Due to Motion in a Gradient Field
-180o
t 0
43
Phase Contrast Imaging
Velocity Encoded Image
180o
Phase Difference
PHASE
time
Velocity Compensated Image
-180o
Motion Compensation Gradient (Bipolar) Applied
180o
PHASE
time
Velocity Encoded Image
-180o
TISSUE phase equals zero in BOTH images
BLOOD phase is DIFFERENT in each image
44
Magnetic Field Gradients in MRI(Two More
Functions)

• Slice Selection
• Phase Encoding
• Frequency Encoding
• Sequence Timing (Dephase/Rephase)
• Motion Compensation
• Motion Encoding

45
2D Phase Contrast MRA Features

• Can use minimum TR
• doesnt rely on T1 effects
• Good for slow flow
• Motion is imaged in only one direction
• usually slice select
• Requires 2 images
• Velocity compensated / velocity encoded

46
2D Phase Contrast MRA Advantages

• Short acquisition times
• Variable velocity sensitivity
• Good background suppression
• Minimal saturation effects
• Short T1 tissues do not show up on images

47
2D Phase Contrast MRA Limitations

• Single thick section projection
• Vessel overlap artifact
• Sensitive to flow in only one direction
• Unstructured flow may cause problems

48
3D Phase Contrast MRA Features

• Images obtained at higher spatial
• resolution than 2D PC
• 3D PC requires at least four images
• flow compensated
• x-encoded
• y-encoded
• z-encoded
• Low velocity imaging in tortuous vessels
• Takes the most time

49
3D Phase-Contrast MRA Renal Circulation
FP
Coronal, 3D PC TR/TE 33/6 ms 20o flip
Coronal, Gd enhanced TR/TE 7/1.4 ms 40o flip,
false renal stenosis (FP)
50
3D Phase Contrast MRA Advantages

• Thin slices
• Quantitative flow velocity and direction
• Excellent background suppression
• Variable velocity sensitivity
• Short T1 tissues do not appear on images

51
3D Phase Contrast MRA Limitations

• Long acquisition times
• Long TE values

52
Flow Measurement with PC-MRI

• Typically uses 2DFT phase contrast method
• Slice positioned perpindicular to axis of vessel
• ROI drawn to delineate vessel lumen
• Average value in ROI is mean velocity
• Area of ROI is vessel cross-sectional area
• Flow mean velocity Area
• For pulsatile flow, multi-phase cine required

53
Phase Contrast Velocity Images
Magnitude
Phase Contrast
No Flow
Stationary
In Out In Out
Flow Velocity 29 cm/s
54
Velocity Encoding Range (Venc)
Venc
180o
Phase Difference (degrees)
MRI Velocity (cm/s)
-180o
-Venc
True Flow Velocity (cm/s)
55
3D Cerebrovascular Flow
Flow Encoding Right to Left
Magnitude
Saggital Sinus
Flow Encoding Anterior to Posterior
Flow Encoding Cranial to Caudal
Straight Sinus
Ant. Cerebral aa.
Basilar a.
56
Summary

• Two different approaches to MRA are commonly
  used Time-of-Flight (TOF-MRA) Phase Contrast
  (PC-MRA)
• TOF-MRA is easy to implement and is robust but
  has difficulty with slow flow
• 3D TOF can be combined with fast imaging methods
  and Gd contrast agents to obtain improved
  depiction of vascular structures

57
Summary

• PC-MRA requires more time to acquire more images
  but can result in high resolution, fewer flow
  related artifacts, and quantitative measurement
  of flow
• Phase-contrast MRI may provide the most accurate,
  noninvasive method for measuring blood flow in
  vivo