Introduction to Magnetic Resonance Angiography

Geoffrey D. Clarke, Ph.D.Division of Radiological Sciences

University of Texas Health Science Center at San Antonio

Overview

• Flow-Related Artifacts in MRI

• Time-of-Flight MR Angiography

• Contrast-Enhanced MR Angiography

• Phase-Contrast MR Angiography

• Quantitative Flow Imaging

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

Vessel Signal Voids

Early multi-slice spin echo images depicted vessels in the neck as signal voids

FlowingFlowingBloodBlood

Multi-slice Spin Echo

MRIMRISlicesSlices

StationaryTissue

Long TR90o-180o

Fast flow

Slice #1

Slice #2

Slice #3

Spins do not get

refocused by 180o pulse

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

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.

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

The MRI Signal: Amplitude & Phase

Net MagnetizationNet Magnetization

BBoo rf = Brf = B11

RealReal

ImaginaryImaginaryRealReal

ImaginaryImaginary

Dephasing Due to Motion

GGsliceslicetimetime

timetime

t = 0t = 0

BLOOD: phase not zeroBLOOD: phase not zero

TISSUE: phase equals zeroTISSUE: phase equals zero

Phase Shift Phase Shift Due to Motion Due to Motion

in a in a Gradient FieldGradient Field

PH

AS

EP

HA

SE

-180-180oo

+180+180oo

Pulsatile Motion Artifact

AortaAorta

ArtifactArtifact

ArtifactArtifact

ArtifactArtifact

Motion Compensation Gradients

GGsliceslicetimetime

timetime

t = 0t = 0

BLOOD: phase equals zeroBLOOD: phase equals zero

TISSUE: phase equals zeroTISSUE: phase equals zero

Phase Shift Phase Shift Due to Motion Due to Motion

in a in a Gradient FieldGradient Field

PH

AS

EP

HA

SE

-180-180oo

+180+180oo

*Only applies for constant flow. *Only applies for constant flow. More gradient lobes needed for acceleration.More gradient lobes needed for acceleration.

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

Magnetic Resonance Angiography (MRA)

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

Methods of Magnetic Methods of Magnetic Resonance AngiographyResonance Angiography

• Signal Amplitude MethodsSignal Amplitude Methods2D Time-of-Flight3D Time-of-Flight

• Signal Phase MethodsSignal Phase Methods 2D Phase Contrast

(Velocity Imaging = Q-flow)3D Phase Contrast

(Velocity Imaging = Q-flow)

Time-of-Flight MRA Method

RealRealImaginaryImaginary

BBoo

MM

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

2D Time-of-Flight MRA2D Time-of-Flight MRAConditionsConditions

•Field Echo ImagingField Echo Imaging

•Short TEShort TE

•Partial Flip Angle Partial Flip Angle

•generally largegenerally large

•keeps stationary tissues saturatedkeeps stationary tissues saturated

•TR and flip angle TR and flip angle

•adjusted to minimize stationary tissue adjusted to minimize stationary tissue

•adjusted to maximize bloodadjusted to maximize blood

2D Time-of-Flight MRAAdvantages

•Good stationary tissue to bloodGood stationary tissue to blood flow contrastflow contrast

•Sensitive to flowSensitive to flow

•Minimal saturation effectsMinimal saturation effects

•Short scan times Short scan times

•Can be used with low flow rateCan be used with low flow rate

2D Time-of-Flight MRALimitations

•Relatively poor SNRRelatively poor SNR

•Poor in-plane flow sensitivityPoor in-plane flow sensitivity

•Relatively thick slicesRelatively thick slices

•Long echo times (TE)Long echo times (TE)

•Sensitive to short TSensitive to short T11 species species

Improving Contrast in Time-of-Flight MRA

1. Venous Pre-saturation 1. Venous Pre-saturation

(spatial suppression)(spatial suppression)

2. Magnetization Transfer Contrast 2. Magnetization Transfer Contrast

(frequency selective irradiation)(frequency selective irradiation)

3. Fat Saturation 3. Fat Saturation

(frequency selective irradiation)(frequency selective irradiation)

4. Cardiac Gated MRA4. Cardiac Gated MRA

5. Spatial variation of flip angle5. Spatial variation of flip angle

Spatial Pre-saturation in Time-of-Flight MRA

•Saturates and dephases spins before Saturates and dephases spins before they enter imaging slicethey enter imaging slice

•Can be used to isolate arteries or veinsCan be used to isolate arteries or veins

•Can be used to identify vessels feedingCan be used to identify vessels feeding

a given territorya given territory

•Can be used to establish the direction of Can be used to establish the direction of flow in a particular vessel flow in a particular vessel

Magnetization Transfer Contrast

““Free” WaterFree” Water

LipidsLipids ““Bound” WaterBound” Water

PROTON SPECTRUMPROTON SPECTRUM

Frequency (Hertz)Frequency (Hertz)

Frequency (Hertz)Frequency (Hertz)

217 Hz217 Hz 1500 Hz1500 Hz

00

00

Gradient Echo with MTC Pulse

TX

RX

Gsl

Gro

Gpe

RF excitation

Field Echo

Crushers or Spoilers

Phase Encode

Dephasing

Rephasing

Slice Select

Read Out

Digitizer On

Off-resonance rf pulse

Spoilers

Maximum Intensity Projections

MIP #1MIP #1

MIP #2MIP #2

OBJECTOBJECT

2D TOF Application

AbdominalAneurysm

3D Time-of-Flight MRAConditions

•Uses two phase encode gradients and volume excitation

•Maximum volume thickness limited by flow velocity

•Use minimum TR, adjust flip angle for best contrast

Three Dimensional Gradient Refocused Echo Imaging

TXRF pulse (short time)

Field Echo

RX

Gsl

Gro

Gpe

Crusher

Primary Phase

Encoding

Phase Rewinder

Dephasing

Rephasing

Slab Select

Read Out

Digitizer OnSecondary Phase Encoding

3D Time-of-Flight MRAAdvantagesAdvantages

• Higher resolution (thinner slices) Higher resolution (thinner slices) available allowing for delineation of available allowing for delineation of smoother edgessmoother edges

• Higher signal-to-noise than 2D methodsHigher signal-to-noise than 2D methods

• Lower slice select gradient amplitudes Lower slice select gradient amplitudes results in fewer phase effect artifacts results in fewer phase effect artifacts than 2D methodthan 2D method

• Short duration RF pulses can be used to excite slab – TE can be reduced

3D Time-of-Flight MRALimitationsLimitations

• Blood signal is easily saturated Blood signal is easily saturated with slow flowwith slow flow

• Relatively poor background Relatively poor background suppressionsuppression

• Short TShort T11 tissues may be mistaken tissues may be mistaken

for vesselsfor vessels

3D-TOF Application:Cerebral 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

Circle of Willis

Time of Flight MRA

90o

Cerebral Venous Angiogram

SaggitalSinus

Confluence Of Sinuses

TransverseSinus

StraightSinus

FRONT

TOP

Use of arterial presaturation

allows visualization of cerebral

venous vessels

Cerebven.mpeg

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

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 VOI’s

• Shorter acquisition times allow breath-holding for visualization of central and pulmonary vasculature

MRI Compatible Power Injectors

ProgrammableAutomatic Injection

MRI Compatible

Allows rapid arterial injection of

Gd-DTPA

www.medrad.com

3D CE-MRA of Aortic Aneurysm

DigitalSubtraction

X-rayAngiography

Phase 3 Phase 2 Phase 1

Phase 2 Phase 1

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

Schoenberg SO, et al. JMRI 1999;10:347-356

Bolus Chase 3D MRA

Earlier venous enhancement noted with fast injection

Station 1 Station 2 Station 3

Ho VB et al. JMRI 1999; 10: 376-388

Normal Runoff MRA

Image of tissue surrounding vessel can be manually striped off

http://www.uth.tmc.edu/radiology/publish/mra/gallery.html

Phase-Contrast MRA Method

RealRealImaginaryImaginary

BBoo

Dephasing Due to Motion

GGsliceslicetimetime

timetime

t = 0t = 0

BLOOD: phase not zeroBLOOD: phase not zero

TISSUE: phase equals zeroTISSUE: phase equals zero

Phase Shift Phase Shift Due to Motion Due to Motion

in a in a Gradient FieldGradient Field

PH

AS

EP

HA

SE

-180-180oo

+180+180oo

Phase Contrast Imaging

timetimePH

AS

EP

HA

SE

-180-180oo

+180+180oo

timetime

BLOOD: phase is BLOOD: phase is DIFFERENTDIFFERENT in each image in each image

TISSUE: phase equals zeroTISSUE: phase equals zeroin in BOTHBOTH images images

PH

AS

EP

HA

SE

-180-180oo

+180+180oo Velocity Velocity Encoded ImageEncoded Image

Velocity Velocity Compensated Compensated

ImageImage

Phase DifferencePhase Difference

Velocity Velocity Encoded ImageEncoded Image

Motion Compensation

Gradient (Bipolar) Applied

Magnetic Field Gradients in MRI(Two More Functions)

• Slice Selection

• Phase Encoding

• Frequency Encoding

• Sequence Timing (Dephase/Rephase)

• Motion Compensation

• Motion Encoding

2D Phase Contrast MRAFeatures

•Can use minimum TR Can use minimum TR doesn’t rely on Tdoesn’t rely on T11 effects effects

•Good for slow flowGood for slow flow

• Motion is imaged in only one directionMotion is imaged in only one directionusually slice selectusually slice select

• Requires 2 imagesRequires 2 imagesVelocity compensated / velocity encodedVelocity compensated / velocity encoded

2D Phase Contrast MRA2D Phase Contrast MRAAdvantages

•Short acquisition times

•Variable velocity sensitivity

•Good background suppression

•Minimal saturation effects

•Short T1 tissues do not show up on images

2D Phase Contrast MRALimitations

•Single thick section projection

•Vessel overlap artifact

•Sensitive to flow in only one direction

•Unstructured flow may cause problems

3D Phase Contrast MRAFeatures

•Images obtained at higher spatial resolution than 2D PC

•3D PC requires at least four images:flow compensatedx-encodedy-encoded z-encoded

•Low velocity imaging in tortuous vessels

•Takes the most time

3D Phase-Contrast MRA Renal Circulation

Coronal, Gd enhancedTR/TE = 7/1.4 ms

40o flip, false renal stenosis (FP)

Coronal, 3D PCTR/TE = 33/6 ms

20o flip

FP

3D Phase Contrast MRAAdvantages

•Thin slices

•Quantitative flow velocity and direction

•Excellent background suppression

•Variable velocity sensitivity

•Short T1 tissues do not appear on images

3D Phase Contrast MRALimitations

•Long acquisition timesLong acquisition times

•Long TE valuesLong TE values

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

Phase Contrast Velocity Images

No Flow

FlowVelocity29 cm/s

Magnitude Phase Contrast

StationaryIn Out In Out

Velocity Encoding Range (Venc)

-V-Vencenc

+V+Vencenc

Ph

ase

Differen

ce

Ph

ase

Differen

ce

(de

gree

s)

(de

gree

s)

MR

I Ve

loc

ity

(cm

/s)

MR

I Ve

loc

ity

(cm

/s)

180180oo

-180-180oo

True Flow Velocity (cm/s)True Flow Velocity (cm/s)

3D Cerebrovascular Flow

Magnitude

Flow Encoding Cranial to

Caudal

Flow Encoding Right to

Left

Flow Encoding Anterior

to Posterior

Saggital Sinus

StraightSinus

Ant. Cerebral aa.

Basilar a.

Summary

1. Two different approaches to MRA are commonly used: Time-of-Flight (TOF-MRA) & Phase Contrast (PC-MRA)

2. TOF-MRA is easy to implement and is robust but has difficulty with slow flow

3. 3D TOF can be combined with fast imaging methods and Gd contrast agents to obtain improved depiction of vascular structures

Summary

4. PC-MRA requires more time to acquire more images but can result in high resolution, fewer flow related artifacts, and quantitative measurement of flow

5. Phase-contrast MRI may provide the most accurate, noninvasive method for measuring blood flow in vivo