Flow Artifacts & Magnetic Resonance Angiography•Displacement artifacts 5 of 185 Flow-Related Enhancement Neck Abdomen 6 of 185 Bright Blood in MRI •Prominent in T1-weighted image

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Flow Artifacts & Magnetic

Resonance Angiography

Hsiao-Wen Chung (鍾孝文), Ph.D., Professor

Dept. Electrical Engineering, National Taiwan Univ.

Dept. Radiology, Tri-Service General Hospital

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Blood in MRI

• Sometimes bright (strong signal)

• Sometimes dark (no signal)

• Different from in vitro -- artifacts ?

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Bright Blood & Black Blood in MRI

Gradient echo Spin echo

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Blood Flow Artifacts

• Flow-related enhancement

– Bright blood in gradient echo

• Flow void (spin-echo)

• Displacement artifacts

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Flow-Related Enhancement

Neck Abdomen

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Bright Blood in MRI

• Prominent in T1-weighted image

• Only in through-plane flow

• Slice-dependent in multi-slice

scans

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Through-plane & In-plane Flow

Often bright Not too bright

image slice

vessel

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Multi-slice Brightness

Brighter towards upstream

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MRI Image Formation

• Magnetization

• RF excitation

• Spatial encoding repeat N times

• Signal receiving

• Image calculation

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Bright Blood in MRI




scans

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2D Gradient-echo Sequence

t

t

t

t

...

RF

Gs

Gp

Gr

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MRI Sequence Diagram

Repeat many times

RF t

t

t

...

...

...

TR

Gp

Gr

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Magnetization Changes in Sequence

RF t

t

...

...

short TR

z’

y’ x’

Gr

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Saturation Phenomena at Short TR

TR

Sig

na

ls

short T1

long T1

T1WI : TR ~ 600

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Effects of T1 on Signals

• Short TR no time for T1 recovery

low signal saturation

• “Saturation” effect

• TR ~ 200 msec: more or less

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Saturation Phenomena of Blood

RF excitation

t RF

image slice

vessel blood

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Signal receiving and T1 recovery

t RF

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T1 recovery

t RF

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Next RF excitation

t RF

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“Yellow” part not excited previously !

t RF

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Magnetization Changes in Sequence

RF t

t

...

...

short TR

z’

y’ x’

Gr

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Saturation in Blood Flow

• Static tissue somehow saturated

• Flowing blood less saturated

• Less saturation = bright signal

• Flow-related enhancement (FRE)

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Bright Blood in Abdomen

Flow-related enhancement on T1WI

1.5 Tesla

GE Signa

Gradient-echo

Aorta & IVC

especially bright

on T1WI

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Shortened T1 with Flow ?

• Bright signal at short TR

• Found in 1951 by Suryan G

• 1946: NMR, 1973: MR imaging

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What Affect FRE ?

• TR selection

• Velocity and slice thickness

• Flip angle

• Velocity distribution in vessel

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FRE vs. Velocity & Slice Thickness

Blood flow

slow

fast

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What Affect FRE ?

• TR selection


• Flip angle


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Flip Angle Effects

• FRE generally larger at large flip

angle (T1-weighted)

• Much more complicated in reality

• Will be addressed later

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Flip Angle Controls PD or T1 Contrast

T1 weighting PD weighting

z'

y'

x'

Bo Bo z'

y'

x'

Large angle:

recover from 0

Small angle:

little room for

recovery

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What Affect FRE ?

• TR selection


• Flip angle


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No Bulk Blood Flow in Vessel !

Different FRE in one single vessel

laminar

flow

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Bright Blood in MRI




scans ?

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Multi-slice Saturation Phenomena

Exciting the first slice

t RF

image slice

vessel

TR

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Exciting the second slice

t RF

TR

image slice

vessel

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Exciting the third slice

t RF

TR

image slice

vessel

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Exciting the first slice again

t RF

Prominent FRE

TR

image slice

vessel

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Some blood already saturated due to first RF

t RF

TR

No prominent FRE

image slice

vessel

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Most blood saturated due to two previous RFs

t RF

TR

image slice

vessel

No prominent FRE

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Multi-slice Brightness

Brighter toward upstream

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Multi-slice FRE

• Prominent FRE toward upstream

• Especially the first “entry” slice

– Entry slice phenomena

• True only in sequential scan

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Entry-Slice FRE Phenomena

Entry slice Inner slice

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FRE & Slice Order

• Sequential & interleave different

• FRE too complicated in

interleaving scan omitted

• But it is an artifact anyway !

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Sequential & Interleave Scan

FRE substantially different

image slice Sequential: 4 3 2 1

Interleave: 4 2 3 1

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Other FRE Artifacts

• Flow ~ motion, motion ghosts

• FRE leads to strong blood signal,

hence bright ghosts as well

• How to eliminate FRE ?

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FRE Combined with Motion Ghosts

Bright blood and strong ghosts of aorta

Strong enhancement

leads to obvious

motion ghosts !

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Eliminating FRE Artifacts

• Reduce upstream blood signals

– Inflow saturation

• Ghosts from blood flow motion

also removed

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Inflow Saturation

• Excite upstream of the slice

• Immediate select the slice

• Blood saturated before flowing into

the image slice

• No signals black blood

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Inflow Saturation Principle

90

RF

Gz

Gy

Gx

t

t

t

t

gradient echo saturate

90 a

image slice

SAT bands

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Multi-directional Inflow Saturation

Operator selectable, or pre-selected in a protocol

SAT bands

Image volume

2 bands along each

of the 3 directions

6 SAT bands

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Superior Inflow Saturation

No Sat With Sat

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Inferior Inflow Saturation

No Sat With Sat

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Comparison of Sup/Inf SAT

Aorta & IVC both darkened after SAT

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Note : Not Only These

• We discussed only flow effects

• Not the compositions of blood !

• Fresh blood? Fresh blood clot? Old

blood clot? Scar? Out of the scope of

this course

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Artifact Can Be Useful

• If the blood signal is so bright that

the static tissue looks no signal …

• Image contains only blood vessels ?

• Time Of Flight MR angiography !

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Artifacts Can Be Useful

Image would contain only blood vessels !

Adjust scanning

parameters to emphasize

enhancement and to

suppress static tissue

signals

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Time-of-Flight MR Angiogram

Angiography using bright blood signals

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Time-of-Flight Magnetic





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Vessel Bright, Other Dark

• Time Of Flight MR angiography

– No contrast agents used

– No subtraction used

– Completely noninvasive

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MIP MRA in Circle of Willis

Axial view Sagittal oblique view

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Comparison: Invasive X-ray Angio

Interventional radiology (diagnosis & therapy)

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Blood Behavior in MRI

• Static tissue suppress FRE

– Inflow saturation

• Blood vessels enhance FRE

– TOF MR Angiogram

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From FRE To MRA

• From multiple slices to multi-angle

projection

• Maximum Intensity Projection

– MIP : computer calculation

• Volume (surface) rendering …

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… using bright blood signals

?

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Maximum Intensity Projection

Any orientation, computer calculation

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2D TOF MRA

Carotid arteries at three different angles

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MRA Lecture Over ?

• No. Not that easy !

• To show up vessels faithfully?

– Better contrast

– Artifact removal

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What Affect FRE ?

• TR selection


• Flip angle


• Factors determining MRA success

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Better MRA Contrast

• Adjust scanning parameters

– Slice thickness & orientation

– TR & flip angle

– Saturate unwanted vessels

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Slice Orientation

• FRE only in thru-plane flow

• Slice perpendicular to vessel

• Curved vessels often look

narrow due to less FRE

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Through-plane & In-plane Flow

Often bright Not too bright

image slice

vessel

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Slice Orientation Effects

Narrow proximal anterior tibial artery branch ?

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Slice Thickness

• FRE comes from blood flowing “out

of” the image slice

• FRE obvious for thin slices

• Spatial resolution also higher

• Note gradient & SNR restrictions

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Blood flow

slow

fast

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Better MRA Contrast



– TR & flip angle


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TR Effects

• FRE comes from blood flowing “out of”

the image slice

• Short TR insufficient time to flow out

• Long TR static tissues less saturated

• Optimal TR = thickness / velocity

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Blood flow

slow

fast

optimal TR

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Flip Angle Effects

• Large flip angle favors T1 contrast

• Bright blood leads to flow ghosts

• Large flip angle may partially

saturate slow flow

– Close to static tissue regime

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Flip Angle Effects on Ghosts

Angle = 900 Angle = 600 Angle = 300

A P

A : anterior tibial artery

P : common peroneal trunk artery

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Flip Angle Effects

• Quite complicated in reality

• Anatomical location dependent

– Different flow velocities

• 45 ~ 60 frequently used (2D TOF)

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Flip Angle Effects on MRA


Also note small vessel contrast

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Note: TR & Flip Angle

• More important in slow flow

– Relative to slice thickness

• Often not critical in 2D TOF MRA

• Essential in 3D TOF MRA

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Better MRA Contrast



– TR & flip angle


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Use of Saturation Band

• Suppress unwanted vessels to avoid

overlapping

• SAT band follows the slice

– Tracking SAT

• Order: from downstream to upstream

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Inflow Saturation Principles

90

RF

Gz

Gy

Gx

t

t

t

t

gradient echo saturation

a

image slice

SAT band

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Jugular Vein Suppressed with SAT

Angiogram Arteriogram

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Use of Saturation Band

• Suppress unwanted vessels to avoid

overlapping

• SAT band follows the slice

• Tracking SAT

• Order: from downstream to upstream

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Tracking SAT Principles

Excitation from downstream to upstream

image slice SAT band

SAT band

always follows

the image slice

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Just Like Entry-Slice FRE Phenomena

Entry slice Inner slice

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Better MRA Contrast



– TR & flip angle


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MRA Not Over Yet !


– Better contrast


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MRA Artifacts

• Intra-voxel phase dispersion

causes apparent narrowing of

blood vessels

• Interrupt from bright signal of fat

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Intra-Voxel Dephasing

• Non-uniform or turbulent flow

• Dephasing occurs with gradient

• Dephasing in one voxel low

signal looks like no flow

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No Bulk Blood Flow in Vessel !

Different FRE in one single vessel

laminar

flow

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Turbulent Flow

Carotid Artery Basilar Artery

CCA

ECA

ICA

BA

S Cerebellar

S Cerebellar

Cerebral P Cerebral

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Pseudo Stenosis from Dephasing

MRA XRA

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Reason for Narrowing

• Gradient = local magnetic field

• Flow Bo changed phase

• Different flow = different phase

low signal from dephasing

• Pseudo-stenosis

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Risk of False Positive

• Can’t be regarded as normal

– Stenosis often seen in patients

• Over-estimate of stenosis

– Dephasing due to turbulent flow

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Over-Estimated Stenosis

Stenosis magnified due to dephasing signal loss

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Slice Orientation Is One Reason Too


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What about Aneurysm ?

• Possible slow flow

• Vortex may also be present

• Signal loss like static tissue ?

• Under-estimate of aneurysm

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Under-Estimate of Aneurysm

One drawback of MRA (false negative)

signal loss due to vortex

looks like static tissue

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Remedy : Flow Comp

• Flow Compensation (GE)

• GMR (Siemens), MAST (Picker)

• Pre-calculated gradient waveform to

yield phase independent of flow

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Gradient Moment Rephasing

No GMR With GMR

t

t

t

t

RF

Gs

Gp

Gr

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Intra-voxel Dephasing & Flow Comp

No flow comp With flow comp

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Remedy : Short TE

• Dephasing takes time

• Short TE to reduce dephasing

– Or less signal loss

• Fractional Echo (remember?)

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k-space Data Omission

Half Fourier Fractional echo

kx

ky

kx

ky

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Fractional Echo Sequence

TE can be shortened

z gradient

RF (B1) t

t

y gradient t

x gradient t ...

TE

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Short TE & GMR Effects

Stenosis downstream signal loss at long TE

TE = 1.5 2.5 3.8 5.3 6.2 7.0 8.0

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Flow Comp Leads to Longer TE

No GMR With GMR

t

t

t

t

RF

Gs

Gp

Gr

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MRA Artifacts

• Intra-voxel phase dispersion

causes apparent narrowing of

blood vessels

• Interrupt from bright signal of fat

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Fat Artifacts

• TOF MRA is from T1 contrast

• Fat has a short T1

• Strong fat signal looks like blood

vessels ?

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3D TOF MRA (Circle of Willis)

Note the periorbital fat

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Remove Fat Artifacts

• CHESS fat SAT (remember?)

• Successful fat SAT requires …

– Shimming

– Avoid air/tissue interface …

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CHESS Principles

water

fat

900 fat only

RF

Gz

Gy

Gx

t

t

t

t

spin-echo fat-SAT

ppm

ppm

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Fat SAT Comparison (Original Image)

No Fat SAT Fat SAT

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Fat SAT Comparison (MIP MRA)

No Fat SAT Fat SAT

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Never Forget Though …

• Successful fat SAT requires

– Uniform Bo & B1 …

• Failed fat suppression leads to

confusing TOF MRA !

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Fat SAT in MRA (Courtesy a Friend)

No Fat SAT Fat SAT

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MRA Still Not Over Yet!


– Better contrast


• What about 3D TOF MRA then?

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3D Time-of-Flight

MR Angiography




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… using bright blood signals

?

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Maximum Intensity Projection

Any orientation, computer calculation

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Motivation of 3D MRA

• Multiple 2D images for projection

are already 3D in nature

• Why not just doing 3D then ?

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MRA Needs 3D Volume Anyway

3D MRA has slice resolution advantages

2D MRA 3D MRA

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2D & 3D MRA

• Same FRE principle on 3D scan

– More thinner slices

– Higher SNR

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Review: 3D MRI Principle

• Not too different from 2D

• Another loop for slice encoding

• Thick slab selection

– Scan time much longer

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3D Imaging Pulse Sequence

Gz & Gy form two inner/outer loops

z gradient

RF (B1) t

t

y gradient t

x gradient t

sample

...

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3D MRI Properties

• Scan time concern -- short TR

– Small a, gradient-echo, T1WI

• SNR ~ slice number

– Trade high SNR for thin slice

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Review : Good TOF MRA

• Gradient-echo (avoid flow void)

• Short TR (T1WI & FRE)

• Proper scanning parameters

• Consistent with 3D MRI !

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Bright Blood & Black Blood MRI

Gradient echo Spin echo

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3D MRA Motivation

• Multiple 2D images for projection are

already 3D in nature

• Same FRE principle on 3D scan

– More thinner slices

– Higher SNR

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3D Limitations Though …

• Factors affecting FRE

– Slice thickness & TR

• Extremely complicated in 3D !

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RF excitation

t RF

image slice

vessel blood

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Signal receiving and T1 recovery

t RF

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T1 recovery

t RF

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Next RF excitation

t RF

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“Yellow” part not excited previously !

t RF

experience 2 RF

experience 1 RF

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FRE in 3D MRI

RF excitation

t RF

image slab

vessel blood

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FRE in 3D MRI

Signal receiving, T1 recovery

t RF

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FRE in 3D MRI

T1 recovery

t RF

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FRE in 3D MRI

Next RF excitation

t RF

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FRE in 3D MRI

3D FRE obviously weaker (not too critical)

t RF

experience 2 RF

experience 1 RF

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FRE in 3D MRI

Inconsistent vessel contrast after slicing !

brighter vessel upstream darker vessel downstream

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3D Slab Thickness

• Thick slab leads to complex FRE

• Upstream shows strong FRE,

downstream gradually saturated

• Darkened vessel looks narrowed

• Prominent in slow flow

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Gradually Darkened 3D TOF MRA

Lowered signal at downstream

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2D & 3D FRE (Carotid Bifurcation)

2D TOF : uniform 3D TOF : clear edge

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TR Effects on Vessel Intensity

Short TR shows strong intensity gradient

t RF

experience 2 RF

experience 1 RF

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Solution

• Less RF for the upstream

– Small flip angle excitation

• More T1 weighting at downstream

– Large flip angle excitation

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Solution

• Flip angle varies locally

– Using one single RF pulse

– RF solved with simple math

– TONE (Siemens), RAMP (GE)

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TONE & sinc RF Pulse Comparison

Control saturation for uniform vessel signal

300

200

400

flip angle

sinc RF pulse

TONE RF pulse

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TONE (Ramp) Pulse Comparison

Sinc RF pulse TONE pulse

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Combining Pros of All Flip Angles


Also note small vessel contrast

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3D TOF MRA Properties

• FRE weaker than 2D

– Background suppression

relatively important

• MT saturation & fat suppression

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FRE in 3D MRI

3D FRE obviously weaker

t RF

experience 2 RF

experience 1 RF

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Fat SAT Comparison (Original Image)

No Fat SAT Fat SAT

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Fat SAT Comparison (MIP MRA)

No Fat SAT Fat SAT

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Fat SAT in MRA (Courtesy a Friend)

No Fat SAT Fat SAT

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MT Saturation

• Magnetization transfer saturation

• No time to explain, but basically...

• Suppresses tissues containing

large protein molecules

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MTS for MRA

• Suppress background tissues to

enhance blood signals

– Relatively few proteins in blood

– Serum albumin relatively small

– MTS stronger in static tissues

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MTS Comparison (Original Image)

No MTS with MTS

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MTS Comparison (MIP MRA)

No MTS with MTS

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TONE & MTS Comparison

None MTS MTS + TONE

Note conspicuity of small vessels

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3D TOF MRA Properties

• Less prone to darkened intensity

due to curved vessels

• Obviously an advantage

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Slice Orientation (2D)

• FRE only in thru-plane flow

• Weak signal at curved vessels

– Narrowing, pseudo-stenosis

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Slice Orientation Effects


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FRE & Slice Orientation

• Perpendicular at entrance suffices

– Unsaturated blood flows into

imaging region (slice or slab)

– Unimportant afterwards

• 3D slab thick, easy to handle

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2D MRA Slice Orientations

Not all slices can be made perpendicular

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3D MRA Slab Orientation

FRE from carotid artery, Circle of Willis unaffected

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2D versus 3D

• 2D : think slice stronger FRE

good for slow flow (venogram)

• Insensitive to in-plane flow

• Application : carotid arteries

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Note : “Slow” ??

• About venous flow

• No FRE if too slow

– Example : CSF

• Flow ~ w.r.t. TR & slice thickness

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2D TOF MRA

Carotid arteries at three viewing angles

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2D versus 3D

• 3D : thick slab, weaker FRE

• SNR & resolution both better

• Curved vessels are fine

• Application : circle of Willis

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3D MRA (MIP) of the Circle of Willis

Axial view Sagittal oblique view

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Common 3D TOF MRA Today

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Combining Advantages

• Slow curved flow (e.g., AVM)

• Multiple thinner 3D slabs

• Multiple Overlapped Thin-Slab

Acquisition (MOTSA)

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Arteriovenous Malformation

Two 3D slabs to see slow flow

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MOTSA of AVM

Visualize both arterial/venous sides of AVM

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MOTSA

• Commercialized for aneurysm

and vascular malformation

• Inter-slab boundaries still visible

• Not too perfect yet

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Multi-Slab Pitfalls (but fine)

Artifacts at overlapped slabs unavoidable

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MRA Not Over Yet ...

• Artifacts: flow + gradient ~ phase

– Intravoxel phase dispersion

• Phase for velocity ?

• Next week : phase-contrast MRA

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Flow Quantification at SVC

SVC flow profile in one cardiac cycle

SVC flow

cardiac phase

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MRA Not Over Yet ...

• Artifacts : Fat interrupts MRA

– Short T1 strong signals

• Can we shorten blood T1 ?

• Advanced topic: CE MRA

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Intracranial CE MRA (Aneurysm)

TOF MRA CE MRA

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Body Contrast-Enhanced MRA

GI MRA Breast MRA

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Flow Artifacts & Magnetic





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FRE 在血管上游和下游的差異

TR = 20 TR = 40 TR = 80

Documents

Flow Artifacts & Magnetic Resonance Angiography•Displacement artifacts 5 of 185 Flow-Related Enhancement Neck Abdomen 6 of 185 Bright Blood in MRI •Prominent in T1-weighted image