Artefacts In Clinical MRIKris Armoogum MSc

Department of Medical Physics,Ninewells Hospital

DundeeDD2 1QW

Kris.Armoogum@tuht.scot.nhs.ukStephen.Gandy@tuht.scot.nhs.uk

14th Scottish MRI Seminar, Wednesday 19th November 2003

• Artefacts are parts of reconstructed images that are not present in the true anatomy.

• Artefacts are dependent on a variety of factors from patient movement to magnetic field inhomogeneities.

• Artefacts can lead to misdiagnosis if they are not recognised and/or removed.

• Ideally, we want all image artefacts to be below the level of user's perception.

Main classifications -

1. Movement Artefacts2. Geometrical Artefacts3. Resolution/Sequence Artefacts4. Bo Artefacts5. RF Artefacts6. Noise

Introduction

1. Movement Artefacts• Motion Artefacts – patient

• Flow artefact – inflow/washout effect, diastole, systole, arterial flow

• Reducing flow artefacts – Gradient Moment Nulling

• Respiratory compensation, triggering, ROPE, navigator echoes

Motion Artefact - Patient

• Patient movement – as outer areas of k-space acquired

• May mimic truncation artefact• Difference – truncation artefact

diminishes with distance from the high contrast boundary

• If related to pulsation of vessels, this can be reduced by applying an anterior sat band

T2W SE Thoracic Spine

Patient Movement Artefact• Smearing of image• Particularly in phase directionSolutions• Immobilise the patient more effectively• Reduce the scan time – reduce NSA, breathold, shorter TR, less k-space

lines• Reduce the scan time (reduced k-space acquisition) e.g. HASTE, TSE with

large turbo factor

• T1W GE sequence• RF pulse saturates the blood momentarily in the slice (yellow)• If blood is stationary, long T1 of blood means that no signal available

for successive RF pulses to excite – hypointense signal• If velocity of inflowing blood > z/TR then full inflow occurs and the next

RF pulse ‘sees’ unsaturated spins in the slice – ‘Bright Blood’ signal• Other tissues within the slice are saturated, and therefore suppressed

Flow Artefact – Inflow Effect

90o pulse Bright signal

90o pulse Dark signalNO FLOW

FLOW

Next TR

Next TR

Image

z

Flow Artefact – Washout Effect• Spin-echo sequence - 90o pulse excites spins in the slice• In the absence of flow, bright signal is seen because the spins

experience both the 90o and 180o pulses• In the presence of flow, blood flows out of the slice and does not

experience the 180o pulse – no rephasing, ‘Black Blood’ signal• D.G. Nishimura "Time-of-Flight MR Angiography."

Magn. Reson. Med. 14:194-201 (1990)

Image

90o pulse 180o pulse No signal

90o pulse 180o pulse SignalNO FLOW

FLOW

TE/2 TE

Flow Artefact I - Body

• Diastole (filling phase), systole (emptying phase).• Aortic ghosts in PE direction [because FE step (msec) takes much less

time than a PE (sec) step]• Physiological modulation

Grad

D

S

PhaseShift

-1

+1

TT

Why only the PE Direction?

Why is motion artefact only seen in the PE direction?• A FE step takes much less time (of the order of msec)

than a PE step (of the order of seconds)• Most motion that occurs during clinical MRI is much

slower than the rapid sampling process along the FE axis• However, each PE line is separated by the time interval

TR which is long enough for blood to flow/move/dephasebetween successive phase encodings

Phase Shift Effects

• Systolic-diastolic switching of the flow velocity (at frequency ωm) modulates the MR signal (with frequency ωο)

• Two frequency sidebands (upper and lower frequency sideband components) appear as ghosts either side of the primary image

S

D

ωo

ωm

FT

ωo

ωo+ωmωo-ωm

MR Signal

Flow Artefact II - Brain• Velocity profile – laminar flow

(Re < 2100)• Velocity zero at wall, fastest at

centre of lumen• Continuous spread of velocities

A

B

Grad

PhaseShift

Continuous

• A = flow artefact from eye movement• B = flow artefact from sagittal sinus

+1

-1

TT

Flow From Sagittal Sinus

• Distinguishable from Truncation artefact – artefact propagates across the anatomy.

• Truncation artefact diminishes with distance from the high contrast boundary.

Further Flow Artefacts

• Flow from middle (A) cerebral arteries, (B) eye movement & sagittal sinus, and (C) salivary glands (swallowing)

A B C

Flow – Popliteal Artery

• Popliteal artery flow generates artefacts across the femur.• More prominent when fat suppression removes the marrow signal.• May disturb interpretation of bone bruising or subchondral cysts.

Popliteal Artery

Phase Shift Effects

• Complex modulation frequency, made up of many component waves• FT results in a spread of upper and lower sidebands - artefact• Worse for GE images, due to bright blood inflow effect• SE dark blood inflow effect – less severe phase artefacts

FT

ωo

ωo+ωmaxωo-ωmax

MR Signal

ωo

ωm

Reducing Phase Flow Effects - I

• Use sequence with a high bandwidth – shorter TE• Amount of dephasing generally goes up with TE2

• Shorter TE minimises velocity induced phase effects

Grad Grad

PhaseShift

Large

PhaseShift

Small

Low bandwidth High bandwidth

-G

+G

+2G

-2G

2T2T

TT

Gradient Moment Nulling• Gradient Moment Nulling (GMN) – technique used for flow

compensation

+1

-2

+1

Stationary tissueConstant velocity bloodConstant acceleration blood

• Stationary tissue (0o) – unaffected• Constant velocity blood (1o motion) –

rephased by +1-2+1 gradients• Constant acceleration blood (2o) –

need –1+3-3+1 gradients to rephase• Jerk motion (3o) – need +1-4+6-4+1

gradients to rephase

-1

+3

-3

+1

2o flow comp 3o flow comp

+1

-4

+6

-4

+1

Phase shift

Flow Compensation

• Two cords appear to be present in first T2W TSE image.• The ‘extra cord’ is flow artefact from pulsatile CSF flow.• First order (+1-2+1 gradient) flow compensation

(Gradient Moment Nulling) results in the RHS image.

Vertebral foramenThoracic spine

Swallowing Motion

• Any patient motion during a scan can cause PE artefacts (A-P above).• Left image - artefact generated by patient swallowing during data

acquisition - increased signal intensity in the spinal cord.• Eliminated by applying presaturation RF pulses to the anatomy that

was generating the artefact.• Sat band visible on RHS image.

CIV

CV

Cervical spine

A P

S

A

T

Respiratory ArtefactPeriodic respiratory motion – ghosting above and below the body• Remove by breathold imaging (<18sec scan time).• Increase the NSA (anatomy SNR improved relative to ghosts). • NSA 4 to 6 ~ respiratory compensation.

Respiratory Gating/Triggering

• Reduces respiratory artefact• Bellows placed over abdomen.• Sequence TR ‘gated’ via patient

breathing rate.• Equivalent to TR ~ 4000ms,

(breathing rate 15/min) so only able to generate PDW and T2W scans (long TR reduces T1 effect).

• Signal acquired when chest wall is in same position - minimises ghost images.

Bellows pressure = electrical trigger

Inhalation

Exhalation

Respiratory Compensation• Reduces respiratory artefact• ROPE – Respiratory Ordered Phase Encoding (also uses bellows).• Typical TR for a T1W sequence = 500 msec.• Typical breathing rate = 15 breaths per min, i.e. 4000 msec period.• Can therefore fit 8 TR’s (8 PE steps) per breathing cycle.• Outer k-lines (image boundary detail) acquired at peak inhalation.• Central k-lines (signal, contrast) are acquired at peak exhalation.

+64 -64

K-space

Inhalation

Exhalation

Respiratory Gating or ROPE ?

• Gating: + simple technique• Gating: – effective TR very long (cannot do T1W)• ROPE: + shorter scan times• ROPE: – residual ghosts if patient breathes

deeply

Navigator Echoes• Two slice selective directions and FE in the third direction

(of motion).• Small column of tissue excited across the diaphragm.• Spin echo sequence – acquires series 1D images of the

diaphragm boundary over time.• Stack images side-by-side - intensity difference between

diaphragm and lung indicates respiratory motion.• Navigator echo is interleaved within main scan sequence.• Data for main image can then be adjusted for respiratory

motion by using data acquired during specific range of diaphragm motion.

2. Geometrical Artefacts• Phase wrap

• Partial volume

• Cross talk

• Magic Angle artefact

Phase Wrap - Aliasing• Regions outside FOV still produce a signal if in proximity to receiver coil.• Anatomy outside FOV is mapped inside FOV.• Corrected by - larger FOV or apply presat pulses to undesired tissue.• ‘No Phase Wrap’ – double the FOV; but because PE steps is doubled need to half

number of averages to keep scan time constant.• Aliasing in FE direction can occur, but eliminated by filters (no time waste).

+200o

= -160o

0

180o

+-

-180o +180o0

+200o-160o EQUIVALENT

Partial Volume Effect• Partial volume occurs if slice thickness > thickness of tissue of interest• If small structure is entirely contained within the slice thickness along with

other tissue of differing signal intensities then the resulting signal displayed on the image is a combination of these two intensities. This reduces contrast of the small structure.

• If the slice is the same thickness or thinner than the small structure, only that structures signal intensity is displayed on the image.

• Typically would use 3mm slices for cranial nerves and 5-10mm slices for liver.

VII (Facial) and VIII (Acoustic) cranial nerves

Cross Talk• Perfect RF pulse is a sinc function (FT = ‘top hat’)• Real RF pulse is a truncated sinc (FT = ‘top hat with rounded edges’)• Inter-slice cross talk could cause increased T1 weighting and

reduced SNR.

How Does Cross Talk Occur?

• Typical TR for T1W scan = 600ms, typical TE = 20ms.• Theoretically possible to acquire 30 slices within the TR.• Cross talk region between slices 1 and 2 – experiences RF excitation from

slice 1, then slice 2.• Effective TR is 20ms giving loss of signal due to lack of T1 recovery.• Solution - ‘interleave’ slices.• A 3D sequence avoids the problem altogether – contiguous slices.

TR = 600SLICE 1

SLICE 2

SLICE N

90o 180o

TE = 20

PE2

PE2

PE2

90o 180o

90o 180o

10-20% interslice gap

Magic Angle Artefact (54.7o)• Collagen fibril orientation w.r.t. B0 field.• T2 lengthening at Magic Angle.• Result is that the T2W image becomes hyperintense at the

magic angle.• Magic Angle is solution to: 3cos2θ-1=0 (from dipolar

Hamiltonian mathematical theory)• Magic angle imaging of the median nerve (brain) which has a

high collagen content.

At Magic AngleMedian Nerve in brain

3. Resolution/Sequence Artefacts• Truncation artefact

• Chemical Shift artefact (Types I and II)

Truncation Artefact - Brain

• Also known as Gibbs (‘ringing’) artefact.• Usually occurs in the PE direction at high contrast borders.• Due to undersampling of high spatial frequencies (sharp edged borders)• Remedied by taking more samples (e.g. 256 PE steps).• Truncation artefact causes ring-down effect because F.T. of truncated sinc

function has ripples at the edges.

128x256

256x256

F.T.

tω

Truncation Artefact or Syrinx?

Syrinx (fluid filled cavity in spinal cord)

C V

• Problematic down centre of spinal cord –could be misinterpreted as a syrinx

Chemical Shift Artefact – Type I

• T1W image of lumbar spine.• Low BW sequence used.• Frequency shift of a few pixels

is visible at the base of each vertebra (black line).

• Vertebra-disc boundary detail is lost at the top of each vertebra.

• Observation of small disc herniations in L spine difficult.

T1W Lumbar spine

L III

L IV

Chemical Shift (I) - Explained

• Protons from different molecules (eg: fat & water) precess at different frequencies.

• Protons in H2O precess slightly faster than those in fat, (diff. is 3.5 ppm)• Chemical shift = 3.5ppm = 224Hz at 1.5T [ ω0 = γ.B0 :: (42.6MHz/T)(1.5T) ::

64MHz :: 3.5ppm x 64MHz = 224Hz ]• LHS = 12.5kHz (low BW), 256 resolution.• Chemical shift is 4.6 pixels [ 224 / (12.5kHz/256) ]• Chemical shift also occurs between silicone & fat/water (Breast MRI)• Modify CS by using fat suppression, increase the bandwidth, swap freq and

phase directions, or lower the Bo field (impractical)!

Egg (low BW) Egg (high BW)

Displacement of yolk (fat) on LHS image

Chemical Shift – Type II Artefact

Worked example• Applies to Gradient Echo techniques, (not in SE because of 180º refocusing

pulse).• Fat and water proton resonant frequencies differ by 3.5ppm.• For an imaging field strength of 1.5T, ω=λ= 64 MHz (from ω0 = γ.B0 ).• Difference between fat and water proton resonant frequencies is therefore

about 224 Hz, ( ω diff ).• The phase of the fat and water spin vectors will thus coincide at 1/ωdiff, which is

4.6 ms.• If a TE of 4.6 ms is used, then the fat and water components of the signal will

be in phase. If a TE of 6.9 ms (4.6 + 2.3) is used then the fat and water components of the signal will be out of phase.

Out of Phase In

PhaseLiver Thoracic

aorta

Chemical Shift Type II Artefact

• Phase cancellation artefact – gradient echo sequences• Water precesses slightly faster than fat (phase difference between them)• Phase differences accumulate between water and fat signal• Vary the TE, f+W (in phase), f-w (out of phase – black boundary artefact)• At 1.5T, f-w occurs in 4.6ms multiples, starting at about 2.3ms (then 6.9,

11.5, 16.1 ms) - artefact• At 1.5T, f+w occurs at 4.6ms (then 9.2, 13.8, 18.4 ms) – no artefact• Dixon technique – ip+op images = water image, ip-op = fat image• The artefact can occur in both encoding directions• Not a problem in SE images since 180o pulse refocuses chemical shift

FW

4.6msF

W

2.3ms

F W3.5 PPM

ωo

4. Bo Artefacts• Susceptibility artefacts

• Metallic artefacts

• Bo Inhomogeneity

Susceptibility Artefacts• Occur when two materials with different magnetic susceptibility (χ)

lie together, (tissue-air & tissue-fat).• Local Bo changes cause spin dephasing at the boundary causing

signal loss.• Haemosiderin (end stage of haemorrhage) deposits (high χ) – local

susceptibility changes in tissue.• Susceptibility artefacts can be useful - bony trabeculae (low χ).• Use a FSE and keep TE short to minimise susceptibility artefacts.

Metallic Artefacts• Similar to susceptibility artefacts.• Metals have much higher susceptibility than tissue.• Large Bo inhomogeneities around object causing signal loss and

distortion.• Implants absorb RF energy, so local field varies.• RF problems affect SE sequences as well as GE.

Metallic Artefact

Small metal flake in lumbar spinal canal

Bo Inhomogeneity and FatSat

• Unsuccessful Fat suppression in T2W breast images.• Result of poor Bo field homogeneity.• Artefacts arise because of inability to distinguish fat and water

frequencies locally.• Usually more prominent in images with a large FOV or off-axis.• Solution – improve the magnet shimming.• Modern magnets – auto shimming for very reliable fatsat.

5. RF Artefacts• Ghosting

• RF interference

• Stimulated Echoes

• RF Coil artefacts

• Steady State artefacts

Ghosting• Arises from any structure that moves during

acquisition of data eg: chest wall, pulsatile movement of vessels, swallowing etc.)

• Ghosts displaced along PE axis due to inherent time delay between phase encoding and readout.

• Number and intensity depends upon period of modulation and the TR.

Inhalation

Exhalation

A

B

Chest wall

P1

P2

• Moving anatomy is mismapped into the FOV.

Quadrature Ghost

• Occurs due to differences in the gain of real and imaginary receiver channels

• Phase errors between the two quadrature RF receive channels can also cause this

• Ghost is displaced diagonally across the centre in both PE and FE directions

• Solution - ? phase alternating

RF Interference• Zipper artefact appears as bright and dark zipper lines along PE.• External RF picked up by coils (e.g RF breakthrough waveguide filters).• Pulse oximeters (monitors the percentage of haemoglobin saturated with

oxygen) use RF – can be picked up by MR coils.• RF from within the MR system may be coherent – bright spot on image.• Mains RF – modulated by 50Hz – regularly spaced faint zipper artefacts

across image.

RF breakthrough Zipper artefact

Herring-Bone Artefact• Occurs due to the presence of a spike of noise (or an ‘arc’ from

a static discharge) in the raw data.• FT (series of spikes) which is convolved with the image data.• Probably due to breakdown of RF system (poor RF decoupling).• Best solution – rescan the image.

Halo Artefact• Results from signal clipping caused by overflow on the

ADC’s.• Occurs if receiver gain is incorrectly set.• Signal becomes too large for the ADC range and

information in the centre of k-space is lost.• Unusual - unless receiver gain is manually set.

Stimulated Echoes (STE)

• 1st pulse forms transverse magnetisation• 2nd pulse – remaining transverse components form Hahn echo• 3rd pulse converts longitudinal magnetisation to transverse

magnetisation, and components re-phase to form stimulated echo

yx

zHahn Echo1st 90o pulse 2nd 90o pulseDephasing

Lag

Lead

x

y

3rd 90o pulse Stim Echo

STE – Coherence Pathways

• STE has different spatial encoding and contrast

• Avoid STE by using ‘spoiler’ gradients to destroy residual transverse magnetisation, or use ‘rewinder’ gradients to prevent the STE occurring in the sampling window

• Can also widen the bandwidth, or alter the TE to avoid STE

Phase 90o 90o 90o

H STE

H=Hahn EchoS=Stimulated Echo

(Longitudinal)

SHOULDER

RF Coil Artefacts• One of the arrays of a

phased array coil is out of phase with the other coils.

• Bands of signal addition and cancellation.

• Solution – call engineer!

Sagittal Pelvis

Surface Coil Flare

• The result of signal saturation at edge of surface coil.

• Optimal signal is further in from edge.• Solution – Surface Coil Intensity

Correction (SCIC) – algorithm that reduces the high intensity fat signal nearest the coil for improved visualisation.

• SCIC is very useful for correcting sagittal and axial spine images.

Axial abdomen

Steady State Imaging - Artefacts

• Common on True FISP, balanced FFE, FIESTA (fully balanced gradients).• Related to variation of steady state condition due to Bo inhomogeneities.• Aliasing of one side of the body to the other results in superimposition of signals

of different phases that alternatively add and cancel.• Equivalent to introducing a systematic error to the flip angle.• Require a short TE and good shimming – otherwise bands ~ 1/B0• Solution – phase alternation of RF pulse

Coronal abdomen

Moire Fringes

Noise

Random Noise• Noise can be considered an artefact

since it is unwanted.• Grainy, snowy, no recognisable

pattern.• Solution – improve the SNR• Increase slice thickness, increase TR,

reduce TE, decrease bandwidth, decrease pixel resolution, increase the FOV, increase phase steps, increase the number of averages

• Remember ‘trade-offs’ (scan time [2D] = TR x NY x NEX).

And finally…

Observer Artefact• Self explanatory• Otherwise known as “Upside-down Error”

• Solution – apply for time off !

References

• MRI from Picture to Proton: Donald W. McRobbie, Elizabeth A.Moore, Martin J.Graves and Martin R.Prince Cambs Uni Press

• All you need to know about MRI Physics: Moriel NessAiver

For further information

Kris.Armoogum@tuht.scot.nhs.ukStephen.Gandy@tuht.scot.nhs.uk