MR Artifacts

SusceptibilityGradient FieldRFK-SpaceMotionChemical ShiftGibbs (Ringing, Truncation) ArtifactsAliasing (Wraparound)Partial VolumeHigh Speed ImagingEffect of Field Strength

Spike (Herringbone)

Bad data point/noise spike in k-space Either very high / low intensity compared w/ rest of image Spike is convolved with all other image info during FT

Since each image pixel is a weighted sum of all individual points in k-space

Results in dark stripes overlaid on image

Occurs with high duty cycle gradients sequences Loose connection/breakdown of connections in RF coil

Uses

Can be used in cardiac imaging

Prep. Pulses applied before imaging sequence Forms echoes in different parts of k-space FT produces tags in grid-like pattern

Tags applied at start of each cardiac phase Images acquired at multiple phases of cardiac

cycle Follow changes in tag position during cycle assess

cardiac motion

Spike Artifact

Image space K-space

Herringbone Artifact

Zipper Artifact Caused by leakage of e-m energy into magnet

room

Results in region of increased noise Width of 1-2 pixels extends in frequency encode direction Through entire series

Room shielded from outside e-m signals Signals from equipment brought into room OR RF shield compromised

Zipper

Motion-related Patient Motion, either:

Voluntary non periodic Eye movement Swallowing Smearing of image

Involuntary Periodic Respiratory Cardiac Pulsatile movement of vessels & CSF Bowel motion Coherent ghosts formed

Blurring/ghosting in phase-encode direction Time difference in adjacent points in PE direction relatively long

= TR Introduces phase difference between adjacent k-space lines

Phase Mis-mapping

• PEG has different amplitude every TR, unlike FEG/SSG

• As anatomy moves, misplaced in PE direction as PEG changes• Anatomy given different

phase values depending on its position along gradient

• Time delay between PE & readout

anatomy may have moved between PEG & FEG when placed into k space

Swap PE & FE Directions Artifact occurs only in PE direction

Change axis/direction Pick axis Produce least interference w/ ROI

Example: Sagittal C Spine Usually FE performed in z-axis (head to foot)

Longest axis PE would then be AP (Y axis)

However: Artifacts in AP direction Swallowing Carotids pulsatile motion Ghosting over spinal cord

Swap PE & FE axes Y gradient (AP) performs FE Z gradient performs PE Artifacts now harmless in head to foot direction

Use Pre-Sat Pulses Placing pre-sat volumes over areas producing

artifacts Nullifies signal Reduces artifact

Example: Sagittal C Spine Pre sat pulse over throat

reduces swallowing artifact Reduces artifact from flowing nuclei in blood vessels

Respiratory MotionREMEDIES

Breath hold Patient cooperation req’d May take multiple breath holds

Respiratory gating Image acquisition only at certain phases in resp. cycle Acquisition time ↑

Respiratory compensation/phase reordering ROPE (Resp. Ordered Phase Encoding) PE steps ordered on basis phase in resp. cycle Difficult if resp. not regular

Real-time navigator echo gating Echo from diaphragm determines its position

Navigator echo interleaved with actual imaging sequence Real-time monitoring

data only acquired during specific range of diaphragmatic motion

Patient Motion

Without breath-holding With breath-holding;

With Cardiac pulsation artifacts

Resp. Compensation & K Space

Respiratory Motion Compensation

Without compensation With compensation

Navigator Placement

Aqua overlay shows navigator section from which displacement info obtained

Graph shows diaphragmatic movement (white wave & green line).

Yellow boxes: best time to image

Cardiac Pulsation

ECG gating

Time acquisition to occur @ same phase of each cardiac cycle Coordinate excitation pulse w/ R wave of systole

Further artifact suppression if breath hold

Segmented K-Space

Echo-planar imaging Single RF pulse/excitation Continuous reversal of echoes using gradient pulses Acquire all lines in k-space to form a single image

Alternate lines in k-space read in opposite direction Prior to FT lines must be reversed Introduces phase errors in alternate k-space lines

Errors from: Nonlinear gradient reversal Eddy currents Poor shimming

Results in image ghosts

EPI

SS- vs. MS EPI

Single Shot EPI Single additional ghost

image Reduced intensity Shifted by ½ FOV

½ k-space lines are different from other half

“N/2 Ghost”

Reduce artifact Minimize phase errors

Multi-Shot EPI # ghost images ↑

As # of discontinuities in k-space ↑

Errors # echoes / shot & # of segments in k-space

Minimize artifact May be necessary to

obtain additional navigator echo

Fast Spin Echo

Segmented k-space artifacts can also occur

Minor timing errors in sequence Between multiple RF pulses Between data collection windows

Eddy currents

Call Service Support

Ghost Artifacts

Initial ImageFSE

Single Shot EPIN/2 ghost

multi Shot EPI

Flow Artifacts

Flowing blood source of artifacts

Ghosting in phase-encoding direction

SE sequences not as susceptible Flow appears dark (no signal) Blood exposed to 90° excitation pulse flows out of imaging

section before 180° refocusing pulse Blood that moved into imaging section never exposed to

90° pulse

GRE Sequences susceptible In-flow effect bright blood

Pre-saturation Pulse

Attenuate signals upstream of imaging volume

Reduces intensity of fluid flowing into FOV

Apply saturation band adjacent to imaging section 90° pulse

All spins tilted towards axial plane Spoiled with gradient crusher pulses before image

acquisition Saturated spins exhibit no signal when moving into

imaging volume

Reducing Flow Artifacts

Cardiac Pulsation Flow-Related Artifact Suppression

Cardiac pulsation artifact After saturation band

applied

Reducing Flow Artifacts

Flow Compensation/Gradient Moment Nulling

Flowing spins not in phase with static spins when echo forms Flowing spins brought back into phase by

motion-compensating gradient pulses No effect on static spins

Penalty is increased echo time

Reducing Flow Artifacts

Gradient Moment Nulling

Cardiac Pulsation

Cardiac pulsation artifact After Motion Compensation

Flow ArtifactFlow artifact in right to left direction (phase encoding) from the popliteal vessel, seen as a small bright artifact along the middle of the femoral bone.

Susceptibility Artifact

Tissues placed in magnetic field become temp. magnetized

Slightly alters local magnetic field

Difference in susceptibility between tissues Field inhomogeneity at tissue boundaries field gradient Spins dephase faster

Signal loss Low signal intensity

Signal loss worst for bone-soft tissue & air-tissue boundaries Air, bone much lower magnetic susceptibility than most tissues Geometric distortions introduced

SE vs. EPI SE sequences less affected

180° refocusing pulse cancels susceptibility gradients

EPI more severely affected Echoes are refocused by using gradients over long time period Minimize by orienting PE gradient along same axis as

susceptibly gradients

Reduce artifact by: Use SE sequences Reduce echo time Increase acquisition matrix Proper shimming over VOI to improve local field inhomogeneity

Reorienting PE Gradient

Left-Right PE direction Anterior-Posterior PE(same axis as susceptibility gradients)

SE vs. GRE Susceptibility

Spin echo

Gradient echo

Metal Implants Most severe susceptibility artifact

Metal > magnetic susceptibility than tissues

Typically areas of complete signal loss

Minimize effect: Large receiver bandwidth Decreased echo time Fast SE with high bandwidth Watch heating of adjacent tissue

Dental Fillings

Chemical Shift Molecular protons surrounded by clouds of e-

In external magnetic field electric current induced This current will induce a magnetic moment

Antiparallel to external field Reduce local magnetic field felt by proton

“electronic shielding”

Protons in water vs. protons in fat Significantly different chemical environments Resonance frequencies different Precess @ different frequencies

Chemical Shift Larmor frequency shift between water protons & fat protons

Chemical Shift Artifacts in frequency encode direction

Slight mis-registration of fat content slight shift in frequency of fat protons Amount of shift depends on:

# samples in FE direction Receiver bandwidth Gx

x

Chemical Shift

Reduce Chemical Shift

Less important in FSE imaging Higher bandwidth receiver window used

EPI very susceptible Long duration of sampling affects/shifts any off-resonance signal (fat) Minimized by:

Applying frequency-selective RF pulse to nullify fat signal before imaging sequence

Successful fat sat only if magnetic field homogeneous throughout ROI Proper shimming2 distinct signal for fat & water

STIR sequences can also be used Fat T1 short Can be used to suppress fat signal with inversion recovery

sequence TI needed to null signal from any tissue = 0.7 x T1

EPI & Chemical Shift

Severe chemical shift artifact;Insufficient fat suppression

Reduced chemical shift artifact;fat saturation

Fat Saturation vs. Inversion

Chemical Shift Artifact

Chemical Shift of the 2nd Kind

In-phase @ points A, C, E

Out-of-phase@ points B, D

• Applies to GE sequences• Since H2O precesses faster than fat it gets 360° ahead of fat in

short time period• Thus times when fat & H2O are totally in phase, times when

totally out of phase• Dark boundaries when out of phase

• Does not only appear in FE direction as CS Artifact of 1st kind

In- vs. Out-Of-Phase

In PhaseMean signal intensity in mass = 115

Out of PhaseMean signal intensity in mass = 66

Fat present in lesion Benign adrenal adenoma

3-cm left adrenal mass

In- vs. Out-Of-Phase

In Phase Out of Phase

No differences in lesion signal between 2 images

No Fat present in lesion Metastases generally do not contain fat

metastatic non-small cell and squamous cell carcinoma of the lung demonstrate a left adrenal mass

Aliasing (Wraparound)

Overlapping opposite side of image of signals outside FOV Spatial encoding of objects outside FOV cannot be

distinguished from inside FOV

Imaging FOV smaller than anatomy being imaged

PE direction FOV = distance along gradient to complete 1 cycle Gradient will move from -180° to +180° across FOV

If RF transmission coil sensitivity extends beyond FOV Spins outside FOV excited Will be part of next cycle in PE direction (i.e. -360° to +360°

etc. )

Phase Equivalence

Phase angles of spins outside FOV essentially equivalent to spins within FOV But on opposite sides of image

For FT: Spins at x° = x+360° Results in overlap of signal outside FOV with

signal within FOV

Phase-Encoding

PE: Modifying phase of spins in a direction of slice planeStep 1: Phase shifts range from -180° to +180°Step 2:Shifts increased by multiple of 180 (360, 540 Etc.)

.

.

.Step N

Meaningful phase range

Phase shift Outside FOV

Phase = 200°

Equivalent phase = -160°

Inside FOV:Phase shifts range from -180° to +180° Outside FOV: Phase shifts < -180° or > +180°

Mismapped to equivalent phase inside image.

FOV ≥ imaged anatomy

FOV < imaged anatomy

Wrap around artifacts

FOV

Aliasing

Gradient field doesn’t stop @ end of FOVComputer cannot recognize frequencies above fmax or below –fmaxAnatomy outside FOV that have higher than fmax frequencies will be misplaced as if it had lower frequencies on opposite side

Remedies

Increase FOV

No Phase Wrap

Aliasing-Moiré Artifact

Wrap AroundPh

ase

enco

de

frequency encode

freq

uenc

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code

Phase encode

Gibbs/Truncation

Bright/Dark lines near borders of abrupt signal change Parallel & adjacent to border

Insufficient sampling in either phase/readout direction Usually seen in PE direction PE matrix often < readout matrix in order to ↓ acquisition

time

Minimized by ↑ matrix size with same FOV ↑ acquisition time ↓ per-pixel SNR

Gibbs Ring Artifact

Desired object profile

Object profile

128 x 128

256 x 256

↑Spatial resolution minimizes artifact

Example

Gradient Field Distortion

Coil current

Magnetic field

Coil current

Magnetic field

Magnetic field variation

Superimposed magnetic fields

Linear region

Nonlinear regions

Gradient Field Distortion

Gradient linearity ↓ with ↑ distance from isocenter

Results in images getting compressed Mis-mapping spins from true

locations @edges in large FOV

Distortion-correction algorithms compensate

Gradient Distortion Artifact

Before correction algorithm applied

After correction algorithm applied

RF Artifacts-Section Cross talk

Occurs @ imaging of contiguous sections

Results in all sections having reduced intensity except edge sections

Selective RF pulses yield imperfect slice profiles Partially excite neighboring slices

Subjected to RF pulse more than once during 1TR Partial saturation

↓ Signal intensity

Remedies: Produce RF pulses with sharper profiles ↑ gap between sections Interlace sections 3D acquisition

Slice Gap

Perfect slice profile

Imperfect slice profile

Cross talk

Imperfect slice profile

Slice gapNo Cross talk

Slice Interleaving

Imperfect slice profile

1st set of excitations 2nd set of excitations

Cross Talk

Parallel Imaging -Reduced FOV

Decreasing FOV

Increasing aliasing as expectedIncreasing noise & ghosting in center of FOV

Parallel Imaging-Increased Acceleration

No reduction Reduction factor = 2

Reduction factor = 3

Increasing noise (both magnitude & inhomogeneity)

Summary

Artifact Axis Remedy Penalty

Flow Motion PE

Swap PE & FE May need anti-aliasing

gatingVariable TRVariable image contrast↑Scan time

Pre-saturation May lose a sliceGradient moment rephasing ↑Minimum TE

Chemical Shift FE

↑bandwidth ↓Minimum TE available↓SNR

↓FOV ↓SNR↑resolution

Use chemical saturation ↓SNRMay lose slices

Out of Phase FE & PE Select TE @ periodicity of

fat & waterMay lose a slice if TE significantly ↑

SummaryArtifact Axis Remedy Penalty

Aliasing FE & PE

No frequency wrap None

No phase wrapMay ↓SNRMay ↑scan time↑Motion artifact

Enlarge FOV ↓resolutionZipper FE Call engineer

Magnetic Susceptibility FE & PE

Use spin echoNot flow sensitiveBlood product may be missed

Remove metal none

Shading FE & PECheck shim noneLoad coil correctly none

Patient Motion PE

Use anti-spasmoticsCostly

invasiveImmobilize patient noneCounsel patient noneAll remedies for flow motion See Previous

sedation

Possible side effectsinvasivecostlyRequires monitoring

SummaryArtifact Axis Remedy Penalty

Cross Talk SS none none

Cross Excitation SS Interleaving Double scan timeSquaring off RF pulses ↓SNR

Moiré FE & PE Use SE nonePatient not to touch bore none

Magic Angle FE & PE Change TE slightly noneAlter position of anatomy none

Quality Control

ACR Accreditation

ACR Tests

1. Geometric accuracy2. High-contrast spatial resolution3. Slice thickness accuracy4. Slice position accuracy5. Image intensity uniformity6. Percent-signal ghosting7. Low-contrast object detectability

ACR Accreditation

It is highly recommended that all sites apply for ACR accreditation, for the simple reason that the process of accreditation provides a means of establishing a quality-assurance or quality-control program that is recognized by leading authorities.

An even more important and more practical reason to seek accreditation is that more and more insurance companies (starting with Aetna in 2001) require that sites be accredited before claims are submitted for insurance payment.

ACR Phantom

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