Download - MR Artifacts
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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