Aliasing - Questions And Answers ​in MRI

Questions and Answers ​in MRI

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• Complete List of Questions
• ...Electromagnetism & NMR
  • Basic Electromagnetism >
    • What causes magnetism?
    • Radiofrequency waves?
    • Wave properties?
    • What is a tesla?
    • Who was Tesla?
    • What is a gauss?
    • How strong is 3.0T?
    • What is a gradient?
    • Aren't gradients coils?
    • What is susceptibility?
    • Susceptibility causes?
    • How to levitate a frog?
    • What is ferromagnetism?
    • Superparamagnetism?
  • Spin >
    • What is spin?
    • Why I = ½, 1, etc?
    • Proton = nucleus = spin?
    • Predict nuclear spin (I)?
    • Magnetic dipoles?
    • Gyromagnetic ratio (γ)?
    • "Spin" vs "Spin state"?
    • Quantum "reality"?
    • Energy splitting?
    • Fall to lowest state?
    • Does M instantly appear?
    • MR vs MRI vs NMR?
  • Precession >
    • Why precession?
    • Who was Larmor?
    • Energy for precession?
    • Chemical shift?
    • Net magnetization (M)?
    • Does M also precess?
    • Does precession = NMR?
    • Rotating frame?
  • Resonance >
    • Who discovered NMR?
    • Why is RF input needed?
    • Why at Larmor frequency?
    • How does B1 tip M?
    • Phase coherence?
    • Where does RF energy go?
    • Release of RF energy?
    • Sample Q&A
    • Off-resonance?
    • Why is RF a pulse?
    • Partial flip angles?
    • How is signal higher?
    • Optimal flip angle?
    • Free Induction Decay?
    • Origin of MR signal?
    • Not radio waves?
  • Relaxation - Physics >
    • Bloch equations?
    • What is T1?
    • What is T2?
    • Relaxation rate vs time?
    • Why is T1 > T2?
    • T2 vs T2*?
    • Causes of Relaxation?
    • Dipole-dipole interactions?
    • Chemical Exchange?
    • Spin-Spin interactions?
    • Macromolecule effects?
    • Which H's produce signal?
    • "Invisible" protons?
    • Magnetization Transfer?
    • T1-rho?
    • Bo effect on T1 & T2?
    • How to predict T1 & T2?
  • Relaxation - Clincial >
    • T1 bright? - fat
    • T1 bright? - other oils
    • T1 bright? - cholesterol
    • T1 bright? - calcifications
    • T1 bright? - meconium
    • T1 bright? - melanin
    • T1 bright? - protein/mucin
    • T1 bright? - myelin
    • Magic angle?
    • MT Imaging/Contrast?
• …Scanners & Hardware
  • MR Magnets - Part I >
    • Types of magnets?
    • Brands of scanners?
    • Which way does field point?
    • Which is the north pole?
    • Low v mid v high field?
    • Advantages to low-field?
    • Disadvantages?
    • What is homogeneity?
    • Why homogeneity?
    • Why shimming?
    • Passive shimming?
    • Active shimming?
  • MR Magnets - Part II >
    • Superconductivity?
    • Perpetual motion?
    • How to ramp?
    • Superconductive design?
    • Room Temp supercon?
    • Liquid helium use?
    • What is a quench?
    • Is field ever turned off?
    • Emergency stop button?
  • Gradients >
    • Many kinds of coils?
    • Gradient coils?
    • How do z-gradients work?
    • X- and Y- gradients?
    • Open scanner gradients?
    • Eddy current problems?
    • Active shielded gradients?
    • Active shield confusion?
    • What is pre-emphasis?
    • Gradient heating?
    • Gradient specifications?
    • Gradient linearity?
  • Radiofrequency Coils? >
    • RF Coil function(s)?
    • Radiofrequency waves?
    • RF-transmit coils?
    • LP vs CP (Quadrature)?
    • Multi-transmit RF?
    • Receive-only coils?
    • Array coils?
    • AIR Coils?
  • Site Planning >
    • MR system layout?
    • What are fringe fields?
    • How to reduce fringe?
    • Magnetic shielding?
    • Need for vibration testing?
    • What's that noise?
    • Why RF Shielding?
    • Wires/tubes thru wall?
• …Making an Image
  • MR Signals >
    • Free Induction Decay (FID)?
    • Gradient echo?
    • TR and TE?
    • Spin echo?
    • 90°-90° Hahn Echo?
    • Stimulated echoes?
    • STEs for imaging?
    • 4 or more RF-pulses?
    • From Signals to Images >
      • Signal squiggles?
      • Real v Imaginary?
      • Fourier Transform (FT)?
      • What are 2D- & 3D-FTs?
      • Who invented MRI?
      • How to locate signals?
  • Frequency Encoding >
    • Frequency encoding?
    • Receiver bandwidth?
    • Narrow bandwidth?
    • Slice-selective excitation?
    • SS gradient lobes?
    • Cross-talk?
    • Frequency encode all?
    • Mixing of slices?
    • Two slices at once?
    • Simultaneous Multi-Slice?
  • Phase Encoding >
    • Phase-encoding gradient?
    • Single PE step?
    • What is phase-encoding?
    • PE and FE together?
    • 2DFT reconstruction?
    • Choosing PE/FE direction?
  • Performing an MR Scan >
    • What are the steps?
    • Automatic prescan?
    • Routine shimming?
    • Coil tuning/matching?
    • Center frequency?
    • Transmitter gain?
    • Receiver gain?
    • Dummy cycles?
    • Where's my data?
    • MR Tech qualifications?
  • Image Quality Control >
    • Who regulates MRI?
    • Who accredits?
    • Mandatory accreditation?
    • Routine quality control?
    • MR phantoms?
    • Geometric accuracy?
    • Image uniformity?
    • Slice parameters?
    • Image resolution?
    • Signal-to-noise?
    • Ghosting?
  • ...RF & Gradient Details >
    • Timing Diagram: Basic
    • Timing Diagram: Advanced
    • Pulse Sequence Modules
    • Pulse sequence families?
    • Crushers vs spoilers?
    • MR template
    • Adiabatic excitation?
    • Adiabatic pulses?
    • MR template
    • MR template
• …Pulse Sequences
  • Spin Echo >
    • SE vs Multi-SE vs FSE?
    • Image contrast: TR/TE?
    • Opposite effects ↑T1 ↑T2?
    • Meaning of weighting?
    • Does SE correct for T2?
    • Effect of 180° on Mz?
    • Direction of 180° pulse?
  • Inversion Recovery >
    • What is IR?
    • Why use IR?
    • Phase-sensitive IR?
    • Why not PSIR always?
    • Choice of IR parameters?
    • TI to null a tissue?
    • STIR?
    • T1-FLAIR
    • T2-FLAIR?
    • IR-prepped sequences?
    • Double IR?
  • Gradient Echo >
    • GRE vs SE?
    • Multi-echo GRE?
    • Types of GRE sequences?
    • Commercial Acronyms?
    • Spoiling - what and how?
    • Spoiled-GRE parameters?
    • Spoiled for T1W only?
    • What is SSFP?
    • GRASS/FISP: how?
    • GRASS/FISP: parameters?
    • GRASS vs MPGR?
    • PSIF vs FISP?
    • True FISP/FIESTA?
    • FIESTA v FIESTA-C?
    • DESS?
    • MERGE/MEDIC?
    • GRASE?
    • MP-RAGE v MR2RAGE?
  • Susceptibility Imaging >
    • What is susceptibility (χ)?
    • What's wrong with GRE?
    • Making an SW image?
    • Phase of blood v Ca++?
    • Quantitative susceptibility?
  • Diffusion: Basic >
    • What is diffusion?
    • Iso-/Anisotropic diffusion?
    • "Apparent" diffusion?
    • Making a DW image?
    • What is the b-value?
    • b0 vs b50?
    • Trace vs ADC map?
    • Light/dark reversal?
    • T2 "shine through"?
    • Exponential ADC?
    • T2 "black-out"?
    • DWI bright causes?
  • Diffusion: Advanced >
    • Diffusion Tensor?
    • DTI (tensor imaging)?
    • Whole body DWI?
    • Readout-segmented DWI?
    • Small FOV DWI?
    • IVIM?
    • Diffusion Kurtosis?
  • Fat-Water Imaging >
    • Fat & Water properties?
    • F-W chemical shift?
    • In-phase/out-of-phase?
    • Best method?
    • Dixon method?
    • "Fat-sat" pulses?
    • Water excitation?
    • STIR?
    • SPIR?
    • SPAIR v SPIR?
    • SPIR/SPAIR v STIR?
• …K-space & Rapid Imaging
  • K-space (Basic) >
    • What is k-space?
    • Parts of k-space?
    • What does "k" stand for?
    • Spatial frequencies?
    • Locations in k-space?
    • Data for k-space?
    • Why signal ↔ k-space?
    • Spin-warp imaging?
    • Big spot in middle?
    • K-space trajectories?
    • Radial sampling?
  • K-space (Advanced) >
    • K-space grid?
    • Negative frequencies?
    • Field-of-view (FOV)
    • Rectangular FOV?
    • Partial Fourier?
    • Phase symmetry?
    • Read symmetry?
    • Why not use both?
    • ZIP?
  • Rapid Imaging (FSE &EPI) >
    • What is FSE/TSE?
    • FSE parameters?
    • Bright Fat?
    • Other FSE differences?
    • Dual-echo FSE?
    • Driven equilibrium?
    • Reduced flip angle FSE?
    • Hyperechoes?
    • SPACE/CUBE/VISTA?
    • Echo-planar imaging?
    • HASTE/SS-FSE?
  • Parallel Imaging (PI) >
    • What is PI?
    • How is PI different?
    • PI coils and sequences?
    • Why and when to use?
    • Two types of PI?
    • SENSE/ASSET?
    • GRAPPA/ARC?
    • CAIPIRINHA?
    • Compressed sensing?
    • Noise in PI?
    • Artifacts in PI?
• …Contrast Agents
  • Contrast Agents: Physics >
    • Why Gadolinium?
    • Paramagnetic relaxation?
    • What is relaxivity?
    • Why does Gd shorten T1?
    • Does Gd affect T2?
    • Gd & field strength?
    • Best T1-pulse sequence?
    • Triple dose and MT?
    • Dynamic CE imaging?
    • Gadolinium on CT?
  • Contrast Agents: Clinical >
    • So many Gd agents!
    • Important properties?
    • Ionic v non-ionic?
    • Intra-articular/thecal Gd?
    • Gd liver agents (Eovist)?
    • Mn agents (Teslascan)?
    • Blood pool (Ablavar)?
    • Iron-based contrast?
    • Lymph node agents?
    • Feridex & Liver Agents?
    • Ferumoxytol?
    • Ferumoxytol relaxivity?
    • Ferabright™?
    • Bowel contrast agents?
  • Contrast Agents: Safety >
    • Ferumoxytol Safety?
    • Gadolinium safety?
    • Allergic reactions?
    • Renal toxicity?
    • What is NSF?
    • NSF by agent?
    • Informed consent for Gd?
    • Gd protocol?
    • Is Gd safe in infants?
    • Reduced dose in infants?
    • Gd in breast milk?
    • Gd in pregnancy?
    • Gd accumulation?
    • Gd deposition disease?
• …Cardiovascular and MRA
  • Flow effects in MRI >
    • Defining flow?
    • Expected velocities?
    • Laminar v turbulent?
    • Predicting MR of flow?
    • Time-of-flight effects?
    • Spin phase effects?
    • Flow void?
    • Why GRE ↑ flow signal?
    • Slow flow v thrombus?
    • Even-echo rephasing?
    • Flow-compensation?
    • Flow misregistration?
  • MR Angiography - I >
    • MRA methods?
    • Dark vs bright blood?
    • Time-of-Flight (TOF) MRA?
    • 2D vs 3D MRA?
    • MRA parameters?
    • Magnetization Transfer?
    • Ramped flip angle?
    • MOTSA?
    • Fat-suppressed MRA?
    • TOF MRA Artifacts?
    • Phase-contrast MRA?
    • What is VENC?
    • Measuring flow?
    • 4D Flow Imaging?
    • How accurate?
  • MR Angiography - II >
    • Gated 3D FSE MRA?
    • 3D FSE MRA parameters?
    • SSFP MRA?
    • Slab-selective IR MR?
    • MRA with ASL?
    • Other MRA methods?
    • Contrast-enhanced MRA?
    • Timing the bolus?
    • View ordering in MRA?
    • Bolus chasing?
    • TRICKS or TWIST?
    • CE-MRA artifacts?
  • Cardiac I - Intro/Anatomy >
    • Cardiac protocols?
    • Patient prep?
    • EKG problems?
    • Magnet changes EKG?
    • Gating v triggering?
    • Gating parameters?
    • Heart navigators?
    • "Remote" cardiac gating?
    • Dark blood/Double IR?
    • Why not single IR?
    • Triple IR?
    • Polar plots?
    • Coronary artery MRA?
    • Cardiac AI?
    • Cardiac MR artifacts?
  • Cardiac II - Function >
    • Beating heart movies?
    • Cine parameters?
    • Real-time cine?
    • Ventricular function?
    • Tagging/SPAMM?
    • Perfusion: why and how?
    • 1st pass perfusion?
    • Quantifying perfusion?
    • Dark rim artifact
  • Cardiac III - Viability >
    • Gd enhancement?
    • TI to null myocardium?
    • PS (phase-sensitive) IR?
    • Wideband LGE?
    • T1 mapping?
    • Iron/T2*-mapping?
    • Edema/T2-mapping?
    • T1-rho mapping?
    • Why/how stress test?
    • Stess drugs/agents?
    • Stress consent form?
• …MR Artifacts
  • Tissue-related artifacts >
    • Chemical shift artifact?
    • Chemical shift in phase?
    • Reducing chemical shift?
    • Chemical Shift 2nd Kind?
    • In-phase/out-of phase?
    • IR bounce point?
    • Susceptibility artifact?
    • Metal suppression?
    • Dielectric effect?
    • Dielectric Pads?
  • Motion-related artifacts >
    • Why discrete ghosts?
    • Motion artifact direction?
    • Reducing motion artifacts?
    • Saturation pulses?
    • Gating methods?
    • Respiratory comp?
    • Camera-based systems?
    • Navigator echoes?
    • Remote respiratory gating?
    • PROPELLER/BLADE?
    • SAMER?
  • Technique-related artifacts >
    • Partial volume effects?
    • Slice overlap?
    • Aliasing?
    • Wrap-around artifact?
    • Eliminate wrap-around?
    • Phase oversampling?
    • Frequency wrap-around?
    • Spiral/radial artifacts?
    • Gibbs artifact?
    • Nyquist (N/2) ghosts?
    • Zipper artifact?
    • Data artifacts?
    • Surface coil flare?
    • MRA Artifacts (TOF)?
    • MRA artifacts (CE)?
• …Functional Imaging
  • Perfusion I: Intro & DSC >
    • Measuring perfusion?
    • Meaning of CBF, MTT etc?
    • DSC v DCE v ASL?
    • How to perform DSC?
    • Bolus Gd effect?
    • T1 effects on DSC?
    • DSC recirculation?
    • DSC curve analysis?
    • DSC signal v [Gd]
    • Arterial input (AIF)?
    • Quantitative DSC?
  • Perfusion II: DCE >
    • What is DCE?
    • How is DCE performed?
    • How is DCE analyzed?
    • Breast DCE?
    • DCE signal v [Gd]
    • DCE tissue parmeters?
    • Parameters to images?
    • K-trans = permeability?
    • Utility of DCE?
  • Perfusion III: ASL >
    • What is ASL?
    • ASL methods overview?
    • CASL?
    • PASL?
    • pCASL?
    • ASL parameters?
    • ASL artifacts?
    • Gadolinium and ASL?
    • Vascular color maps?
    • Quantifying flow?
  • Functional MRI/BOLD - I >
    • Who invented fMRI?
    • How does fMRI work?
    • BOLD contrast?
    • Why does BOLD ↑ signal?
    • Does BOLD=brain activity?
    • BOLD pulse sequences?
    • fMRI Paradigm design?
    • Why "on-off" comparison?
    • Motor paradigms?
    • Visual?
    • Language?
  • Functional MRI/BOLD - II >
    • Process/analyze fMRI?
    • Best fMRI software?
    • Data pre-processing?
    • Registration/normalization?
    • fMRI statistical analysis?
    • General Linear Model?
    • Activation "blobs"?
    • False activation?
    • Resting state fMRI?
    • Analyze RS-fMRI?
    • Network/Graphs?
    • fMRI at 7T?
    • Mind reading/Lie detector?
    • fMRI critique?
• …MR Spectroscopy
  • MRS I - Basics >
    • MRI vs MRS?
    • Spectra vs images?
    • Chemical shift (δ)?
    • Measuring δ?
    • Backward δ scale?
    • Predicting δ?
    • Size/shapes of peaks?
    • Splitting of peaks?
    • Localization methods?
    • Single v multi-voxel?
    • PRESS?
    • STEAM?
    • ISIS?
    • CSI?
  • MRS II - Clinical ¹H MRS >
    • How-to: brain MRS?
    • Water suppression?
    • Fat suppression?
    • Normal brain spectra?
    • Choice of TR/TE/etc?
    • Hunter's angle?
    • Lactate inversion?
    • Metabolite mapping?
    • Metabolite quantitation?
    • Breast MRS?
    • Gd effect on MRS?
    • How-to: prostate MRS?
    • Prostate spectra?
    • Muscle ¹H-MRS?
    • Liver ¹H-MRS?
    • MRS artifacts?
  • MRS III - Multi-nuclear >
    • Other nuclei?
    • Why phosphorus?
    • How-to: ³¹P MRS
    • Normal ³¹P spectra?
    • Organ differences?
    • ³¹P measurements?
    • Decoupling?
    • NOE?
    • Carbon MRS?
    • Sodium imaging?
    • Xenon imaging?
• ...Artificial Intelligence
  • AI Part I: Basics >
    • Artificial Intelligence (AI)?
    • What is a neural network?
    • Machine Learning (ML)?
    • Shallow v Deep ML?
    • Shallow networks?
    • Deep network types?
    • Data prep and fitting?
    • Back-Propagation?
    • DL 'Playground'?
  • AI Part 2: Advanced >
    • What is convolution?
    • Convolutional Network?
    • Softmax?
    • Upsampling?
    • Limitations/Problems of AI?
    • Is the Singularity near?
  • AI Part 3: Image processing >
    • AI in clinical MRI?
    • Super-resolution?
• ...Tissue Properties Imaging
  • MRI of Hemorrhage >
    • Hematoma overview?
    • Types of Hemoglobin?
    • Hyperacute/Oxy-Hb?
    • Acute/Deoxy-Hb?
    • Subacute/Met-Hb?
    • Deoxy-Hb v Met-Hb?
    • Extracellular met-Hb?
    • Chronic hematomas?
    • Hemichromes?
    • Ferritin/Hemosiderin?
    • Subarachnoid blood?
    • Blood at lower fields?
  • Cartilage mapping?
  • MR Elastography?
  • Synthetic MRI?
  • Amide Proton Transfer?
  • MR thermography?
  • MR Fingerprinting?
  • Electric Properties Imaging?
• ...Safety and Screening
  • Overview >
    • Status of the 5G line?
    • ACR Safety Zones?
    • MR safety screening?
    • Incomplete screening?
    • Passive v active implants?
    • Conditional implants?
    • Common safety issues?
    • Projectiles?
    • Metal detectors?
    • Pregnant patients?
    • Postop, ER & ICU patients?
    • Temperature monitoring?
    • Orbital foreign bodies?
    • Bullets and shrapnel?
  • Static Fields >
    • "Dangerous" metals?
    • "Safe" metals?
    • Magnetizing metal?
    • Object shape?
    • Forces on metal?
    • Most dangerous place?
    • Force/torque testing?
    • Static field bioeffects?
    • Dizziness/Vertigo?
    • Flickering lights?
    • Metallic taste?
  • RF Fields >
    • RF safety overview?
    • RF biological effects?
    • What is SAR?
    • SAR limits?
    • Operating modes?
    • How to reduce SAR?
    • RF burns?
    • Estimate implant heating?
    • SED vs SAR?
    • B1+rms vs SAR?
    • Personnel exposure?
    • Cell phones?
  • Gradient Fields >
    • Gradient safety overview
    • Acoustic noise?
    • Nerve stimulation?
    • Gradient vs RF heating?
  • Safety: Neurological >
    • Aneurysm coils/clips?
    • Shunts/drains?
    • Pressure monitors/bolts?
    • Deep brain stimulators?
    • Spinal cord stimulators?
    • Vagal nerve stimulators?
    • Cranial electrodes?
    • Carotid clamps?
    • Peripheral stimulators?
    • Epidural catheters?
  • Safety: Head & Neck >
    • Additional orbit safety?
    • Cochlear Implants?
    • Bone conduction implants?
    • Other ear implants?
    • Dental/facial implants?
    • ET tubes & airways?
  • Safety: Chest & Vascular >
    • Breast tissue expanders?
    • Breast biopsy markers?
    • Airway stents/valves/coils?
    • Respiratory stimulators?
    • Ports/vascular access?
    • Swan-Ganz catheters?
    • IVC filters?
    • Implanted infusion pumps?
    • Insulin pumps & CGMs?
    • Vascular stents/grafts?
    • Sternal wires/implants?
  • Safety: Cardiac >
    • Pacemaker dangers?
    • Pacemaker terminology?
    • New/'Safe" Pacemakers?
    • Old/Legacy Pacemakers?
    • Violating the conditions?
    • Prep/Consent for CIEDs
    • Epicardial pacers/leads?
    • Cardiac monitors?
    • Heart valves?
    • Miscellaneous CV devices?
  • Safety: Abdominal >
    • PIllCam and capsules?
    • Gastric pacemakers?
    • Other GI devices?
    • Contraceptive devices?
    • Foley catheters?
    • Incontinence devices?
    • Penile Implants?
    • Sacral nerve stimulators?
    • GU stents and other?
  • Safety: Orthopedic >
    • Orthopedic hardware?
    • External fixators?
    • Traction and halos?
    • Bone stimulators?
    • Magnetic rods?
• Copyright/Legal
  • Copyright Issues
  • Legal Disclaimers
• Forums/Blogs/Links
• Self-test Quizzes
  • Magnets & Scanners Quiz
  • Safety & Screening Quiz
  • NMR Phenomenon Quiz
  • Pulse Sequences Quiz
  • Making an Image Quiz
  • K-space & Rapid Quiz
  • Contrast & Blood Quiz
  • Cardiovascular & MRA Quiz

Aliasing What is aliasing?

Aliasing refers to the incorrect measurement of a signal's frequency due to an inadequate digital sampling rate. If a signal is not sampled using enough data points, its true frequency will be underestimated. This falsely estimated signal will be indistinguishable from (i.e., be an "alias" to) another signal having that true lower frequency. To properly measure a signal, the digital sampling rate must be at least twice the highest frequency contained within that signal. This is known as the Nyquist sampling theorem.

The aliasing phenomenon is not confined to MRI but is present in all types of technology, explaining audible distortions of sound, moire patterns in photos, and unnatural motion in cinema. The "wagon wheel effect" is a familiar example of aliasing. In this optical illusion, spokes on a wheel appear to rotate at different rates or even backwards depending on the digital frame rate of the video.

Aliasing is an important phenomenon in MR. It is the source of a common artifact (phase wrap-around) discussed in the next several Q&A's. Other types of aliasing artifacts include contamination of spectra from distant voxels in MRS and false measurement of flow velocities or flow direction in MRA. The aliasing phenomenon can used to advantage, however, forming the basis of parallel imaging techniques such as SENSE and GRAPPA.

Advanced Discussion (show/hide)»

No supplementary material yet. Check back soon! References "Aliasing". Wikipedia, the Free Encyclopedia. Oldhasusen BA. Aliasing. (Lecture notes for Vision Science course, UC Davis, 10 Oct 2000, available on-line) Related Questions Why does the phase wrap-around artifact occur?

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