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MRI Artifacts (introduction, classification, types)
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- 2. Introduction • There arenumerous kinds of artifacts that can occur in MRI. • Some effect the quality of the MRI exam. • Others may be confused with pathology. • Some artifacts can be mitigated by the MR Tech while others require an engineer.
- 3. Sources of Artifacts •Hardware Issues e.g. calibration, power stability • Software problems e.g. programming errors • Physiological phenomena e.g. blood flow • Physics limitations e.g. Gibbs and susceptibility
- 4. • MRI hardwareand room shielding • zipper artifact • herringbone artifact • zebra stripes • Moiré fringes • central point artifact • RF overflow artifact • inhomogeneity artifact • shading artifact • aliasing artifact (also known as wrap around artifact) • starry sky artifact • MRI software • slice-overlap artifact (also known as cross-talk artifact) • cross excitation
- 5. • Patient andphysiologic motion • phase-encoded motion artifact • ventricular CSF pulsation artifact • entry slice phenomenon • Tissue heterogeneity and foreign bodies • black boundary artifact • magic angle effect • magnetic susceptibility artifact • blooming artifact • chemical shift artifact • dielectric effect artifact • Fourier transform and Nyquist sampling theorem • Gibbs artifact/truncation artifact • zero-fill artifact • aliasing/wrap around artifact
- 6. Chemical Shift Artifact •Frequency-encoding direction • The different resonant frequency of fat & water is transformed into spatial difference. • Common in vertebral bodies, orbits, solid organs surrounded by fat. • Worst at higher field strength, less with stronger gradients.
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- 8. Aliasing or "Wrap-around" •Occurs when the field of view (FOV) is smaller than the body part being imaged causing the region beyond to project on the other side of the image. •Caused by undersampling in the phase or (rarely) frequency direction. •May occur in end slices of a 3D acquisition.
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- 11. Aliasing or "Wrap-around" • Correction: • Increase the FOV (decreases resolution). • Oversampling the data in the frequency direction (standard) and increasing phase steps in the phase-encoded direction – phase compensation (time or SNR penalty). • Swapping phase and frequency direction so phase is in the narrower direction. • Use surface coil so no signal detected outside of FOV.
- 12. Black Line Artifact •Anartificially created black line located at fat-water interfaces such as muscle-fat interfaces. •Occurs at TE when the fat and water spins located in the same pixel are out of phase, cancelling each other’s signal. Particularly noticeable on GE sequences. Both freq and phase direction. •At 1.5 Tesla, occurs at 4.5 ms multiples, starting at about 2.3 ms.
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- 14. Black Line Artifact •Mitigation: • Use in-phase TE’s • Fat suppression • Increase bandwidth or matrix size. • Aids in the diagnosis of benign conditions : • lipomatous hypertrophy of the interatrial septum • focal pancreatic fat • fat-rich renal angiomyolipoma • Aids in the identification of inflammation: • acute pancreatitis • mesenteric panniculitis • omental infarction
- 15. Gibbs or TruncationArtifact •Bright or dark lines that are seen parallel & next to borders of abrupt intensity change. May simulate a syrinx on sagittal image of spinal cord. •Related to the finite number of encoding steps used by the Fourier transform. •Mitigation: More encoding steps lessen the intensity and narrows the artifact.
- 16. Gibbs or TruncationArtifact 128x256 256x256
- 17. Zipper Artifacts • Mostare related to hardware or software problems beyond the radiologist control. May occur in either frequency or phase direction. • Zipper artifacts from RF entering room are oriented perpendicular to the frequency direction.
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- 19. Motion Artifacts • Brightnoise or repeating densities usually oriented in the phase direction. • Extend across the entire FOV, unlike truncation artifacts that diminish quickly away from the boundary causing them. • Examples: Arterial pulsations, CSF pulsations, swallowing, breathing, peristalsis, and physical movement.
- 20. Motion Artifacts • Mitigation: •Arterial and CSF pulsation artifacts can be reduced with flow compensation and cardiac gaiting. • Spatial presaturation can reduce some swallowing and breathing artifacts and arterial pulsations. • Surface coil localization can reduce artifacts generated at a distance from the area of interest.
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- 22. Slice-overlap (cross-slice) Artifacts •Loss of signal seen in an image from a multi-angle, multi-slice acquisition. • Same mechanism as spatial presaturation for reduction of motion and flow artifacts. • Example: Two groups of non-parallel slices in the same sequence, e.g., L4-5 and L5-S1. The level acquired second will include spins that have already been saturated.
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- 25. Slice-overlap Artifacts • Correction: •Avoid steep change in angle between slice groups. • Use separate acquisitions. • Use small flip angle, i.e. GE sequence
- 26. Cross-talk Artifact • Resultof imperfect slice excitation, i.e. non-rectangular, of adjacent slices causing reduction in signal over entire image. • May be reduced by using gap, interleaving slices and optimized (but longer) rf pulses.
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- 28. Magic Angle Effects •Seen most frequently in tendons and ligaments that are oriented at a 55o angle to the main magnetic field. • Normal dipolar interactions between the H+’s in water molecule aligned in tendons shortens T2, causing loss of signal. • The dipolar interactions go to zero at about 55o increasing the signal.
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- 30. Entry slice (Inflow)artifact • Unsaturated spins in blood or CSF entering the initial slices results in greater signal than reduces on subsequent slices. • May be confused with thrombus. • Can use spatial saturation to reduce. • Mechanism for TOF angiography.
- 31. Entry slice (Inflow)artifact
- 32. Field inhomogeneity •Types: •Main magneticfield •RF coil inhomogeneity •Dielectric effects – worst at 3T+ •May cause variation in intensity across image •May cause non-uniform fat suppression
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- 34. Field inhomogeneity- RFcoil •
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- 36. Field inhomogeneity • Mitigation: •Shimming, area of interest in near isocenter • Use STIR for Fat sat vs. Chess. Caution with Gad. • Coil – Use volume vs. surface coil, allow space between coil and body. • Dielectric – use phased array coils, software compensation
- 37. RF Overflow Artifacts(Clipping) • Causes a nonuniform, washed-out appearance to an image. • Occurs when the signal received from the amplifier exceeds the dynamic range the analog-to-digital converter causing clipping. • Autoprescanning usually adjusts the receiver gain to prevent this from occurring.
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- 39. Moire Fringes •Moire fringesare an interference pattern most commonly seen when doing gradient echo images. •One cause is aliasing of one side of the body to the other results in superimposition of signals of different phases that add and cancel. Can also be caused by receiver picking up a stimulated echo. •Similar to the effect of looking though two window screens.
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- 41. Central Point Artifact •A focal dot of increased or decreased signal in the center of an image. • Caused by a constant offset of the DC voltage in the amplifiers.
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- 43. Central Point Artifact •Correction: • Requires recalibration by engineer • Maintain a constant temperature in equipment room for amplifiers.
- 44. Quadrature ghost artifact •Another amplifier artifact caused by unbalanced gain in the two channels of a quadrature coil. Combining two signals of different intensity causes some frequencies to become less than zero causing 180 degree “ghost.”
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- 46. Susceptibility Artifacts • Variationsin the magnetic field strength that occurs near the interfaces of substance of different magnetic susceptibility such as ferromagnetic foreign bodies. • Causes dephasing of spins and frequency shifts of the surrounding tissue.
- 47. Susceptibility Artifacts • Worstwith long echo times and with gradient echo sequences. • Worst at higher magnetic field strength. • Less with fast/turbo spin echo sequences.
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- 50. Zebra Artifacts • Band-like,usually oblique stripes. • Data in the K-space array will be missing or will be set to zero by the scanner or an electrical spike may occur as from static. • The abrupt change from signal to no signal or normal signal to high signal results in artifacts in the images.
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- 52. Eddy Current Artifacts •Varying magnetic field from gradients can induce electrical currents in conductors such as the cryostat causing distortion of the gradient waveforms. • Particularly a problem with echo-planar imaging that uses strong, rapidly changing gradients.
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- 54. Eddy Current Artifacts •Mitigation: • Precompensation- A “distorted” gradient waveform is used which corrects to normal with the eddy current effects. • Shielded gradients – Active shielding coils between gradient coils and main gradients.
- 55. Diastolic Pseudogating •Change inintensity of blood in large vessel such as the aorta from slice to slice when there is synchronization of the cardiac cycle and the pulse sequence, i.e., repetition rate = heart rate (TR=1/HR) •Synchronization of the cardiac cycle and the pulse sequence results in high signal in the artery during diastole when blood is relatively stationary and loss of signal during systole when flow is high.
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- 57. Gadolinium “Pseudolayering” • Threedensity layers in the bladder after Gd • Low conc. Gd top layer = dark • Med conc. Gd middle layer = bright • High conc. Gd lower layer near ureters = dark • T2 shortening overshadows normal T1 effects at high concentrations.
- 58. Gadolinium “Pseudolayering” Left imagefrom A. Elster’s book