Artifacts & Flow Phenomena

Spatial Inversion Recovery (SPIR)

A hybrid fat-suppression technique that combines chemical pre-saturation and STIR. A 180 pre-saturation RF pulse at the precessional frequency of fat is applied to the imaging volume. After a time TI corresponding to the null point of fat, the 90 excitation pulse is applied. Can be used after Gad is given (unlike STIR) Reduces aliasing

Out of Phase artifact (Chemical Misregistration)

A ring of dark signal around certain organs where fat and water interfaces occur within the same voxel Occurs because of the precessional frequency difference between fat and water Remedy: Select a TE that matches the periodicity of fat and water at your field strength

SENSE ghost artifact

A type of aliasing artifact that occurs in parallel imaging when the reconstructed FOV is smaller than the imaged object Fold-over appears in the middle (and sometimes edges) of the image along the phase-encoding direction Looks similar to wraparound/ aliasing, but aliasing starts at the edge of the FOV and works in, while SENSE ghost starts in the center of the FOV

Moire artifact

Black and white banding artifact on the edge of the FOV Always seen in gradient echo Cause: Inhomogeneities; interference of aliased signals at different phases Remedies: Use spin echo, ensure pt's arms are within the FOV

Vortex flow

Blood flow that is initially laminar, then intersects a vessel stenosis or stricture, becoming high velocity central flow and spiral near the walls of the vessel

Remedies for phase mismapping artifact

Change phase encoding direction Use pre-saturation pulses Use gradient moment nulling Use respiratory compensation techniques and/or cardiac gating Increase NEX

Entry slice phenomenon

Contrast difference between flowing and stationary nuclei because the flowing nuclei are 'fresh' - they haven't been exposed to repeated RF pulses Fresh blood flowing into a volume of tissue results in high intravascular signal at the end slices of a multisection acquisition (vessels appear bright at the beginning slices of a 'stack') Remedies: Thicker slices, shorter TR, use spatial pre-saturation

Flow-related enhancement

Decrease in TOF phenomenon (decrease in signal void) Occurs when protons flowing into the imaging volume have received both RF pulses → increased signal intensity → vessel appears bright

Compensation for eddy currents

Design methods that interrupt potential current loops (use of slotted coils and shields) Active RF shielding Pre-emphasis (modifying the input current to the gradients to account for expected eddy-current distortions) Image post-processing (to correct for spatial nonlinearities and frequency/phase shifts due to eddy currents)

Aliasing / Wrap-Around artifact

Direction: frequency and phase Caused by undersampling (Occurs when a signal is sampled at a rate that is not at least 2x greater than the band of frequencies contained within that signal (Nyquist theorem))

Zipper artifact

Direction: phase and frequency Caused by a leak in the RF shielding Remedy: Close the door

Magnetic Susceptibility artifact

Distortion of the image with large signal voids Caused by metal within the imaging volume and/or naturally-occurring iron content of hemorrhage.

Remedies for magnetic susceptibility artifact

Don't use gradient echo Use long ETLs in FSE Use STIR instead of fat suppression Don't use parallel imaging Decrease the TE Use wide receive bandwidth (to also reduce the TE) Decrease slice thickness Increase matrix/decrease pixel size Increase NEX

Eddy currents

Electrical currents induced by rapidly-changing gradient and RF fields Always produced when imaging is performed (bc MR uses rapidly changing magnetic fields to generate and spatially define the signal) Are induced in any nearby conducting media, such as within the gradient coils themselves, the main magnet and shim coil windings, cryoshields, liquid helium vessel, and RF shields. Create two undesired phenomena: unwanted time-varying gradients and shifts in the main magnetic field (inhomogeneity) May also produce biological effects such as tissue heating or peripheral nerve stimulation

Saturation pulse

Eliminates signal from specific regions of unwanted anatomy Depicted as signal voids A 90* pulse followed by a spoiler pulse

Even-echo rephasing

Flow compensation technique/phenomenon that that occurs in spin echo sequences. It uses multiple evenly-spaced echoes to reduce intra-voxel dephasing. Even-numbered echoes demonstrate less dephasing (and have higher vascular signal) than odd-numbered echoes (which have decreased vascular signal)

Which of the following, in an MRA sequence, aids in minimizing the loss of signal due to dephasing within a voxel: I. Long TE II. Short TR III. Smaller voxel size IV. Short TE

III and IV - smaller voxels and shorter TE

Remedies for aliasing artifact

Increasing FOV size Placing pre-saturation bands onto areas outside the FOV that may wrap into the image Use SPIR Anti-aliasing software along the frequency and/or phase axis

Turbulent flow

Irregular flow with random variations in pressure/velocity

Truncation / Gibbs artifact

Multiple fine lines parallel to sharp tissue interfaces (interfaces of high and low signal) Caused by undersampling and using Fourier transforms Remedy: avoid undersampling → increase the number of phase encoding steps.

Parallel imaging artifacts

Occur when the FOV is too small for the acceleration factor selected / when the acceleration factor is too high for the parameters and FOV selected. Artifacts include noise, motion, and SENSE ghost. Motion looks slightly different compared to non-parallel imaging. Direction: phase or slice select Remedies: reduce the acceleration factor, increase FOV, change the phase encoding direction

Cross excitation artifact

Occurs in subsequent, consecutive slices in a multislice acquisition; occurs primarily in STIR Caused by signal loss due to partial saturation in adjacent slices Remedy: use interleaving/concatenation

Partial volume averaging artifact

Occurs when multiple tissue types are contained within a single voxel. Direction: slice select Remedy: *Decrease voxel size* → *thinner slices*, smaller FOV, higher image matrix; Avoid using "fast-RF" pulse options

Crosstalk / Slice Overlap artifact

Occurs when slices overlap or are too close together; imperfect slice profiles Remedies: increase slice gap, acquire images as a stack, use interleaving/ concatenation

In what directions can each of the artifacts occur? Parallel imaging Partial volume averaging Zipper Chemical shift Wrap Ghosting

Parallel imaging - phase & slice select Partial volume averaging - slice select Zipper - phase & frequency Chemical shift - frequency Wrap - phase & frequency Ghosting - phase

Dieletric effect

Produces inhomogeneity (shading) artifacts Abnormal bright and dark areas Especially seen in 3T body imaging with large FOV Caused by Bo field inhomogeneity; Caused by eddy currents due to the increased conductivity of body tissue Remedies: *use lower field strength systems*, use dielectric pads, decrease FOV

Intra-voxel dephasing

Reduction of total signal amplitude from a voxel due to the phase difference between flowing and stationary nuclei within the voxel.

Phase Mismapping / Ghosting artifact

Replications of moving anatomy across the image in the phase encoding direction Caused by anatomy moving along the phase encoding gradient during the pulse sequence (i.e. blood flowing through vessels, breathing, etc.)

Remedies to reduce intra-voxel dephasing

Smaller voxels Shorter TE Use gradient echo (bc of the shorter TE and rephasing pulse) Use even-echo rephasing Use gradient moment rephasing/nulling

Remedies for chemical shift artifact

Swap the phase-encode and frequency-encode directions Scanning at lower field strengths Keep FOV to a minimum Use widest receive bandwidth possible

Anti-aliasing / Anti-foldover / No Phase Wrap

Used to eliminate aliasing/wrap artifact Oversamples along the phase encoding axis by increasing the number of phase encodings performed. Increases FOV in phase direction and increases scan time

Gradient moment rephasing/nulling / Flow Compensation

Uses additional gradients to correct altered phase values of flowing nuclei → compensates for intra-voxel dephasing Only effective on constant velocity flow (peripheral small vessels, CSF, etc). Does not work on larger arteries with faster, turbulent flow (veins and small arteries will be bright, but larger arteries with faster, turbulent flow will still be dark)

Spatial pre-saturation

Using additional 90* RF pulses to null the signal from flowing nuclei and/or specific tissues (two types of this are chemical pre-saturation and spatial inversion recovery (SPIR)) RF pulses can be applied to tissue inside the FOV (to reduce artifact from aorta) or outside the FOV (to reduce artifact from nuclei flowing into the FOV) Reduces the effects of entry slice phenomenon, TOF phenomenon, phase mismapping/ghosting, and aliasing/wrap

Chemical pre-saturation

Using pre-saturation pulses to null signal from a specific tissue, usually fat or water A 90* RF pulse matching the precessional frequency of a specific tissue is applied to the entire imaging volume/FOV before excitation to null that tissue

Time of flight phenomenon

Variations in signal / brightness of a vessel due to the flow of protons in & out of the imaging volume

Chemical Shift artifact

White and dark bands flanking an object along the frequency-encoding direction Caused by the different precessional frequencies of fat and water

Magic angle artifact

Appears as high signal intensity in tissues that contain collagen (i.e. tendons) in sequences with a short TE. May mimic pathology Occurs when structures containing collagen lie at a 55* angle to the main field, and short TEs are used (doesn't occur in T2W images) Remedy: Alter the position of the anatomy, use sequences with longer TE (T2W)

Flow phenomena

Artifacts produced by flowing nuclei

Spiral flow

Blood flow where the direction of flow is spiral

Laminar flow

Flow that is at different but consistent velocities across a vessel Smooth flow

High velocity signal loss

Increase in TOF phenomenon (increase in signal void) Occurs when nuclei flow out of the imaging volume before receiving both RF pulses → decreased signal intensity → vessel appears dark Remedy: thicker slices