Artifacts

Lower field strength systems are defined as those operating below

1 T

High field strength systems are defined as those operating below

1-3T

Ultra-High field strength systems are defined as those operating below

7T

Digital magnetic resonance imaging systems realize an SNR boost of

30-40%

Those at higher risk for SAR thermal heating include

obese, hypertensive, geriatric, and diabetic patients, patients on sedatives, beta blockers, and diuretics

function of the gradient magnetic field

create the image orientation and the spatial resolution (detail) used to distinguish adjacent or abnormal structures

slice select gradient

main function of determining slice thickness, scan plane and orientation of a pulse sequence

frequency encoding gradient

responsible for the encoding in the long axis of anatomy, is not a direct contributor of scan time, and is on during the sampling of the MR signal echo

frequency encoding gradient is on when

during the sampling of the MR signal echo

phase encoding gradient

plays a pivotal role in shaping the image detail and scan time, as this gradient switches on and off to fill lines of k-space.

Common methods of scan time shortening include

fast spin echo sequences, halfscan, rectangular field-of-view (rFOV), and parallel imaging

each additional 180° RF pulse in this sequence implements ______ times the SAR power compared to the initial 90° RF excitation pulse

four

Halfscan

scan time shortening technique whereby the phase encoding process is shortened using assumed filling of k-space. ~60%

specific location where aliasing artifacts arise

the outer edges of each image

where the data misregistration of anatomical location in the phase direction takes place, outermost edges of the image, working inward based on the severity of the phase direction undersampling

aliasing corrective measures

“no phase wrap” or “anti-aliasing” options, or an increase in phase dimension sampling

Parallel Imaging

takes advantage of RF phased array coil elements and channels to sample data in parallel

how does parallel imaging reduce scan time

how does reducing number of times phase encoding steps performed reduce scan time

Higher parallel imaging factors result in

shorter scan times, slightly reduced SNR and greater potential for artifacts

The two main advantages of selecting parallel imaging options are

the reduction in gradient switching and the lack of additional RF power required performing the acceleration technique

Two main approaches to parallel imaging are rooted in

the method of data collection

Image based Methods of parallel imaging

that reconstruct images from each coil element within a reduced FOV then merge the images with the knowledge of collected coil sensitivities (reference scan (GRE))
ASSET, SENSE, mSENSE, Speeder, Rapid

K-space based methods of parallel imaging

that explicitly calculate missing kspace lines before Fourier transformation of the raw data
SMASH, GRAPPA/iPat,

Vendor Parallel Imaging GE

ASSET: Image

SMASH: K-space

Vendor Parallel Imaging Phillips

SENSE: Image

Vendor Parallel Imaging Siemens

GRAPPA / iPat: K-space

mSENSE: Image

Vendor Parallel Imaging Toshiba

Speeder: Image

Vendor Parallel Imaging Hitachi

Rapid: Image

prerequisites exist for parallel imaging techniques to be considered

a gradient echo based reference scan or calibration scan is prerequisite to collect tissue data within phased array coil element, dedicated, RF phased array coil

dedicated, RF phased array coil

contains multiple elements of small RF coils, generating larger anatomic coverage capabilities

parallel imaging artifcsats look like

resulting in a mis-representation of the patient’s tissue, typically in the central portions of the image

Parallel imaging artifacts occur when

the FOV is too small for the parallel imaging factor selected,

parallel imaging reduction factor too high for the parameters and FOV selected

Parallel imaging artifacts occur in what direction

phase encoding direction, slice select dimensions

corrective measures for parallel imaging

Increase FOV or reduce parallel imaging factor

what cases should you be careful when reducing parallel imaging factor

abdominal MRI because of breath holds

Parallel Imaging artifact Corrective Measures

increase FOV maintain voxel size, increase FOV maintain phase matrix, decrease parallel imaging factor

In parallel imaging if you Increase FOV and maintain voxel size

increased scan time, no change on detail, longer scan time with increased matrix

In parallel imaging if you increase FOV maintain phase matrix

no change scan time, less detail, Maintained scan time with decreased spatial resolution (detail)

In parallel imaging if you Decrease Parallel Imaging Factor

Increased scan time, Unchanged detaill, Increased SAR absorption, Longer scan time

what intrinsic factor does not affect chance of parallel imaging factor from showing up

Using a system with lower field strength or fewer channels does not increase the chance for artifact potential

parallel imaging artifacts in DWI look like

reduce the susceptibility effects in diffusion weighted images, especially around air-tissue interfaces

Parallel imaging artifacts in the slice select direction are found where

in the first and last slices of image acquisition in a 3D volume data set

Parallel imaging artifacts in the slice select direction are reduced by and what are the tradeoffs

reduction in Slice direction parallel imaging factor, which will result in longer scan times and improved signal-to-noise ratios

Any reduction in the parallel imaging R factor will result in

more signal, less noise and fewer centrally located artifacts.

what prohibits use of parallel imaging

stimulators that need T/R coils, metal artifact reductio

Certain patients that have surgically implanted stimulators (such as a vagus nerve stimulator, or certain spinal cord stimulators) can only be imaged with and why

a dedicated, transmit-receive coil
its not equipped with multiple RF coils, nor can parallel

bariatric pts and using parallel imaging

increased SAR possibilities

artifacts

signal misrepresentations that do not correspond to the spatial location of the specific tissue imaged.

primary contributor of noise in an MRI system

pt

secondary contributor of noise in an MRI system

system equipment

motion

smearing across the image in the phase encoding direction

what can reduce motion artifact

Triggering, radial k-space acquisition techniques, reduction in signals averaged, or utilization of parallel imaging techniques

Chemical shift artifact occurs because

fat and water precess at different frequencies

chemical shift occurs where

along the frequency axis

what is used for the reduction of chemical shift artifacts

higher bandwidth

Chemical shift misregistration artifact looks like

bright or dark outlines predominantly at fat / water interfaces

chemical shift is present in what body part mostly

kidney, optic nerve or small disc herniation

Higher field strengths will create more chemical shift artifacts due to

the increased separation of fat and water frequencies.

Inhomogeneities can lead to

poor fat suppression, limits in field of view and challenges in off-isocenter imaging especially in extremity MRI procedures.

Increasing parallel imaging factors can decrease ________ where ______

susceptibility artifacts around air tissue interfaces, such as temporal bone/auditory canal regions.

why is GRE more susceptible to metal artifacts

due to the lack of 180° refocussing pulse that would reduce field inhomogeneities.

When the TE is shortened, susceptibility artifacts ____ bc _______

decrease because shortened TE's allow less time for dephasing thus reducing signal loss.

To decrease magnetic susceptibility

utilize smaller voxels, shorter TE, and increased receiver bandwidth, use FSE and SE bc lots of 180 pulses correct for inhomogeneities and local field distortions

Gradient moment nulling (or flow compensation)

correcting for constant velocity (first order) motion distortion in standard spin echo or gradient echo pulse sequences

flow compensation used where

used in a cervical spine MRI to minimize CSF flow artifacts

how does GMN work

In spin echo and gradient echo sequences, the first moments of the imaging gradients are zeroed at the echo time to cancel out the accumulation of phase due to movement

goal of GMN

minimize the phase shifts obtained by the transverse magnetization of excited nuclei moving along the gradients (including the effect of refocusing RF pulses), useful in reducing image artifacts due to fluid motion

first order motion exs.

constant velocity (peripheral small vessels, CSF, etc).

zipper artifact

leak in the RF shielding can appear as a zipper artifact in the phase or frequency encoding direction. Ensure adequate door suction in the MR scan room

to correct for Gibbs artifact

increase the number of phase encodings or reduce the FOV (while maintaining matrix)

truncation can mimic what pathology

syrinx in cervical spine

gibbs truncation or ringing artifact

seen as bright or dark lines parallel and adjacent to borders of distinct intensity change

how does gibbs artifact occur

when a small matrix is used, incompletely digitizing the echo by the end of the acquisition.

Partial volume averaging

when multiple tissue types are contained within a single voxel

wshat slices contribute to partial volume averaging

Thicker slices contribute more heavily to partial volume averaging

where does partial volume averaging take place

slice select direction of the image

A ____ in the voxel volume yields a _____ in partial volume averaging artifacts, with ______ slice thickness being the easiest solution.

decrease, decrease, thinner

Dielectric effects caused by

caused by local eddy currents due to the increased conductivity of body tissue

dielectric effect looks like

areas of shading or signal loss

Cross talk artifact, slice overlap artifact, occurs when

slices overlap or are too close together

what does cross talk create and where is it found

“band” of signal loss, usually found posteriorly and working its way anterior towards the spinal canal in a lumbar spine, multi-stack axial acquisition

The severity of the cross talk is directly proportional to

the amount of slice overlap.

Corrective measure for cross talk

increase gaps between the slices, to reduce the slice overlap, or utilize a “stacks as packages” function on the scanner

how can BLADE / Propeller / MultiVane help with artifacts

technique of filling k-space aids in correcting patient motion, flow artifacts, and, according to this article, correct for crosstalk artifacts.

Cross excitation artifacts deal with

subsequent, consecutive slices in a multi-slice acquisition

Cross excitation artifacts due to

signal loss due to partial saturation in adjacent slices. This is due to the partial excitation and subsequent saturation of adjacent slices in multi-slice imaging.

what sequences does cross excitation occur most

STIR

guidelines for metal artifact reduction

 Use Fast Spin Echo in place of Gradient Echo sequences, the 180º RF refocusing pulses will help to correct for metal susceptibilities

 Do Not Use Parallel Imaging Techniques or Image Uniformity correction options as they require Gradient echo reference scans

 Use high TSE Factors (long ETL) in Fast Spin Echo, the 180º RF refocusing pulses will help to correct for metal susceptibilities [27, 28]

 Use a wide receiver Bandwidth (rBW) / small Water Fat Shift to reduce the chemical shift, minimize the metal artifact [27, 28]

 Use a thin slice thickness to reduce the partial volume artifact, thus reducing the amount of metal artifact

 Use STIR sequences in place of Fat Suppression to further enhance the reduction of metallic susceptibility artifacts

 Maintaining good SNR is key to adequate MARS imaging; increase NSA/NEX to add signal, MARS sequences are not fast!!

Metal Artifact Reduction Summary

• Use TSE over Gradient Echo

• Use STIR instead of Fat Suppression techniques

• Utilize Wide rBW (receiver Bandwidth)

• No parallel imaging or image uniformity correction

• Thin slices

• Small pixel size/large matrix

• High TSE factor/ ETL

• Add NEX/NSA to buy back signal, restore IQ

moire fringes artifact caused by

interference of aliased signals at different phases

moire fringes artifact seen in

3D gradient echo images at higher field strengths, when larger field of views are utilized

moire fringe aka

zebra artifact

what can correct for FID artifacts

Increase in NEX and/or the enabling of Flow Compensation

fine-line artifacts look like and originate from

give the appearance of Gibbs-like artifacts,
originate from an incompletely crushed FID signal from an RF refocusing pulse

where are FID artifacts common in

FSE / TSE sequences because of their pulse sequence design and their high SNR

contributing factors to fine-line artifacts

reduced refocusing pulses, increases in FOV, and decreases in slice thickness

magic angle arttifact occurs when

a tendon or ligament is positioned approximately 55° to the direction of the main magnetic field, and a short TE value pulse sequence is utilized., sometimes longer flip angles

confirmation of magic angle artifact

confirmation of magic angle artifact, instead of tendinopathy, is made by the utilization of a long TE sequence, where the long T2 relaxation time of the collagen fibers will not yield high signal in the tendon at the 55° angle to the magnet.

annefact or cusp artifacts come from

active RF coil channels or elements outside the scanning field of view

annefact artifacts happen in what imaging

spine