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Torques Artifacts

Not Compatible

MR Safe

MR Compatible

Yes

No

Yes

Large

No Small

MR CompatibilityMR Compatibility

There are three basic levels of MR compatibility, as shown in the table below. Incompatible devices are dangerous to take into the MR suite. MR safe devices can be taken into the suite, but may have large artifacts on MR images. MRcompatible devices have neither torques nor large artifacts.

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Arrowhead artifact obtained when the needle is | B0 and the image plane is | to the needle.

Artifacts obtained when the imaging plane is || to the needle.

Typical artifactsTypical artifacts

These images show typical needle artifacts for three differentneedles. Also shown is an egg (left) and a bottle of oil (right).The needles are placed through a perforated plastic sheet, seen in the lower images.

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Needle artifacts overviewNeedle artifacts overview

Factors that affect the artifact on MR images include:• Magnetic susceptibility of the material ()• Size of device• Field strength of the MR scanner• RF refocusing• Orientation of the needle wrt Bo

• Orientation of frequency encoding direction• Receive bandwidth, if an RF-refocused sequence is used• Echo Time, if a gradient recalled echo sequence is used

In the following pages, you can find examples of most of these effects.

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DiamagneticDiamagnetic ParamagneticParamagnetic FerromagneticFerromagnetic

TitaniumTitanium

'Nonmagnetic'Stainless Steel'Nonmagnetic'Stainless Steel

Magnetic Stainless SteelMagnetic Stainless Steel

Pure IronPure Iron

-1-1 -10-1-10-1 -10-2-10-2 11 105105

10-510-5-10-5-10-5

Magnetic susceptibilityMagnetic susceptibility

Water

Copper

All materials have a property called magnetic susceptibility (). Devices of ferromagnetic materials are generally incompatible. Some non-magnetic stainless steels are MR safe. Paramagnetic materials such as titanium are MR compatible. Alloys such as Inconel have a net magnetic susceptibility in the mid para-magnetic range. A longer list of magnetic susceptiblities can be found in Schenk et al. Med Phys.

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Material and sizeMaterial and size

22 20 18 16 14Gauge:

Stainless Steel

High Ni, low alloy

Inconel

Titanium alloy

This picture is pretty selfexplanatory.

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Field strengthField strength

Artifacts are bigger at higher field strength, as you can see in the images below. These were taken with the same receive bandwidth. Not only are the in-plane shifts larger at the higherfield stregth, but there is also more potato-chipping of the slicearound the needles at high field. This results in signal modulationin the lower left image.

0.5T

1.5T

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GRE

FSE

Freq | Needle

FSE

freq || Needle

RF refocusingRF refocusing

With gradient echo sequences, there is

signal dropout. With RF refocused

sequences, the artifact is more

complicated and interesting.

Mostly, images from an RF

refocused sequence (FSE) are shown in

this presentation.

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Orientation wrt/ BOrientation wrt/ B00

The needle orientation wrt/ B0 has a huge effect on the artifact. In the top row, the needle is perpendicular to B0, while in the bottom row, the needle is parallel to B0. In the needle parallel to B0 orientation, the needle shaft is perfectly depicted, while there is artifact off the end of the needle. Off resonance spins are shifted in the direction of frequency encoding.

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Frequency encoding directionFrequency encoding direction

When frequency encoding is along the direction of the needle, the spins are distorted along the length of it (grey arrows). This is evident by the distortion of the perforated plastic sheet. When frequency encoding is perpendicular to the needle, the artifact is broadened.

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Receive bandwidthReceive bandwidth

Higher receive bandwidths reduce the artifact on RF-refocusedsequences.

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Again, higher receive bandwidths result in a smallerartifact on rf-refocused sequences. In gradient echo sequences, there is signal dropout dependent on echo time andindependentof the BW.

Receive bandwidthReceive bandwidth

FSE

SE

SPGR

4 8 16 32 64Bandwidth

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Echo time in GREEcho time in GRE

These images mimic echo times of .5 ms, 1 ms and 2 ms. This was done by offsetting the gradient recalled echo from the spin echo. The piling up artifact is progressively more dephased with echo offset, just as it would be with longer echo time GRE.

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Example incompatible needle artifactExample incompatible needle artifact

Incompatible21g needle

MR CompatibleTrocar

Compatible21g needle

These examples were acquired with a GRE sequence.

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View angle tiltingView angle tilting

View angle tilting (VAT) compensates for shifts of off-resonance spins during readout with shifts during slice selection. Signal appears to be registered properly. VAT does not compensate for potato chipping of the slice. That’s why the needle artifact isn’t

perfectly circular in the lower right picture.