1. MR Compatibility Torques Artifacts Not Compatible Yes Yes MR Safe No
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. MR
compatible devices have neither torques nor large artifacts.
Torques
Artifacts
Not Compatible
Yes
Yes
MR Safe No
Large
MR Compatible No
Small

2. Typical artifacts 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.
These images show typical needle artifacts for three different
needles. 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.

3. Needle artifacts overview
Factors that affect the artifact on MR images include:
• Magnetic susceptibility of the material (c)
• 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.

4. Magnetic susceptibility
All materials have a property called magnetic susceptibility (c).
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.
Diamagnetic
Diamagnetic
Paramagnetic
Paramagnetic
Ferromagnetic
Ferromagnetic
Magnetic T
itanium
Magnetic
Water T
itanium
Stainless Steel
Stainless Steel 5 -1 -1
-10
-10 -1 10 5 -1
-10
-10 -2 -2
-10
-10 -5 10 -5 10 -5 -5 1 1 10
'Nonmagnetic'
'Nonmagnetic'
Copper
Stainless Steel
Stainless Steel
Pure Iron
Pure Iron

5. Material and size Gauge: 22 20 18 16 14 Stainless Steel
This picture is pretty self
explanatory.
Gauge: 22 20 18 16 14
Stainless Steel
High Ni, low c alloy
Inconel
Titanium alloy

6. Field 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 higher
field stregth, but there is also more potato-chipping of the slice
around the needles at high field. This results in signal modulation
in the lower left image.
0.5T
1.5T

7. RF refocusing GRE FSE Freq | Needle FSE freq || Needle
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.
GRE
FSE
Freq | Needle
FSE
freq || Needle

8. Orientation wrt/ B0
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.

9. Frequency 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.

10. Receive bandwidth
Higher receive bandwidths reduce the artifact on RF-refocused
sequences.

11. Receive bandwidth Bandwidth 4 8 16 32 64 SE FSE SPGR
Again, higher receive bandwidths result in a smaller
artifact on rf-refocused sequences.
In gradient echo sequences,
there is signal dropout
dependent on
echo time and
independent
of the BW.
Bandwidth 4 8 16 32 64 SE
FSE
SPGR

12. Echo 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.

13. Example incompatible needle artifact
These examples were acquired with a GRE sequence.
MR Compatible
Trocar
Incompatible
21g needle
Compatible
21g needle

14. View 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.