Transcript Managing the Magnet:

Managing the Magnet:
A Prequel to Pressing the Scan Button
Authors:
J Patel MD, CM Glastonbury MBBS, D Arnold RT (MR),
J Morel RT (MR), A Srinivasan MD
Control #: 492
eEdE#: eEdE-80
ASNR 2015 Annual Meeting
Disclosures
The authors have no financial disclosures.
Philips hardware is demonstrated in several cases because of
the authors’ experience with Philips equipment at their
institution.
For detailed information regarding hardware set-up, usage and
safety of their own scanner, viewers should refer to vendor
product manuals and/or consult support staff.
Plan

Introduction to MR safety

Discussion of coil types and advances

Elaborate on time saving strategies in MR
imaging
The MR Environment



The MR environment is the area around an MR scanner
and can be hazardous, even deadly, if unsafe materials
or devices enter it.
It poses safety risks because of:
 Static magnetic field & spatial gradients
 Rapidly changing magnetic fields
 Rapidly changing radiofrequency (RF) pulses
The highest risk areas are those within a 5 gauss (0.5
mT) magnetic field line of the scanner(s).
FDA primer document (1997)
How Can MR Harm?
Static
Magnetic Field
(Always on)
Rotational Forces
i.e. Torque
Translational Force
(from spatial gradient)
Gradient
Magnetic Field
Induced Currents
TEARING OF
TISSUE from
device rotation
Tearing of tissue
or acceleration
of object
(MISSILE
EFFECT)
DEVICE
MALFUNCTION
FDA primer document (1997)
How Can MR Harm?
Induced Currents
causing Heating
BURNS
(thermal or electric)
Electromagnetic
Interference
(in objects with a power source)
POSSIBLE
DEVICE
MALFUNCTION
Radiofrequency
Pulses
MR Safety

To delineate the area and maximize patient and
personnel safety, the MR environment is divided into
4 safety zones.

All patients and any non-MR personnel entering the
MR environment must go through MR safety screening
before passing beyond Zone 2.
ACR guidance document on MR safe practices (2013)
Zones 1 and 2
Zone 1



Freely accessible to general public.
e.g. Hallway outside reception area.
Magnetic fringe fields are < 5 gauss (0.5 mT), so there
is no magnet-related hazard.
Zone 2



Separates general public area (Zone 1) from areas
under stricter control.
Includes reception, waiting, and changing rooms.
Patients are screened here, and are under the general
supervision of MR personnel.
Zones 3 and 4
(“The MR Environment”)
Zone 3



Only authorized personnel and screened patients can
enter - access should be physically restricted.
MR control room is in this zone.
Magnetic field may exceed 5 gauss (0.5 mT).
Zone 4


DANGER
This is the magnet room, and the most hazardous area
due to the magnetic field strength.
Screened patients must be under the constant
supervision of personnel trained for Zone 4.
Zone 4 Labeling

All portable objects and devices taken into Zone 4 must
be labeled following criteria developed by the American
Society for Testing and Materials (ASTM) and used by
the U.S. Food & Drug Administration (FDA).

They should be classified as MR safe, MR conditional
or MR unsafe before entering Zone 3.

Never assume MR safety/compatibility without written
documentation!
Zone 4 Labeling
MR SAFE
*ASTM labeling used by FDA
Objects that are wholly non-metallic and made from
material known to be safe in the MR environment.
MR CONDITIONAL
Metallic objects that pose no hazard only if specific
conditions are met. There are numerous condition
categories.
MR UNSAFE
Objects that are hazardous due to ferromagnetism
or other MR environment interactions (e.g. induced
currents in looped leads)
ACR guidance document on MR safe practices (2013)
What is MR Conditional?

Determining MR conditional status must be meticulous.

There are multiple determinants of conditional safety
beyond field strength (i.e. 1.5T vs. 3T).

References for safety documentation should be used
(e.g. www.mrisafety.com and www.magresource.com).
What is MR Conditional?
Specific Determinants of MR Conditional Status can include:
 Magnetic field strength
 Directionality of field (“open” vertical vs. closed horizontal)
 Time rate of change of the magnetic field
 Duration of active scanning
 Radiofrequency (RF) fields
 Specific absorption rate (SAR)
 Configuration of device and time since implantation
www.mrisafety.com
Specific Absorption Rate

Specific absorption rate (SAR) is a measure of the
deposition of electromagnetic energy in the body
(units = watts/kilogram).

FDA warns of significant risk for harm for whole
body SAR > 4 W/kg averaged over 15 minutes, and
head SAR > 3.2 W/kg averaged over 10 minutes.

Devices can have different SAR conditional safety
requirements.
FDA guidance document (2014)
Specific Absorption Rate
For the Reveal XT Insertable Cardiac Monitor, head
SAR must be ≤ 3.2 W/kg (matching FDA requirement).

Image reproduced with permission of Medtronic, Inc.

But note that whole body SAR must be ≤ 2.0 W/kg
(stricter than FDA figure).
Reveal XT product manual (2013)
Safety: Closed vs Open Magnets

Different inherent characteristics of closed and open
magnets can have implications for safety.

Closed and open magnets differ in the directionality of
their maximum field gradient lines and spatial
distribution of the field.

A device could be conditionally safe for a 1.5 or 3
Tesla closed magnet, but actually unsafe for a 1.0
Tesla open magnet.
Field orientation: Closed vs Open
Classic cylindrical “closed” magnets
have a horizontally-oriented maximum
field gradient line centrally through the
isocenter.
“Open” magnets have a verticallyoriented maximum field gradient
line centrally.
Implication:
Ferromagnetic objects might experience more
rotational force (torque) in an open magnet
Closed vs. Open Magnet
The Reveal XT Insertable Cardiac Monitor is conditionally
safe for closed bore, cylindrical magnets with static fields
of 1.5 or 3 Tesla.
Image courtesy of chestdevices.com
It is unsafe for open vertical field magnets.
Reveal XT product manual (2013)
Spatial Gradients: Closed vs Open
Closed cylindrical magnets:
Maximum spatial gradient that could be
encountered by a device varies between
concentric circles from the isocenter.
Open magnets:
Maximum spatial gradient encountered
varies along the height of the horizontal
plane at which the object is positioned.
Spatial Gradients: Closed vs Open
The Multilink Ultra OTW coronary stent is confirmed
conditionally safe for a
maximum spatial gradient of 3.3 T/meter.
In a 1.5 T closed Philips Achieva
magnet a device positioned 20 cm
from isocenter axis would
encounter a 2.6 T/meter spatial
gradient.
This stent is
Conditionally SAFE
for the gradient
at 20 cm from isocenter
in this closed 1.5 T magnet
In a 1.0 T open Philips Panorama
magnet, the minimum spatial
gradient would be at 20-22 cm
above the table top, which would
be 4.4 T/meter.
This stent is
UNSAFE for this
open magnet b/c
minimum gradient is
too high
www.magresource.com
Philips technical description (2013)
Managing the Magnet
MR Radiofrequency
(RF) Coils
Radiofrequency Coils

If the MR scanner is considered like a camera, then the
RF coil would be the lens.

RF coils determine the diagnostic field of view.

The closer the coil is to the area of interest, the better the
signal-to-noise ratio (SNR) will be.

Generally, the smaller the coil, the better the SNR.
Radiofrequency Coils

Many RF coils used in neuroimaging only receive
signals; they are “receive-only” coils.

Receive-only coils use the body scanner to transmit
signal; this gives uniform excitation throughout the area
of interest, but it has the implication of higher whole
body SAR.

“Transmit/receive” RF coils can transmit and receive
signal; they generate less whole body SAR, but have
lesser field uniformity in the area of interest.
Radiofrequency Coils
Neuroimaging generally requires a combination of
volume and surface RF coil configuration
depending on anatomy being studied.
Head
Neck
Cervical
Thoracic
Lumbar
RF Coils: Brain
Philips Achieva
SENSE Head coil
A volume design head coil
permits imaging of the brain,
and special protocols involving
the orbits, pituitary, and
brainstem, etc.
Head
Photo courtesy of Philips Healthcare
RF Coils: Neurovascular + H/N
A volume RF coil
configuration (requiring
an anterior component)
permits adequate
signal receipt for
neurovascular and
head/neck studies.
Achieva SENSE Neurovascular coil
Base
Anterior component
Photos courtesy of Philips Healthcare
Neck
C-spine
RF Coils: Neurovascular + H/N
Brachial plexus protocol. Coronal postcontrast T1-weighted images demonstrate a
neoplastic lesion infiltrating the left lower
MRA of the neck
cervical nerve roots and trunks.
Neck
C-spine
RF Coils: Spine
A surface design posterior coil can be used for spine imaging.
Achieva SENSE Spine coil
Base component of neurovascular
coil for cervical spine
Photo courtesy of
Philips Healthcare
Spine
RF Coils: Special Circumstances

There can be circumstances where a routine RF coil
set-up may be insufficient.

Complete exam requires a configuration that provides
optimal signal detection in the area of concern.

For example, sacral pathology extending significantly
into the pre-sacral region would be best performed with
a dedicated pelvic coil rather than with a surface spine
coil (e.g. sacrococcygeal teratoma or sacral chordoma).
Special Circumstance: Pelvis
The example below (left) shows a sacral chordoma that
was imaged with a dedicated body coil (right) allowing the
necessary visualization more anteriorly in the pelvis.
Anterior part
Axial T2-weighted fat-suppressed
Posterior part
Achieva SENSE Body Coil
Photo courtesy of Philips Healthcare
Open Magnet RF Coils
Philips ST Body/Spine M Coil
Open magnets (like the
Philips Panorama HFO) can
require volume design, with
anterior and posterior
components, for spine
imaging.
Photo courtesy of Philips Healthcare
Integrated RF Coils
Newer generation systems
may have RF coils built-in,
which decreases equipment
set-up time.
On the right is an example
of a posterior surface coil
built into the table.
Ingenia Integrated Posterior Coil
Photo courtesy of Philips Healthcare
Multichannel RF Coils

Multichannel receive-only coils have multiple
elements arranged in a way that obtains signal
uniformly from the imaged region.

They provide improved image quality from
increased signal-to-noise ratio and improved spatial
resolution.
Multichannel RF Coils
32-channel head coil
used for a functional MRI exam
Axial T1-weighted images demonstrate distinct optic tracts
and sharp gray-white matter differentiation.
Managing the Magnet
Saving Time
MR Scan Time

Acquiring diagnostic quality images in a timely manner
is essential due to increasing patient volumes and for
patient comfort.

Protocoling should be performed so that only the
necessary sequences are performed to save time.

However, technical improvements are ongoing to
facilitate faster imaging and throughput.
Parallel Imaging

Parallel imaging (PI) technique is now a commonplace
strategy for accelerated image acquisition.

PI relies on the arrangement of multiple coil elements
to obtain spatial information (from a reference scan)
that is used for subsequent sequences.

This enables an undersampling of k-space, and
therefore faster imaging.
Multichannel RF Coils

Therefore, multichannel RF coils can provide increased
image quality, but they also enable parallel imaging.
Axial post-contrast T1-weighted images from a
preoperative Stealth exam using a 32-channel RF coil.
Compressed Sensing

Compressed sensing (CS) is an emerging technique
that accelerates imaging time by significantly
undersampling k-space.

It “compresses” the image with a unique coding
transform that uses a sparse representation of the
desired image.

It requires that undersampling-related artifacts that
would ordinarily be distinctly seen using a standard
transform be instead noise-like (and indistinct) in the CS
sparsifying transform domain.
Lustig M et al. (2008)
Compressed Sensing

CS is ideal for MR sequences
that are already “sparse” in their
appearance.

For example, MR angiography
focuses on vascular structures,
with poor (“sparse”) signal
representation of the background
tissue and structures, making it a
good candidate for CS.
Lustig M et al. (2008)
MR Scan Time

Streamlining equipment can also substantially reduce
exam time.

For example, integrated RF coils can obviate the laborintensive transfer, set-up, and removal of delicate heavy
equipment, and therefore increase facility efficiency and
throughput.
The Philips Ingenia Integrated
Posterior Coil within the scanner
table is an example of a built-in
“ready-to-go” RF coil that would
save set-up time.
Photo courtesy of Philips Healthcare
Conclusion

Thank you for your time.

We hope that by viewing this exhibit, you were
able to enhance your understanding of the
essentials of MR coil technology, safety issues
and timely scanning, all of which happen before
the scan hits our workstations for interpretation.
References
Kanal E, Barkovich AJ, Bell C, et al. ACR guidance document on MR safe practices: 2013. J Magn Reson Imaging 2013; 37:501-530
"Information and terminology regarding The List." Retrieved from http://www.mrisafety.com/GenPg.asp?pgname=InfoAndTerminology
U.S. Food and Drug Administration, Center for Devices and Radiological Health. (2014). Criteria for significant risk investigations of
magnetic resonance diagnostic devices - Guidance for industry and Food and Drug Administration staff. Retrieved
from http://www.fda.gov/RegulatoryInformation/Guidances/ucm072686.htm
U.S. Food and Drug Administration, Center for Devices and Radiological Health. (1997). A primer on medical device interactions with
magnetic resonance imaging systems. Retrieved from http://www.fda.gov/RegulatoryInformation/Guidances/ucm107721.htm
Abbott. (2009). MULTI-LINK RX ULTRA and MULTI-LINK OTW ULTRA Coronary stent systems - Information for prescribers.
Retrieved through www.magresource.com from http://www.doctordoctor.biz/pdf/abbott/EL2040635.pdf
Medtronic. (2013). Reveal XT 9529 insertable cardiac monitor. Retrieved from http://manuals.medtronic.com/wcm/groups/mdtcom_sg/
@emanuals/@era/@crdm/documents/documents/contrib_092102.pdf
GE Healthcare. (2005 Spring). RF coils…They’ve come a long way. MR Field Notes, 1(2), 1-4. Retrieved from http://mri-q.com/uploads/
3/2/7/4/3274160/ge_fieldnotes_volume1-2_coils.pdf
Philips Healthcare. (2013). Technical description: Achieva release 3.2 series, Panorama 3.2 series.
Parikh PT, Sandhu GS, Blackham KA, et al. Evaluation of image quality of a 32-channel versus a 12-channel head coil at 1.5T for MR
imaging of the brain. Am J Neuroradiol 2011; 32:365-373.
Lustig M, Donoho DL, Santos JM, Pauly JM. (2008 March). Compressed sensing MRI. IEEE Signal Processing Magazine, 72-81.
*Philips Healthcare, Medtronic and www.chestdevices.com have given their permission for their photographs/images to be used in this
exhibit.