managing the magnet: a prequel to pressing the scan button authors: j patel md, cm glastonbury mbbs,...
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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-80ASNR 2015 Annual Meeting
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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.
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Plan
Introduction to MR safety
Discussion of coil types and advances
Elaborate on time saving strategies in MR imaging
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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:
The highest risk areas are those within a 5 gauss (0.5
mT) magnetic field line of the scanner(s).
The MR Environment
Static magnetic field & spatial gradients Rapidly changing magnetic fields Rapidly changing radiofrequency (RF) pulses
FDA primer document (1997)
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Static Magnetic Field
(Always on)
Rotational Forcesi.e. Torque
Translational Force(from spatial gradient)
TEARING OF TISSUE from device rotation
Tearing of tissue or acceleration
of object (MISSILE EFFECT)
Gradient Magnetic Field Induced Currents
DEVICE MALFUNCTION
How Can MR Harm?
FDA primer document (1997)
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How Can MR Harm?
Radiofrequency Pulses
Induced Currentscausing Heating
ElectromagneticInterference
(in objects with a power source)
BURNS (thermal or electric)
POSSIBLE DEVICE
MALFUNCTION
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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)
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Zones 1 and 2
Zone 1
Zone 2
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.
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.
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Zones 3 and 4
Zone 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).
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.
DANGER
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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!
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Zone 4 Labeling
MR SAFE
MR CONDITIONAL
MR UNSAFE
*ASTM labeling used by FDA
Objects that are wholly non-metallic and made from material known to be safe in the MR environment.
Metallic objects that pose no hazard only if specificconditions are met. There are numerous condition categories.
Objects that are hazardous due to ferromagnetismor other MR environment interactions (e.g. induced currents in looped leads)
ACR guidance document on MR safe practices (2013)
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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).
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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
What is MR Conditional?
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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)
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For the Reveal XT Insertable Cardiac Monitor, head
SAR must be ≤ 3.2 W/kg (matching FDA
requirement).
Specific Absorption Rate
Reveal XT product manual (2013)
Image reproduced with permission of Medtronic, Inc.
But note that whole body SAR must be ≤ 2.0 W/kg
(stricter than FDA figure).
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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.
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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 vertically-oriented maximum field gradient line centrally.
Implication: Ferromagnetic objects might experience more rotational force (torque) in an open magnet
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The Reveal XT Insertable Cardiac Monitor is conditionally safe for closed bore, cylindrical magnets with static fields of 1.5 or 3 Tesla.
Closed vs. Open Magnet
Reveal XT product manual (2013)
Image courtesy of chestdevices.com
It is unsafe for open vertical field magnets.
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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.
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This stent is Conditionally SAFEfor the gradientat 20 cm from isocenter in this closed 1.5 T magnet
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.
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.
The Multilink Ultra OTW coronary stent is confirmed conditionally safe for a
maximum spatial gradient of 3.3 T/meter.
This stent is UNSAFE for this open magnet b/c minimum gradient is too high
Spatial Gradients: Closed vs Open
www.magresource.com Philips technical description (2013)
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Managing the Magnet
MR Radiofrequency (RF) Coils
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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.
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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
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Radiofrequency Coils
Neuroimaging generally requires a combination of
volume and surface RF coil configuration
depending on anatomy being studied.
Head Neck
Cervical Thoracic Lumbar
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RF Coils: Brain
A volume design head coilpermits imaging of the brain, and special protocols involving the orbits, pituitary, and brainstem, etc.
Head
Philips Achieva SENSE Head coil
Photo courtesy of Philips Healthcare
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RF Coils: Neurovascular + H/N
Neck
C-spine
Base Anterior component
Achieva SENSE Neurovascular coil A volume RF coil configuration (requiring an anterior component) permits adequate signal receipt for neurovascular and head/neck studies.
Photos courtesy of Philips Healthcare
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Brachial plexus protocol. Coronal post-contrast T1-weighted images demonstrate a neoplastic lesion infiltrating the left lower cervical nerve roots and trunks.
MRA of the neck
Neck
C-spine
RF Coils: Neurovascular + H/N
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RF Coils: Spine
Spine
A surface design posterior coil can be used for spine imaging.
Achieva SENSE Spine coil
Base component of neurovascular coil for cervical
spinePhoto courtesy of Philips Healthcare
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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).
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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 Posterior part
Axial T2-weighted fat-suppressed Achieva SENSE Body CoilPhoto courtesy of Philips Healthcare
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Open magnets (like the Philips Panorama HFO) can require volume design, with anterior and posterior components, for spine imaging.
Philips ST Body/Spine M Coil
Open Magnet RF Coils
Photo courtesy of Philips Healthcare
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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
Integrated RF Coils
Photo courtesy of Philips Healthcare
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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
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32-channel head coil used for a functional MRI exam
Axial T1-weighted images demonstrate distinct optic tracts and sharp gray-white matter differentiation.
Multichannel RF Coils
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Managing the Magnet
Saving Time
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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.
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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.
Parallel Imaging
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Multichannel RF Coils
Axial post-contrast T1-weighted images from a preoperative Stealth exam using a 32-channel RF coil.
Therefore, multichannel RF coils can provide increased image quality, but they also enable parallel imaging.
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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)
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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)
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MR Scan Time
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.
Streamlining equipment can also substantially reduce exam time.
For example, integrated RF coils can obviate the labor-intensive transfer, set-up, and removal of delicate heavy equipment, and therefore increase facility efficiency and throughput.
Photo courtesy of Philips Healthcare
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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.
Conclusion
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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
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*Philips Healthcare, Medtronic and www.chestdevices.com have given their permission for their photographs/images to be used in this exhibit.
References