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

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 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)

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)

How Can MR Harm?

Radiofrequency Pulses

Induced Currentscausing Heating

ElectromagneticInterference

(in objects with a power source)

BURNS (thermal or electric)

POSSIBLE DEVICE

MALFUNCTION

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

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.

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

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

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)

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).

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?

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)

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).

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 vertically-oriented maximum field gradient line centrally.

Implication: Ferromagnetic objects might experience more rotational force (torque) in an 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.

Closed vs. Open Magnet

Reveal XT product manual (2013)

Image courtesy of chestdevices.com

It is unsafe for open vertical field magnets.

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.

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)

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.

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

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

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

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

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

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

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 Posterior part

Axial T2-weighted fat-suppressed Achieva SENSE Body CoilPhoto courtesy of Philips Healthcare

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

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

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.

Multichannel RF Coils

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 (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

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.

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

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

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

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.

References