Download 3t labeling b

3T LABELING
B1+RMS
The following terms and definitions will be utilized throughout the document.
Term
Definition
B0
The static magnetic field produced by the scanner. Units are Tesla (T)
B1 field
The rotating RF magnetic field produced by the MRI scanner. Units are micro-Tesla (µT).
B1+ field
The particular (positively rotating) component of the B1 Field useful for imaging
B1+RMS
The root-mean-square value of the MRI effective component of the B1 field. Units are micro-Tesla (µT).
RF
Radiofrequency
SAR
Specific absorption rate. Units are watts per kilogram (W/kg)
SNR
Signal-to-noise ratio
Background
Medtronic’s offering of MR-Conditional implantable
cardiac device systems (SureScan™ Systems) includes
pacemakers (IPG), defibrillators (ICD), and cardiac
resynchronization defibrillators (CRT-D) that allowed
for full body scans with 1.5T closed bore MR, a maximum
whole body specific absorption rate (SAR) of 2W/kg, and
a maximum head SAR of 3.2 W/kg.
Medtronic has expanded the MR-Conditional labeling
for selected SureScan Systems to 3T in order to provide
SureScan device patients with broader access to MRI
with improved diagnostic imaging.
The new Medtronic MRI labeling for 3T specifies the
maximum patient RF exposure in terms of the RF magnetic
field used to create the image. This parameter is called
‘B1+RMS’ and is displayed on the console of newer MRI
scanner models, or systems with updated software. For
scans below C7 the maximum allowed B1+RMS is ≤ 2.8µT.
What is the advantage of 3T over 1.5T?
3T provides superior Signal to Noise Ratio (SNR) which can
result in improved image quality compared to 1.5T MRI
imaging.1,2 This improved imaging quality has potential
to provide superior diagnostics compared to 1.5T MRI
imaging.1-5 In addition, the superior SNR can reduce the
number of imaging averages required to produce images,
which will reduce the overall scan duration.6,7
1
What is SAR?
SAR (Specific Absorption Rate) is a measure of the rate at
which energy is absorbed by the body when exposed to a
radiofrequency (RF) electromagnetic field. It is measured
in units of Watts per kilogram of body weight. These are
the different modes available for the SAR level:
§ Normal operating mode: Whole body SAR less than
or equal to 2 W/kg, head SAR less than or equal to
3.2 W/kg. In the normal operating mode, no physiologic
stress is expected.
§ First level controlled operating mode: Whole body SAR
greater than 2 W/kg but less than 4 W/kg, head SAR less
than or equal to 3.2 W/kg. In the First Level Controlled
mode, some patients who are unable to tolerate a
thermal challenge may experience physiologic stress.
Examples include: elderly, frail, obese, diabetic, etc.
SAR is very patient dependent; it varies depending on a
patient’s size and mass (weight) and there is no absolute
direct measure of SAR that can be performed during
an MRI scan. As a result, MRI manufacturers rely on
numerical models to conservatively estimate the SAR
for a particular scan. Each scanner manufacturer builds
conservative assumptions into their SAR models to
ensure that no patient exceeds the specified SAR limits.
The MRI system can measure the B1+ field (the positively
rotating RF magnetic field produced by the MRI scanner)
needed for an imaging sequence, and uses the time
averaged B1+ field, or B1+RMS, to predict the estimated SAR
that will occur due to that imaging sequence.
What is B1+RMS?
B1+RMS is the time-averaged RF magnetic field component
relevant for creating an MR image that is generated by
the scanner during a scan and is measured in units of
micro-Tesla (µT). Understanding the importance of B1+RMS
to 3T MR-Conditional labeling requires a brief overview of
some basic MRI physics.
When a patient enters an MRI magnet, protons in the
body align in the direction of the B 0 magnetic field similar
to a compass aligning with the earth’s magnetic field.
An MR imaging sequence is composed of a series of
RF pulses that produce a magnetic field that interacts
with these magnetically aligned protons and rotates
them through a specific angle typically called the ‘flip
angle’ or ‘tip angle.’ The RF magnetic field produced
by the scanner is called the ‘B1’ field of which only one
part known as the positively rotating or ‘+’ component is
useful for ‘flipping’ the magnetically aligned protons and
allows images to be created. The maximum 10-second
time averaged B1+ field strength8 of the RF pulses in the
imaging sequence is the root-mean-square or ‘RMS’ B1+
value of the imaging sequence. Figure 1 defines each of
the elements of the symbol B1+RMS.
B1+RMS
The magnetic
field generated
by the RF coil
Root-mean-square
(maximum
10-second time
average RF pulses)
The particular (positively rotating)
component of the B1 field useful
for imaging
Figure 1: Definition of B1+RMS
What is the Advantage of using B1+RMS over SAR?
B1+RMS is a more precise RF exposure metric than SAR.
§ One reason for this is because B1+RMS is the time
averaged fundamental RF field parameter related to
MR image creation. The scanner calibrates the RF pulse
B1+ field strength during pre-scan and the B1+RMS value
for an imaging sequence is determined by the scan
parameters needed to produce the desired
tissue contrast.
2
§ Another reason that B1+RMS is a more precise RF
exposure metric than SAR is that it is patient
independent. In contrast, SAR is a conservative
estimate of the RF power deposited in a specific region
of the patient under examination (head, whole-body,
and partial-body) for a particular B1+RMS value. Predicting
SAR from the known B1+RMS value is a complicated
function of patient weight, morphology, tissue
composition, posture, landmark location and averaging
time. MRI scanners estimate the SAR for each scan
and account for patient specific attributes using realtime RF power supervision combined with proprietary
computational algorithms that have unknown safety
margins.
The B1+RMS limit of ≤ 2.8µT identified in Medtronic
SureScan Systems’ MRI labeling is independent of such
manufacturer specific approaches to SAR estimation
and represents the actual RF field exposure that is safe
for all patients implanted with Medtronic full-body eligible
SureScan System using any 3T scanner that meets the
requirements specified in the labeling.
Last but not least, imaging sequences that have been
configured for a certain B1+RMS can be saved for future use
and will be relatively consistent when recalled since B1+RMS
isn’t patient dependent. However, if an imaging sequence
was configured for a particular SAR value, when that
sequence is recalled the SAR can often vary by large
amounts depending on the patient.
“From my point of view, considering how troublesome
SAR is for RF safety “predictions,” the sooner we switch
our approach throughout the industry away from SAR and
to a more quantifiable and reproducible unit such as B1+RMS
— the better.”
Emanuel Kanal, MD, FACR, FISMRM, MRMD, AANG
Professor of Radiology and Neuroradiology
Director of Magnetic Resonance (MR) Service
University of Pittsburgh
“Setting parameters based on B1+RMS instead of SAR is a
significant advance for patients and clinicians because
B1+RMS is a more accurate and reproducible measure of
potential implant heating in the MRI scanner. Utilising
B1+RMS for implant labeling allows for the greatest possible
performance of MRI scanning protocols while also
ensuring patient safety.”
Yair Safriel, MBBCh, MD
Interventional & Diagnostic Spine and Neuroradiologist
Chief Medical Officer, Pharmascan Clinical Trials
Asst. Clin Professor, University of South Florida
If B1+RMS is a better measure of the RF field and SAR is
based on this measure, why did Medtronic use SAR for
their initial labeling?
Prior to 2010, manufacturers of MRI scanners weren’t
required to display the value for B1+RMS . Therefore, when
Medtronic’s initial SureScan MRI cardiac systems were
released in 2008, it was not practical to label these
devices to B1+RMS since the MR technologist would have
no means of verifying that they were conforming to
the labeling.
What is the maximum RF output allowed under the
3T labeling?
As of May 2016, selected Medtronic SureScan cardiac
systems are approved for a scan in a 3T scanner that
can display B1+RMS with the following requirements:
§ 3T closed bore clinical MRI used for hydrogen
proton imaging
§ Maximum spatial gradient of ≤ 20T/m (2,000 gauss/cm)
§ Maximum gradient slew rate performance per axis
≤ 200 T/m/s
§ No restriction if the isocenter is at or superior to the
C7 vertebra
§ B1+RMS must be ≤ 2.8µT when isocenter is inferior to the
C7 vertebra
Why did Medtronic choose B1+RMS labeling for 3T MRI?
Medtronic’s 3T labeling (which utilizes B1+RMS) allows for
the greatest access to 3T MRI, without compromised
performance. Medtronic arrived at the specified 3T
labeling using B1+RMS in order to allow the greatest possible
MR performance of 3T systems and reduce the impact
on radiology staff, while simultaneously ensuring patient
safety from implant heating.
Does this mean I need to change my scan sequences
to stay within this limit?
The 2.8µT B1+RMS limit is not expected to limit current
3T scan protocols for scans below C7, thus no
modification of scan sequences should be required.
In most scenarios, basic SAR restrictions that are applied
for all patients (associated with the limits imposed by
First Level Controlled Operating Mode9) will prevent scan
sequences from operating above 2.8µT when positioned
inferior to the C7 vertebra. Scan sequences superior
to the C7 vertebra have only First Level Controlled
Operating Mode limits. The general principles that an
MR technologist uses to reduce SAR can be used to
reduce B1+RMS .
If a scan sequence were to be encountered that is greater
than 2.8µT for a landmark position inferior to the C7
vertebra, the general principles used to create reduced
SAR protocols also apply to B1+RMS reduction.
Is the B1+RMS Display available on all 3T Scanners?
No. The regulations that govern MRI equipment require
that B1+RMS must be displayed on all new MRI scanners
beginning in 20109 allowing a grace period until 2013.
Some major manufacturers added this feature to
scanners sold prior to 2013 as part of regular software
upgrades. Medtronic estimates that more than 75%
of installed 3T MRI scanners display B1+RMS , and this
percentage will continue to grow in the coming years.
Should the 3T scanner console not display the value for
B1+RMS then the patient should not be scanned in said
scanner. Consider finding a different 3T scanner with
B1+RMS or scanning the patient at 1.5T instead. If you are
unsure whether a scanner displays B1+RMS , or is eligible for
a software upgrade that enables this feature, contact the
scanner manufacturer.
Where is B1+RMS Displayed on the Scanner Console?
The location of the B1+RMS display on the scanner console
is different for each manufacturer but is likely to be
displayed in close proximity to SAR.
3
References 1
Kamada K, Kakeda S, Ohnari N, Moriya J, Sato T, Korogi Y. Signal intensity of motor and
sensory cortices on T2-weighted and FLAIR images: intraindividual comparison of 1.5T and
3T MRI. Eur Radiol. December 2008;18(12):2949-2955.
2
Stankiewicz JM, Glanz BI, Healy BC, et al. Brain MRI lesion load at 1.5T and 3T versus clinical
status in multiple sclerosis. J Neuroimaging. April 2011;21(2):e50-e56.
3
Luccichenti G, Giugni E, Péran P, et al. 3 Tesla is twice as sensitive as 1.5 Tesla magnetic
resonance imaging in the assessment of diffuse axonal injury in traumatic brain injury
patients. Funct Neurol. Apr-Jun 2010;25(2):109-114.
4
Craven I, Griffiths PD, Hoggard N. Magnetic resonance imaging of epilepsy at 3 Tesla.
Clin Radiol. March 2011;66(3):278-286.
5
Alvarez-Linera J. 3T MRI: advances in brain imaging. Eur J Radiol. September 2008;67(3):415-426.
6
http://www.ncbi.nlm.nih.gov/pubmed/16555259Multicontrast black-blood MRI of carotid
arteries: comparison between 1.5 and 3 tesla magnetic field strengths.
7
http://www.biomedsearch.com/article/Cardiovascular-MRI-at-3T/209239236.html and
http://www.medscape.com/viewarticle/566817 Cardiovascular MRI at 3T.
8
As specified in IEC 60601-2-33, “the maximum 10 second time averaged…”
9
IEC 60601-2-33 (Third edition – 2010).
Brief Statement
Medtronic SureScan™ Portfolio for 1.5T and 3T MR Conditional Use Medtronic SureScan products and systems are MR Conditional, and as such are designed to
allow patients to undergo MRI under the specified conditions for use.
Pacing, ICD and CRT-D Systems: When programmed to On, the MRI SureScan feature allows
the patient to be safely scanned while the device continues to provide appropriate pacing.
A complete transvenous SureScan system, which is a SureScan device with appropriate
SureScan lead(s), is required for use in the MR environment. When a single coil SureScan
defibrillation lead is used, a Medtronic DF-1 pin plug must be secured in the SVC port to make
a complete SureScan DF-1 defibrillation system. To verify that components are part of a
SureScan system, visit http://www.mrisurescan.com/. Any other combination may result in a
hazard to the patient during an MRI scan.
Indications: The SureScan Advisa MRI™ and Revo MRI™ pacing systems are indicated for
rate adaptive pacing in patients who may benefit from increased pacing rates concurrent
with increases in activity. Dual chamber SureScan pacing systems are also indicated for dual
chamber and atrial tracking modes in patients who may benefit from maintenance of AV
synchrony.
The SureScan Evera MRI™ and Visia AF MRI™ defibrillation systems are indicated to provide
ventricular antitachycardia pacing and ventricular defibrillation for automated treatment of
life-threatening ventricular arrhythmias. In addition, the dual chamber devices are indicated for
use in the above patients with atrial tachyarrhythmias, or those patients who are at significant
risk of developing atrial tachyarrhythmias.
SureScan Amplia MRI™ and Compia MRI™ CRT-D systems are indicated for ventricular
antitachycardia pacing and ventricular defibrillation for automated treatment of lifethreatening ventricular arrhythmias and for providing cardiac resynchronization therapy in
heart failure patients on stable, optimal heart failure medical therapy if indicated, and meet
any of the classifications detailed in the specific device manuals. New York Heart Association
(NYHA) Functional Class III or IV and who have a left ventricular ejection fraction ≤ 35% and a
prolonged QRS duration. Left bundle branch block (LBBB) with a QRS duration ≥ 130 ms, left
ventricular ejection fraction ≤ 30%, and NYHA Functional Class II. NYHA Functional Class I, II, or
III and who have left ventricular ejection fraction ≤ 50% and atrioventricular block
(AV block) that are expected to require a high percentage of ventricular pacing that cannot
be managed with algorithms to minimize right ventricular pacing. Optimization of heart
failure medical therapy that is limited due to AV block or the urgent need for pacing should
be done post implant. Amplia only: Some CRT-D system are also indicated for use in patients
with atrial tachyarrhythmias, or those patients who are at significant risk for developing atrial
tachyarrhythmias.
Micra™ Model MC1VR01 is indicated for patients with symptomatic paroxysmal or permanent
high grade AV block in the presence of AF. It is also indicated in the absence of AF as an
alternative to dual chamber pacing, or symptomatic bradycardia-tachycardia syndrome
or sinus node dysfunction (sinus bradycardia/sinus pauses) when atrial lead placement is
considered difficult, high risk, or not deemed necessary for effective therapy.
The Reveal LINQ™ Insertable Cardiac Monitor (ICM) is an implantable patient-activated and
automatically-activated monitoring system that records subcutaneous ECG and is indicated
for patients with clinical syndromes or situations at increased risk of cardiac arrhythmias, or
patients who experience transient symptoms such as dizziness, palpitation, syncope and chest
pain that may suggest a cardiac arrhythmia.
Contraindications: The SureScan transvenous pacing systems are contraindicated for
implantation with unipolar pacing leads (Revo MRI only), concomitant implantation with another
bradycardia device or an implantable cardioverter defibrillator.
Micra IPG is contraindicated for patients who have the following types of medical devices
implanted: an implanted device that would interfere with the implant of the Micra device in
the judgment of the implanting physician, an implanted inferior vena cava filter, a mechanical
tricuspid valve, or an implanted cardiac device providing active cardiac therapy that may
interfere with the sensing performance of the Micra device or for patients who have the
following conditions: femoral venous anatomy unable to accommodate a 7.8 mm (23 French)
introducer sheath or implant on the right side of the heart (for example, due to obstructions
or severe tortuosity), morbid obesity that prevents the implanted device from obtaining
telemetry communication within ≤ 12.5 cm (4.9 in), or known intolerance to the materials
listed in the Instruction for Use, or to heparin, or sensitivity to contrast media that cannot be
adequately pre-medicated.
Medtronic
710 Medtronic Parkway
Minneapolis, MN 55432-5604
USA
Toll-free in USA: 800.633.8766
Worldwide: +1.763.514.4000
medtronic.com
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Minneapolis, MN. All Rights Reserved.
Printed in USA. 10/2016
SureScan defibrillation and CRT-D systems are contraindicated for patients experiencing
tachyarrhythmias with transient or reversible causes, or patients with incessant VT or VF.
For dual chamber and CRT-D devices, the device is contraindicated for patients whose
primary disorder is chronic atrial tachyarrhythmia with no concomitant VT or VF. For single
chamber devices, the device is contraindicated for patients whose primary disorder is atrial
tachyarrhythmia.
Reveal LINQ: There are no known contraindications for the implant of the Reveal LINQ
ICM. However, the patient’s particular medical condition may dictate whether or not a
subcutaneous, chronically implanted device can be tolerated.
Warnings and Precautions: Changes in patient’s disease and/or medications may alter the
efficacy of the device’s programmed parameters. Patients should avoid sources of magnetic
and electromagnetic radiation to avoid possible underdetection, inappropriate sensing
and/or therapy delivery, tissue damage, induction of an arrhythmia, device electrical reset,
or device damage. Do not place transthoracic defibrillation paddles directly over the device.
Additionally, for CRT-D devices, certain programming and device operations may not provide
cardiac resynchronization. Use of the device should not change the application of established
anticoagulation protocols.
Patients and their implanted systems must be screened to meet the following requirements
for MRI:
§ SureScan transvenous pacing, ICD and CRT-D systems: no lead extenders, lead adaptors
or abandoned leads present; no broken leads or leads with intermittent electrical contact as
confirmed by lead impedance history; and the system must be implanted in the left or right
pectoral region. For pacemaker-dependent patients, it is not recommended to perform an
MRI scan if the right ventricular (RV) lead pacing capture threshold is greater than 2.0 V at
0.4 ms. A higher pacing capture threshold may indicate an issue with the implanted lead.
No diaphragmatic stimulation at a pacing output of 5.0 V and at a pulse width of 1.0 ms in
patients whose device will be programmed to an asynchronous pacing mode when MRI
SureScan is on. It is not recommended to perform MRI scans during the lead maturation
period (approximately 6 weeks).
§ Pacemaker specific: pace polarity parameters set to Bipolar for programming MRI
SureScan to On (Advisa MRI only); or a SureScan pacing system with a lead impedance value
of ≥ 200 Ω and ≤ 1,500 Ω. Revo MRI patients must have pacing capture thresholds of ≤ 2.0 V
at a pulse width of 0.4 ms and a SureScan pacing system that has been implanted for a
minimum of 6 weeks.
§ Micra: no abandoned leads are present; device is operating within the projected service life;
pacing amplitude is ≤ 4.5 V at the programmed pulse width; no diaphragmatic stimulation is
observed when MRI SureScan is programmed to On.
MR Scanning Conditions:
§ Transvenous system patients may be scanned using a horizontal field, cylindrical bore, clinical
1.5T or 3T MRI system for hydrogen proton imaging, maximum spatial gradient ≤ 20 T/m,
and maximum gradient slew rate performance per axis ≤ 200 T/m/s. 1.5T scanners must be
operated in Normal Operating Mode (whole body averaged specific absorption rate (SAR)
≤ 2.0 W/kg, head SAR ≤ 3.2 W/kg). 3T scanners must be operated in First Level Controlled
Operating Mode or Normal Operating Mode. B1+RMS must be ≤ 2.8 μT when the isocenter
(center of the bore) is inferior to the C7 vertebra. Scans can be performed without B1+RMS
restriction when the isocenter is at or superior to the C7 vertebra. Revo MRI pacemakers can
only be scanned using 1.5T systems.
§ Micra and Reveal LINQ patients may be scanned using a horizontal field, cylindrical bore,
clinical 1.5T or 3T MRI system for hydrogen proton imaging, maximum spatial gradient
≤ 25 and T/m, and maximum gradient slew rate performance per axis ≤ 200 T/m/s. The Whole
Body Specific Absorption Rate (SAR) as reported by the MRI equipment must be ≤4.0 W/kg;
the head SAR as reported by the MRI equipment must be ≤ 3.2 W/kg.
§ Reveal LINQ: Do not use local transmit coils on the chest, trunk, or shoulder region.
§ For all SureScan pacing systems, proper patient monitoring must be provided during the
MRI scan. For SureScan defibrillation and CRT-D systems, continuous patient monitoring
is required while MRI SureScan is programmed to On. Do not scan a patient without first
programming MRI SureScan to On and do not leave the device in MRI SureScan mode after
the scan is complete. While MRI SureScan is programmed to On, arrhythmia detection and
therapies are suspended, leaving the patient at risk of death from untreated spontaneous
tachyarrhythmia. In addition, if the device is programmed to an asynchronous pacing mode,
arrhythmia risk may be increased.
Potential Complications: Potential complications include, but are not limited to, rejection
phenomena, device migration, infection, or erosion through the skin. Potential complications
associated with cardiac rhythm devices include muscle or nerve stimulation, oversensing,
failure to detect and/or terminate arrhythmia episodes, acceleration of tachycardia, and
surgical complications such as hematoma, inflammation, and thrombosis. Potential lead
complications include, but are not limited to, valve damage, fibrillation, thrombosis, thrombotic
and air embolism, cardiac perforation, heart wall rupture, cardiac tamponade, pericardial
rub, infection, myocardial irritability, and pneumothorax. Other potential complications
related to the lead may include lead dislodgement, lead conductor fracture, insulation failure,
threshold elevation, or exit block. Other potential complications related to Micra are access
site hematoma, AV fistulae, and vessel spasm, Potential MRI complications include, but are
not limited to, lead electrode heating and tissue damage resulting in loss of sensing or capture
or both, or MR-induced stimulation on leads resulting in continuous capture, VT/VF, and/or
hemodynamic collapse. Potential complications of the Reveal LINQ device include, but are
not limited to, device rejection phenomena (including local tissue reaction), device migration,
infection, and erosion through the skin.
See the appropriate product MRI SureScan Technical Manual before performing an MRI Scan and
see the device manuals for detailed information regarding the implant procedure, indications,
contraindications, warnings, precautions, and potential complications/adverse events. For
further information, call Medtronic at 1-800-328-2518 and/or consult the Medtronic website at
www.medtronic.com or www.mrisurescan.com.
Caution: Federal law (USA) restricts these devices to sale by or on the order of a physician.