Kagayaki Kuroda1
1School of Into Sci & Tech, Tokai University, Japan
Synopsis
In order to achieve best practice in MR
examination of patients with IED's, essences for considering safety will be
described, in particular from the view point of RF-heating. Those include physical
interactions of devices with electromagnetic fields of MR scanners, tips to deal
with SAR and B1+rms in the light of reliability and diagnosability, and
recent progresses in database and guidance.
Target audience
- Clinicians and technologists examining patients
with IMD's including IED's
- Researchers working on MR safety
- Engineers of medical device manufactures
- Company employees working for regulatory
affairs
- Officers in regulatory body
Objectives
To understand the followings;
- Physical interactions of
IED's with electromagnetic fields of MR
- Gap between console-displayed
SAR and actual-measured SAR
- Variability of the SAR and B1+rms among different scanners
- Mechanisms why those
gap and variety are allowed
- Effect of SAR and B1+rms
limits on image quality and radiologists' impression
- Pogresses in MR-safety
database and guidance
Introduction
Magnetic
resonance (MR) safety of implantable medical devices are becoming general
knowledge not only for health professions but also for patients and general
citizens. In addition to the cardiac devices, some other IED products such as deep
brain stimulators (DBS's), cochlear implants (CI's), spinal cord stimulators
(SCS's), and vagus nerve stimulators (VNS's) have also been approved as
MR-conditional based on the manufactures' intensive efforts for testing. However,
the physical mechanisms and hidden information about MR conditionality are
still difficult to be fully understood in clinical practice. In particular, the
tips to secure patients' safety and yet maintain diagnostic effectiveness under
the electromagnetically induced heating seem to be one of the most important challenges
for clinicians. In this lecture, the essence of these challenges will be
imparted, starting with a few tips from the view point of electromagnetics.Physical interactions of IED's with electromagnetic fields of MR
Tissue heating by RF magnetic field is
inductive heating in that the electric motive force or electric field induced
by the alternation of B1-field generates Joule heat. The specific absorption
rate (SAR) is an expression of Joule's law per a unit weight of tissue. When a
conductive rod or lead is placed in the tissue, the electric current made by
the electric motive force will flow into the conductor making the current
density at the tip parts of the conductor much denser than that in the
surrounding tissue. If the conductor is insulated remaining its bare tips, the
current density will easily reach to dozens of times denser(1) making SAR, which is in proportion to a square of current, hundreds
of times higher. When an
insulated lead is in the electric field, the amount of the tangential component
of the electric field determines the degree of heating. When a conductor
has blunt and sharp parts, the electric field at the sharp part will be stronger
than the blunt with a reverse ratio of the curvature radiuses of the two parts(2). Accurate understanding of these basic phenomena may help
clinicians for protecting patients from heating.SAR and B1+rms: How variable and reliable are the values on consoles?
In order to see the variability and
reliability of the console-displayed SAR, calorimetric studies were performed in
two different 1.5-T and in one 3-T clinical scanners(3). The imaging conditions in this study were set to have a
console-reported whole body averaged SAR (Console SAR) as 2.0 W/kg, 1.0 W/kg,
or a console-displayed B1+rms (Console B1+rms) as 3.2 μT.
Fluoroptic temperature measurements were performed along with the ASTM method(4, 5) for 30 minutes during fast spin echo sequence of TR, 3275-4424 ms;
TE, 63 ms; and ETL 7 or 14. The console SAR and console B1+rms
were plotted against measured SAR, which was obtained with the net weight of
the phantom (25kg). In the two 1.5-T scanners, the console SAR was smaller than
the measured SAR, meaning that the console SAR indicated underestimation. The inter-scanner
variation of the measured SAR was of a factor of 2. In the 3-T scanner, the
console SAR was close to the measured SAR. When applying the NEMA (2) strategy,
in that the "equivalent body mass" (EBM) of 50-75kg(6) or 70-90 kg(7) was applied, all of the console SAR values became same or higher
than the measured SAR. There was also a variation of a factor of 1.2 between
the console B1+rms values at the two 1.5T.SAR and B1+rms: How do they impact on image quality and diagnosability?
Image quality and diagnosability were assessed
for 30 healthy volunteers (15 male and 15 females) at both 1.5T and 3T with the
aforementioned console SAR and B1+rms settings. The brain, heart,
liver, lumber, and pelvic (uterine for female and prostate for male) regions
were examined by routinely used sequence parameters. The scan durations and
spatial coverage were maintained consistent for the different settings. The
trends of image quality (SNR and CNR) against the different SAR and B1+rms
settings varied among the body regions and sequences. Diagnosability scored by
three radiologists was similar for the different console settings, although
individual difference among the radiologists was recognized. At 3T, there was recognizable
image quality degradation at lower SAR setting.Recent progresses in database and guidance
Another aspect of MR safety of IED's are the
construction and maintenance of the database and guidelines. In 2019, Ministry
of Health, Labor and Welfare (MHLW) and Pharmaceuticals and Medical Devices
Agency (PMDA) in Japan issued an official announcement that promotes the MR
safety labeling in any type of medical device that have probability to enter to
MR environments. As a response to this announcement, MR-safety data search
system(8) for both IED's and non-electric IMD's as well as data sources of
IED's have been opened to public with no charge by industries and academic
societies.
Summary
The key points of the physical background and
clinical impact of the MR conditionality were described mainly for MR-induced heat.
Improvements in the database and guidance by industries, regulatory bodies and
the academic society are rapidly ongoing.Acknowledgements
This work was supported in parts by Ministry of Health, Labor and Welfare (MHLW) Japan, and Japanese Society of Magnetic Resonance in Medicine (JSMRM).References
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