Medical Design Briefs - July 2021 - 8

Capacitor Selection
MRI coils to help get the signal just right
as shown in Figure 1. This precise signal
that needs to be measured is known as
the Larmor frequency, which varies with
magnetic field strength. To identify this
small signal with precision, the trimmer
capacitor is used to make capacitance
adjustments so that the frequency of the
electrical resonance of the coil's LC circuit
matches the frequency of the nuclei
in the tissue. Think of this process much
like you would turn the dial on a radio to
fine-tune the signal. This process is necessary
to overcome variations in the values
and tolerances of other components utilized
in the coil.
These capacitors are generally available
in half-turn and multi-turn options.
For low-power receive coils, half-turns can
be a suitable solution, but to provide the
performance needed for high-power and
body-coil applications, multi-turn trimmers
are the best option. Multi-turn trimmers
boast higher Q and DWV and higher
precision. More specifically, the space
between metalized surfaces in a trimmer
capacitor can be filled with a variety of
dielectrics. Multi-turn trimmer capacitors
that use air, sapphire, or polytetrafluoroethylene
(PTFE) dielectric materials
will provide the lowest loss and best overall
performance.
Furthermore, the amount of insulation
provided by the dielectric material contributes
to the voltage rating of the trimmer
capacitor, usually given as its DC
withstanding voltage. PTFE exhibits a
higher dielectric constant than air (which
is equal to unity) and can support trimmer
capacitors with a much higher DC
withstanding voltage rating. Therefore,
multi-turn trimmers that use PTFE as a
dielectric are an excellent option for use
in MRI coils.
Conclusion
Beyond the part-specific considerations
discussed above, be sure to evaluate
the suppliers you are considering as well.
You will want to ensure that they are
using high-purity metals that will in fact
exhibit no measurable magnetism over
the life of the application. Compliance
with this parameter starts with strict traceability
and testing regimens and a foundation
of materials science expertise. So,
be sure to ask these types of tough questions
as well.
In practice, irregular detections caused
by excessive signal noise or random variations
in signal intensity produce granular
images that can lead to inaccurate diagnosis.
While component section for parts
such as capacitors is typically viewed as
simple or uncomplicated, life-critical
applications demand specialized attention
in every aspect of design. Therefore,
when designing an MRI coil and evaluating
components such as trimmer capacitors
or MLCCs, as discussed, there are a
number of factors to think about to
ensure you can consistently produce
high-quality images in a safe manner.
This level of care on the component level
can prevent distortion and minimize the
potential need for image corrections.
Reference
1. Bernhard Gruber, et al., " RF coils: A practical
guide for nonphysicists, " J Magn Reson
Imaging. 2018 Sep. 48(3): 590-604, https://
www.ncbi.nlm.nih.gov/pmc/articles/
PMC6175221.
This article was written by Matt Ellis,
Product Manager - Trimmers and NonMagnetic
Components, Knowles Precision
Devices, Cazenovia, NY. For more information,
visit http://info.hotims.com/79415-342.
International Manufacturing Services, Inc.
www.ims-resistors.com | (401) 683-9700
8
Free Info at http://info.hotims.com/79415-706
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Medical Design Briefs, July 2021
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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6175221 http://info.hotims.com/79415-342 http://info.hotims.com/79415-706
Medical Design Briefs - July 2021

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