Medical Design Briefs - July 2021 - 7

Condutive Wire Loop
(e.g. Copper)
Active Detuning
Circuit
DC Bias
CTune
F
Single
Coil
Element
Inductor
L
Pin Diode
D
Ground (GND)
CMatch
RF Signal
CV
Passive Detuning
Circuit
Fig. 1 - This is an example schematic for a receive-only coil. The trimmer capacitors are represented
by two lines with an arrow while the MLCCs are represented by just two lines. (Source: Journal of
Magnetic Resonance Imaging1)
Q Factor
Influences power handling; higher Q lessens self-heating under RF conditions
Important in filter circuits; impacts insertion loss
Typically, >2,000 for trimmer capacitors
εr Value (Dielectric
Constant)
Nonmagnetic
Properties
Determines the capacitance density in conjunction with dielectric withstand
voltage
Higher the εr value, smaller the component can be
Essential for MRI components, especially body coil and surface coils
Close control of raw materials and processes required to ensure MRI accuracy
and performance
Need a relative permeability (µr) of 1.0005 or lower, the closer to the coil the
lower it needs to be
Dielectric Withstand
Voltage (DWV)
The maximum DC voltage the part can withstand without failure
Looking for ~7 kV for high-performance devices
Table 1. When selecting capacitors for MRI coils specifically, these characteristics are important to consider:
Q factor,
r value (dielectric constant), nonmagnetic properties, and dielectric withstand voltage.
coils that are applied to the part of the
body being imaged. These signals disrupt
that portion of the uniform field, which
breaks the single-line formation of these
protons. After this interruption, the protons
return to their state of alignment,
emitting a small amount of energy that
can be measured and used to distinguish
between different types of molecules and
their locations.
This is a delicate process where even the
slightest variation in the homogeneity of
the magnetic field will cause the protons
to align differently. These differences can
confuse the detection algorithms, which
means these subatomic particles will not
respond the same way to the stimulus,
resulting in a low-quality image. This is a
big issue because distorted MRI images
may lead to a mistaken diagnosis and, consequently,
misguided treatment selections.
Medical Imaging with
Nonmagnetic Components
In general, MRI machines are demanding
applications that operate at high voltMedical
Design Briefs, July 2021
Intro
Cov
ages yet require extreme precision when it
comes to transmitting and receiving signals
between the human body and the
MRI coils. Therefore, these high-power
machines need components for the MRI
coils that can transmit and receive highfrequency
RF signals up to 300 MHz, have
high Q and low loss, and come in a small
form factor.
In addition to meeting these requirements,
since the quality of an MRI image is
heavily dependent on the uniformity of
the magnetic field, components used in an
MRI machine must not exhibit any measurable
magnetism. This can be a challenge
since many parts, such as capacitors, are
traditionally designed with materials that
possess magnetic properties, such as a
nickel barrier finish to maintain solderability
or commercial brass connections.
Thus, to summarize, when selecting
capacitors for MRI coils specifically, the
following characteristics are important to
consider: Q factor, εr value (dielectric constant),
nonmagnetic properties, and di -
electric withstand voltage (DWV) (see
www.medicaldesignbriefs.com
ToC
+
-
A
µ
What to Look for in
an MLCC
Let's look at the qualities you should
look for in the MLCCs used in an MRI
coil. It is imperative that these MLCCs
have high Q and, in turn, low equivalent
series resistance (ESR) so that the MLCCs
do not exhibit energy and heat loss.
Additionally, these MLCCs must have a
high DWV to handle voltage buildup and
prevent a failure that could impact the
safety of patients and technicians operating
the equipment.
But, beyond these characteristics, there
are several other important considerations
many circuit designers may not be used to
thinking about. First, these MLCCs cannot
exhibit any magnetic qualities; therefore,
they cannot be made with a typical nickel
barrier finish, and a nonmagnetic termination
is needed. Historically, silver/palladium
(Ag/Pd) has been a commonplace
solution to meet this requirement.
But, just ensuring that the MLCC is void
of magnetic properties is not enough.
Since these MLCCs are used in the coils
that transmit and receive the RF signals
and are placed on or in close proximity to
patients, MRI machine component manufacturers
also need to meet the Restriction
of Hazardous Substances (RoHS) directive
requirement for using lead-free solders.
This requirement can be achieved without
introducing magnetism into the capacitor
by using a copper barrier with a tin finish
on top. This alternative is lead-free while
also avoiding increased soldering temperatures
and leaching problems.
To meet all of these requirements,
Knowles Precision Devices, for example,
makes a line of nonmagnetic, ultra-low
loss, high Q ceramic capacitors with
C0G/NP0 characteristics. MLCCs can also
be made using class 2 dielectrics (X7R)
with a FlexiCap™ termination to withstand
greater levels of mechanical strain.
While not used in MRI coils, these MLCCs
are suitable for other areas of the MRI
machine such as the low-noise amplifier
(LNA) circuity.
What to Look for in a Trimmer
Capacitor
Since the signals emitted from the protons
in the human body are very small,
trimmer capacitors are also needed in
7
Table 1). Because of the safety and precision
requirements for the coils used for
MRI, both multilayer ceramic capacitors
(MLCCs) and trimmer capacitors are
needed as shown in Figure 1.
PreAmp
Capacitance
C
RF Choke
È
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Medical Design Briefs - July 2021

Table of Contents for the Digital Edition of Medical Design Briefs - July 2021

Medical Design Briefs - July 2021 - Intro
Medical Design Briefs - July 2021 - Cov4
Medical Design Briefs - July 2021 - Cov1
Medical Design Briefs - July 2021 - Cov2
Medical Design Briefs - July 2021 - 1
Medical Design Briefs - July 2021 - 2
Medical Design Briefs - July 2021 - 3
Medical Design Briefs - July 2021 - 4
Medical Design Briefs - July 2021 - 5
Medical Design Briefs - July 2021 - 6
Medical Design Briefs - July 2021 - 7
Medical Design Briefs - July 2021 - 8
Medical Design Briefs - July 2021 - 9
Medical Design Briefs - July 2021 - 10
Medical Design Briefs - July 2021 - 11
Medical Design Briefs - July 2021 - 12
Medical Design Briefs - July 2021 - 13
Medical Design Briefs - July 2021 - 14
Medical Design Briefs - July 2021 - 15
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Medical Design Briefs - July 2021 - 17
Medical Design Briefs - July 2021 - 18
Medical Design Briefs - July 2021 - 19
Medical Design Briefs - July 2021 - 20
Medical Design Briefs - July 2021 - 21
Medical Design Briefs - July 2021 - 22
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Medical Design Briefs - July 2021 - 24
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Medical Design Briefs - July 2021 - 36
Medical Design Briefs - July 2021 - 37
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Medical Design Briefs - July 2021 - 39
Medical Design Briefs - July 2021 - 40
Medical Design Briefs - July 2021 - Cov3
Medical Design Briefs - July 2021 - Cov4a
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