Today, magnetic resonance imaging (MRI) technology is widely used by healthcare professionals to examine soft tissues and organs in the body. MRI is an excellent diagnostic tool because it can be used to detect a variety of potentially life-threatening issues ranging from degenerative diseases to tumors in a noninvasive manner. To understand the design challenges involved in developing MRI equipment, specifically when it comes to the selection of radio-frequency (RF) and electrical components such as capacitors, it’s first important to understand the basic physics behind the way MRI machines operate.
At a high level, the MRI machines used today are based on the principle of nuclear magnetic resonance (NMR). This means the machines use a strong magnetic field and computer-generated radio waves to produce cross-sectional images. This is possible because the molecules that make up the human body contain hydrogen, and the nucleus of the hydrogen atom has a single proton that behaves like a magnet with a north and south pole. When a strong magnetic field is applied to the body, such as the one used with MRI, the spins of the protons arrange uniformly.
Next, a short computer-generated RF signal is sent out to the transmit/receive 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.
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.
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 voltages 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 high-frequency 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 dielectric withstand voltage (DWV) (see 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.
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 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.
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.
- Bernhard Gruber, et al., “RF coils: A practical guide for nonphysicists,” J Magn Reson Imaging. 2018 Sep. 48 (3): 590 – 604.
This article was written by Matt Ellis, Product Manager – Trimmers and Non-Magnetic Components, Knowles Precision Devices, Cazenovia, NY. For more information, visit here .