The following are frequently used commercial off-the-shelf (COTS) electronic components and materials. These lists are not endorsements of the corresponding products and are provided exclusively for information purposes.
Fast Rectifier Diodes
These types of diodes are used in pairs ("crossed") to implement passive switch circuits such as detuning traps, T/R switches, etc. Some are already packaged in pairs to assist use in the crossed configuration. Schottky diodes consist of a junction between a semiconductor and metal and consequently have a lower turn-on voltage than standard PN diodes.
The main parameters to consider are
- recovery time (ns): determines maximum frequency of operation of the switch of the switch
- capacitance (pF): capacitance seen when the switch is OFF
- forward current (A): maximum current handling of the switch
- forward voltage (V): turn-on voltage of the switch
- power dissipation (W)
Consult the corresponding data sheets for accurate information. The following are few examples (of likely hundreds of diodes available) that may be used in MRI applications.
|Part Number||Configuration||Recovery Time (ns)||Capacitance (pF)||Forward Current (A)||Forward Voltage (V)||Power Dissipation (W)|
|BYW29||single with heatsink||25||10 (to heatsink)||8||0.9||~10|
PIN diodes use a junction with an undoped (intrinsic) region between the P and N regions. The intrinsic region stores charge and makes the PIN diode an excellent (low-resistance) switch at high frequencies. Carrier lifetime determines the ON resistance of the switch and influences the lowest frequency of operation.
PIN diodes are typically controlled with an external DC bias to act as switches or current-controlled resistors (e.g., in attenuators).
The diodes in the following table are commonly used in MRI applications. Consult the corresponding data sheets for accurate information.
|Part Number||Carrier Lifetime (µs)||Resistance (ohm)||Capacitance (pF)||Power Dissipation (W)||Reverse Voltage (V)|
Transistor Cross Reference
Here are some popular transistors used in low-noise preamplifiers and their equivalents from alternate manufacturers.
|Manufacturer||Part Number||⇔||Manufacturer||Part Number|
* now obsolete according to this notice
These are some inductors known to have a non-ferromagnetic core and otherwise negligible ferromagnetic materials. Measured self-resonant frequency (SRF) is reported where available. The SRF is important when the inductor is used as an RF choke because at the SRF the series impedance of the inductor is maximal.
It is also common also to use self-wound inductors when the required inductance values are modest (e.g., below 100 nH).
Supplier of numerous air-core and other nonmagnetic inductors. Tunable versions can be ordered with aluminum core instead of ferrite.
|Part number||Nominal Inductance (nH)||Measured SRF (MHz)|
Non-magnetic (phenolic) core up to 18µH and otherwise no measurable magnetism.
Non-magnetic core and otherwise no measurable magnetism.
|Part number||Nominal Inductance (nH)||Measured SRF (MHz)|
Non-magnetic core up to 0.82µH.
Non-magnetic core up to 2.2µH.
Magnetic susceptibility in capacitors
The following diagram shows a cross section of a typical multilayer ceramic capacitor (MLCC):
Although magnetic susceptibility in MLCCs is commonly associated with the dielectric, the dielectric itself is not actually magnetic. The magnetic properties of MLCCs are determined almost entirely by the materials used for their terminations (the part we solder to) and the electrodes (the interdigitated plates buried inside the dielectric, which form the plates of the capacitor).
NME vs BME electrodes
Up through the 1990s, MLCCs generally used materials like Palladium or Silver for electrodes, and were reffered to as "Noble metal electrodes" (NME). These materials happened to be nonmagnetic as well. Then in the 90s, breakthroughs occurred which allowed cheaper "base" materials like Nickel and copper to be used for electrodes (BME). In practice, BME electrodes are usually entirely Nickel, which is what gives most MLCCs there strong magnetic susceptibility.
Manufacturers prefer to use BME wherever it can be justified. But it turns out that BME electrodes tend to degrade the stability of capacitors compared to NME electrodes. Because of this, BME dominates class 2 dielectrics (X5R, X7T, Y5R, etc) where stability is not such a concern, while NME is still used with most class 1 dielectrics (C0G/NPO). This is why a NPO/C0G capacitor will typically have much less magnetic susceptibility than an X7R cap of the same case size, capacitance, and voltage rating. It's only indirectly related to the dielectric.
Terminations for MLCCs are selected primarily for cost and solderability. Nickel is very commonly used as an intermediate layer in MLCCs between the electrodes and the terminal finish (usually Tin). This is also true for many other surface mount components as well. Unfortunately for MRI engineers, Nickel is very cheap and effective, so it shows up everywhere.
Usually for BME capacitor, the electrodes are responsible for most of the magnetism. But for very small case sizes (0402, 0201, etc), terminations may actually contribute more.
Termination material is generally unrelated to dielectric. Therefore most NPO/C0G capacitors will have Nickel in their terminals, giving them some magnetism. Again, this is most noticeable with very small case sizes.
Some capacitors are available with nickel-free terminations, but these should be chosen with caution. Often they are not suitable for repeated rework, due to solder leeching. Parts with alternative terminations are also more difficult to find in stock from vendors.
Vishay VJ series capacitors
The VJ Commercial Series from Vishay is notable in that it contains class 2 dielectrics built with NME. They're also offered with nonmagnetic terminations, but these are difficult to find in stock. If you really need large capacitor with low susceptibility, these are a good bet.
In cases where large capacitances are needed for DC supply filtering and decoupling, tantalum capacitors can be a good alternative to MLCCs. The dielectric and electrode materials used in solid tantalum capacitors not ferromagnetic, but the terminations will usually contain a nickel layer. The MICROTAN 298D from Vishay is one series which offers a good tradeoff between magnetism and capacitance. Kemet also advertises tantalum capacitors which are completely nonmagnetic, but these can likely only by ordering directly from the manufacturer.
These are some capacitors known to contain negligible ferromagnetic materials. Important parameters to consider include
- maximum voltage handling (V)
- equivalent series resistance (ESR, ohm)
- temperature coefficient of capacitance change (ppm/ºC)
- capacitance tolerance (F)
SMD Commercial High Voltage C0G
Surface mount, multilayer ceramic chip capacitors with low ESR and high voltage (kV range) rating. Confirmed very small magnetic attraction (C1812C151JHGACTU, 150pF).
ECC and ECK Series
Radial ceramic disk capacitors with leads, high voltage rating. Confirmed nonmagnetic (various part numbers beginning with ECCNVS).
Radial ceramic disk capacitors with leads, high voltage (kV range) rating. Confirmed very small magnetic attraction (CC45SL3AD821JYNNA, 820pF).
Surface mount, multilayer ceramic chip capacitors with low ESR and high voltage (kV range) rating. Confirmed very small magnetic attraction (CGA8L1C0G3F151K160KA, 150pF).
Radial ceramic disk capacitors with leads, high voltage (kV range) rating. Confirmed nonmagnetic (F102K43Y5RP6UK5R, 1000pF).
Fuses are used as second fail-safe devices to detune RF coils in case the trap circuits containing Diodes above do not work.
The table below lists some fuses that have been used in RF coils.
|Part Number||Manufacturer||rated current (A)||mounting||cold resistance (ohm)|
The following lists include materials commonly used in the construction of MRI equipment along with their properties.
Magnetic susceptibility (\chi, in the SI system) is important because of its effects on static magnetic field (B0) homogeneity.
|Material||Susceptibility (ppm)||Density (g/mL)|
|FR4 PCB board||-3.742||1.952|
|FR3 PCB board||-7.671||1.601|
|PET (Mylar A)||-8.252||1.421|
|POM (Sustarin C)||-9.357||1.412|
|vinyl ester based CRFP||-9.422||1.49|
|FR2 PCB board||-10.112||1.352|
Most data adapted from Table 1 of http://dx.doi.org/10.1016/j.jmr.2014.02.005 (reference susceptibility of water was taken to be -9.032 ppm).
* from Table 4 https://doi.org/10.1002/(SICI)1099-0534(1998)10:3<133AID-CMR1>3.0.CO;2-Y. This reference also contains valuable information such as susceptibility of numerous solvents, chemical resistance, water absorption, etc.
Permittivity of 3D Printing Materials
The following data was compiled from several sources. Materials for 3D printing (FDM) vary widely as does the final density of the deposited material. It is recommended that properties be verified experimentally.
|Material||Frequency / MHz||Volume Fraction||Relative Permittivity||tan \delta|
The presence of NMR signal is important for items in close proximity to the imaging region, such as RF coils, where extraneous signals could lead to radiological misinterpretation of the images. This is especially important for acquisitions using ultra-short or zero echo time sequences because they can capture short-lived signals from solid materials. Converesely, the presence of NMR signal (e.g., RGD525 below) is desirable to create MR-visible solid phantoms.
For materials used in 3D printing we have the following data.
|Material||B0 / T||T2 / ms||T2* / ms|
The following table indicates the presence (+) or absence (-) of signal from four different nuclei for each of the listed polymers or composites. Data were obtained using spectroscopic methods. Note that the two references sometimes reach inconsistent results, and that different versions of the same polymer can also give different results.
|Commercial Name||Chemical Name||Abbreviation||1H signal||19F signal||13C signal||31P signal|
|acrylonitrile butadiene styrene||ABS||+||-||+|
|Acrylic / Plexiglas / Perspex / etc.||poly(methyl methacrylate)||PMMA||-||-||+|
|Delrin AF||polyoxymethylene / polytetrafluoroethylene||POM/PTFE||+||+||+|
|*Ertalyte||polyethylene terepthalate||PET, PETE||-||-||-|
|Estralon||polyethylene terepthalate||PET, PETE||-||-||-|
|Extruded Acrylic||poly(methyl methacrylate)||PMMA||-||-||+|
|*||high-impact polystyrene||HIPS, PS||+||+||-|
|high-impact polystyrene||HIPS, PS||+||-||+|
|*KYDEX 100||polyvinyl chloride /
|Noryl||polyphenylene ether / polystyrene||PPE/PS||+||-||+|
|Nylatron GS||polyimide / molybdenum disulphide||PI/MoS2||+||-||-|
|Phenolic XX||paper / phenolic||+||-||-|
|Polyester Impact Resin||-||-||-|
|*Polyurethane (Innovative Polymers)||polyurethane||PU||+||-||-|
|*Royalite R52||acrylic / polyvinyl
https://doi.org/10.1002/mrm.1910130317 (4.7T data)
* https://doi.org/10.1002/mrm.21566 (9.4T data)
° For polycarbonate and polyethylene,Table 2 of https://doi.org/10.1002%2Fmrm.22558 gives apparent spin densities of 85% and 2% (relative to water), and T2* of 7.5µs and 40µs, respectively (4.7T data).
Nonmagnetic Coaxial Cables
Many coaxial cables utilize a steel inner conductor (plated with copper or silver), or multiple strands, to increase tensile strength and durability. Luckily, nonmagnetic versions made of copper conductors are made by a few manufacturers and are readily available. Some nonmagnetic coaxial cables are listed in the table below. Check the manufacturer's data sheets for up-to-date information.
|Manufacturer||RG Designation*||Description||Dielectric OD (mm)||Part Number|
|RG-174||1.5||7805 and 7805R|
|similar to RG-223||2.95||G03262D|
* note that RG designations are obsolete and open to manufacturers' interpretations.
Many connectors contain ferromagnetic parts or platings. For example, BNC connectors must be carefully inspected because they often contain a ferromagnetic spring for the bayonet locking mechanism.
Add cross reference between cable types and compatible nonmagnetic coax connectors.