Non-Magnetic Spring Probe Solutions for MRIs

By Contributed Article | October 29, 2024

Using a spring probe grid array allows designers to customize the number of ground pins used to optimize the performance of connectors used in MRI systems, serving specific frequencies/coil designs, saving size, weight, and cost while maintaining the performance of a coax channel where needed.

MRI (Magnetic Resonance Imaging) systems provide a clear and detailed view of structures inside the body in a non-invasive procedure used for complex medical diagnostics. This equipment applies the principal of nuclear magnetic resonance within a powerful magnetic field to create images of body structures without the use of potentially harmful X-rays. MRI technology continues to refine, providing healthcare providers ever more valuable insights.

MRIs depend on a variety of sophisticated interconnect technologies. Within an MRI machine, an RF coil is used as an antenna, encircling the specific body part to be imaged. This coil communicates with the RF gradient within the magnetic field and is typically connected into the larger MRI system with a high frequency cable and connectors.

However, electronic components are typically comprised of magnetic materials, which may create interference that distorts the image. In order to prevent that, the coil cables and connectors used in MRI systems must be non-magnetic. Additionally, the signal, power, and RF contacts used in these connectors must be highly reliable to endure the tens of thousands of mating cycles required by this in-demand equipment. As coaxial RF contacts can be large, costly, and fragile, there is pressing need for smaller, more robust, and more cost-efficient interconnects to serve these systems.

In MRIs, coils are often in direct contact with the patient. This calls for design that centers on patient comfort as well as technical performance. Making coils from lightweight materials helps prevent the need for excessive pressure on the patient being imaged. Minimizing the size and weight of the cable and connector is important for the same reason.

Each RF channel in an MRI coil normally requires one coaxial connection. By replacing coaxial

contacts with a contact grid array, designers can reduce the size, weight, and cost of the connector. However, a more effective solution may be replacing pin and socket contacts with spring probes. This makes cleaning the cable connector faster and easier, since flat target contacts are simpler to clean than coaxial sockets. This helps improve the overall throughput in an MRI suite.

Adding a connector at the coil end of the cable makes it easier to replace the cable if it becomes damaged, instead of sending the entire coil out for repair, which reduces downtime for equipment and lowers the cost of equipment ownership. To do so requires a very small connector since it is in even closer proximity to the patient. Connectors that feature an array of contacts make this possible.

Spring probe PGA technology

Spring probes have a very high mating cycle life, often higher than traditional pin-and-socket contact systems. The use of spring probes keeps very expensive MRI systems operating to their full capacity. Smiths Interconnect has developed non-magnetic spring probes with mating cycles exceeding 60,000, specifically for MRI applications.

Using a spring probe grid array also provides greater versatility and customization options. The designer can customize the number of ground pins used and optimize the performance of the connector for specific frequencies/coil designs. This strategy helps save size, weight, and cost while maintaining the performance of a coax channel. Spring probe grid arrays also simplify cable termination activities by allowing mass solder termination to printed circuit boards.

Comparing the relative size of the PGA and coaxial contact connectors in select applications, the PGA contacts occupy only about half the space of traditional coaxial contacts, as shown in the examples below for 16 and 32 channel connector pin fields

Screenshot

Features & Benefits of Spring Probe PGA (Pin Grid Array)

Features and benefits of connectors specified for typical MRI system applications:

– High Density: Smaller footprint for patient comfort

– 60K mating cycles: Increase in MTBF

– Stable contact resistance – Consistent image quality

– Damage-resistant target contacts – Increase in MTBF

– Sealed target contacts – Cleanable interface / Increase in MTBF

– Non-magnetic components – Prevent “artifacts” that could distort the image

Typical Interconnect Requirements for RF MRI Connectors

Relative Magnetic Permeability (Less than 0.00005µr)
Insertion Loss (Greater than -0.2dB up to 12.5MHz; Greater than -0.3dB up to 135MHz)
Return Loss (Less than -33dB up to 12.5MHz; Less than -20dB up to 300MHz)
Crosstalk Ratio (Less than -45dB up to 12.5MHz)
Low Level Circuit Resistance (Less than 20mΩ per contact (before and after mating cycle life)
Mating Cycle Life (60k mating cycles)

A typical 16 channel PGA layout, shown in Figure 3, incorporates 32 probe contacts, in which 16 are signal and 16 are returns. Each RF channel utilizes 1 signal contact and an adjacent contact for the return.

To validate the interconnect technology of spring probes in a PGA pattern, Smiths Interconnect commissioned a series of tests, including tests for insertion loss vs. frequency, magnetic permeability, insertion loss and return loss, crosstalk ratio, low level contact resistance, connector durability/contact life, and more. The configuration tested was a circular connector with a ring array of 24 equally spaced spring probes carrying 12 channels.

The testing exercise indicates that spring probe grid array connectors provide the

mechanical, electrical, and optimizable performance required for a typical MRI coil

interconnect system, as well as internal connections within the system. Additionally, this technology provides a non-magnetic interface, reliable mating up to 60,000 cycles, with insertion/return loss, cross talk, and contact resistance levels required by MRI system designers, making it a highly effective choice for the design of next generation MRI equipment.

To see the detailed analysis of the test results, including tests for insertion loss vs. frequency, magnetic permeability, insertion loss and return loss, crosstalk ratio, low level contact resistance, connector durability/contact life, and more, visit Smiths Interconnect’s technical library to read this white paper in its entirety.

To learn more about spring probes, visit Smiths Interconnect.