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How to ensure coaxial cables and connectors for critical MRI applications are truly nonmagnetic

As magnetic resonance imaging scanner fields get stronger, materials, processes and testing are crucial for nonmagnetic interconnects.

By Kai Loh, Times Microwave Systems®

Originally published with Medical Design & Outsourcing

 

Magnetic resonance imaging (MRI) uses powerful magnets and radio waves to generate images of the body’s internal structures. The strength of the magnetic field in MRI machines is one of the primary factors in determining the quality of the images produced. Currently, most MRI machines have a magnetic field strength between 1.5 Tesla (T) and 3T. However, there are now scanners approved for clinical use that go up to 7T and experimental systems that reach up to 11.7T.

MRI machines rely on extensive arrays of radio frequency (RF) interconnects — coaxial cables and connectors — to send and receive the pulsed RF signals used to image patients. It is critical these components be nonmagnetic, as MRI machines rely on the precise and accurate alignment of magnetic fields to produce high-quality images. The presence of any magnetic material can interfere with the process and result in reduced accuracy, distorted images, and potentially even harm to patients.

The term “nonmagnetic” is often used with respect to coaxial cable assemblies. The differences in magnetic properties among common materials used in RF interconnects can be subtle but significant in their impact, especially in potentially life-critical applications such as MRI machines. Therefore, careful consideration must be paid to the base materials used, material processing, component finishing, and testing of completed coaxial cable assemblies to ensure no magnetic properties are inadvertently introduced.

Material selection

Although RF interconnects such as coaxial cables and connectors are integral to MRI systems, their potential to introduce magnetic materials can be easily overlooked. Therefore, when selecting coaxial cables for MRI machines, it is vital to consider the base material and to what degree it can be magnetized. For example, ferrous materials such as iron and most types of steels should be avoided. Alternatively, nonmagnetic materials such as copper, brass, beryllium copper, aluminum alloys, and austenitic stainless steels are proven solutions in many applications.

The connectors used in MRI coaxial cable assemblies should also be made of nonmagnetic materials and designed to minimize the magnetic field they generate. It is not uncommon for many cable assembly suppliers to use components from various sources. Key components are constructed from a number of intricate piece parts often fabricated by different vendors. Without comprehensive control or a vertically integrated manufacturing environment, it can be challenging for a supplier to effectively meet the critical nonmagnetic requirement for MRI machines.

Processing and surface finishing

Even with careful selection of base materials and control of manufacturing processes, the extrusion process and machining of materials can introduce elements that may change magnetic properties, especially on a production line that handles a variety of materials.

Even nonmagnetic stainless steel can become magnetic after it has been machined into a connector. It is crucial to ensure the process is carefully regulated.

The finishing of components is also an essential consideration, as a nonmagnetic substrate can become magnetic if plated with the wrong material. For example, using a small amount of silver plating over a nonmagnetic substrate would typically produce an end product considered nonmagnetic.

However, with the increasing field strength of modern MRI machines, even a micro-inches-thick amount of magnetic material becomes increasingly relevant. Therefore, it is critical to carefully select and clearly specify material for each layer of the plating stack-up.

Testing

Testing and validating the finished materials used for coaxial cables and connectors to confirm they are nonmagnetic is essential. Use an in-depth test process aligned with industry standards such as ASTM F2052 or ASTM F221 to ensure the components are nonmagnetic. These standards define the requirements for materials, magnetic properties, and performance.

This can be done using a Gauss meter or a magnetometer,  two tools that measure the strength and direction of magnetic fields. The cables and connectors should be tested in their final configuration, including any adapters or extensions. Thorough testing of the materials and final products can ensure they will not interfere with the MRI imaging process.

Finding the right partner

Given the life-saving diagnostics provided by MRI, it is imperative to choose a partner with a proven track record of developing solutions for mission-critical applications such as the military, spaceflight and aerospace industries,. The partner should be able to leverage time-tested standards such as quality, cleanliness, and traceability from those industries.

Manufacturers must take these factors seriously and should offer not only materials but also technical expertise to solve complex problems from an industry-standard perspective. A third party that provides engineering services in addition to coaxial cable assemblies can become a part of the design team from the beginning and collaborate on the right interconnect solutions for a specific MRI application.

Whether designing a product or helping with processing and techniques, the partner must be committed to providing technical solutions and answers to deliver high-quality results.

Takeaways

In conclusion, the fundamental requirement to ensure coaxial cables and connectors used in MRI applications are truly nonmagnetic is increasingly important to ensure the images produced are accurate as MRI magnetic fields strengthen.

Careful consideration of the base materials, materials processing, and component finishing ensure coaxial cable assemblies used in critical MRI applications remain nonmagnetic. Once complete, testing the assembled cables and connectors for magnetic fields and verifying compliance with industry standards confirms the assembly will not interfere with the MRI imaging process.