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Test Field Diversity Method Using a Tabletop E-field Generator: In vitro Transfer Function Model Validation for Implantable Medical Device Safety Assessment in 1.5T and 3T MRI

Catalog of Regulatory Science Tools to Help Assess New Medical Devices 

This regulatory science tool describes a method which can be utilized in the Magnetic Resonance Imaging (MRI) safety assessment of implantable medical devices to assist in the prediction of potential RF-induced heating in the human body (or induced voltage in the device) in clinical 1.5T and 3T MRI scanners.

Technical Description

This tool provides an in vitro transfer function model validation method required to assess the RF-induced heating of implantable medical devices in the human body (or induced voltage in the device) while exposed to clinical MRI scanners using Tier 3 method defined in the International Standard Organization (ISO) Technical Specification (TS) 10974 [1]. This tool results in more than 50% reduction in TF model validation time while accomplishing the equivalent TF model validation tasks compared to traditional methods. This tool does not require a shielded room or costly high-power amplifier, making it simpler, faster, and cheaper than the current standard approach.

Intended Purpose 

The tool is intended to assist with assessing RF-induced power deposition in implantable medical devices and subsequent tissue heating (or induced voltage in the device) when exposed to clinical MRI scanners. This tool may support implantable medical device development, design optimization, and device intercomparison to establish MR-conditional labeling. This tool is intended for users such as medical device manufacturers and device test labs to validate the transfer function model of the elongated implantable medical devices’ response inside tissue simulating medium along with recommended procedures and options for:

  • Scaling factor calculation required for the Tier 3 method defined in ISO/TS 10974.
  • In vitro transfer function model validation using minimum necessary trajectories (i.e., two routings) inside tissue simulating liquid for RF-induced heating in human bodies (or induced voltage in the device) in 1.5 T and 3 T clinical MR scanners.

Testing

This method was compared with traditional in vitro transfer function model validation method using a whole-body sized birdcage coil [2]. The proposed method only required two device positions for the test while producing three times more incident test field conditions. Its advantages were that it did not require a shielded room, testing took less than three hours compared to more than eight hours for the traditional method, and produced an equivalent agreement in predicted RF power deposition.

Limitations

  • The method has only been evaluated for MRI field strengths of 1.5T and 3T.
  • The method can validate the transfer function model of implantable device trajectories of length up to 105 cm.
  • This tool does not fulfill all requirements for Tier 3 assessment of RF-induced power deposition inside human tissue (or RF-induced voltage in the device) per ISO TS 10974.
  • The method does not include bench testing of clinically relevant device routings with simulation studies in various virtual human models inside whole-body transmit coils.

Supporting Documentation

Full descriptions of the in vitro transfer function model validation method [1] and the test procedure using a test field diversity with tabletop E-field generator [2], [3], are provided in the article below: 

[1] ISO/TS 10974:2018, Assessment of the safety of magnetic resonance imaging for patients with an active implantable medical device. [Online]. Available: https://www.iso.org/standard/65055.html 

[2] H. Jeong, J. W. Guag, and A. Kumar, “RF-induced Heating Estimation of a Stent in a 3T MRI using Transfer Function Approach with a Tabletop E-field Generator,” IEEE Access, vol. 12, pp. 191945–191954, 2024, doi: 10.1109/ACCESS.2024.3518974. 

[3] Jeong, Hongbae, Transfer function measurement system validation with SAIMD-U. [Online]. Available: https://github.com/dbp-osel/Transfer-function-measurement-system-validation-with-SAIMD-U 

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