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A Transfer Function Measurement System for MRI Safety Assessment of Implantable Medical Devices in Curved Trajectories for MRI Radio Frequency Safety Assessment

Catalog of Regulatory Science Tools to Help Assess New Medical Devices 

This regulatory science tool presents an apparatus to measure transfer functions (TF) of implantable medical devices in curved trajectories for MRI safety assessment.

Technical Description

The tool comprises a robotic arm with six degrees of freedom and an attached probe that measures induced current on the attached implantable medical leads in any arbitrary, clinically relevant trajectories fully or partially immersed in a tissue simulating phantom made according to the ASTM 2182 recipe.  TF measurements are made based on the reciprocity theorem of electromagnetics (EM) and the measurement is recorded by a vector network analyzer (VNA) in 2-port scattering mode (S21). The port-1 of the VNA is attached to a tiny monopole antenna via coaxial cable and the antenna excites the distal electrode end of the implanted lead for the heating TF measurement or proximal impulse generator (IPG) end of the active implantable medical device (AIMD), while the port-2 of the VNA is connected to the current sensing probe which traverses along the curved lead path and measures the induced current along the lead. A computer system controls the robotic arm and records and stores the TF magnitude and phase measured by the VNA through the current sensor.

Intended Purpose 

The tool is intended for the EM characterization of implantable medical device responses to MRI radiofrequency (RF) fields to assess MRI thermal safety following the Tier-3 approach described by ISO/TS 10974, “Assessment of the safety of magnetic resonance imaging for patients with an active implantable medical device”.  TF measurements can be used to estimate induced currents in leads and voltages in internal pulse generators under MRI scanner operation.  While TF is traditionally measured with leads in a straight-line configuration and fully immersed in tissue phantom, this tool enables TF measurements for clinically relevant curved lead trajectories as well as leads placed partially-in-partially-out (PIPO) of the body.

Testing

The tool has been thoroughly tested to verify its robust repeatability for accurate of measurement in following curved trajectories, with results compared to the traditional straight line configuration method: 

  1. Semi-circular arc trajectory
  2. Sinusoidal trajectory
  3. U-shaped trajectory with 5 cm separation from parallel lead segments (bend radius of 2.5 cm)
  4. Three different clinically relevant trajectories derived from XYZ images of patients with implanted leads.

A 55 cm long commercial lead with a complex winding configuration and a 60 cm simplified straight wire lead were used in the TF measurement validation and the average relative error (ARE) % based on deviation from TF measurements done in straight line configuration was performed on commercial lead.

To facilitate the placement of flexible implantable leads, rigid 3D printed trajectories in 2 mm diameter were developed to support the implantable leads for TF measurements. 

The maximum ARE for the TF magnitude measurements made on the commercial lead was for the U-shaped trajectory at 16.5% at 128 MHz. For the clinically relevant trajectories the maximum ARE was 15% at 64 MHz. These ARE % are lower than the percentage of uncertainty of TF measurement in traditional TF measurement in straight-line configuration, therefore the system is valid for TF measurements in curved trajectories. 

Scaling and validation of the measured TF would be performed like the traditional TF measurement scheme. 

Limitations

  • The tool does not specify test acceptance criteria for perform risk assessment
  • The method was only demonstrated and evaluated using a current probe with an overall width of 15 mm, which necessitates a minimum bend radius of 7.5 mm for all trajectories.
  • TFs of closely wound loops cannot be measured with this method.
  • TF measurement above 128MHz for partially-in-partially leads are not possible. 

Supporting Documentation

  • Z. Zuo, L. Yang, J. Zheng, Q. Wang, H. Jeong, S. Long, A. Kumar, J. Chen, “On the Validity of the AIMD Transfer Function Model Over Different Implantation Trajectories” IEEE Transactions on Electromagnetic Compatibility, vol. 66, no. 6, pp. 1698-1705, Dec. 2024, doi: 10.1109/TEMC.2024.3464125 

The following supporting documents are compiled in the User Manual. 

  • SOP
  • Instructional Guide
  • Installation instructions for the application software with graphical user interface (GUI)
  • Software code for application GUI
  • References

Contact

Tool Reference