National Physical Laboratory

Assessing the compatibility of medical implants with hospital MRI scanners

Research into the safety of patients with medical implants in the high magnetic fields of MRI scanners.

This project is looking at the measurement techniques to determine whether it is safe to perform a scan of a patient with an implant using MRI.

During an MRI scan, the patient is exposed to a high static magnetic field (typically 1.5T), a gradient field that is switched at approximately 1 kHz and a radio frequency field, having a frequency related to the static field, B0, by

SAR MRI Equation
SAR MRI Diagram

The scan sequence is carefully controlled to prevent peripheral nerve simulation (PNS) due to the switched fields, or excessive heating of tissues due to absorption of the RF energy.

Computed distribution

The image to the right shows a simulation of a 16-rung whole-body MRI coil, showing the RF power absorption in the patients legs. Significant enhancements of the absorption can occur due to the presence of medical implants, if these are electrically conducting. Thus, to ensure patient safety, such devices must be tested for MR compatibility.

MRI CoilLaboratory based system

NPL has a laboratory based system that recreates the RF field from a 1.5 T MRI scanner. The implant under test is placed in a tissue equivalent phantom, and electro-optic temperature probes are used to determine the temperature rises that occur around the implant during a imaging scan. The research is addressing the measurement uncertainties for this technique and accuracy of predicting the tissue temperature rises in the patient, the effect of the phantoms complex permittivity on the results. Also it will establish best-practice for the design of implanted devices for MR compatibility. At this stage, the work is focusing on measurements of passive medical implanted devices. We encourage the involvement of industrial collaborators in this work.

For more information please contact Benjamin Loader  

Last Updated: 11 Sep 2012
Created: 11 Sep 2012

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