MRI-based transfer function determination for the assessment of implant safety.
OBJECTIVE
We introduce a new MR-based method to determine the transfer function (TF) for radiofrequency (RF) safety assessment of active implantable medical devices. Transfer functions are implant-specific measures that relate the incident tangential electric field on an (elongated) implant to a scattered electric field at its tip. The proposed method allows for TF determination with a high spatial resolution in relatively fast measurements without requiring dedicated bench setups from MRI images.


METHODS
The principle of reciprocity is used in conjunction with the potential to measure currents with MRI to determine TF. Low-flip angle 3D dual gradient echo MRI data are acquired with an implant as transceive antenna, which requires minimal hardware adaptations. The implant-specific TF is determined from the acquired MRI data, with two different postprocessing methods for comparison.


RESULTS
TFs of linear and helical implants can be determined accurately (with a Pearson correlation coefficient R ≥ 0.7 between measurements and simulations, and a difference in field at the tip ΔEtip ≤ 19%) from relatively quick (t < 20 minutes) MRI acquisitions with (several) millimeter spatial resolution.


CONCLUSIONS
Transfer function determination with MRI for RF safety assessment of implantable medical devices is possible. The proposed MR-based method allows for TF determination in more realistic exposure scenarios and solid media. Magn Reson Med 78:2449-2459, 2017. © 2017 International Society for Magnetic Resonance in Medicine.
Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
J P
JP
Tokaya
Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.|Biomedical Image Analysis, Eindhoven University of Technology, Eindhoven, The Netherlands.
A J E
AJE
Raaijmakers
Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands.
P R
PR
Luijten
Medtronic Eindhoven Design Center, Eindhoven, The Netherlands.
J F
JF
Bakker
Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
C A T
CAT
van den Berg
D000818:Animals; D003196:Computer Graphics; D003198:Computer Simulation; D046690:Deep Brain Stimulation; D004574:Electromagnetic Fields; D060733:Electromagnetic Radiation; D006801:Humans; D007091:Image Processing, Computer-Assisted; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging; D019047:Phantoms, Imaging; D019736:Prostheses and Implants; D011846:Radio Waves; D015203:Reproducibility of Results; D059629:Signal-To-Noise Ratio
EM simulations; RF heating; active implantable medical device (AIMD); safety; transfer function