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Your search keyword '"Raaijmakers, Alexander J.E."' showing total 2 results
Start Over Author "Raaijmakers, Alexander J.E." Topic safety
2 results on '"Raaijmakers, Alexander J.E."'

1. MRI-based, wireless determination of the transfer function of a linear implant : Introduction of the transfer matrix

Author

Tokaya, Janot P., Raaijmakers, Alexander J.E., Luijten, Peter R., and van den Berg, Cornelis A.T.

Subjects
Full Paper, RF heating, Phantoms, Imaging, Radio Waves, Transfer function, Temperature, EM simulations, Active implantable medical device (AIMD), Equipment Design, Prostheses and Implants, Magnetic Resonance Imaging, Electrodes, Implanted, Electromagnetic Fields, Radiology Nuclear Medicine and imaging, Humans, Computer Simulation, Safety, Full Paper—Hardware and Instrumentation, SAR
Abstract

Purpose: We introduce the transfer matrix (TM) that makes MR-based wireless determination of transfer functions (TFs) possible. TFs are implant specific measures for RF-safety assessment of linear implants. The TF relates an incident tangential electric field on an implant to a scattered electric field at its tip that generally governs local heating. The TM extends this concept and relates an incident tangential electric field to a current distribution in the implant therewith characterizing the RF response along the entire implant. The TM is exploited to measure TFs with MRI without hardware alterations. Theory and Methods: A model of rightward and leftward propagating attenuated waves undergoing multiple reflections is used to derive an analytical expression for the TM. This allows parameterization of the TM of generic implants, e.g., (partially) insulated single wires, in a homogeneous medium in a few unknowns that simultaneously describe the TF. These unknowns can be determined with MRI making it possible to measure the TM and, therefore, also the TF. Results: The TM is able to predict an induced current due to an incident electric field and can be accurately parameterized with a limited number of unknowns. Using this description the TF is determined accurately (with a Pearson correlation coefficient R≥0.9 between measurements and simulations) from MRI acquisitions. Conclusion: The TM enables measuring of TFs with MRI of the tested generic implant models. The MR-based method does not need hardware alterations and is wireless hence making TF determination in more realistic scenarios conceivable.

Published
2018

2. MRI‐based transfer function determination through the transfer matrix by jointly fitting the incident and scattered B1+ field.

Author

Tokaya, Janot P., Raaijmakers, Alexander J.E., Luijten, Peter R., Sbrizzi, Alessandro, and Berg, Cornelis A.T.

Subjects
TRANSFER matrix, TRANSFER functions, BESSEL functions, COPPER wire, SPHERICAL harmonics
Abstract

Purpose: A purely experimental method for MRI‐based transfer function (TF) determination is presented. A TF characterizes the potential for radiofrequency heating of a linear implant by relating the incident tangential electric field to a scattered electric field at its tip. We utilize the previously introduced transfer matrix (TM) to determine transfer functions solely from the MR measurable quantities, that is, the B1+ and transceive phase distributions. This technique can extend the current practice of phantom‐based TF assessment with dedicated experimental setup toward MR‐based methods that have the potential to assess the TF in more realistic situations. Theory and Methods: An analytical description of the B1+ magnitude and transceive phase distribution around a wire‐like implant was derived based on the TM. In this model, the background field is described using a superposition of spherical and cylindrical harmonics while the transfer matrix is parameterized using a previously introduced attenuated wave model. This analytical description can be used to estimate the transfer matrix and transfer function based on the measured B1+ distribution. Results: The TF was successfully determined for 2 mock‐up implants: a 20‐cm bare copper wire and a 20‐cm insulated copper wire with 10 mm of insulation stripped at both endings in respectively 4 and 3 different trajectories. The measured TFs show a strong correlation with a reference determined from simulations and between the separate experiments with correlation coefficients above 0.96 between all TFs. Compared to the simulated TF, the maximum deviation in the estimated tip field is 9.4% and 12.2% for the bare and insulated wire, respectively. Conclusions: A method has been developed to measure the TF of medical implants using MRI experiments. Jointly fitting the incident and scattered B1+ distributions with an analytical description based on the transfer matrix enables accurate determination of the TF of 2 test implants. The presented method no longer needs input from simulated data and can therefore, in principle, be used to measure TF's in test animals or corpses. [ABSTRACT FROM AUTHOR]

Published
2020
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