MRI-based transfer function determination through the transfer matrix by jointly fitting the incident and scattered field.

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

doi:10.1002/mrm.27974

Janot P Tokaya, Cornelis A.T Van Den Berg + Show 3 more

Open Access

https://doi.org/10.1002/mrm.27974

Copy DOI

Abstract

PurposeA 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 MethodsAn 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.ResultsThe 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.ConclusionsA 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.

Highlights

The transfer function (TF) has been introduced to characterize the potential for radiofrequency (RF) heating of elongated linear medical implants like pacemaker and deep brain stimulator leads.[1]
The ratio between the complex electric field at the tip and the complex field at the excitation device as a function of position along the implant gives the value of the transfer function at the location of the excitation
If a B+1 map is acquired together with a transceive phase distribution, φtr, with the implant to be tested in a phantom, the transfer matrix can be determined by fitting this expression to the measured field distribution

Summary

Introduction

The transfer function (TF) has been introduced to characterize the potential for radiofrequency (RF) heating of elongated linear medical implants like pacemaker and deep brain stimulator leads.[1]. The excitation device and measurement probe swap position: The induced field along the wire as a function of position is measured while the implant is excited at the tip.[4] Both methods are equivalent because of the principle of reciprocity. In previous work, this principle of swapping excitation and measurement position was used to develop a fully MR‐based method where the implant is modified into a transmit‐receive antenna that is driven at its tip. The current induced in the implant is proportional to the TF and is calculated from acquired MR images.[5]

Methods

Findings

Discussion

Conclusion