An improved method to measure transfer functions using MRI

• Published in: Magnetic resonance in medicine Vol. 92; no. 5; pp. 2246 - 2260
• Main Authors: Eijbersen, Michael A., Steensma, Bart R., Berg, Cornelis A. T., Raaijmakers, Alexander J. E.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.11.2024
• Subjects: Algorithms; Computer Simulation; Copper wire; Electric fields; Humans; Image Processing, Computer-Assisted - methods; Jefimenko's equation; Magnetic resonance imaging; Magnetic Resonance Imaging - methods; magnitude squared least squares (MSLS); Maxwell's equations; Phantoms, Imaging; Reproducibility of Results; RF heating; safety; Thrombolytic drugs; transfer function/matrix; Transfer functions; magnitude squared least squares (MSLS); Jefimenko's equation; Maxwell's equations; RF heating; transfer function/matrix; safety
• ISSN: 0740-3194; 1522-2594; 1522-2594
• DOI: 10.1002/mrm.30179

Purpose A previously published method for MRI‐based transfer function assessment makes use of the so‐called transceive phase assumption (TPA). This limits its applicability to shorter leads and/or lower field strengths. A new method is presented where the background electric field is determined from both B1+$$ {\mathrm{B}}_1^{+} $$‐ and B1−$$ {\mathrm{B}}_1^{-} $$‐field distributions, avoiding the TPA and making it more generally applicable. Theory and Methods These B1$$ {\mathrm{B}}_1 $$‐distributions are determined from a spoiled gradient echo multiflip angle acquisition. From the separated B1$$ {\mathrm{B}}_1 $$‐components the background electrical field and the induced current are computed. Further improvement is achieved by recasting the B1$$ {\mathrm{B}}_1 $$‐field model as a “magnitude squared least squares” problem. The proposed reconstruction method is used to determine transfer functions of various copper wire lengths up to 40 cm inside an elliptical ASTM phantom. The method is first tested on EM‐simulated data and subsequently phantom and bench measurements are used to determine transfer functions experimentally. Results In silica reconstructions demonstrate the validity of the proposed B1$$ {\mathrm{B}}_1 $$‐field model resulting in highly accurate reconstructed B1$$ {\mathrm{B}}_1 $$‐fields, currents, incident electric fields and transfer functions. The experimental results show slight deviations in the field model, however, resulting transfer functions are accurately determined with high similarity to simulations and comparable to bench measurements. Conclusion A more generally applicable method for MRI‐based transfer function assessment is presented. The proposed method circumvents phase assumptions making it applicable for longer objects and/or higher field strengths. Additional improvements are implemented in the B1$$ {\mathrm{B}}_1 $$‐mapping method and the solution algorithm.