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Analytical Path to Improved RF Field Homogeneity for High Field MRI

Chen, Xin
http://rave.ohiolink.edu/etdc/view?acc_num=case1237482374

Abstract Details

2009, Doctor of Philosophy, Case Western Reserve University, Physics.
The focus of this dissertation is on the finite RF (radio frequency) wavelength effect in high field MRI (magnetic resonance imaging). As the RF frequency increases proportionally with the main magnetic field strength, and due to the high dielectric permittivity of human tissue, the wavelength of the electromagnetic (EM) field produced by the MRI RF coil is substantially shortened in that tissue. The shortened RF wavelength is comparable with or even shorter than the size of the imaging subject (for example, the human head or torso.) The oscillation of the RF field in an imaging subject with such a short wavelength is the underlying physical cause of RF field and image inhomogeneities. Additionally, due to the short wavelength, the EM field can no longer be considered static or quasi-static, and full wave calculations must be applied to determine the field. In this work, we develop analytical calculations to simulate the RF field for several different types of RF coil models. Since the oscillation of the RF field is derived from Maxwell equations and cannot be eliminated, we first demonstrate that RF field with a high planar homogeneity in a specific plane can be achieved by completely restricting the oscillation of the field to the spatial axis perpendicular to that plane. Furthermore, RF coil models with a number of independent current sources are simulated. By virtue of efficient analytical calculations, optimizations are applied to search for optimal current sources that can produce desired RF field patterns with significantly improved homogeneity. Last but not least, the current distribution and the RF field patterns of a circular loop antenna operating at ultra-high RF frequency are studied. Calculations demonstrate that the shortened RF wavelength has an impact on not only the field pattern, but also the source current distribution on the RF coil. Through the comparison with FDTD (finite difference time domain) numerical simulations, we find that our analytical calculations based on simplified models can accurately simulate the field for a more realistic situation. The optimization significantly improves the RF field homogeneity. Additionally, the extremely high computation efficiency of analytical calculations is also shown.
Robert Brown (Committee Chair)
David Farrell (Committee Member)
Hiroyuki Fujita (Committee Member)
Mark Griswold (Committee Member)
Harsh Mathur (Committee Member)
Shmaryu Shvartsman (Committee Member)
Birdcage/TEM; RF FIELD; Rung; Coil Model; FDTD

Recommended Citations

Citations

  • Chen, X. (2009). Analytical Path to Improved RF Field Homogeneity for High Field MRI [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1237482374

    APA Style (7th edition)

  • Chen, Xin. Analytical Path to Improved RF Field Homogeneity for High Field MRI. 2009. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1237482374.

    MLA Style (8th edition)

  • Chen, Xin. "Analytical Path to Improved RF Field Homogeneity for High Field MRI." Doctoral dissertation, Case Western Reserve University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=case1237482374

    Chicago Manual of Style (17th edition)

case1237482374
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© 2009, all rights reserved.
This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.