Simulations of Radiofrequency Magnetic Fields

Numerical Simulations of Radiofrequency Magnetic Fields for MRI

In Magnetic Resonance Imaging (MRI) a variety of magnetic fields are applied to the human body to manipulate the hydrogen nuclei throughout the body. These fields interact with the body in a variety of ways, producing both the desired signal and favorable image contrast, but also side effects including image distortions and heating of tissues.

We develop a variety of numerical methods and tools considering anatomical models of the human body in the MRI environment and use them to characterize electromagnetic field behavior, image appearance, and temperature increase due to field/tissue interactions in MRI. Such tools are increasingly becoming an integral part of engineering and safety assurance in MRI.

Radiofrequency Magnetic Fields

In order to excite nuclei to a state in which they produce a detectable signal in MRI, a radiofrequency (RF) magnetic field, B1+, must be applied. The available signal will depend in part on the magnitude and phase of this applied B1+ field throughout space and time. To receive signal it is necessary to have one or more receive coils (or antennas). By the principle of reciprocity, the received signal is proportional to the RF field B1- produced when driving the receive coil(s). The frequency of these fields is directly proportional to the strength of the static field, B0. It is advantageous to use a strong B0 field in MRI, but as the frequency of the B1 fields increases, two things happen electromagnetic wavelength effects make it more difficult to maintain homogeneous RF fields, and stronger currents are induced in conductive tissues, potentially leading to heating of the patient. Also, due to the nature of electromagnetics there are RF electric fields associated with the magnetic fields pertinent to excitation and reception. The electric fields produced during transmission can cause heating of tissues and those associated with reception determine the sensitivity to thermal noise for each coil.

We have developed several methods for calculating the RF fields during MRI. We have developed methods for calculating signal-to-noise ratio, signal intensity distributions on reconstructed images, and many other important entities for MRI as functions of the RF fields and coils. Our calculation methods regularly show excellent agreement with experiment, and our calculations have been valuable in many applications, including coil design, explanation of observed phenomena in MRI, and heating of tissues due to RF fields in MRI (1-43).

 

Principal Investigator: 
Christopher Collins

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Philanthropic Support

We gratefully acknowledge generous support for radiology research at NYU Langone Medical Center from:
 
• The Big George Foundation
• Raymond and Beverly Sackler
• Bernard and Irene Schwartz

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