Simulation of Static Magnetic Fields

Numerical Simulations of Static 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.

Static Magnetic Fields

Figure 1: Image was acquired with a fast echo-planar imaging sequence on a plane just above the nasal cavity. The large black spot is due to distortions because of the boundary here.

In order to align all of the hydrogen nuclei in the body, very strong static magnetic fields are used in MRI. Because body tissues have a different magnetic permeability than that of air, unwanted distortions in a static magnetic field can occur near air-tissue boundaries (1). This can cause problems with imaging methods very sensitive to distortions in static magnetic fields, such as methods used in functional MRI. For example, Figure 1 was acquired with a fast echo-planar imaging sequence on a plane just above the nasal cavity. The large black spot is due to distortions because of the boundary here.

 

We have performed thorough calculations of the static field distortions in a finite element model of the human head and verified the results experimentally (2). Figures 2 and 3 show the finite element model and some results of some of these calculations. The model contains frontal, nasal, ethmoid, and maxillary sinuses. The permeability of all sinuses and the air surrounding the model were set to that of air and the permeability of all tissue was set to that of water.

Figure 2: Finite element model of the human head. The model contains frontal, nasal, ethmoid, and maxillary sinuses. The permeability of all sinuses and the air surrounding the model were set to that of air. The permeability of all tissue was set to that of water (2).

More recently we have developed code to calculate the magnetic fields throughout more finite difference models of the human head and breast, with many tissues having different permeabilities (2, 3). Figure 4 shows the results of some of these calculations with units in parts-per-million deviation from the applied field strength. The calculations were made with a finite difference method and a mesh of about a few million cells. The numerically-calculated field distributions matched both analytically-calculated and experimentally-measured distributions very well (2).

 

Figure 3: Results of some of finite difference calculations of the static magnetic field. Units are in parts-per-million deviation from the applied field strength. The mesh consisted of a few million cells and many different tissues. The calculated field distributions matched experimentally-measured distributions very well (3)

 

Principal Investigator: 
Christopher Collins

Sponsors

Latest Updates

12/04/2017 - 18:10
11/30/2017 - 11:13
11/29/2017 - 08:31

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

Go to top