Simulation of Magnetic Fields in MRI Machines
The invention of Magnetic Resonance Imaging (MRI) machines in medical diagnosis is very interesting. Before MRI was MRI, it was called Nuclear Magnetic Resonance (NMR) that was initially demonstrated in 1945. The transformation of medical imaging from NMR to MRI is attributed to three Physicists: Dr Raymond Damadian, Paul Lauterbur and Sir Peter Mansfield.
In 1969 Dr. Damadian hypothesized that cancerous cells can be differentiated from healthy ones using magnetic resonance. As cancer cells hold more water, they have more hydrogen atoms that would show up in the MR image. His theory was proved through experiments at NMR specialities company.
In 1971 Paul Lauterbur made similar findings and he also came up with the first MR image of water filled test tube. His idea of having 3D MR images of living tissues was only a hypothesis till then. In 1972, Peter Mansfield was studying chemical anisotropy and suggested that exposing a chemical to a magnetic field gradient can help to image its atomic structure. This would allow Scientists to create three dimensional images of tissues based on their underlying biochemical constituents.
The race to create first whole body MR scanner began shortly thereafter with these three Scientists participations. Damadian created the first whole body scanner in 1977. This system was named “indomitable”. The images taken from this scanner were far clearer and detailed compared to X-Rays and CT Scans. In 1978, Damadian founded his first MRI scanner company called Fonar that was later approved by FDA.
The physics based simulation technologies came in early 1970’s which is before the arrival of MRI technology. However, simulation tools of those times were not robust enough to model phenomenon such as magnetic fields and its effect on human tissues. Today, Simulia offers such modeling capabilities through its world class technology in the product called Opera.
There are three aspects of simulation in MRI machines: Main field, shielding and quenching.
The mail field is a very strong (perhaps 60,000 times the earth magnetic field intensity) and homogenous axial or z-axis aligned DC magnetic field that is formed over the image volume causing protons in the body to align parallel or perpendicular to the field. Higher DC fields increase the proton resonant frequency thereby producing images of finer resolutions and shorter scan times.
The shielding is an FDA enforced safety protocol to ensure that the personnel operating the MRI machines within its close vicinity are not exposed to dangerous levels of radiation. The magnetic intensity in public field should not exceed 10 times the earth magnetic field. The Opera formulation of the electromagnetic fields achieves accuracy in the imaging volume that exceeds parts per million – necessary for successful MRI design and correction.
While the gradient coils are conducting in nature made of sheets or wires, the main and shield coils are superconducting in nature. Low Temperature Superconducting (LTS) coils usually operate at around 4 K, carrying large currents without resistance to produce the intense magnetic fields required for high MRI resolution. If the part of superconductor becomes conducting, the coils will quench. That will produce resistance in the medium thereby generating heat that will further quench the coil.
SIMULIA Opera can simulate the superconducting quench phenomenon as well as recommend measures to prevent it. Quenching can be caused by relatively small amount of heat so appropriate thermal shielding of coils is also critical. This can be achieved by the Multiphysics capabilities of Opera using coupled magnetic and thermal simulation.
Visualizing the invisible and measuring the immeasurable is a challenging task. The electromagnetic aspect of simulation belongs to such category as the quantities of interest are invisible to human eye, unlike deformation in structural analysis which is visible to observers.