History of Magnetism and MRI

Historical Context of Magnetism

• Magnetism originated from Magnesia, an ancient village in Western Turkey
• Magnetism is a fundamental property of some forms of matter
• Magnetite, an iron oxide, is a rodlike stone that rotates when suspended
• At rest, magnetite points towards water and is called a lodestone or leading stone
• The word "lodestone" is derived from "to find one's way"
• Lucretius Cerus first described magnetism's ability to attract and repel

MRI Historical Background

• Jasper Jackson is credited with producing the first MR signal from a live animal in 1967
• Wolfgang Pauli discovered the magnetic properties of atomic nuclei in 1924
• The first human image was obtained in 1977
• The first superconductive whole-body MRI scanner, constructed at Downstate Medical Center in New York, had a field strength of 5000 Gauss
• Edward Purcell and Michael Faraday won the Nobel Prize for their NMR research
• Edward Purcell led the Harvard University group that conducted the first magnetic resonance experiments with solids.
• Paul Lauterbur described Zeugmatography, an NMR imaging technique
• Dr. Raymund Damadian produced the first crude NMR image showing a rat tumor.
• The image appeared on the cover of Science Magazine
• The first whole-body MRI scanner was called Indomitable and used by Dr. Raymund Damadian
• Petrous Peregtimus de Maricourt named the ends of a magnet north and south poles
• William Sturgeon, a British scientist, constructed the first electromagnet in 1825
• Michael Faraday developed the law of magnetic induction, used in MRI
• Felix Bloch led the group of scientists from Stanford University that conducted the first MR experiments
• In 1989, Dr. Raymund Damadian was inducted into the National Inventors Hall of Fame for inventing the first human scanner
• Neils Bohr received the 1943 Nobel Prize in Physics for his theory of atomic structure
• Otto Stern received the 1943 Nobel Prize in physics for developing a method to study the magnetic moments of atoms
• Nikola Tesla developed the most effective way to generate and transmit AC; the magnetic unit of measurement, Tesla, was named after him
• Jean Baptiste Joseph Fourier developed complex mathematical formulas used in MRI for signal reconstruction into images

Basic Principles of Magnetism

• Any charged particle in motion creates a magnetic field
• The intensity of a magnetic field is represented by imaginary lines which always form closed loops
• When a charged particle moves in a circular or elliptical path, a perpendicular magnetic field moves with it
• Electron spin creates a magnetic field, which is neutralized in electron pairs
• Atoms with unpaired electrons in any shell exhibit a small magnetic field
• Spinning electric charges induce a magnetic field
• A proton in a hydrogen nucleus spins on its axis, creating a magnetic dipole called a magnetic moment
• Magnetic field lines do not start or end like electric field lines.
• These fields are bipolar or dipolar, having both a north and south pole
• A magnetic dipole is a small magnet created by the electron's orbit
• A magnetic domain is an accumulation of many magnets with aligned dipoles
• Randomly oriented dipoles align with the magnetic field when acted upon by an external magnetic field
• Magnetic field lines always form closed loops

Types of Magnets

• Naturally occurring magnets, such as the Earth itself, are a principal magnet type
• Lodestones exhibit strong magnetism because they remained undisturbed within the Earth's magnetic field
• Artificially induced permanent magnets are available in numerous sizes and shapes, such as bar or horseshoe magnets
• Compasses are a prime example of artificially induced permanent magnets
• Permanent magnets are typically produced by aligning their domains in the field of an electromagnet
• Electromagnets consist of a wire wrapped around an iron core.
• The intensity of the magnetic field is proportional to the electric current, and the iron core greatly increases the intensity

Magnetic Susceptibility

• Magnetic Susceptibility is the degree to which a material can be magnetized
• Diamagnetic materials are weakly repelled by both poles of a magnetic field, like water and plastic
• Paramagnetic materials lie between ferromagnetic and nonmagnetic materials.
• They are slightly attracted to a magnet and loosely influenced by an external magnetic field, for example, contrast agents used in MRI
• Ferromagnetic materials are strongly attracted to a magnetic field like Cobalt, iron, nickel, alnico, and rare earth ceramics
• Superparamagnetic materials consist of individual domains of elements that have ferromagnetic properties in bulk
• Superparamagnetic materials' magnetic susceptibility lies between ferro- and paramagnetic materials, for example, iron-containing contrast agents for bowel, liver, and lymph node imaging

Magnetic Laws

• Poles are the ends of a magnet
• Every magnet has 2 poles: North and South
• If a magnet is broken into smaller pieces, each piece becomes a baby magnet
• Like magnetic poles repel each other
• Unlike magnetic poles attract each other
• Imaginary lines of the magnetic field leave the north pole and return to the south pole of a magnet

Magnetic Induction

• Magnetic lines of induction are imaginary magnetic field lines
• The density of these lines is proportional to the intensity of the magnetic field
• Ferromagnetic material (soft iron) brought into an intense magnetic field alters the lines of induction by attraction
• The iron becomes temporarily magnetic

Maxwell's Field Theory

• The field created by a magnetic field behaves similarly to an electric field
• Magnetic force is similar to electrostatic and gravitational forces
• These forces are inversely proportional to the square of the distance

Magnetic Field Units

• The Earth behaves as though a large bar magnet is embedded in it
• Tesla (T) is the SI unit of magnetic field strength
• Gauss (G) is an older unit of magnetic field strength
• 1 T = 10,000 G
• An MRI field is 3T
• Earth's magnetic field ≈ 0.0001 T