Magnetic resonance imaging generates detailed body scans using magnetic fields

Credit: Sandra Kang/ Credit: Sandra Kang/ Credit: Kimberley Williams via Flickr Creative Commons Credit: Kimberley Williams via Flickr Creative Commons Credit: Ram_Issaquah via Flickr Creative Commons Credit: Ram_Issaquah via Flickr Creative Commons Credit: Manish Shrestha via Flickr Creative Commons Credit: Manish Shrestha via Flickr Creative Commons

If you’re fortunate enough never to have had one, you might not know what MRI means. If, however, you were unlucky enough to have experienced a medical examination involving one, or you just know a few acronyms, you might know that it stands for magnetic resonance imaging. Magnetic resonance imaging (MRI) is a medical imaging technique that uses magnetic fields to build images of various parts of the body.

The process relies on the electronic properties of atoms to collect snapshots of the body by manipulating these atoms’ electron structures. An MRI is used primarily to examine soft tissue in the body, and it is capable of examining anatomical structures such as the brain, spinal cord, and major muscle groups. MRI machines use very powerful magnets and rely on dense computational tools.

To begin our discussion of MRIs, we must first review the underlying physical and chemical principles that govern MRI. It is a basic principle of physics that a moving charge generates a magnetic field.

For those who are unfamiliar with the concept of a magnetic field, consider the effect of a gravitational field such as Earth’s gravity. A gravitational field produces a force that acts on all bodies with mass within that field. Earth, for example, has a gravitational field, which is why objects fall to the Earth rather than float away. Magnetic fields are similar to gravitational fields, but instead of acting on mass, they act on charges in motion.

Because every atom contains electrons that are constantly in motion, every atom has what is called a ‘spin magnetic moment’ in quantum mechanics. This is the magnetic moment in an atom induced by the spin of its electrons around the nucleus.

The magnetic moment is a measure of how magnetized an atom becomes due to the motion of its electrons, and thus how it will move when placed in a magnetic field. In the presence of a magnetic field, the spin magnetic moment of an atom can align with or against the direction of the field; this alignment can change the magnetic spin and produce an energy gap in the atom. The size of the energy gap relies on the strength of the magnetic field to which the atom is exposed.

To understand how this energy gap is utilized in MRI, we must introduce another fundamental idea in chemistry, called the electromagnetic spectrum. This spectrum refers to the spectrum of all possible types of radiation. Radiation is the emission of energy in the form of waves or particles, and comes in forms ranging from from radio waves, to visible light, to gamma rays.

Radiation is defined by its frequency and wavelength, and the frequency determines how much energy the radiation has. For the strength of a magnet typically used in an MRI machine — 1.5 to 3.0 tesla — the corresponding energy gap produced in atoms exposed to the field is equal to that of radiation in the frequency range of radio waves.

In order to measure the energy gap produced by the magnet in the MRI machine, the machine projects radio waves onto the patient to excite the magnetized atom and cause it to essentially jump the energy gap.
When the atom falls back down from the excited state of the energy gap, it emits the energy it absorbed as radiation in the form of radio waves. It is this re-radiation that the MRI machine detects using antennas placed close to the patient’s body.

Another interesting property of an MRI machine is that the magnetic field produced is not constant, but rather a gradient. If the field were constant, the spin magnetic moments produced in the atoms of the patient would all be the same, and there would be no differentiation between different parts of the body.

In order to produce a gradient magnetic field, coils are placed throughout the machine with different properties to create varying magnetic fields along the length of the machine. The gradient magnetic field produces gradient spin magnetic moments along the patient’s body, which creates a gradient of re-radiation radio wave signals picked up by the MRI. Knowing what part of the machine corresponds to what part of the magnetic field allows computer programs to generate an image from the re-radiation signals.