Before the development of magnetic resonance imaging, X-rays were the only way to image inside the body. X-rays are too energetic and show hard structures such as bones while they penetrate connective tissue and organs that show only as ghostly images.

In essence magnetic resonance tricks hydrogen atoms into revealing their location.

More than 60 percent of the human body is composed of hydrogen atoms. They are attached to carbon and oxygen atoms in myriad compounds that make up the various types of tissue. MRI uses a property of hydrogen atoms called "spin" to distinguish differences between tissues such as muscle, fat and tendon.

There are three main components to an MRI machine: the primary coil, a radio frequency coil and gradient magnets. The superconducting wires of the primary coil are kept cold enough to maintain their superconductivity by a bath of liquid helium at a temperature of 450 degrees Fahrenheit below zero. Without superconductors the wires would have too much resistance and would not be able to carry enough electrical current to produce the necessary magnetic field, more than 20,000 times stronger than Earth’s magnetic field.

Normally the single proton nuclei of hydrogen atoms are randomly precessing on their axes in all different directions like spinning tops. Under the influence of the primary coil, the protons align their spin in one of two parallel directions pointing either toward the patient’s head or feet. In principle the strong magnetic field aligns the protons in equal numbers in both directions, but because the process is random there is always a slight excess in one direction or another. It is only a few atoms out of every million, but it is enough.

A tuned radio pulse forces the protons to alter their magnetization alignment relative to the field. This "twisting" is the "resonance" part of the system. When this field is turned off, the protons return to the original magnetization alignment and create a signal that can be detected by the scanner. The specific resonant frequency depends on the particular tissue being imaged and the strength of the primary magnetic field.

At the same time three smaller gradient magnets, about one-thousandth as strong as the primary, cycle off and on rapidly. The arrangement of the three gradient magnets is such that when they’re turned on and off rapidly, they alter the main magnetic field on a local level. This allows targeting of a specific area, referred to as a "slice" as thin as a few millimeters. The rapid on/off cycling of the gradient magnets produces the hammering noise that many patients find uncomfortable during a typical 45-minute scan.

The protons release the energy absorbed from the resonant frequency pulses as they return to their natural alignment within the magnetic field when the pulse is turned off. This is the signal that is processed by the computer to create the images on the monitor.

Since the development of MRI scans from the first crude machines in the 1970s, the technology has become more efficient and has aided health professionals in identifying soft-tissue injuries that would be impossible by any other means.

Richard Brill is a professor of science at Honolulu Community College. Email questions and comments to rickb@hcc.hawaii.edu.