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How it Works: MRI scanner

By DuPage Medical Group Radiology

Water molecules are the most abundant molecules in the human body, so these are the very molecules the MRI machine uses to create an image. When an MRI machine looks inside the body, what it really sees are water molecules. Because all parts of the body contain some water, MRIs can examine any part of a patient's body. Water molecules consist of oxygen and hydrogen atoms, and the core of a hydrogen atom—its nucleus—is a single proton.

These protons have a basic, inherent property called nuclear spin causing them to spin like a gyroscope or a top. Because the protons also have an electrical charge, the spin makes them act like tiny magnets. A magnetic field will make these spinning protons wobble, like a spinning top that isn't quite vertical. The stronger the magnetic field, the faster the wobble.

If a proton is given some extra energy by a radio frequency, it will send that energy back out as a signal that’s faintly detectable. The proton's signal is determined by its wobbling rate, which for water is at a rate of about 50 million cycles per second.

An MRI machine is basically just a strong magnet and a radio transmitter and receiver, plus a lot of electronics to coordinate their operation. The MRI magnet creates a strong magnetic field, hundreds of thousands of times stronger than the earth's magnetic field; the radio transmitter beams an intense burst of radio waves into the patient to excite the wobbling protons; and the receiver detects the protons' faint radio signal.

To create an image, the MRI machine must determine which radio signals are coming from which protons and plot them in their proper locations. To distinguish protons from one another, the electronic component manipulates both the magnetic field and the radio energy so that the protons in different parts of the patient emit slightly different radio signals. The MRI machine automatically knows where each of the different signals originated from and can then build a three-dimensional image.

The MRI machine also reads how strong the radio signals are in each area. Where there is a lot of water, for example in muscle, the signal is strong, and where there is less water, for example in bone, the signal is weak. The different strengths of the radio signal create the shades of gray in the final image based on the patient’s anatomy.

History of MRI

A Rotating Magnetic Field was first discovered in 1882 in Budapest, Hungary, by Nikola Tesla, which is why the magnetic field strength used for MRI scanners is measured in Tesla.

In 1937, Columbia University Professor Isidor I. Rabi noted that atomic nuclei absorb or emit radio waves when exposed to a strong magnetic field.

Raymond Damadian, a physician and experimenter working at Brooklyn's Downstate Medical Center discovered in 1969 that the hydrogen signal in some tissues is different from that of other tissues because the differences in the amount of water contained in them. Hydrogen atoms, which are the most abundant atoms in the human body, are more abundant in water than any other substance so water molecules are targeted in MRI scanning. Some credit him as being the “father of MRI”, a claim that is considered controversial.

In 1973, Paul Lauterbur, a chemist and an NMR (Nuclear Magnetic Resonance) pioneer at the State University of New York, Stony Brook, produced the first NMR image. In the late 1970s, the procedure became magnetic resonance imaging (MRI) due to the negative connotations the word “nuclear” invited.

On July 3, 1977, the first MRI scanner was made for human scanning as a prototype.

MRI scanners started showing up in hospitals in the early 1980’s for research purposes. By 1985, the first outpatient facilities with MRI scanning equipment opened-up in the Chicagoland area.  

Topics and Subtopics: Diagnostic Testing & Radiology

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