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## Physics Tutorials

### Electromagnetism and MRI

Electromagnetism is electric and magnetic fields interacting.

For example, when an electric current passes through a straight wire, a magnetic field is produced. The magnetic field created will consist of lines of force that circle the wire. When the wire is coiled around an iron bar, the lines of force reinforce to form an electromagnet. In other words, electrons spiraling around a core produce a magnetic field straight through the center of the core. An electromagnet is the same as a regular magnet, except it is temporary: the magnetic field only exists when electric current is flowing.

#### Applications

MRI -- Magnetic resonance imaging is an imaging devise that uses nuclear magnetic resonance of atoms within the body induced by the application of radio waves.

MRI provides an unparalleled view inside the human body. An MRI is called noninvasive because you see into the body with minimal damage. MRI's are used in most medical facilities such as hospitals to produce computerized images of internal body tissues.

MRI works by the patient being placed first in a strong magnetic field. Most of the hydrogen atoms that make the tissues of the body contain one proton in the nucleus. Protons are always spinning. Because they are charged and spin, they produce a tiny magnet field, something like the electrons spiraling around the core of an electromagnet. In a magnet field the hydrogen protons act like tiny toy tops that wobble as they spin. The rate of the wobbling is called resonance. Since the proton makes up the nucleus of an atom, the wobbling effect is called nuclear magnetic resonance.

A short pulse of radio waves is sent through the body. Radio waves are signals made of pulsating electric and magnetic fields. When the frequency of the signal is the same as the resonance, the proton flips and spins on its opposite end. To do this the proton absorbs some of the energy of the radio wave. The change in energy is detected and converted to an image. The combination of magnetic field strength and radio signal that flips the proton depends on what substance the hydrogen atom is in. Scanning through particular combinations provide an diagnosis of what substances are in what locations.

#### Question

What are some differences between MRI and X-ray?

Answer: MRI involves lower energy and is less damaging. Xrays are used to make images of bone, while MRI provides images of the hydrogen-rich soft tissues.

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Michael Jassal