Physics

How MRI Works



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Magnetic Resonance Imaging (MRI) operates using the physical properties of the hydrogen nucleus to create an image of internal body parts without damaging radiation. Previously called Nuclear Magnetic Resonance Imaging the naming convention was changed to reduce fears about nuclear radiation. Those fears are erroneous as MRI uses high power radio waves to work on the nucleus of atoms unlike the ionizing radiation of X-rays.

Atoms with an odd number of protons and neutrons exhibit a property that physicists call spin. While they don't really spin like a top the property is such that they appear to have an angular momentum that can be manipulated through an electromagnetic or radio frequency field. By applying a radio frequency to the nucleus of such atoms some energy will be absorbed, changing the spin of the atomic nucleus. When that field is removed the energy is released and can be detected and measured.

Since hydrogen is one of the most common elements in the human body this is the one that is mostly detected by an MRI scanner. However, by tuning the radio frequency of the scanner and detector other atoms such as carbon-13, nitrogen-15 and many other isotopes can also be detected. When the hydrogen atoms in different parts of the body are subjected to the magnetic field of the MRI their energy levels are changed. When the magnetic field is relaxed they go back to their original state releasing energy. Hydrogen atoms in differing types of molecules show slightly different properties in how they release their energy allowing computers to analyze the responses creating an image.

By taking two-dimensional images of thin slices through the body the MRI scanner can generate a three-dimensional image of the entire body, or just the body part that needs to be examined. Each two-dimensional image is a cross section of the body and when they are stacked together by the computer doctors can look deep inside for help in diagnosing abnormalities. This is a powerful tool in detecting tumors, problems with heart and blood vessels and diagnosing joint problems.

Unlike X-rays, and the related application of X-rays, Computed Tomography, MRI allows careful study of soft tissues. Since X-rays are absorbed by bone and generally pass through soft tissue, applications requiring the study of muscle, arteries and other such are limited. MRI, by working with the nuclei of hydrogen atoms, can be used for the applications where X-ray technologies are less capable.

Further advances in MRI technology have allowed for a number of specialized applications. Scientists studying brain activity can use MRI to show the response in the brain to certain stimuli and activities. Cardiologists can use MRI to study heart, artery and vein activity. Further advances in MRI combine the ability of nuclear magnetic resonance to identify specific molecules with the imaging capabilities of MRI to give information about what is actually happening biochemically inside the body.

Magnetic Resonance Imaging is a powerful tool for doctors and scientists. Allowing the study of the inside of the body without dangerous invasive surgery can save time, money and patient lives.

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