The picture becomes clear for magnetic resonance imaging
Box 1 | How magnetic resonance imaging works
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You will get more from this topic if you have mastered the basics of atoms and molecules and electromagnetic radiation – these links will take you to an annotated list of sites with helpful background information. |
Magnetic resonance imaging (MRI) makes use of an intriguing fact of nature. When exposed to a strong magnetic field, atoms tend to fall into alignment with it – just like a compass needle aligns itself with the Earth’s magnetic field by always pointing north. The MRI machine uses a superconducting magnet to generate a very strong magnetic field – as you move into the bore of the machine you move into this magnetic field.
A very strong magnetic field
The strength of the magnetic field inside the MRI machine (0.5-2.0 tesla) is up to 20,000 times stronger than the Earth’s magnetic field. Despite the strength of the field, it has no detrimental effects on the human body. Nevertheless, such a strong magnetic field can still be dangerous: any loose, metallic object inside the MRI examination room will be attracted to the machine, often at high speed. All sorts of hospital equipment, as well as spectacles, watches and earrings, have all ended up stuck to MRI machines by mistake. The danger is not limited to external metallic objects: tiny metal fragments that may be lodged in a patient's eye can be ‘sucked’ out by the magnet, damaging the tissue – and the patient’s eyesight. Pacemakers and bionic ears may also be adversely affected.
Atoms line up with a magnetic field
The strong magnetic field has its most interesting effect at the subatomic level. Your body is composed of countless billions of atoms, each one of which contains a nucleus (made up of protons and neutrons) and at least one electron in orbit around it. Atomic nuclei that have an unequal number of protons have a tendency to line up with a magnetic field. This is the atom's magnetic moment or magnetism.
MRI can make use of elements such as phosphorous, sodium, nitrogen, carbon and fluorine. But hydrogen, which has a high magnetic moment, a small mass and is very common throughout the body, is the element most commonly targeted in MRI procedures.
When subjected to the strong magnetic field inside the MRI machine, the hydrogen nuclei inside your body are induced to align either in the direction of or against the direction of the magnetic field. Slightly more will line up with the magnetic field than against it, producing a small net nuclear magnetisation in the direction of the magnetic field.
To obtain a magnetic resonance measurement, researchers apply a pulse of energy to the patient in the form of radio waves at a frequency that is specific to hydrogen. This imposes electric and magnetic fields for a very short time, causing the magnetic moments of the hydrogen nuclei to flip through 90°, so they are now pointing at right angles to the magnetic field. As the magnetic moments of the hydrogen nuclei precess (rotate) in the new plane, they induce a signal that can be recorded by the MRI equipment. After the pulse of energy, the signal decays with time as the hydrogen nuclei return to their original orientation.
Different tissues give different radio signals
The density of hydrogen atoms will vary depending on the nature of the tissue being examined, and the density of tumours and other abnormalities will usually differ from the surrounding tissue. Differences in the density of hydrogen atoms are reflected in differences in the induced radio signal; the higher the density, the higher the strength of the signal. A computer interprets the radio signal data and generates a visual image of each slice of tissue based on differences in the density of hydrogen atoms. Each slice is only a few millimetres thick. Two-dimensional images of each slice can be produced or the computer can put them together to form a three-dimensional image of the tissue (and abnormality) in question.
Gradient magnets and contrast agents
Additional magnets, called gradient magnets, are also used. They switch on and off rapidly, altering the main magnetic field in the exact area of interest inside the body and allowing the medical team to focus in on particular areas – one of MRI’s great advantages over other scanning technologies. (The rapid switching on and off causes the very loud clicking sound referred to in the Key text.)
The images are often improved by the use of contrast agents or dyes – substances that penetrate the tissue of interest and alter the magnetic field there. Abnormal tissue will usually absorb the agents to a lesser degree, and this will be reflected in a different response to the imaging process and will therefore be easier to observe.
Related site
How MRI works (How Stuff Works, USA)
Posted June 2001.