The picture becomes clear for magnetic resonance imaging

Key text

In the mid-1980s doctors added another technology to their medical kits: magnetic resonance imaging (MRI). It may prove to be the most useful imaging tool yet in the diagnosis of a wide range of diseases and injuries. Not only is it safer than X-rays (prolonged exposure to X-ray radiation may cause tumours), it provides more detailed images of soft tissue than X-rays, ultrasound, computed tomography scans, or any other imaging technique.

The MRI procedure

If you are to undergo an MRI examination you will be asked if you have any metallic implants and to remove your watch and jewellery. (The MRI machine generates a strong magnetic field, which will attract any metallic objects.) The MRI examination room itself can be intimidating. A conventional MRI machine looks like a large cube – perhaps 2 metres high – with a tunnel, or bore, through the middle. Protruding from the bore is a bench: the patient lies down on this bench and something called a surface coil is positioned around the area of interest, such as the head (if, for example, the patient is having a brain scan).

The patient is then moved into the bore and the examination begins. This can be uncomfortable – about 3 per cent of patients suffer claustrophobia inside the bore of a conventional MRI machine and the examination has to be abandoned. The noise, too, can be distressing – a clicking sound, registering up to 90 decibels (similar to a power lawn-mower), continues throughout the procedure. Earplugs or headphones help reduce any discomfort that this might cause, and vacuum technology is being developed to reduce the noise. The examination takes between 10 minutes and an hour, depending on the type of examination being performed and the MRI system being used.

Magnetic fields and radio waves

So what is happening while a patient is inside the bore of an MRI scanner? The machine generates a very strong magnetic field inside the bore, and hydrogen nuclei in the patient's body act like tiny magnets and align with the field. By altering the magnetic field and sending pulses of radio waves, MRI operators can determine what type of tissue exists at a particular point inside the patient's body. For example, tumours can be distinguished from normal tissue. As the machine scans different points inside the body, it sends this information to a computer that generates a map of the different tissue types (Box 1: How magnetic resonance imaging works).

What MRI can do

MRI is becoming increasingly popular among the medical profession for diagnostic information – more than 10,000 machines are in use worldwide, just over 100 of which are in Australia. Of all the scanning technologies, MRI provides the most detailed images of soft tissues such as the brain; eyes; inner ear; blood vessels; organs such as the heart, liver, kidney, spleen and pancreas; the female reproductive system; the bladder and prostate; and joints such as the shoulder, knee, wrist, ankles and feet.

It is being used now for the detection of stroke and different forms of cancer. There is reasonable evidence, for example, that MRI is effective in detecting and establishing the size and extent of cancerous tumours in the breast, and it can help identify stroke victims who have damaged but viable nerve tissue and may therefore respond to particular types of therapy.

MRI can be used to provide ‘real time’ images of the heart and blood circulatory system in action, assisting the diagnosis of heart disease. It has a role in the diagnosis and treatment of sports injuries such as ligament and cartilage damage to the knee. It is also being used in brain research, because it can provide information on brain activity as the patient performs different tasks. This last use of MRI is called ‘functional’ MRI, because it helps scientists determine the functions of different parts of the brain.

MRI technology is evolving rapidly and becoming more user-friendly. For example, the bore of a conventional MRI machine is now being replaced by C-shaped machines that are open on one side, and by short-bore devices that are less claustrophobic. Smaller machines are being developed for particular parts of the body, and the speed at which they can produce images is increasing.

Safety

The main risk associated with MRI is that posed by the effects of the strong magnetic field on metallic implants inside a patient’s body (eg, pacemakers or bionic ears). Neither the magnetic field nor the radio waves are harmful.

Another risk is that MRI sometimes returns ‘false positives’. This means that the scan ‘reveals’ a disorder for which there are no other symptoms, potentially leading to incorrect diagnoses and treatment for ailments that don’t exist or are benign and therefore not needing treatment. A key part of a specialist’s job, then, is to ensure that the data provided by MRI is consistent with other symptoms and is not the only tool used in diagnosis.

Cost

A disadvantage of MRI is its price tag. MRI machines cost several million dollars and the unit cost of scans is higher than for X-rays and ultrasound. In Australia these costs are subsidised by the medical system, but people in poorer countries generally have much less access to such expensive procedures.

The future of MRI

There seems little doubt that MRI technology will continue to improve and, in more developed countries at least, will become an indispensable and increasingly reliable diagnostic tool. Smaller MRI units dedicated to imaging specific parts of the body should help reduce cost while also improving the quality of images. The picture is becoming clear: our ability to look inside the body without damaging it will continue to improve, increasing the accuracy of diagnoses and enabling more effective treatment of disease and injury.

Box 1. How magnetic resonance imaging works

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)

Activities

• The Educator's Reference Desk (USA)
  • Electromagnets – students learn how to construct an electromagnet and understand its basic properties.

• In the classroom (Public Broadcasting Service, USA)
  • 21st century medicine: Image-guided surgery – students magnetise objects then measure their magnetic force.

• Newton's Apple (USA)
  • Brain mapping – students read about different ways to map brain activity then observe and make a map of a city block.

• The Visible Human Project (National Library of Medicine, USA)
  • MRI scans – six MRI scans of a human male showing head, thorax, abdomen, pelvis, thigh and feet. These are a small sample of the images used to create a complete, anatomically detailed, three-dimensional representation of a normal male body.

• Multi-dimensional human embryo (University of Michigan, USA)
  • Sample images listed by Carnegie stages – MRI images of selected stages of human embryos. Slices through different planes – coronal, sagittal and axial – are available.
  Note: These animations require QuickTime.

• CSIRO Science Education Centres (Australia)
  • CSIRO Science Education Centres in each state and territory offer exciting hands-on electricity and magnetism activities for students. Contact the Centres to organise a visit.

Further reading

Useful sites

An explanation of how MRI uses magnetic fields to 'see' inside a body or another object. Scanning a chick embryo developing inside an egg is used as an example.
http://chickscope.beckman.uiuc.edu/about/overview/mrihs.html

Brain imaging (Neuroscience for Kids, University of Washington, USA)

Briefly describes different procedures for imaging brains (computed tomography, positron emission tomography, magnetic resonance imaging, and functional magnetic resonance imaging) and gives the advantages and disadvantages of each.
http://faculty.washington.edu/chudler/image.html

A life saving window on the mind and body: The development of magnetic resonance imaging (MRI) (Beyond Discovery, National Academy of Sciences, USA)

Explores how basic physics research over the last 70 years contributed to the development of MRI and functional MRI. (A text file of the complete article is available.)
http://www.beyonddiscovery.org/content/view.article.asp?a=129

Picturing the body – fMRI: watching the brain at work (The Exploratorium Magazine, USA)

Explains that functional MRI allows researchers to visualise the brain's functions and processes.
http://www.exploratorium.edu/exploring/bodies_mag/fmri_1.html

Nobel Prize.org, Sweden

• Press release: The 2003 Nobel Prize in Physiology or Medicine
  Describes the discoveries concerning magnetic resonance imaging that led to the 2003 Nobel prize in physiology or medicine.
  http://nobelprize.org/medicine/laureates/2003/press.html

• Poster: The Nobel Prize in Physiology or Medicine 2003
  Presents the research on magnetic resonance imaging that won a Nobel Prize.
  http://nobelprize.org/medicine/educational/poster/2003/index.html

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  History of MRI (University of Manchester, UK)

  Describes the development of MRI as a medical imaging technique from initial nuclear magnetic resonance experiments in the 1940s.
  http://www.isbe.man.ac.uk/personal/dellard/dje/history_mri/history%20of%20mri.htm

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  Glossary

  atom. The fundamental unit of all matter consisting of a nucleus of protons and neutrons surrounded by orbiting electrons (or in the case of hydrogen, just one electron). For more information see Back to Basics: Atoms and molecules (Australian Academy of Science).

  decibel (dB). A logarithmic scale used to denote the intensity, or pressure level, of a sound relative to the threshold of human hearing. A step of 10 dB is a ten-fold increase in intensity or sound energy and actually sounds a little more than twice as loud.

  The quietest sound we can hear is 0 dB; a soft whisper has about 100 times more sound energy and so is about 20 dB. A power lawn-mower has a factor of 10⁹ more sound energy and is about 90 dB. A rock band may be as high as 110 dB. Above 120 dB the sound produces discomfort and even pain. The scale is often adjusted to take account of the reduced sensitivity of human hearing to high and low frequencies and is then specified as dB(A). On this adjusted scale (the A-weighted scale), the range of human hearing is about 3 to 140 dB(A).

  For more information see What is a decibel? (How Stuff Works, USA); What is a decibel? (University of New South Wales, Australia); and Intensity and the decibel scale (The Physics Classroom, USA).

  element. A substance made up of only one type of atom. For more information see our Back to basics topic, Atoms and molecules.

  protons and neutrons. Small particles that form the nucleus of an atom. Protons have a small positive charge; neutrons have no charge.

  radio waves. Low frequency electromagnetic radiation. Radio waves have wavelengths ranging from less than a centimetre to as long as 100 kilometres. The hertz (Hz) is the unit of frequency and means one complete oscillation per second. Many frequencies are much higher than this so other units are used (eg, 1 megahertz (1MHz) = 1,000,000Hz).

  We divide the radio wave part of the electromagnetic spectrum into bands that are allocated to different uses. These include AM radio (amplitude modulation), FM radio (frequency modulation) and CB radio (citizens' band), television, aircraft communications, satellites, mobile phones and pagers. Within each band, no two transmissions can use the same part of the spectrum – or frequency – at the same time. For this reason, each band within the radio wave spectrum, itself a part of the broader electromagnetic spectrum, must be managed carefully to ensure the best use of this limited resource.

  The frequency of radio waves used in magnetic resonance imaging range from 1-100 megahertz, depending on the strength of the magnetic field in the scanner. This is close to the range of frequencies used for FM radio (88-108 megahertz). For more information see How the radio spectrum works (How Stuff Works, USA).

  superconducting magnet. A type of electromagnet (a temporary magnet formed when an electric current is conducted through a coil of wire). In superconducting magnets, the wire is cooled to a temperature close to absolute zero. At this temperature, there is virtually no resistance to the flow of electricity through the wire. For more information see How electromagnets work (How Stuff Works, USA).

  surface coil. A device that closely fits against the surface of the part of the body being imaged. It acts like a radio antenna, receiving radio signals from the patient's body.

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  Page updated August 2006.

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