The picture becomes clear for magnetic resonance imaging
Key text
This topic is sponsored by the Australian Government's National Innovation Awareness Strategy.
Magnetic resonance imaging is increasing in importance as a tool for diagnosing illness and injury. Regulations in Victoria now require professional boxers to have an MRI brain scan every 3 years.
To do their job properly, doctors need to know what is going on inside a patient’s body. One of the earliest instruments to help with this was the stethoscope, which since the early 1800s has allowed medical practitioners to listen to the heart, lungs and other organs. More recently, visualising tools such as X-ray, ultrasound and computed tomography scanning (a technique based on X-rays) have been developed.
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.
Posted June 2001.