|
MRI is a unique imaging method because it does not use radiation as an energy source. It requires specialized equipment and expertise and allows evaluation of some body structures that may not be visible with other imaging methods.
How the MRI Scanner Works
Your body is composed of tiny particles called atoms. Under normal conditions, the protons inside of these atoms spin randomly. The magnet creates a strong, steady magnetic field. This causes the protons to line up together and spin in the same direction. A radio frequency signal is beamed into the magnetic field. This makes the protons move out of alignment. When the signal stops, the protons move back to their aligned position and release energy. A receiver coil measures the energy released by the disturbed protons. The time it takes the protons to return to their aligned position is also measured. A computer uses this information to construct an image on a TV screen, showing the distribution of protons of certain atoms (usually hydrogen, as in the body protons are most abundant in the hydrogen atoms of water).
The screen images are then recorded on film so there is a permanent copy.
Preparation for an MRI Examination
Prior to an MRI scan you should eat normally and take any medication as usual, unless you were specifically instructed otherwise at the time of booking your appointment.
When you arrive for your scan you will be required to complete a questionnaire regarding your medical history, and the scanning procedure will be explained to you. Because the strong magnetic field used for MRI will pull on any ferromagnetic metal object implanted in the body you will be asked whether you have any prosthetic joints, aneurysm clips in the brain, a heart pacemaker or artificial heart valve, IUD (intra-uterine device) or metal plates pins or screws, or surgical staples. You will be asked if you have ever had a bullet or shrapnel in your body or have worked with metal and may have metal fragments in your body, particularly your eyes. Patients with any of these conditions may not be suitable for MRI scanning.
You will be asked to remove any metallic objects such as jewellery, hairpins, glasses, clips, hearing aids and non-permanent dentures.
Tooth fillings generally are not affected by the magnetic field, but they may distort images of the facial area or brain, so the radiologist should be aware of them.
If you are pregnant, you should inform staff. Although there is no evidence that a hazard exists for pregnant women, it is best not to take chances. In some cases an MRI scan may be recommended in pregnancy since it is safer than x-rays or surgery.
The MRI Equipment
The MRI unit is a cylindrical magnet with a table, which slides in and out of the centre of the magnet. The patient is positioned on the table and the table is then moved into the correct position for scanning within the magnet.
The MRI Procedure
The staff will help you onto the scanning table and you will be positioned for your examination. The scanning table will slide into the magnet. An advantage of MRI is that many pictures can be taken without repositioning the patient. You will need to keep absolutely still during the scanning process.
The Technologist will leave the room during the scan, but can see you and will communicate with you through an intercom.
During the scan you will not feel anything. Some patients may find it uncomfortable to remain still during the examination, and others experience a feeling of being “closed in” or claustrophobic.
During the scan you will hear some peculiar noises:
- The hum of the machine
- A thump when the radio waves are turned on and off
- Whirring, grating and other machine-like noises
Depending on the part of the body being examined, a contrast material may be used to enhance the visibility of certain tissues or blood vessels. A small needle is inserted into the arm or the back of the hand and the contrast material is administered.
Common Uses of MRI
MRI is an especially valuable tool for helping to diagnose:
- Brain and nervous system disorders
Multiple sclerosis can be seen in its earliest stages.
Tumours, malformations, strokes and inflammation can be distinguished from surrounding normal tissue.
- Spinal disease
Degeneration, tumours or other abnormalities of the bone, discs and spinal cord can be easily detected.
- Cancer and other Organ diseases
MRI may be used to detect cancer and other diseases of the uterus, ovaries, prostate liver, pancreas lymph nodes, bladder, kidneys, vocal cords and other organs.
- Bones and Joints
Abnormalities in bones, muscles, joints, cartilage and ligaments are well seen including knee, shoulder, ankle injuries and degeneration.
MRI can be used to diagnose many other conditions and can also be used to evaluate the effect of treatment of disease.
|
