MRI stands for Magnetic Resonance Imaging. MRI scans were introduced in the 1980's and are now used frequently in the diagnosis of soft tissue injuries and other medical conditions.
What are MRI scans?
Unlike X-rays which only show bone, MRI scans show all tissues within the body, including bones, muscles, cartilage and fatty tissue. They are especially useful for viewing soft tissue which is enclosed or surrounded by bone, such as the brain and spinal cord. MRI's produce far more detailed pictures than x-rays or CT scans and so they are often used for detecting conditions such as brain tumours, strokes and heart defects. Sports injuries such as meniscus tears, ACL ruptures and labrum tears are often scanned by MRI to confirm diagnosis prior to surgery.
What happens when you go for your scan?
MRI scans are performed in hospitals, usually on an outpatient basis. When you arrive, all of the necessary paperwork must be completed and your radiographer should explain the procedure to you. You will then be asked to change into a hospital gown and remove all jewelry, hearing aids and make-up etc.
Depending on the reason for your scan, a dye may be required to highlight some tissues. This will be injected into a vein in your arm or hand and will be safely removed from your system via your urine over the next 24 hours.
The scanner itself is a cylindrical shaped, open ended machine, in which you lay on your back. The machine itself is quite noisy whilst the scan is being taken and so you may be asked to wear earplugs or headphones. The radiographer will be in a separate room but will always be able to see you inside the scanner. You will be given an alarm which you can press at any time throughout the scan if you need assistance.
Throughout the scan you will be asked to stay as still as possible and breathe very gently. It can take several minutes to take even one image and so the time your scan takes depends upon the number of images to be taken.
After the scan you can normally go straight home and the results will be sent to the Doctor who requested them on your behalf.
How do they actually work?
MRI's use magnetic fields and radio waves to produce an image of your body. These are not harmful, unlike x-rays. Passing these strong waves through the body affects the atoms within the body. The nuclei within each atom is forced into a different position, and when it returns to its normal position this produces a second radio wave. The machine picks this up and translates the information into an image. The image produced is due to the position and strength of these radio waves.
All of the tissues within our bodies contain water (H20) and water contains Hydrogen (H2). The Hydrogen atoms have a large part to play in how the image appears. Those structures with a high proportion of hydrogen atoms appear much brighter than those with a lower proportion. For this reason, fatty tissue appears much brighter then bone.