Magnetic resonance imaging (MRI) scan is a common procedure around the world.
MRI uses a strong magnetic field and radio waves to create detailed images of the organs and tissues within the body.
Since its invention, doctors and researchers continue to refine MRI techniques to assist in medical procedures and research. The development of MRI revolutionized medicine.
This article looks specifically at MRI scans, how they work, and how doctors use them.
Fast facts on MRI scanning
- MRI scanning is a non-invasive and painless procedure.
- Raymond Damadian created the first MRI full-body scanner, which he nicknamed the Indomitable.
- The cost of a basic MRI scanner starts at $150,000 but can exceed several million dollars.
- Japan has the most MRI scanners per capita, with 48 machines for every 100,000 citizens.
An MRI scan uses a large magnet, radio waves, and a computer to create a detailed, cross-sectional image of internal organs and structures.
The scanner itself typically resembles a large tube with a table in the middle, allowing the patient to slide in.
The development of the MRI scan represents a huge milestone for the medical world.
Doctors, scientists, and researchers are now able to examine the inside of the human body in high detail using a non-invasive tool.
The following are examples in which an MRI scanner would be used:
- anomalies of the brain and spinal cord
- tumors, cysts, and other anomalies in various parts of the body
- breast cancer screening for women who face a high risk of breast cancer
- injuries or abnormalities of the joints, such as the back and knee
- certain types of heart problems
- diseases of the liver and other abdominal organs
- the evaluation of pelvic pain in women, with causes including fibroids and endometriosis
- suspected uterine anomalies in women undergoing evaluation for infertility
This list is by no means exhaustive. The use of MRI technology is always expanding in scope and use.
There is very little preparation required, if any, before an MRI scan.
On arrival at the hospital, doctors may ask the patient to change into a gown. As magnets are used, it is critical that no metal objects are present in the scanner. The doctor will ask the patient to remove any metal jewellery or accessories that might interfere with the machine.
A person will probably be unable to have an MRI if they have any metal inside their body, such as bullets, shrapnel, or other metallic foreign bodies. This can also include medical devices, such as cochlear implants, aneurysm clips, and pacemakers.
Individuals who are anxious or nervous about enclosed spaces should tell their doctor. Often they can be given medication prior to the MRI to help make the procedure more comfortable.
Patients will sometimes receive an injection of intravenous (IV) contrast liquid to improve the visibility of a particular tissue that is relevant to the scan.
The radiologist, a doctor who specializes in medical images, will then talk the individual through the MRI scanning process and answer any questions they may have about the procedure.
Once the patient has entered the scanning room, the doctor will help them onto the scanner table to lie down. Staff will ensure that they are as comfortable as possible by providing blankets or cushions.
Earplugs or headphones will be provided to block out the loud noises of the scanner. The latter is popular with children, as they can listen to music to calm any anxiety during the procedure.
Once in the scanner, the MRI technician will communicate with the patient via the intercom to make sure that they are comfortable. They will not start the scan until the patient is ready.
During the scan, it is vital to stay still. Any movement will disrupt the images, much like a camera trying to take a picture of a moving object. Loud clanging noises will come from the scanner. This is perfectly normal. Depending on the images, at times it may be necessary for the person to hold their breath.
If the patient feels uncomfortable during the procedure, they can speak to the MRI technician via the intercom and request that the scan be stopped.
After the scan, the radiologist will examine the images to check whether any more are required. If the radiologist is satisfied, the patient can go home.
The radiologist will prepare a report for the requesting doctor. Patients are usually asked to make an appointment with their doctor to discuss the results.
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It is extremely rare that a patient will experience side effects from an MRI scan.
However, the contrast dye can cause nausea, headaches, and pain or burning at the point of injection in some people. Allergy to the contrast material is also seldom seen but possible, and can cause hives or itchy eyes. Notify the technician if any adverse reactions occur.
People who experience claustrophobia or feel uncomfortable in enclosed spaces sometimes express difficulties with undergoing an MRI scan.
An MRI scanner contains two powerful magnets. These are the most important parts of the equipment.
The human body is largely made of water molecules, which are comprised of hydrogen and oxygen atoms. At the center of each atom lies an even smaller particle called a proton, which serves as a magnet and is sensitive to any magnetic field.
Normally, the water molecules in the body are randomly arranged, but on entering an MRI scanner, the first magnet causes the water molecules to align in one direction, either north or south.
The second magnetic field is then turned on and off in a series of quick pulses, causing each hydrogen atom to change its alignment when switched on and then quickly switch back to its original relaxed state when switched off.
Passing electricity through gradient coils, which also cause the coils to vibrate, creates the magnetic field, causing a knocking sound inside the scanner.
Although the patient cannot feel these changes, the scanner can detect them and, in conjunction with a computer, can create a detailed cross-sectional image for the radiologist.
Functional magnetic resonance imaging (fMRI)
Functional magnetic resonance imaging or functional MRI (fMRI) uses MRI technology to measure cognitive activity by monitoring blood flow to certain areas of the brain.
The blood flow increases in areas where neurons are active. This gives an insight into the activity of neurons in the brain.
This technique has revolutionized brain mapping, by allowing researchers to assess the brain and spinal cord without the need for invasive procedures or drug injections.
Functional MRI helps researchers learn about the function of a normal, diseased, or injured brain.
fMRI is also used in clinical practice. Standard MRI scans are useful for detecting anomalies in tissue structure. However, an fMRI scan can help detect anomalies in activity.
In short, fMRI tests what tissues do rather than how they look.
As such, doctors use fMRI to assess the risks of brain surgery by identifying the regions of the brain involved in critical functions, such as speaking, movement, sensing, or planning.
Functional MRI can also be used to determine the effects of tumors, stroke, head and brain injuries, or neurodegenerative diseases, such as Alzheimer’s.
How long will an MRI scan take?
MRI scans vary from 20 to 60 minutes, depending on what part of the body is being analyzed and how many images are required.
If, after the first MRI scan, the images are not clear enough for the radiologist, they may ask the patient to undergo a second scan straight away.
I have braces or filings, should I still undergo the scan?
Although braces and fillings are unaffected by the scan, they may distort certain images. The doctor and technician will discuss this beforehand. The MRI scan may take longer if additional images are required.
Can I move while I am in the MRI tunnel?
It is important to stay as still as possible while in the MRI scanner. Any movement will distort the scanner and, therefore, the images produced will be blurry. In particularly long MRI scans, the MRI technician may allow a short break halfway through the procedure.
I am claustrophobic, what can I do?
The doctor and radiologist will be able to talk the patient through the whole procedure and address any anxieties. Open MRI scanners are available in some locations for certain body parts to help patients who have claustrophobia.
A person can take medication prior to the test to ease anxiety.
Do I need an injection of contrast before my MRI scan?
A contrast dye can improve diagnostic accuracy by highlighting certain tissues.
Some patients may need to have a contrast agent injected before the scan.
Can I have an MRI scan if I am pregnant?
Unfortunately, there is no simple answer. Let a doctor know about the pregnancy before the scan. There have been relatively few studies on the effect of MRI scans on pregnancy. However, guidelines published in 2016 have shed more light on the issue.
Typically, doctors do not recommend contrast material for women who are pregnant.
MRI scans should be restricted during the first trimester unless the information is considered essential. MRI scans during the second and third trimester are safe at 3.0 tesla (T) or less. The tesla is a measurement of magnetic strength.
The guidelines also state that exposure to MRI during the first trimester is not linked to long-term consequences and should not raise clinical concerns.
Medically reviewed by Judith Marcin, M.D. on July 24, 2018 — Written by Peter Lam