MRI is a non-invasive imaging technique used in radiology, which can show anatomical structures and physiological processes in your body in high resolution using strong electromagnetic waves, without the use of ionizing radiation.
How does MRI work?
The physics behind MRI can get pretty complicated, but we’ll try to explain the basis of it as simply as possible.
As you probably already know, water makes up about two thirds of your total body volume. Water molecules are composed of oxygen and hydrogen atoms. Fat also contains a large amount of hydrogen atoms. Hydrogen (H+) atoms (which are basically made out of a single proton, a particle with a positive electric charge) have natural magnetic properties, so a proton essentially acts as a tiny magnet.
When put in a strong electromagnetic field like the MRI uses, these protons align themselves just like compass needles. Then, the main magnetic source is turned off, and these protons go back to their regular state, while emitting signals of different intensities, which are detected by the MRI. Those signals are then digitally transformed into a visual image.
Other than "standard" MRI, there's also a so-called functional MRI (fMRI), which can detect changes in tissue activity by measuring changes in the blood flow, making MRI ideal for diagnosing diseases in the brain, spinal cord, and the heart, among others. MRI can be used to view changes in tissue metabolism, too, with the help of a technique called MR spectroscopy.
What is MRI used for?
Because MRI only "sees" protons, it is great for evaluating structures that are mainly composed of water and fat. MRI is predominantly used to diagnose diseases in the following structures:
- Brain and spinal cord (including strokes and aneurysms)
- Vertebras and bone marrow
- Joints (most commonly knee and elbow)
- Abdominal and pelvic organs (such as liver and uterus)
Besides diagnosing pathologic changes in tissues, MRI can also be used to evaluate the effects of different therapy procedures, and more recently, cancer (especially breast cancer) screening, which is still in its early stages.
How do I prepare for the MRI exam?
You don’t need to make any special preparations for this type of exam, unless your doctor specifically tells you otherwise. So, if you are on any kind of regular treatment, you can take your medications as you normally would. It would be smart to bring your medical documentation (the prior results from the radiology department would be ideal) so whoever reads the scan can compare their findings to the old ones.
These are some kinds of things you can’t bring to a room with an MRI machine:
- Jewelry (including piercings, watches, glasses, bobby pins)
- Mobile phones
- Credit cards
- Hearing aids
You will then answer a short questionnaire about your lifestyle and medical history, such as previous operations, injuries, tattoos and piercings. This is a safety precaution which helps the staff to minimize any potential damage that may happen by using the MRI incorrectly.
What does the MRI exam look like?
The MRI machine looks a lot like a CT scanner, meaning that it has a moving table that you lay on, and a donut-shaped part where the main parts of the machine are placed.
When you lay on the top of the table, you will be equipped with something called an RF (radio frequency) coil. It is a special frame fitted over the part of the body that is being scanned, and basically acts as an antenna, in order to properly receive the signals that the protons in your body emit. There is a different type of coil for every body part. For example, some coils are wrapped around your arms and legs, whereas you put your head inside of a coil that looks a lot like a helmet.
It’s important to mention that MRI machines are extremely loud, so much that you can compare it to the noise jackhammers make on a construction site. Luckily, you will be given earplugs (or even special headphones) that will ease the discomfort made by the banging sounds.
Because of all this, spending time inside of an MRI machine can make some patients feel scared or anxious, especially because the MRI exam can last from 15 minutes to one hour. The fact that you need to be completely still, and sometimes even hold your breath for short periods of time only complicates this process. This is why it’s important to tell the staff if you suffer from claustrophobia, or you're feeling nervous. As a precautionary measure, you will be given a little “alarm” pump, which you can press if you’re feeling uneasy, thereby stopping the examination. The staff will communicate with you via the intercom system at all times.
The MRI uses several sequences, and the length of the examination depends on how many and which ones the radiologist chooses. Three to five different sequences are usually performed.
Sometimes a contrast agent is needed so that the doctors can see some structures with more detail. If that’s the case, a technician will inject those contrast agents through your veins.
When all of the sequences are finished and the examination is over, you are free to go, and your radiologist can start interpreting your results, which you will get relatively fast.
Is MRI dangerous?
So far, there is no data to suggest that the MRI causes any kind of damage to your body, if done properly. This is why it’s extremely important to listen to the staff preparing you for the exam. Nevertheless, if you are pregnant, MRI is not recommended in the first three months, although that’s more of a precautionary measure, Similarly. women who breastfeed are encouraged not to feed their babies for up to two days after the exam.
For example, if you happen to cross your arms or legs during the exam, your skin might get burned. If you feel any kind of temperature change, you need to tell the staff immediately.
However, MRI examination is not possible in patients who:
- Have a pacemaker or an insulin pump implanted.
- Have an artificial heart valve.
- Have artificial implants made of metals (artificial hips, for example).
- Have had an operation which involved putting any metallic devices inside your body (aneurysm clips or a cochlear (hearing) implant).
- Have had injuries which resulted in metallic foreign body stuck somewhere in the body.
- Are obese and can’t fit into the machine or coil properly.
Technology has come a long way since the first MRIs, so many patients with artificial implants are now able to enter the MRI, although you need to have proof that the material the implant is made of has no magnetic properties whatsoever. Dental implants and fillings, on the other hand, have no magnetic properties, and are allowed inside the MRI.