What is an MRI?

I have been through a few of them and I am a physicist, so I hope I can explain you what is it, how it works, why it works and all questions related to the patient’s point of view… First of all, my personal experience is related to head MRIs, I have no idea what a knee MRI is like!

If after reading this Q&A you have further questions, please ask me!

What is it like? You’ll be lying on your back and your head and shoulders will be inside the cylinder part:


The inside of the cylinder is white and bright. Your head will be covered by the coil which emits a radiofrequency signal that allows the MRI to work.


You must not move during the scan, and sometimes the nurse will place large foam cubes at both sides of your head to avoid any movement. The scan is noisy, lasts for about half an hour and after about 15 minutes the nurse will inject you a contrast agent.

So, what should I know before?

A magnetic field is applied, this means you have to report any metal implant you have in your body. Some are compatible with MRI, some are not.

This also means you have to undress at least above your hips (you’ll get a gown, don’t worry), sometimes your own T-shirt can be worn. Remember most bras have metal parts. Just ask the staff if you don’t know what (not) to wear.

You’ll have to remove all jewellery (earrings, necklaces, any body piercings…), hairclips, etc. Ask if you hesitate.

Don’t forget to leave your credit cards and cell phone outside the room where the MRI scanner is!

You’ll be given written information that explains a few things about the MRI, read it carefully and sign it if you agree. It has probably also a few questions (about your health, your allergies, metal implants, etc.) you’ll have to answer.

And when I am in there, what happens?

Not much. You have filled in the form, you are wearing the gown, you have removed all metal parts, you have informed the staff about any metal implants… Pretty much all you can do now is lie and rest.

You lie on the bed, they place the coil, the foam to prevent you from moving, earplugs for the noise… and then you are pushed in. You’ll be holding a panic button that you can press if you panic and that will stop the procedure. The cylinder is bright and white and some have a mirror and you can see outside. The staff is in a nearby room and will be watching you through a large glass window.

The scan lasts about half an hour. After the first part the nurse will inject a contrast agent in your bloodstream and carry on with the scan.

I have claustrophobia, I don’t want to get in there. No way.

Ok, calm down. The cylinder is bright and white. Some models have a mirror and you can see the outside.  If you lower your eyes you can see your toes. Sometimes you are wearing headphones instead of earplugs and can listen to music. You’ll be holding a panic button in case you panic. Press it and the cavalry will appear. They will stop the procedure. If none of this helps, some hospitals are equiped with other MRI models that are open, the imaging is not as good as in closed ones, but still. Other posibility is sedation. But really, it is all right. I, myself, I am afraid of lifts and I have survived a lot of MRI scans!

What is that contrast agent you keep talking about?

It is gadolinium. It looks clear and water-like. It is not radioactive. It provides better contrast between normal tissue and abnormal tissue in the brain (and body). It accumulates in the abnormal tissue and thus the images are enhanced and more accurate and the abnormal spots (tumors) are very easy to identify and measure. It is also rapidly cleared from the body.

Ok, lets get scientific. How does MRI work?

The human body is primarily made of hydrogen atoms (remember most of our body is water, and water is two atoms of hydrogen and one of oxygen). MRI uses properties of hydrogen atoms to distinguish between different tissues. The protons in the nuclei of atoms have a property called spin. Some atoms have no spin. Some have two spin states (hydrogen, for example).

Without the presence of a magnetic field, the spins are randomly oriented. If you apply a strong magnetic field (the M in MRI), all spins will align with it (imagine them as tiny magnets) and will be oriented in one of two directions. This two directions or states are commonly called “up” and “down” and a certain amount of energy is needed or released to go from one to the other (just as if you were on a step jumping “up” or going “down”). The coil can emit a radiofrequency to make the spin switch to the opposite direction (to make “up” go “down” and “down” go “up”). When this happens the atoms/molecules release/absorb energy (the exchange of energy is called resonance, hence the R in MRI) and this energy can be translated in to an image (hence the I).

Does it harm my body?

No. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range (yes, of the same type used by the radio you have in the kitchen or by your TV). It doesn’t alter the molecules at all. Regarding harm to the body, it is better than a CT scan or an X-Ray which involve inonizing radiation. Nevertheless, current guidelines recommend that pregnant women undergo MRI only when essential.

Are there any after-effects?

No. You can return to your normal life as soon as the scan is over. A radiologist (a doctor specialised in this diagnosis) will see the pictures and write a report for the doctor who requested the scan.


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