Physics of Medical Scans
What is an MRI?
An MRI scan is a medical scan that uses magnets to see inside your body.
MRI can be used to monitor brain activities, blood flow, heart problems, as well as joint and ligament damage.
Why MRI?
-
MRI produces more detailed images, and is particularly good at imaging soft tissues, whereas other scans such as CT are better suited for imaging bones.
-
MRI is non-ionising, meaning it does not emit harmful radiation. A MRI examination is not believed to increase the patient's risk of cancer.
What to expect?
An MRI scan typically lasts for 30 minutes to an hour, sometimes even longer, depending on the area to be examined.
Patients must lie still in the scanner (approximately 60cm diameter) during the examination. This could cause mild discomfort for larger patients.
The scanners also produce noises, due to the magnetic field being constantly switched on and off. Patients are given headphones or ear plugs to block out the noises created by the scanner, and there will be an emergency button next to them, should they experience any discomfort during the scan. MRI scans are completely painless!
​
How does it work?
Magnets and Water Molecules
The magnets used in this scan are typically 1500 to 3000 times stronger than the magnet on your fridge.
When you place a kitchen magnet close to a compass, the needle on the compass moves.
An MRI scan works in the same way.
Our body is primarily made up of water molecules. These water molecules consist of smaller particles called “protons”, which act as tiny magnets.
When a strong magnetic field is applied to the protons, they align themselves in the direction of the field. The magnetic field is constantly being switched on and off to cause the protons to move. The scanner can detect the motion of these protons.
The protons in different parts of your body such as fat and muscle move at different speeds, and this enables the computer to distinguish between different tissues.
The images produced by this scan help doctors see if the patient has any heart problems, brain or spinal cord injuries, or even tumours.
Can I have an MRI scan if I have...
-
a pacemaker
-
a cochlear implant
-
a dental implant
Always inform your healthcare provider if you have had any metallic implant.
Patients with a pacemaker or a cochlear implant cannot have an MRI scan, as the strong magnet inside the scan will interact with the implants.
Dental implants use materials that are not ferromagnetic - meaning they are not attracted to magnets, and hence are perfectly safe.[4]
I suffer from claustrophobia and am worried about the scan...
Your doctor may be able to prescribe oral sedation medication.
Inside the scan, there is a "emergency button", if you experience severe anxiety during the scan. [3]
Can I eat and drink before an MRI exam?
Yes, you should be able to eat and drink normally, unless your doctor tells you otherwise. [3]
How much does an MRI scanner cost?
A 1.5T scanner costs about £1M. 1.5T is the most common magnetic field strength due to a compromise between performance, patient comfort, and ease of installation.
For a more detailed and scientifically involved explanation, click here for the advanced level
References:
[1] Eheman E et al, "PET/MRI: Where Might It Replace PET/CT?", Journal of Medical Research and Innovation, vol 46, Issue 5 (2017)
[2] Dementias Platform UK, "News: Dementias Platform UK trebles MRI-PET scanner resource for the UK", https://www.dementiasplatform.uk/news/dementias-platform-uk-trebles-mri-pet-scanner-resource-for-the-uk [Accessed: 21/02/19]
[3] NHS, "MRI: How is it performed?", https://www.nhs.uk/conditions/mri-scan/what-happens/ [Accessed: 22/02/19]
[4] Mathew C. A, Maller S, and Maheschwaran, "Interactions Between Magnetic Resonance Imaging and Dental Materials", Journal of Pharmacy and Bio-allied Sciences, vol 5 (2013)