4. When a person lies in a scanner, the hydrogen nuclei (i. e
When a person lies in a scanner, the hydrogen nuclei (i.e., protons) found in abundance in the human body in water molecules, align with the strong main magnetic field
A second electromagnetic field, which oscillates at radiofrequencies and is perpendicular to the main field, is then pulsed to push a proportion of the protons out of alignment with the main field
These protons then drift back into alignment with the main field, emitting a detectable radiofrequency signal as they do so
Since protons in different tissues of the body (e.g., fat vs. muscle) realign at different speeds, the different structures of the body can be revealed.

10. Diagnose tumours

11. Multiple sclerosis

12. Sports injuries

16. Advantages
MRI doesn't use ionising radiation, and magnetic fields are thought to be safe
MRI images have excellent spatial resolution (i.e. you can see very small details in the images)

17. Disadvantages MRI scanners are very big and expensive
Because of the strong magnetic fields, all metal objects have to be kept out of the room or they would get pulled into the scanner
People with pacemakers or other implanted devices can't have MRI scans as the magnetic field would stop them working

18. Disadvantages
An MRI scan can take up to 20 minutes to complete and you have to be still the whole time as any movement would blur the image
The changing magnetic fields also produce a lot of noise, which can be scary!