How does an MRI work (really simple)?
Protons are normally oriented in random directions in the absence of an external magenetic field.
So we apply a magnetic field externally in order to orient the protons along a single axis – all in the same direction. This requires a very strong magnet, which is why we have to be absolutely sure that patients do not take metals into the MRI scanner.
A radiofrequency pulse (which is a type of electromagnetic pulse) is then applied to these aligned protons causing them to become excited and orient themselves in a plane that is perpendicular to the plane of the magnetic field. All the protons are spinning in phase relative to each other. The radiofrequency pulse is then turned off, giving the protons that changed their orientation to re-orient themselves back in the same direction as the external magenetic field.
T1 relaxation describes the time it takes for the protons to relax or recover back to the same orientation as the magnetic field (after the RF pulse is turned off).
T2 decay describes the time or rate it takes for a proton to spin out of phase in comparison to other neighboring protons (complex topic with a complex definition that is trying be simplified). Different tissues will take different amounts of time to get out of phase once the RF pulse is removed.
More readings/videos:
- https://casemed.case.edu/clerkships/neurology/Web%20Neurorad/MRI%20Basics.htm
- https://www.youtube.com/watch?v=Z2xpY_hkSBY
- https://mri-q.com/why-is-t1–t2.html
- https://neuro.wustl.edu/Portals/Neurology/Education/PDFs/Localizing-Imaging-Workshop-Lecture-3-Handout-MRI-for-Neurology.pdf
Clinical relevance of T1 and T2
Tissues that are bright on T1
Tissues that have a short T1 will be bright on T1 weighted images.
- Fat
- Hemorrhage
- Melanin (melanoma)
- Proteinaceous fluid (found in certain types of cysts or tumors)
- Gallodinium contrast
- Gallodinium is a rare-earth heavy metal ion
- Gallodinium’s main purpose is to shorten T1 (and not as much T2) which as we know will cause tissues on a T1 weighted image to appear brighter
Tissues that are bright on T2
Tissues with a long T2 will be bright on T2 weighted images. H2O (including CSF) will appear bright on T2 weighted images.
- Fat
- Hemorrhage
- Water, edema (includes cysts)
- Inflammation/infection
What is FLAIR?
FLAIR is very similar to T2 except CSF (water) will appear dark, which allows for the easier detection of abnormalities. Especially good for the detection of abnormalities that are located in areas bathed by CSF like sulci or ventricles.