Whenever the Neurology team comes down and starts asking for special MRI scans with T2-this and gadolinium that, my eyes usually glaze over. So here’s a very brief guide to MRI’s. Very brief.
General Principles
The MRI works by applying a magnetic field to the body. This magnetic field aligns the protons in water nuclei. Next, external radio frequency energy is applied which displaces the protons from that alignment. When this RF energy is removed, the energy emitted is measured.
There are two different ways to apply this RF energy, each with different time constants. Each differ based on how often the pulses are applied (TR) or the time between the RF pulse and reading the emitted energy (TE).
- T1: longitudinal, short TE and TR times, and
- T2: transverse, longer TE and TR times.
- T1 can be used using gadolinium (a non-toxic paramagnetic contrast agent) which changes the signal intensities in T1. This makes vascular structures very bright and is good for looking at the breakdown of the blood-brain barrier (tumors, abscesses, MS, among others).
- FLAIR is a kind of T2 that uses longer times. T2 FLAIR is good for differentiating between CSF and other abnormalities.
- Diffusion weighted imaging (DWI) detects the random movement of water protons in the space it has available to it. Normally, extra cellular water is able to move around freely. In ischemic tissue, diffusion becomes limited as the NA/K pump shuts down and water gets trapped in the smaller intracellular space.
| Weighting | Which tissue appears brighter |
|---|---|
| T1: T1-weighted MRI | Tissue with short T1 relaxation time appears brighter (hyperintense). Grey matter is bright, CSF is dark.
This is good for looking at anatomy. |
| T2: T2-weighted MRI | Tissue with long T2 relaxation time appears brighter (hyperintense). Edema lights up. Air will be black. |
| T1C: T1-weighted MRI after administration of contrast media | Many tumors show signal enhancement after administration of contrast agent.
So contrast is good for looking at any structural abnormality. |
| FLAIR: fluid-attenuated inversion-recovery MRI | Bright signal of the CSF (cerebrospinal fluid) is suppressed which allows a better detection of small hyperintense lesions.
This is good for looking at pathology in the white matter. |
| DWI: Diffusion weighted imaging | Bright signal in ischemic tissue, good for looking for acute stroke. |
And there you have it. I’ll find some good open source pictures to include, as well.