add fMRI documentation glossary

docs/glossary.md

@@ -25,6 +25,16 @@ Each record consists of the same number of fields, and these are separated by co
 
 - [wikipedia](https://en.wikipedia.org/wiki/Comma-separated_values)
 
+#### fMRI
+
+**fMRI** stands for *Functional Magnetic Resonance Imaging*.
+
+It measures brain activity by detecting changes associated with blood flow.
+This technique relies on the fact that cerebral blood flow and neuronal activation are coupled.
+When an area of the brain is in use, blood flow to that region also increases.
+
+- [wikipedia](https://en.wikipedia.org/wiki/Functional_magnetic_resonance_imaging)
+
 #### FWHM
 
 **FWHM** stands for *Full Width at Half Maximum*.