What do the different colors on an MRI mean?

Question

I've always wondered how the different shades show up. Why are some things grey as oppsed to white? What shows up as grey or black or white?

Answer 1

When you look at the images from an MRI, you're looking at what are called "sequences."  The physics of MR is extremely complicated, but suffice it to say, that the levels of brightness you see are an indication of SIGNAL characteristics of certain tissues in the body.  We can manipulate different parameters to make either water appear bright (high signal intensity) or make fat look bright.

On T1 weighted images, things that have macroscopic fat look relatively "bright," whilst things with water look intermediate in intensity.

On FLAIR or some T2 weighted sequences, it's the opposite, we make water look bright (except in FLAIR where we deliberately attenuate pure water to aide in conspicuity of certain edematous pathology).  On T2 fat will look intermediate-signal.

Without belaboring the matter, all the different ways we have in acquiring images of the body allow us to non-invasively look at human anatomy in sheer splendor and allow us to look for abnormatlities in ways we were never able to do before.

Absence of colour (signal) can be the result of air, metals (e.g. calcium deposits) or residual blood products.

Answer 2

What ever they want them to mean. The x-ray system for CAT scans and the MRI systems all just see in amounts of specific density of radiation absorbed or given off. The computer furnishes an artificial color to the desplay and can be set to just show density changes, changes with time, dye enhanced areas, etc. Just to provide a picture that the coctors can relate to.