Question: Do you use contrast when performing an MRI on he head or is it not needed?

Jack Miller answered on 16 Jun 2013:

Hi Rhiannaberry,

I’m not sure if you mean “contrast” or “contrast agent” in your question — getting contrast is really the name of the game with MRI, as different tissues have different properties that can make them appear very different from one-another on different scans. For example, two different types of scans (called spin echo and gradient echo) can make blood appear black or white, and can give images that show up other regions of disparity (such as tumours). Contrast _agents_ are substances that provide more contrast in MRI. The best example of this is a substance used clinically, called chelated gadolinium, which appears as bright white on gradient echo scans. The reason this is useful is because it’s removed from the blood by the kidneys after you inject it — so, in a healthy patient, you see the blood and tissues turn bright, and then decay again about five-ten minutes after injection. If something’s wrong — for instance, if someone has had a heart attack, or, in my case, has a more advanced form of cancer in the brain — then the tissues aren’t connected up properly to the circulation, and the gadolinium doesn’t get pumped away again properly afterwards. The end result is a “late gad[olinium] enhancement” in the heart, where regions of tissue that have died following heart attack appear white. In the case of cancer, what’s usually done is to take a scan before and after gadolinium, and look at the difference in the images — regions which are bright white correspond to where the gadolinium has been “left behind”. This occurs as the cancers become promote the growth of blood vessels (“become angiogenic”) in a very chaotic, disordered way, and, frankly, they leak a bit. Hence, you end up with gadolinium left behind — which you can see in the clinic.

This is great, and gadolinium has saved many lives, but the trouble with cancers metastases in the brain is that, by the time they’ve become angiogenic, you’re pretty much unable to do anything else to help the patient — the cancer cells grow along the vasculature in the brain, and only promote the breakdown of the blood brain barrier (which lets the gadolinium through) at a relatively late stage, when they’ve formed large colonies. The technique I work on is essentially using another molecule, one found within the body (pyruvate) to act as a contrast agent — and one that hopefully will show up cancer cells much earlier than they can be seen with gadolinium.

So in short, you don’t need a contrast agent to get a picture of the brain, as the brain’s tissues themselves provide the contrast on the right type of scans — (look at http://en.wikipedia.org/wiki/File:Sagittal_brain_MRI.JPG — you can see the difference between the white matter (white), grey matter (grey), and bone (black)). However, to get something more than a picture of the brain (which is itself very useful!) you sometimes have to use additional contrast agents, like gadolinium or (hopefully!) 13C hyperpolarised pyruvate.

Hope this helps — do ask if it doesn’t!

— Jack