Science Questions

Why shouldn't you drink before a CAT scan?

Sun, 24th Jun 2007

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Question

Pete Soule asked:

In an eleven month period recently I had 3 CAT scans and one PET scan. For the last two CAT scans they said “no contrast”, which was, apparently something to drink 15 minutes before the scan. I thought it might be something with a large scattering cross section for X-radiation but I don’t know – 15 minutes seemed like a short time to get to the blood. What is it and what does it bring out??

Answer

Contrast is a substance you inject (or administer) into people, which is usually something like Barium or Iodine (hopefully you wouldn’t inject Barium – it’s not very nice) these have big nuclei which scatter x-rays.  They circulate in the blood stream and if you have a cancer or something, the blood vessels which supply cancer are leakier than blood vessels supplying healthy tissue, so you tend to get a build up of contrast agent in the abnormal area.

When the scanner comes on, the x-rays go right through normal, healthy tissue, but where the contrast is they get scattered and soaked up.  This means they don’t get through, and this makes the tumour or abnormality glow up which lets you pinpoint areas of abnormality using the contrast agent.

Simply put, it makes areas of damage easier to see.

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