Nuclear medicine - PET and SPECT

Radioactive decay is usually seen as pollution. With some clever chemistry it can make pictures of cancer.












We met this picture before.

Bad dog.

Remember that the reason Holmes could see the dog clearly, was because someone had painted it with phosphorus. In other words the dog glowed with light and therefore was easy to see in the dark. Suppose we could do this with human beings? Clinicians often want to know whether you’ve got cancer in the first place ... But as important as this is, finding out something about your cancer is just as important. Does it have a good oxygen supply? How much sugar fuel is it burning? Will it respond to a particular drug, e.g. herceptin? If we can make the cancer glow then we can do two things: we can see it highlighted against the rest of the body; and we might be able to use how brightly it glows to learn something about it – for example, brighter glow might mean the tumour is burning more sugar.

Gamma rays pass through our bodies fairly easily. If you look at the photo below, the light passes through the curtain. You can see the scene beyond almost completely. So if something inside our bodies glowed with gamma rays, we should be able to see it because the rays would pass out of our bodies without getting disturbed – like sunlight and net curtains. Problem: most of the things we see around us do not glow with gamma rays, and we avoid the ones that do!

It's a bit hazy, but the light still comes through the net curtain.

A PET scan. The bright spots show where the radioactivity has collected.

If we attached a radioactive atom to a substance that is attracted to cancer cells, then we could inject the now radioactive cancer seeking substance and wait. After a while the cancer would have attracted most of the radioactive substance. If we took a photo with a camera that sees gamma rays then we would see a dim outline of the body with a bright spot where the cancer is. As it turns out this is possible and the results are called PET and SPECT scans. You can see the results on the right. The bright spot is a tumour growing in the chest.

Normally the PET/SPECT image is combined with an X-ray image to give a better idea of where the bright spot is in the body. The X-ray image is on the left and the combined image is shown next to it. These images are very powerful. The bright spot is not an image telling you where the tumour is – it’s telling you how active the tumour is in terms of fuel consumption. The location of the tumour is almost a by-product. As we treat the tumour we’d expect the fuel consumption to go down. If we kill some of the tumour it should burn less fuel. By taking pictures of activity we can monitor treatment as well as finding the tumour's location.

A CT - X-ray scan
A combined image - CT + PET.

Go back and explore the other imaging techniques.