About… PET scanning

I said I’d write a little about PET scans and the procedures involved. As always, this comes with the disclaimer that I am neither medically trained, nor a specialist in nuclear medicine/nuclear physics!

PET and CT scanning are all derivatives of basic X-ray technology, so let us recap with the basics. A standard X-ray picture is obtained by shining X-rays (an electomagnetic radiation – much higher in frequency than light, but lower than gamma radiation) through the body to obtain a 2 dimensional shadow picture of the internal organs of the body. Dense tissue (that is, more radio-opaque tissue such as bone) appearing dark in colour because few X-rays pass through it, with less dense tissue appearing lighter.

This rechnique has been standard practise almost since the discovery of X-rays, with the main developments being more sensitive photographic film (so that radiation doses can be reduced), and the application of electronic sensors so that dynamic TV pictures of the internal organs can be obtained. This last techniques is particularly useful for examining blood flow in the heart, or the fluid movement in the gut, or even for minor surgery such as inserting a Hickman line. The use of electronic imaging also allows the possibility of computer enhancement of the images, by adding colour to emphasise the different tissue densities.

However, the basic limitation remains in that the pictures are two dimensional. This limitation was overcome by the next advance, the development of the Computerised Tomography or CT scanner. This uses a rotating continuous low level X-ray beam, that scans the body. The patient lies in the centre of rotating X ray emitters and sensors and a series of two dimensional pictures is taken through one narrow part of the body. These images are combined using computer algorithms to obtain a three dimensional image of a ‘slice’ through the patient’s body. By moving the patient past the rotating sensors at a controlled rate, a whole series of ‘slices’ can be obtained to produce a three dimensional picture of the internal organs.

While this is useful, it doesn’t actually give any information about what is going on inside the organs – it is still a passive picture. So a CT scan might show up an enlarged lymph node, but it won’t indicate if that enlargement is caused by scar tissue, or active disease. Enter the PET scan!

PET (or Positron Emission Tomography) is a relatively new (10-15 years old?) technique that allows the clinician to non-invasively see what is happening inside the body organs at a cellular level. This is done by injecting the patient with a radioactively tagged biologically active substance, such as glucose. All cells use glucose for fuel, but cancer cells take up glucose faster than normal cells. The radioactive marker is selected so that it emits positrons. Positrons are antimatter electrons, and when a positron meets with an electron in the body, the positron and electron mutually destruct, giving a burst of gamma radiation.

This gamma radiation can be detected, and by using a series of sensors placed in a ring around the patients body, the location of the source of the gamma radiation can be determined, as in a CT scan, using a computer to do the complex calculations. Again, by moving the patients body though the detectors a series of scans is obtained that covers the area of interest in the patient’s body. There is a more detailed description here which is also repeated under the lymphoma links on the left.

In practise, the PET scanner also incorporates a CT scanner so that the two images can be superimposed – the CT scanner giving the outline of the major organs – the PET scan showing the biological activity. The PET scan image is often enhanced with colour to improve the contrast between active areas.

So that is the theory – what happens (to me) during a scan?

The first point is that becase the cells take up radioactive glucose, the blood sugar levels have to be low – so there is no eating or drinking for at least 6 hours before the scan takes place. On arrival at the unit, the radioactive tracer is injected into the blood stream and then a period of about 90 minutes resting (or attempting the ‘Telegraph’ crossword) while the cells take up the tracer. After the waiting time is over, the scan takes place.

This involves lying on a couch built into the scanner, stretched out with ones arms above ones head. The bed then moves through the machine (it looks like an archway) while first the CT scan, then the PET scan is conducted. Each ‘slice’ of the PET scan takes about 5 minutes, and (in my case) 6 slices are taken, so the whole procedure takes half an hour. The most uncomfortable part is having ones arms above the head, which can get very uncomfortable after 20 minutes or so 0 and you cannot move during the procedure!

The PET scanners are located at St Thomas’s, and it is worth noting that when I had my first scan in 1999, I was very much a guinea pig – it wasn’t known if the technique would be of any use in my particular lymphoma. 13 scans on, I think the answer is probably ‘yes’! The scanners have also changed – my first scans took over 45 minutes to complete, although the resting time was shorter. I suppose the new scanners are more sensitive, and the radiation levels lower, which might account for the longer resting time but shorter scanning period.

There are very few PET scanners available in the country, so I am fortunate that the Guy’s and St Thomas’s trust have two. The imaging centre with the two new machines is a new facility, opened by Jonathon Dimbleby last year as it was partly funded by the Richard Dimbleby Cancer trust (there is a link on the right of the page) and I was luckily enough to be invited to the opening as a ‘regular customer’!

So (almost) everything you might have wanted to know about PET scans!