PET-CT

The evidence for metastatic disease in my case was provided by a PET-CT scan in early February. This was my second PET-CT , the first being July 2009, when no fluorodeoxyglucose avidity was detected. In the section of scan shown here, you can see my largest tumour just under my diaphragm in the upper right abdominal cavity close to the anterior (front of body) surface.

PET works as follows. The patient has radioactive (Fluorine 18) labelled glucose (fluorodeoxyglucose) injected into a vein and then lies still for about an hour. During this time, parts of the body with high metabolic activity preferentially take up the glucose. This includes the brain, the tongue, kidneys, heart, liver, and any tumour with rapidly dividing cells. Hence, a bright spot showing up in addition to the expected brightness for these organs, is a likely tumour.

At the same time that the PET scan is done, and in positional registration, an X-ray CT scan is carried out. This means that the PET bright spots can be correlated with anatomical details revealed by the X-ray. That is what you can see in my scan, the colour being the PET and the background grayscale image being the X-ray CT.

PET, or Positron Emission Tomography, is a remarkable technique. Fluorine-18 is a positron emitter. This antiparticle to the electron immediately annihilates on meeting an electron in the vicinity of the emission, resulting in two 511 keV gamma rays firing out in opposite directions from the point of annihilation, as shown in the diagram (thanks to Wikipedia). An array of surrounding gamma detectors registers when two gammas arrive at the same time (we physicists call that a coincidence event) and at almost 180 degrees to each other. The source is then located along the straight line of coincidence (the line of response). For detectors with resolving time of 500 picoseconds, it is even possible to localize the event to a segment of a chord. By carrying out a sequence of these reconstructions in the computer, each coincidence event adds to the overall intensity along the lines of response, and by a reconstruction technique known as filtered back projection (the same method use to reconstruct in X-ray CT) the source brightness can be localised in 3 dimensions.

In my Doctoral work in Oxford I worked with several positron emitters, amongst the most remarkable of these experiences being with the 17 hr half-life isotope, Cobalt 55. I will tell that story in a later blog. And in my later scientific life I worked extensively with Filtered Back Projection reconstruction, when we developed the first NMR microscope. Hence, for me the experience of PET-CT was one of fascination and appreciation. The image shows a machine pretty much like the one I have been in.

Fluorine-18 has a half life of about 6 hours. It is currently made in a cyclotron in Melbourne, attached to the glucose, and sent by plane to Wellington, the site of New Zealand's only PET-CT instrument. In the meantime, at least a couple of half-lives are lost, making the sensitivity poorer than if the isotope were generated "on-site". Construction of a cyclotron for Fluorine-18 production has just begun near Wellington airport, ironically close to the sign which proudly declares that New Zealand is "Nuclear Free". Ah well, what's a bit of egregious hypocrisy when lives are at stake?