[Joan M]

Richie,

As Chrisy noted, the PET looks only at metabolic activity. That is, it measures glucose metabolism.

Cancer cells take in large amounts of glucose.

The PET scan show a signal by lighting up on the scan. Anything over the standard uptake value (SUV) 3.0 is considered suspicious for cancer, but other things can light up, like tuberculous or infection. For example, in my case, the lightening up in the lung of my PET/CT in January now seems related to infection (hence the aspergillioma and coughing up blood, etc.).

Also, there was no correlation on CT. That is, even though the PET was lighting up at levels as high as 6.0 (which didn't even happen when the cancer nodules lit up), the CT scan did not show any morphological correlation as in something looking like a nodule. CT scans show morphology, as Chrisy noted. The attenuation part has to do with lining up the patient to give the PET signals a more specific location on a map.

A PET/CT is bascially a PET scan in that the patient is given a radioactive isotope (FDG 18). Sometimes the patient is also given a contrast medium to drink for the CT part, but that's just to get a better map of the abdomen and pelvis area.

The patient is given the isotope first and made to sit in the "guiet room" in the near dark for about an hour (I remember at first thinking, What the hell is the "quiet room."). They don't even want you to read or move a muscle. If you move muscles around a lot the isotope accumulates in those areas of the body and could light up a false positive.

During the scan, the patient is first put through the machine quickly a few times. That's the CT attenuation, correlation part. Then they start the PET part, which is slower.

Also, since PET scans measure metabolic activity, they're not used to scan the brain for cancer.

Joan