PET/CT vs CT

Rich66 · 06-03-2009, 04:07 PM

Can a PET/CT judge lung nodule size changes based on a previous dedicated CT? My understanding of the CT included in a PET/CT is that it isn't "diagnostic". I know teh PET/CT doesn't allow IV contrast. But..is there enough sum information in a PET/CT to track size change?

jones7676 · 06-03-2009, 04:15 PM

When I have what is titled a "PET/CT" where my regular oncology occurs I always get dye at the end, so your question confuses me. When I go to Chicago they give me a CT and make thier judgement based on that. I wish I could be of more help!

Rich66 · 06-03-2009, 04:19 PM

Maybe they do a PET, followed by a CT?
What mom has tomorrow is a PET preceded by a mini CT without contrast.

chrisy · 06-03-2009, 05:48 PM

I get a Pet/CT, with contrast. It is all in the same machine. They do the CT first, then immediately follow with the Pet. This is so they can match up the locations of the CT and the Pet. They both provide information, the CT does show "size" and is measurable, the Pet as you know shows metabolic activity. So, either one gives you information but in combination they give you BETTER info.

The CT shows what structurally is going on - things growing, or shrinking, or appearing or disappearing. The Pet shows how metabolically active "it" is. So you can have metabolic activity due to inflammation or something that has NO correlate on the CT. Or you can have a lesion on the CT that is or is not metabolically active - the latter inferring that it is necrosis rather than an active tumor.

There are 2 things that might be injected - for the pet it is a radioactive sugar (layman's term!) and you always need that for the pet. For the CT there may or may not be contrast.

I can't tell from your post if they are doing them separately, or just in the opposite sequence. Either way, the best eval comes from using the 2 in conjunction.

hope that helps.

And, hope the information is not only useful, but favorable.

Believe51 · 06-03-2009, 07:49 PM

Well said Chrisy, I so love your explanation and could have not said it better if you paid me (smiling). That should go down as a medical dictionary definition!>>Believe51

Rich66 · 06-03-2009, 08:18 PM

It will be CT, then PET. it was explained thoug that the CT in a PET isn't "diagnostic". Something about attenuation correction. Hopefully it is sensitive enough for tracking nodules size without contrast.

jml · 06-04-2009, 01:34 PM

I've been getting non-contrasted CT/PET scans for the past 6 years & it seems in the reports the Radiologist is able to describe changes in size/dimension, as well as SUV uptake.
That being said, prior to my liver resection, I did have to have an additional CT w/contrast to give the docs precise information about the lesion size.
I think a non-contrasted CT/PET is sufficient for re-staging & as part of regular follow up, but for more specific information, a contrasted CT or MRI may be a better option.

Good luck to you & your mom tomorrow after ScanDay.
Remember lots & lots of water post-scan!

Joan M · 06-08-2009, 07:03 PM

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