Anyone else had PET scan to detect recurrance?

I was wondering if anyone has had a PET scan to detect recurrance?
Two body scans came back negative, regular bloodwork shows Tg at a 1 (It was always a 0.2 post surgery)

Tg level after thyrogen injection is a 5.5

Ultrasound in neck showed small mass in neck, had biopsy done and that dr didn't think it was anything.
The biopsy came back stating the samples were mostly blood samples but one had a small tissue sample that came back negative. My dr wasn't very happy that it was mostly blood samples.

I think the PET scan doesn't include the CT scan w/it. My dr didn't say anything about that.

It also sounds from some of the posts that even a PET may not show something. Is it that the cancer is so small that tests can't detect it?

It is frustrating to have some things come back negative but my dr said w/my age, 42 and that when I had surgery 2 years ago it was in my lymph nodes in my neck and w/my Tg after thyrogen was 5.5, made him concerned, but he wanted to monitor it.
I asked for the PET because I've already met my deductable w/everything else I had done this year (We have a high deductable 2500.00 per person per year) and this would probably be covered. I don't think he appreciated that but he went ahead and ordered it.
I'm waiting to see if insurance does indeed cover it.