PET shows up as bright when there is increased uptake of glucose in a part of the body, which can happen with cancer, infection and other conditions. Cancer tends to have higher SUV than infection, but if the lesion is small, the SUV is not reliable. Although the SUV value can favor one diagnosis over another, tissue sampling is the only way to confirm.
CT is based on how dense something is -- how much does it "attenuate" the X-ray beam.
MRI is based on proton spin. It is better at differentiating soft tissue. In the liver, with the right sequences, it can help differentiate between hemangiomas, focal nodular hyperplasia, adenomas, hepatocellular cancer and other neoplasms (including metastasis), which may look similar on CT.
These are all different modalities. Some things show up better on one test compared to another.
A lesion has to be at least 8 mm to show up on a PET scan. CT has better spatial resolution so tiny things can be seen on CT and not PET. Same with MRI, which needs a larger size voxel than CT.
Medical imaging is complex. There are lots of factors. Even just breathing can affect how good one scan is. MRI sequences take longer than CT, so motion is more of an issue. You can ask your doctor when you have doubt. If you don't get a specific enough answer (to your satisfaction), ask the radiologist.
I try to ask myself these questions when I have to decide on something:
- Will this test/procedure change clinical management of my disease? Will it change outcome?
- What are the risks, benefits and alternatives?
- What do my doctors recommend?
- What does my independent research show?
Then I decide. At least I know I made an informed decision. The rest is really out of my control.
In your specific case, you can ask if the needle biopsy will change management (chemo vs no chemo, RFA or liver section vs no surgery, change in chemo, longer chemo, etc). Is the alternative to go straight to an invasive procedure? Is it better to do minimally invasive biopsy first? Hopefully once you have discussed these things, you will feel confident in whichever option you choose.
Stage IIA rectosigmoid CC (T3N0M0)
Dx 6/5/17 @age 41, mom to 5 & 2yo girls
Workup: c-scope, EUS, rectal MRI, CT C/A/P
AdenoCA 5.5cm, WHO Grade 2, 0/22 LN, no distant mets
CEA 1.9 (6/5/17), 0.8 (2/28/18)
No lymphovasc/perineural invasion, clear margins
MSI intact, OncotypeDx RS 7
Lap sig colectomy 6/23/17, no ileo/colostomy
Genetics counseling negative for mutations, 4 VUS
Xeloda monotherapy 8/13/17-1/22/18
PET/CT 3/21/18 NED