menreeq wrote:I may have replied to this in the past, but I'll do a brief response. I am a radiologist, but this is NOT medical advice. I saw some information in the other posts that I wanted to comment on.
CT has better spatial resolution than MRI, but MRI has better contrast resolution. They have different pros/cons. CT slices can be as thin as 0.625 mm. MR slices tend to be significantly bigger (5 mm for example) -- depending on the protocol that is in use, which varies from place to place. But "spots" will show up as bright on the MRI on certain sequences, so they can be easier to see. Regarding "regular" CT vs "multiphase" CT --> it really depends what is meant by "regular." If it is without contrast, mets are very difficult to see. CT for CRC mets should be done with contrast, but sometimes people cannot have contrast for other reasons. Multiphase CT means you do multiple CT exams in different phases of where the contrast is in your body (no contrast, contrast in the arteries, contrast in the portal veins, contrast in the systemic veins, delayed phases, etc) -- and this also depends on the protocol being used by your hospital or imaging center. Seeing the liver in different phases makes it easier to see "spots." Ultrasound is less sensitive than CT and MRI, so this is not typically recommended in the US, but may be recommended in other healthcare settings. Ultrasound is less expensive and has no radiation. PET/CT requires that a lesion be at least 8mm to be detected (sometimes smaller spots can be detected if they are very active).
If a "spot" is less than 8-10mm, it can be difficult to tell on CT if it contains water only (a cyst). The phrase "too small to characterize" is often used. If it's bigger than 10mm, looks like water (this is determine by density on CT and signal on MRI), and does not enhance (meaning the contrast gets into it and it gets brighter), it is called a cyst (a benign finding). Anything that does not meet the criteria for water or that enhances is looked at much more carefully --> maybe it is a complex cyst, a benign liver tumor, or cancer. There are many enhancing tumors in the liver that are not cancer, and they have certain patterns on CT and MRI (especially the latter) that a radiologist can evaluate. But if your CT is for colon cancer surveillance, the radiologist is looking with special attention to where CRC mets go (liver, lungs, brain, peritoneum, ovaries, bones, etc). Fatty infiltration looks different than a "spot" - it is shaped differently and not as well defined. It looks different from typical metastases. Small mets will be indistinguishable from benign lesions, which is why surveillance and looking at change over time is necessary, as well as following CEA levels and any other markers that are helpful for that cancer.
I hope everyone's spots stay small and benign!
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