Neuroimaging

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iBS1972

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So, could someone explain the order of imaging/type of imaging that you use when locating a lesion in a patient? It was explained to me a couple times on rotation, but there it was a lot (with CT noncon, MRI T2, flare/DWI,... dark/bright...) and I got lost near the beginning.
Say a 50 year old patient who came in with a sudden onset left sided weakness, what order of tests would you look at?
Also, a more specific question, why can't you use CT contrast to look for the location of a stroke? (I know people say that because of the contrast you can't distinguish between ischemic vs. hemorrhage, but I don't get that. If it's ischemic, wouldn't it not enhance anyways, since there's no perfusion there, meaning no blood, and thus no contrast and would look just the same as a noncon; while if it's hemorrhagic, well you would see enhancement...?) Can someone be so kind as to explain this to me?

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Preface — just an M3, not a resident/attending. Would be happy to be corrected.

CT is usually the first modality ordered. (Apparently in some rare cases DWI is obtained almost as quickly, but that seems institution-specific.) There are a few reasons to not use contrast:
1) Vessels will become bright. The point of CT in context of suspected stroke is to rule out bleeding. If you light up all the vessels, it becomes difficult to tell if a white spot is a vessel, calcification, or bleeding. This makes interpretation harder.
2) Not much utility. After you rule out bleeding, MRI is better for evaluating potential stroke. Getting a CT+C is not going to give you much useful information, and the soft tissue detail is inferior. Even if you get it, you'd probably end up ordering an MRI. No reason to get two things when you only need one.

In other words, you want to rule out bleeding, and contrast muddles the picture (literally). Further, contrast wouldn't be useful to enhance the ischemic region anyway. Remember that contrast indicates disruption of the BBB. Acutely, the ischemic region should not enhance. After days-1 week, it will (cytotoxic edema and BBB disruption). Here's a good page on that: Contrast enhancement following infarct | Radiology Case | Radiopaedia.org

OK, so then what? You have a negative CT non-con. Now you should get MRI. Some time later, you get all the sequences back. What do you look at first? DWI. Strokes will be bright spots. If you find one, pull up the ADC sequence. If the spot is DARK on ADC, that suggests true diffusion restriction, e.g. a stroke in the correct clinical context. You can then take a look at the other sequences (my rotation's residents drilled that T2 FLAIR is the go-to if you had to choose only one), like T1, T2, FLAIRs, GRE/SWAN, so on and so forth...

EDIT: Often vessel imaging is also ordered, again depending on what you're suspecting on your DDx. My rotation told us that CTA was preferred over MRA, but I don't have a full understanding of the relative strengths and weaknesses of each.
 
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