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So, we're going to talk a little bit about MR perfusion.

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These are raw data images from MR perfusion.

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And what it is

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is a gradient echo-echo planar technique.

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You inject the gadolinium over time.

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You see the signal go down because of the

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T2 star effects of gadolinium,

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and then it washes out.

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So, that's what the raw data looks like.

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And then what happens is you have signal,

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it goes down,

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and then it goes us back to baseline.

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We do some mathematics and get a

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signal versus time curve.

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The area under the curve is

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proportional to the CBV.

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You can create cerebral blood flow maps

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by fusing arterial input function,

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usually from the MCA or ACA.

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And then when you divide those two,

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you get a mean transit time.

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There are other transit time maps that

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you can get the time to peak,

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which is the time to peak of contrast concentration,

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and the Tmax,

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which is the time at which the deconvoluted residue

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function reaches its maximum.

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The Tmax, the TTP, the MTT,

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all show approximately the same thing.

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A lot of studies have been done,

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and now people are pretty much using the

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Tmax maps for the transit time maps.

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They have the most robust signal to noise.

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I'm going to show you an example

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of how these maps are used.

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Basically,

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you're looking for a target mismatch.

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This is a patient who has a left MCA

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stem embolus, has good collaterals on the CTA,

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has a little bit of hypodensity on the non-contrast CT.

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On the MRI,

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there's an infarction in the left basal ganglia

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and the left corona radiata.

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There's a small defect.

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So, that's the core of the infarction

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we talked about in DWI before.

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And what the perfusion images

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show is the penumbra,

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or the tissue at risk of infarction.

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So, you can see this much bigger

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abnormality on the Tmax maps.

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So, there's a big core penumbra mismatch

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that's a target mismatch.

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So, patient has good collaterals,

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core penumbra mismatch,

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patient has an M1 cutoff on this angiogram,

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and then basically,

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you open up the vessel,

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the IR team does that, neuro IR,

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and basically on follow-up,

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the infarct has not extended.

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So, that's what we're using perfusion for,

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to look for a target mismatch,

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to identify patients who will benefit from thrombolysis.

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It's very similar to CTP.

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And then, here's another patient who did not have

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a proximal embolus.

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FLAIR looks normal

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because it's a very early infarct.

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And you can see that there's an infarct

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in the left posterior temporal lobe.

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And on the perfusion maps, on the Tmax map,

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that area is matched.

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So, that area won't extend.

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So if you have a match defect,

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it's not going to extend.

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But in the left frontal lobe,

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there's an area that's normal on DWI

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but abnormal on Tmax.

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So, this patient wasn't an IA thrombectomy candidate

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because he didn't have a proximal embolus,

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but he has tissue at risk.

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So this is a patient

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you're going to keep the blood pressure up

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and hope that area reperfuses.

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Lastly,

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sometimes perfusion is helpful for telling you

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whether the patient's symptoms

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are ischemic or not.

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So, this patient had intermittent right-sided weakness,

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normal DWI, normal CTA.

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But you can see on all these transit time maps

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that there's prolonged transit time

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in the left posterior temporal and occipital lobes

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and low CBF, cerebral blood volumes normal,

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but it's telling you that this patient's symptoms

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were from some ischemia,

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distal stenosis,

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or embolus, and not being caused

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by something else.

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And so, they were worried

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she was going to have an acute stroke.

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And again, blood pressure checks

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and appropriate medications.