Arterial Spin Labelling Perfusion - Usage Examples
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So, now we're going to talk a little bit
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about arterial spin labeling perfusion,
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which is a noncontrast technique.
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Basically, you get a control image, and
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then magnetization of blood in the NAC
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is inverted using radiofrequency pulses,
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and then you subtract the two images
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and you're able to create ASL maps.
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This is from Dr.
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Saharachuk's article in Neuroimaging Clinics
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of North America, and it's labeled CBF map.
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It is a measurement of CBF
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and transit time together.
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So that's arterial spin labeling in a nutshell.
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So how do we use it?
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So it's, it has a high
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negative predictive value.
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So these patients come into the ED
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and they have right-sided weakness.
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For example, this patient did, but
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you had normal DWI and normal ASL.
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So then you think it has high
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negative predictive value for stroke.
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So it means if she's got a stroke, she's got
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a tiny stroke, or she's got some stroke mimic,
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and this turned out to be hemiplegic migraine.
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So normal ASL has a high
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negative predictive value.
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Here's another example.
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Patients come in with right-sided paresis.
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They come in with hemiplegia.
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You have a normal MRI of the neck and MRI
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of the head and DWI is read as normal.
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Maybe there's slight hyperintensity.
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They're hard to tell.
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If you have this elevated signal in the ASL
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and in the thalamus, and that's kind of classic
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of seizures because the thalamus is a relay
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station between cortex and the brainstem.
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So that's another classic example.
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And then, just like other perfusion
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techniques, you can have a matched defect.
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So this patient had a left homonymous
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hemianopsia, has an acute stroke in the
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posterior temporal occipital region, and has a
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defect on the ASL images that's the same size.
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No proximal vessel embolus on the MRA and the
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follow-up CTA just shows that the impact didn't
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extend because you have a matched defect.
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So you see, does not require
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aggressive treatment.
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And then the other scenario, here's a
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patient who has a carotid occlusion,
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normal cross-filling across on the CTA,
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normal bilateral MCAs and ACAs, normal
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DWI, but there's a big ASL defect.
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Uh, this patient also had CBF and MTT maps
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and you can see there's low CBF and marked
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increase in the transit time in that region.
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A couple of days later, she got a follow-up DWI
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and you can see some of that area infarcted.
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So, just like other perfusion,
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it can show you tissue at risk.
Pamela W Schaefer, MD, FACR
Professor of Radiology, Vice Chair of Education
Massachusetts General Hospital
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