Lea Alhilali, MD Profile picture
Feb 7, 2023 25 tweets 10 min read Read on X
1/I always tell my fellows, “Anyone can see the bright spot on diffusion—what sets you apart is if you can tell them why it’s there!”

Can you tell a stroke’s etiology from its appearance on MRI?

Here’s a #tweetorial to show you how!

#medtwitter #neurotwitter #stroke #neurorad
2/First a review of the vascular territories.

I think the vascular territories look a butterfly—w/the ACA as the head/body, PCA as the butt/tail, and MCA territories spreading out like a butterfly wings.
3/Of course, it’s more complicated than that.

Medially, there are also small vessel territories—the lenticulostriates & anterior choroidal.

I think they look like little legs, coming out from between the ACA body & PCA tail.
4/Brain arterial system is like a road system transporting blood/oxygen to all over the brain via different sized roads.

Large vessels are the interstates, branch vessels are state highways, & perforators are county roads. But they are interconnected—just like a road system
5/When trying to remember the etiologies of stroke, it's helpful to think of the arteries like a road system

The same road problems that keep traffic from getting to their destination are analogous to the problems that keep blood from reaching where it needs to go in the brain
6/The first stroke etiology is thromboembolism. This occurs when a vulnerable plaque ruptures & causes local platelet aggregation & clot formation. This occludes the artery and prevents distal blood flow
7/Rupture of the plaque is like a multicar accident that completely blocks the road. Nothing can past the giant pile up—just like nothing can get past the clot formation at the site of plaque rupture
8/If this happens on a highway—& there is no other road serving that area, then no one can reach that whole territory

This is the way it is for northern Arizona & the I 17—if it is blocked, no one is getting to Flagstaff in the north. Thromboembolism causes territorial infarcts
9/Next etiology is embolism.

Emboli can come either from a plaque that ruptures or breaks—but instead of occluding the artery, it spits out emboli downstream.

Alternatively, it can come from the heart, from stasis (Afib, CHF) or vegetations
10/I think of emboli as trouble from out of town. Thrombus from elsewhere invading an innocent artery.

It’s like motorcycle biker gangs from out of town—coming in & disrupting traffic in an innocent city
11/So where do emboli go?

Like biker gangs, emboli go wherever they want. If they end up in large vessels, you get a territorial infarct, or they can block smaller vessels & give smaller infarcts.

They can even give you just one tiny infarct if you catch it soon enough
12/Next etiology is distal hypoperfusion. This is where the plaque is not so large that it occludes the vessel entirely, but large enough that it attenuates the flow distally—and tissue distal to the stenosis does not get enough blood as a result
13/Hypoperfusion is like bad traffic.

You can get through, but waste so much gas sitting in traffic that you end up having to stop before your final destination.

As a result, no one gets to the distal cities on the highway—and certainly not all the way to the BORDER.
14/These are called BORDERZONE infarcts, as blood flow runs out like gas & doesn’t make it to the distal borders between the territories

How to remember the borders? They’re the border between the butterfly parts. So picture the butterfly & you’ll always remember the borderzones
15/A common borderzone infarct is between the butterfly body (ACA) & wing (MCA). This borderzone infarct commonly has several small infarcts along the border.

It is sometimes called the string of pearl signs, b/c this row of small round infarcts looks like a string of pearls
16/I remember that a string of pearls is worn around the NECK.

So if I see a string of pearls on diffusion imaging, I immediately check the NECK, b/c this border zone infarct is commonly from a carotid stenosis in the neck
17/Next etiology is impingement on perforators. This is when the plaque in a large vessel covers up the opening of a small perforator emerging from its wall. This obstructs flow to the perforator
18/This is like when traffic is bad on the highway & blocks your exit. There’s no traffic on your exit—but you just can’t get to it b/c of traffic on the main highway.

There’s nothing more frustrating than seeing no traffic on your home exit—but being unable to reach it
19/These perforator infarcts usually result in subcortical infarcts.

I remember this b/c a single exit is being blocked. Like your exit to the street leading to your neighborhood or SUBDIVISION.

SUBdivision block means SUBcortical infarct.
20/Next etiology is vasculitis.

Vasculitis is an inflammatory condition of the vessel wall, that could be idiopathic, autoimmune, or infectious.

Regardless of the reason, the inflammation leads to vessel wall damage, stenosis, & focal occlusions or thrombosis
21/Vasculitis is like poor road conditions. It is like having potholes everywhere. These potholes cause car accidents wherever they may appear & result in traffic back up.
22/Usually potholes are on smaller roads—b/c the government always takes care to make sure highways are maintained first, so they’re usually less like to have potholes than smaller streets. Similarly, infarcts are usually from smaller rather than larger vessels in vasculitis
23/Last, but certainly not least, is small vessel disease.

This is a kind of wastebasket that encompasses many different pathologies that all have in common that they cause damage to & occlusion of small, unnamed vessels in the brain
24/You can remember this bc unnamed vessels are like the unnamed country roads that go to places larger roads don’t go to

These are usually dirt roads, so they’re very vulnerable to slow traffic, potholes, mud, etc

They are tiny, so their infarcts are usually tiny as well
25/So now you understand the different etiologies of stroke & how different etiologies have different distributions on MRI.

Remember, catching the stroke on the diffusion imaging isn’t the end of your job—it’s the beginning!

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More from @teachplaygrub

Aug 1
1/They say form follows function!

Brain MRI anatomy is best understood in terms of both form & function.

Here’s a short thread to help you to remember important functional brain anatomy--so you truly can clinically correlate! Image
2/Let’s start at the top. At the vertex is the superior frontal gyrus. This is easy to remember, bc it’s at the top—and being at the top is superior. It’s like the superior king at the top of the vertex. Image
3/It is also easy to recognize on imaging. It looks like a big thumb pointing straight up out of the brain. I always look for that thumbs up when I am looking for the superior frontal gyrus (SFG) Image
Read 12 tweets
Jul 29
1/Talk about bad blood!

Do you know when a hematoma is going to expand?

Read on for month’s @theAJNR SCANtastic on all you need to know about imaging intracranial hemorrhage!

ajnr.org/content/46/7/1…Image
@TheAJNR 2/Everyone knows about the spot sign for intracranial hemorrhage

It’s when arterial contrast is seen within a hematoma on CTA, indicating active
extravasation of contrast into the hematoma.

But what if you want to know before the CTA? Image
@TheAJNR 3/Turns out there are non-contrast head CT signs that a hematoma may expand that perform similarly to the spot sign—and together can be very accurate.

How can you remember what they are? Image
Read 9 tweets
Jul 25
1/Time to go with the flow!

Hoping no one notices you don’t know the anatomy of internal carotid (ICA)?

Do you say “carotid siphon” & hope no one asks for more detail?

Here’s a thread to help you siphon off some information about ICA anatomy! Image
2/ICA is like a staircase—winding up through important anatomic regions like a staircase winding up to each floor Lobby is the neck.

First floor is skullbase/carotid canal. Next it stops at the cavernous sinus, before finally reaching the rooftop balcony of the intradural space.Image
3/ICA is divided into numbered segments based on landmarks that denote transitions on its way up the floors.

C1 is in the lobby or neck.

You can remember this b/c the number 1 looks elongated & straight like a neck. Image
Read 10 tweets
Jul 23
1/My hardest thread yet! Are you up for the challenge?

How stroke perfusion imaging works!

Ever wonder why it’s Tmax & not Tmin?

Do you not question & let RAPID read the perfusion for you? Not anymore! Image
2/Perfusion imaging is based on one principle: When you inject CT or MR intravenous contrast, the contrast flows w/blood & so contrast can be a surrogate marker for blood.

This is key, b/c we can track contrast—it changes CT density or MR signal so we can see where it goes. Image
3/So if we can track how contrast gets to the tissue (by changes in CT density or MR signal), then we can approximate how BLOOD is getting to the tissue.

And how much blood is getting to the tissue is what perfusion imaging is all about. Image
Read 18 tweets
Jul 21
1/Do you know all the aspects of, well, ASPECTS?

Many know the anterior circulation stroke scoring system—but posterior circulation (pc) ASPECTS is often left behind

25% of infarcts are posterior circulation

Do you know pc-ASPECTS?!

Here’s how to remember pc-ASPECTS! Image
2/Many know anterior circulation ASPECTS.

It uses a 10-point scoring system to semi-quantitation the amount of the MCA territory infarcted on non-contrast head CT

If you need a review: here’s my thread on ASPECTS: Image
3/But it’s only useful for the anterior circulation.

Posterior circulation accounts for ~25% of infarcts.

Even w/recanalization, many of these pts do poorly bc of the extent of already infarcted tissue.

So there’s a need to quantitate the amount of infarcted tissue in these ptsImage
Read 12 tweets
Jul 2
1/The medulla is anything but DULL!

Does seeing an infarct in the medulla cause your heart to skip a beat?

Does medullary anatomy send you into respiratory arrest?

Never fear, here is a thread on the major medullary syndromes! Image
2/The medulla is like a toll road.

Everything going down into the cord must pass through the medulla & everything from the cord going back up to the brain must too.

That’s a lot of tracts for a very small territory. Luckily you don’t need to know every tract Image
3/Medulla has 4 main vascular territories, spread out like a fan: anteromedial, anterolateral, lateral, and posterior.

You don’t need to remember their names, just the territory they cover—and I’ll show you how Image
Read 18 tweets

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