Lea Alhilali, MD Profile picture
Dec 19, 2022 18 tweets 9 min read Read on X
1/My hardest #tweetorial 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!
#stroke #neurotwitter #neurorad #meded #FOAMed #radtwitter #medtwitter
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
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.
4/Clinically, there are 2 main perfusion parameters used: (1)Cerebral blood flow (CBF), which is how FAST blood gets to the tissue & (2) Tmax or time to max residue function. Everyone knows Tmax is used to estimate penumbra, but does anyone know what it really is??? You will now
5/Let’s start w/CBF. CBF is how FAST blood gets to tissue. We could estimate it by measuring how fast contrast accumulates in tissue—make a curve of the amount of contrast in a tissue over time. If the curve has a steep slope, contrast/blood is being delivered fast & CBF is high
6/Unfortunately, it’s not that simple. You can’t just measure the slope of the contrast curve in tissue to get CBF. Many things change how fast contrast travels besides just blood flow. If you inject more contrast or inject it faster—these increase how fast contrast washes in
7/If we can’t measure how fast contrast washes in to get CBF, we’ll measure how it washes out! If you want to measure river velocity, dropping in dye & measuring how fast it washes out gets the same answer as watching it wash in. But we can’t drop contrast directly in the brain!
8/So we must back calculate. Pretend we want to know how fast a kitchen prepares food—Restaurant Continental Breakfast Flow or rCBF. If we know when ingredients arrive & we know when food gets on our table, we can back calculate kitchen speed--& that’s what we do for the real CBF
9/When the ingredients arrive is the arterial input function. We measure over a cerebral artery to see when the blood first arrives. It’s equal to how long it takes the restaurant to get the ingredients from the supplier—how long it takes the artery to get blood after injection
10/How fast food is building up on our table is the tissue concentration. We measure in brain parenchyma to detect the buildup of contrast. How long it takes for blood to get from injection to tissue is equal to how long it takes ingredients to be turned into food on our table
11/Time for the kitchen to turn ingredients to food for the table is CBF. We want to find CBF by dropping contrast right in a brain artery & see how fast it washes out to tissue. This is kitchen time--the time for a blood drop to wash out from artery (kitchen) to tissue (table)
12/If we know:
1)Time for blood to get from injector to artery
2)Time to get from injector to tissue
We can then back calculate time it takes to get from artery to tissue.

So we use the arterial input function & tissue concentration to back calculate the artery to tissue time
13/This back-calculated artery to tissue time simulates dropping blood into a brain artery & watching it wash out—like our dye & river—the best way to find CBF

This back-calculated function is the "residue function"—not a real measurement in the brain, but a calculated entity
14/Residue function is what you would get if you dropped a perfectly tight bolus of blood into an artery & then watched it washout into tissue as it is replaced by fresh blood. It is exactly what we wanted to do w/dye in the river
15/The function is maximized the second you drop all that blood into the artery—before any washes out.

This is equal to the time it takes for blood to hit the artery—none has washed out.

So Tmax (time to max residue function) is the time it takes blood to reach the artery
16/The height of the residue function is CBF.

This is b/c the residue function represents the blood being dropped right into the artery & timing how long it takes to wash out.

So we calculate CBF by measuring the height of the residue function
17/Since Tmax is the time it takes for blood to reach the artery, it doesn’t take into account the time it takes blood to travel through the microvasculature to the tissue. So it isn’t affected by microvascular pathology—making it a great indicator of large vessel occlusion (LVO)
18/So now you know all the inner workings of the kitchen behind the numbers and names you seen in perfusion imaging.

May this be the Tmax of your knowledge function and leave you hungry for more!

• • •

Missing some Tweet in this thread? You can try to force a refresh
 

Keep Current with Lea Alhilali, MD

Lea Alhilali, MD Profile picture

Stay in touch and get notified when new unrolls are available from this author!

Read all threads

This Thread may be Removed Anytime!

PDF

Twitter may remove this content at anytime! Save it as PDF for later use!

Try unrolling a thread yourself!

how to unroll video
  1. Follow @ThreadReaderApp to mention us!

  2. From a Twitter thread mention us with a keyword "unroll"
@threadreaderapp unroll

Practice here first or read more on our help page!

More from @teachplaygrub

Mar 14
1/The 90s called & wants its carotid imaging back!

It’s been 30 years--why are you still just quoting NASCET?

Do you feel vulnerable when it comes to identifying plaque vulnerability?

Here’s a thread to help you identify high risk plaques with carotid plaque imaging Image
2/Everyone knows the NASCET criteria:

If the patient is symptomatic & the greatest stenosis from the plaque is >70% of the diameter of normal distal lumen, patient will likely benefit from carotid endarterectomy.

But that doesn’t mean the remaining patients are just fine! Image
3/Yes, carotid plaques resulting in high grade stenosis are high risk.

But assuming that stenosis is the only mechanism by which a carotid plaque is high risk is like assuming that the only way to kill someone is by strangulation. Image
Read 25 tweets
Mar 12
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
Mar 10
1/I always say you can tell a bad read on a spine MR if it doesn’t talk about lateral recesses.

What will I think when I see your read? Do you rate lateral recess stenosis?

Here’s a thread on lateral recess anatomy & a grading system for lateral recess stenosis Image
2/First anatomy.

Thecal sac is like a highway, carrying the nerve roots down the lumbar spine.

Lateral recess is part of the lateral lumbar canal, which is essentially the exit for spinal nerve roots to get off the thecal sac highway & head out into the rest of the body Image
3/Exits have 3 main parts.

First is the deceleration lane, where the car slows down as it starts the process of exiting.

Then there is the off ramp itself, and this leads into the service road which takes the car to the roads that it needs to get to its destination Image
Read 21 tweets
Mar 3
1/Does PTERYGOPALATINE FOSSA anatomy feel as confusing as its spelling?

Does it seem to have as many openings as letters in its name?

Are you pterrified of the pterygopalatine fossa (PPF)?

Let this thread on PPF anatomy help you out. Image
2/The PPF is a crossroads between the skullbase & the extracranial head and neck

There are 4 main regions that meet here:

(1) Skullbase itself posteriorly, (2) nasal cavity medially, (3) infratemporal fossa laterally, and (4) orbit anteriorly. Image
3/At its most basic, you can think of the PPF as a room with 4 doors opening to each of these regions: one posteriorly to the skullbase, one medially to the nasal cavity, one laterally to the infratemporal fossa, and one anteriorly to the orbit Image
Read 18 tweets
Feb 28
1/Feel like a fish out of water when it comes to water on the brain?

Read on for this month’s @Radiographics summary of what you need to know about hydrocephalus!!



@cookyscan1 @RadG_editor #RGphx doi.org/10.1148/rg.240…Image
2/To understand hydrocephalus, think of CSF like the flow of traffic

3 main ways traffic backs up:

(1) Obstruction on the road:
For hydrocephalus, this is an obstruction along CSF in the ventricle Image
3/

(2) Obstruction of an off ramp
For hydrocephalus=obstruction at its off ramp into the venous system

(3) Rush hour
For hydrocephalus=over production Image
Read 8 tweets
Feb 27
1/Do scans for dizziness make your head spin?

Need to know what to look for?

Just hear me out!

This month’s @theAJNR SCANtastic will show what to look for:

ajnr.org/content/46/2/3…Image
2/I always remember the rhyme of the big three for dizz-ee!

First, are vestibular schwannomas

These give an ice cream cone shape in the internal auditory canal! So scoop up that finding! Image
3/Next is labyrinthitis

Labyrinthitis can look like night & day, depending on the timing

Late labyrinthitis is dark—loss of bright fluid signal on FIESTA

Early labyrinthitis is bright—enhances on post-contrast Image
Read 12 tweets

Did Thread Reader help you today?

Support us! We are indie developers!


This site is made by just two indie developers on a laptop doing marketing, support and development! Read more about the story.

Become a Premium Member ($3/month or $30/year) and get exclusive features!

Become Premium

Don't want to be a Premium member but still want to support us?

Make a small donation by buying us coffee ($5) or help with server cost ($10)

Donate via Paypal

Or Donate anonymously using crypto!

Ethereum

0xfe58350B80634f60Fa6Dc149a72b4DFbc17D341E copy

Bitcoin

3ATGMxNzCUFzxpMCHL5sWSt4DVtS8UqXpi copy

Thank you for your support!

Follow Us!

:(