1/Were you today years old when you learned ASL wasn’t just short for American Sign Language?

A #tweetorial about a key perfusion method: arterial spin labeling (ASL) in collaboration w/@RadioGraphics!

Featuring this current issue article: doi.org/10.1148/rg.220…
#RGPHx Image
2/In perfusion imaging, we want to know how blood is flowing

Usually, we do that by adding IV contrast to blood—to go along for the ride. We can track contrast by changes in MR signal

So if contrast runs w/blood, we can track blood by extension & know how it’s flowing. #RGPhx Image
3/But what if we want to do perfusion imaging & don’t want to use contrast?

For example, in kids, we’d prefer not to give contrast.

Also, if there is an allergy, we REALLY don’t want to give contrast.

There must be another way.
#RGPhx Image
4/If we want to know how fast something is traveling—be it blood or a whale—we need a way to keep track of it. We need to TAG it

For whales, they literally shoot a tag into a whale to keep track of it. They track the tagged whale to see how fast the whole herd is moving
#RGPhx Image
5/Tagging is important, especially if you’re trying to keep track of 1 whale in a sea of whales

Same w/blood. If you’re trying to track how fast blood is flowing, you need to make sure you’re tracking the same blood the whole time—otherwise you get lost in a sea of blood
#RGPhx Image
6/So since we can’t harpoon blood—how do we tag it?

We can do it w/magnetization. We essentially zap some of the blood w/a radiofrequency pulse.

This changes the magnetic properties of the blood we zap—making them different or TAGGED compared to the rest of the blood
#RGPhx Image
7/It’s like in “Spiderman.” Being bitten by a radioactive spider transformed Peter Parker into something different than everyone else—Spiderman

The spider “tagged” him

Same w/blood. It’s “bitten” by a radiofrequency pulse & becomes different from the remaining blood
#RGPhx Image
8/Tagged blood is like a dye to track blood flow

It’s like finding river velocity w/dye

Tagging blood is like dropping dye at a start line. You wait a minute & then check how much dye got to the finish

You know distance & time, so that gives you river (blood) velocity. #RGPhx Image
9/This is what we do in ASL

We tag blood at the start line (in the neck), then wait a little bit, & then check how much dyed/tagged blood made it to the finish line (the head)

This gives cerebral blood flow or CBF. CBF is the only perfusion parameter ASL can measure
#RGPhx Image
10/Sadly, ASL has poor signal to noise

Tagging blood in our vessels isn’t like dropping dye into a canal—it’s dropping it in a mountain river

Dye gets diluted by other contributing streams & also washes out into other vessels, so very little actually gets to the brain #RGPhx Image
11/To increase signal to noise, we subtract out the background

We take a background image w/no tagged blood & subtract it from the image w/tagged blood

This way, background noise is subtracted out & only tagged blood signal remains--like digital subtraction angiography #RGPhx Image
12/Tagging blood is like dyeing water. How do we pour in the dye?

We can be like a little kid & pour all our dye into the river at once

Or we can be like an adult & patiently distribute it over time—like pouring small glasses of koolaid to serve all the kids at a party. #RGPhx Image
13/Going from the neck to the head is like running a marathon for blood

Pouring all the dye in at once is like all-out sprinting the start of a marathon—you’ll get drained

Tagging blood all at once is called continuous ASL. It runs out of steam & has poor signal to noise #RGPhx Image
14/Instead of all-out sprinting, you could save your energy. Run a little, rest a little, run a little

This surely gives more endurance—you won’t exhaust yourself, but you won’t be fast

This is pulsed ASL—tagging in short bursts. Good signal to noise, but not efficient #RGPhx Image
15/Let’s combine the two approaches.

All out sprint for a bit, but also take a short rest before all out sprinting again. This way, you have speed & endurance.

This is pseudocontinous ASL—tag for long periods but take a break in between. It’s best for SNR & efficiency #RGPhx Image
16/Best way to run a marathon is to go hard as long as you can, but also have short rests so you don’t exhaust yourself (pseudocontinuous running).

Same w/ASL. Best way to tag blood is to tag for a long period of time & take small breaks. This is pseudocontinous ASL. #RGPhx Image
17/So remember—you don’t need contrast for perfusion! ASL can transform blood into a superhero that doesn’t need contrast!

Be sure to check out the excellent review by Iutaka et. al. on ASL, featured in the current issue of @RadioGraphics: doi.org/10.1148/rg.220…
#RGPhx Image

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More from @teachplaygrub

Jan 27
1/Do questions about brainstem anatomy cause you to suddenly get a case of locked in syndrome?!

Do you try to localize the lesion or just wait for the MR?

Here’s a #tweetorial to help you w/brainstem #anatomy & localization!
#medtwitter #meded #neurotwitter #neurorad #radres Image
2/First some basic gross anatomy.

Brainstem from the side looks like a "dad bod"--the body you get after you have kids & the kids wear you down & you don’t exercise anymore.

Head & shoulders are the midbrain, potbelly is the pons, & fat thighs are the medulla Image
3/Midbrain is the head & shoulders.

This makes sense b/c the name “midbrain”—brain should be in the head.

Midbrain also has the cerebral peduncles which look like classic Mickey Mouse ears—and dads classically have big, usually hairy, ears Image
Read 24 tweets
Jan 23
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 #tweetorial to help you w/ICA #anatomy!

#medtwitter #meded #neurotwitter #neurorad #radres #FOAMed 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
Jan 18
1/Time to FESS up! Do you understand functional endoscopic sinus surgery (FESS)?

If you read sinus CTs, you must know what they’re doing to make the helpful findings

Here’s a #tweetorial to help you!
#medtwitter #meded #FOAMed #FOAMrad #radres #neurorad #HNrad #radtwitter
2/The first step is to insert the endoscope into the nasal cavity.

The first two structures encountered are the nasal septum and the inferior turbinate.
3/So on every sinus CT you read, the first question is whether there is enough room to insert the scope. Will it go in smoothly or will it be a tight fit?
Read 19 tweets
Jan 14
1/Is your ability to remember temporal lobe anatomy seem, well, temporary?

Here’s a #tweetorial to help you remember the structures of the temporal lobe!

#medtwitter #meded #neurotwitter #radtwitter #radres #neurorad #FOAMed #neurosurgery #medstudenttwitter #neurology
2/Temporal lobe can be divided centrally & peripherally. Centrally is the hippocampus. It’s a very old part of the brain & is relatively well preserved going all the way back to rats. Its main function is memory—getting both rats & us through mazes—including the maze of life
3/Peripherally is the neocortex. Although rats also have neocortex, theirs is much different structurally than humans.

So I like to think of neocortex as providing the newer (neo) functions of the temporal lobes seen in humans: speech, language, visual processing/social cues
Read 12 tweets
Jan 10
1/Talk about twisting your back! Do spine vascular lesions make your brain feel tangled like the dilated vessels you see?

Here’s a #tweetorial on #spine vascular #anatomy & dural arteriovenous fistulas (dAVF)

#medtwitter #meded #FOAMed #neurotwitter #neurosurgery #neurorad Image
2/To understand spinal dural AVFs, you need to understand basic spinal vascular anatomy.

The spine is LONG—to get blood from the top of the cord to the bottom is like going through the length of a marathon course Image
3/So we will need to tackle it like you tackle running a marathon.

When you run a marathon, you replenish yourself at aid/water stations along the way so you can make it all the way through.

Same w/spinal arterial vasculature—it needs to be replenished on the way down. Image
Read 19 tweets
Jan 5
1/Nothing strikes fear into the heart of a radiologist like the question,“Is it safe to do an MRI on this pt w/an implanted device?”

Never fear again! Here’s a #tweetorial on how to navigate implanted devices & #MRI
#medtwitter #meded #radtwitter #radres #neurotwitter #neurorad
2/MRI & CT are like nuclear & coal power, respectively. Everyone knows CT is worse for you & usually MRI is very safe & better for your body

But like nuclear power, when things go bad in MRI, they can go horribly wrong. Flying chairs into the magnet wrong. So, people are afraid
3/The trouble is from the magnetic attractive forces. There are 3 ways these attractions can wreak havoc. First is translation. Magnet literally pulls an object, like a chair, towards itself. This is the strongest attraction—like two lovers who literally can’t stay apart.
Read 22 tweets

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