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
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
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
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
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
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
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
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
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
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
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
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
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
17/So remember—you don’t need contrast for perfusion! ASL can transform blood into a superhero that doesn’t need contrast!
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
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
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?
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
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.