2/ fMRI is based on a principle called “neurovascular coupling.” This is the principle if there is increased neuronal activity in a region, there will be increased blood flow to that region to meet the increased demand
3/ Think of it like a baby crying because it is hungry—parents immediately rush to feed it. The increased oxygen demand of the neurons immediately brings increased fuel to feed it.
4/ However, the body actually overreacts to that demand—it is like going McDonald’s when you are starving—you are going to walk away with way more food than you need and end up feeling incredibly stuffed. The neurons end up getting way more oxygenated blood than they need.
5/ This changes the oxygenated to deoxygenated blood ratio. Initially deoxygenated blood is increased b/c activated neurons are using up oxygen, but this is soon overwhelmed by supply. So counterintuitively—oxygenated blood is more with this metabolic activity.
6/ This is important b/c deoxygenated blood⬇️fMRI signal & oxygenated blood⬆️it. Initially, a signal drop occurs as neurons use up oxygen, but the tidal wave of oxygenated blood coming in overwhelms this & you get increased signal w/neuronal activity.
7/ So if you perform an activity, say finger tapping, the regions involved in finger tapping (motor cortex) will experience increased blood flow compared to regions of the brain that are not involved in that activity.
8/ B/c of increased blood flow, oxygenated blood & fMRI signal will increase in regions involved in a task compared to those not involved. This is how we map what brain regions are associated with an activity—not just finger tapping, but language, memory, etc.
9/ fMRI images are made by subtracting images taken during baseline (no activity) from images taken during activity. All that will left after the subtraction is the increased flow/signal over baseline--and this will only be in regions activated by the task.
10/ For the baseline image, no activity is performed, and so no regions are activated, so all regions will show low signal.
11/ When a task begins, blood flow only increases to regions involved in the task, so only those regions will have increased blood flow/signal over baseline. This example is finger tapping, but we can map which regions are associated w/more complex brain activities.
12/ Here is an example w/finger tapping. At baseline, the motor cortex is not activated & has low flow. But w/finger tapping, signal increases w/increased flow. So when we subtract baseline images from activity images, the increased signal over baseline remains.
13/ On the fMRI images, we see the increased signal over baseline as the colored blobs you all recognize. These just mean there is increased blood flow in this region over baseline with a given activity, and so that specific activity maps to that region.
14/ Now let’s look at a region not activated by finger tapping. At baseline, it is not activated & has low flow. W/finger tapping, it is also not activated & flow is same as baseline. So w/subtraction, the 2 images are identical & cancel out, so signal is 0.
15/ Since signal is zero, there are no colored blobs in this region and so we know this region is not associated with the task.
16/ So those fMRI colored blobs just mean there is⬆️flow in a region w/an activity & so that region is involved in performing that task. That's how we map the different "functions" of brain regions
So next time if someone asks you if you understand fMRI you can say “F--- yeah!”
2/Aneurysm rupture is a devastating even, as it results in subarachnoid hemorrhage & complications such as hydrocephalus, vasospasm, infarcts, & death.
Preventing it by treating aneurysms before they rupture is key. But you also don’t want to overtreat.
3/To remember what features make an aneurysm more likely to rupture, think what makes that guy at the bar that you angered more likely to rupture & start a fight.
What makes him more likely to rupture are the same things that make aneurysms more likely to rupture
1/Need help reading spine imaging? I’ve got your back!
It’s as easy as ABC!
A thread about an easy mnemonic you can use on every single spine study you see to increase your speed & make sure you never miss a thing!
2/A is for alignment
Look for: (1) Unstable injuries
(2) Malalignment that causes early degenerative change. Abnormal motion causes spinal elements to abnormally move against each other, like grinding teeth wears down teeth—this wears down the spine
3/B is for bones.
On CT, the most important thing to look for w/bones is fractures. You may see focal bony lesions, but you may not
On MR, it is the opposite—you can see marrow lesions easily but you may or may not see edema associated w/fractures if the fracture is subtle
1/Asking “How old are you?” can be dicey—both in real life & on MRI! Do you know how to tell the age of blood on MRI?
Here’s a thread on how to date blood on MRI so that the next time you see a hemorrhage, your guess on when it happened will always be in the right vein!
2/If you ask someone how to date blood on MRI, they’ll spit out a crazy mnemonic about babies that tells you what signal blood should be on T1 & T2 imaging by age.
But mnemonics are crutch—they help you memorize, but not understand. If you understand, you don’t need to memorize
3/If you look at the mnemonic, you will notice one thing—the T1 signal is all you need to tell if blood is acute, subacute or chronic.
T2 signal will tell if it is early or late in each of those time periods—but that type of detail isn’t needed in real life
1/Do radiologists sound like they are speaking a different language when they talk about MRI?
T1 shortening what? T2 prolongation who?
Here’s a translation w/an introductory thread to MRI.
2/Let’s start w/T1—it is #1 after all! T1 is for anatomy
Since it’s anatomic, brain structures will reflect the same color as real life
So gray matter is gray on T1 & white matter is white on T1
So if you see an image where gray is gray & white is white—you know it’s a T1
3/T1 is also for contrast
Contrast material helps us to see masses
Contrast can’t get into normal brain & spine bc of the blood brain barrier—but masses don’t have a blood brain barrier, so when you give contrast, masses will take it up & light up, making them easier to see.