Lea Alhilali, MD Profile picture
Jun 27, 2022 22 tweets 10 min read Read on X
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/a #tweetorial introduction to MRI
#medtwitter #FOAMed #FOAMrad #medstudent #neurorad #radres @MedTweetorials #neurosurgery Image
2/When it comes to bread and butter neuroimaging—MRI is definitely the butter. Butter makes everything taste better and packs a lot of calories. MRI can add so much information to a case Image
3/In fact, if CT is a looking glass into the brain—MRI is a microscope. It can tell us so much more about the brain and pathology that affects the brain. So let’s talk about the basic sequences that make up an MRI and what they can show us. Image
4/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. Image
5/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. Image
6/So to review, T1 is for anatomy and contrast. I remember this bc anatomy is the number 1 thing a radiologist needs to know and a mass is the number 1 thing a radiologist doesn’t want to miss. Image
7/Now to T2! T2 sequences are water sensitive sequences. What is pathologic water in the brain? Edema! My attending once said, “Everything bad in this world is trying to turn you back into what you came from—water. So T2 shows you edema—but this edema can be from many things. Image
8/To review—T1 is for anatomy and contrast, T2 (and FLAIR, which is a type of T2) is for water—which is bright on T2. I remember this bc H20 has a 2 in it—T2 is for H20. Image
9/Next to diffusion or DWI. Diffusion is primarily to detect stroke. Acute strokes are bright on diffusion. But just as all that glitters is not gold, not all that is bright on DWI is an acute stroke. Image
10/This is bc all diffusion does is detect how difficult it is for water to move. Anything that makes the space around water crowded and difficult to move will be bright on diffusion imaging Image
11/So classically, it from a stroke. When cells run out of ATP, the Na/K pump stops working & immediately water rushes in from osmotic pressure & the cells swell. These swollen cells fill the interstitium & restrict the movement of water. This is why strokes are bright on DWI! Image
12/But other things can make it crowded and difficult for water to move. For example, tightly packed cells in aggressive tumors will also fill the spaces & make it difficult for water to move—it is trapped between the tumor cells! So highly cellular tumors are often bright on DWI Image
13/Here is an example. Here is a mass that is as bright as stroke on diffusion bc of its densely packed cells. On contrast images, we see it avidly enhance, as we would expect for a mass. On CT, the tumor is very dense bc of the densely packed cells. Image
14/Hematomas are also bright on DWI. In normal blood, water flows happy & free—but once the clotting cascade starts & fibrin & thrombin & whatever stuff I don’t remember as a radiologist clumps everything together, things get tight—water is trapped in the clot interstices! Image
15/Here is an example. The hemorrhage is bright on CT bc it is clotted, and thus more dense than the brain and CSF, which are closer in density to water. For this same reason, the hemorrhage is bright on diffusion—bc the dense clot traps the water. Image
16/Pus is also bright on diffusion. As a radiologist I don’t often see pus, but as a mom, I sure do. It is thick and gooey and you can just imagine how difficult it is for water to travel through that gelantinous blob of pus. Image
17/Here’s an example. There is a ring enhancing lesion w/a lot of edema on T2. Centrally, there is restricted diffusion, meaning that there is something gooey or thick or dense centrally. Bc this central stuff doesn’t enhance, we know it’s not a mass. This is pus in an abscess! Image
18/So to review--while not everything that is bright on diffusion is a stroke, the most important use is for strokes. I remember this bc it's called DWI--which I joking say stands for Diagnose With Infarct! Image
19/Last but not least is gradient imaging. Gradient imaging is sensitive to metals. And what’s the most important metal in body? Iron—bc iron is in blood. So gradient is our blood sensitive sequence Image
20/Blood is black on gradient. I remember this bc gradient is for metal—and when I think of metal, I think of blacksmiths forging metal products. So BLACKsmith=metal is BLACK on gradient. Image
21/But other metals will be black too. Notably, calcium, which is in our bones and in many other lesions. So remember, just all that glitters is not gold, not all that is black on gradient is blood—other metals are black too Image
22/So now you know the basic MRI sequences and what they are used for. So hopefully now, the radiologist won’t sound like they are speaking a different language when they talk to you—they will just be nerdy and socially awkward when they do! Image

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More from @teachplaygrub

Sep 13
1/Hate it when one radiologist called the stenosis mild, the next one said moderate--but it was unchanged?!

How do you grade it?

Do you estimate? Measure? Guess???

Here’s a thread about a lumbar grading system that’s easy, reproducible & evidence-based! Image
2/Lumbar stenosis has always been controversial.

In 2012, they tried to survey spine experts to get a consensus as to what are the most important criteria for canal & foraminal stenosis.  

And the consensus was…that there was no consensus

So what should you use to call it? Image
3/Well, you don’t want just gestalt it—that is a recipe for inconsistency & disagreement

But you don’t want to measure everything either—measurements are not only cumbersome, they introduce reader variability & absolute measurements don’t mean the same thing in every patient. Image
Read 15 tweets
Sep 9
1/Nothing is more CENTRAL to reading an MRI than finding the CENTRAL sulcus?

How do YOU find it?

Are you just using gestalt when you say “frontal” or “parietal”? 

Time to recenter your search pattern with this thread on how to find the central sulcus on a brain MRI! Image
2/On axial images, at the very top of the brain, the superior frontal gyrus & precentral gyrus combine to look like a bent knee

You can remember that bc precentral is the motor strip & you move by bending your knee! Image
3/Hand motor region is here as well.

You can remember this bc superior frontal gyrus & precentral gyrus together look like a letter L.

And you make the L loser L sign w/your hand! Image
Read 12 tweets
Sep 3
1/Sometimes the tiniest thing can be the biggest pain—that’s microvascular compression of the trigeminal nerve!

But seeing such a tiny finding can be hard!

Here’s a thread about how to look at the trigeminal nerve on MRI! Image
2/The most important thing to remember is that the nerve is 3D so you have to look at it in all three planes.

So what is the normal and abnormal appearance of the trigeminal nerve in each plane? Image
3/Let’s start w/the axial plane.

On a normal axial, the trigeminal nerves should look like the arms of an alien sticking out of the pons. Image
Read 13 tweets
Aug 28
1/Feeling mad every time they ask you if it’s safe to give gad?

Do you know which gadolinium contrast material is safe to give?

At an impasse about the contrast class?

Read on as this month’s @theAJNR SCANtastic answers the question: Is Gad Bad?

ajnr.org/content/45/8/1…
Image
2/Whether or not to give gadolinium is a common question, as almost 1/2 of all MRI studies use gadolinium contrast.

Contrast-enhanced imaging can help elucidate many different pathologies, including tumors, demyelinating disease, & infection Image
3/What is gadolinium, that we're injecting into so many people?

It’s actually a metal, like iron or calcium. Pure gadolinium looks just like a shiny rock that you would find in a mine

But what makes gadolinium special for MRI is that it has 3 unpaired electrons Image
Read 11 tweets
Aug 19
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?

Wait no more!

Here’s a thread about the brainstem Rule of Four to help you localize brainstem lesions! Image
2/The hallmark of a brainstem lesion/syndrome is:

(1) Ipsilateral cranial nerve deficit

(2) Contralateral body deficit (be it weakness, sensory loss, or ataxia) Image
3/You can remember this because often your head has certain feelings that opposite the feelings in your heart/body.

Similarly, the cranial nerve deficit can be the opposite of the body deficit

This split between head and body is key for recognizing brainstem syndromes Image
Read 12 tweets
Aug 16
1/Is your understanding of medial temporal anatomy, well, temporary?

If only there was a way to make hippocampal anatomy memorable!

Here is a thread of the basics of hippocampal anatomy that will hopefully stay in your hippocampus! Image
2/Its name “hippocampus” comes from its shape on gross anatomy.

Early anatomists thought it looked like an upside down seahorse—w/its curved tail resembling the tail of a seahorse.

Hippocampus literally means seahorse. Image
3/In cross section, it has a spiral appearance, leading to its other name, Cornu Ammonis, translated Ammon’s Horn.

Ammon was an Egyptian god w/spiraling rams horns.

The hippocampal subfields are abbreviated CA-1, CA-2, etc, w/CA standing for “Cornu Ammonis” Image
Read 17 tweets

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