Transcription
Now let us talk about this very, very important topic which is sequences of MRI. We already know these are the two important sequences of MRI: T1 and T2. All right?
Now let us see these images. Now instead of two images, now I have got three images. This is very easy for everyone to identify. We know that on T1, fat is white and CSF is black. On T2, fat is white and CSF is white. All right? Now what is my problem? My problem is if there is a pathology on T2, it is white, and the CSF is also white on T2. And a lot of time, it is difficult to see a pathology on a T2-weighted image, specifically when we are talking about the brain, because the background is all of CSF.
If I have to explain it to you, I can easily explain it with the help of this example. Now see this. It is so easy to see my name written over here, right? Because on a white background, I am writing with a black color. Now if I write my name again, we can't see it. But if I change the background here, now if I change the background here, see this. Now it is so easily seen. So that means everything is dependent on contrast, your ability to see something in relationship to the background.
What we do now, see this image carefully. See this image. In this image, CSF is black, but white matter is gray, and the gray matter is white. So white matter gray, gray white matter white makes it T2. But the CSF is black. So this is a special sequence of MRI, and I would like to call it as T2 why T2? Because gray is white, white is gray, but it does not have white water. So I call it as T2 minus water. And T2 minus water has been given a name which is called as FLAIR. T2 minus water has been given a name which is called as FLAIR. And if somebody asks me what is FLAIR, I will say it has got all the properties of T2 except water is black. Except water is black.
What is the significance? Now, if I see a white color on MRI, it is not CSF, but pathology on T2. If it is white, it is either water or pathology. Right? Do a FLAIR. If it remains white, it is pathology. If it becomes black, it is CSF. So FLAIR is a very, very important sequence to differentiate. It is a very, very important sequence to differentiate.
Now let us see an image. You have got a T2-weighted image and you have got a FLAIR image. Right? Now see, it is so easy to see a pathology in the bilateral thalamus. It is very difficult to see the pathology here because white pathology over a black background, white pathology over a white background. So T2, T2, FLAIR. When somebody asks me, sir, how to differentiate? Look at the CSF. If the CSF is white, it is T2. If the CSF is black, this could be either T1 or FLAIR. Look at the gray matter, white matter. If gray is gray, white is white, it is T1. If gray is white, white is gray, then it is FLAIR. Right? T1, T2, and FLAIR. Very, very simple point.
Now let us see this image. What do you see? So T1, T2, FLAIR. I hope these are clear. This is very easy to identify. Now let us see the MRI of the knee joint, correct? In the coronal plane, MRI of the knee joint in the coronal plane. Now please see this. Please see this. This is cortical bone. This is cortical bone. This is the central spongy part of the bone. The central spongy part of the bone contains bone marrow, and in adults, bone marrow contains fat. Now fat is white on both T1 and T2 weighted images. Fat is white on both T1 and T2 weighted images.
Now if there is a pathology on T2 which is white, and there is a fat which is also white, white pathology over a white background. And that is another same problem that I see. So what I should do? I should do something so that the pathology remains white, but the background becomes black, as we did in FLAIR. But we use FLAIR in the brain because the problem was water. Here the problem is not water. Here the problem is fat. So scientists said, when you can have T2 minus water, why can't you have a T2 minus fat? So that means you can subtract fat from T2 also. And a T2 minus fat is broadly called as STIR. So this is not actually STIR. There are different ways to suppress the fat. But the easiest way to remember the sequence for a PG entrance exam is T2 minus water is FLAIR. T2 minus fat is STIR.
Now what is the significance? What was the confounding factor in the brain? Water. What do you use in the brain? FLAIR. What is the confounding factor in the rest of the body? Fat. What do you use in the rest of the body? STIR. So most important sequence in the brain is FLAIR. Most important sequence of MRI in the rest of the body to differentiate.
Next sequence. What do you see? A very, very blurry image of the MRI, unlike of a normal MRI. But pathology is appearing very, very bright. A very blurry image of the MRI with an extremely wide pathology. Let's not go into too much of the detail. Just remember this is a special sequence of MRI which is called as DWI. What does DWI stand for? Diffusion Weighted Imaging. Diffusion Weighted Imaging is a special sequence of MRI which assesses the Brownian motion of the water molecules. It assesses the Brownian motion of water molecules, which is a normal phenomenon, right? In certain diseases, Brownian motion is blocked. And this is called as restricted diffusion. Restricted diffusion. Areas of restricted diffusion. Areas of restricted diffusion are seen as bright on DWI. Areas of restricted diffusion are seen as white on DWI. Most common cause of restricted diffusion. Remember this. Most common cause of restricted diffusion is acute ischemic infarct. Most common cause of restricted diffusion is acute ischemic infarct. So it's a very, very useful sequence for acute ischemia. CT does not show you acute ischemia up to even six hours. And Diffusion Weighted Imaging can show you ischemia within 30 minutes. Within 30 minutes.
However, you also need to remember this is not the only cause of restricted diffusion. It can also be seen in high-grade tumors. It can also be seen in epidermoid cysts. It can also be seen in brain abscess. And it can also be seen in encephalitis. So high-grade tumors, epidermoid cysts, brain abscesses, and encephalitis. T1, T2, FLAIR, STIR, DWI are the five important sequences of MRI that you should be aware of.
What next? What is this? You can easily see this is the Circle of Willis. This Circle of Willis. You have got what are the vessels? You have got the two vertebral arteries, then this is the basilar artery, then this is the posterior cerebral artery. These are the two internal carotid arteries. This is MCA. This is ACA. And in between them, right, are the various vessels of the Circle of Willis. Please remember this sequence is called as angiography. Angiography. The point to be noted is that angiography can be done using an MRI, using a CT, or catheter angiography. This is an MR angiography. Okay. Why do we learn it? In why? But before that, please remember MR angiography can be done with contrast. And if you are doing it with contrast, the commonest contrast that we are using is Gadolinium. MR angiography is a special type of angiography which can be done even without contrast. And if it is done without contrast, it is done with a special sequence of MRI which is called as Time of Flight imaging. Time of Flight imaging is a special type of angiographic sequence. So angiography can be done even without contrast. Even without contrast. Right?
Okay. So now what happens? A lot of time, we get a question in the exam where you have to pick up an angiography, and you do not know whether it is catheter angiography, it is CT angiography, or MR angiography. A lot of people get confused here. I will tell you a very simple way, and it will be a cakewalk for you in the exam. Now let us see this angiography. Just remember this. It is so simple. I do not know why people think it is so complicated. Angiography is what? It is the study of blood vessels. Study of the blood vessels is called as angiography. It is of two types: invasive angiography and non-invasive angiography. Invasive angiography and a non-invasive angiography. What is the example of an invasive angiography? The most important example of an invasive angiography is a catheter angiography. Is a catheter angiography. What are the examples of non-invasive angiography? MR angiography, CT angiography, and for the simplicity, I am writing Doppler also. Right?
Catheter angiography. Now when do you think we should do a catheter angiography? It is so simple. Catheter angiography is the most accurate way to look at the blood vessel. It is the gold standard to look at the blood vessel. So that means whenever in an MCQ, it says gold standard, most accurate, these words are given, mark the answer: catheter angiography. Direct visualization of the blood vessel. Right? If diagnosis and treatment both have to be done in the same setting, the beauty of catheter angiography is it is diagnostic as well as therapeutic. So if the treatment and diagnosis both can be done in the same setting, it is a catheter angiogram. It is a catheter angiogram. All right?
Now you have an aneurysm, right? You have an aneurysm. Always remember, a surgeon coils, a radiologist coils. So you can do an angiography and you can coil the aneurysm from within. You can coil the aneurysm from within. So this is called as catheter angiography. So whenever in an acute emergency, whenever in an acute emergency, treatment is more important, whenever it is most accurate, whenever it is gold standard, my answer will be catheter angiography. In any case, when the angiography has to be done for diagnostic purpose, whenever it has to be done for diagnostic purpose, the preferred angiogram will be a non-invasive angiogram. What are the examples of non-invasive angiogram? MR angiography, CT angiography, and Doppler. MR angiography, we just did it. It can be done with Gadolinium or with ToF. In a CT angiography, we have to give iodinated dye. You have already learned about the risks associated with iodinated dye, including allergic reactions, contrast nephropathy. And Doppler? No dye. Think about it and tell me, if you want to give a dye, you don't want to give a dye? Sir, I don't want to give a dye. I say, if you don't want to give a dye, use Doppler. But can you do it for a deep vessel or a superficial vessel? Superficial vessel. Large or small? Large vessel. So superficial, large vessel, the first investigation to see that is Doppler. However, if it is a deep vessel, small vessel, it is a CT angiography or an MR angiography. The problem with the MR angiography is time-consuming. CT angiography is quick. CT angiography is quick. MR angiography is time-consuming.
So this is very, very simple. Always remember, when you want to do an angiography in the head and neck region, I would like to go for MR angiography. Whenever you want to do an angiography in the chest and abdomen, I would like to go with the CT angiography. For pulmonary embolism, ischemia, CT angiography. Carotid artery disease, MR angiography. Always remember, there should not be any exception or contraindication. You want to see a carotid artery stenosis, I would like to do an MR angiography. But if a patient has a pacemaker, I will do a CT angiography. Renal artery stenosis, gold standard is catheter angiography. But the investigation of choice should be CT angiography. But the answer is MR angiography. Why MR angiography? Because I don't want to give a nephrotoxic dye in a renal artery stenosis. So sometimes it's an exception. Sometimes it's a contraindication. And you have to take it with a pinch of salt. So be careful. This is right.