Hello. My name is Mahan Mathur and this is the fourth video in our lecture series on Introduction to Radiology. And in this video, we'll cover Magnetic Resonance Imaging or MRI. The objective that I'd like to get out of this presentation is to be able to identify the difference between basic MRI sequences, specifically, what a T1 weighted sequence looks like and what a T2 weighted sequence looks like. And we'll go about doing this by covering a few basic principles of MRI imaging and we'll then dive into the imaging sequences themselves. So here's some basic principles for MR imaging. Now, it turns out that the body is composed of multiple tissues. Many of these tissues contain hydrogen. We are made up of a lot of water, so there's a lot of hydrogen there, there's fat, there's carbohydrates, there's amino acid. We have a lot of hydrogen in our body. Now, hydrogen itself is made up of one proton and one electron. The proton actually rotates around its axis, as you can see in this diagram over here. If we go back to principles of electric magnetic imaging, you'll recall that any moving electric charge creates a magnetic field. So if we think about it in some other sense, our body is actually made up of many little tiny magnets each of which is a hydrogen atom, and normally, these are randomly distributed throughout our body and that's the net magnetic moment cancels each other out. So how can we use these principles to explain how we get MR images? Well, we insert the patient into a giant circle. In this circle, there is a strong external magnetic field, we're talking about 1.5 Tesla or 3 Tesla magnetic field. As a result of this external magnetic field, most of these hydrogen atoms will align with the field. Field will align against it, but most of them will align with the field. Now what we're able to do is then impose a temporary electric current through a radiofrequency pulse. This allows us to scatter the hydrogen protons in all different directions. Now, if we take this radiofrequency pulse away, the hydrogen protons will once again align with the external magnetic field. This will happen differently based on the tissue that this hydrogen proton is contained in. Again, remember that the hydrogen proton is like a little electric charge and as it moves back to its equilibrium with the strong external magnetic field will create an electric charge and this can be measured. And once we measure these electric charges, we can convert that to a signal that gives us the MRI images. So the basic measurements that we make result in T1 and T2 weighted sequences. There are a few different names based on the different vendors that supply these MR magnets, but these basic sequences are T1, T2 weighted looking at fat-saturated images where everything attains fat in the body is made to have zero signal, null signal which appears black and we can do imaging sequences after we give intravenous contrast. What's the basic difference between T1 and T2 weighted images on MRI? Well, fluid looks bright or white on T2 weighted images. Let's use these principles to go through a few imaging sequences. So if we evaluate the following two images, image one and image two, which one do you think is T1 weighted? And which one do you think is T2 weighted? Well, for this, we need to look at structures that we know contain simple fluid. And a good way to go about doing this very quickly is to look at the cerebrospinal fluid that bathes the spinal cord. That's going to be simple fluid, it'll always appear very white or bright on T2 weighted images hence image one is the T2 weighted image and image two is the T1 weighted image. We can also look for other structure that contain fluids such as the gallbladder which contains bile which is just simple fluid here, as well as the bowel which contains fluid as seen in the genome over here. How about these two images? Are these T1 or T2 weighted? What is different about image number two? So both these images are T2 weighted images, we can see that the CSF, the cerebrospinal fluid is white or bright on both these images. The difference here however, is if we look at the subcutaneous fat within the patient, all of that is much darker on the image number two. And so this is what we call a T2 weighted image with fat saturation. So we can apply an external signal to make sure that all the fat in the body becomes very dark and we can use this in order to detect pathology in the patient. How about these two images, are they T1 or T2 weighted? What is different about image number two? So if we look at these two images, both of them are T1 weighted. If we look toward our cerebrospinal fluid, it has a grey or relatively dark signal on both these images. These are both T1 weighted images. The difference between this image and this image is that once again, the fat in the second image is dark and so we simply say this is a T1 weighted image with fat saturation. How about these sets of images, are they T1 or T2 weighted? And what is different about image number two? So if we look at both these images, we see that the cerebrospinal fluid once again is dark and signals, so we know these are T1 weighted images. If we look at the fat it's also dark, so we know both of them are T1 with fat saturation, but the big difference between number one and number two is that the aorta is much brighter on image number two and that the kidney and the spleen and all the other organs are much brighter on image number two. Therefore, this is a T1 weighted fat saturated image after we've given intravenous contrast. This is what post contrast images look like on MRI. So what are some ways in which we can differentiate MRI imaging from CT images? Well, for one thing, MRI imaging is very good at depicting the soft tissues. The soft tissue contrast resolution is much better than on CT imaging. And what that means from a practical perspective is that you have an ability to differentiate the internal architecture of different soft tissue structures, the muscles from the tendons from the bones. For example here, we have a sagittal MR image of the knee and we have side-by-side a CT image of the knee in a sagittal plane as well. In general we can see that the cortex of osseous structures is very dark as a black signal or the marrow which contains a variable amount of fat can have relatively bright signal on most MRI imaging sequences. Compare that to CT scans because the bone is so dense, it appears much brighter than any of the other structures on the CT images. When we examine the subcutaneous fat around the patient in sequences that are not fat saturated that fat signal will be relatively bright, while on CT imaging, subcutaneous fat has a very dark signal with a houseful unit lower than minus 20 in general. If we look at some other structures, we can see very nicely differentiation of muscles from the tendon of muscles, and if we look at the knee joint itself, we can see the different ligaments around the knee joint as well as the cartilage which is very difficult to differentiate on the CT images. If we look at this actual T2 weighted image of the brain, and compare it side-by-side to an axial non-contrast CT scan of the brain, we can see that on the MRI images the internal architecture of the brain itself is much better seen with the different grey matter and white matter tracks much better delineated as opposed to the images of the CT scan where everything appears relatively grey and homogeneous in its appearance. We can also note that on this T2 weighted image, the cerebrospinal fluid is bright as it would be expected on T2 weighted imaging, on non-contrast CT scan is dark, and if we look at the bones the cortex, once again, on the MR images is very dark or the marrow demonstrate a very low amount of bright signal depending on the amount of fat it has within it, on the CT scan, it appears very bright as it is made of a denser material. So in summary, our body is made up of multiple hydrogen atoms and each of these hydrogen atoms act like little magnets. When we place our body inside a strong external magnetic field, majority of these hydrogen atoms will align with that field. Now, we're able to deliver an electric pulse that scatters these atoms will go in all different directions, if we take that pulse away, once again, these hydrogen atoms will align with the strong external magnetic field. The rates that will stay align result in electric signal that can be detected by a computer and this results in T1 and T2 weighted images. Of course, different tissues that contain these hydrogen atoms will realign differently. Our basic sequences are T1 and T2 weighted and if a quick way to differentiate these two sequences is to look at the fluid. Fluid is bright or white in appearance on T2 weighted images. Thank you for your attention.
