The best way to confirm whether you have a lumbar disc herniation is to go to the hospital and have a CT or MRI (magnetic resonance imaging) film taken of your lower back. However, doctors generally do not have ample time to explain the film report to you in detail, and patients often have to hold the “concise” CT or MRI report card to understand their condition. Don’t worry, we’re going to show you how to read CT and MRI films yourself. This tutorial does not require previous medical knowledge, just learn a few simple steps in this tutorial, you can read the “advanced” CT/MRI films yourself in ten minutes. The lumbar disc consists of three parts: the nucleus pulposus in the middle, the fibrous ring surrounding the nucleus pulposus on the outside, and the cartilage plate on the top and bottom. For more information on the composition of the lumbar disc, please see the article in the Lumbar Spine Knowledge Center. Here we will focus on a few key anatomical structures related to lumbar disc disease on MRI/CT films. Look at the axial view of the lumbar disc film. These key structures are important for diagnosis: ・Tendon sac, also called the dural sac ・Exiting Spinal Nerve Roots (L5) ・Traversing Spinal Nerve Roots (S1) We The key “culprits” to look at in this diagram are: ・Nucleus pulposus ・Posterior longitudinal ligament ・Posterior fibrous ring ・Small joints Any problem with any of these structures can lead to low back pain and lower extremity pain (sciatica). The correct way to see where the pressure is and what is pressing on it is to look at the axial view (top to bottom) of the MRI/CT. A sagittal view (from the side) will only give you a general idea of whether there is compression, bulging or protrusion. As you learn more, you may feel that CT and MRI images are not as clearly distinguishable from each other as they are in this diagram, and that you sometimes have to use your “imagination” when looking at real CT/MRI films ^_^ Lesson 2: Find the disc we want to look at. Without a localization map of the lumbar spine, it is almost impossible for the “layman” to tell the difference between the 5 discs of the lumbar region. The localization chart is like a road map that tells us exactly where to scan at each level of the lumbar spine, so it is essential even for the very experienced physician. This localization map shows the 20 MRI layers of the lumbar spine from a sagittal view. Each number represents an MRI image taken through that plane. The image layers in this slice cover only the L3, L4, and L5 segments. For example, layer 11 (the layer above layer 10 marked in red) goes right through the L4 disc. If you have a problem with the L4 disc, look at the image of this layer. The layer 18 information is also very useful, as it corresponds to the L3 disc. For patients with very thin discs, the layer thickness should ideally be thinner (6 mm) to ensure that it goes right through the thinned disc. So, if you go back to your MRI or CT films of the lumbar spine now, you have learned how to find it by “locating the map”. On many MRI films, each large image is accompanied by a small localization map, which makes it easy to know which disc we are looking at. Lesson 3: Learn to see the axial view of a lumbar MRI/CT film. Figures (a) and (b) show the axial view of the L5 disc. Although this patient has moderate lumbar disc degeneration (black disc seen on the film) and a small non-compressive 4mm central disc herniation, he has a large “central canal” that provides a good representation of axial MRI anatomy. The nucleus pulposus is not visible in these two images because the disc is too dehydrated to separate the annulus fibrosus from the nucleus pulposus, and because these images are T1-weighted (higher resolution), so the watery nucleus pulposus cannot be separated from the drier annulus fibrosus. However, on a normal, non-degenerative disc T2-weighted image, it is easy to see the nucleus pulposus region and the fibrous annulus region on a T2-weighted image (see Figure 3). The “posterior nerve structures” include the Traversing Nerve Roots, the Dural Sac, and the Exiting Nerve Roots. The Exiting Nerve Roots are located in the intervertebral foramen (see the pink area of the IVF in this image) and are not visible in this image. If you use your imagination, you can find a “Mickey Mouse”-like image in the image, with the dural sac being the head of Mickey Mouse and the two exiting nerve roots being the ears of Mickey Mouse. To repeat, although this disc has a 4mm herniation, there is no contact between the passing nerve root S1 and the herniation in this patient. In most cases, the disc herniation or scar tissue will obscure one of the passing nerve roots (Mickey Mouse ear), which is usually a sign of nerve root compression. Figure (iii) shows an axial view of the L4 disc in another healthy 45-year-old male. We can now distinguish between the nucleus pulposus region and the surrounding ciliofacial annulus. Note that the “Mickey Mouse” will not be visible in this plane. Also note the concavity of the posterior edge of the disc near the nerve root at L5, a sign of a normal healthy disc, with the nerve root emanating from L4 a little more laterally. It is easy to see why if there is a large disc herniation or spinal stenosis, both the emitting nerve root L4 and the passing nerve root L5 will be compressed. In this image, the tiny nerve roots (L5 and S1) hanging in the dural sac can be clearly seen, arranged in an incomplete order. Note that the neural foramina are very open (light yellow areas), indicating the absence of existing spinal stenosis due to adjacent intervertebral joint problems. Lesson 4: Learn to look at the sagittal CT/MRI image of the lumbar spine. Figure (d) is a lateral, or sagittal, view of the lumbar region of the spine. Note that this image is between T2 and T1 and is called a proton density image and is the best image for determining whether a herniated disc has penetrated the posterior longitudinal ligament (PLL). Like the T1 image, it is highly magnetic, so the subtleties are surprisingly well represented. First, look at its basic structure: the discs located between the vertebral bodies should be white (with a lot of water). Note the black-colored (dehydrated) L5 disc (the disc between L5 and the sacrum), which represents moderate to severe degenerative disc disease. The posterior longitudinal ligament (PLL small blue arrow) is shown in the image as a black line running vertically down the posterior edge of each vertebral body and disc. Interestingly, although this patient has a 9 mm disc herniation (HNP) with some bone buckling and visible nucleus pulposus material above the disc plane, the posterior longitudinal ligament still contains the herniated nucleus pulposus material without freeing it out. This condition is known academically as a large inclusive disc herniation. The dural sac (red star) appears as a “super-white” structure that fills the central spinal canal posterior to the vertebral body. This sac contains a free-floating spinal nerve root (cauda equina) composed of both motor nerve fibers and sensory nerve fibers. The ligamentum flavum (green star) is located between each vertebra and enhances the stability of the spine. This structure can become enlarged or thickened, contributing to the formation of central spinal stenosis that older individuals are prone to. Lesson 5, finding the herniated area of the lumbar disc: the anterior epidural space Now let’s use some CT axial views to learn about the different areas of the anterior epidural space where lumbar disc herniations occur. If you’ve ever read an MRI report, some of the terminology may sound familiar because these areas are often used by radiologists to describe the exact location of a herniated disc. Blue zone: This is the “central zone,” which is immediately posterior to the disc and encircles the anterior aspect of the dural sac. Because the posterior longitudinal ligament is thickest in this area, a disc herniation is usually either slightly to the left or to the right in this area. Pink zone: This is the “paracentral zone” or lateral saphenous fossa, located immediately lateral to the central zone. Because the posterior longitudinal ligament is not as thick in this area as it is in the central area, disc herniation is often seen in this area. In fact, this is the number one location where disc herniations occur. It is common to see passing nerve roots being contacted, displaced and compressed by the herniated disc in this area. (Remember, the L5 disc herniation in the lateral saphenous fossa is compressing the passing nerve root S1, not the emanating nerve root L5 in the foramen) Green zone: This is the “intraforaminal zone”, also called the “subarticular zone”, which is located within the foramen. It is rare for a disc to herniate into this zone or further out. In fact, only 5% to 10% of disc herniations occur in this zone or further out. When a disc does herniate in this area, it is usually more problematic for the patient. This is because the “dorsal root ganglion” (DRG), which has a superfine neural structure, is located in this zone. Any compression of the DRG will result in severe sciatica and neuronal damage. Yellow zone: This is the “extraforaminal zone”, located immediately outside the foramen. Again, this is a rare area for a herniated disc to occur, but if it does, it can be difficult for both the patient and the physician. A herniated disc in this zone can also stimulate the “sympathetic nervous system” and lead to reflex sympathetic dystrophy (RSD)-like symptoms in the lower extremities. Lesson 6: Observing the axial CT myelogram. Now let’s look at some CT myelograms. The one in Figure (VI) is from the layer immediately above the L5 disc and below the vertebral body (remember that both CT and MRI are thin sections through different planes of the spine). Because this layer is a layer on the horizontal plane of the disc, only the posterior neural structures can be seen and not the disc itself. Note the bright white annulus (not labeled), which indicates the outer contour of the vertebral body (upper part of the image). The cauda equina (dural sac) is completely filled with “white” contrast (injected during the myelogram) making both the dural sac and the dural sheath appear bright white. Because the contrast only filled the root sheath below the dorsal root ganglion, the dorsal root ganglion of L5 was not well visualized. I have drawn a black line in the center of the dorsal root joint on each side. Note also the intervertebral joint (angled black split), which resembles a sandwich between the supra-sacral articular process and the inferior articular process of L5. The CT slice in Figure (vii) is from below the left layer and shows the posterior part of the intervertebral disc quite well. We can clearly see that the posterior disc is both bulging and protruding to the left over the nerve root S1, so obscuring (flooding) the S1 nerve root (the color is not as white as the right S1). A line (thin white smiley line) is now drawn posterior to the annulus to illustrate how the diseased disc is bulging outward. Any time disc tissue is seen outside the posterior annulus of the vertebral body, the disc is considered to be bulging. The bulging disc is usually no more than 2 to 3 mm in size and is concentric or nonfocal in shape. The bulging disc in this picture is an outward-facing pocket and eccentric in shape, partially reaching the left “lateral fossa”. This outward pocket is the bulge of the disc after injury, allowing the left S1 nerve root to disappear (flood) because it cannot be properly filled with contrast due to compression at this level. You may notice a white, surface-to-air missile-like launch beam below the affected left S1 nerve root. This is the result of an “accidental” leakage of contrast into the epidural space after the myelogram, not an intentional semi-epidural contrast effect. The same image of the L5 intervertebral disc, but without labeling. Try to look at it yourself without the auxiliary lines and labels. Do you see the herniation? I’m sure you see it now! It is a large basal herniation with a base larger than the bulge. One thing to remember: CT myelography is looking for “filling defects” and is not the best image to evaluate the human disc. MRI is much better at showing the details of the disc. On a CT myelogram (left side), if the disc herniation is large, it will darken or disappear the nerve root. This is because the compression exerted on the nerve root prevents the contrast from filling the nerve, so the bright white nerve root is not visible. This means there is a problem! In Lesson 7, look at an example of an MRI view of a 9mm lumbar disc herniation . Figure (9) shows a T1 axial and sagittal view of a large 9mm herniation (red star). The herniation has completely obscured (not visible) the right overlying nerve root S1 (left side of the image) and is extruding it into the vertebral plate (small green arrow). Moderate to severe compression of the dural sac from this large protrusion can be observed in the axial and sagittal views (between the blue arrow and red five stars). This patient is a 24 year old who has avoided surgery and is doing well. Note that his spinal canal is much smaller than that of the young man in Figures 10 and 11, and that a large spinal canal is far more tolerant of lumbar disc herniation than a small spinal canal. Lesson 8, it’s time for the test ~ let’s see what we know now! Please use what you have learned earlier to answer the following five questions: 1. Name the structure represented by each figure in the MRI axial view of Figure (x). 2, Name the location of the lumbar disc herniation using the correct regional delineation. 3.Which nerve root is replaced by the protrusion of the intervertebral disc? 4, What type of MRI image is this? Hint: T1, T2, or proton density? 5, Which two tissue structures are contacted by the disc protrusion? Think first, don’t read the following answers right away. (3) Left S1 nerve root. (4) Dural sac. (5) Epidural space. (6) Right vertebral plate (7) Spinous process (8) Left intervertebral joint 2. It is a 9mm large non-inclusive right paracentral disc herniation located in the right lateral saphenous fossa. 3, Right S1 nerve root. 4, T1-weighted image 5, right S1 nerve root and dura Diagnosis of tui na massage point therapy