[ Axial Anatomy | Scout Image | Axial Anatomy | Sagittal Anatomy | Herniation Zones | CT Myelogram | Disc Herniation | Quiz | MRI False-Positive Rates ]

Basic Anatomy:

I've already covered the axial disc anatomy at nauseam on my 'Disc Anatomy Page'. Please go there first if you are not comfortable with the structures of the disc.

The key structures of the axial (over-head) MRI are as follows: The Thecal Sac, the Exiting Spinal Nerve Roots (L5), and the Traversing Spinal Nerve Roots (S1 ). All of the preceding structures all have the capability to generate PAIN within the patient, if they become irritation (from biochemical which leak from the disc - cytokines) and/or compressed.

The axial view is the only true way to visualize just what structures are being compressed and by what, although the sagittal view can 'grossly' demonstrate compression and herniation/bulging. Unfortunately, as you will learn below, MRI images are not as well demarcated as my drawing to the right of us! Sometimes you have to "hallucinate" a little. (All you MDs and DCs will know just what I'm talking about!)

Who's On First? How To Tell What Disc is What: The Scout Image

Without the "scout image", it will be near impossible for the layperson to distinguish the difference between any of the 5 discs of the lumbar spine. The scout image is like a road map that tell you which slice is what and is almost mandatory for even the most experience doctor to have.

Here we have a "scout image" that identifies 20 'cuts' or slices through the sagittal lumbar spine. Each number is assigned to the MRI image that was taken along that plane. These images only cover discs L3, L4, and L5 in this example.

For example, slice #11 (one line above 10 which is colored red to help you) goes right through the L4 disc and is the image to look for if you have a L4 disc problem.

Image 18 (line 18) would correspond with the L3 disc and Image 5 is the best cut through the L5 disc.

On a patient with a greatly thinned disc, It would be better to ask for 'thinner slices' (6mm should do) in order to insure that you get a cut through the thinner disc. In this case, my L5 disc

So, if you want to e-mail me an image of your disc, now you know how to find the image by using the scout image. On some MRI films, a tiny scout image is built right in to each bigger image. This makes it very easy to tell what disc you are looking at.

MRI Anatomy: The Axial Views.

Figures #10 & #11 are over-head views (aka: axial views) of the L5 disc. Although this patient does have a moderate degree of disc degeneration (black disc) and a small non-compressive 4 millimeter central disc herniation, he had a very large 'central canal' which nicely demonstrates the axial MRI anatomy.

The nucleus pulposus is not visible on these images. One reason for this is because the disc has too much desiccation to distinguish between the anulus and nucleus, and the other reason is that this is a T1 Weighted image (higher resolution) with will not differentiate between the high water content of the nucleus and the more arid anulus; however, on a normal, non-degenerated disc, you can easily see a nuclear region and a anular region on the T2 Weighted image (see Figure #7).

The 'posterior neural structures' include the Traversing Nerve Roots, the Thecal Sac (dura and arachnoid mater) and the Exiting Nerve Roots that lie within the IVF (pink zone) and not very visible on this particular image. If you 'hallucinate' a bit, you can see a 'Mickey Mouse' like orientation, where the thecal sac is the head and the traversing spinal nerve roots are the ears. Again, note that despite the 4mm disc protrusion, there is no contact between the traversing S1 roots and the herniation in this patient. (I wish I had this much room in my central canal!) Often times a disc herniation or scar tissue will 'blot-out' one of the traversing roots (Mickey's Ear); this is often a sign of nerve root compression.

Figure #7 demonstrates another axial view of a normal health L4 disc from a 45 year-old-male.

Now we can see a distinct nuclear region and a surrounding anular region. Note that 'Mickey Mouse' is no longer visible at this level. Also note the concavity of the posterior disc adjacent to the traversing L5 nerve roots. This is the sign of a healthy disc. The exiting L4 nerve roots can be seen farther laterally. It is very important to understand that in the lumbar spine, BOTH the exiting nerve roots (L4 roots at this level) AND the traversing nerve roots (L5 roots at this level) may become compressed via large disc herniation and/or severe stenosis.

In this particular image, you can clearly see the tiny nerve roots hanging in a semi-organized fashion within the thecal sac (L5 roots and S1 roots).

Note the neuroforamina are wide open (light yellow zone) and demonstrate no evidence of major stenosis from the adjacent facet joints.

T2 Weighted images are always best for visualizing Degenerative Disc Disease. This is because the T2 image shows off watery structures as bright white and shows off desiccated regions (areas with low fluid concentrations) as black.

Okay, lets move on to some other stuff. Remember, if you need a over-view of the anatomy, please visit my 'Disc Anatomy Page'.

The Sagittal Image Anatomy: View from the Side.

Figure #8 is a view of the lumbar spine from the side, or a Sagittal Image. Note that this particular image is, contrast wise, in between the T2 image and T1 image; it's called a Proton Density Image and is the absolute best image to denote whether or not a disc herniation has 'extruded' through the posterior longitudinal ligament (PLL). It, like the T1 image, is done at high magnification,hence it's detail is amazing.

First the basic structures: The discs, which are stationed between the vertebrae, should be a white color (hydration). Note the 'blackness' (desiccation) of the L5 disc (disc between L5 and sacrum); this represents moderate to severe degenerative disc disease. The PLL (tiny blue arrows) appears as a black vertically orientated line running down the posterior surface of the vertebral bodies and disc. Note that, amazingly, despite this patients 9mm disc herniation (white HNP), the PLL is still 'containing' the protruding nuclear material and has not appeared to have ruptured, although it is certain uplifted from the bone, as nuclear material is seen above level of the disc space. This would technically be called a large contained disc extrusion.

The thecal sac (red stars) is the 'super white' structure that fills the central spinal canal just behind the posterior vertebral bodies. This sac house the free-floating spinal nerve roots (cauda equina) and is made up of both motor and sensory nerve fiber.

The ligamentum Flavum (green star) courses between each of the vertebrae and adds stability to the spine. This structure can hypertrophy or thicken in some patients and help to cause the dreaded central canal stenosis, which is the nemesis of the elderly.

Zones of the Anterior Epidural Space: Herniation Zones.

Now, let's use some CT axial images (my own) to learn the different regions of the anterior epidural space; the regions where disc herniation occur in. If you have ever read your MRI report, some of these terms may sound familiar, for these are regions (zones) are often used by radiologists to describe the exact location of your disc herniation.

BLUE: This is the 'Central Region' and is located directly behind the disc and encompasses the anterior aspect of the thecal sac. Since the PLL (posterior longitudinal ligament) is at its thickest in this region, the disc usually herniates slightly to the left or right of this central zone. (As a side note, I would add that if you do have a centrally located disc herniation, the chances of a successful discectomy are reduced.)

PINK: This is the 'Paracentral Region' or 'Lateral Recess' and is located just outside of the Central Region. Because the PLL is not as thick in this region, disc herniations are frequently found here; in fact, this is the number one region for disc herniations to occur in. The Traversing Nerve Roots, which are the neural structures found in this zone, are frequently contacted, deviated and compressed in this zone. (Remember, an L5 disc herniation that occur into the lateral recess with compress the traversing S1 nerve root, not the exiting L5 nerve root that lives within the IVF.)

GREEN: This is the 'Intraforaminal Zone', also known as the 'Subarticular Zone', and is located within the intervertebral foramen (IVF). It is fairly rare for a disc to herniate into this region or beyond; in fact, only 5% to 10% of all disc herniation occur here or farther out. (241-244) When herniations do occur in this zone, they are often very troublesome for the patient. This is because a super-delicate neural structure called the 'Dorsal Root Ganglion' (DRG) lives in this zone. Any compression of the DRG can result in severe pain, sciatica (aka: radiculopathy) and nerve cell body (neuron) damage.

YELLOW: This is the 'extraforaminal zone' and, as the name implies, is just outside (lateral to) of the IVF. Again, it is very rare for a disc to herniate into this region, but when it does happen, it is often very troublesome for the patient and surgeon. A herniation in this zone may also irritate the 'Sympathetic Nervous System' and cause RSD like symptoms in the lower limb.

Axial CT Myelogram: Anatomy and Herniation.

Let us now look at some CT Myelogram images (mine). The first image (left) is from a 'slice' (remember, CT and MRI images are all thin cuts through the spine in different planes) just above the L5 disc, through the inferior region of the vertebral body.   Since the 'slice' is above the level of the disc, you will only see the posterior neural structures and not the disc itself.

Note the bright white 'ring apophysis' (not labeled) that out-lines the vertebral body - more so anteriorly (top of the picture).

Note that the cauda equina (thecal sac) is completely filled with 'white' contrast material (injected during my myelogram) that makes the thecal sac (dura and arachnoid mater that surrounds the cauda equina) and the 'Dural Sleeves' (which contain the spinal nerve roots) bright white. Since the contrast only fills the root sleeves up to the DRG, the DRG of L5 is not well visualized. I've placed a black line in the center of each DRG (dorsal root ganglion).

Also note the facet joints (oblique black slits - labeled) which are sandwiched between the superior articular process of the sacrum and the inferior articular process of L5.

The next CT slice (right) is a little be lower than the slice above and demonstrates the posterior of the disc quite nicely.   Now you can plainly see the posterior disc has both bulged and herniated into the left traversing S1 nerve root, hence blotting it out (not white in color like the right S1 is).

I've high-lighted the posterior ring apophysis (white thick 'smile' line) to demonstrated how a diseased disc can 'bulge' outward. Anytime disc tissue is seen outside of the posterior vertebral body (ring apophysis), the disc is considered to be 'bulging'. Bulging discs are usually no greater than 2 or 3 millimeters (mm) and are 'concentric' or 'non-focal' in shape.

My disc bulge has an out pouching or eccentric component to its shape, as noted by the portion that has moved into the left 'Lateral Recess'. This out-pouching's is the herniation component to my disc lesion, and it has made my left S1 root disappear because its failed to properly fill with contrast - probably due to compression of that root at this level. (Note the white Nike swoosh just under the "S1" on the left, affected root. This is contrast material that "accidentally" leaked into the epidural space following my myelogram. This wasn't suppose to happen be created a semi-epidurogram effect.)

Here is that same image - the L5 disc level - without all the markings.   Try and practice looking at the disc yourself without the aid of my boundary markers.   Can you see the herniation? I bet you can now! This is considered a broad based herniation for its base it bigger than its anterior to posterior projection.

One thing to keep in mind:   CT myelogram's are designed to look for 'filling-defects', and are not really the best imaging system for evaluating the human disc. The MRI is much better at visualizing the disc.

On a CT myelogram (left), if the disc herniation is large enough it will make the nerve roots turn black, or disappear (like it does to my left S1 root - only its confusing for you still see that tilted 'C' of white - which is only the outline of the black, contract-free, left S1 root). This is because the compression upon the nerve root will NOT allow the contract material to 'fill' into the nerve, hence, no bright white nerve root.   That spells TROUBLE!

9MM DISC HERNIATION: A Real Case.

Figure #6. demonstrates a large 9mm disc extrusion (red star) as visualized on both the T1 Axial (over-head) and Sagittal (side) views.

Note that this extrusion has completely blotted out (can't see) the right traversing S1 nerve roots (left side of image) and has pinched it against the lamina (tiny green arrow). Note the thecal sac is moderately to severely compressed by this large herniation, as noted on both the axial and sagittal images (between blue arrow and red star).

This young man (24 years) has avoid surgery and is doing fairly well, although his days of heavy work are probably over for good.Note the central canal of this young man is much smaller that that of the young man in figure 10 and 11. That just pure genetic folks; bigger central canals are much more forgiving for disc herniation, that smaller ones.

Practice test: Let's see what you have learned!

1) On the below MRI axial image of the L5 disc, name the structures that are numbered.

2) Name me the location of the 'disc lesion' using the correct zone.

3) Which nerve root is being displaced by this disc lesion?

4) bonus: What type of MRI image are we looking at??
hint: T1, T2, or Proton density??

5) What two structures are being contacted by this disc lesion?

GOOD LUCK! Make sure you look at the answers via the link below. I've got the case history to go along with the quiz MRI, including his lateral view, and the official MRI report.

Get the Quiz Answers 'Here'.

References:

241) Epstein NC, et al. "Far lateral lumbar disc herniations and associated structural abnormalities: an evaluation in 60 patients of the comparative value of CT, MRI, and myelo-CT in diagnosis and management. Spine 1990; 15:534-49

242) Abdullah AF, et al. "Surgical management of extreme lateral lumbar disc herniations: review of 138 cases." Neurosurgery 1988;22:648-53

243) Kunogi J, Hasue M, "Diagnosis and operative treatment of intraforaminal and extraforaminal nerve root compression." Spine 1991;16:1312-20

244) Lejeune JP, et al. "Foraminal lumbar disc herniation: experience with 83 patients." Spine 1994;10:1905-8

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