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

Basic Anatomy:

I have already covered the axial disc anatomy ad nauseam on my
"Disc Anatomy Page." So, please go there first if you are not comfortable with disc anatomy.

The key pain-sensitive structures to look for on the lumbar over-head view (axial view) are the thecal sac, exiting spinal nerve roots (labeled L5 spinal nerve), and the traversing spinal nerve roots (labeled S1 nerve root). As a side note, all of the preceding structures have the capability to generate spine / extremity pain if they become irritated by either leakage of cytokines from a torn disc (grade-5 annular tear) or physically compressed by a protruding or herniated disc. Also note that the L5 disc, for example, sits between the L5 vertebra and sacrum (S1). The exiting nerve roots of L5 (just like the other lumber segments) leave through the L5 IVF, which again is between L5 and the sacrum. The S1 roots, which usually get compressed from a typical disc protrusion, do not exit at the L5 level; they exit below through the S1 sacral foramen. The key "culprits" to look at are the nucleus pulposus, the contour of the posterior longitudinal ligament (aka: PLL), the posterior annulus fibrosus (aka: Annulus Fibrosus and labeled Annulus Fibrosus) and the bony facet joints. Dysfunction of the foregoing structures can all cause back pain and/or posterior extremity pain (i.e., sciatica) by various mechanisms.

Unfortunately, as you will see below, MRI-viewed structures are not nearly as clearly demarcated as the above figure and you sometimes have to "hallucinate" a little. (All you M.D.s and D.C.s will know just what I'm talking about!)

How To Tell What Disc You're Looking At: 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 tells you by number just which slice is which and is almost mandatory for even the most experience doctor to have.

Here we have a "scout image" that demonstrates twenty-cuts or slices through the sagittal (i.e., view from the side) lumbar spine. (Note the white numbers (1-20) on the right side of the image that each connect to twenty white near horizontal lines).

Each number / line represents an individual MRI picture that was taken down the plane of that line. These pictures are then put in order on a large black rectangular piece of film that the doctor looks at--picture by picture.

Note that in the image to the right, only the 11th cut (the one above line 10 [which is red]) gives the doctor a good view of the L4 disc, for it is a perfect slice down the exact plane of the disc. The 18th cut is also very good as it is down the plane of the L3 lumbar disc. On the other hand, the L5 disc image (line #5) is not very diagnostic, for it does not course down the plane of the disc very well--although a moderate of larger herniation will still be seen.

Now, if you want to e-mail me an image of your disc, you will know how to find the image by using the scout images. On some MRI films, a tiny scout image is built right into each bigger image. This makes it very easy to tell what disc you are looking at and whether or not the cut was made.

Real MRI Anatomy: The Axial Views.

Figures #10 & #11 as you may have deduced secondary to the above lesson, are T1 axial views of the L5 disc of the lumbar spine. Although this patient does have a moderate degree of disc degeneration (black disc) and a small non-compressive 4 millimeter central disc herniation (labeled 4mm HNP), he had a very large central canal, which nicely demonstrates the axial MRI anatomy. Note that neither the exiting L5 nerves (can't really see) nor the traversing S1 roots have been physically compressed by the herniation.

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

The posterior neural structures include the Traversing Nerve Roots (labeled S1), the Thecal Sac (dura and arachnoid mater) and the Exiting Nerve Roots (not really visible) that lie within the IVF (pink zone). If you 'hallucinate' a bit, you can see a "Mickey Mouse" like orientation, where the thecal sac is the head and the traversing S1 spinal nerve roots are the ears.

Figure #7 demonstrates another axial T2 view of a normal healthy L4 disc from a 45 year-old-male. Now we can see a distinct nuclear region (labeled nucleus) and a black circular annular region, which corrals or encloses the Jell-O like nucleus. Note that "Mickey Mouse" is no longer visible at this level (it's not seen well on the T2 images). Also note the concavity of the posterior disc adjacent to the traversing L5 nerve roots--there is no disc bulges or protrusions sticking out here. These are the signs of a healthy disc. Also note the exiting L4 nerve roots can be seen farther laterally--still very hard to see (the L4 root might be the DRG). 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, which can in turn cause pain.

Also note the alignment of the the traversing roots: you can see the L5s, S1s, and even the S2s in this image--this is not always seen.

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.

Look for those Nerve Roots on the Sagittal Image too:

This is a far lateral cut (way off to the edge) of the lumbar spine that shows the exiting nerve roots (red arrows) within the intervertebral foramen. All the discs are black, which indicates this is a T1 series.

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.

T1 vs. T2: the differences:

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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