Disc lesions

Disc lesions could be classified into three sub divisions.

1-Degeneration

2-Disc bulge.

3-Disc herniation.

-Normal appearance of the lumbar disc by CT is characterized by the following:

* Disc is similar in density to the para vertebral muscles.
 
* Posterior edge of the disc is normally concave.

*In the cervical region, you should not find any disc material posterior to the posterior edge of the bone.

*If you find a disc material posterior to the posterior edge of the bone, this means presence of a cervical disc lesion.

*In MRI, to evaluate the disc you should go to the sagital image T2 in the midline, normal disc is normally containing water, so it will appears as a hyper intense in T2, in addition to that, the posterior edge of the disc should not exceeds the posterior edge of the vertebra.

*If you see the disc black, this means that the disc lost its water content which means occurrence of disc degeneration, in such a case it could be associated with bulge or herniation according to the appearance discussed later.

-Normal appearance of the cervical disc by MRI is characterized by the following:

*Normally, the amount of water in the disc is not sufficient to give frank hyper intensity in T2 image, so to diagnose degenerative disc lesion in cervical disc we should depend mainly upon the height of the disc rather than degree of hyper intensity, so if you find the disc narrowed you can say that it is degenerated.

*Manifestation of disc degeneration in imaging is composed of three steps, the first step is the loss of water content, in this stage, the only modality can diagnose it is T2 image as mentioned above.
Then, the next step after loss of water is loss of height of the disc leading to narrowing of the disc space, in this stage all modalities can diagnose it including x-ray, CT and MRI ( nucleus loose water leading to reduction of the height of the disc ) .
The last step of degeneration is presence of gas inside the disc which is known as vacuum phenomena.

-Now you should differentiate between dis bulge and disc herniation as follow:

*Disc bulge = Intact annulus but weak = resulting in diffuse bulge.

*Disc herniation = Torn annulus(complete) = Focal pathology      ( go to the axial image to see if the disc herniation is midline, to the right or to the left side of the spinal canal ).

*Disc protrusion = Torn annulus(partial) = Focal pathology.


So if the appearance of the disc in the spinal canal is diffuse, considered it bulge, but if it is focal with more than 2 mm extension into the spinal canal, considered it herniation and if it is focal with less than 2 mm extension into the spinal canal, considered it protrusion.

Click the image to enlarge

*In case of focal pathology of the disc, measures the distance between the base of the disc to its apex, if this distance is less than 2 mm, consider it disc protrusion due to partial tear of the annulus, but if the disc length is 2 mm or more, consider it a disc herniation.

*Now we know that the disc is herniated into the spinal canal through a defect in the annulus, if this defect is horizontal, the herniation will be horizontal through spinal canal, while if the tear is in the upper surface of the annulus, the herniation will pass upward and if it is in the lower surface of the annulus, it will go inferiorly. 
In another word, if the annulus tear is in the superior surface, herniation will migrate cephalic while if it is in the inferior surface, herniation will migrates caudally.

*During migration of the disc herniation, if the distal part of the herniated disc is separated, this will lead to what is known as sequestrated disc.

* Usually sequestrated fragment is coming from the nearest disc.

*Usually sequestrated fragment is moving and stopping in the neural foramen giving appearance of neurofibroma, in such a case give contrast, if it is not enhanced it will be a sequestrated fragment, but if it is enhanced, it will be a neurofibroma.

-Disc and granulation:

*Disc could be associated with granulation tissue with or without surgical history as the herniated material of the nucleus make severe irritation in the epidural space resulting in formation of a type of granulation tissue.

*This granulation tissue appears as material similar to that of the herniated disc in imaging giving false impression about the size of this herniation.

*If the patient receive corticosteroid, this granulation tissue will resolve, and if the imaging is done again, the disc will appear smaller in size due to treatment.

*So it is essential to mentioned that in the report, so surgeon do not go immediately to surgery, he must give first treatment of the granulation tissue then making another imaging to evaluate the exact size of herniated disc and if it is worse an operation or not.

*It is easly to differentiate between disc lesion and granulation tissue by MRI T2 image,where you can see the disc material followed by granulation tissue. You can also differentiate between them by giving contrast and making MRI T1 image where the granulation tissue will take enhancement while disc is not taking.