CME article 13

MRI and disc disease: Review
Introduction

MRI today has become the modality of choice in the evaluation of spinal degenerative disease. MR is superior even to contrasted CT in distinction of bone, disc, ligaments, nerves, thecal sac, and spinal cord. On T1WI, the disc is a fairly homogenous structure isointense to muscle. On long TR images, the disc becomes brighter due to its water content (Fig 1). The nucleus palposus which is more hydrated than the annulus fibrosis becomes more brighter than the annulus on T2 (Fig 2). The appearance of the disc on gradient echo sequences is variable depending upon the pulse sequence and flip angle used.

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Fig 1
Fig 2

Pathogenesis


The pathogenesis of disc degeneration is complex and not well understood. Endplate and disc degeneration is probably a normal sequence of the ageing process. By age 50, 85-95% of adults show evidence of degenerative disc disease at autopsy. It is mainly the nucleus palposus that loses its water content. The disc tissue also undergoes chemical changeswith respect to its collagen and prptepglycan content. The entire disc loses its volume, and the annular fibres get stretched, leading to disc bulging; this may not be significant, unless the spinal canal is small. These disc changes take place in those parts of the lumbar spine most subjected to mechanical stress, L4/5 and L5/S1

Imaging

The earliest radiologically visible changes of intervertebral disc degeneration are those that occur at the endplate, and are best seen on MR (Fig 3). Three types of endplate changes have been described:
Type I: low signal on T1W and high signal on T2WI (replacement of endplate marrow with vascular fibrous tissue)
Type II: High signal on T1WI and low signals on T2WI (replacement of endplate marrow with fatty tissue).
Type III: low signal on T1WI and T2WI ( bony sclerosis)

The prognostic significance of the endplate changes is not completely understood, as they could be part of a normal aging process, and are not to be confused wiht tumor or infection

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Fig 3

Classification/ Types

No universally accepted classification of disc herniation exists. Abnormal discs can be classified as bulging or herniated. A herniated disc can be sub classified as protruded, extruded or sequestered. Disc herniations are more common postero-laterally since the posterior longitudinal ligament and the annular fibers are thickest in the middle and thinner laterally (Fig 4 & Fig 5).

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Fig 4
Fig 5

An annular bulge represents extension of disc margin beyond the confines of the adjacent vertebral endplate (Fig 6). The annular fibres in this case are stretched, but intact. The disc bulge is usually central- (posterior), but can be lateral, and usually does not lead to clinical symptoms unless the spinal canal is congenitally narrow.

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Fig 6


When some of the fibres (usually inner) of the annulus fibrosus tear, the nucleus can focally herniate through the tear, this is called a disc protrusion, and is distinguished from annular bulge by its focality Fig 7).
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Fig 7


Disc extrusion occurs when the nucleus palposus herniated through a complete tear of the annulus fibrosus and is contained only by the posterior longitudinal ligament (Fig 8). This herniated portion of the disc can move caudad or cephalad, but remains attached to the parent disc.

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Fig 8

When this extruded segment breaks free from the parent disc, it is called disc sequestration (Fig 9). Disc sequestration may not be confined by the posterior longitudinal ligament, and may migrate superiorly or inferiorly. These sequestered discs tend to look like the parent disc on T1WI, and may show higher signal on T2WI.
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Fig 9

Post Operative Spine

In the immediate postlaminectomy period, signal from normal bone and ligament is replaced by edema at the resection site. This signal is isointense to muscle on T1WI, and increased on T2WI. If a discectomy has been performed with a laminectomy, in the early post-operative period, there is edema in the anterior epidural space along with disruption of the posterior annular margin due to disc currettage. This may show an intermediate signal on T1 that may blend with the remaining native disc, and hyperintense on T2WI. IV Gado scans should not be used in evaluation of scar vs recurrent disc in the early post-operative period, as enhancement may be seen in para spinal muscles, and nerve roots in the immediate post-op setting. MR should be used only if complications like hematoma, pseudomeningocele or infection are suspected. It is only after 6 months or so, that the edema and mass effect may subside.


Recurrent Disc vs Post-Op Scar

Distinction between recurrent disc and post-operative scar shouldbe made as reoperation on scar leads to a poor result with formation of more scar tissue. Scar tissue generally tends to conform to the shape of and often tends to enwrap the dural sac and exiting nerve roots, and has no definable margin. Mass effect may or may not be present, usually not as the dura may actually get retracted toward the scar. On non contrast MR, anterior epidural scar is hypointense to disc on T1WI, and hyperintense on T2WI. Herniated discs tend to be contiguous to native disc, unless they are sequestered, will have a definable margin, may show mass effect, and tend to show low signal on all pulse sequences (Fig 10). Sequestered discs however, can be hyerpintense on T2WI, making distinction between disc, scar and CSF difficult. Gado MR shows consistent scar enhancement, even months to years after surgery; this is true for anterior epidural scars, and maybe not for posterior epidural scars whose enhancement may decline over years. Herniated discs do not enhance on immediate postinjection scans, but may enhance on delayed scans after 30 min due to contrast diffusion. Hence scans should be completed within 30 min of injection.

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Fig 10

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Dr Deepak Patkar, MRI Centre, Nanavati Hospital, Mumbai