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MRI of the Lumbar Spine

MRI of the Lumbar Spine: A Sports Medicine Approach

Phillip F.J. Tirman, National Orthopedic Imaging Associates

INTRODUCTION

finding as a marker for pain rests in the fact

MRI of the spine has shown significant advancement over the last 5 years. This is in

part

explained

by

improvements

in

technology,

including

improved

coil

technology, leading to

improved imaging sequences more rapid scanning and

increased resolution of spinal anatomy. The more significant advance, however, has been in our increasing ability to be more sensitive and specific in diagnosing spine pathology. Just as treatment of spinal injuries and degeneration has evolved with a basis in conservative sports medicine therapy, spinal imaging is taking the same course. First and foremost, there must be an understanding of the basic anatomy, including intradiscal anatomy. The imager must have an understanding of the mechanisms of spinal injury and the subsequent degenerative changes that can be seen by MRI. Recognition of the body´s inflammatory and reparative response de- monstrated on MRI images should lead to specificity of diagnosis.

DISC DEGENERATION AND HERNIATION

When using MRI to study the degeneration of the spine, it is useful to think in term of Kirkaldy-Willis´ model of the degenerative cascade. In that model, the earliest stage of the degenerative process involves tears of the annulus fibrosus, beginning first with small concentric outer annular tears and progressing to radial tears. All annular tears that communicate with the nucleus can be demonstrated by discography, and all discs that show hypointensity on MRI (“black discs“) will be shown to have annular tears. A subset of annular tears has been described that are hyper intense on heavily T2 weighted MRI sequences. This subset of annular tears has been called a high intensity zone (HIZ) [4,6,7]. This finding on MRI has been shown to have an 86% positive predictive value for predicting concordant pain at discography, although the finding is still somewhat controversial [1,4,6,7,8,9, 27,41] In a recent study, Weishaupt et al. did not find the HIZ to be valuable when comparing MRI to discography [42]. They were able to show only a 56% PPV and a sensitivity of 27%. They concluded that that the HIZ could not be used to identify a painful disc. The anatomic and biochemical basis for this

that with outer annular tears as the disc attempts to repair itself neovascularization extends into the tear, dragging nocioceptive pain fibers along. There are two pieces of evidence that the bright signal represents neovascularization and not just fluid which would also be bright on a T2 sequence. First, most HIZs enhance following the intravenous administration of gadolinium, which requires the presence of vascularization. Second, an HIZ has been biopsied, and granulation tissue was observed [19,21]. While originally HIZs were described only in the midline of the posterior annulus, and not differentiated as to type, experience has taught us that they may occur anywhere in the outer annulus, and also may be subdivided into three types [2,3,12]. A radial tear extends linearly from the nucleus to the outer annulus; a transverse tear is a focal tear at the insertion of the annulus to the adjacent vertebral margin (most frequently inferiorly), and a concentric tear, which extends in a curvilinear fashion transversely along the outer annular fibers.

In the original work by Yu, et al., the transverse tear was felt to be most associated with pain on discography, and we are currently evaluating whether this distinction holds in comparing MRI to

provocative discography.

pressure

monitored

While most HIZs enhance following intravenous gadolinium, this does not help one differentiate between painful and non- painful HIZs [10]. While some observers feel that the HIZ is a marker for internal disc disruption and “discogenic“ pain, the jury is still out on this question.

With the coalescence of concentric annular fissures a portal is created for the extension of nuclear material beyond its normal confines beneath the inner annulus. Any focal abnormal annular contour measuring

Protrusion (sagittal)

Small transverse HIZ

at least 3 millimeters will be shown at autopsy to have nuclear material underneath it. One of the great problems in the reporting of spinal MRIs has been the the lack of standardized terminology. Such terms as “discal bulge“, “prolapse“, and “bulging disc“ should be discarded to be replaced by specific terms to describe specific pathoanatomy that can now be demonstrated with high resolution MRI. The term “annular bulge“ should be used when as a result of disc dessication and loss of height the annulus concentrically extends beyond the vertebral margin. The term protrusion should be reserved for a focal or broad based smoothly marginated extension of the annulus beyond the vertebral margin where one of the following findings is clearly evident: the outer annulus/posterior longitudinal ligament complex is intact; or the base of the abnormal annular morphology is broader than the apex.

An extrusion on the other hand, should have the opposite findings: the outer annulus/posterior longitudinal ligament complex is clearly disrupted, and/or the base is narrower than the apex. It should be noted that because the PLL is narrower between the discs and broadens out at the disc, that an extruded disc fragment may migrate cranially or caudally and remain in front of the PLL.

Finally, a sequestration refers to a disc fragment that has broken off from the parent nucleus and is free in the epidural space.

Protrusion (axial)

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