Almost every medical imaging modality has

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1 John A. Carrino and William B. Morrison Almost every medical imaging modality has been applied to the evaluation of degenerative lumbar disc disease, including radiography, fluoroscopy, and techniques modified with contrast (myelography discography), computerized tomography (CT), magnetic resonance imaging (MRI), and scintigraphy (isotope bone scan). The medical image serves as a surrogate record of morphology and sometimes physiology. A probe, or energy source, is applied to a patient whereby there is a physical interaction that alters the probe (changes the energy output), and a detector records this. Some modalities use ionizing radiation, either from electrons (radiography, CT) or from the nucleus (scintigraphy), while others use nonionizing sources, such as radiofrequency (MRI) or sonication (ultrasound). Acquisition may be projectional (a shadowgram ), such as with the radiographic techniques, or cross-sectional ( bread loafing ), such as with CT and MRI. The 3 pertinent parameters of spatial resolution, contrast resolution, and temporal resolution are generic descriptors of performance capability of an imaging system. Spatial resolution refers to the ability to see spatial detail (ie, resolve 2 points as different). Radiography has the highest spatial resolution (5 to 10 line pairs per mm), with intermediate resolution for CT and MRI (0.5 line pairs per mm, or approximately 1 mm resolution). Although with the advent of multi-detector CT, isotropic submillimeter voxels can be acquired. Scintigraphy typically has the least resolution, approximately 1 to 2 cm, unless single photon emission computerized tomography (SPECT) techniques are used. Contrast resolution refers to the ability to distinguish between signal values at different locations and requires some change in luminance or signal intensity over the background. The usual range is from 0.5% to 10% of some reference signal. MRI is advantageous because of its ability to perform different pulse sequences to exploit different types of soft tissue contrast that are not available by other modalities (eg, T1-weighted, T2- weighted, and intermediate weighted images). MRI has superb soft tissue contrast resolution, being able to detect 5% to 10% differences in nuclear magnetic relaxation times. CT can detect 0.5% differences in radiograph attenuation regarding water, which is the reference standard (the Hounsefield Unit of water is calibrated to zero). The physical interaction is based on the linear attenuation coefficient, and this is roughly proportional to density (that is why ligamentous structures such as the anulus fibrosus are hyperattenuating and subcutaneous fat is hypoattenuating). Therefore, for CT, contrast is best between very dense structures (ie, bone), highly compact soft tissues (eg, tendons, ligaments, anulus fibrosus), water-containing tissue (eg, muscle, thecal sac), low-density tissues (eg, fat) and gas. This is an improvement over projectional radiography, which requires approximately a 10% change of full scale to detect contrast differences. One mechanism to improve contrast resolution is to administer a contrast agent, which can be performed through several different routes. The most commonly used routes for spine imaging are intravenous, intrathecal, and intradiscal. Nuclear medicine techniques can also detect approximately 10% difference in radioactivity as an emission phenomenon from the administered agent. The agent most commonly used for isotope bone scanning is technetiumlabeled methylene diphosphonate (MDP). This agent works by chemisorption onto the bone matrix. MRI deserves a more in-depth description because it is widely used in the evaluation of spine degenerative disc disease, and it has more complex features than other modalities. There are several different types of MRI platforms, which may be referred to as closed and open. From the Departments of Radiology, Harvard Medical School, Brigham and Women s Hospital, Boston, MA; and Jefferson Medical College, Thomas Jefferson University Hospital, Philadelphia, PA. Address reprint requests to John A. Carrino, MD, MPH, Assistant Professor of Radiology, Harvard Medical School, Brigham and Women s Hospital, Department of Radiology, 75 Francis Street, Boston, MA jcarrino@partners.org Elsevier Inc. All rights reserved /03/ $30.00/0 doi: /s (03) Seminars in Spine Surgery, Vol 15, No 4 (December), 2003: pp

2 362 Carrino and Morrison Closed MRI systems are high field strength (1.5 T or higher) with a signal-to-noise ratio that produces images of overall better quality than open low field strength magnets (usually from 0.3 to 1.0 T). In addition, frequency selective fat suppression can be used. However, to obtain images using a high field strength scanner, the bore needs to be smaller to accommodate the gradients and some other hardware. Open MRI systems may produce diagnostic quality studies of the lumbar spine but are best reserved for patients with severe claustrophobia or a body habitus that precludes the use of a closed system because the image quality is generally significantly inferior to a closed system. Overall, the installation and maintenance of open MR systems is simpler, making open systems more economical. A coil of wiring acts as an antenna to receive the signal from the patient s body. A dedicated spine coil is useful to improve the signalto-noise 3 to 5 times over the scanner s built-in body coil. The type of image weighting used during image acquisition determines MR contrast. The most commonly used images for lumbar spine applications are T1-weighted, T2-weighted, intermediate-weighted (proton density), and short tau inversion recovery (STIR). These types of images may be generated using various sequence designs. Commonly used mechanisms are (1) conventional spin echo (CSE); (2) fast spin echo (FSE), also known as turbo spin echo, depending on the magnet vendor; and (3) gradient echo (GE). CSE is the traditional way of acquiring images and is the standard way to obtain T1- weighted images. Traditional T2-weighted images have been largely supplanted by FSE. The pulse sequences reflect different ways of interrogating tissue, and are related to different patterns of applied radiofrequency pulses and signal collection. They extract different magnetic properties that infer specific tissue composition. It should be noted that there is some variability in protocols, and this depends on several factors, including the operator s familiarity with certain pulse sequences. However, a general-purpose protocol for lumbar spine imaging is commonly used (Table 1). Axial sections are often used parallel to the disc, to help facilitate identification of contour abnormalities. In addition, socalled stacked axial sections through the lower lumbar spine are used to facilitate detection of other pathology and/or displaced or sequestered disc fragments. T1-weighted images are useful for detection of fat, and fat acts as a natural contrast agent for detection of epidural or paraspinal lesions, marrow infiltration or replacement, focal bone lesions, and also the diagnosis of lipid-containing lesions, especially hemangiomas. T1-weighted images are also used after the administration of contrast material. In this case, fat suppression is often used to increase conspicuity of the contrast agent deposition. On T1-weighted images, fat is bright (unless it has been suppressed, such as in postcontrast studies), and fluid is dark. Therefore, one strategy for identifying T1-weighted images is to identify a known fluid containing structure, such as the cerebrospinal fluid (CSF) in the thecal sac or fluid in the urinary tract or gall bladder, and note the signal intensity for the known fluid. T1- weighted images make fat conspicuous and help identify anatomic cleavage planes, an essential part to study bone marrow. In addition, T1 weighted images provide a high intrinsic signalto-noise ratio because of the short echo time, which allows anatomic detail. Intermediateweighted or proton density weighted images are called such because they minimize T2-weighting (by having a short echo time) and minimize T1- weighting (by having a long repetition time), and, thus, have a contrast that is intermediate to T1 and T2. Intermediate-weighted images have the highest signal-to-noise ratio, but, because the contrast properties often do not add much to other sequences, it is not commonly used for routine lumbar spine imaging. Fluid appears bright on T2-weighted images, so, again, one strategy is to look for known fluidcontaining structures, such as CSF or urinary bladder, and identify the bright hyperintense fluid signal. Fat is variable, and can depend on whether the image is acquired as CSE versus FSE and whether spectral fat suppression is used. Spectral fat suppression can only effectively be used at moderate to high field strengths (1.0 T or higher). T2-weighted images are useful for the detection of areas of bone marrow edema and are critical for showing disc pathology. Specific uses in the spine would be to show disc desiccation, hyperintensity zones, and Modic end plate findings. The fat suppression increases the dynamic range, although there is a loss in anatomic detail

3 363 Table 1. Lumbar Spine MRI Protocol Section Plane Weighting Sequence Repetition time (ms) Echo time (ms) Sagittal T1 CSE (minimum) Echo Train Length Fat Suppression Section Thickness (mm) Section Gap (mm) Miscellaneous None No Prescription should cover lateral extraforaminal aspect of pedicles. Sagittal T2 FSE 3,000-4, Yes Fat suppression is variably used.* Axial oblique T2 FSE 2,000-4, None No Prescribed parallel to the intervertebral discs. Axial T1 CSE (minimum) 8-16 No Prescribed as a stack through the lower lumbar spinal canal. *Fat suppression is variably used for sagittal T2-weighted images of the spine for routine imaging depending on site and magnet. If fat suppression is used, then the echo time should be reduced (closer to 60 milliseconds) so that the background architecture signal is preserved. Without fat suppression, a long echo time is needed to maintain fluid sensitivity of an FSE sequence. because of the loss of cleavage planes. This effect can be somewhat compensated for by maintaining a moderate echo time (approximately 60 to 80 milliseconds). This modification to the echo time still allows for a fluid sensitive sequence, but improves the anatomic background detail. On any field strength scanner, fat suppression and fluid sensitive sequences can also be obtained using STIR, chemical shift imaging (in and out of phase imaging), and Dixon techniques. On STIR imaging, fluid is bright because the contrast is a combination of T1 and T2-weighted images. Therefore, structures that have a long T1 and a long T2 are particularly bright, such as fluid. Fat is always suppressed. STIR has poor intrinsic signal-to-noise characteristics when compared with other pulse sequences. However, it is the most robust fat-suppression technique that is readily available on all scanners. It is very sensitive for detection of edema, and works well as a screening sequence for many neoplastic, infectious, and traumatic pathologies. However, it is not as useful for degenerative conditions due to a high degree of noise and low resolution that characterize this sequence. For this reason, STIR has been referred to as the bone scan of MRI. Gradient-recalled echo (GRE or GE) sequences use a very short repetition time and echo time, and the signal is strongly influenced by another parameter called the flip angle. A low flip angle (eg, 5 to 20 ) results in an image with bright fluid, while a larger flip angle (eg, 40 to 90 ) results in a T1-weighted like image with bright fat. Unlike all the other sequences mentioned previously, a GE sequence is acquired without a refocusing pulse ; signals from the patient (ie, echoes ) are generated by rapidly altering the magnetic gradients. The refocusing pulse accounts for magnetic heterogeneity in the patient s body, so GE images are very susceptible to artifact. For example, a metal plate or screws will black out much more surrounding anatomy than a similar spin echo sequence (CSE or FSE). However, calcium and blood products also result in a mild degree of low signal artifact, which can be used to an advantage. For example, a disc

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5 365 herniation is bright (like the adjacent disc) compared with a calcium-containing spur. Also, in the setting of trauma, GE images are useful to detect small amounts of cord hemorrhage. These applications are especially useful in the cervical spine. Because of the rapid nature of this technique, variations of it are used for imaging blood flow of the vertebral arteries (MR angiography). Another advantage of GE sequences is that they can be acquired using very thin slices with high spatial resolution, and can even be acquired as a 3-dimensional dataset that can potentially be reconstructed in different planes. In summary, regarding the MR pulse sequences, the following points should be emphasized. On T1-weighted images, fluid is dark and fat is bright unless fat-suppression is used, which is only typically done when contrast administration is performed. Proton density or intermediate weighted images have the highest signal-to-noise ratio, and fluid is somewhat bright, depending on the echo time. On T2-weighted images, fluid is bright; fat signal depends on whether spectral fat-suppression has been used or not. STIR images are the most fluid-sensitive. Fluid is bright, and other structures are typically dark to intermediate signal intensity. GE sequences are used mostly in the cervical spine to generate rapid, high spatial resolution images over a smaller area but are not commonly used for lumbar spine imaging. Imaging Findings of Degenerative Disc Disease The intervertebral disc is a composite structure consisting of 3 distinct components: (1) the nucleus pulposus, (2) the anulus fibrosus, and (3) the cartilaginous end plates. They are cartilaginous joints and, in this sense, reflect intervertebral symphysis. The anulus fibrosus is the limiting capsule of the nucleus pulposus, and is attached superiorly and inferiorly to the vertebral body ring apophysis by Sharpey fibers. It is confluent with the anterior longitudinal ligament and posterior longitudinal ligament. The anulus fibrosus is made predominantly of type 1 collagen and, because of the absence of free protons and dense lamellar structure, it is normally hypointense on all MRI pulse sequences (Fig 1). In the lumbar spine, the anulus fibrosus tends to be thicker ventrally than dorsally. The nucleus pulposus is made up of glycosaminoglycans (GAG) and has approximately 85% to 90% water content under normal conditions. Its signal intensity is intermediate on T1-weighted image and hyperintense on T2-weighted images, reflecting the high water binding of the GAG (Fig 1B). The intervertebral disc height reflects the status of the nucleus pulposus and typically increases gradually as one goes from cephalad to caudal, with the exception of the lumbosacral junction, which may be narrower than the remainder of the lumbar intervertebral discs. The posterior disc margin tends to be concave in the upper lumbosacral spine (Fig 1D), and is straight or slightly convex at L4-5 and L5-S1. The posterior margin typically projects no more than 1 mm beyond the end plate. Often, there is a horizontally oriented developmental cleft present, best identified on the T2-weighted images. The bilocular appearance of the adult nucleus resulting from the development of a central horizontal band of fibrous tissue is considered a sign of normal maturation. This cleft is also well shown on discography. The end plates are covered by hyaline cartilage, serving as the biomechanical and metabolic interface between vertebral body and nucleus pulposus. Disc degeneration begins early in life. The etiologies may be related to normal aging, genetic predisposition, or environmental factors. Component changes can occur in the nucleus 4 Figure 1. Normal magnetic resonance imaging (MRI) appearance of the lumbar intervertebral disc. (A) Sagittal T1-weighted. (B) Sagittal T2-weighted without fat suppression. (C) Sagittal T2-weighted with fat suppression. (D) Axial T2-weighted through the intervertebral disc level. Note that on T1-weighted images, the disc is hypointense to the lumbar vertebral body, while on T2-weighted images it is hyperintense, reflecting normal water content of the nucleus pulposus. Small intervertebral clefts may be present (arrow). On axial imaging, the posterior margin should have a concavity (arrowhead), with the exception of the lumbosacral junction, which may normally have a slight convexity. The disc margins should project no more than 1 or 2 mm beyond the vertebral end plate. Note that the marrow is slightly hyperintense on the nonfat suppressed images and dark on the fat suppressed pulse sequences.

6 366 Carrino and Morrison pulposus, anulus fibrosus, cartilaginous end plates, and subjacent marrow. The nucleus pulposus typically shows desiccation, fibrosis, or a vacuum phenomenon, while the anulus fibrosus undergoes mucinous degeneration. Fissuring may occur from radial tearing in the vertical or transverse direction (ie, rupture of the Sharpey fibers near the ring apophysis). The cartilaginous end plates show marginal osteophytes and subarticular marrow signal alteration. There is some confusion over the terminology of degenerative joint disease in general. Osteoarthritis or osteoarthrosis is a process of synovial joints. Therefore, in the spine, it is appropriately applied to the zygoapophysial (Z-joint, facet), atlantoaxial, costovertebral, and sacroiliac joints. Degenerative disc disease is a term applied specifically to intervertebral disc degeneration. The term spondylosis is often used in general as synonymous with degeneration, which would include both nucleus pulposus and anulus fibrosus processes, but such usage is confusing. It is best that degeneration be the general term and spondylosis deformans be a specifically defined subclassification of degeneration characterized by marginal osteophytosis without substantial disc height loss that reflects predominantly anulus fibrosus disease. Intervertebral osteochondrosis is the term applied to the condition of mainly nucleus pulposus and the vertebral body end-plates disease, including anular fissuring (ie, tearing). There is a widely endorsed nomenclature supported by many subspecialty groups, which should be the basis for describing disc related pathology between different types of providers. 1 It is important to recognize that the definitions of diagnoses should not define or imply external etiologic events, such as trauma, should not imply relationship to symptoms, and do imply need for specific treatment. The terminology used in this article is supported by the Nomenclature and Classification of Lumbar Disc Pathology document available on the Internet. 2 The disc derives its structural properties largely through its ability to attract and retain water. Internal disc disruption is a term that was coined in the 1970s to describe pathologic changes of the internal structure of the disc. Decreased tissue cellularity and altered matrix architecture characterize intervertebral disc degeneration. The physiochemical change of diminished water binding capacity in the GAG is heralded on MRI by the loss of T2 signal and has been called the desiccated disc. Thus, some refer to this condition as dark disc disease or black disc disease (Fig 2). Osteophytosis is a hallmark of degenerative disc disease and should be differentiated from paravertebral calcification/ossification, syndesmophytes, and longitudinal ligament calcification/ossification. Marginal osteophytes tend to be horizontal and parallel to the disc margin, as if to be creating additional articular surface. However, they can be bridging, from one level to the next. Anterior and lateral marginal osteophytes have been found in 100% of skeletons of individuals over 40 years old and are thought to be consequences of normal aging, while posterior osteophytes have been found in only a minority of skeletons of individuals over 80 years old, so are not inevitable consequences of aging. 3 The claw osteophyte of McNabb is defined as the bony outgrowth occurring very close to the disc margin from the vertebral body apophysis, directed with a sweeping configuration, towards the corresponding part of the vertebral body opposite the disc. It is said to be associated with instability. Paravertebral calcification/ossification tends to come off the mid portion of the vertebral body and can be seen in HLA B27 seronegative spondyloarthropathies, such as psoriasis and reactive arthritis, formerly known as Reiter disease. There is often a paucity of degenerative disc disease, which can be helpful in the differential diagnosis. Syndesmophytes are calcifications along the outer margin of the anulus fibrosus and have a thin vertical orientation from one disc margin to the next. This is a hallmark of ankylosing spondylitis and occurs in the context of young men with only minimal disc disease. Calcification may occur in the anterior or posterior longitudinal ligament. Ossification of the posterior longitudinal ligament is a degenerative related condition typically seen in the cervical spine and not often seen in the lumbar spine. Anterior longitudinal ligament mineralization is predominantly seen in the thoracolumbar spine. This is thought to be a senescent related condition, usually with only minimal disc height loss, and, when it involves more than 4 contiguous segments, it is referred to as diffuse idiopathic skeletal hyperostosis.

7 367 Figure 2. Disc desiccation. Magnetic resonance image (MRI) shows loss of the normal intervertebral disc hydration. (A) Sagittal T1-weighted image. (B) Sagittal T2-weighted image. This result is manifested by low nucleus pulposus signal intensity on the T2-weighted images at the L4-5 and L5-S1 level (arrows). There is an associated disc herniation at L5-S1. Schmorl nodes are intervertebral disc herniations and may be considered a transosseous disc extrusion. Herniation of the nucleus pulposus occurs through the cartilaginous end plate into the vertebral marrow space. They often have a characteristic round or lobulated appearance. They may enhance after contrast administration, with ring-like enhancement being most common. They are often incidental and likely to be developmental or post-traumatic rather than purely degenerative or adaptive. 4 There is now imaging evidence of a significant genetic association among the COL9A3 tryptophan allele (Trp3 allele), Scheuermann disease, and intervertebral disc degeneration in patients who are symptomatic. 5 Intradiscal calcification is most often incidental and can be seen in the pediatric population, but it is also frequently seen in the setting of degenerative disc disease or simply as senescent changes. 6 However, when associated with predominantly nucleus pulposus disease (ie, loss of disc height and present at virtually every lumbar segment), this is pathonomonic for alkaptonuria (ochronosis). Ochronosis is a hereditary disorder of amino acid metabolism consisting of the accumulation of a dark pigment (homogentisic acid) in connective tissues. The imaging manifestations are marked height loss, vacuum, and sclerosis. There is minimal osteophytosis because this is primarily a nucleus pulposus disease. Dystrophic calcification universally presents in all discs is the radiographic hallmark. Disc contour changes are part of the degenerative process, and have been broadly characterized as bulges and herniations. The following is a summary of the accepted nomenclature for abnormal disc contours, typically referred to as bulges and herniations. MRI is well suited to show the severity and characterize contour ab-

8 368 Carrino and Morrison Figure 3. Disc contour abnormalities. (A-C) Axial T2- weighted images at the level of the intervertebral disc. (A) Annular bulge. Generalized displacement of more than 180 of the disc margin beyond the normal margin of the intervertebral disc space is evident (arrowheads) and is the result of disc degeneration with an intact anulus. (B) Disc protrusion. The base against the parent disc margin is broader than any other diameter of the herniation. Extension of nucleus pulposus through a partial defect in the anulus is identified (arrow). The herniated disc is contained by some intact anular fibers. (C) Disc extrusion. The base against the parent disc margin is narrower than any other diameter of the herniation. There may be extension of the nucleus pulposus through a complete focal defect in the anulus. Substantial mass effect is present, causing moderate central canal and severe left lateral recess stenosis (arrowhead). normalities: (1) size, (2) morphology, and (3) location. Mass effect on the spinal cord and nerve roots can also be shown. However, this needs to be put into the context of the clinical syndrome. Hence, the following are pathoanatomical descriptors that do not imply a specific pathoetiology or syndrome. An anular bulge is described as a generalized displacement (more than 180 ) of disc margin beyond the normal margin of the intervertebral disc (Fig 3A). The normal margin is defined by the vertebral body ring apophysis exclusive of osteophytes. It can be the result of disc degeneration with a grossly intact anulus. Disc margins tend to be smooth, symmetric, or eccentric and nonfocal, and may have a level-specific appearance in the lumbar spine. Disc herniation is a localized displacement (less than 180 of the circumference) of disc material beyond the normal margin of the intervertebral disc space (Fig 3B). This material may consist of nucleus pulposus, cartilage, fragmented apophyseal bone, or fragmented anular tissue. It is often the result of disc degeneration, with some degree of focal anular disruption. The types of disc herniation are designated as protru-

9 369 sion, extrusion, and free fragment (sequestration). Protrusion refers to a herniated disc in which the greatest distance, in any plane, between the edges of the disc material beyond the disc space is less than the distance between the edges of the base in the same plane. Protrusions are characterized by the following: (1) the base against the parent disc margin is broader than any other diameter of the herniation, and (2) extension of nucleus pulposus may occur through a partial defect in the anulus but is contained by some intact outer anular fibers and the posterior longitudinal ligament. The types of protrusions may be broad-based (90 to 180 circumference) or focal (less than 90 circumference). Extrusion refers to a herniated disc in which, in at least one plane, any one distance between the edges of the disc material beyond the disc space is more than the distance between the edges of the base in the same plane, or when no continuity exists between the disc material beyond the disc space and that within the disc space (Fig 3C). An extrusion is characterized by the following (1) the base against the parent disc margin tends to be narrower than any other diameter of the herniation and (2) extension of the nucleus pulposus through a complete focal defect in the anulus fibrosus. Extruded discs in which all continuity with the disc of origin is lost may be further characterized as sequestrated. Disc material displaced away from the site of extrusion may be characterized as migrated. It may stay subligamentous, contained by the posterior longitudinal ligament or may migrate widely. A chronic disc herniation may show a calcification, ossification, or gas and vacuum phenomenon. There are no formal staging systems for lumbar degenerative disc disease, and most observers will report findings commonly using the designations of mild, moderate, and severe. However, these designations will hold different meaning among observers, especially regarding the degree of disc degeneration. The following scheme is used to define the degree of canal compromise produced by disc displacement based on the goals of being practical, objective, reasonably precise, and clinically relevant. Measurements are typically taken from an axial section at the site of the most severe compromise. Canal compromise of less than one third of the canal at that section is mild, between one and two thirds is moderate, and more than two thirds is severe. This scheme may also be applied to foraminal (neural canal) narrowing, with the sagittal images also playing a very useful role for defining the degree of narrowing. Observer interpretations are also made with various degrees of confidence. The statement of the degree of confidence is an important component of communication. The reporter should characterize the interpretation as Definite if there is no doubt, Probable if there is some doubt but the likelihood is more than 50%, and Possible if there is reason to consider but the likelihood is less than 50%. Modic Changes Modic and coworkers initially described vertebral marrow end plate findings in association with degenerative disc disease, and this spectrum of findings is popularly referred to as Modic changes. 7 Type 1 is fluid-like, and shows T1 hypointensity and T2 hyperintensity (ie, follows fluid signal). Type 1 Modic changes show bone marrow edema (Fig 4), have mild enhancement that may involve the disc, and are identified in 4% of patients scanned. On contrast-enhanced MRI, the enhancement is proportional to reactive granulation tissue present at the peripherally herniated nucleus pulposus, anular tear, or degenerated end plate. With a degenerated end plate, this tends to be linear, parallel with the disc being linear, spotty, or diffuse. Type 2 Modic change is fat-like and follows fat signal intensity on all pulse sequences (Fig 5). Therefore, type 2 changes show T1 and T2 hyperintensity without fat suppression (Fig 5B) or T2 hypointense with fat suppression (Fig 5C). Type 2 Modic changes are identified in 16% of patients scanned for lumbar disease. Type 3 Modic change is sclerosis-like and shows hypointensity on all pulse sequences (Fig 6). Type 3 can also be identified on radiography as a rounded area of sclerotic opacity abutting the end plate and is known as discogenic vertebral sclerosis (Fig 6C). The characteristic findings for Modic changes are that they are related to the end plate. They can be round or hemispherical but do not have to be. The disc shows degeneration, meaning that there is at least some desiccation of the nucleus pulposus. The differential diagnosis may include infection and, one way to distinguish this, is that infection tends to have intradiscal fluid-like signal and end plate erosions. Modic findings are thought to be along a

10 370 Carrino and Morrison Figure 4. Vertebral marrow signal alteration (Modic type 1 change). (A) Sagittal T1-weighted magnetic resonance imaging (MRI). (B) Sagittal T2-weighted MRI. Disc height loss and desiccation at multiple levels is evident. At the L3-4 level, this is associated with rounded areas of signal alteration that abut the end plate and follow fluid-like signal with T1 hypointensity and T2 hypointensity (arrows). spectrum from type 1 through type 3. However, mixed end plate findings are often present and are typically associated with more severe degenerative disc disease. The significance of Modic end plate findings for predicting a clinical syndrome beyond being simply a marker for degenerative disc disease (painful or painless) is indeterminate. There is conflicting evidence in terms of predicting a positive response to provocative discography. One investigation showed no significant relationship between vertebral end plate signal changes at MRI and discography. 8 Another investigation showed that moderate and severe end plate abnormalities of the Modic type 1 and type 2 varieties are useful for predicting discography positive pain response in patients with symptomatic low back pain. 9 In support of this, others have found that Modic changes are relatively specific but an insensitive sign of a painful lumbar disc in patients with discogenic low back pain. 10 High Intensity Zone High intensity zone (HIZ) is the term coined to denote the finding of an area of hyperintense signal without the periphery of the disc in the region of the anulus fibrosus on T2-weighted MRI (Fig 7A). Posterior tends to be more common than anterior. In the patient population having MRI for lumbar back pain, this finding may be noted in approximately 25%. The presence of a HIZ correlates with an anular tear and about an 85% chance that there will be concordant pain reproduction at discography. 12 A follow-up investigation found similar results with only one HIZ found in control subjects. Therefore, the initial understanding was that for patients with symptomatic low back pain, the HIZ was a reliable marker of painful outer anular disruption. 13 Others have also concluded that the lumbar disc HIZ in patients with low back pain is likely to represent painful internal disc disruption. 14

11 371 Figure 5. Vertebral marrow signal alteration (Modic type 2 change). Magnetic resonance images (MRI) show disc desiccation at multiple levels. (A) Sagittal T1-weighted. (B) Sagittal T2-weighted without fat suppression. (C) Sagittal T2-weighted with fat suppression. At the L5-S1 level, this is associated with a rounded area of signal abnormality in the anteroinferior aspect of L5 abutting the end plate. This follows fat signal on all pulse sequences, and is hyperintense on T1-weighted and T2-weighted images without fat suppression. On the fat suppression image (C), the area is signal void. This predominantly consists of fat. The normal marrow usually has some hematopoietic elements and, thus, is not as hypointense as the Modic type 2 changes on fat suppressed images (arrows).

12 372 Carrino and Morrison Figure 6. Vertebral marrow signal alteration (Modic type 3 change). (A) Sagittal T1-weighted. (B) Sagittal T2-weighted with fat suppression magnetic resonance imaging (MRI). Marked degenerative disc disease with disc osteophyte complex formation and a prominent bulge at the lumbosacral junction are evident. The anterior aspects of L5 and S1 show areas of T1 and T2 hypointensity abutting the end plate. (C) A characteristic radiographic pattern is identified with a rounded area of sclerotic opacity involving the L4 vertebral body abutting the end plate at a disc level where there is narrowing and vacuum phenomena. This has been referred to as discogenic vertebral sclerosis (arrowhead). Also note sclerotic findings in the inferior aspect of L3.

13 373 Figure 7. Hyperintense zoned (HIZ). (A) Sagittal T2- weighted image shows a small focus of hyperintensity (arrow) within the posterior anulus fibrosus. (B) It is inconspicuous on the sagittal T1-weighted image without contrast. (C) Intravenous contrast enhanced sagittal T1-weighted image shows enhancement within the posterior anulus fibrosus (arrow) corresponding the HIZ identified on the T2-weighted image. This phenomenon of enhancement is thought to reflect the ingrowth of fibrovascular tissue to the area. Reprinted with permission. 11

14 374 Carrino and Morrison However, disagreement exists in the literature as to the significance of the HIZ shown on MRI as a potential pain indicator in patients with low back pain. Although the HIZ is present within the posterior anulus of some abnormal discs, it is not necessarily associated with a concordant pain response at provocative discography. 15 So, although several investigations confirm that the HIZ is a marker of a posterior anular tear, the usefulness of this as a prediscography predictor of pain is limited by low sensitivity. 16 Studies comparing symptomatic to asymptomatic people having both MRI and discography have revealed that as in other disc related MRI findings, asymptomatic HIZ may also be encountered. The presence of a HIZ does not reliably indicate the presence of symptomatic internal disc disruption; it is a marker of pathoanatomy and not a specific painful syndrome. Although a higher percentage of HIZ exists in symptomatic patients, the prevalence in asymptomatic individuals with degenerative disc disease (25%) is too high for meaningful clinical use. When injected during discography, a similar percentage of asymptomatic and symptomatic individual discs with a HIZ were painful. 17 Therefore, merely the presence of a HIZ does not define a group of patients with particular clinical features. 18 The nature of the HIZ finding remains unknown, but it probably represents an area of secondary inflammation as a result of an anular tear. As has been well shown, HIZ correlates with peripheral anular tears shown at discography (painful or not). The focal T2 hyperintense areas may indicate fragmentation of the outer collagenous anulus fibrosus. The preferred term for such lesions is fissures rather than tears because of the connotation of a traumatic etiology with the term tear. However, tear is so entrenched in medical practice that it is likely to persist. A HIZ may enhance after contrast administration, reflecting the fibrovascular tissue ingrowth into the region of the anular fissure (Fig 7C). In addition, nerve tissue has also been seen by histology in this lesion and is the purported mechanism by which peripheral anular fissures generate pain. Given the current data, the prognostic or therapeutic significance of this finding has not yet been elucidated. Role of CT Myelography CT myelography continues to be requested extensively. MRI is not only limited in specificity but, in some instances, accurately depicts the pathoanatomic state. CT myelography is equally accurate to MRI and can be more specific because of the ability to distinguish bone osteophytes from soft tissue. The advantages of MRI include providing excellent visualization of regions proximal and distal to severe stenosis or a block. It often avoids the need for contrast, although contrast improves conspicuity. The main reasons cited for using CT myelography in conjunction with or in lieu of MRI are improved visualization of the definition of the extent of disc herniations, showing of focal neural compression by small herniations, and clarifying abnormalities of the facets, including synovial cysts. However, there is still an opportunity for refinement of the indications for CT myelography, given the wide range in variability of use. MR myelography can also be obtained using heavily T2-weighted images with fat suppression. The disadvantages can be poor ability to differentiate desiccated disc from osteophyte. MR myelography yields images that resemble conventional myelography and may be used to help confirm abnormalities seen on conventional MR in selected cases. However, there are a large number of false-positive and false-negative findings. 19 Although MR myelography does not significantly improve the diagnostic accuracy of MRI, it allowed a better overall view of the dural sac and root sleeves, therefore making it easier to diagnose spinal stenosis and disc herniation in a minority of cases. 20 The development of better 3-dimensional pulse sequences with isotropic voxels combined with improved signal and spatial resolution available on higher field strength systems (eg, 3 or 7 T) may make MRI competitive with the spatial resolution and anatomic detail that surgeons seem to favor in CT myelography. These datasets may also allow the ability to develop a virtual spinoscopy application, allowing an operator to navigate through the spinal canal and its contents. Provocative Discography There is anatomic evidence and, hence, concept validity that the disc can be a source of pain

15 375 (nociceptor) because of the innervation from the ventral nerve roots that provide branches anteriorly and posteriorly. 21 Although the concept of discogenic pain represents a reasonable paradigm, poorly performed discography can assuage the importance of making this diagnosis. There are also concerns regarding whether intradiscal injection, which produces a tensile load, is comparable pathophysiologically to the compressive load that is exerted by virtue of humans bipedal existence. The primary purpose for discography is for documentation of the disc as a pain source. For patients who have chronic predominately axial and nonmyelopathic and nonradicular pain, imaging may be insufficient or equivocal for determining the nature, location, and extent of symptomatic pathology. A position statement regarding lumbar discography from the North American Spine Society (NASS) was published in Specific indications include patients with persistent pain in whom noninvasive imaging and other tests have not provided sufficient diagnostic information. In preoperative patients who are to undergo fusion, discography can be used to determine if discs within the proposed fusion segment are symptomatic and if the adjacent discs are normal. Surgeons concerned with limiting the extent of fusion are interested in obtaining more evidence beyond MRI abnormalities to document what intervertebral disc levels are contributing to the painful syndrome. In postoperative patients who continue to have significant pain, discography can be used to assist in differentiating between postoperative scar and recurrent disc herniation (when MRI or CT is equivocal); or to evaluate segments adjacent to the arthrodesis. Discography can also be used to confirm a contained disc herniation or internal disc disruption as a prelude to minimally invasive intradiscal therapy. Discography is also being used as part of the selection criteria for many clinical trials assessing lumbar interbody fusion devices or percutaneous intradiscal treatments. Interpretation of a discogram includes a morphologic and functional evaluation. The fundamental tenet of discography is that injection into the discs and subsequent increased intradiscal pressure will elicit a concordant pain response (ie, one that mimics the patient s typical pain) if that disc is a significant nociceptor. A scale of subjective pain severity from 0 (no pain) to 10 (maximal pain) can be determined during the procedure by asking the patient to relate what his/her level of pain is during each injection. The patient is also asked whether the pain mimics his/her typical pain (ie, is concordant ) or a component thereof. To evaluate the patient s pain response more objectively, multiple vertebral levels around the suspected pain generator are injected during the procedure; the patient is not told which level is being injected or when the injection is starting. Before the procedure, patients are instructed regarding the reporting of pain and monitoring for spontaneous pain elicited during the examination. It is important to establish a reference level or relatively painfree level with injection. For discography to be considered positive, there should be at least one reference level, which is defined by the absence of pain or lack of concordant symptoms on injection. An unequivocally positive discogram consists of a single concordantly symptomatic intervertebral disc, with control discs above and below that level if it is not the lumbosacral junction. Manometric measurement of intradiscal pressure is an attempted refinement recently applied to lumbar discography. There is an interest in characterizing and segmenting patients based on the results of pressure-controlled manometric discography. This technique may help stratify patients into categories who are more likely to improve from interbody fusion. 23 It is believed that with the use of pressure-controlled manometric discography, improved and more specific diagnostic categorization is possible. Some have advocated that pressure-controlled, provocative discography should be considered for athletes with chronic constant lumbar discogenic pain. 24 The goal is to categorize precisely and prospectively positive discographic diagnoses to predict outcomes from treatment, surgical, or otherwise. Although retrospective analysis has shown promise, there is no validation of this schema, and the use of pressure-controlled manometric discography is variably used in clinical practice. Detailed technical descriptions of lumbar discography are available elsewhere. 11,25,26 However, a few technical points are worth emphasizing. The tip of the disc puncture needle should be positioned as close as possible to the center of the disc so that injection is into the nucleus pulposus (Fig 8A) instead of the innervated anular fibers, which can result in a false-positive pain response

16 376 Carrino and Morrison Figure 8. Lumbar spine discogram morphology. (A) A morphologically normal disc shows a central globule of contrast collection and may have opacification around the horizontally oriented intervertebral cleft, which is typically identified by magnetic resonance imaging (MRI). (B) Internal disc derangement (IDD) and degenerative disc disease are indicated by the irregular linear distribution of contrast that continues posteriorly beyond the vertebral body margin. Reprinted with permission. 11 (Fig 9C). After all needles are placed, contrast material is injected at each level, with fluoroscopic monitoring and evaluation of elicited pain, if any. A morphologically normal disc shows a central globule of contrast collection or hamburger-bun configuration (Fig 8A), and degeneration is indicated by a horizontal, linear distribution of contrast (Fig 8B). An anular tear is diagnosed if contrast extends into the periphery of the disc in the expected region of the anulus fibrosus. Transaxial CT is often used as a complementary study to fluoroscopy spot images or radiographs after injection (Fig 9A). CT provides useful additional information to confirm and characterize anular pathology. The typical candidate lesion for intradiscal therapy (eg, nucleoplasty, electrothermal anuloplasty) is to identify an intervertebral disc level that has a contained anular fissure or contained protrusion (Fig 9B) without substantial disc height loss and generated a concordant pain response at the time of contrast injection (ie, a positive discogram). Anular injections can be readily differentiated from nuclear injections (Fig 9C). There is a scheme for anular tear classification (Dallas Discogram Description) using CT, 27 which has undergone some modification. The scheme goes from 0 to 5, with the following grades: (1) 0 contrast entirely within the nucleus pulposus; (2) 1 contrast within the inner third of the anulus fibrosus; (3) 2 contrast in the middle third of the anulus fibrosus; (4) 3 contrast in the outer third of the anulus fibrosus; (5) 4 a radial dissection, which means there are also some concentric components; and (6) 5 full thickness tear with contrast extravasation through the outer anulus fibrosus. Although this scheme is a useful morphologic construct, it can be difficult to

17 377 Figure 9. Computerized tomography (CT) characterization after intradiscal contrast injection. Transaxial CT after discography. (A) A normal nucleogram characterized by central globule of contrast material that remains within the expected confines of the nucleus pulposus. (B) Anular fissure. Contrast material is noted within the nucleus pulposus but also extends in a radial fashion posteriorly beyond the expected confines of the nucleus pulposus into the region of the anulus fibrosus (arrow). (C) The prior 2 patterns should be compared with this collection of contrast material, which roughly parallels the nucleus/anulus junction without central collection of contrast material (arrowheads). This pattern is indicative of an anular injection, and may create a false-positive pain response. The CT appearance should not be confounded for an anular tear. apply consistently, and there are few data regarding prognostic information. The demand for discography is increasing as a diagnostic tool to determine levels of pain generation for patients who are being considered for surgical treatment (eg, interbody arthrodesis) or another type of procedure. 28 Although the diagnostic use of discography is quite evident, the treatment use based on the patient outcome is paramount. Therefore, the value added feature that discography should provide, is to identify patients amenable to available therapies and not to contribute to the treatment dilemma. Meanwhile, less invasive forms of intradiscal therapy are also evolving, which may make discography more relevant. Therefore, a spine specialist who is considering instituting disc-specific therapy most often requests discography. This is not considered a diagnostic test used in the primary care provider setting. However, either for patient driven or other reasons, it may be necessary to establish the disc as a nocicepter despite no change in therapeutic treatment. Discography is performed on an outpatient basis. Guidance for needle placement is preferably done with a C-arm, floating image intensifier,

18 378 Carrino and Morrison or with biplane fluoroscopy. Patients must be informed ahead of time that the purpose of the procedure is to generate a pain response, which, in some circumstances, can be severe. Complications include persistent pain, infection, bleeding, and injury to exiting nerve roots. To minimize the risk of disc infection, the procedure should be performed with a surgical-type preparation and drape of the patient, and surgical scrub, gown, mask, and gloves for the physician. Discitis following discography is an uncommon occurrence (ie, 1% to 4%), 29,30. It can be debilitating for the patient and can pose a diagnostic dilemma. Signs and symptoms are not always apparent, and the diagnosis is often delayed secondary to inconclusive laboratory and imaging studies early in the course of the illness. Preliminary data show that uncomplicated discography does not produce MRI abnormalities following intradiscal injection. 31,32 Therefore, MRI is suitable for evaluation of potential complications after discography. The frequency of discitis after discography is minimized by prophylactic antibiotic administration, either by intravenous or intradiscal administration. Intradiscal administration of antibiotic mixed with the contrast media is widely used, however, this is not an approved route of administration. Imaging Strategies: Indications and Guidelines Currently, there are many options available for spine imaging evaluation, which contribute to the quandary of how to use them best. Radiography is typically the first line imaging of the lumbar spine and often is used as a screening test, in part because it is readily available, has a rapid acquisition time, and provides a reasonable global assessment. CT is used predominately for trauma, when MRI is not available or contraindicated, or for a specific problem solving application related to osseous integrity. Scintigraphy is useful for a global physiologic assessment. MRI has become the mainstay for advanced imaging of the spine and offers complementary features to radiography so most patients with chronic symptoms will have these 2 imaging modalities. Discography is a provocative examination performed under image guidance, and is most useful for establishing a discogenic pain origin and confirming if there is an anular tear or contained protrusion often as a prelude to intradiscal therapy or fusion. CT myelography is also predominantly used as a preoperative test to provide a roadmap to surgical planning. MRI can underestimate root compression caused by degenerative changes in the lateral recess, while conventional and CT myelography are more accurate when using surgery as the reference standard to confirm degenerative root impingement in the lateral recess as the cause of radiculopathy. 33 The role of the scintigraphy in patients with acute low back pain is limited. The bone scan is a moderately sensitive test for detecting the presence of tumor, infection, or occult fractures of the vertebrae but not for specifying the diagnosis. The yield is very low in the presence of normal radiographs and laboratory evaluation, and highest in known malignancy. 34 High-resolution isotope imaging, including SPECT, may localize the source of pain in patients with articular facet osteoarthritis before therapeutic facet injection. 35 Similar scans may be helpful for detecting and localizing the site of painful pseudarthrosis in patients following lumbar spinal fusion. 36 The isotope bone scan remains invaluable when a survey of the entire skeleton is needed. Imaging costs have been cited as a major reason for increases in health care expenditures. Actual cost information for delivering radiology services is difficult to quantify accurately using traditional methods. Activity-based costing focuses on processes that drive cost. By tracing health care activities back to events that generate cost, a more accurate measurement of financial performance is possible. However, this is not available for lumbar spine imaging. Charges by institutions and reimbursements by insurers are not true reflections of cost. However, to gain an appreciation of how imaging modalities are valued by the US government, Medicare global reimbursement (circa 2000) was as follows (in US dollars) (1) radiography ($36), (2) scinitigraphy ($204), (3) CT ($280), (4) discography ($335), (5) myelography ($352), and (6) MRI ($542). These dollar values have to be put into the context of information gained, risk to the patient, and downstream relevance to treatment. The value of information to a provider or a patient, albeit often negative or exclusionary, has not been emphasized but has likely been a substantial driving force. Given the high incidence and prevalence of back symptoms, a reduction in imaging expendi-