Criteria for classifying normal and degenerated lumbar intervertebral disks

The purpose of this study was to develop criteria for distinguishing normal and degenerated lumbar intervertebral disks. Cryomicrotome sections from and magnetic resonance images of 31 cadavers were reviewed. The immature nucleus pulposus, found up to age 2 years; the transitional nucleus pulposus, found in teenagers; and the adult nucleus pulposus were associated with an intact anulus fibrosus without tears except for small concentric or transverse tears in the periphery of the anulus fibrosus. Discoloration of the nucleus pulposus, diminishing disk height, and diminishing signal intensity were associated consistently with a radial tear of the anulus fibrosus. The severely degenerated nucleus pulposus was associated also with a radial tear or complete disruption of the anulus fibrosus. The results suggest that intervertebral disk normally progresses from the immature to the transitional to the adult nucleus pulposus. When a radial tear develops in the anulus there is shrinkage and disorganization of fibrocartilage in the nucleus pulposus and replacement of the disk by dense fibrous tissue and cystic spaces.     

Comparison of MR and diskography in detecting radial tears of the anulus: a postmortem study

Radial tears of the anulus fibrosus, which anatomic studies suggest are a primary event in disk degeneration, can be detected by diskography or MR imaging. We compared the sensitivity of MR and diskography in the detection of anular tears. MR, diskography, and cryomicrotomy anatomic sectioning were performed in eight cadaver lumbar spines. Diskography demonstrated 15 radial tears in 36 intervertebral disks. MR demonstrated 10 of the 15, a sensitivity of 67%. MR (T2-weighted images) in each of the diskographically normal disks showed the high signal intensity characteristic of normal disks. Thirteen of 15 disks from which contrast medium extravasated at diskography had diminished signal intensity in MR images. We conclude that although MR may demonstrate some radial tears of the anulus, and associated changes in the disk, it cannot be used as effectively as diskography to visualize a radial tear.     

Tears of the anulus fibrosus: correlation between MR and pathologic findings in cadavers

Tears of the anulus fibrosus, which have been implicated in back pain, have not been studied systematically with MR imaging. We correlated MR images with cryomicrotome sections to study the lumbar anulus fibrosus in 20 cadavers. Three distinct types of tears of the anulus were identified: concentric tears, characterized by fluid-filled spaces between adjacent lamellae; radial tears, characterized by a rupture of all layers in the anulus between the nucleus and the surface of the disk; and transverse tears, characterized by a rupture of Sharpey's fibers in the periphery of the anulus, near the ring apophysis. In each type, fluid or mucoid material was present in the tear. MR demonstrated the transverse and radial tears. We conclude that MR imaging provides an accurate means for investigating tears of the anulus.     

Intervertebral disk appearance correlated with stiffness of lumbar spinal motion segments.

BACKGROUND AND PURPOSE: Because it diminishes the stiffness of the intervertebral disk, disk degeneration results in abnormal motions of the spine. Therefore, disk degeneration associated with back pain may indicate spinal fusion. The purpose of this study was to correlate the MR appearance and stiffness of lumbar intervertebral disks. METHODS: Eighty-two lumbar spinal segments were imaged with MR. The intervertebral disks were classified as: 1) normal, 2) having transverse or concentric tears of the annulus fibrosus if MR imaging showed only these changes, 3) having radial tears of the annulus fibrosus if MR imaging showed high-intensity zones in the annulus fibrosus or reduced signal intensity in the disk characteristic of radial tears, or 4) having advanced degeneration if MR imaging showed markedly reduced height, large osteophytes, or both. The rotation occurring from the application of a 6.6-Newton-meter (Nm) moment of axial rotational torque was measured kinematically. Average stiffness, in Nm/degree, was calculated as the ratio of the torque to the rotation. RESULTS: Stiffness averaged 7.0 Nm/degree for the normal group; 1.9 Nm/degree for the disks with concentric or transverse tears; 1.7 Nm/degree for disks with radial tears; and 3.1 Nm/degree for disks with advanced degeneration. The differences were statistically significant. CONCLUSION: Concentric, transverse, and radial tears of the intervertebral disk indicate reduced stiffness of the intervertebral disk and increased motions for a unit of applied torque. The most severely reduced stiffness was found in disks with radial tears of the annulus fibrosus. With collapse of the disk space, stiffness increases.     

Progressive and regressive changes in the nucleus pulposus. Part II. The adult

The spines of 28 cadavers were studied with magnetic resonance (MR) imaging, correlated with anatomic sections, to characterize the progressive and regressive changes in the nucleus pulposus. Four additional types of nucleus pulposus were identified in this study. The transitional type (type II) of childhood had fibrous tissue that developed near the anulus fibrosus anteriorly or posteriorly. MR images showed a notch of reduced signal intensity in the nucleus pulposus where the fiber was developing. The adult type (type III), which is predominant after age 30 years, had an indistinct boundary between the nucleus pulposus and anulus fibrosus. MR images showed the fibrocartilage in the nucleus pulposus and inner anulus fibrosus as areas of high signal intensity. Adult disks with a tear of the anulus fibrosus (type IIIB) had a lower signal intensity on MR images and slightly reduced disk heights. This type may represent an early stage of degeneration. Degenerated intervertebral disks (type IV) had either dense disorganized fibrous tissue or fluid replacing the normal fibrocartilaginous structure of the nucleus pulposus. MR images showed narrow intervertebral disk spaces with either reduced signal intensity if collagen was present or increased signal intensity if fluid was present. This study suggests that the nucleus pulposus may progress normally through types I, II, and III.     

Degenerative lumbar disk disease: pitfalls and usefulness of MR imaging in detection of vacuum phenomenon

Magnetic resonance (MR) images of the lumbar spine from 150 patients were retrospectively reviewed. In 14 of these patients, at 18 disk levels, a vacuum phenomenon (VP) had been identified on plain radiographs and/or computed tomographic scans. The MR imaging appearance of these gas collections in 17 disks was an area without signal, best seen on spin-echo sequences with short repetition time and echo time in the sagittal view. MR imaging precisely located the VP in the anulus fibrosus, the nucleus pulposus, and Schmorl nodes. In all but one case, degeneration of the disk was complete and associated with adjacent changes in vertebral bone. Pitfalls of MR imaging detection of VP included chemical shift artifact, calcifications, and tears without gas in the disk.     

Canine intervertebral disks: correlation of anatomy and MR imaging

Because the structure, maturation, and degeneration of canine intervertebral disks resemble those of humans, the authors developed a model of acute intervertebral disk degeneration in dogs. Herniated disks of 18 dogs were examined with magnetic resonance (MR) imaging and then sectioned with a freezing microtome. On the basis of their morphologic appearance in the freezing microtome sections, most of the lumbar intervertebral disks could be categorized as one of six types. Each type has characteristic features and a distinctive appearance on MR images. Findings of this study--including decreased signal intensity from the intervertebral disk, altered signal intensity in contiguous bone marrow, bulging of the anulus fibrosus, herniation of the nucleus pulposus, and contrast enhancement after intravenous administration of gadolinium diethylenetriaminepentaacetic acid (DTPA)--resemble observations from human clinical studies. Classification of degenerating disks and identification of MR imaging characteristics of each type may improve the interpretation of MR images and recognition of early disk degeneration in humans.     

Effect of degeneration of the intervertebral disk on the process of diffusion.

PURPOSETo test the hypothesis that diffusion of contrast medium into the intervertebral disk is affected by the integrity of the nucleus pulposus and annulus fibrosus.METHODSIn canine intervertebral disks, defects were made in the annulus fibrosus and nuclear material was removed from the disk with a nucleotome. MR imaging was performed with intravenous contrast medium at 15, 30, 60, and 90 days after the procedure. The diffusion of contrast medium in the intervertebral disk was studied by visual inspection and by measuring changes in signal intensity. The intervertebral disk were classified at each MR study as normal or abnormal on the basis of the signal intensity on T2-weighted images.RESULTSIn untreated disks after intravenous injection of contrast medium, a band of increased signal intensity was observed near the endplates that became wider with time and approached the center of the disk. In six of the 12 treated disks, the band of increased signal intensity was visibly diminished or less discrete compared with that in the control disks. Weeks later, these treated disks showed diminished signal intensity on T2-weighted images and bulging of the annulus fibrosus.CONCLUSIONSIntervertebral disks with defects in the annulus fibrosus and reduced cartilage content were characterized by abnormal diffusion of contrast medium into the disk, and changes characteristic of early disk degeneration were detected subsequently.     

High-dose i.v. contrast in CT scanning of the postoperative lumbar spine

Evaluation of the postoperative lumbar spine is sometimes difficult because of obliteration of epidural fat by hypertrophic scarring. We examined 70 patients using a high-dose intravenous contrast technique in an attempt to distinguish hypertrophic scarring from herniated disk. The CT interpretation was confirmed in all 17 patients who had follow-up operations. Thirteen had herniated disk material associated with scar and four had hypertrophic scarring only. The latter four patients underwent reoperation because of concomitant foraminal or spinal canal stenosis seen on CT. Twelve of the herniated disks had the expected appearance of a nonenhancing mass surrounded by a rim of enhancing scar tissue. In the 13th patient, homogeneous enhancement of the herniated disk was seen. It is thought that chronically herniated disks, such as this one, may incite enough surrounding scar to give the CT appearance of an enhancing disk. Finally, marginal enhancement in the anulus fibrosus region was seen in over 90% of disk spaces examined. Although an anatomical explanation cannot be given at present, this phenomenon is thought to represent a normal finding.     

Tears of the anulus fibrosus: assessment with Gd-DTPA-enhanced MR imaging

T2-weighted images have been shown to be capable of defining anular tears in vitro as increased signal intensity within the normal low-signal-intensity anulus fibrosus. Since growth of granulation tissue into anular tears has been described as part of the healing process, it seemed likely that gadolinium-DTPA should enhance anular tears as it does scar tissue in other parts of the spine. We retrospectively reviewed spinal MR images from 30 previously unoperated patients and correlated areas of increased signal intensity within the anulus on T2-weighted images with areas of enhancement on T1-weighted images, and to a limited extent, with surgical findings. Eighteen separate areas of anular enhancement were found in 12 patients (six cervical, 12 lumbar). Only five of these enhancing areas showed increased signal intensity on T2-weighted images, four of a type II tear pattern and one of a type III tear pattern. Contrast enhancement within the anulus was in a pattern of type II tear in 14 and type III in four. Histology from an enhancing type II anulus demonstrated vascularized granulation tissue within the avascular anulus, without focal herniation. Anular tears may be imaged in vivo not only with T2-weighted images but also with gadolinium-DTPA-enhanced T1-weighted images by virtue of their vascularized granulation tissue.     

Tears of the anulus fibrosus: assessment with Gd-DTPA-enhanced MR imaging

T2-weighted images have been shown to be capable of defining anular tears in vitro as increased signal intensity within the normal low-signal-intensity anulus fibrosus. Since growth of granulation tissue into anular tears has been described as part of the healing process, it seemed likely that gadolinium-DTPA should enhance anular tears as it does scar tissue in other parts of the spine. We retrospectively reviewed spinal MR images from 30 previously unoperated patients and correlated areas of increased signal intensity within the anulus on T2-weighted images with areas of enhancement on T1-weighted images, and to a limited extent, with surgical findings. Eighteen separate areas of anular enhancement were found in 12 patients (six cervical, 12 lumbar). Only five of these enhancing areas showed increased signal intensity on T2-weighted images, four of a type II tear pattern and one of a type III tear pattern. Contrast enhancement within the anulus was in a pattern of type II tear in 14 and type III in four. Histology from an enhancing type II anulus demonstrated vascularized granulation tissue within the avascular anulus, without focal herniation. Anular tears may be imaged in vivo not only with T2-weighted images but also with gadolinium-DTPA-enhanced T1-weighted images by virtue of their vascularized granulation tissue.     

Progressive and regressive changes in the nucleus pulposus. Part I. The neonate

Magnetic resonance (MR) imaging, correlated with anatomic sections, was used to characterize the progressive and regressive changes in the nucleus pulposus in neonates. The spines of five fetuses and five full-term infants between 16 and 40 weeks old were studied. In anatomic sections, the nucleus pulposus was sharply demarcated from the anulus fibrosus, Sharpey fibers were conspicuous, and a plate of primitive notochord was evident in the equator of the disk. On long repetition time (TR)/long echo time (TE) or long TR/short TE MR images, Sharpey fibers (low signal intensity) and notochord (low signal intensity) could be differentiated from the high-signal-intensity nucleus pulposus and anulus fibrosus. The major differences between the fetal and infant spines were the amount of notochord in the disk and ossification in the vertebral body.     

Observer variation in MRI evaluation of patients suspected of lumbar disk herniation

OBJECTIVE: Our objective was to assess observer variation in MRI evaluation in patients suspected of lumbar disk herniation. SUBJECTS AND METHODS: Two experienced neuroradiologists independently evaluated 59 consecutive patients with lumbosacral radicular pain. Per patient, three levels (L3-L4 through L5-S1) and the accompanying roots were evaluated on both sides. For each segment, the presence of a bulging disk or a herniation and compression of the root was reported. Images were interpreted twice: once before and once after disclosure of clinical information. Interobserver agreement was expressed as unweighted kappa values. RESULTS: Without clinical information, interobserver agreement for the presence of herniation or bulging disk was moderate (full agreement, 84%; kappa = 0.63; 95% confidence interval [CI], 0.53-0.72). Of a total of 352 segments evaluated, there was disagreement on 58 segments (17%): bulging disk versus no defect in 26 (7.4%), bulging disk versus herniation in five (1.4%), and hernia versus no defect in 27 (7.7%). With clinical information, twice as many bulging disks were reported but no new herniations were detected. Agreement slightly decreased, but not significantly (full agreement, 77%; kappa = 0.59; 95% CI, 0.49-0.69; p = 0.12). CONCLUSION: On average, more than 50% of interobserver variation in MRI evaluation of patients with lumbosacral radicular pain is caused by disagreement on bulging disks. Knowledge of clinical information does not influence the detection of herniations but lowers the threshold for reporting bulging disks.     

Prevalence of annular tears and disc herniations on MR images of the cervical spine in symptom free volunteers

STUDY DESIGN: Prospective MR analysis of the cervical spine of 30 asymptomatic volunteers. OBJECTIVES: To evaluate the prevalence of annular tears, bulging discs, disc herniations and medullary compression on T2-weighted and gadolinium-enhanced T1-weighted magnetic resonance (MR) images of the cervical spine in symptom free volunteers. SUMMARY OF BACKGROUND DATA: Few studies have reported the prevalence of cervical disc herniations in asymptomatic people, none have reported the prevalence of cervical annular tears on MR images of symptom free volunteers. MATERIALS AND METHODS: Thirty symptom-free volunteers (no history or symptoms related to the cervical spine) were examined using sagittal T2-weighted fast spin-echo (SE), sagittal gadolinium-enhanced T1-weighted SE imaging and axial T2(*)-weighted gradient echo (GRE). The prevalence of bulging discs, focal protrusions, extrusions, nonenhancing or enhancing annular tears and medullary compression were assessed. RESULTS: The prevalence of bulging disk and focal disk protrusions was 73% (22 volunteers) and 50% (15 volunteers), respectively. There was one extrusion (3%). Eleven volunteers had annular tears at one or more levels (37%) and 94% of the annular tears enhanced after contrast injection. Asymptomatic medullary compression was found in four patients (13%). CONCLUSION: Annular tears and focal disk protrusions are frequently found on MR imaging of the cervical spine, with or without contrast enhancement, in asymptomatic population. The extruded disk herniation and medullary compression are unusual findings in a symptom-free population.     

Occult lumbar lateral spinal stenosis in neural foramina subjected to physiologic loading.

PURPOSETo measure the effect of extension, flexion, lateral bending, and axial rotation loads applied to the spine on the anatomic relationship of the spinal nerves in the neural foramen to the ligamentum flavum and the intervertebral disk, anc to determine the effect of disk degeneration on the response to loading.METHODSCadaveric lumbar motion segments were examined with CT and MR imaging, loaded with pure moment forces, frozen in situ, reexamined with CT, and sectioned with a cryomicrotome. The morphology of the intervertebral disks was classified on the basis of the appearance of the cryomicrotome sections. The neural foramina were classified as having no evident stenosis, as being stenotic, as having occult stenosis, or as showing resolved stenosis on the basis of the images and sections before and after loading. The stenotic and nonstenotic foramina were stratified by disk level, intervertebral disk classification, and type of loading applied. The effect of spinal level, disk type, and load type on the prevalence of stenosis was studied.RESULTSOn average, extension, flexion, lateral bending, and axial rotation resulted in the ligamentum flavum or intervertebral disk contacting or compressing the spinal nerve in 18% of the neural foramina. Extension loading produced the most cases of nerve root contact, and lateral bending produced the fewest cases. Each of the loading types resulted also in diminished contact between the spinal nerve and the intervertebral disk or ligamentum flavum in some cases. Disk degeneration significantly increased the prevalence of spinal stenosis. All foramina associated with advanced disk degeneration and half of the foramina associated with disks having radial tears of the annulus fibrosus either developed occult stenosis or were stenotic before loading.CONCLUSIONSThe study supports the concept of dynamic spinal stenosis; that is, intermittent stenosis of the neural foramina. Flexion, extension, lateral bending, and axial rotation significantly changed the anatomic relationships of the ligamentum flavum and intervertebral disk to the spinal nerve roots.     

Can MRI signal characteristics of lumbar disk herniations predict disk regression?

PURPOSE: To assess whether or not MRI signal characteristics of lumbar disk herniations can predict subsequent disk regression. MATERIALS AND METHODS: Medical and radiology records from 1999-2003 were reviewed, and 123 patients who had more than one lumbar MRI during the study interval were identified. Of these, 42 patients had a disk herniation (protrusion, extrusion, or free fragment) identified on their first examination. Six of the 42 patients were not included because of prior lumbar surgery, or inadequate examinations. The remaining 36 patients had a total of 77 examinations to evaluate 44 disk herniations. The herniated disks were evaluated by two CAQ neuroradiologists for size, morphology and a qualitative assessment of the T2 signal. RESULTS: Between the first and last examination, 25 of 44 (57%) herniated disks decreased, 17 (39%) were unchanged, and two increased in size. 9 of 11 (82%) of disk extrusions improved. The mean size of the disks that regressed was significantly larger than those that were unchanged (8.6 mm vs. 6 mm, p=.001). On average, the disks decreased 3.2 mm (37%). Of the disks that decreased in size, 15 (63%) had an area of increased signal on T2-weighted images (T2WI) compared to the parent disk on the initial study. Of the disks that were unchanged, 6 (35%) had increased signal on the T2WI's. CONCLUSION: 57% of herniated disks in this study group decreased in size over time. Larger herniations and extrusions were more likely to regress than smaller herniations. Disks that regressed were more likely to have high signal on T2WI's than those that were stable.     

Disk processes in MR

Disk processes can be differentiated in MR with the introduction of high resolution coils and the use of a multi slice technique with a minimal slice thickness of 5 mm. By using a sensitive double echo sequence we found that in the 80 patients examined the size and extent of the posterior herniation could be well demonstrated. Sagittal slices show the relationship of the herniation to the spinal cord. The pathomechanical parts of a disk process such as ruptured or preserved anulus fibrosus, perforated or intact posterior longitudinal ligament can be differentiated. For lateral lying processes in connection with the foramen intervertebral and the spinal roots an axial slice parallel to the intervertebral space is recommended. All CT findings especially in the cervical spine region were shown and further qualified.     

Multidetector CT in patients suspected of having lumbar disk herniation: comparison of standard-dose and simulated low-dose techniques

PURPOSE: To compare standard-dose and simulated low-dose multidetector computed tomography (CT) in patients suspected of having lumbar disk herniation. MATERIALS AND METHODS: The institutional review board approved the research protocol with a waiver of patient informed consent. Sixty consecutive patients underwent multidetector CT with four detector rows at 1 mm collimation at 140 kVp, with tube current-time product adapted to body mass index (BMI): 200 (BMI< 22 kg/m(2)), 300 (BMI > or =22 to <30 kg/m(2)), and 400 effective mAs (BMI > or =30 kg/m(2)). Simulated doses at 65%, 50%, 35%, and 20% of the dose were used for acquisition. During two separate sessions, three independent radiologists coded each of three caudal disks as normal, bulging, or herniated and graded canal and foramen compromise. Median numbers of discrepancies between the standard and reduced doses were compared with Friedman and Wilcoxon tests. Agreements within and between readers were evaluated through kappa statistics. RESULTS: Dose reduction had no effect on a reader's ability to identify bulging disks (P = .128) and left and right foramen compromises (P = .413 and .665, respectively). However, for normal disks (P = .002), herniated disks (P = .004), and canal compromise (P = .002), dose reduction did have a significant effect. For normal disks and canal compromise, a reduction dose effect was not detected at 65% (P = .121 and .250, respectively) but appeared at 50% (P = .004 and .008, respectively). For herniation, a dose reduction effect was detected at 35% (P = .031). Agreements within and between readers ranged from poor to excellent and tended to decrease with dose reduction. CONCLUSION: For patients suspected of having lumbar disk herniation, tube charge settings could be reduced to 65% of the standard dose adapted to the BMI.     

A model of unloaded human intervertebral disk based on NMR relaxation.

NMR relaxation rates were related to the composition of the nucleus pulposus from 11 and anulus fibrosus from six human intervertebral disks. Tissue water was proportional to glycosaminoglycan (GAG) and residue, the noncollagen, non-GAG portion of the dry weight (R2 = 0.74). The solid signal fraction depended on collagen and residue protons (R2 = 0.89). 1/T1 was proportional to collagen and residue (R2 = 0.97). T2 showed 2-4 components labeled A, B, C, and D, with means +/- standard deviations of 3.1 +/- 1.6, 17.5 +/- 9.5, 64 +/- 22, and 347 +/- 162 msec. Signal fractions of A and B depended on the collagen-associated water protons (R2 = 0.94 and 0.85), C on residue-associated water protons (R2 = 0.82), and D on GAG-associated water protons (R2 = 0.74). The data led to a model of disk architecture in which the collagen and residue were largely solid, forming distinct water compartments; the remaining water was present in a proteoglycan gel.     

The value of T2 relaxation times to characterize lumbar intervertebral disks: preliminary results

BACKGROUND AND PURPOSE: The present standard for staging intervertebral disk degeneration is a discrete scale, consisting usually of 5 stages. The purpose of this pilot study was to investigate the use of T2 measurements as a continuous measure of intervertebral disk degeneration. METHODS: We obtained images in 5 volunteers with a 3D fast spin-echo sequence modified for the purpose of calculating T2 relaxation times from multiple echoes in the echo train. Disks were classified on the basis of conventional criteria into one of the 5 stages of disk degeneration. Average T2 values were calculated for stage II, III, and V disks, which were identified in the volunteers. Differences between the disk levels were analyzed with analysis of variance and differences between stages tested with a Student t test with significance set at the 0.01 level. RESULTS: In the 5 volunteers, 20 stage II, 4 stage III, and a single stage V disk were found. Contour plots showed the highest T2 values in the nucleus pulposus near the vertebral endplates and lower T2 values in the intranuclear cleft region and peripheral annulus fibrosus. Average T2 values were significantly lower in the type III and V disks than in the normal disks. CONCLUSIONS: The study suggests that intervertebral disks can be characterized and classified accurately by means of T2 values. More studies are warranted to determine the range of T2 values for normal disks.