Typical MRI Findings in Sports Medicine Evaluation for Degenerative Disc Disease

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1 Typical MRI Findings in Sports Medicine Evaluation for Degenerative Disc Disease Sports medicine patients are a subgroup who may be prone to low back pain. CT evaluation, while accurate, exposes patients to ionizing radiation, especially critical in the younger age group. A large number of general orthopaedic patients with low back pain unresponsive to conservative therapy were examined with conventional lateral sagittal and the more flexible oblique multislice axial images. Conventional lateral views alone were found to be insufficient in the accurate diagnosis of degenerative disc disease and must be complemented by oblique axial imaging if MRI is to be used as the primary spine imaging modality, for sports medicine imaging. This paper presents normal and pathologic anatomy as seen with oblique axial surface coil MR lumbar spine imaging. Recent publications (2, 3) have not sufficiently emphasized the need for oblique axial imaging of the spine and are content with mostly lateral sagittal views. (4,7). In this investigation of a large number of patients presenting with low back pain, magnetic resonance imaging (MRI) scanning with multislice multiangle high resolution axial scanning proved more flexible in the diagnosis and management of low back pain. In the author's opinion, MRI scanning has revolutionized imaging of the spine and is complementary to and may well replace much of CT and myelography in the future. The marked and rapid improvement in signal/noise ratio, spatial resolution, speed and flexibility makes multislice multiplanar magnetic resonance (MR) imaging our first line for lumbar spine imaging in many patients. The advantages of MR spine imaging include: 1) No ionizing radiation. 2) Lateral sagittal images of intervertebral discs can't be performed with conventional CT without additional radiation. 3) Survey views of entire lumbar spine, visu- ' Director of Neuroradiology. Bronx Municipal Hospital Center. Albert Einstein College of Medicine. Pelham Parkway 8 Eastchester Road. Bronx. NY Requests for reprints may be directed to Dr. Barasch at this address. Application Scientist. Fonar Corporation. Melville. NY. alizing spondylolisthesis, with axial and sagittal visualization of neural foramina. 4) Assessment of disc bound water content. 5) Visualization of nerve roots and dorsal root ganglia without contrast. 6) Direct observation of the site of the conus. 7) Obtaining true axial slices in patients with steep lumbosacral angles. 8) Patients with "roentgenphobia." 9) Obese patients who cannot be moved with mechanized CT tables. SUBJECTS Over 400 patients were initially examined from a general orthopaedic and sports medicine practice who presented with symptoms of low back or radicular leg pain. These patients had been unresponsive to routine conservative management including medication, prolonged bed rest, or physical therapy. The patients ranged in age from years of age. After consultation with the primary physician, any neurologic involvement was known before scanning and the appropriate imaging sequences and scanning modifications implemented. All patients were followed for at least 3-6 months. Postoperative patients were not included in the study /89/1008/0290$02.00/0 THE JOURNAL OF ORTHOPAEDIC AND SPORTS PHYSICAL THERAPY Copyright Q 1989 by The Orthopaedic and Sports Physical Therapy Sections of the American Physical Therapy Association TECHNICAL METHODS All examinations were performed using the FONAR BETA-3000 MRI (Fonar Corp. 100 Mar- 290 BARASCH AND DE MA10 JOSPT February 1989

2 cus Drive, Melville, NY) permanent magnet.3 Tesla system with a 256 matrix. After it was ascertained the patient had no cerebral aneurysm clips, cardiac pacemakers, neurotransmitters, or potentially mobile ferromagnetic foreign bodies (absolute contraindications to scanning), informed consent was obtained. The patients were positioned supine in the scanner with knees flexed. It became obvious rather early that while implanted metallic fixation devices or prosthetic joints degraded the images immediately adjacent, they posed no hazard. A patient laser alignment system was used with the cross-hairs centered midline and approximately 3 cm above the iliac crest. When pathology was found on the initial scan sequence, the site of abnormality was then positioned in the center of the field for the most optimal imaging. Because of patient comfort, we sought to limit the total examination to under 40 minutes. [We have found that for most unsedated patients, more than 40 minutes of scanning time led to patient discomfort with subsequent motion degrading the scans, particularly the long echo sequence (T-2) weighted scans.] The first examination performed was a 7- slice lateral sagittal "T-2" weighted [long echo time (TE) and long repetition time (TR)] with a TE of 84 msec and TR of 1240 msec (Fig. 1). The slice thickness was 7 mm and slice interval of 10 mm; resolution was 1 mm per pixel. This sequence optimized the T-2 weighting, visualizing the bound water in the intervertebral disc as well as any sites of edema or mass. All degenerated discs exhibited variably decreased signal on the above mentioned T-2 weighted image (Fig. 2). It has been reported that only an acutely herniated disc (in the author's experience, an infected disc as well) can temporarily increase its water content, possibly second- ary to edema (6). The second lateral sagittal scan (often performed as part of a multiecho examination) is a "T-1" weighted examination with a TE of 28, TR of 500 msec, slice thickness of 4 mm, and slice interval of 5 mm; resolution 1 mm/pixel. This short echo examination was performed for optimal anatomic information (Fig. 3). This short echo sequence maximized contrast between cerebrospinal fluid in the thecal sac (long T-1: black), fat (short T-1: white), and disc (medium to low signal). Cortical bone is visualized as a black line on all sequences because of few mobile hydrogen atoms and produce few echos. Cortical bone is usually best visualized on conventional radiographs and CT scans, but medullary bone has considerable fat and emits a normally bright signal easy to assess. Because of better image resolution and faster imaging time, the authors used short echo T-1 Figure 2. Lateral sagittal long echo sequence demonstrating a focally dehydrated disc (and bulging) at L5/S1 disc. Note rudimentary disc at lower transitional vertebrae. Figure 1. Lateral sagittal long echo sequence midline image demonstrating the normal high signal intervertebral discs as a bright signal separated by the normal low signal of the intervertebral cleft. Figure 3. Lateral sagittal short echo midline sequence demonstrating five 4 mm slice oriented precisely parallel with the disc space. JOSPT February 1989 MRI OF LUMBAR SPINE 29 1

3 sequences for the oblique axial images. From either of the lateral sagittal images, oblique axial slices were performed absolutely parallel to the disc space (Fig. 3). This slice cursor software provided rapid optimal disc visualization. FINDINGS MR diagnoses of herniated discs were strictly defined as a focal, often angular, usually paramedian, lateral and less often central, disc protrusion (medium disc signal) with effacement of epidural fat and displacement of thecal sac or nerve root causing focal radicular, objective, neurological, and EMG findings. This strict definition was often on a continuum with diffusely bulging discs and required close correlation clinically. Any significant decrease of bound water leads to a decreased signal on the T-2 weighted image (Fig. 2). This decreased signal indicates a dehydrated degenerated disc, highlighting this disc for axial evaluation and confirmation. However, a decreased signal can be considered as a normal finding with age; we have observed that above the age of 50, many asymptomatic patients will have a decreased signal of one or more lumbar discs on the long echo sequences. The appearance of a low disc signal in a younger sports medicine patient appeared to have more significance. Of course, any modality must be able to accurately image all other disease entities. In the evaluation of younger patients with lower extremity neurologic findings, the location and configuration of the conus is critical. We have found the T-1 short echo sequence to be the best noninvasive way to visualize the termination of the spinal cord in coronal and lateral sagittal views (Figs. 4, A and B and 5). Although cortical bone is not directly imaged because of a lack of mobile hydrogen atoms, it can be seen as a negative defect, visualizing many congenital abnormalities (Fig. 4B). MR is extremely sensitive for lipomas, because of the high signal produced by fat. The lateral sagittal views usually demonstrate at least four discs simultaneously, making it an ideal screening agent for both the vertebral bodies, intervertebral discs, posterior elements, bony spinal canal, and thecal sac (Fig. 6). Any disc space narrowing, loss of the normal high disc signal, spondylolisthesis (Fig. 7), space occupying lesions within the neural foramina (Fig. 8), thecal sac or metastatic disease in medullary bone (Fig. 9), were also well visualized. The sequential parallel axial images excelled in demonstrating the low signal of facet joints, configuration of the neural foramina and nerve roots, and ganglia contrasted by the bright signal of epidural fat. Typical patterns of normal and pathologic anatomy were easily demonstrated, Figure 4, A and B. Coronal TI weighted images demonstrating the normal conus, demonstrated as a high signal area contrasted against the low signal of cerebrospinal fluid and low signal cortical bone (note hemivertebrae). simulating an axial slice water soluble contrast myelogram. The dorsal root ganglion presented as a bulging area of low signal just beneath the pedicle and lateral to the lateral recess (Fig. 10A) contrasted with the high signal (short T-1) of epidural fat. On sequential views both descending nerve roots and exiting nerve roots were seen as well (Fig. 10, B and C). The typical pattern we have observed with disc herniation is an asymmetric angular, paramedian (Fig. 11, A, 6, and C) midline or lateral protrusion of the medium signal disc, displacing epidural fat (white signal), thecal sac, descending or exiting nerve root (black signal). The axial images can then be correlated with the short or long echo lateral sagittal views, confirming the axial findings. Spinal stenosis was demonstrated (with MR) (Fig. 12) as a small bony canal with diffuse loss of 292 BARASCH AND DE MA10 JOSPT February 1989

4 Figure 5. Lateral sagittal short echo sequence demonstrating size and position of normal conus. Figure 6. Lateral sagittal short echo sequence, off midline view, demonstrating low signal nerve roots exiting neural foramina contrasted against the high signal fat of neural foramina and low signal of cortical bone. epidural fat and inability to distinguish thecal sac from nerve roots, often associated with bulging or herniated disc and articular facet disease. A note of caution should be stated here. The authors' experience has been that older patients have a generalized loss of signal from disc, and low signal bony overgrowth from osteophytes may occur normally and lead to difficulty in interpretation. Accordingly, in older patients we have relied often on the short echo lateral sagittal and oblique views for optimal delineation of anatomy. Since volume averaging (well known to CT) can cause difficulty in evaluation of central disc bulging, lateral sagittal views alone are often misleading and can be mistaken for a focal disc herniation (Figs. 13 and 14). The lateral sagittal images alone are suboptimal for diagnosis and Figure 7. Lateral sagittal TI weighted image demonstrating posterior spondylolisthesis and a markedly narrowed dehydrated disc with a mild impingement on the thecal sac at L4/5 disc. The high signal subcortical bone surrounding the disc space is thought to be replacement of the normal red marrow with yellow marrow. Figure 8. Lateral sagittal short sequence view demonstrating partial obliteration of the L4/5 neural foramina from low signal degenerative changes from the posterior articular facets. treatment, and must be correlated with axial images. DISCUSSION The complex water content of the nucleus pulposus is 85-90% in the young, but decreases to 70% as a function of normal aging (5, 6) (Fig. 1). The water content of the annulus fibrosis in the young is 78%, dropping to 70% with normal aging (1, 6). With aging, trauma (perhaps exces- JOSPI' February 1989 MRI OF LUMBAR SPINE 293

5 Figure 9. Lateral sagittal TI weighted images demonstrating focal low density area in the posterior vertebral body at L3 representing a metastasis from breast malignancy. sive in sports medicine patients), infection or vascular compromise, the final common pathway is that the flexible hydrophilic disc is changed to a dehydrated fibrotic remnant with markedly decreased bound water content. On MRI the intervertebral discs are normally imaged as lenticular areas of high signal with a surrounding area of low signal (Fig. 1). A horizontal line of low signal separates the disc into superior and inferior sqments. It is believed (8) that the central bright area is nucleus pulposus and the annulus fibrocartilage, while the peripheral low signal represents the dense collagen of the annulus continuous with the hyaline cartilage of the vertebral end plate. The central low density may well be annulus invagination into the disc, but this theory has not yet been universally accepted. In patients with acute lumbosacral angles, MRI was superior to CT. Most CT units have gantries which angle only to 20-25'. When the patient's lumbosacral angle is very acute, it is impossible to obtain a parallel slice through the disc space for optimal axial disc evaluation. Computer reformations from the CT axial slices must then be performed. These can be unsatisfactory because they can be markedly degraded by only a slight amount of patient motion, as well as markedly increasing the radiation dose by requiring more frequent and thinner slices. With the oblique axial cursor, any angle through discs can be obtained without difficulty (Figs. 3 and 15). Since fat on the T-1 short echo sequences normally has a very high signal, epidural fat is very well visualized on all views, serving as high contrast against low signal cortical bone and medium signal intervertebral disc. MR on both the lateral sagittal and oblique axial views give an unparalleled perpendicular view of contents of the neural Figure 10, A, B, and C. Sequential (cephalad to caudad) oblique axial images at LS/Sl (TI) demonstrating the descending and exiting nerve roots as low signal area contrasting against the high signal epidural fat. The thecal sac, since containing CSF, always has low signal on TI providing tissue contrast. Note Figure 11A visualizing the dorsal root ganglia just under the pedicle. foramina, which is difficult to visualize without computer reconstructions (Fig. 6). Although it is a controversial subject, it is believed axial CT-myelography, performed with 294 BARASCH AND DE MA10 JOSPT February 1989

6 Figure 11, A, B, and C. Sequential (cephalad to caudad) oblique axial images demonstrating right paramedian, and right lateral disc herniations seen as obliteration of bright epidural fat, displacement of low signal nerve root and/or thecal sac. water soluble myelographic agents, to be the most sensitive and specific modality to evaluate degenerative disc disease (7). Therefore, MRI must be equal or better than CT to become a Figure 12. Typical appearance of focal spinal stenosis with complete loss of the epidural fat, and inability to distinguish thecal sac from low signal cortical bone with a congenitally small bony spinal canal. T, * Figure 13. Lateral sagittal Ti weighted images demonstrated diffuse disc bulging, most prominent at L5/S1. Note only very minimal impingement on the low signal of the thecal sac. primary lumbar spine imaging modality. The authors believe that, with the addition of rapid oblique multislice axial surface coil technology (6, 7) now available, lumbar spine MR is equivalent to CT, especially in young patients with abundant epidural fat, and very likely is a better modality for JOSPT February 1989 MRI OF LUMBAR SPINE 295

7 r CT chanaes are noted. The wtentia\ use of MR\ I for che~onuc~eol~sis and counseling athletes regarding the functional capacity of their! discs, remains to-be considered. The contrast and spatial resolution provided by multislice oblique axial MRI imaging compares very favorably with CT-myelography. The lack of ionizing radiation alone makes MRI very valuable in patients of child-bearing age and it is precisely those ~atients in whom MRI is most likely to be I I With the addition of multislice oblique angled Figure 14. Oblique axial view of patient in Figure 13, confirming a mild midline anterior indentation on the thecal sac, not conforming to the usual definition of paramedian focal disc herniation. Figure 15. Lateral sagittal short echo sequence demonstrating flexibility of MRI in obtaining a series of slices parallel to a very angulated (53O) disc. depicting the thecal sac without use of contrast media. With normal aging, trauma, degenerative disc disease, and other lesions, both the blood supply and the complex hydrophilic capacity of the mucopolysaccharide portion of the intervertebral disc are reduced (1, 2). This is the physiologic basis for the reduced signal of the intervertebral disc on the T-2 long echo sequence. It is believed, although not proved, that intervertebral discs often lose their bound water content and degenerate before herniation. Therefore, physiologic and pathologic disc degeneration before any x-ray or axial images of the intervertebral disc, MRI is a primary modality in the diagnosis of disc disease, complementing and at thles totally replacing CTmyelography. Lateral sagittal MRI views are insufficient for the diagnosis and treatment of degenerative disc disease and must be complemented with axial oblique views. The advantages of MRI (no ionizing radiation, unlimited angles of scanning, assessment of disc bound water content, no intrathecal contrast needed, etc.) as well as some MRI liabilities, have been discussed. In our experience, demonstrating and confirming patterns of normal and pathologic anatomy demonstrates the marked improvement in contrast, spatial resolution and early visualization of disease through the use of MRI. 0 Acknowledgment to Dr. Carl Weiss of Garden City. New York and Dr. Calvin Rumbaugh. Director of Neuroradiology. Brigham and Women's Hospital. Harvard Medical School. Boston. MA. REFERENCES 1. DePalrna AF, Rothman RH: The Intervertebral Disc. Philadelphia: WB Saunders Co Edelman R. Shoukimas GM. Stark DD, Davis KR. New PFJ. Saini S. Rosenthal Dl. Wismer GL. Brady TJ: High resolution surface wil imagmg of lumbar disc disease. AJNR 6: Edelman R. Stark DD, Saini S. Ferrucci JT Jr. Dinsmore RE, Ladd W. Brady TJ: Oblique planes of section in MR imaging. Radiology 159: , Maravdla K. Lesh P, Weinreb JC. Selby DK. Mooney V: Magnetic resonance imaging of the lumbar spine with CT correlation. AJNR 6: , Mcdic MT. Masaryk TJ. Ross JS. Mulopulos GP, Bundschuh CV, Bohlman H: Cervical radiculopathy: value of oblique MR imaging. Radiology 163: Mdic MT. Pavlicek W. Weinstein MA, Boumphrey F. Ngo F. Hardy R. Duchesneau PM: Magnetic resonance of intervertebral disc disease. Radiology 152: Newton TH. Potts DG: Computed tomography of the spine and spinal cord. San Anselmo. CA: Clavadell Press, Pech P, Haughton VM: Lumbar intervertebral disc: correlative MR and anatomic study. Radiology 156: BARASCH AND DE MA10 JOSPT February 1989