Spinal stenosis is a progresive narrowing of the spinal canal that occurs most commonly in the cervical and lumbar areas.  The prevalence of lumbar spinal stenosis is about 9.3%, with people in their 60s and 70s most commonly affected. 
Consensus criteria are needed to define and classify spinal stenosis. The means by which spinal stenosis is diagnosed is also changing. Concerns related to radiation risk of CT scans and the recognized limitations of spinal radiography suggest that MRI represents the best imaging modality in many cases. MRI helps avoid myelography and the associated risk and pain. In a recent study, electrodiagnostic studies showed no superior accuracy compared with MRI. Electrodiagnostic studies, when needed, should be limited to such specific conditions as radiating leg pain. 
In a study comparing CT and MRI reliability, CT overestimated the degree of stenosis 20-35% of the time, and MRI overestimated the degree of stenosis 2-11% of the time. 
Central spinal stenosis results from a developmental or degenerative narrow spinal canal. In certain cases, the spinal pedicles are shorter than normal. This is more common in the lumbar spine. Recognition of this primary defect in patients with short pedicles has led to pedicle-lengthening osteotomy as a treatment. 
Spinal stenosis rates and the money spent to treat spinal stenosis and back pain has increased in the recent past. Many suggestions have been offered regarding the increased occurrence. Obesity has been offered as the most likely factor. 
The selection of an initial screening examination in patients who are suspected of having spinal stenosis depends on the age of the patient. Older patients in whom spinal stenosis is suspected should be examined initially using conventional spinal radiographic imaging. Cervical stenosis assessment in older patients should include anteroposterior (AP), lateral, oblique, and swimmer’s lateral views. Assessment of the lumbar spine should include a lower lumbar centered, AP, lateral, and oblique projections. Lateral views are most sensitive for central spinal stenosis, whereas oblique views of the cervical and lumbar areas better demonstrate lateral stenosis syndromes. In older patients, radiography of the cervical, thoracic, and lumbar spine have value for screening for compression fractures or spondylosis. If the patient is symptomatic with radiculopathy, weakness or altered gate, MRI of the spine is the preferred method. CT of the spine should be limited to trauma and to those patients who are have contraindications for MRI. [3, 7, 8, 9, 10, 11, 12]
Younger patients and all patients in whom conventional radiology findings are negative should be evaluated using either spinal CT scanning with multiplanar reformatted images or spinal MRI. Nuclear medicine, including single-photon emission computed tomography (SPECT) bone scintigraphy, is generally reserved for the presumptive evaluation of infections, metastatic and primary neoplasms, and complex occult trauma. The amount of radiation exposure to patients should be carefully monitored, especially in the case of children and young adults. 
In cases that require additional investigation, spinal MRI is the most universally suitable technique for the diagnosis of spinal stenosis. The examination should be performed using thin sections (3 mm) and high resolution (256 × 192 matrix). Spinal MRI should include imaging sets obtained in the axial and sagittal planes using T1-weighted, proton-density, and T2-weighted fast spin-echo and gradient-echo techniques. The bony and osteophytic components of the spinal stenosis pattern are seen best using a T2-weighted gradient-echo technique. [13, 14, 15, 16]
CT scanning of the cervical, thoracic, and/or lumbar spine may be useful in certain patients, often during the performance of CT myelography. Indications for CT myelography include contraindications for MRI, implanted metal devices, and postoperative patients with suggested complications. CT scanning of the spine should be followed by multiplanar reformatted images and 3-dimensional (3D) imaging techniques in selected patients. 
CT and MRI studies in patients who are asymptomatic and younger than 40 years demonstrate a 4-28% occurrence of spinal stenosis. Most persons older than 60 years have spinal stenosis to some degree. Progressive narrowing of the spinal canal may occur alone or in combination with acute disc herniations.
Nuclear medicine's SPECT bone scintigraphy is valuable primarily in differentiating spondylosis with stenosis from medical disease, infections, and tumors.
The goal of spinal imaging is to localize the site and level of disease and to help differentiate between conditions in which patients require surgery and conditions in which patients recover following conservative treatment. It is important to correlate imaging findings with the signs and symptoms of the spinal disease under consideration.
Limitations of techniques
Radiography of the spine is insensitive for detection of spinal stenosis based on changes in soft tissues of the spine. Superimposed structures limit the accuracy of measurements of the spinal canal.
CT scanning of the spine is provides excellent detail of the bony tissues and some information about central soft-tissue abnormalities. The use of intravenous contrast agents improves the soft-tissue resolution of CT significantly. The use of intrathecal contrast as a CT myelogram introduces some increased risk of injury, risk of infection, and expense.
MRI provides excellent soft-tissue differentiation but has somewhat limited spatial resolution. MRI contrast agents further improve soft-tissue visualization but have no effect on spatial resolution. The definition of cortical bone lesions and fractures is less with MRI than with CT.
SPECT bone scintigraphy is sensitive to diseases that actively affect bone pathophysiology, but spatial resolution is limited. Positive results are more likely for lesions that have osteoblastic activity.
See the images below displaying spinal stenosis.
The AP diameter of the normal adult male cervical canal has a mean value of 17-18 mm at vertebral levels C3-5 (see the image below). The lower cervical canal measures 12-14 mm. Cervical stenosis is associated with an AP diameter of less than 10 mm, whereas diameters of 10-13 mm are relatively stenotic in the upper cervical region.
In the central cervical spinal region, hypertrophy of the ligamentum flavum, bony spondylitic hypertrophy, and bulging of the disc annulus contribute to development of central spinal stenosis. In each case, the relative significance of each structure contributing to the stenosis is variable.
Movement of the cervical spine exacerbates congenital or acquired spinal stenosis. In hyperextension, the cervical cord increases in diameter. Within the canal, the anterior roots are pinched between the annulus margins and spondylitic bony bars. In the posterior canal, hypertrophic facet joints and thickened infolded ligamentum flavum compress the dorsal nerve roots. In hyperflexion, neural structures are tethered anteriorly against the bulging disc annulus and spondylitic bars. In the event of a vertebral collapse, the cervical spine loses its shape, which may result in anterior cord compression.
In cases of severe central cervical spine stenosis, the possibility of thoracic or lumbar stenosis should be carefully considered. Stenotic cauda equina syndrome has been reported following surgical decompression of cervical stenosis.  Lateral cervical stenosis results from encroachment on the lateral recess and the neuroforamina of the cervical region, primarily as a result of hypertrophy of the uncovertebral joints, lateral disk annulus bulging, and facet hypertrophy.
The thoracic spinal canal varies from 12 to 14 mm in diameter in the adult. Primary central thoracic spinal stenosis is rare. Occasionally, hypertrophy or ossification of the posterior longitudinal ligament results in central canal stenosis.
Lateral thoracic stenosis may result from hypertrophy of facet joints with occasional synovial cyst encroachment. The diameter of the normal lumbar spinal canal varies from 15 to 27 mm. Lumbar stenosis results from a spinal canal diameter of less than 12 mm in some patients; a diameter of 10 mm is definitely stenotic.
Differentials and other problems to be considered
The differential diagnosis of spinal stenosis includes many conditions, including congenital conditions such as achondroplasia and osteogenesis imperfecta . Degenerative conditions such asosteoporosis and osteogenetic arthritis are common. Inflammatory diseases may include rheumatoid arthritis of the spine and spondylodiskitis. Other conditions to be considered include metastatic breast cancer, prostate cancer, and Paget disease.
Certain diseases and conditions such as Morquio syndrome manifest spinal stenosis as a major cause of morbidity. 
Unlike acute lumbar disk herniation, spinal stenosis is not typically treated using interventional radiologic techniques. Pain management, including facet injections, may provide temporary relief for patients. Biopsy of metastatic spinal disease is performed easily using CT scan guidance to aid in the diagnosis. Spinal stenosis associated with compression fractures has been successfully treated using percutaneous kyphoplasty and percutaneous vertebroplasty. [19, 20, 21, 22, 23]
As the population ages, the frequency and severity of spinal stenosis is likely to continue to increase. In recent years, an attempts have been made to development less invasive means of treating spinal stenosis and the related pain.  Minimally invasive spinal fusion using bilateral transforaminal lumbar interbody fusion for high-grade isthmic defects has been reported.  Other percutaneous interventions, including adhesiolysis, have been reported.  Nonsurgical pain management techniques may provide relief from pain following failed back surgery. 
The surgical introduction of pedicle screws and intervertebral disk space implants may be associated with injury to nerve roots and, rarely, the spinal cord. Postoperative patients should be carefully evaluated for hardware failure, mistaken introduction, and migration. Subdural hemorrhage has been reported during spinal decompression surgery. 
Standard radiographs remain the recommended initial imaging study of choice in spinal stenosis.  In patients with severe stenosis, cervical spine radiographs are useful; however, radiographic studies are insensitive to soft-tissue hypertrophy and other nonosseous causes of spinal stenosis. In the older patient, standard radiographs help exclude more serious conditions, such as pathologic compression fracture. Anterior osteophytes, even when they become very large, may not be related to spinal symptoms.
The lateral view of the spine is the most useful. Spondylosis appears as curvilinear bony outgrowths from the lateral and posterior margins of the vertebral body endplates. The general outline of each vertebral body should be reviewed to exclude possible compression injuries or pathologic compression fractures.
Hypertrophic facet joints are best seen on oblique views in which narrowing of the neuroforaminal spaces in the cervical spine and lumbar spine regions are commonly visualized. In the cervical spine, uncovertebral hypertrophy is best visualized on oblique and lateral views. The AP view is useful for the assessment of alignment and uniformity of the interspinous process distances. The soft tissues surrounding the lumbar spine can be evaluated on the standard AP radiograph. Disruption of the psoas muscle stripe may indicate a paravertebral abscess or tumor. 
Radiographic images of spinal stenosis are provided below.
Although few false-positive findings exist, occasionally, even marked anterior spondylosis is not associated with significant central spinal canal narrowing. Diseases associated with bone softening may be related to significant spinal canal narrowing without obvious radiographic findings.
On CT scans, spinal stenosis is well defined as the diminished diameter(s) and cross-sectional area of the spinal canal. CT scanning of the cervical spine can be improved using intravenous contrast agents to enhance the epidural veins, thus better defining the margins of the epidural space. Enhancement of epidural fibrosis is greatest soon after surgery. Paraspinal masses may present with associated calcifications or may appear as cystic or fluid collections in the case of abscess. In all cases, the relationship of the mass to the central spinal canal, the lateral spinal canal recess, and the neuroforamen should be determined. All images should be reviewed with both a standard soft-tissue window and a narrow window to evaluate bone disease and calcifications. [10, 1]
CT images of spinal stenosis are provided below.
Degree of confidence
Osseous and calcified features are well outlined on CT scans. Findings in epidural soft-tissue diseases rely on the displacement of epidural fat or contrast enhancement, which may vary. In general, the use of intravenous contrast agents improves the visualization of soft-tissue diseases, masses, and abscesses.
False-positive findings related to epidural scarring result from a failure of fibrotic tissue to enhance years after surgery. False-negative CT examinations occur because of far lateral lesions, which become averaged together with the surrounding bone of the neuroforaminal space. The evaluation for an abscess is difficult immediately following surgery. The residual blood and gas in the tissues may appear similar to an infectious process. Delayed follow-up examinations are recommended.
Magnetic Resonance Imaging
Spinal stenosis is best diagnosed using MRI. In the lumbar spine, MRI sequences may include sagittal T1-weighted, T2-weighted, STIR, and proton density-weighted, as well as axial T1-weighted and T2-weighted sequences.  Measurements taken from sagittal images are particularly useful and, in most patients, can be accepted as accurate. Although measurement of the cervical canal are important, interpretation of the diagnosis of spinal canal stenosis must be made carefully. The clinical significance of spinal canal stenosis in children is probably less important than the increased mobility of the child's neck compared with that of the adult. Due to susceptibility artifacts related to osseous and calcified structures, gradient-echo images tend to result in slight overestimations of the degree of stenosis in the lateral recesses and neuroforaminal spaces.
On MRI, findings of spinal stenosis have a variable presentation depending on the specific disease causing the stenosis as well as associated edema of the related vertebral bodies. [30, 31, 7, 8, 9] The following describes some of these findings:
Osteophytes and calcified bulging disc structures are dark on T2-weighted fast spin-echo and T1-weighted spin-echo imaging.
Osteophytic tissues are dark on T2-weighted gradient-echo images.
Epidural soft tissues and related disease processes are typically isointense on T1-weighted noncontrast images, compared with muscle, synovial tissues, and most spinal ligaments.
Vertebral endplates present a variable degree of increased or even decreased brightness on T2-weighted images, depending on the degree of inflammation and the chronic nature of the degenerative changes.
Following intravenous administration of gadolinium diethylenetriaminepentaacetic acid (DTPA), fibrosis or surgical inflammatory changes, extra-axial tumors, and some stress reactions within vertebral body endplates present an increased signal brightness on T1-weighted images.
Postcontrast T1-weighted images are most useful in the detection of neoplasms and spinal infections if a fat-suppression technique is used.
The assessment of cervical spine stenosis is improved by careful evaluation of cerebrospinal fluid (CSF) flow in the region of the stenosis. Spatial modulation of magnetization allows the degree of stenosis to be correlated with restriction to the CSF flow. In high-grade stenosis both the diastolic and the systolic CSF flow velocities are reduced. The degree of symptoms has been correlated to the compromised cervical cord and the related effects upon CSF flow.
Magnetic resonance images of spinal stenosis are provided below.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.
On MRI, a false-positive finding of the spine rarely occurs except in patients with central spinal canal stenosis. Gradient-echo images may lead to overestimation of the degree of lateral recess and neuroforaminal space stenosis. CSF-pulsation artifacts seen on sagittal T2-weighted fast spin-echo images may give rise to a false impression of dorsal stenosis. False-negative MRI results are generally related to movement artifacts and the presence of metal in the region of interest.
Spinal stenosis may be reflected on single-photon emission computed tomography (SPECT) nuclear medicine images as areas of increased activity related to the vertebral body endplates, facet joints, and uncovertebral joints. Medical diseases related to the bones of the vertebral bodies, such as Paget disease, present with markedly increased nuclide uptake (see the image below). Metastatic disease, which may cause spinal canal stenosis, is usually associated with increased uptake of the nuclide agent in the areas of abnormal bone.
Degree of confidence
Most causes of spinal stenosis have nonspecific findings on nuclear medicine studies. Paget disease, osteomyelitis, and spinal metastasis have strongly positive focal findings.
Many cases of spinal stenosis are not identified using SPECT nuclear medicine as a primary diagnostic method. Nuclear medicine scans may demonstrate positive findings in the absence of spinal stenosis. SPECT spinal imaging should be reserved for patients in whom osteomyelitis, Paget disease, or other specific disease conditions exist. Some diseases in which only bone destruction occurs may not have increased uptake in the areas of metastatic disease.
Angiography is rarely indicated except in patients with arteriovenous malformations, dural fistulas, and vascular spinal tumors. In these patients, the degree of spinal canal narrowing can only be inferred on the basis of venous or arterial displacement or neovascularity.
Spinal angiography can indicate spinal canal narrowing only indirectly, based on epidural enhancement and vascular (venous) dilatation.
Spinal angiography should be reserved for specific indications related to arteriovascular malformations, arteriovascular fistulae, and highly vascular tumors. Epidural venography was performed before the availability of MRI. As a result of the variability of the epidural venous plexus, use of epidural vein displacement as an indication of a lateral disc herniation is subject to both false-positive and false-negative diagnoses.