Plain Radiography and Its Variants (Stress Radiography, Arthrography, Myelography, Discography, Fluoroscopy, and Videofluoroscopy)

Plain radiographs are obtained when an x-ray beam is directed through the body part being imaged to x-ray film with amplification via a rare-earth film screen, to image plates with photostimulable crystals (computed radiography), or to solid state detectors that convert x-ray photons into electrical charges (direct radiography).⁷⁶ Part of the beam is absorbed by the body, producing a shadow image. Five different types of tissues can be imaged with plain radiography: gas, fat, soft tissue and water, bone, and metal (metals, barium, and iodinated contrast material). The differentiation of tissue within each of these five groups is limited, however, which makes it difficult to differentiate entities such as edema from blood, or muscle from tumor. Despite these limitations, plain radiographs are a relatively inexpensive way to assess fractures or bony abnormalities.

It is crucial to have plain radiograph protocols for each body part. The protocols should specify the number of views, technique, and film-screen combination or computed radiography, or direct radiography settings. To exclude a fracture, at least two orthogonal views perpendicular to each other are necessary, and often three or more are needed, depending on the body part. Patient history and skin markers placed on the region of interest can help identify abnormalities and might alter the patient positioning or imaging technique.

Stress radiography is a procedure in which stress is placed on a given joint to assess for any change in joint width or alignment caused by ligamentous laxity or disruption, usually in comparison with the asymptomatic normal side. Examples of stress radiography include acromioclavicular joint views holding weights, Telos stress examination of the ankles with varus or posterior stresses,²⁶ and valgus stress on the elbow.¹²⁹ Flexion and extension lateral views or open-mouth odontoid views with side-bending of the cervical spine to assess transverse or alar ligament laxity can also be considered stress views, although the stress is achieved passively using the weight of the head and the tension of the cervical muscles.

Arthrography is a procedure in which iodinated contrast material or air (or both) is instilled into a joint before plain radiographs are obtained. This outlines the joint space as well as structures within or surrounding the joint. Arthrography can be performed on virtually any synovial joint, but at present it is used less often than in the past because of the development of newer, noninvasive modalities. The risks of arthrography are those of a needle puncture, including hemorrhage, infection, and drug reaction. Tenography involves injection of iodinated contrast material into a tendon sheath to assess for tendon pathology or rupture of a ligament and abnormal communication with an adjacent joint space.

Myelography is plain radiography performed after instillation of iodinated contrast material into the thecal sac. Nonionic iodinated myelographic contrast material is usually injected by posterior upper lumbar puncture, but can be injected via a lateral C1–C2 approach. Although myelography has largely been supplanted by magnetic resonance imaging (MRI), there are some advantages of myelography over MRI. Myelography and postmyelography computed tomography (CT) better show bony detail and subtle impressions on the nerve roots. Myelography also allows imaging of the lumbar spine in the upright weight-bearing position as well as in flexion and extension. The risks of myelography include hemorrhage, infection or meningitis, drug reaction, nerve damage, and cerebrospinal fluid (CSF) leak or spinal headache. These risks can be minimized with careful technique.

Discography is a procedure in which plain radiography is performed after instillation of iodinated contrast material into the intervertebral disk spaces. Suspected symptomatic disks are injected, along with a “control” disk. The most important aspect of discography is whether pressurization of the disk space during injection reproduces the location and quality of the patient’s symptoms.^79,108,140 Unequivocal concordant symptoms during the injection correlate with that disk being the pain generator. The risks of discography are similar to those of myelography, except for a slightly higher risk of infection, thought to be due to the low vascularity of the intervertebral disk space (which can be prophylactically treated by admixture of an antibiotic with the disk injectate). In a series of 12,634 examinations comprising 37,135 disk injections, only 2 cases of confirmed diskitis were seen.¹²²

Fluoroscopy is the real-time x-ray visualization of structures, and is used during spinal diagnostic and therapeutic procedures and in the instillation of contrast medium for arthrography, myelography, and discography. Fluoroscopy might or might not involve obtaining plain radiographs.

Videofluoroscopy entails recording fluoroscopic images to study the motion of joints. It can demonstrate dynamic abnormalities during motion, such as in the cervical spine and especially in the atlantoaxial occipital region. When there is a question of vertebral fusion in a postoperative patient, dynamic videofluoroscopy can sometimes be helpful.

Computed Tomography

CT is the production of cross-sectional images of the body by selective absorption of a rotating x-ray or electron beam. Multiple detectors measure the transmission of the beam at multiple angles, and computer algorithms are used to form images from the data. Contrast between different tissue types is significantly higher with CT than with plain radiography, and there is more precise localization of structures on the cross-sectional imaging. The imaging plane is usually axial or axial oblique, although direct coronal images of the foot and ankle and sagittal or coronal images of the wrist and elbow can be obtained with variations in patient positioning. Axial images can be reformatted into sagittal, coronal, oblique, or complex planes,¹²⁶ but the resolution depends on the section thickness of the original images and is degraded if there is patient motion during the scan. Multidetector-row CT (MDCT) allows for simultaneous generation of multiple images or a volume of data, which allows for more rapid acquisition, thinner slice thickness, and improved multiplanar reformations. Three-dimensional reformatted images with surface rendering can also be obtained and are occasionally helpful for surgical reconstruction of complex fractures or to assess bony impingement on adjacent structures (Figure 7-1).^22,89

FIGURE 7-1 Sagittal reformation and surface rendering of multidetector-row computed tomography (MDCT) images of the cervical spine. A, Sagittal reformation of axial data shows C3-4 foraminal narrowing (arrow) from facet joint bony hypertrophic changes (F). Note normal foraminal caliber at adjacent C4-5 and C2-3 (arrowheads). B, Surface rendering of axial data in the sagittal oblique position shows foraminal narrowing (arrow) and severe facet hypertrophic changes (F) at C3-4.

CT has a definite advantage over MRI in the imaging of cortical bone. CT can also better image chondroid and osteoid matrices. The detection of fractures and delineation of positioning of fracture fragments are achieved well with CT, but a fracture tangential to the imaging plane can be missed, in part because of partial voluming artifact (see explanation below). This potential pitfall is reduced by review of multiplanar reformations.

Computed Tomography Myelography and Postdiskography Computed Tomography

Postmyelography CT is a requisite adjunct to myelography. The bony intervertebral foramina and spondylosis are best seen on axial MDCT images and sagittal reformations. Intraforaminal or far lateral disk abnormalities can be invisible on the plain film myelogram and are best shown on CT. Disk abnormalities at L5–S1 might be invisible on myelography (because of the ample ventral epidural fat at this level), but visible on CT. The postmyelography CT levels should include any levels with abnormality detected on myelography, any levels of clinical abnormality, and the L5–S1 level.

Postdiskography CT is an adjunct to diskography to better demonstrate the anatomy of an annular tear (Figure 7-2).

FIGURE 7-2 Postdiskography computed tomography (CT) scan of a posterior central annular tear, demonstrating iodinated contrast material extending from the nucleus through a midline tear (arrows) to a subannular location posteriorly (arrowheads). No focal convexity to the posterior disk margin is present; this abnormality would not be seen on plain CT or postmyelography CT.

Magnetic Resonance Imaging

MRI is the production of cross-sectional images of the body through placement of the imaged body part in a large, static magnetic field with a varying magnetic gradient pulsed in such a way as to allow the resonance of hydrogen to be detected.¹²⁴ The data obtained are then converted by computer algorithms into cross-sectional images. These images depend on the number of mobile hydrogen atoms and specific tissue characteristics of the hydrogen. Pulse sequence parameters can be adjusted to accentuate certain inherent qualities of tissues, allowing for much higher contrast between different types of tissue (Table 7-1). For example, fat-containing tissues can be accentuated or suppressed, and water-containing tissues can be accentuated or suppressed.

Table 7-1 Relative Advantages and Disadvantages of Magnetic Resonance Imaging and Computed Tomography

CT, Computed tomography; MRI, magnetic resonance imaging.

Because the patient is placed in proximity to a large magnetic field, there are contraindications to MRI. Patients with pacemakers, pacemaker wires, implanted electronic devices, ferromagnetic cerebral aneurysm clips, and metal around or within the orbits should not be scanned. Some centers are scanning select patients with pacemakers or pacemaker leads under controlled protocols. Some other metallic devices are contraindications to MRI, and if there is a question of compatibility with the scanner, the consulting radiologist should be contacted before the examination.

MRI has multiple available imaging planes, including complex imaging planes. Multiple magnetic gradient pulse sequences are also available to accentuate different characteristics of tissues (Table 7-2). Standard pulse sequences include T1-weighting, proton density, T2-weighting, short inversion time–inversion recovery (STIR), and fat suppression imaging. Numerous pulse sequences are available on any given magnetic resonance (MR) scanner, and different manufacturers typically use different abbreviations for the sequences. The advent of fast spin-echo sequences has shortened imaging times. However, the natural fat signal suppression on T2-weighted spin-echo images is partially lost on fast spin-echo T2-weighted images unless additional fat suppression techniques are included.

Table 7-2 Magnetic Resonance Signal Characteristics of Different Tissues

∗ Low signal with routine spin-echo imaging. Fast spin-echo T2-weighted images do not show as much loss of fat signal.

The signal-to-noise ratio and image quality of an MR image depend on multiple factors, including magnetic field strength, surface coil design, field of view, matrix size, number of repetitions of the pulse sequences, other pulse sequence parameters, patient size, and body habitus.

STIR imaging shows additive T1 and T2 characteristics and has a high sensitivity for edema and many types of tumors. There is also suppression of the signal from fat, which causes the fat to appear dark, although some nonfat tissues can be suppressed if they have a short T1.⁷⁷ Kinematic MR images are obtained as a joint is moved stepwise through a range of motion. This is useful to assess patellar tracking abnormalities.¹⁵¹ Kinematic imaging of the temporomandibular joint and shoulder¹⁴³ can also be performed for specific clinical indications.

Nuclear Medicine Studies

Radionuclide bone scintigraphy is performed after intravenous injection of a bone-seeking isotope, for example, technetium-99m–methylene diphosphonate to detect areas of increased bone turnover. Multiple lesions throughout the skeleton can be demonstrated in a single study, but radionuclide scintigraphy often has a low specificity. It can be useful for whole-body screening for bony metastases, but bony metastases in a given area can also be detected with MRI, which has a higher specificity and spatial resolution. A bone scan of the foot and ankle for chronic foot pain can help isolate the location of the abnormality, which might then be studied with MRI or CT. Bone scanning is often used to detect stress or insufficiency fractures, but MRI might actually show these lesions earlier and provide better spatial resolution and specificity.

Single photon emission CT (SPECT) is an adjunct to the planar bone scan. It provides cross-sectional images of the body (axial, coronal, sagittal) using the same radioisotope emissions as a bone scan, but with a moving gamma camera. This is especially useful in the spine to show whether activity is greatest at the vertebrae anteriorly or around the facet joints or other posterior elements. The signal-to-background ratio is also improved with SPECT imaging. However, SPECT imaging takes additional time and adds expense, so it is used only for specific indications. In the patient with mechanical back pain, a bone scan can help show the level of facet joint abnormality, although the facet joint with abnormal activity might not necessarily be the one that is painful. Often the contralateral facet is painful from abnormal stresses caused by the “hot” facet joint. Bone scan with SPECT has been shown to be helpful in the identification of patients who would benefit from facet joint injection and has been shown to reduce the number of facets to be injected by clinical determination alone.¹²¹

Radiolabeled white blood cell imaging, gallium imaging, or both are sometimes used to identify areas of osteomyelitis or infection. Some noninfected areas, however, such as around the tip of an orthopedic prosthesis or an amputated bone end, can also show increased activity.

Positron emission tomography (PET)/CT scanning, which combines radiotracer uptake in the form of a radiolabeled glucose analog with the anatomic detail of CT, shows promise in localizing infection in difficult-to-diagnose cases such as Charcot joints⁷⁰ and possibly in patients with joint prostheses⁸⁶ or orthopedic hardware.

Ultrasound

Targeted musculoskeletal ultrasound can be a useful addition in the diagnosis of musculoskeletal pathology. It is highly operator dependent and should only be performed at sites where the physicians and sonographers are specialized in musculoskeletal imaging and aware of common imaging pitfalls.⁶⁸ Ultrasound can be used in patients when MRI is contraindicated secondary to MRI-incompatible devices such as pacemakers and when patients have claustrophobia in the MRI unit. Ultrasound does not use any ionizing radiation. High-resolution linear array transducers with a broad bandwidth should be used for musculoskeletal ultrasound. Ideal transducer frequency is between 7.5 and 12 MHz. Ultrasound also has the advantage of being able to dynamically visualize tendons, ligaments, and superficial structures (Figure 7-3).¹³¹

FIGURE 7-3 Transverse ultrasound images of the extensor carpi ulnaris tendon (arrows) at the distal ulna in pronation (left) and supination (right) show changes in tendon position. DU, Distal ulna.

Targeted ultrasound can also be used to evaluate soft tissue masses. The main utility of ultrasound in this regard is to differentiate a solid mass from a cyst. It can also be helpful to determine whether a mass is vascular using Doppler interrogation. Ultrasound can be helpful to determine whether a foreign body is present. It is especially helpful for identification of nonradiopaque foreign bodies such as wood, plastic, and certain types of glass. Most foreign bodies will be associated with artifact such as acoustic shadowing or a comet tail artifact.

Ultrasound can also be a valuable tool for interventional procedures. It can be used to localize fluid collections for drainage and sampling such as in the evaluation of the prosthetic hip. It is also excellent for localization, drainage, and therapeutic injections of popliteal cysts. As always care must be taken to use sterile technique and specialized ultrasound probes. Probe covers should be used for needle-guided procedures. Ultrasound allows direct visualization of the needle for aspiration and injection procedures. For tendon sheath injections the needle placement is verified to avoid intratendinous injection and inadvertent injection of vascular structures or nerves. Before any ultrasound-guided interventional procedure, a thorough diagnostic examination should be performed to characterize the target and surrounding structures. Doppler interrogation can be used to delineate vascular structures to be avoided. Needle path should be in the same longitudinal plane as the transducer so that the needle is visualized during its entire course. Slight back-and-forth motion of the needle is helpful to determine the location of the distal tip of the needle.

Plain Radiography Artifacts

On plain radiographs a common artifact is the Mach line, which, occurs when a bony edge overlaps another bone. A thin, dark line appears just adjacent to the overlapping bone and can be mistaken for a fracture (Figure 7-4).

FIGURE 7-4 Mach line simulating a dens fracture. (A) Lateral plain film of the cervical spine demonstrates a curvilinear lucency traversing the dens (arrows), but this parallels the undersurface of the C1 ring and mastoid bones as well as extending past the margins of the dens. (B) Repeat extension lateral view of the same patient demonstrates no fracture line at the same site, and there is a fainter Mach line, now located more caudad (arrows).

Computed Tomography Artifacts

The three artifacts seen most commonly with CT are those of partial voluming, streak, and beam hardening. Helical and MDCT scanners can show additional artifacts.⁷A partial voluming artifact occurs because a CT section has a finite thickness, such as 0.625, 1.25, 2.5, or 5 mm. If a structure extends only through a portion of the section, the attenuation is averaged with that of the structure beside it in the section. For this reason, partial voluming is more likely to occur with thicker sections. Partial voluming can result in missing a fracture in the axial plane, where it is averaged with the solid bone on either side of the fracture. Use of a thinner section thickness minimizes this artifact, which is why cervical spine CT images are obtained with thin-section thickness. CT of ankle or foot fractures may be performed with a relatively thin-section thickness in both the coronal and the axial planes, which minimizes the likelihood of missing a fracture from partial voluming. Helical volumetric CT imaging can also reduce partial voluming.

Reconstruction of the CT data to form an image assumes a constant energy of the x-ray beam as it circles around the patient. An area of increased density, such as thick bone, can attenuate the lower energy portion of the x-ray spectrum and cause a relatively higher energy beam to pass through. This difference in energy over a portion of the data stream can result in beam-hardening artifact, with variable attenuation central to the high-density bone (Figure 7-5).

FIGURE 7-5 Streak and beam-hardening artifact on computed tomography (CT). A, Postmyelography CT demonstrates low- and high-attenuation anteroposterior streaks at the air–soft tissue interfaces of the piriform sinuses (arrows). This image, obtained at the midcervical level, does not show beam-hardening artifact from the shoulders, and the spinal cord cross-section is well delineated, surrounded by intrathecal iodinated contrast material (arrowheads). B, A more caudal image in the same patient shows beam-hardening artifact from the shoulders, with multiple transverse lines degrading the image and making it more difficult to detect the left posterior extrusion tilting the cord (arrowheads). Streak artifact is seen at the air–soft tissue interface of the trachea (arrows).

Streak artifact occurs where there is an interface between tissues of very different attenuation, such as bone and air, resulting in linear streaks extending along the plane of the interface. This can be seen at the bone-air interface of sinuses or at the interface of a metal prosthesis and bone. Presence of a metal prosthesis can result in both beam-hardening and streak artifacts. Newer CT scanners with multidetector-row arrays can significantly reduce or eliminate artifact from internal fixation hardware or prostheses in concert with improved reconstruction algorithms, and allow for detection of orthopedic hardware complications.^48,81,110

Magnetic Resonance Imaging Artifacts

Partial voluming can occur with MRI, in that there is a finite thickness of tissue sample to make an image, and there can be averaging of signal from tissue components within the thickness of the section. This effect can be reduced with thinner section thickness.¹⁶³ Partial voluming is routinely seen on sagittal images obtained through the spine at the lateral edge of the thecal sac, where there is partial voluming of the CSF with the epidural fat. Partial voluming of the edge of the spinal cord with the adjacent CSF on sagittal images can artifactually increase the signal intensity of the cord on the most lateral images of the cord.

Magic angle artifact is a phenomenon seen on imaging of anisotropic structures that course 55 degrees (the “magic angle”) relative to the main magnetic field in the MR scanner.^46,47 There is an artifactually increased signal within the structure at this angle. This artifact most commonly occurs during imaging of tendons that are anisotropic and course at a 55-degree angle to the main magnetic field, such as in the rotator cuff supraspinatus tendon¹⁶⁸ or the ankle tendons as they course around the malleoli. The artifact is especially problematic in the rotator cuff, where increased T1 and proton density signal in the critical zone (which might course 55 degrees relative to the main magnetic field) can represent tendinopathy. A partial- or full-thickness tear of the supraspinatus should not be confused with the increased signal intensity arising from imaging at the magic angle, as T2-weighted images show more signal abnormality with tears and less magic angle effect. Signal intensity of peripheral nerves on MR neurography can increase as the nerve courses at the magic angle.²⁸

Chemical shift artifact is seen because the resonance frequency of hydrogen varies with the structure that the hydrogen is within.¹⁶³ The resonance frequency of fat is slightly different from that of water because of the different hydrogen bonds. Consequently the reconstruction algorithm can position fat slightly differently than water-containing structures, leading to artifacts in the frequency encoding direction. This can cause misregistration of fatty bone marrow in relation to soft tissues adjacent to the bone, giving an asymmetry and inaccuracy of cortical bone thickness in the extremities⁴² or at the vertebral endplate or cortex.

Motion artifact is usually visible on MR images as blurring or double images.¹⁶³ Flow artifact from vessels or CSF can cause artifacts, either within the vessel or CSF or in a line in the phase encoding direction.⁸⁵ These artifacts can often be minimized with flow compensation or saturation bands in the imaging protocol. However, if there is an unusual round focus of signal not expected within a structure, it is prudent to check if it lies in a horizontal or vertical line with a blood vessel and if it is of the same caliber.

Metal artifact occurs when either microscopic or macroscopic metal fragments cause a localized change in the homogeneity of the magnetic field. This can result in a focus of signal void with an adjacent high signal intensity ring.¹⁶³ These artifacts are dramatically evident when a prosthesis or internal fixation device is present, and they appear as small foci in the postoperative patient if microscopic fragments of metal break off the drills or other instruments during surgery. A small high-signal ring or partial ring near the signal void helps differentiate this artifact from a calcification or hemosiderin. Artifact from metal can be reduced by using T2-weighted fast spin-echo techniques, rather than conventional spin-echo techniques,¹⁶⁵ and by careful attention to scanning parameters and alignment of a prosthesis in the magnetic field.⁸¹

Ultrasound Artifacts

One of the most common sources of artifact in musculoskeletal ultrasound is from anisotropy. This phenomenon occurs because of the parallel arrangement of tendon fibers. Optimal echoes result from the ultrasound probe being oriented at 90 degrees to the tendon being scanned. When the probe is not perpendicular, decreased echoes return, and the images will show decreased echogenicity within the tendon that can be misinterpreted as a tear or tendinopathy.¹³⁹ Acoustic shadowing is another potential cause of false-positive tears in tendons and results when areas overlying the tendons have a differing density or thickness, which then causes an acoustic shadow over the tendon.

Trauma

Imaging for spinal trauma depends on the clinical situation and presence of symptoms, neurologic deficit, and sensorium of the patient. The National Emergency X-Radiography Utilization Study prediction rule (NEXUS), the Canadian C-Spine Rule (CCR) criteria, or both are used to determine when cervical imaging is not indicated.¹³ For suspected cervical spine trauma the ACR appropriateness criteria have eight clinical scenarios depending on clinical criteria. Some of these include myelopathy, mechanically unstable spine, unevaluable for greater than 48 hours, suggested arterial injury, and suggested ligamentous injury. Depending on the clinical situation, when cervical imaging is indicated, CT with multiplanar reformations is usually the best initial diagnostic procedure. MR is recommended as a complementary procedure in the setting of myelopathy, instability, or ligamentous injury.

Helical CT can better delineate a fracture shown on plain radiograph, and can also disclose other vertebral fractures not seen on plain radiographs.^92,109 MRI can best show any traumatic disk extrusion or spinal cord abnormality if the patient has myelopathic symptoms.⁴⁵ Fast spin-echo T2-weighted images with fat suppression can show soft tissue edema or hemorrhage associated with ligamentous tearing in whiplash injuries in the acute setting.

The ACR appropriateness criteria for blunt trauma meeting the criteria for thoracic or lumbar imaging rate CT with multiplanar reformations highly. MR is also rated highly if neurologic abnormalities are present.

MRI performed before and after intravenous gadolinium instillation can help differentiate vertebral collapse resulting from osteoporosis from that caused by malignancy.³⁶

Intramedullary Abnormalities

MRI is the procedure of choice for assessing the intramedullary space–spinal cord. Six MRI patterns of intramedullary abnormalities have been defined by their appearance on T1-weighted images, before and after contrast injection, and on T2-weighted images, with a short differential diagnosis for each.¹⁷

The ACR appropriateness criteria are available for different clinical variants of myelopathy, including traumatic, painful, sudden onset, stepwise progressive, slowly progressive, seen in an infectious disease patient, and seen in an oncology patient. All of these earn high ratings for MRI and high ratings for CT in several variants. The addition of postcontrast enhancement MRI is considered appropriate in several of the variants. This can be done to find the abnormality or to better characterize a known abnormality.

Intramedullary primary and metastatic neoplasms are well shown on MR T2-weighted images. Most intramedullary spinal tumors enhance with gadolinium contrast agents,¹¹⁵ although some intramedullary astrocytomas do not enhance.¹⁵⁰ Metastatic tumors can show a very focal enlargement of the cord as opposed to the more diffuse enlargement with primary gliomas.

The abnormalities of multiple sclerosis can be located entirely in the cervical spinal cord without brain involvement. Spinal cord multiple sclerosis plaques are characteristically peripherally located, are less than two vertebral segments in length, and occupy less than half the cross-sectional area of the cord.¹⁶⁶ If a cord lesion is suspicious for multiple sclerosis, either by imaging or by clinical criteria, MRI of the head should be performed to look for additional lesions and to strengthen the diagnosis.

MRI can well demonstrate enlargement of the cord from syringomyelia and can demonstrate an associated Chiari 1 malformation. If a syrinx involves the entire cervical region with no Chiari malformation to explain it, consideration should be given to imaging the rest of the cord. Cord tumors located more caudally can be associated with a holocord syrinx.

Increased T2 signal within the cord can be seen in areas of chronic compression from degenerative disk disease and from spondylosis. The likelihood of detecting increased cord signal is proportional to the severity of the clinical myelopathy and the degree of spinal canal compression. The response to surgery is less favorable in patients with an intense, well-defined, increased cord signal than in those with a faint, poorly defined signal or those with a normal signal.²⁹

Intradural Extramedullary Abnormalities

MRI with intravenous gadolinium contrast is the most sensitive imaging study for assessing abnormalities within the dural sac, including drop metastases, hematogenous leptomeningeal metastases, meningitis, and arachnoiditis (Figure 7-6).⁵⁴ T2-weighted axial images without contrast can demonstrate very well the three different types of arachnoiditis seen on MRI. These include nerve clumping, tumefactive mass-like arachnoiditis, and the “empty sac” sign of the roots being attached to the thecal sac.¹³⁶ Residual Pantopaque, a possible cause for arachnoiditis, can be seen as a fat signal on MRI because of its oily base.

FIGURE 7-6 Postmyelography computed tomography of arachnoiditis after laminectomy, fusion, and dural tear demonstrates clumping of the right-sided roots (arrow), with more evenly spaced left-sided roots (arrowhead). This clumping was seen just cephalad to the site of dural repair and dural surgical clips.

Nerve root tumors such as schwannomas or neurofibromas can actually be shown on myelography, as they can move with the roots with upright and prone positioning, indicating their origin. MRI with contrast agent enhancement, however, makes intradural or intradural-extradural root sheath tumors and their relation to the nerve root more conspicuous.

Extradural Abnormalities

Degenerative Disk Disease and Spondylosis

MRI is probably the single best examination to assess the intervertebral disk and surrounding structures. Plain CT and postmyelography CT, however, can both also demonstrate any morphologic abnormalities of the disk. Plain CT or postmyelography CT can show gas within the epidural space from extension through a full-thickness annular tear when the degenerated disk space contains gas, the “vacuum phenomenon” (Figure 7-7). There is little correlation between plain radiograph findings and the presence or absence of a disk extrusion.

FIGURE 7-7 Non–contrast-enhanced computed tomography of the spine, demonstrating “vacuum” phenomenon, with gas in the disk space (arrows) and extension of gas into the left ventral epidural space (arrowheads), indirect evidence of an annular tear. The ventral epidural gas is just posterior to the left paracentral endplate osteophyte contributing to acquired spinal stenosis.

The high incidence of asymptomatic imaging abnormalities in the general population makes it difficult to prove that an imaged abnormality is the pain generator. Diskography with pressurization of the disk space might be the most accurate method of determining whether an abnormal-appearing disk is a generator of low back pain,¹⁰⁸ or a generator of pain radiating to the lower limbs, in a patient with no MRI evidence of nerve root compression,^79,98 if the patient has unequivocal concordant symptoms during pressurization different from a control disk level. Controversy remains regarding the utility of diskography.^104,108

The ACR appropriateness criteria for chronic neck pain include ratings for 10 variants, including first study, previous malignancy, previous surgery, no neurologic findings, neurologic signs or symptoms, spondylosis (without or with neurologic signs or symptoms), old trauma (without or with neurologic signs or symptoms), and bone or disk margin destruction. Five-view plain films including obliques are the first study of choice, and MRI is indicated when neurologic signs or symptoms, or bone or disk destruction is present.

The ACR appropriateness criteria for low back pain include ratings for the following six clinical variants: uncomplicated acute low back pain and/or radiculopathy, nonsurgical presentation with no red flags; low back pain in the setting of low-velocity trauma, osteoporosis, or age greater than 70 years; low back pain with suspicion of cancer, infection, or immunosuppression; low back pain and/or radiculopathy, surgery or intervention candidate; low back pain with prior lumbar surgery; and cauda equina syndrome. For uncomplicated lower back pain with no red flags, all imaging modalities are assigned a low appropriateness rating. MRI becomes more appropriate in the other clinical settings. Postcontrast MR is given a higher appropriateness rating in the setting of cancer, infection, immunosuppression, previous surgery, and cauda equina syndrome.

The normal intervertebral disk has a low T1 and high T2 signal, with a lower T2 signal cleft centrally and a surrounding low-signal annulus. With disk degeneration the T2 signal of the nucleus begins to decrease as the nucleus dehydrates. Once the disk has lost the T2 signal, the signal does not return. Loss of the T2 signal can be seen with either intervertebral disk space narrowing or normal disk height, but more commonly with the former.

Combined task forces of the North American Spine Society, American Society of Spine Radiology, and American Society of Neuroradiology have developed a standardized nomenclature and classification of lumbar disk pathology.² Other than normal and disk desiccation, there are four general descriptions of disk disease (Figure 7-8)¹⁹:

1. Circumferential bulging of the disk, suggesting laxity of the annulus fibrosus

2. Protrusion of the disk, in which a focal convexity has a width wider than depth, consistent with a partial-thickness tear through the annulus fibrosus. Protrusions can be described as focal, less than 25% of the circumference, and broad-based, between 25% and 50% of the circumference. These partial tears can also show a focus of increased T2 signal that represents fluid or granulation tissue extending through the annular tear. These annular lesions can sometimes appear more like a radial tear, and in some cases more like a partially concentric tear, shaped like a bucket handle tear.

3. Extrusion of the disk, in which a focal convexity has a depth greater than the width, consistent with the nucleus extending through a full-thickness tear of the annulus and extending extraannularly. Other criteria for extrusion that can be used are extension of the nuclear material cephalad and caudad past the levels of the endplates, or visible extension through the annulus and posterior longitudinal ligament.

4. Sequestered or free fragment, in which the extruded disk material is not connected with the native nucleus pulposus. These fragments can be located well cephalad or caudad from the donor site and can extend into the intervertebral foramen. Often these sequestered fragments have different signal characteristics than the native disk.

FIGURE 7-8 Various disk abnormalities. A, T2-weighted sagittal magnetic resonance imaging (MRI) of the lumbar spine demonstrates normal height and hydration of the L3–L4 disk (thick arrow). The L5–S1 disk space is severely narrowed, with loss of hydration and low T2 signal intensity as well as a circumferential bulging of the disk (arrowheads). The L4–L5 disk space shows moderate loss of height and hydration, with a focal convexity to the posterior disk having a focus of increased T2 signal intensity (thin arrow), consistent with annular tear and protrusion. B, Sagittal T1-weighted MRI obtained more laterally at the intervertebral foramina shows a focal convexity to the L4–L5 disk, which extends cephalad into the intervertebral foramen (arrowheads) and deviates the exiting L4 root superiorly (thick white arrow), representing an intraforaminal extrusion. The adjacent normal intervertebral foramen shows a low-signal exiting root (black arrow) surrounded by high T1 signal foraminal fat. C, Axial T2-weighted MRI of the lumbar spine demonstrates a focal convexity to the disk extending caudad from the left posterior aspect of the disk, consistent with a focal extrusion, as the depth is greater than the width (arrowheads). This extrusion impinges on the exiting left root just posterior (thick arrow) and deviates the descending root within the thecal sac just adjacent (arrow). D, Postdiscography computed tomography demonstrates extension of contrast material into a left central subannular region (arrow), consistent with an annular tear and protrusion. This disk lesion is similar to that seen at L4–L5 in A. E, Axial T1-weighted image of the lumbar spine demonstrates a focal convexity to the right far lateral disk (arrowheads) deviating the exited nerve root in comparison with the normally exiting root on the contralateral side (arrow). The disk abnormality has a depth similar to width, best described as an extrusion. F, Sagittal T1-weighted image of the lumbar spine after intravenous injection of gadolinium contrast agent demonstrates narrowing of the L5-S1 disk space and degenerative changes of the adjacent endplate (arrowheads). Just caudad to the posterior aspect of the disk space is a nonenhancing mass (arrows) that is separate from the native disk space and best described as a free fragment or sequestered disk.

Imaging findings of degenerative disk disease must be correlated with clinical history, physical examination findings, and possibly diagnostic injection results. Many abnormal imaging findings can be asymptomatic. In 60 asymptomatic patients aged 20 to 50 years, the prevalence of lumbar disk bulge was 20% to 28%. For protrusion it was 38% to 42%; for annular tears, 32% to 33%; for extrusion, 18%; and there were no disk sequestrations.¹⁷⁶ Disk extrusion, sequestration, nerve root compression, endplate abnormalities, and moderate to severe facet joint osteoarthritis were rare in asymptomatic patients younger than 50 years when the prevalence among all 300 lumbar intervertebral disk levels in the study was considered.¹⁷⁶ In 36 patients (ages 17 to 71 years) without lower back pain or sciatica, the prevalence of disk bulge was 81%, protrusion 33%, and annular tears 56%, with no extrusions noted.¹⁶² Annular tears showed contrast enhancement in 96%. Assessment of T2 high-intensity zones in the disk (annular tears) by other authors in other studies, however, showed a high correlation with pain at diskography and a low prevalence in asymptomatic patients.^5,145 A high prevalence of abnormal findings on cervical MRI of asymptomatic individuals is also seen, and this increases with age.¹⁴

Intervertebral disk contour abnormalities can occur anywhere along the circumference of the disk, and can be described by location as central zone, subarticular zone (posterolateral), foraminal zone, and extraforaminal (far lateral). Foraminal zone disk abnormalities can be further described as occurring at the entrance zone, within the foramen, or at the exit zone of the foramen. The level of a herniation can be described as disk level, suprapedicle level, pedicle level, and infrapedicle level.^2,179 MRI criteria to differentiate subligamentous from transligamentous disk extrusion, such as the presence of a continuous low signal intensity line posterior to the extrusion, disk extrusion size less than 50% of the size of the spinal canal, and absence of disk fragments, are unreliable.¹⁵²

Small epidural hematomas can be associated with disk extrusions⁵⁸ and cause a larger mass effect than can be accounted for by the extrusion itself. If the extradural mass effect trails along a root sheath toward the foramen, or has signal characteristics more like those of fluid or hemorrhage, then a small epidural hematoma should be considered.

Endplate degenerative changes associated with disk degeneration have been classified into type 1 (low T1 and high T2 signal), edema/inflammation and fibrovascular change; type 2 (high T1 and high T2 signal), fatty marrow; and type 3 (low T1 and low T2 signal), consistent with diskogenic sclerosis. Type 1 changes are more associated with lower back pain and segmental instability.¹²⁷ Some of these endplate abnormalities can be associated with painful disks at discography in patients with low back pain.^167,177

Facet Joint Abnormalities

Facet and pars interarticularis abnormalities can often be seen with plain radiography. Oblique views are necessary to assess for a pars defect (spondylolysis). Thin-section CT with bone detail algorithm and sagittal reformations is the most accurate means of assessing for a pars defect, and can demonstrate any hypertrophic bone formation at the facet or pars contributing to foraminal narrowing (Figure 7-9). MRI is relatively insensitive to cortical bone defects, and so 30% of cases of lumbar spondylolysis might be undiagnosed if the physician relies on direct visualization of pars interarticularis defects.¹⁷² However, 97% of levels of spondylolysis have been shown to yield one or more secondary MRI signs, including increased sagittal diameter of the spinal canal, wedging of the posterior aspect of the vertebral body, and reactive marrow changes in the pedicle distinct from normal adjacent levels.¹⁷² Spondylolysis without spondylolisthesis can appear as widening of the sagittal dimension of the spinal canal because of dorsal subluxation of the posterior elements.¹⁷¹ Fluoroscopy during facet joint injection below a pars defect can show flow of the contrast agent into the pars defect and often then to the facet joint above the pars defect.

FIGURE 7-9 Multidetector-row computed tomography (MDCT) of pars interarticularis defects. A, Axial CT scan demonstrates discontinuity of the lumbar vertebral ring (arrows). B,

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