Magnetic Resonance Imaging in Acute Spinal Injury By Adam E. Flanders, Lisa M. Tartaglino,
N ESTIMATED 10,000 people in the United States sustain a spinal cord injury (SCI) each year with a prevalence of approximately 200,000 people in any given year.Q The total lifetime cost for medical treatment and rehabilitation per individual can exceed one million dollars depending on the type of injury. In addition, the obvious social, psychological, and emotional burdens to the patient, family, and society are enormous. Injury to the cervical spinal cord can result in quadriplegia (paralysis involving both the arms and the legs), whereas injury to the spinal cord below the cervical region can result in paraplegia (paralysis of the lower extremities only). More than half of all SCIs induce quadriplegia, and a similar percentage of all SCIs are complete (no motor or sensory function below the neurological level). Although more than 95% of paraplegic individuals can return to an independent lifestyle, this is achievable only to a variable degree with quadriplegic injuries. Because SCIs are endemic to teenagers and young adults, the impact on society in terms of loss of productive years is substantial. The proportion of injuries to the spinal axis varies by location. The cervical spine is prone to injury because of its supportive function of the head and the extended flexibility of the neck. Injuries to the cervical axis account for more than half of all spinal injuries and are usually the result of motor vehicle accidents, diving injuries, or contact sports.3 The thoracic cage offers additional stability to the thoracic spine, and injuries to this area are relatively uncommon. Tremendous force is required to distract the thoracic segments. Intrinsic abnormalities to one or more thoracic vertebral bodies can predispose to an injury at this location. Fracture dislocations at the thoracolumbar junction (lapbelt injury) and lumbar spine are frequently associated with high speed motor vehicle accidents or falls from heights. The radiological evaluation of spinal injury has undergone a remarkable evolution with the development of magnetic resonance imaging (MRI). Although plain radiographs, myelography, and computed tomography (CT) were once
A
Seminars in Roentgenology, Vol XXVII, No 4 (October),
David P. Friedman,
and Lucille F. Aquilone
the mainstay of spine imaging, MRI has recently become a necessity in the evaluation of SCI. The depiction by MRI of the soft tissue injuries associated with SC1 is unrivaled by any other imaging modality and has supplanted myelography in most instances.4-11Moreover, MRI is the only method available that allows for direct evaluation of the spinal cord parenchyma.h*x~“J2-i~ This information has radically changed our abilities to assessthe patient in the emergent period and has altered our understanding of the pathophysiology and prognosis of SCI.7,16-” TECHNIQUE
The ability to safely perform MR in the emergent period on the spinal injury patient is complicated by additional technical factors including minimizing the movement of the patient, monitoring the patient’s vital signs, and maintaining the patient’s ventilator-y support. It is therefore appropriate to address these technical factors individually. Equipment Adequate images of the spinal axis can probably be attained with any commercial magnet currently in use. There are distinct advantages to using the ultra-low to low static field strength units over high field strength in the emergent
ABBREVIATIONS ALL, anterior longitudinal ligament; CSF, cerebrospinal fluid; CT, computed tomography; 3-DFT, threedimensional Fourier transformation; FSE, fast spin echo; MRA, magnetic resonance angiography; MRI, magnetic resonance imaging; PLL, posterior longitudinal ligament; PPM, posttraumatic progressive myelopathy; RARE. rapid acquisition with relaxation enhancement; SCI, spinal cord injury
From the Department of Diagnostic Radiology. Thomas Jefferson University Hospital, Philadelphia, PA. Address reprint requests to Adam E. Flanders? MD, Department of Diagnostic Radiology, Thomas Jefferson Universil) Hospital, Suite 1072 Main Building, 10th and Sansom Sts. Philadebhia, PA 19107. Copyright 0 1992 by W.B. Saunders Cornpan) 0037-198X19212704-0006$5.OOiO
1992: pp 271-298
271
272
period in that there is a relative lack of siteing limitations (installation in the emergency room for example), availability of larger bore sizes, and ability to incorporate conventional lifesupport equipment (ie, ventilators and monitors) directly into the imaging suite. Although the high field strength units have strict siteing limitations and can only subsist with special MRI-compatible monitoring equipment, they have the overwhelming advantage of producing images of superior quality with higher overall resolution in a shorter period of time. Decreased sensitivity to SCIs reported in early studies using low field strength units have been largely overcome with the prevalence of high static field strength imagers.13The demonstration of acute hemorrhage (deoxyhemoglobin) in the spinal cord is more readily detected in high static field strength.22-24 Flat profile surface coils are most easily adaptable for use in the spinal injury patient. In the cervical region, a posterior neck coil or a standard 3- to 5-in circular receives only surface coil and offers the best results. The anteroposterior (AP) neck coil cannot be fitted over a halo fixation device or tongs. The 5 x 11 in “license plate” surface coil is used for the thoracic or lumbar area. The coil is often contained within a hollow acrylic board that allows the coil to be positioned without moving the patient. The recent development of the phased-array surface coil allows for easy acquisition along the entire spinal axis with improved signal to noise characteristics and better resolution. As with any critically ill patient, direct monitoring of vital signs is an essential part of the MRI examination. Several manufacturers produce MRI-compatible monitoring devices which register pulse, respiration, blood pressure, and pulse oximetry to a remote display in the control area. Because many of the spinal injury patients require endotracheal intubation and ventilation, an MRI-compatible ventilator is an important additional piece of equipment to have available. The installation of these accessories makes the MRI facility readily accessible to most critical care situations. Patient handling is an important issue after SCI. Patient movement should be minimized, therefore, the number of transfers should be limited. Transfers should be supervised by
FLANDERS
ET AL
trained personnel who are familiar with safe transfer techniques. A detachable MRI table is most helpful in this regard. The patient can be released from traction and transferred to the MRI table outside of the MRI suite and then placed directly into the magnet bore. Some centers have developed simple devices to maintain traction on the spine during the MRI examination.25,26Under no circumstances should traction weights be inadvertently brought into the MRI suite. Many of these “sandbags” contain iron pellets which can become projectiles in the peripheral static field. For cervical injuries, the patient will arrive in a simple fiberglass collar or in an MRIcompatible graphite halo vest after closed reduction of any subluxation. Thoracic or lumbar injuries often cannot be reduced with traction alone and will undergo open reduction and fixation of the injury at a later time. The thorax may be contained in a fiberglass vest and/or femoral traction may be incorporated. Any metallic fixation device that has not been predetermined to be MRI compatible should be tested before placing the patient in the scan area. Imaging Objectives
Plain-film radiography is the imaging procedure of choice for the initial evaluation of any spinal injury.14 CT can be used to supplement plain films or to better define complex fractures.*’ MRI is not a substitute for either of these modalities; rather, it offers supplemental information about the spinal cord and paraspinal soft tissues that are beyond the capabilities of conventional radiography.15,*’ This is especially important in instances of a posttraumatic neurological deficit and normal appearing radiographs, as described with hyperextension injuries.28,29 Spinal canal narrowing, cord compression, and epidural processes(eg, hematoma and herniated disc) are better visualized with MRI than with CT or radiographs.6,9 As a general rule, for patients with clinical evidence of SCI, the less the plain radiograph shows the more an MRI examination is needed.9J8T29 The MRI examination should be performed after closed reduction and stabilization and before any surgical intervention. A complete MRI study of the spinal axis
MRI AND SPINAL
INJURY
requires both Tl- and TZweighted information in at least two orthogonal planes.14 Sagittal images are mandatory, whereas axial images are best obtained only through the region of abnormality. Coronal images are not necessary in most instances; however, they are helpful in patients with scoliosis or in thoraco-lumbar injuries that have a large translational component. Tl-weighted sagittal images provide excellent anatomic definition of the spinal axis and provide a basic localizer for the remainder of the examination.6 These images also yield precise definition of the spinal cord caliber.15,30Using a 4-mm-thick slice at a 1 mm gap, only about nine to eleven images are necessary to traverse the entire spinal canal and the lateral bony elements. Dual echo, T2-weighted sagittal images give excellent depiction of spinal cord edema and acute hemorrhage (deoxyhemoglobin).6J4J7,27,31 They are also superb for evaluating disc herniation, ligamentous rupture, and epidural/paraspinal fluid collections.8~‘“~16 The field of view can vary from 20 cm in the cervical spine to 24 cm in the lumbar spine. A gradient echo sagittal sequence with a small flip angle is beneficial in the cervical and thoracic region because of the inherent “myelogram” effect it produces. Disc herniations and spinal cord hemorrhage can be confirmed on these images. There is a more sharply defined interface between the bone and the soft tissue that allows for better definition of fracture lines in the cortical bone.i2x16The boundaries of the thecal sac are well defined by the hyperintense cerebrospinal fluid (CSF). Compression of the theta and its contents is therefore often best visualized with use of this sequence. In those instances where motion degrades the images, the gradient echo sequence and the Tl-weighted images may provide the only useful diagnostic information. Axial imaging is necessary for assessingcompromise of the thecal sac and its contents, localizing bony fragments and herniated disc material, and ascertaining the integrity of the posterior elements. In the cervical spine, threedimensional Fourier (3-DFT) gradient echo data acquisitions are quite useful for obtaining multiple contiguous thin section axial images.
273
At low flip angles cerebrospinal fluid (CSF) is rendered markedly hyperintense contrasted to the low signal intensity spinal cord and bony elements. Stacks of 1.5 to 2 mm axial images are useful for interrogating the level of injury. The TE should be minimized to less than 15 milliseconds to prevent dephasing artifacts that tend to exaggerate stenoses.32 Lateral herniated disc fragments are more easily detected as are bony fractures, particularly those involving the posterior elements. In a similar fashion, the use of a 3-DFT gradient echo data acquisition with a spoiler gradient allows the use of larger flip angles to achieve relative Tl weighting. This type of imaging sequence may be useful in evaluating spinal cord cysts, posttraumatic syrinx, and hematomyelia. These volumetric data offer the additional advantage of being used for postprocessing on a workstation for the creation of reformatted images. Magnetic resonance angiography (MRA) can be used in the cervical region to evaluate for clinically occult damage to the extracranial carotid or vertebral arteries. The fixation of the vertebral arteries within the transverse foramina predispose them to injury by stretching or from direct compression from adjacent bone fragments (Fig l).’ A stack of contiguous 1.S-mm sections of the neck using a two-dimensional time-of-flight technique and superior saturation to suppress venous flow produces multiple projection images that show an overview of the cervical vascular tree. Although subtle intimal irregularities will probably remain undetected, vascular narrowing or occlusion can he shown with a large degree of reliability. The recent development of rapid spin-echo techniques (RARE or fast spin-echo [FSE], etc.) have tremendous application in the area of spinal trauma. This technique produces true spin-echo images in one half to one sixteenth of the time required to acquire a conventional T2-weighted spin-echo sequence. Improvements in signal and acquisition time enable use of larger matrix sizes and thinner sections, which results in higher resolution images acquired in less time. Motion artifacts from CSF pulsation are noticeably reduced and depiction of spinal cord edema is improved. A significant drawback to this technique is a decrease in
274
FLANDERS
ET AL
Fig 1. Traumatic dissection of the left vertebral artery. A 2g-year-old man who sustained a C5 quadriplegia while body surfing. (A) Mid-sagittal T2-weighted image of the cervical spine. There is a fracture of the C5 vertebral body and extensive edema and hemorrhage of the spinal cord. Prevertebral edema is identified (arrow) as well as injury to the interspinous ligaments between C4 and C5 (asterisk). (B) Patient deteriorated suddenly 4 days later. AP view of an arteriogram shows a large intraluminal thrombus (arrows) in the left vertebral artery. (C) The basilar artery Is occluded at the vertebrobasilar junction secondary to an embolus (arrow).
susceptibility artifact caused by acute spinal cord hemorrhage (deoxyhemoglobin). Therefore, this species of hemorrhage is often less conspicuous than on conventional TZweighted spin-echo or gradient echo images. This mandates the use of gradient echo images to confirm the existence of acute hemorrhage. The relative insensitivity of the FSE technique to magnetic field inhomogeneity can be exploited when evaluating the postoperative patient. Artifacts from metallic appliances may be reduced and a useful image may be obtained.
IMAGING
OBSERVATIONS
AND SIGNIFICANCE
Bony injury
The initial radiographic evaluation of bony spinal injury should always begin with plain radiography.r4 Cross table lateral films of the cervical, thoracic, and lumbar spine are essential tools for assessing fractures and subluxations. A complete radiographic series (AP, lateral, and oblique) often cannot be conveniently performed on a patient with an unstable spinal injury. Fortunately, the development of
MRI AND SPINAL
INJURY
CT has made it much easier to “clear” obscured areas of the bony spine for fracture and dislocation. Classification and description of all bony injuries to the spinal axis will not be provided herein and excellent reviews on this subject can be found elsewhere.“3-38 Traditionally, MRI has not been considered a sensitive enough technique to detect and describe fractures. Whereas cortical bone contains few mobile protons and therefore generates no visible signal, the adjacent marrow elements contain enough protons of sufficient mobility to yield a signal and provide a representation of the vertebral body.6 Although MRI is insensitive to cortical fractures relative to CT or conventional tomography, it is exquisitively sensitive to changes in composition of the marrow elements39 (this property is exploited when using MRI to detect infiltrative processesof the bone marrow such as metastases or infection). Minor cortical infractions, although not routinely visible on MRI examination, characteristically induce a mild response in the adjacent marrow elements which are easily detected with
275
MRI.8J2,‘3,23,30 Similarly, a compressive injury to a vertebral body imparts a characteristic alteration in the relative proton density of the marrow elements. This is thought to represent actual hemorrhage into the marrow elements from damage to small trabeculae. The presence of additional free water (plasma) within the marrow elements renders the involved vertebral body of lower signal intensity on the Tlweighted images and of relative hyperintensity on the TZweighted images.sJ2J”J3In the setting of trauma, these signal alterations within the marrow serve as a clue that sufficient compressive force (axial loading) has induced damage in the vertebral body. This finding may be present even when there is no significant alteration of the configuration of the vertebral body itself (Figs 2 and 3) or even in instances where the plain radiographs and CT are unremarkable.30 This should alert the radiologist to search for associated soft tissue injuries to the same vertebral segment. Major compression or comminuted fractures
Fig 2. Multiple thoracic compression injuries. Middle-aged male after a fall from a height. (A) Tl-weighted mid-sagittal image of the thoracic spine. There is loss of height of the T5 vertebral body (open arrow) with a dorsally displaced fragment (small arrow) which is compressing the spinal cord. Note the hypointensity of the vertebral marrow at the fractured level and in the remainder of the thoracic segments. (B) The T2-weighted sagittal image shows abnormal hyperintensity of the vertebral marrow at multiple levels (arrows). These signal characteristics are related to microfractures and internal hemorrhage within the marrow spaces of the vertebral bodies.
276
FLANDERS
ET AL
Fig 3. Extensive hemorrhagic contusion. (A) Tl-weighted sag&tat image of the cervical spine. There is flexion deformity of the cervical spine centered at the C5 vertebral body. The vertebral body is hypointense relative to the adjacent segments (arrow) suggesting that it has received a compressive injury. The spinal cord is massively enlarged in caliber from C2 to Tl. The spinal cord parenchyma is heterogeneous in signal intensity. (B) The damaged C5 vertebral body reverts to high signal intenslty on the T2-weighted image. A large focus of low signal intensity is present within the spinal cord (deoxyhemoglobin). This area is surrounded by hyperintense edema which extends longitudinally in the spinal cord for many levels above and below the hemorrhage. (C) The paramagnetic effects of deoxyhemoglobin are exploited on this gradient echo sagittal image. (D) Gradient echo axial image. The characteristic appearance of bilateral hemorrhage in the central gray matter of the spinal cord known as the “snake eyes” or “pig snout” sign (arrows).
of the vertebral bodies are plainly depicted with sagittal MRI images because of deformity of the involved segment and relative loss of alignment with the adjacent segments.7,8Complex fracture
fragments are often better resolved on the gradient echo sequences because of sharper interfaces between cortical bone and soft tissue (Fig 4). Vertebral alignment and traumatic
MRI AND SPINAL
INJURY
Fig 4. Fracture, subluxation, and ligamentous rupture C7/Tl. (A) A proton density sagfttal image of the cervical spine shows a large degree of subluxation (curved arrow) of C7 on Tl following closed reduction. The ALL is ruptured (arrow) and there is a fracture of the superior corner of Tl. (6) The T&weighted image shows hyperintense fluid ventral to the vertebral bodies [small whiie arrow) probably containing hemorrhage and disc material. The PLL is intact (open arrow) and bounded by epidural hemorrhage. The ligamentum flavum is ruptured (small black arrow) and the spinous processes of C7 and Tl are opposed (black arrows). (C)An axial CT image shows fracture and separation of the vertebral body of C7 from the posterior elements. The facet articulations are disorganized bilaterally. (D) Four contiguous (1.5 mm) gradient echo axial images. Although the osseous damage is not depicted to the same detail as in C, most of the key fractures and separated elements are shown.
275
subluxation are better appreciated with MRI than with CT.13 Posterior element fractures are notoriously difficult to image effectively with MRI.6,8J2J3,28
FLANDERS
ET AL
This is because of the relative small size of the posterior column structures compared with the vertebral bodies, the spatial limitations of routine slice thicknesses (4 to 5 mm), and the
Fig 5. Unilateral facet dislocation. (A) Tl-weighted sagittal image of the cervical spine shows subluxation of C4 on C5 (open arrow). There is deformity of C5 and the body is hyperintense secondary to compression. Epidural fluid is present dorsal to the vertebral body (arrow). (B) The T&weighted sagittal image shows an extensive amount of prevertebral edema and hemorrhage secondary to rupture of the anterior longitudinal ligament (small arrows). There is discontinuity of the posterior longitudinal ligament (black arrow). Spinal cord edema is present (asterisk). There is damage to the interspinous ligaments (curved arrows) and ligamentum flavum (arrowhead). (C) The left parasagittal proton density image reveals the “perched” C4 facet on C5 (arrow). A small bone fragment (arrowhead) has avulsed off the inferior corner. (D) Axial CT image shows the degree of subluxation and rotation of the C4/5 facet articulation (arrows).
MRI AND SPINAL
INJURY
inherent low sensitivity of MRI in depicting cortical fractures. Posterior element fractures are more visible when there is sufficient distraction of the fragments. In general, MRI has low sensitivity but reasonable accuracy to posterior element fractures when compared with CT.12,13 The orientation of the facet articulations can be scrutinized with reliability on the parasagittal Tl-weighted spin-echo or gradient echo images in cervical and lumbar injuries. All of the facet surfaces are seen in one or two sagittal images that incorporate the contents of the lateral masses. Facet subluxations or fractures can be detected on these images (Figs 5 and 6).
Fig 6. Fracture/subluxation with facet distraction. (A) Sagittal proton density image of the cervical spine using fast spinecho technique shows a fracture of the inferior corner of the C5 vertebral body (large arrow) and avulsion of the ALL. There is associated subluxation relative to C6 with dorsal displacement of the PLL (small arrow). (8) Parasagittal images of the left articular pillar shows distraction of the C6/C6 facet joint on the left (arrow). (C) Gradient echo pararagittal image shows hyperintense fluid (arrow) in the facet joint capsule secondary to injury.
279
In addition, the integrity of the neural foramina and contents can also be assessed. A key element in the evaluation of spinal injury is the degree of compromise of the spinal canal and its contents by subluxed or retropulsed bone and/or soft tissue.13 The same degree of canal reduction can result in minor or severe neurological deficits depending on the natural dimensions of the spinal canal and the level of canal involved. The individual roots of the cauda equina are much more tolerant to direct impact injury, compression, or tension than the spinal cord itself. Traumatic compromise of the spinal canal by more than 50% of its
280
FLANDERS
Fig 7. Congenital spinal stenosis and SCI. (A) Tl-weighted sagittal image of the cervical spine. There is developmental the spinal canal in the AP dimensions with effacement of the subarachnoid space and flattening of the spinal cord from C2 that there is no obvious evidence of fracture, subluxation or disc herniation. (8) On the TZ-weighted sagittal image, a long intramedullary edema is shown (black arrows). The C5 vertebral body (open arrow) and C5/6 intervertebral disc (arrow) hyperintense suggesting that they are damaged as well.
normal dimensions is often tolerated neurologitally at levels below the termination of the conus medullaris (T12-Ll). The spinal cord is much more sensitive to direct compressive injury than the individual roots of the cauda equina, therefore a subluxation or compression fracture in the thoracic and cervical region can have profound neurological consequences. MRI is unique in its ability to depict not only the fracture/deformity of the spinal axis but also at the same time to show details of the soft tissue injury and how these changes affect the neural elements (spinal cord and nerve roots). 6~8~13,28 Developmental or acquired spinal stenosis or spondylosis of the spinal canal has been implicated as a cause of spinal injury (particularly central cord injury) in instances of relatively trivial trauma (Fig 7). 13,21,28,29,40
ET AL
narrowing to C6. Note segment of are slightly
The surgeon’s initial objective in treating spinal injury is to restore the alignment of the spinal axis through closed reduction. Where there is permanent instability of the spinal axis from fracture or ligamentous disruption, open fixation/fusion of the involved spinal elements must be performed. 41 The timing of surgical intervention in the acute phase (early v late) remains controversial even today.41-48 Investigations at our center have suggested that residual compression of the cervical spinal cord (visualized on MRI) after closed reduction is associated with a much higher incidence (by a factor of 5) of permanent spinal cord damage.12Although this does not imply a direct casual relationship between traumatic compression of the spinal cord and the severity of permanent neurological deficit, it may support the argument for open reduction, decompression, and
MRI AND SPINAL
INJURY
fixation in the early period (
N ESTIMATED 10,000 people in the United States sustain a spinal cord injury (SCI) each year with a prevalence of approximately 200,000 people in any given year.Q The total lifetime cost for medical treatment and rehabilitation per individual can exceed one million dollars depending on the type of injury. In addition, the obvious social, psychological, and emotional burdens to the patient, family, and society are enormous. Injury to the cervical spinal cord can result in quadriplegia (paralysis involving both the arms and the legs), whereas injury to the spinal cord below the cervical region can result in paraplegia (paralysis of the lower extremities only). More than half of all SCIs induce quadriplegia, and a similar percentage of all SCIs are complete (no motor or sensory function below the neurological level). Although more than 95% of paraplegic individuals can return to an independent lifestyle, this is achievable only to a variable degree with quadriplegic injuries. Because SCIs are endemic to teenagers and young adults, the impact on society in terms of loss of productive years is substantial. The proportion of injuries to the spinal axis varies by location. The cervical spine is prone to injury because of its supportive function of the head and the extended flexibility of the neck. Injuries to the cervical axis account for more than half of all spinal injuries and are usually the result of motor vehicle accidents, diving injuries, or contact sports.3 The thoracic cage offers additional stability to the thoracic spine, and injuries to this area are relatively uncommon. Tremendous force is required to distract the thoracic segments. Intrinsic abnormalities to one or more thoracic vertebral bodies can predispose to an injury at this location. Fracture dislocations at the thoracolumbar junction (lapbelt injury) and lumbar spine are frequently associated with high speed motor vehicle accidents or falls from heights. The radiological evaluation of spinal injury has undergone a remarkable evolution with the development of magnetic resonance imaging (MRI). Although plain radiographs, myelography, and computed tomography (CT) were once
A
Seminars in Roentgenology, Vol XXVII, No 4 (October),
David P. Friedman,
and Lucille F. Aquilone
the mainstay of spine imaging, MRI has recently become a necessity in the evaluation of SCI. The depiction by MRI of the soft tissue injuries associated with SC1 is unrivaled by any other imaging modality and has supplanted myelography in most instances.4-11Moreover, MRI is the only method available that allows for direct evaluation of the spinal cord parenchyma.h*x~“J2-i~ This information has radically changed our abilities to assessthe patient in the emergent period and has altered our understanding of the pathophysiology and prognosis of SCI.7,16-” TECHNIQUE
The ability to safely perform MR in the emergent period on the spinal injury patient is complicated by additional technical factors including minimizing the movement of the patient, monitoring the patient’s vital signs, and maintaining the patient’s ventilator-y support. It is therefore appropriate to address these technical factors individually. Equipment Adequate images of the spinal axis can probably be attained with any commercial magnet currently in use. There are distinct advantages to using the ultra-low to low static field strength units over high field strength in the emergent
ABBREVIATIONS ALL, anterior longitudinal ligament; CSF, cerebrospinal fluid; CT, computed tomography; 3-DFT, threedimensional Fourier transformation; FSE, fast spin echo; MRA, magnetic resonance angiography; MRI, magnetic resonance imaging; PLL, posterior longitudinal ligament; PPM, posttraumatic progressive myelopathy; RARE. rapid acquisition with relaxation enhancement; SCI, spinal cord injury
From the Department of Diagnostic Radiology. Thomas Jefferson University Hospital, Philadelphia, PA. Address reprint requests to Adam E. Flanders? MD, Department of Diagnostic Radiology, Thomas Jefferson Universil) Hospital, Suite 1072 Main Building, 10th and Sansom Sts. Philadebhia, PA 19107. Copyright 0 1992 by W.B. Saunders Cornpan) 0037-198X19212704-0006$5.OOiO
1992: pp 271-298
271
272
period in that there is a relative lack of siteing limitations (installation in the emergency room for example), availability of larger bore sizes, and ability to incorporate conventional lifesupport equipment (ie, ventilators and monitors) directly into the imaging suite. Although the high field strength units have strict siteing limitations and can only subsist with special MRI-compatible monitoring equipment, they have the overwhelming advantage of producing images of superior quality with higher overall resolution in a shorter period of time. Decreased sensitivity to SCIs reported in early studies using low field strength units have been largely overcome with the prevalence of high static field strength imagers.13The demonstration of acute hemorrhage (deoxyhemoglobin) in the spinal cord is more readily detected in high static field strength.22-24 Flat profile surface coils are most easily adaptable for use in the spinal injury patient. In the cervical region, a posterior neck coil or a standard 3- to 5-in circular receives only surface coil and offers the best results. The anteroposterior (AP) neck coil cannot be fitted over a halo fixation device or tongs. The 5 x 11 in “license plate” surface coil is used for the thoracic or lumbar area. The coil is often contained within a hollow acrylic board that allows the coil to be positioned without moving the patient. The recent development of the phased-array surface coil allows for easy acquisition along the entire spinal axis with improved signal to noise characteristics and better resolution. As with any critically ill patient, direct monitoring of vital signs is an essential part of the MRI examination. Several manufacturers produce MRI-compatible monitoring devices which register pulse, respiration, blood pressure, and pulse oximetry to a remote display in the control area. Because many of the spinal injury patients require endotracheal intubation and ventilation, an MRI-compatible ventilator is an important additional piece of equipment to have available. The installation of these accessories makes the MRI facility readily accessible to most critical care situations. Patient handling is an important issue after SCI. Patient movement should be minimized, therefore, the number of transfers should be limited. Transfers should be supervised by
FLANDERS
ET AL
trained personnel who are familiar with safe transfer techniques. A detachable MRI table is most helpful in this regard. The patient can be released from traction and transferred to the MRI table outside of the MRI suite and then placed directly into the magnet bore. Some centers have developed simple devices to maintain traction on the spine during the MRI examination.25,26Under no circumstances should traction weights be inadvertently brought into the MRI suite. Many of these “sandbags” contain iron pellets which can become projectiles in the peripheral static field. For cervical injuries, the patient will arrive in a simple fiberglass collar or in an MRIcompatible graphite halo vest after closed reduction of any subluxation. Thoracic or lumbar injuries often cannot be reduced with traction alone and will undergo open reduction and fixation of the injury at a later time. The thorax may be contained in a fiberglass vest and/or femoral traction may be incorporated. Any metallic fixation device that has not been predetermined to be MRI compatible should be tested before placing the patient in the scan area. Imaging Objectives
Plain-film radiography is the imaging procedure of choice for the initial evaluation of any spinal injury.14 CT can be used to supplement plain films or to better define complex fractures.*’ MRI is not a substitute for either of these modalities; rather, it offers supplemental information about the spinal cord and paraspinal soft tissues that are beyond the capabilities of conventional radiography.15,*’ This is especially important in instances of a posttraumatic neurological deficit and normal appearing radiographs, as described with hyperextension injuries.28,29 Spinal canal narrowing, cord compression, and epidural processes(eg, hematoma and herniated disc) are better visualized with MRI than with CT or radiographs.6,9 As a general rule, for patients with clinical evidence of SCI, the less the plain radiograph shows the more an MRI examination is needed.9J8T29 The MRI examination should be performed after closed reduction and stabilization and before any surgical intervention. A complete MRI study of the spinal axis
MRI AND SPINAL
INJURY
requires both Tl- and TZweighted information in at least two orthogonal planes.14 Sagittal images are mandatory, whereas axial images are best obtained only through the region of abnormality. Coronal images are not necessary in most instances; however, they are helpful in patients with scoliosis or in thoraco-lumbar injuries that have a large translational component. Tl-weighted sagittal images provide excellent anatomic definition of the spinal axis and provide a basic localizer for the remainder of the examination.6 These images also yield precise definition of the spinal cord caliber.15,30Using a 4-mm-thick slice at a 1 mm gap, only about nine to eleven images are necessary to traverse the entire spinal canal and the lateral bony elements. Dual echo, T2-weighted sagittal images give excellent depiction of spinal cord edema and acute hemorrhage (deoxyhemoglobin).6J4J7,27,31 They are also superb for evaluating disc herniation, ligamentous rupture, and epidural/paraspinal fluid collections.8~‘“~16 The field of view can vary from 20 cm in the cervical spine to 24 cm in the lumbar spine. A gradient echo sagittal sequence with a small flip angle is beneficial in the cervical and thoracic region because of the inherent “myelogram” effect it produces. Disc herniations and spinal cord hemorrhage can be confirmed on these images. There is a more sharply defined interface between the bone and the soft tissue that allows for better definition of fracture lines in the cortical bone.i2x16The boundaries of the thecal sac are well defined by the hyperintense cerebrospinal fluid (CSF). Compression of the theta and its contents is therefore often best visualized with use of this sequence. In those instances where motion degrades the images, the gradient echo sequence and the Tl-weighted images may provide the only useful diagnostic information. Axial imaging is necessary for assessingcompromise of the thecal sac and its contents, localizing bony fragments and herniated disc material, and ascertaining the integrity of the posterior elements. In the cervical spine, threedimensional Fourier (3-DFT) gradient echo data acquisitions are quite useful for obtaining multiple contiguous thin section axial images.
273
At low flip angles cerebrospinal fluid (CSF) is rendered markedly hyperintense contrasted to the low signal intensity spinal cord and bony elements. Stacks of 1.5 to 2 mm axial images are useful for interrogating the level of injury. The TE should be minimized to less than 15 milliseconds to prevent dephasing artifacts that tend to exaggerate stenoses.32 Lateral herniated disc fragments are more easily detected as are bony fractures, particularly those involving the posterior elements. In a similar fashion, the use of a 3-DFT gradient echo data acquisition with a spoiler gradient allows the use of larger flip angles to achieve relative Tl weighting. This type of imaging sequence may be useful in evaluating spinal cord cysts, posttraumatic syrinx, and hematomyelia. These volumetric data offer the additional advantage of being used for postprocessing on a workstation for the creation of reformatted images. Magnetic resonance angiography (MRA) can be used in the cervical region to evaluate for clinically occult damage to the extracranial carotid or vertebral arteries. The fixation of the vertebral arteries within the transverse foramina predispose them to injury by stretching or from direct compression from adjacent bone fragments (Fig l).’ A stack of contiguous 1.S-mm sections of the neck using a two-dimensional time-of-flight technique and superior saturation to suppress venous flow produces multiple projection images that show an overview of the cervical vascular tree. Although subtle intimal irregularities will probably remain undetected, vascular narrowing or occlusion can he shown with a large degree of reliability. The recent development of rapid spin-echo techniques (RARE or fast spin-echo [FSE], etc.) have tremendous application in the area of spinal trauma. This technique produces true spin-echo images in one half to one sixteenth of the time required to acquire a conventional T2-weighted spin-echo sequence. Improvements in signal and acquisition time enable use of larger matrix sizes and thinner sections, which results in higher resolution images acquired in less time. Motion artifacts from CSF pulsation are noticeably reduced and depiction of spinal cord edema is improved. A significant drawback to this technique is a decrease in
274
FLANDERS
ET AL
Fig 1. Traumatic dissection of the left vertebral artery. A 2g-year-old man who sustained a C5 quadriplegia while body surfing. (A) Mid-sagittal T2-weighted image of the cervical spine. There is a fracture of the C5 vertebral body and extensive edema and hemorrhage of the spinal cord. Prevertebral edema is identified (arrow) as well as injury to the interspinous ligaments between C4 and C5 (asterisk). (B) Patient deteriorated suddenly 4 days later. AP view of an arteriogram shows a large intraluminal thrombus (arrows) in the left vertebral artery. (C) The basilar artery Is occluded at the vertebrobasilar junction secondary to an embolus (arrow).
susceptibility artifact caused by acute spinal cord hemorrhage (deoxyhemoglobin). Therefore, this species of hemorrhage is often less conspicuous than on conventional TZweighted spin-echo or gradient echo images. This mandates the use of gradient echo images to confirm the existence of acute hemorrhage. The relative insensitivity of the FSE technique to magnetic field inhomogeneity can be exploited when evaluating the postoperative patient. Artifacts from metallic appliances may be reduced and a useful image may be obtained.
IMAGING
OBSERVATIONS
AND SIGNIFICANCE
Bony injury
The initial radiographic evaluation of bony spinal injury should always begin with plain radiography.r4 Cross table lateral films of the cervical, thoracic, and lumbar spine are essential tools for assessing fractures and subluxations. A complete radiographic series (AP, lateral, and oblique) often cannot be conveniently performed on a patient with an unstable spinal injury. Fortunately, the development of
MRI AND SPINAL
INJURY
CT has made it much easier to “clear” obscured areas of the bony spine for fracture and dislocation. Classification and description of all bony injuries to the spinal axis will not be provided herein and excellent reviews on this subject can be found elsewhere.“3-38 Traditionally, MRI has not been considered a sensitive enough technique to detect and describe fractures. Whereas cortical bone contains few mobile protons and therefore generates no visible signal, the adjacent marrow elements contain enough protons of sufficient mobility to yield a signal and provide a representation of the vertebral body.6 Although MRI is insensitive to cortical fractures relative to CT or conventional tomography, it is exquisitively sensitive to changes in composition of the marrow elements39 (this property is exploited when using MRI to detect infiltrative processesof the bone marrow such as metastases or infection). Minor cortical infractions, although not routinely visible on MRI examination, characteristically induce a mild response in the adjacent marrow elements which are easily detected with
275
MRI.8J2,‘3,23,30 Similarly, a compressive injury to a vertebral body imparts a characteristic alteration in the relative proton density of the marrow elements. This is thought to represent actual hemorrhage into the marrow elements from damage to small trabeculae. The presence of additional free water (plasma) within the marrow elements renders the involved vertebral body of lower signal intensity on the Tlweighted images and of relative hyperintensity on the TZweighted images.sJ2J”J3In the setting of trauma, these signal alterations within the marrow serve as a clue that sufficient compressive force (axial loading) has induced damage in the vertebral body. This finding may be present even when there is no significant alteration of the configuration of the vertebral body itself (Figs 2 and 3) or even in instances where the plain radiographs and CT are unremarkable.30 This should alert the radiologist to search for associated soft tissue injuries to the same vertebral segment. Major compression or comminuted fractures
Fig 2. Multiple thoracic compression injuries. Middle-aged male after a fall from a height. (A) Tl-weighted mid-sagittal image of the thoracic spine. There is loss of height of the T5 vertebral body (open arrow) with a dorsally displaced fragment (small arrow) which is compressing the spinal cord. Note the hypointensity of the vertebral marrow at the fractured level and in the remainder of the thoracic segments. (B) The T2-weighted sagittal image shows abnormal hyperintensity of the vertebral marrow at multiple levels (arrows). These signal characteristics are related to microfractures and internal hemorrhage within the marrow spaces of the vertebral bodies.
276
FLANDERS
ET AL
Fig 3. Extensive hemorrhagic contusion. (A) Tl-weighted sag&tat image of the cervical spine. There is flexion deformity of the cervical spine centered at the C5 vertebral body. The vertebral body is hypointense relative to the adjacent segments (arrow) suggesting that it has received a compressive injury. The spinal cord is massively enlarged in caliber from C2 to Tl. The spinal cord parenchyma is heterogeneous in signal intensity. (B) The damaged C5 vertebral body reverts to high signal intenslty on the T2-weighted image. A large focus of low signal intensity is present within the spinal cord (deoxyhemoglobin). This area is surrounded by hyperintense edema which extends longitudinally in the spinal cord for many levels above and below the hemorrhage. (C) The paramagnetic effects of deoxyhemoglobin are exploited on this gradient echo sagittal image. (D) Gradient echo axial image. The characteristic appearance of bilateral hemorrhage in the central gray matter of the spinal cord known as the “snake eyes” or “pig snout” sign (arrows).
of the vertebral bodies are plainly depicted with sagittal MRI images because of deformity of the involved segment and relative loss of alignment with the adjacent segments.7,8Complex fracture
fragments are often better resolved on the gradient echo sequences because of sharper interfaces between cortical bone and soft tissue (Fig 4). Vertebral alignment and traumatic
MRI AND SPINAL
INJURY
Fig 4. Fracture, subluxation, and ligamentous rupture C7/Tl. (A) A proton density sagfttal image of the cervical spine shows a large degree of subluxation (curved arrow) of C7 on Tl following closed reduction. The ALL is ruptured (arrow) and there is a fracture of the superior corner of Tl. (6) The T&weighted image shows hyperintense fluid ventral to the vertebral bodies [small whiie arrow) probably containing hemorrhage and disc material. The PLL is intact (open arrow) and bounded by epidural hemorrhage. The ligamentum flavum is ruptured (small black arrow) and the spinous processes of C7 and Tl are opposed (black arrows). (C)An axial CT image shows fracture and separation of the vertebral body of C7 from the posterior elements. The facet articulations are disorganized bilaterally. (D) Four contiguous (1.5 mm) gradient echo axial images. Although the osseous damage is not depicted to the same detail as in C, most of the key fractures and separated elements are shown.
275
subluxation are better appreciated with MRI than with CT.13 Posterior element fractures are notoriously difficult to image effectively with MRI.6,8J2J3,28
FLANDERS
ET AL
This is because of the relative small size of the posterior column structures compared with the vertebral bodies, the spatial limitations of routine slice thicknesses (4 to 5 mm), and the
Fig 5. Unilateral facet dislocation. (A) Tl-weighted sagittal image of the cervical spine shows subluxation of C4 on C5 (open arrow). There is deformity of C5 and the body is hyperintense secondary to compression. Epidural fluid is present dorsal to the vertebral body (arrow). (B) The T&weighted sagittal image shows an extensive amount of prevertebral edema and hemorrhage secondary to rupture of the anterior longitudinal ligament (small arrows). There is discontinuity of the posterior longitudinal ligament (black arrow). Spinal cord edema is present (asterisk). There is damage to the interspinous ligaments (curved arrows) and ligamentum flavum (arrowhead). (C) The left parasagittal proton density image reveals the “perched” C4 facet on C5 (arrow). A small bone fragment (arrowhead) has avulsed off the inferior corner. (D) Axial CT image shows the degree of subluxation and rotation of the C4/5 facet articulation (arrows).
MRI AND SPINAL
INJURY
inherent low sensitivity of MRI in depicting cortical fractures. Posterior element fractures are more visible when there is sufficient distraction of the fragments. In general, MRI has low sensitivity but reasonable accuracy to posterior element fractures when compared with CT.12,13 The orientation of the facet articulations can be scrutinized with reliability on the parasagittal Tl-weighted spin-echo or gradient echo images in cervical and lumbar injuries. All of the facet surfaces are seen in one or two sagittal images that incorporate the contents of the lateral masses. Facet subluxations or fractures can be detected on these images (Figs 5 and 6).
Fig 6. Fracture/subluxation with facet distraction. (A) Sagittal proton density image of the cervical spine using fast spinecho technique shows a fracture of the inferior corner of the C5 vertebral body (large arrow) and avulsion of the ALL. There is associated subluxation relative to C6 with dorsal displacement of the PLL (small arrow). (8) Parasagittal images of the left articular pillar shows distraction of the C6/C6 facet joint on the left (arrow). (C) Gradient echo pararagittal image shows hyperintense fluid (arrow) in the facet joint capsule secondary to injury.
279
In addition, the integrity of the neural foramina and contents can also be assessed. A key element in the evaluation of spinal injury is the degree of compromise of the spinal canal and its contents by subluxed or retropulsed bone and/or soft tissue.13 The same degree of canal reduction can result in minor or severe neurological deficits depending on the natural dimensions of the spinal canal and the level of canal involved. The individual roots of the cauda equina are much more tolerant to direct impact injury, compression, or tension than the spinal cord itself. Traumatic compromise of the spinal canal by more than 50% of its
280
FLANDERS
Fig 7. Congenital spinal stenosis and SCI. (A) Tl-weighted sagittal image of the cervical spine. There is developmental the spinal canal in the AP dimensions with effacement of the subarachnoid space and flattening of the spinal cord from C2 that there is no obvious evidence of fracture, subluxation or disc herniation. (8) On the TZ-weighted sagittal image, a long intramedullary edema is shown (black arrows). The C5 vertebral body (open arrow) and C5/6 intervertebral disc (arrow) hyperintense suggesting that they are damaged as well.
normal dimensions is often tolerated neurologitally at levels below the termination of the conus medullaris (T12-Ll). The spinal cord is much more sensitive to direct compressive injury than the individual roots of the cauda equina, therefore a subluxation or compression fracture in the thoracic and cervical region can have profound neurological consequences. MRI is unique in its ability to depict not only the fracture/deformity of the spinal axis but also at the same time to show details of the soft tissue injury and how these changes affect the neural elements (spinal cord and nerve roots). 6~8~13,28 Developmental or acquired spinal stenosis or spondylosis of the spinal canal has been implicated as a cause of spinal injury (particularly central cord injury) in instances of relatively trivial trauma (Fig 7). 13,21,28,29,40
ET AL
narrowing to C6. Note segment of are slightly
The surgeon’s initial objective in treating spinal injury is to restore the alignment of the spinal axis through closed reduction. Where there is permanent instability of the spinal axis from fracture or ligamentous disruption, open fixation/fusion of the involved spinal elements must be performed. 41 The timing of surgical intervention in the acute phase (early v late) remains controversial even today.41-48 Investigations at our center have suggested that residual compression of the cervical spinal cord (visualized on MRI) after closed reduction is associated with a much higher incidence (by a factor of 5) of permanent spinal cord damage.12Although this does not imply a direct casual relationship between traumatic compression of the spinal cord and the severity of permanent neurological deficit, it may support the argument for open reduction, decompression, and
MRI AND SPINAL
INJURY
fixation in the early period (
