The role of magnetic resonance imaging on spinal trauma

Pictorial

Review The Role of Magnetic Resonance Imaging on Spinal Trauma PETER

Depart,lwrlt.s

of *Radiology

and

fOrthopardics.

Received: I2 January 1999

CORR*,

S. GOVENDER?

University

of Natal.

Private

Bag

7, Congella

4013,

Durban,

S.

Africa

Revised: 9 April 1999 Accepted: I3 May 1999

Spinal MR has an increasingly important role in the assessment of spinal trauma. The ability to visualise clearly the spinal cord, nerve roots, ligaments, intervertebral discs and adjacent vascular structures allow a more accurate assessment of the extent of injury, and necessity for further management and provide a prognosis for recovery. Corr, P. and Govender, S. (1999) Clinicul Radiology 54, 629-635. 0 I999 The Royal College of Radiologists Key words: MR. spinal imaging, spinal injury. spinal cord injury.

The role of MR imaging in the diagnosis and management of spinal trauma is controversial [l-3]. The role is essentially three-fold: to assess the spinal injury, to direct appropriate management, and to predict neurological outcome [I-3]. MR accurately demonstrates injury to the spinal cord, intervertebral discs, spinal ligaments, and surrounding vascular structures. MR is used to assess the need for urgent spinal decompression and fixation. Monitoring an injured patient within an MR system is often problematic. Patients often have spinal shock and are haemodynamically unstable in the acute phase [3]. Access to MR imaging is often unavailable at short notice in many trauma centres, especially after hours. A recent review from North America found that in practice only 6% of trauma centres used emergency MR [3]. SPINAL

INJURY

ASSESSMENT

MR is useful in assessing the extent of soft tissue, ligamentous and spinal cord injury [I]. Adequate plain radiographs should always be the initial investigation to assess bony injury. It is critical to visualise the whole spine, especially the craniocervical and cervicothoracic junctions as these regions are the most mobile and most likely to be injured. The other reason to visualise the whole spine is that second fractures are not uncommon, occurring in 5% of patients with spinal injuries, especially in the patients sustaining polytrauma from road traffic accidents [4]. CT is particularly useful in detecting subtle fractures and is, in this regard, far superior to MR [5]. MR protocols should include fast spin echo TI and T2 weighted sagittal and axial images. as well as gradient echo sequences to OOOY-Y260/99/100629+07

$12.0010

detect acute and chronic haemorrhage within the spinal canal and spinal cord [I ,2,6]. Patients who are acutely injured will require cardiorespiratory monitoring during MR imaging. It is important to have MR compatible monitoring available and for staff to be familiar with its use. VERTEBRAL

ALIGNMENT

Sagittal MR images are useful for the detection of malalignment, which may indicate serious ligamentous and or bony injury. The following four parallel longitudinal lines may be assessed on the sagittal MR image: a line joining the anterior margins of the vertebral bodies, the posterior margin of the vertebral bodies, the spinolaminar line joining the junction of the laminae with the anterior margin of the spinous processes and a line joining the tips of the spinous processes. VERTEBRAL

BODY

Following trauma, there may be bone marrow oedema and haemorrhage which has a low intensity on Tl-weighted and high intensity on T2-weighted images. There is often loss of cortical margins and vertebral body deformity indicating a fracture (Fig. I). However, in some fractures the bony margins and marrow signal are preserved. The MR anatomy is generally not as well demonstrated as on CT. Without doubt, CT is the preferred investigation to assess the bony anatomy in conjunction with good quality radiographs of the cervical spine [5]. CT is of value in detecting clinically suspected fractures when the 0 1999 The Royal College of Radiologists

630

CLINICAL

RADIOLOGY

LIGAMENTOUS

INJURY

Ligaments are commonly injured resulting in spinal instability. The ligaments, which are of very low signal on all sequences, are best visualized on fat saturated sagittal images. Ligamentous injury is detected when there is an increased signal or loss of the integrity of the ligament (Fig. 2) [9]. There is usually an associated malalignment in the sagittal plane. However in some patients with serious ligament injury, alignment is normal. There is often an associated prevertebral haematoma which is a pointer to serious ligament injury.

INTERVERTEBRAL

DISC

HERNIATION

Traumatic disc hemiation of the cervical spine, originally considered uncommon, is actually fairly frequent in cervical spine injuries. occurring in up to 35% of patients in one series [IO]. Hemiation is more common with flexion compression and anterior subluxation injuries than hyperextension and axial compression injuries [lo]. Disc hemiation associated with trauma is not usually seen on conventional CT studies [IO]. The presence of disc herniation may alter the surgical approach

Fig. I - Sag&al E-weighted MR image of a patient with a C6/7 flexion subluxation injury with bony deformity and high intensity marrow oedema (arrows) and narrowing of the spinal canal with extensive cord contusion (expanded cord and yielding high signal).

radiographs are reported as normal. which can occur in up to 57% of patients with craniocervical junction injuries [5]. MR can accurately detect focal bony fragments within the spinal canal [l]. MR is particularly valuable in differentiating between osteopenic fractures and those due to metastates. In acute wedge fractures, marrow oedema and or haemorrhage may be seen in both conditions however, without gadolinium enhancement, marrow signal is more likely to be normal intensity in osteoporotic fractures [6]. With chronic metastatic wedge fractures with malignant infiltration, there is marked reduction in signal on Tl-weighted images in the marrow compared to osteopenic fractures where the marrow signal is usually maintained [7]. On fracture healing, osteopenic fractures have the same bone marrow signal intensity as the adjacent vertebral bodies. In a series of 30 patients, diffusion MR of the spine has recently been shown to be more specific in differentiating benign from malignant compression fractures when compared to spin echo sequences [8]. However diffusion associated artefacts from CSF flow and respiratory motion will probably limit its routine use in the acutely injured patient until better spinal coils and motion compensation techniques are developed.

Fig. 2 - Sagittal n-weighted MR image of a patient with subluxation with disruption of the posterior longitudinal ligament ligamentum flavum (curved arrows).

a CY6 and the

THE ROLE OF MAGNETIC

from posterior to anterior. Disc herniation may occur in all directions, including into the end plate. Acute disc hemiation may cause significant spinal cord compression, requiring anterior surgical decompression. Traumatically herniated discs often have a focal hyperintensity on T2 and gradient echosequences, presumably due to disc oedema, unlike degenerative discs which are invariably hypointense on Tl and T2 images (Fig. 3). Chronic annular tears may also have an increased signal on T2-weighted images and these should not be confused with traumatized discs.

RESONANCE

631

IMAGING

compression, the collection can be observed and should resolve over 2 weeks. Using MR, a well defined elliptical collection in the dorsal extradural space is detected. In the acute phase, the collection has a low signal on Tl- and T2-weighted images due to deoxyhaemoglobin, after 1 week the collection shows increased signal on Tl- and TZweighted images due to change in methaemoglobin (Fig. 4). SPINAL CORD

Haematomas are uncommon in spinal injury and usually follow venous bleeding from tom veins in the epidural space [ 111. Extradural haematoma is more common in patients with pre-existing coagulopathies or patients on anticoagulants, and occurs particularly in males over the age of 50 years with a history of previous trauma in the thoracic spine [12]. The patient presents with a delayed paraparesis with severe root pain. Spinal cord and cauda equina compression requires urgent surgical decompression. If there are no clinical signs of cord

MR accurately determines spinal cord integrity following injury. The presence and extent of focal cord contusion, and intramedullary haematoma are readily detected in acute injury [ 131. Contusion is diagnosed when there is cord swelling with focal decreased signal intensity on Tl, and increased signal on T2 (Fig. 5). Cord contusion resulting in a central canal syndrome is common in hyperextension injuries in older patients with acquired canal stenosis in the cervical spine from multiple osteophytes and apophyseal joint hypertrophy. Demonstration of cord transection by MR, however, is of no clinical value in the management of the patient. Focal haemorrhage within the cord may range from petech&I haemorrhage to intramedullary haematoma (haematomye&). There is usually evidence of blood products, usually deoxyhaemoglobin or methaemoglobin. Gradient echo sequences are

Fig. 3 - Sagittal TZ-weighted MR image of a patient with a surgically proven traumatic disc hemiation at the C6/7 level (arrow) with spinal cord compression and contusion.

Fig. 4 - Sagittal T2-weighted MR image of a patient with a dorsal extradural haematoma (arrow) compressing the thecal sac and spinal cord anteriorly.

INTRASPINAL

HAEMATOMA

632

CLINICAL

Fig. 5 - Sag&al n-weighted MR image of a patient with a C5/6 dislocation with a focal cord hyperintensity resulting from contusion (arrow).

useful in confirming haemorrhage (Fig. 6) [ 1,5,13]. The presence of intramedullary haemorrhage is indicative of permanent injury and suggests a poor prognosis for recovery [ 131. In the chronic phase, infarcts and haematomas result in regions of intramedullary gliosis and cyst formation. This is called myelomalacia. On MR there are focal ill defined low intensity lesions within the cord on Tl-weighted images with hyperintensity on n-weighted images. Myelomalacia must be differentiated from cord atrophy. This is best achieved by reviewing the axial images of the spinal cord to determine the presence of intramedullary low intensity lesions of myelomalacia and to assess cord contour which is markedly decreased and irregular in patients with cord atrophy. Intramedullary cavitation may occur in more severe injuries and be progressive, forming a post traumatic syrinx or cord cavity (Fig. 7). The importance of detecting a post traumatic syrinx is that they may enlarge with deterioration in the patient’s neurological symptoms and signs [14]. Shunting of the syrinx and or performing a duraplasty to release dural adhesions may halt neurological deterioration in these patients [ 14,151. MR can demonstrate spinal cord tears following birth injuries. These cord injuries occur at the cervico thoracic junction and have a widely separated tear in the cord and associated dural tear from traction during delivery (Fig. 8).

RADIOLOGY

Fig. 6 - Sagittal gradient echo MR image of a patient with a low intensity focal haematoma resulting from deoxyhaemoglobin (arrow) with marked surrounding cord oedema following a C2/3 subluxation. SPJNAL

NERVE

ROOTS

MR demonstrates nerve root avulsion following injury to the bra&al or rarely the lumbosacral plexus. These injuries usually follow severe traction to the shoulder with nerve root avulsion at their origin with the spinal cord. The diagnosis is suggested by demonstrating pseudomeningocoeles at the site of the root avulsion on heavily TZweighted coronal scans (Fig. 9a, b). Coronal MR myelography demonstrates the intrathecal nerve roots as they exit the cord and gives an image very similar to conventional myelography [16]. In our experience, MR myelography is as sensitive as conventional myelography in assessing nerve root avulsion.

VASCULAR

INJURY

Occult vertebral artery injury is fairly common in cervical trauma, with an incidence of 15-20% [17]. MR angiography has an accuracy comparable to conventional angiography in the detection of arterial occlusions and dissections [17]. It is important to routinely check the flow voids of both vertebral and carotid arteries on axial MR images, and to perform a 2D time-of-flight MR angiogram to confirm patency of these

THE ROLE OF MAGNETIC

RESONANCE

IMAGING

Fig. 7 - Sagittal Tl-weighted MR image of a patient who sustained a C6/7 Rexion injury 12 months previously demonstrates focal cord atrophy and a syrinx proximally (arrow).

arteries. Usually there is occlusion of the artery, less commonly dissection of the artery (Fig. lOa,b). There are usually no clinical signs of vertebrobasilar occlusion or insufficiency in most patients with unilateral occlusions. Usually the middle third of the artery is involved. Patients with bilateral vertebral occlusions have a poor prognosis with posterior fossa or brain stem infarction [17]. Carotid artery injury is far less common [ 181. Dissection is however more commonly being detected with the application of MR imaging [ 191. Usually common carotid artery dissection results in arterial occlusion [19]. Patients with unilateral arterial occlusions or dissections are at risk of delayed posterior circulation stroke from embolism [20]. Although no long term follow up is available from published series, these patients will require long term anticoagulation, which may be problematic in an injured patient [20]. Therefore the detection of these vascular lesions will alter long term management, requiring long term anticoagulation and follow up. PENETRATING

INJURY

MR is particularly useful in demonstrating focal cord injuries from penetrating trauma from gun shot and knife

injuries. Partial and complete spinal cord transection are well shown particularly when gradient echo images are used to confirm haemorrhage [21]. This has prognostic significance although such information is unlikely to change clinical management. POST-OPERATIVE

SPINE MR IMAGING

The post-operative spine presents unique challenges to the radiologist. Wires and fixation devices can cause ferromagnetic artefacts making imaging difficult [22]. The use of fast spinecho and spin-echo techniques as opposed to gradient echo or recalled sequences will minimise ferromagnetic artefacts. Patients can usually be examined safely with spinal fixation devices such as Harrington’s rods in situ [22]. Many spinal wires are now manufactured from titanium which is paramagnetic and has reduced the problem of ferromagnetic artefacts. The assessment of bony fusion using MR is difficult, as postoperative bone marrow and soft tissue oedema persist for many months. Fatty change in the marrow with an increased signal on Tl weighted images is probably the only reliable sign of complete fusion and bony stability [23].

CLINICAL

(b) Fig. 9 - (a, b) Coronal and axial T2-weighted MR images of a patient who sustained a traction injury to the right shoulder and brachial plexus demonstrates a right cervicothoracic traumatic pseudomeningocoele following nerve root avulsion (arrows).

RADIOLOGY

(b) Fig. IO - ((1. b) Axial TZ-weighted MR image of a patient who sustained blunt neck trauma demonstrates a dissected right distal vertebral artery (arrow). Compare this to the normal signal void of the opposite vertebral artery. A ZD time-of-flight angiogram confirms occlusion of the right vertebral artery.

THE ROLE

PREDICTING

OUTCOME

OF SPINAL

OF MAGNETIC

INJURY

There have been attempts to correlate the degree of recovery with the MR features [ 1S, 131. There are two findings that predict a poor recovery: the presence of intramedullary haematoma or haemorrhagic contusion and the longitudinal extent of the cord contusion [ I, 131. Gradient echo sequences greatly assist in the identification of cord haemorrhage and should be employed routinely [1,5,13].

CONCLUSION MR has a positive role in the early imaging of the patient with spinal trauma. However problems of emergency access to the MR unit, time delays, cost and monitoring problems have limited its potential value. Advances in MR technology, especially open magnets, will allow easier monitoring of the acutely spinal injured patient. However the low field strength of open magnets will result in reduced signal to noise and poorer spatial resolution of the images compared to closed high field units. We believe that MR imaging of the acute spine injured patient is worthwhile and the information obtained will assist the surgeon in early management of these patients.

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RESONANCE

IMAGING

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