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149

Review

MR Imaging Advances Gordon

of the Spinal

Cord:

Status

and Future

Sze1

The advent of MR imaging has dramatically altered the evaluation of suspected myelopathy. MR is far less invasive than traditional imaging techniques and often offers a degree of un-

derstanding

of an abnormality

achievements vances, such introduction

not previously

have closely followed recent as the development of contrast

of sequences

that

permit

a reduction

possible.

These

technologic agents and in both

adthe

imaging

time and artifacts. The current role of MR in the imaging of spinal cord disorders, including intramedullary tumors, infectious and inflammatory myehitis, demyehinating diseases, vascular lesions, trauma, synngomyehia/hydromyehia, and neurodegenerative disorders, is reviewed. While further improvements will undoubtedly occur, the field of spinal MR imaging appears to be entering a

maturing

Current

Article

Technique Technically, assessment of the spinal cord presents one of the greatest challenges in the use of MR to evaluate the neuraxis. Early images were markedly degraded by motion artifacts, especially from respiration, cardiac pulsation, and CSF pulsation [2, 3]. Several techniques have evolved to compensate for involuntary motion, and high-quality images of the For

cord

are now

the evaluation

intramedullary

lesion,

routinely.

of a suspected, both

short

and long TR (e.g., 2200/30,80)

and possibly

TR (e.g.,

sagittal

600/30

images

small, [TR/TE])

are essential.

Short TR sagittal sequences generally contain excellent morphohogic detail. Long TR sagittal sequences often show subtle

phase.

The advent of MR imaging has permitted direct visualization of the spinal cord for the first time [1 ]. Although the presence or absence of cord lesions could be inferred on myelography and postmyehographic CT by the shape of the surrounding contrast column, MR imaging allows specific assessment of the cord parenchyma itself. This review focuses on the MR imaging findings in diseases of the spinal cord, including tumors, cysts, demyehinating diseases, infections, vascular diseases, and trauma. The article concludes with a discussion of recent advances in the techniques available for MR imaging of the cord.

lesions that are not visualized on the short TR sagittal Sequences, and also provide further characterization of lesions. These acquisitions are best obtained with cardiac gating and/ or gradient-moment nulhing techniques in order to decrease the effect of flow artifacts, especially on high-field-strength units [2, 3]. For both short TR and long TR images, it is necessary to obtain sufficiently thin sections, preferably 3 mm or less with minimal interslice gaps. The use of a higher matrix improves resolution and helps to reduce the effect of the Gibbs artifact, which may produce a thin linear longitudinal line down the center of the cord that can be mistaken for a syrinx [4].

Received December 6, 1991 ; accepted after revision February 3, 1992. Presented at the annual meeting of the American Roentgen Ray Society, Boston, May 1991. I Department of Diagnostic Radiology, Yale University School of Medicine, 333 Cedar St. , P.O. Box 3333, G. Sze. AJR 159:149-159,

obtained

July 1992 0361 -803X/92/1

591/0149

© American

Roentgen

Ray Society

New Haven,

CT 0651 0. Address

reprint

requests

to

SZE

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150

Although much of the information in an MR study of the cord is obtained in the sagittal plane, axial images can also be vital. In general, if further morphologic detail is desired, axial short TA sections are usually acquired. If further characterization of lesions or if confirmation of a subtle lesion seen on the long TR sagittal images is desired, then short TR sequences are not optimal. Rather, gradient-echo (e.g., 800/ 35/20#{176} [TR/TE/flip angle]) or long TA axial sections may be necessary. In the cervical spine, volumetric gradient-echo sequences that allow one to obtain sections as thin as 1 mm are used. These sequences are very sensitive to subtle disturbances

of the cord parenchyma,

such

as multiple

sclerosis

(MS) plaques. Intramedullary

Tumors

The advent of MR has tremendously increased the ability to detect and characterize tumors of the cord. Short TA sagittal sequences readily disclose the marked cord widening often associated with intramedullary neoplasms. Long TA sagittal scans reveal high signal within the substance of the cord, consistent with either tumor or surrounding edema. Because of MR’s sensitivity to hemorrhage, areas of bleeding are easily detected. A number of different appearances may be seen. If hemorrhage into a tumor cyst occurs, fluid levels may result. Owing to the evolution of hemoglobin breakdown products

and to other

factors,

the relative

signal

intensities

of

the inferior and superior components may vary. In addition, hemorrhagic cord tumors are often associated with hemosidem deposition, which appears as peripheral marked hypointensity on long TA sequences. While unenhanced MR images generally can be used to detect tumors of the cord accurately, gadopentetate dimeglumine can help in further

characterization

and delineation

[5-

9]. Enhancement with gadopentetate dimeghumine is most useful in cases of focal masses, especially hemangioblastomas and metastases [6] (Fig. 1). Both of these lesions tend

Fig. 1.-Spinal A and B, Short

AJR:l59,

July 1992

to be fairly well circumscribed and produce extensive surrounding edema. Because of this, cord swelling far beyond the region of the actual tumor is often seen. The use of gadopentetate dimeghumine can be very effective in pinpointing the exact location of the lesion [5, 6]. Although the area of cord enlargement can be extensive, the actual lesion may be small, less than one vertebral body in height. Results with primary cord ghiomas are less dramatic. First, as in the brain, it is likely that areas of enhancement do not coincide with the actual boundaries of these infiltrative tumors. Second, unlike that seen with hemangioblastomas and metastases, enhancement is often variable. It does appear, however, that primary tumors tend to enhance in the cord much more often than in the head [6-9]. In ghiomas of the cord, enhancement again tends to be fairly focal [5-9] (Fig. 2). Areas of enhancement may be representative of more active tumor and good areas for biopsy, as is the case in the brain. Often, these tumors are associated with cysts. Cysts can be either intratumoral or rostral and caudal. Rostral and caudal cysts tend to be benign [1 0, 1 1]. Even though the fluid within them might be proteinaceous or hemorrhagic, these cysts are usually not lined by tumor nor do they contain tumor cells. Unlike tumor cysts, they do not require excision but are merely drained at surgery. Contrast enhancement has proved useful in identifying the nature of cysts associated with tumors [6-9]. Tumor cysts are generally surrounded by enhancement, while the walls of benign

cysts

lack associated

contrast

uptake.

Although 40-50% of ghiomas of the brain do not enhance, the large majority of ghiomas of the cord do, regardless of grade [5-9]. In fact, unenhancing ghiomas of the cord are unusual,

although

they

have

been

documented.

Even

very

how-grade tumors tend to show some enhancement. Of the 55 reported cases of cord ghioma in which contrast-enhanced MA was used, enhancement was seen in 54 [5-9, 12]. Because of this, the use of contrast material can be helpful in differentiating suspected neoplasms from other disorders, for

cord hemangioblastoma.

TR (600/30) and gradient-echo (800/35/20#{176}) sagittal MR images show an enlarged cervical cord with central edema. A small nodule Is noted along wall of cystic cavity posteriorly. C, Short TR (600/30) sagittal MR image after administration of contrast material shows enhancing nodule along posterior

cystic aspect

cavity

and surrounding

of cyst.

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AJR:159,

MA OF SPINAL

July 1992

Fig. 2.-Cord

MR image shows a diftusely expanded cord. In addition, area of cystic change is noted in upper portion of thoracic cord. MR image confirms superior cyst. Also noted Is area of heterogeneity within cord inferior to cyst. MR image after administration of contrast material shows marked enhancement of main tumor nidus, with rostral cyst.

example, infection or benign syrinx. If no enhancement is seen, the likelihood of a ghioma of the cord is significantly reduced. and Inflammatory

Myelitis

As with the brain, multiple infectious agents can affect the cord and cause myelopathic symptoms, including bacterial, granulomatous, viral, and even parasitic diseases. As a rule, however, inflammatory diseases that affect the cord tend to be seen earlier than those in the brain. Therefore, certain lesions that require time to develop, such as the well-formed abscess capsule, are comparatively unusual in the cord. Viral agents are probably the most common ones to directly involve the cord. Often, the exact virus may be difficult to determine. Regardless of the specific agent, the response of the cord parenchyma is stereotypical [1 3, 14]. An inflammatory cellular infiltrate, usually composed of lymphocytes and plasma cells, predominates, although neutrophils may be numerous early in the disease. Edema, both vasogenic and cytotoxic, occurs. Cellular destruction may be limited to mdividual neurons and small foci, or it may be extensive and fail to respect anatomic boundaries. Vessel thrombosis may lead to secondary ischemia. Short TA images generally reveal mild enlargement of the cord, often quite extensive. Long TA images show high intensity within the cord, often extending beyond the area that might be expected from the degree of cord swelling. The presence of a skip area separating multiple regions matory

151

astrocytoma.

A, Short TR (600/20) saglttal B, Long TR (2136/80) saglttal C, Short TR (600/20) sagittal

Infectious

CORD

of enlarged cord is strongly suggestive of an inflamcondition rather than tumor [1 5]. After the administra-

tion of contrast material, enhancement may be nodular and resemble that of tumor. Alternatively, enhancement may be diffuse, peripheral, or even punctate and speckled [15] (Fig. 3). All of these patterns of enhancement are unusual for tumor.

Granuhomatous diseases, such as tuberculosis or sarcoid, also affect the cord. In these cases, short and long TA images

generally show focal enlargement of a limited segment of the cord. After the administration of contrast material, nodular enhancement is usually seen [1 5]. This pattern strongly resembles that of tumor, and differentiation between the two may be difficult.

Demyehinating

Diseases

MS is the most common demyehinating disease to affect the cord. Eighty percent of cases involve the cord, and 2033% show symptoms referable to the cord alone [1 6]. Lesions may involve any region of the cord, regardless of anatomy, although the cervical cord is involved more often. Devic’s disease, also known as neuromyehitis optica, represents the association of transverse myehitis with acute visual loss due to optic neuritis. Pathologically, the typical regions of demyehination are seen [1 6]. In acute plaques, the axons are relatively preserved while the myehin is fragmented. These changes are often associated with microghial and perivascular hymphocytic infiltration. In chronic plaques, dense fibrillary ghiosis is typical. In severe cases, atrophy is common. The majority of patients with cord lesions documented with the use of MR imaging will also have brain lesions, occurred at different times. amination may be performed

although these lesions may have Therefore, a screening head exin lieu of a spinal examination if

the diagnosis is in doubt [1 7]. However, if evaluation of the cord is desired, MS plaques in the cord can be detected. Good-quality cardiac-gated long TA images, with flow-cornpensation techniques, are essential for the visualization of cord plaques in the sagittal plane. In the axial plane, gradientecho images are often very sensitive in showing MS plaques that disrupt the otherwise easily visualized border between gray and white matter in the cord. As in the head, MS plaques generally appear of high intensity compared with the remainder of the cord on long TA or gradient-echo images. Large and acute plaques may have

Fig. 3.-Inflammatory myelitis. A, Swollen areas of spinal cord show diffuse high signal on long TR (2200/80) MR image. B, After contrast administration, long TR (2200/

80) MR image shows diffuse enhancement

with

unenhancing

that

skip area.

This pattern

is unlike

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usually seen in tumor.

mass effect and resemble tumors. Acute plaques generally enhance after the administration of contrast material while chronic plaques do not [1 8] (Fig. 4). While the natural history of plaques in the brain has been described both without and with treatment, this research has not been done in the spine. Presumably, acute cord plaques also enhance for several weeks initially before repair of the blood-brain barrier takes place. Radiation change is another entity that may cause myelopathy. The development of myehopathy is dependent on the dose of radiation received, the interval at which the radiation was administered, and the number of fractions [1 9-23]. Radiation myelopathy may be divided into acute and chronic forms. The acute form is best known as the transient myelopathy of Lhermitte and is manifested by shockhike sensations down the back when flexing the neck. It usually occurs 3 months after radiation treatment and resolves spontaneously. Findings on MA imaging are normal. Chronic radiation myehopathy has been described in three forms [1 9-23]. First, lower motor neuron disease can occur as a result of injury to the anterior horn cells. Second, acute paraplegia due to radiation-induced vascular changes and secondary cord ischemia has been seen. Third, and by far the most common, is chronic progressive radiation myelopathy. It primarily involves the lateral columns. These patients have sensory or motor changes and frequently Brown-Sequard syndrome. Symptoms are often slowly progressive and may result in death. The cause of these changes is unclear, but they may result from either radiation-induced vascular change or primary neuronal damage. Chronic radiation myehopathy of all types usually occurs approximately 1 year after

radiation therapy and has been described as late as 13 years after irradiation. As with MS, radiation myelopathy may have a variety of appearances on MR [24]. When it is fulminant, chronic progressive radiation myelopathy often can be seen as an area of focal cord swelling on short TA images with associated high intensity on long TA images. In particularly severe cases, contrast enhancement can occur [24]. The appearance can markedly resemble that of tumor. In the late stage of chronic radiation myehopathy, the cord may appear totally normal on both short TA and long TA images or may even appear atrophic.

Acute disseminated encephahomyehitis, also known as postvaccinal or postinfectious encephalomyehitis, typically occurs several weeks after an antecedent innoculation or viral syndrome. Patients have focal lesions, often in the white matter; as in MS, these lesions appear random in distribution [16]. Pathologically, although the syndrome is associated with previous infection, no infectious agents can be found. Most often, one sees a perivascular lymphocytic infiltration, associated with areas of demyehmnation that follow the course of the vessels. In severe cases, hemorrhage may occur. The reaction is presumed to be immune in nature, possibly owing to antigen-antibody

deposition,

complement-mediated

endothe-

hal damage, and vascuhitis. The brain is most often involved, but a very small proportion of cases involve the cord. Imaging findings, in general, resemble those seen in the acute phase of MS. Short TA images may show a normal cord or a slightly enlarged cord. Long TA images show corresponding

areas

severe

enhancement

cases,

of high signal

intensity

(Fig. 5). In more

may be seen. Since acute

dis-

seminated encephahomyelitis is a monophasic disease, unlike MS, it has been speculated that the enhancement pattern should also be monophasic, unlike MS, in which different lesions may enhance, subside, and reenhance at different intervals.

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Vascular

Lesions

Vascular lesions of the cord can be separated into infarcts and malformations. Cord infarcts generally present acutely. Although they may have a wide array of causes, they are most often associated with surgery for aortic aneurysms [25]. As in the head, mass effect on short TA images and high signal on long TA images is generally seen. In the subacute phase, contrast enhancement, generally diffuse in nature, can be demonstrated. Presumably, the time course of enhancement reflects that seen in brain infarcts, although there is a hack of research on this topic. Early exaggerated enhancement due to incomplete ischemia has not been described in the cord. Many types of malformations may involve the cord, including intramedullary arteriovenous malformations (AVM5), cayernous hemangiomas, and extramedullary vascular malformations [26]. Intrameduhhary AVM5 generally occur in young adults, are fed most often by the anterior spinal artery, and present acutely as cord hemorrhage or as subarachnoid hemorrhage

[26].

The

thoracic

cord

is involved

most

often.

Short TA and long TA images may reveal a normal or swollen cord, often with areas of hemorrhage. Serpiginous signal voids representing the nidus are seen in the cord, consistent with the actual malformation [27, 28] (Fig. 6). Large serpiginous signal voids may also be present in the subarachnoid space owing to feeding and draining vessels. Atrophy or syrinx formation can be caused by the AVM. Hemosiderin deposition is usually present, indicative of previous hemorrhage. Cobb syndrome is a metameric angiomatosis associated with an intramedullary AVM, an epidural AVM, vertebral or superficial angiomas, or any combination of these abnormahities. Extramedullary arteriovenous fistuhas, also known as dural arteriovenous fistulas or as radiculomeningeal fistuhas, generahhy occur in older patients, especially men, and are due to a fistula between a dural branch of a radicuhar artery and the perirnedullary veins [26]. These fistuhas are generally fed by the posterior circulation, and symptoms are usually those of

Fig. 4.-Multiple

sclerosis.

A, Long TR (2235/30) sagittal MR image shows high intensity in cord over a short segment. B, Short TR (500/12) saglttal MR Image after administration of contrast material shows en-

hancement.

cord claudication or a progressive myeloradiculopathy, often a conus medullaris syndrome. These lesions can be very difficult to visualize on MA (Fig. 7). Conus enlargement and intramedullary

high signal

on long TA images

may be present.

surface of the cord may be caused by the enlarged perimedullary veins. If gadopentetate dimeglumine is given, punctate enhancement of the posteriorly draining medullary veins can be seen along the surface of the cord in about one fourth of cases [29, 30] (Fig. 7). On delayed scans, diffuse cord enhancement has been reported, possibly due to venous engorgement and secondary ischemia. Finally, cryptic vascular malformations of the cord may occur, as in the head; the most common of these, cavernous hemangiomas, are seen with increasing frequency [31]. Their appearance in the cord is similar to their appearance in the head (Fig. 8). Generally, these lesions appear either as a central focus of high signal intensity, surrounded by a lowintensity rim, due to the breakdown products of blood, or, in smaller lesions, as how intensity due to hemosiderin deposiScalloping

tion.

Mass

In general,

of the posterior

effect

is unusual

very little contrast

unless

they

enhancement

have

recently

bled.

is seen.

Trauma

Pathologic changes that occur in the injured spinal cord can be divided into acute and chronic [32]. Acutely, the trauma can result in a wide spectrum of iniurv, ranging from mild cord contusion to severe disruption of cord architecture and hemorrhage. Secondary edema can head to vascular compromise of adjacent areas. Most of these changes occur in the first few days following trauma. After the first week, the subacute phase begins, as the body attempts to heal from the results of the injury. Macroscopically, edema and the smaller areas of hemorrhage are resorbed. The acute infiltrate is replaced by mononuclear cells. Astrocytic ghiosis may also be seen at the periphery of the injury. Chronically, fibrous tissue fills in the regions of necrosis and hemorrhage. Secondary atrophy of the cord may be present. In the acute setting, MA imaging permits the visualization of changes in the traumatized cord directly as long as the patient is sufficiently stable and cooperative to be imaged [33, 34]. Imaging may be particularly important when the trauma is incomplete, that is, when some motor and/or sensory function below the level of the injury persists [32]. In these cases, aggressive treatment may be indicated in order to

154

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AJR:159,

Fig. 5.-Acute

disseminated

July 1992

encephalomyeli-

tis.

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A, Long TR (1333/80) sagittal MR image shows focal high intensity at C6-C7. B, Short TR (600/30) sagittal MR image after administration of contrast material shows focal

and marked enhancement

prevent further deterioration. These incomplete injuries may be characterized by varying clinical characteristics. The central cord syndrome is often seen in elderly patients with cervical spinal stenosis, in which even minor trauma can result in hemorrhage into the central gray matter of the cord. These patients have decreased function in the upper extremities but relatively normal function in the lower extremities. Other incomplete cord injuries may present as Brown-Squard syndrome, anterior cord syndrome, or mixed motor and sensory deficits. The MA characteristics of the acutely injured spinal cord reflect the clinical symptoms. If the injury is mild, with only temporary neurologic dysfunction, MA imaging may show a normal cord. With increasing cord injury, short TA images may be normal or may reveal cord swelling [32]. The long TA

A Fig. 6.-Arteriovenous malformation of cord. Short TR saglttah MR image shows marked serpiginous sig. nal voids permeating substance of cord, especially visible

(600/20)

at Cl and on axial image (not shown). In addition, marked large serpiginous signal voids are present along cord periphery superiorly, consistent with large draining veins.

of lesion.

images, however, are the most sensitive in showing cord edema, which is seen as areas of high signal intensity within the substance of the cord (Fig. 9). Areas of hypointensity on both short TA and long TR images, with hater evolution to hyperintensity,

peripheral

at first,

are typical

of hemorrhagic

regions.

The presence of hemorrhage has been associated with a poor prognosis [33]. Also important to note are any retropulsed bone fragments or disk fragments that may impinge on the cord. Rarely, rapidly ascending spinal necrosis can result from thrombosis of the anterior spinal artery or other interruption in the blood supply. In cases of penetrating injury of the spinal cord, such as in knife or gunshot wounds, actual transection of the cord may be visualized. In the chronic stage, an imaging examination may be mdicated if new or progressive signs of neurohogic dysfunction

B

Fig. 7.-Dural arteriovenous fistula. A, Gradient-echo (367/15/15#{176}) sagittal MR image of thoracic ing of posterior surface of cord. B, Short TR (550/20) sagittal MR image after administration subtie punctate enhancement of some focal and serpiginous cord.

spine

shows

subtle

scallop-

of contrast material shows hypointensities posterior to

AJR:159,

MA OF SPINAL

July 1992

CORD

155

Fig. 8.-cryptic vascular malformation of distal thoracic cord in a patient with long-standing fluctuating conus symptoms. A, ShortTR(600/20)sagittal MR Image obtained during a period of exacerbation shows focal high

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signal intensity in conus, consistent with evolving products

of hemorrhage.

Also

seen

are

areas

of

mixed increased and decreased signal intensity extending superiorly, representing hemorrhage at other stages of evolution and extension of the bleeding. B, Long

TR (2368/70)

sagittal

MR image

again

shows multiple areas of mixed signal intensity within cord, consistent with evolving hemorrhage. In this case, because of acute nature of hemorrhage, high signal intensity is also noted in surrounding cord, probably because of edema.

[32, 35]. In these cases, the development of intramedullary cysts, myelomalacia, or adhesions may be treated. Areas of cord injury or transection are well demonstrated (Fig. 1 0). Myelomalacia is generally visualized as a poorly defined area of how intensity within the cord on short TR images [32]. Long TA images disclose high intensity in the corresponding occur

regions.

Progression

to a well-defined

cyst

can be detected

on MA by the presence of a well-defined low-intensity region on short TA images, with a sharp interface between the surrounding normal cord and the cyst itself. The cyst contents appear uniformly hypointense or may show evidence of flow, unlike in myehomalacia. If the lesion is extensive, then a cyst is more likely, since myelomalacia is characterized by relatively short segments of involvement. Clearly, the differentation is clinically important, since myehomahacia cannot be treated with shunting. prominent

Those are

cysts

in which flow artifacts appear especially good candidates for treatment,

particularly

since active pulsations further extension.

within

the cyst

may predispose

to

normal

cord

and the syrinx

itself.

Metameric

haustrations

are

Chiari-associated hydromyehia. The syrinx itself is often well defined and smooth, but occasionally septa are detected. If arachnoiditis is present, extensive adhesions may typical

of

blur the interface

between

the cord and the surrounding

[36]. If the cyst shows any suspicious features loss of definition in a focal region, then contrast is indicated

to ensure

Neurodegenerative

that a neoplasm

Disorders

CSF

at all, such as enhancement

is less likely.

of the Spinal

Cord

A small number of neurodegenerative disorders involve the cord [13]. Different disorders involve different tracts, and involvement of structures other than the cord, such as the cerebellum or other parts of the brain, is frequently associated. The pathogenesis of most of these disorders is unknown. Pathologically, these diseases are typified little inflammatory reaction and by progressive,

by very

symmetric

breakdown

of cells, usually neurons. ataxia is believed to be due to a mutation on chromosome 9 and occurs in both autosomal recessive and Friedreich’s

Syringomyehia/Hydromyehia

dominant

Although cord cysts may be commonly associated with trauma or neoplasm, true syringomyehia or hydromyehia is usually found related to Chiari malformation or is idiopathic in origin [1 3]. Hydromyehia is due to congenital dilatation of the central canal. Syringomyehia represents a cavity in the cord itself. The pathogenesis is highly debatable. The dysraphic theory suggests that a closure defect of the neural tube at the level of the posterior raphe is responsible. The hydrodynamic theory maintains that disturbances of CSF outflow from the fourth ventricle heads to hydromyehia, followed by syringomyelia as a result of rupture of the dilated ependymal canal. Occasionally,

syrinxes

can also occur

as a result

of arachnoid

adhesions with secondary alteration in CSF flow [36-38]. These cases typically involve the thoracic region, unlike other syrinxes, which are usually located in the cervical cord. Symptoms depend on the location of the syrinx. Pathologically, syrinxes may be lined by fibrous astrocytosis. On MA, short TA images accurately show the morphology of the cyst. Typically, there is a sharp interface between the

inheritance

patterns

[39].

Patients

olescents or young adults and present picture of gait ataxia, upper extremity

with

are typically a mixed

ad-

clinical

clumsiness, and dysarloss and ghiosis focused on the posterior columns, spinocerebellar tract, and corticospinal tract are seen, with secondary atrophy. MR imaging reveals an atrophic spinal cord with normal signal intensity [40]. While Friedreich’s ataxia is the prototypical progressive ataxia, several other degenerative disorders of the cord can also occur. These include hereditary spastic paraplegia (Strumpelh-Lorain disease) and hereditary posterior column ataxia (Biemond syndrome) [41]. Other syndromes may involve the brain, especially the cerebellum, as well as the cord and include ohivopontocerebellar atrophy and Shy-Drager syndrome. Other types of neurodegenerative disorders primarily involve the motor neurons. Of these, the prototype is amyotrophic lateral sclerosis. Although most cases are sporadic, thria. Pathologically,

occurring

autosomal

in elderly

dominant

myelin

patients,

fashion

the disease

is transmitted

in approximately

in an

5% of cases

156

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AJR:159,

July 1992

Fig. 9.-Posttraumatic cord contusion in patient with bullet wound to soft tissues of neck and

secondary quadriparesis. Long TR (2308/80) sagthaI MR image shows diffuse high intensity in

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enlarged

region

of cord.

Fig. 10.-chronic disruption of cord after direct stab wound. 5hort TR (600/20) sagittal MR image shows disruption of distal thoracic cord with significant atrophy both above and below area of trauma.

[42]. distal ties, fects

The clinical triad consists of atrophic weakness of the upper extremities, mild spasticity of the lower extremiand generalized hyperreflexia. Innervation atrophy afthe muscles.

Pathologically,

degeneration

of the corticospinal

tract

and

lower motor neurons is seen [1 3, 14]. Because corticospinal tract degeneration can extend superiorly to involve the cerebral peduncles, posterior portion of the posterior limb of the internal capsule, and corona radiata, MR reports have focused on the hyperintense foci seen on long TA sequences in the brain. However, the cord may also appear to be atrophic and flattened in its anterior and lateral portions owing to loss of motor

neurons

in the

anterior

horns

in corticospinal

amyotrophy

of the

distal

upper

extremity,

also

known as monomehic amyotrophy or benign focal amyotrophy. Characteristic unilateral muscular atrophy in the distal upper extremity due to selective anterior horn cell involvement in the lower cervical cord is seen. Unlike amyotrophic lateral sclerosis, this disease usually occurs in boys and stabilizes after a few years. Focal atrophy of the cervical cord, often limited to the anterior horn cell region, has been described [43].

Miscellaneous

Clinically, most metabolic and toxic disorders affect the brain far more than the cord. A very few, however, target the cord. Subacute combined degeneration of the cord occurs in patients with pernicious anemia due to vitamin B12 deficiency, but

has

also

been

associated

with

many

other

malnutritive

and malabsorptive conditions [1 3, 14]. Pathologically, degeneration of myelin results in demyehination of the white matter, especially

the dorsal

Subacute nancy,

usually

and lateral

and axons

syndrome

Recent

Although

usually

transverse

Advances

the cord

been

well

including

technology,

continued

and

new

several

The

pulse

and other

lesions

recent

technohogic

sequences,

methods

of

phased-array

of image

the scope and versatility

to expand

cord.

of the Cord

of tumors

studied,

new

in the thoracic

myehitis.

in MR Imaging

the appearance has

innovations,

coil

occurs,

resembles

display,

have

of MR imaging

of the cord.

tracts.

Hyperintense foci in the cord have not been reported. Other more unusual forms of motor neuron disease have been described clinically but not by imaging. One exception is juvenile

sheaths

columns.

necrotizing myelopathy accompanies mahiga lung carcinoma [1 3, 14]. Necrosis of myelin

New Pulse Sequences

Often, plaques,

cord lesions, especially subtle lesions such as MS are first detected with long TA images. Unfortunately, long TA images of the cord are often time-consuming and marred by motion artifacts. Aecently, fast spin-echo (FSE) techniques have been developed that promise to provide images with a sensitivity to pathology comparable to that of long TA images, but in a much shorter time. These techniques are all variants of the original RARE (rapid acquisition with relaxation enhancement) sequence devised by Hennig et al. in 1 986 [44-47]. Modification of the original sequence allows adaptation for clinical use [48, 49]. Although many variations exist, all of the FSE sequences consist of a single 90#{176} pulse, followed by multiple 180#{176} refocusing pulses. Each refocusing pulse is associated with its own

phase-encoding

gradient.

The TE occurs

lowest-order phase encodes, responsible placed. The length of the echo train is crucial time

of the acquisition,

contrast, typical

length

and

the

number

the presence

contributes

to the

number

of

of certain

the

the

are

in determining

of sections,

or absence

the FSE sequences, of the interecho spacing of

where

for contrast,

the image

artifacts

such as blurring [49]. The is also important and similarly sections,

the

image

contrast,

AJR:l59,

MR OF SPINAL

July 1992

and the presence

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acquisitions,

of certain artifacts.

new parameters,

Thus, in specifying

such as the length

FSE

of the echo

train and the interecho spacing, become important in addition to the familiar TA, TE, matrix size, and number of excitations. FSE images strikingly resemble the routine long TA spinecho images (Fig. 1 1). One crucial difference involves the persistent

high

signal

intensity

of lipid on FSE images,

even

when parameters apparently similar to those of routine spin echo are used. This persistent high signal intensity of lipid has been explained by the homonuclear coupling interaction of adjacent

protons

[49].

Early

reports

indicate

that the FSE

sequences are as sensitive in the detection of intramedullary disease as routine spin-echo sequences are [50]. MR imaging of the spine is characterized by long acquisition times and susceptibility to motion artifacts, factors that are less of a problem in brain imaging. Therefore, FSE images offer tremendous potential in the spine. Early reports on the evaluation of extradurah disease suggest that FSE sequences are equivahent to gradient-echo sequences in the sagittal plane but are still inferior

in the axial plane

The rapidity dimensional

(Sze G, unpublished

of the FSE sequences

volumetric

approach

a volumetric acquisition where N is the number phase-encoding

possible.

Since

a three-

the time

of

is equal to (TA x N x S x S)/P, of excitations, S is the number of

steps,

and P is the number

data).

also makes

S is the number

of slice select

of phase-encoding

steps

steps,

per TA, in

routine spin echo, a volumetric acquisition would be prohibitive in the time required. However, with the FSE sequences,

the volumetric acquisition can be obtained in a reasonable length of time [51 ]. Preliminary clinical evidence indicates that these sequences are also sensitive in the detection of intramedullary

lesions.

A small number sequences:

effect

of disadvantages

(1) FSE sequences

of these

new

artifacts.

The

can be minimized with optimization of all the parameters. (2) The number of sections that can be obtained in a given acquisition is less than that of routine spin-echo imaging for a given TR. In MR imaging of the spine, the limited number of sections is not a clinical problem. (3) There is a theoretical loss of small object definition. Preliminary reports indicate that in the spine, detection of even small lesions, such as MS plaques, has not been compromised. The theoretical loss of small object definition applies to extremely small objects (two pixels), which also have short T2 relaxation times, a phenomenon very different from the majority of cord lesions. (4) Motion compensation techniques, such as gradient-moment nulhing, are more difficult to apply in FSE imaging owing to the very short interecho spacing. Nevertheless, gradient-moment nulhing can be applied to FSE imaging with slight limitations of other parameters, such as interecho spacing or TE.

Phased-Array Increasingly,

artifacts,

do exist with the FSE

introduce

however,

Coils clinicians

request

scans

of

the

entire

cord.

Imaging of the length of the cord may be indicated to assess a diffuse process, such as tumor, MS, or inflammatory or infectious disease, or in cases of myelopathy in which exact localization is difficult. Both unenhanced and enhanced scans

157

CORD

may be necessary.

These clinical demands

capability in spinal must be performed

increased

sequences

head to a need for

MA imaging. more rapidly

Either individual or large portions

of the cord must be imaged on one acquisition, imaging

limiting patient

time.

In the past, only one surface coil could be used at a time, permitting only a limited field of view (FOV) in each acquisition [52, 53]. Increasing the FOV invariably led to decreased

signal-to-noise

ratio

(SNR),

particularly

at the extremes

near

the end of the coil. Increasing the size of the surface coil reduced the advantages of the surface coil and decreased SNA. A single surface coil, therefore, could not always meet clinical

demands.

With phased-array coils, several surface coils can be used together, combining the benefits of the small surface coil with the large FOV. These surface coils are arranged in a linear fashion. Coupled together to act as one, they markedly increase the length of the FOV, up to 48 cm (Fig. 12). The advantages of the phased-array coils are thus twofold: a larger FOV and increased SNA. With phased-array coils, one can routinely image the entire cord in a single acquisition (Fig. 12). The

multiple

surface

of receivers and separate receiver. processed at this

boards

coils

are matched

by an equal

number

memory boards. Each coil feeds into a The information from a particular coil is receiver and stored in one of the memory

processing recoils is thus processed and stored. Because each receiver only processes signal from one specific coil, the noise heard by a particular receiver is limited to that transferred by a single surface coil. This allows for less noise coupling at the level of the receiver, ceiver.

resulting

associated

The

information

in increased

and memory

boards

recombination

type

solely with the particular from

SNA.

the

multiple

surface

The information

is then coupled

from

together

all receivers

by using

a

of algorithm.

Because of their advantages, phased-array coils will likely the most versatile of all coils for routine scanning of the spine. If a large FOV is required, such as a scan of the entire prove

cord to evaluate an extensive syrinx, phased-array coils clearly the method of choice. However, if only a limited is required, phased-array coils still can be used because provide better SNR and resolution than a single surface Phased-array coils, therefore, are extremely versatile in

are FOV they coil. their

capabilities.

Image

Display

Significant advances have been made recently in the field of image display. The development of three-dimensional reformations promises to be one of the most important advances in MA imaging of the cord [54]. The ability to acquire very thin sections with gradient-echo sequences has permitted data acquired in one plane to be reconstructed and displayed in other planes. If isotropic voxels are obtained, minimal distortion or artifact exists in the reconstructions. Three-dimensional reconstructions can be combined with enhancement when spoiled

with

gadopentetate

dimeglumine,

gradient-echo sequences that display tion in a “Ti -weighted” fashion are used. One major

particularly

informaquestion

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158

SZE

Fig. 1 1.-Multiple

AJR:l59,

sclerosis.

TR (2188/80) cardlac-gated sagittal MR image of cervical spine, obtained sec, shows a small high-intensity lesion consistent with multiple sclerosis plaque. B, Fast spin-echo (2000/112) MR image obtained in 3 mm 12 sec also shows lesion. A, Long

Fig. 12.-Syrinx. Short TR (500/12) sagittal MR image with phased-array area of how signal intensity within cord, consistent with a syrinx.

still must be addressed, namely, the sensitivity of spoiled gradient-echo sequences to contrast enhancement. Although spoiled gradient-echo sequences appear markedly Ti weighted, it has been noted that these sequences do not appear to show contrast enhancement after the administration of gadopentetate dimeglumine as clearly as short TA spin-echo sequences do (Augenstein HM et al., presented at the Radiological Society of North America meeting, November 1990; Rand SD et al., presented at the American Society of Neuroradiology meeting, June 1991). Cine MA is another recent advance in the assessment of the cord and may be used to assess the pulsatihity of cord cysts and to evaluate cord motion itself [55, 56]. As mentioned, cord cysts that are pulsatihe are often considered better candidates for surgery, since persistent pulsatihity may be associated with cyst expansion. These changes are optimally

imaged

technique, readily

with

displayed

differentiated

July 1992

the

use

of a cardiac-gated

in cine. from

true

In addition, cyst

gradient-echo

myelomalacia can be formation, since it fails to

in 12 mm 48

coils shows small focal

display pulsatihity or turbulence. Use also facilitate understanding of the Although it has been suggested that connected to the fourth ventricle by flow

void

from

the

ventricle

to

the

of this technique may origin of syringomyehia. congenital

the obex, syrinx

syrinxes

are

a continuous has not been

observed. Cord motion can be assessed with similar techniques. In these studies, a cardiac-gated spin-echo pulse sequence can be used to obtain phase information [56]. Peak caudal velocity of the cord has been noted to occur in early systole and to be 12.4 mm/sec. These velocities are decreased in patients with tethered cord or spinal cord compression, but not in those

with

intrinsic

cord

tumors.

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MR imaging of the spinal cord: current status and future advances.

The advent of MR imaging has dramatically altered the evaluation of suspected myelopathy. MR is far less invasive than traditional imaging techniques ...
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