485

Multiplanar Metrizamide­ Enhanced CT Imaging of the Foramen Magnum

David L. LaM asters 1 Definition of masses and malformations at the foramen magnum has been less than Ted J. Watanabe2 optimal with traditional radiographic techniques. The use of intrathecal metrizamide Evan F. Chambers3 with computed tomographic (CT) scanning improves contrast resolution and facilitates David Norman3 detection of abnormalities that may not be apparent on Pantopaque cisternography, Thomas H. Newton3 plain films, or conventional axial CT alone. Fifty patients with clinically suspected foramen-magnum lesions were evaluated with this technique. Forty were abnormal with the following findings: 14 cases of the Chiari malformation (five cases had associated syringomyelia), five meningiomas, three neuromas, a chondrosarcoma, a chordoma, and five cervical and three brainstem gliomas. Six cases of syringomyelia, a large posterior-fossa subarachnoid cyst, and an anomaly of the cisterna magna were also diagnosed. Sagittal and coronal images were beneficial in defining tonsillar position, configuration of the fourth ventricle, and the relations of mass lesions to the spinal canal and subarachnoid space. Metrizamide CT cisternography provides superior spatial and contrast resolution of lesions at the foramen magnum not obtainable with other radiographic techniques. Morbidity is minimal because of the low dose of metrizamide used. Multiplanar reformations are particularly helpful in assessing ana­ tomic detail not readily recognized on axial scans alone.

Evaluation of lesions at the craniovertebral junction is difficult, both cli nically and radiographically. The " missed " foramen magnum tumor, in particular, has received significant emphasis in the medical li terature [1, 2]. The delay in diagnosis of abnormalities in this location often arises because of a nonspecific or unusual clinical presentation [3]. However, even if the diagnosis of a lesion at the foramen magnum is suspected, it may be missed if the myelographic or computed tomographic (CT) evalu ation is inadequate [4, 5]. Metrizamide CT cisternography improves the diagnostic accuracy and simpli­ fies the radiographic workup of pathology at the foramen magnum. When thi s procedure is combined with CT reformation , the nature, extent, and definition of masses or congenital abnormalities at the craniovertebral junction may be ana­ lyzed in more detail. We believe th at multiplanar reconstructi on with metrizamide CT cisternography obviates myelography in this region. Received October 9, 1981; accepted after re­ vision Apri l 6, 1982. Subjects and Methods 1 Department of Rad iology, Wilford Hall , U.S. A.F. Medical Center, Lac kl and AFB, San An­ tonio , TX 78236. Fifty consecutive patients with symptoms referable to the foramen magnum region were studied using the technique of combined CT scanning and metri zamide cistern ography. 2 Department of Radiology, Kuakini Medical Symptoms in this group in clu ded suboccipital headaches, dysesthesias of the upper Center, Honolulu, HI 96817. extremities, gait disturbance, dizziness, and astereognosis. Ph ysical findings included 3 Departm ent of Rad iology, University of Cali­ upper and lower extremity weakness, hem iparesis, tetraparesis, spasti city, and ataxia. fornia School of Medicine, San Franc isco, CA 94143. Address reprint req uests to T. H. Newto n. Although plain films, tomography, angiography, and conve ntional axial CT had been performed in many in stances, these were oft en non diagnosti c or inconclusive. AJNR 3:485-494, September/ October 1982 Metrizamide CT cisternography was perform ed either as a primary CT study or as an 0195-6108/ 82/ 0305-0485 $00.00 © American Roentgen Ray Society adjunct to metrizamide myelography. In the primary metrizamide CT study, 4-5 ml of 170 486 LaM ASTERS ET AL. AJNR:3, September j Oc tober 1982

TABLE 1: Clinical Entities at Foramen Magnum Satisfactory reformatted images were obtained in 46 of the 50 patients. The rest were degraded by patient motion Abnormal Findings No. during and between axial scans, teeth fillings producing Extraaxial masses: registration artifacts, and residual Pantopaque from prior Meningioma ...... 5 Neurinoma (C2 and cranial nerve XII) 3 myelograms or cisternograms. Reformations were assessed Chondrosarcoma ...... in each case to determine the size of the foramen magnum, Chordoma th e contour of the cord and brainstem, the position of the Intraaxial masses: cerebellar tonsils, and the presence of mass or congenital Cervical glioma 5 Brainstem gli oma 3 lesions. Congenital malformations: Ch iari I ' 8 Ch iari II 6 Normal Anatomy: Reformatted Images Syringomyeli a-r: Traumatic syrinx 1 The anatomy of the craniovertebral junction on axial CT Syringomyeli a 5 cisternograms has been described by several authors Miscellaneous: [6-9]. Multiplanar reformations are less familiar and will be Subarachnoid cyst briefly discussed. Anomalou s cistern a magna Midline structures such as the upper cervical cord, me­ Total 40 dulla oblongata, fourth ventricle, and cerebellar vermis are • One case o f Chiari 1 was associated wit h Kli ppel-Feil and basil ar invagination. best assessed on a median sagittal reformation (fig. 1 A) . t Fi ve additional cases of syrin x were detected in association with th e Chiari malfor­ mati ons. The clivus in its relation to the atlas and axis is apparent. The fourth ventricle is identified between the pons and the vermis. The position of the fourth ventricle may be assessed from the midsagittal reformatted image using the method of mgl ml I metrizamid e was injected into the lumbar subarachnoid Twining (fig. 1 B) [1 0]. In the 10 normal studies, the midpoint space using a 22 gauge spin al needle. Th e pati ent was th en titled of Twining's line was found to lie at or slightly anterior to the 45° head-down in the prone position for 3 min to facilitate diffusion of the contrast agent into the upper cervical subarachnoid space floor of the fourth ventricle. This differs minimally from and c istern s of the posterior fossa. pneumoencephalographic measurements, where the mid­ When metrizamide CT cisternography is performed after myelog­ point of Twining 's line falls at or posterior to the floor of the raphy, the CT scans are obtained about 4-6 hr after the primary fourth ventricle. That position is most likely positional, as myelographic study. This delay prevents untoward side effects. Th e pneumoencephalography requires assessment of the fourth concentrati on of the contrast agent at 4-6 hr remains high enough ventricle with the patient semi prone while in CT cisternog­ to allow an adequate examination of this region. raphy the patient is supine. All 50 patients were scanned on a G.E. 8800 CT I T scanner with Parasagittal images are used to determine the position of at 120 kVp, 320 mA, and a scan speed of 9.6 sec. Axial scans the cerebellar tonsils (fig . 2). This particular reformation was were obtained in a plane parallel to the infraorbitomeatal line. most beneficial in assessing the Chiari type I malformation Collimation of 5 mm with a table motion of 3 mm increments (2 mm overl ap) was used. The patients were scanned from the level of th e and the degree of tonsillar herniation. The vertebral arteries fourth ventricle to the upper cervical reg ion , usuall y to the level of may be seen in the parasagittal plane as they pass around C2, resulting in about 20- 25 axial scans. If an enlarged brainstem the lateral aspect of the medulla. or cervical cord had been detected on th e initial axial scans, the The axis of the brainstem is obliquely oriented to the pati ent was rescanned about 4 hr later in an attempt to demonstrate orbitomeatal plane (usually at 75°). An oblique coronal filling of a central cavity. image generated from a sagittal reformation permits a sec­ After completion of the axial scan, reformatted images were ond view of the entire neural axis from the upper cervical generated using th e GEDIS (General Electric Medical Systems cord through the cerebral peduncles (fig . 3). The neural axis Division) 6.2 1 software package. The planes to be reformatted is narrowest at the cervicomedullary junction. It abruptly depended on the particular clinical problem. Using the GEDIS widens at the level of the pons where the middle cerebellar software package, up to four axial images may be displayed simul­ peduncles diverge laterally into the cerebellar hemispheres. taneously to select the appropriate planes for subsequent sagittal, coronal, paraaxial, or oblique reformations. A trackball cursor sys­ Coronal reformations perpendicular to the orbitomeatal tem was used to designate the appropri ate reformation plane on plane were used with the parasagittal reformations to assess the axial image. Th e software protocol then generates a specified the shape and position of the cerebellar tonsils (fig. 4). The plane from the stored axial data. tonsils normally have a rounded contour and appear as discrete structures posterior to and separate from the lower medulla. While the tonsils may reach the level of the foramen Results magnum, extension below the outer table is abnormal [11]. The axis of the foramen magnum often does not lie in the Of the 50 patients studied using the technique of metri­ orbitomeatal plane. A tangential paraaxial reformation over­ zamide CT cisternography, 40 were abnormal (table 1). The comes this difficulty and allows the foramen magnum to be lesions were divided into extraaxial masses, intramedullary seen in its entirety (fig. 5). The foramen magnum is ellipsoid, tumors, syringomyeli a-syringobulbia, and congenital malfor­ measuring about 3.2-3.6 cm anteroposteriorly and 2.5-3.4 mations. cm from side to side [12]. AJNR :3, September I October 1982 METRIZAMIDE CT IMAGING OF FORAMEN MAGNUM 487

A B

Fig . 1.-A, Midline sagittal reformation. Neural axis and cerebell ar vermis raphy, midpoin t of Twining 's line lies at or slightly anterior to fl oor of fourth defined. B, Position of fourth ventri cle may be assessed by method of ven tricle (vertical white bar). Twining, using midline sagittal reformati on. With metrizamide CT cisternog-

tion. Cerebrospinal fluid (CSF) protein was 400 mg / dl. Initial Pan­ topaque myelography was nondiag nosti c , as the injecti on was partially subdural. Intravenous-contrast-enhanced CT of th e fora­ men magnu m was norm al (fig . 6A). Metrizamide CT cistern ography demonstrated a mass at the craniocervical junc tion (fig. 68). Refor­ matted im ages clearly showed a cleavage plane between th e mass and the upper cervical cord (figs. 6C and 60). The lesion was excised totally and proved to be an extrad ural meningioma arising at the level of th e foramen magnum.

Case 2

A 13-year-old boy had a 7 month history of weakness in th e left arm and hand . Physical examination revealed decreased motor strength in both upper extremities, greater on the left. Pain sensa­ tion was diminished in the hands. Widening of th e cervical cord was noted at myelography (fig. 7 A). Metrizamide CT cistern ography demonstrated enlargement of th e entire cervical cord with extension to th e foramen magnum (fig. 7B). A small cystic cavity filled with contrast material at C3 (fig . 70). A midline sagittal reformation showed the full extent of th e mass (fig. 7C). At biopsy, the lesion proved to be an ependymoma of the cervical cord.

Fig. 2. -Parasagittal reformation. Cerebellar tonsil outlined by metri za­ mide. Case 3 A 15-year-old boy with known Klippel-Feil syndrome developed paresthesias of the right arm and hand associated with weakness Representative Case Reports of gri p about 7 weeks before admission. Metrizamide myelography demonstrated a widened cervical cord from C2 to C7. Delayed Case 1 metrizamide CT cisternography showed filling of a syrinx, tonsill ar A 76-year-old woman had burning dysesthesias in her left hand. ectopia, occipitalization of C1, and basilar invagination. The diag­ Physical examination demonstrated mild left upper and lower ex­ nosis of Ch iari type I with associated syringomyelia was established tremity weakness and diminished sensation to pin-prick and vibra- (fig . 8 ). 488 LaMASTERS ET AL AJNR:3, September j Oc tober 1982

A B

Fig . 3. -A, Midsagittal section with cursor specifies plane of section for peduncles, medulla, pons, mesencephalon, and cerebellar hemispheres. oblique coronal reformati on. B , Oblique coronal demonstrates cerebellar

9A). The conventional CT study suggested that the brainstem may have been invaded by the chordoma. Metrizamide CT c isternogra­ phy was performed to assess the integrity of the dura and exclude invasion of the brainstem. A sagittal section showed the large extraaxial c lival mass deflecting the dura posteriorly without evi­ dence of frank invasion of the brain stem (fig. 98).

Case 5

A 13-year-old girl developed imbalance. Conventional CT dem­ onstrated an enhancing pontine mass, probably representing a brainstem gli oma. The tumor was treated with chemotherapy and 5,440 rad (54.4 Gy) to the posterior fossa. Follow-up CT scan 6 months later suggested that the lesion was larger, although the patient's clinical examination was stable (fig. 1 OA). Metrizamide CT cisternography demonstrated that the mass extended into the right pyramid and inferior cerebellar peduncle. Although the enlargement of the brain stem continued to the level of the foramen mag num, it did not extend beyond the li mits of the previous radiation port (figs. 108-100).

Case 6

A 60-year-old wqman had a 2 year history of increasing numb­ Fig . 4.-Coronal reformation. Tonsil position is above outer table of foramen magnum. ness in both hands, more noticeable on the right. A carpal-tunnel release procedure produced little improvement. Recently, she noted decreased grip strength in both hands. Physical findings on admis­ sion included slight gait ataxia and diminished grip strength on the Case 4 ulnar sid e of the right hand. Cervical myelography demonstrated subtle effacement of the right aspect of the cervical su barac hnoid A 58-year-old man had a clivus chordoma subtotally resected space at C1 (fig. 11 A) . Follow-up metrizamide CT cisternography and subseq uently treated with radiation therapy. Three years later clearl y demonstrated an extradural mass at C1-C2 adjacent to the he developed severe occipital headaches. CT demonstrated recur­ right C2 neural foramen (figs. 118 and 11 C) . At surgery, a nerve rence of the tumor in the sphenoid, sella, and petrous regions (fig . sheath tumor of the C2 root was excised. AJNR:3 , September/ October 1982 METRIZAMIDE CT IMAGING OF FORAMEN MAGNUM 489

A B

Fig . 5 .-A, Sagittat section with paraaxial reformati on defined by curso r. B, Tangential section through foramen magnum demonstrates it s norm al size and contour.

A B

c o Fig. 5.-Meningioma at foramen magnum. A, Unremarkable initial conventional CT. B, Axial sections with metrizamide show mass and devi ation of cord . C and D, Reformatted im ages clearl y defin e lesion as separate from cervical cord. 490 LaM ASTERS ET AL. AJNR:3, September/ October 1982

A

c D

Fig . 7.-Ependymoma of cervical cord extending into lower med ulla. A, foramen magnum. D, Coronal section defines small metrizamide-fill ed cystic Metrizamide myelogram. Fu siform widening of cervical spinal cord . Axial CT cavity. images (B) and midsagittal reformation (C) show ceph alad extent of tumor to

Discussion bral angiography, pneumoencephalography, and plain ra­ diography. Little mention, however, is found in the literature Di agnosis of lesions at the craniovertebral junction poses concerning the utility of computed tomography in evaluating a significant dilemma for the clinical neurologist and neuro­ the craniovertebral junction. surgeon, as well as the neuroradiologist. Both mass lesions Pantopaque cisternography has for many years been the and congenital abnormalities, such as the Chiari type I method of choice in assessing possible lesions near the malformation, may be symptomatic many years before ac­ foramen magnum [16-18]. Both prone and supine exami­ curate diagnosis [13]. Symptoms are often not specific for nations are required to depict the upper cervical subarach­ a foramen magnum process, and early clinical signs may be noid space and cisterns of the posterior fossa. Although this entirely attributed to cervical spondylosis or misdiagnosed technique displays the structures in and around the foramen as multiple sclerosis [1 , 3, 14, 15]. magnum well, it requires considerable operator expertise. Although the clinical diagnosis of a foramen mag num Permanent deposition of Pantopaque in the posterior fossa process is admitted ly difficult, when early physical findings cisterns is undesirable because this agent creates signifi­ and symptoms are identified, the radiologist must undertake cant registration artifacts on CT scans, limiting the effective­ a complete evaluation of this region. Various radiologic ness of CT as a follow-up method. Many investigators do imaging methods have been used to study the cranioverte­ not include supine cisternography as an adjunct to cervical bral junction, including Pantopaque cisternography, verte- Pantopaque myelography to exclude a dorsally placed for- AJNR :3. September/ Oc tober 1982 METRIZAMIDE CT IMAGING OF FORAMEN MAGNUM 491

A B

Fig. 8. -Chiari I with syringomyeli a. A. Parasag ittal reformation. Tonsill ar ectopia. B. Syrin x cavity documented on delayed scan.

A B

Fi g. 9.-Clivus chordoma. A. Conventional CT. Marked destruction of clivus and sph enoid and suggestion of direct invasion of brainstem. B. Midline sagittal metrizamide study. Neural axis is intact.

amen magnum lesion in patients with a nonspecifi c ciir>ical am in ati on of the foramen magnum after myelography has presentation. been suggested by Skalpe and Sortland [20], th ere are few Plain film metrizamide cisternography of the cranioverte­ reports to indicate th at this has been widely adopted. bral junction can be performed, but adverse reactions (nau­ Computed tomography without intrathecal enhancement sea, vomiting, seizures) make this approach undesirable is inadequate in evalu ating most lesions at th e foramen [19]. With cervical myelography, the dorsal aspect of the mag num. Several of the representative cases shown exem­ foramen magnum region is usually not depicted, in an effort plify the difficulty of relying on conventional CT alone to to avoid spilling high-concentration (220-250 mg / mll) me­ define extraaxial masses or the extent of intraaxial lesions trizamide into the basal cisterns. Although delayed CT ex- beyond the level of th e foramen mag num. Although Spall one 492 LaMASTERS ET AL. AJ NR :3, September/ October 198?

A B

c D

Fig. 1 O. -Brain stem glioma. A, Conve ntional CT. Glioma centered at pons. B, Metriza mide CT cistern ogram shows caudal extent to foramen mag num and better estimates overall magnitude of tum or (C and 0).

et al. [2 1] reported two pati ents with tumors defined by CT documented in the evaluation of other central nervous sys­ without metrizamide, both lesi ons were large and one had tem mass lesions. Recent reports describe the use of me­ associated bony changes noted on plain film. The inherently trizamide CT cisternography in evaluating the brainstem [23, poor contrast resolution and concurrent inability to routinely 24], in differentiating partially empty sella from intrasell ar define intraspinal contents with current CT scanners limited tumor [25], and in assessing the spinal canal , particul arly in th e usefulness of CT without intrath ecal enhancement [22]. the diagnosis of syringomyeli a [26 , 27]. In a series of 75 Th e effi cacy of metri zamide CT cistern ography has been patients with a suspected syrinx, the central cavity was AJNR:3. S",ntpmber / October 1982 METRIZAMIDE CT IMAGING OF FORAMEN MAGNUM 493

A B c

Fig . 11 .- Schwannoma C1-C2. A, Myelogram. Subtle effacement of subarachnoid space at C1 - C2. B, Parasagittal reformation. Mass is bett er appreciated. C , Coronal section best demonstrates relati on of tumor to neural foramen.

successfully demonstrated in 67 [28]. The routine use of The superior anatomi c resolution provided by metrizamide metrizamide CT cisternography in the radiologic diagnosis CT cisternography should render Pantopaque cisternogra­ of foramen magnum lesions, however, has not been empha­ phy obsolete in the evaluati on of foramen mag num pathol­ sized. ogy. The combined use of im age reformations further facil ­ Our experience with metrizamide CT cisternography sug­ itates recognition of spatial relations at the crani overtebral gests that it is a sensitive technique in the diagnosis of junction . Untoward side effects are uncommon due to th e craniovertebral pathology. The use of intrathecal metriza­ low doses of metrizamide used. In patients with clinical mide improves contrast resolution, all owing identification of manifestations of a foramen magnum lesion, we recommend structures not routinely seen on conventional CT studies. metrizamide CT cisternography as th e initial radiographic The upper cervical cord, medulla, inferior vermi s, lower examin ation. cranial nerves, vertebral and basilar arteries, posterior fossa cisterns, and fourth ventricles are routinely imaged with this REFERENCES technique. Minor effacements of the cisterns and subtle changes in the contour of the neural axis are more apparent 1. Howe JR , Taren JA. Foramen mag num tumors. Pitfall s in than with conventional computed tomography (cases 1, diag nosis. JAMA 1973;225: 1 061-1 066 4 - 6). 2. Missed foramen magnum tumors (editorial) . Lancet 1973; Reformatted images have proven most useful in demon­ 2: 1482 3. Cohen L, MacRae D. Tumors in the region of the foramen strating the spatial relations of a mass lesion to the neural magnum. J Neurosurg 1962;19:462-469 axis and the osseous canal. In instances wh ere axial sec­ 4. Aring CD . Lesions about th e junction of medulla and spinal tions may show apparent continuity of a mass with the cord , cord . JAMA 1974;229: 1879 reformatted images may allow definition of a cleavage plane 5. Bull J. Missed foramen magnum tumors. Lancet 1974;1 :91 confirming the extraaxial nature of the lesion (case 1). 6. Drayer BP , Rosenbaum AE , Higman HB. Cerebrospinal fl uid Displacement of the cord by extraaxial mass is perceived imaging using seri al metrizamide CT cisternography. Neuro­ with little difficulty on sagittal and oblique coronal reforma­ radiology 1977; 13: 7 - 17 tions. 7. Rosenbaum AE, Drayer BP. CT cistern ography with metriza­ In the Chiari malformation, the position of th e tonsil s and mi de. Ac ta Radiol [Suppl] (Stockh) 1977;355 :323- 337 8. Drayer BP , Rosenbaum AE . Studies of the third circulation. J the contour of the medullary cervical juncti on are best Neurosurg 1978;48: 946- 956 appreciated in sagittal and parasagittal planes. Because of 9. Drayer BP, Rosenbaum AE , Rergel DB, Bank WO, Deeb ZL. the high incidence of associated syrin x, metrizamide CT Metrizami de CT cisternography: pediatric applications. Ra­ cisternography is also ideal for evalu ating a coexisting cen­ diology 1977;124:349-357 tral cavity. Similarly, the degree of cerebell ar tonsillar ecto­ 10. Hilal SK, Tookoran H, Wood BS. Displacement of the aqueduct pia is best appreciated on sagittal and coronal reformatted of Sylvius by posteri or fossa tumors. Acta Radiol [Oiagnj views. The structure may then be vi sualized on a single (Stockh) 1969;9: 1 67 - 182 image as opposed to a series of axial sections. 11 . Wickbom I, Hanafee W. Soft tissue masses immediately below 494 LaMASTERS ET AL. AJNR :3, September I October 1982

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