An Evaluation of Current Diagnostic Radiologic Modalities in the Investigation of Acoustic Neurilemmomas 1 Philip J. Dubois, M.B.B.S., Burton P. Drayer, M.D., William O. Bank, M.D., Ziad L. Deeb, M.D., and Arthur E. Rosenbaum, M.D.

The radiologic investigation of patients withsuspected acoustic neurilemmomas may include specialized plain films, thin-section tomography, CT scanning with intravenous contrast enhancement, Pantopaque cisternography, radionuclide scanning, angiography, and pneumoencephalography. Recently wehave used intrathecally enhanced (Amlpaque)" CT cisternography to study the cerebellopontine angles and fourth ventricle. An assessment is made of the differential diagnostic value and limitations of radiologic modalities both in tumor detection and in accurate localization. A protocol designed to optimize use of the available radioloqlc modalities is projected from 30 patients with acoustic neurilemmomas. INDE'X TERMS: (Acoustic neurinoma, 1 [5] .3641) • Brain neoplasms, diagnosis • Cerebellopontine angle, neoplasms • Neuroma

Radiology 126:173-179, January 1978

EVERAL MODALITIES of investigation are available for the radiologic detection and preoperative evaluation of acoustic neurilemmomas. The newer techniques of cranial computed tomography (CT) (3,5) and selective external carotid arteriography (14) have generated enthusiasm, while radionuclide scanning, pneumoencephalography and even pluridirectional tomography are falling from favor in some departments (9). The expense of performing numerous radiologic investigations, substantial radiation exposure, and the morbidity of more invasive techniques necessitate critical analysis of the various techniques to select their best uses. The efficacy of conventional radiography, complex motion tomography, CT, Pantopaque cisternography, and angiography was reviewed in 30 consecutive patients with acoustic neurilemmomas (TABLE I). Since few of these patients had radionuclide scans or pneumoencephalography, no statistically significant observations of these studies were made. Amipaque CT cisternography was positive in 4 patients and the findings with this new technique are described.







30 14 25 4 9 16 4 1

Plain films Pluridirectional tomography CT (intravenous enhanced) Amipaque CT cisternography Pantopaque cisternography Angiography Technetium scan Pneumoencephalogram





0-1.5 cm 1.5-3.0 cm Over 3.0 cm Plain films Tomography Pantopaque cisternography Angiography Intravenous enhanced CT Amipaque CT cisternography


3/4 3/4 4/4

5/5 2/2 1/1

19/22 7/8 4/4

27/31 (87%) 12/14 (86%) 9/9 (100%)

0/2 1/4



12/12 17/17

14/16 (87%) 23/26 (88%)




4/4 (100%)

auditory canals at polytomography, and bilateral progressive retrocochlear sensorineural hearing loss. Although no surgical proof is available, this patient was included. The final 'series comprised 31 tumors in 30 patients.


Case Selection Thirty-two patients seen at the University of Pittsburgh Health Center between January 1975 and September 1976 had acoustic neurilemmomas removed surgically. Three of these patients had inadequate or missing radiologic studies and are excluded from this report. One patient with neurofibromatosis had bilateral enhancing cerebellopontine angle masses at CT, bilateral erosions of the internal

Technique Specialized radiography (Stenvers, submentovertex, transorbital and Chamberlain-Towne projections) was performed in all cases. When equivocal or normal findings were obtained in patients with audiometric or clinical suspicion of acoustic neurilemmomas, pluridirectional

1 From the Department of Radiology, University of Pittsburgh Health Center, Pittsburgh, Pa. Presented at the Sixty-second Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, 111., Dec. 14-19, 1976. 2 Brand of metrizamide, Sterling-Winthrop Research Institute, Renssalaer, New York. shan






January 1978

Fig. 1. Right acoustic neurilemmoma, approximately 3.5 em. A. Frontal tomographic section. The right internal auditory canal is slightly wider at the porus acusticus than the left. No cortical erosion is present. B. A more posterior section reveals no shortening of the posterior wall of the canal. C. Horizontal tomographic section. No evidence of widening of the right internal auditory canal is seen. There is questionable erosion of the roof of the right porus acusticus. D. Horizontal tomographic section,-2 mm cephalad to Fig. 1,C. The roof of the right porus is less prominent than the left, suggesting early erosion by tumor. E. Intravenous enhanced CT. A large (3.5 em) enhancing cerebellopontine angle mass is seen. Minimal bony erosion (plain radiographs were normal) gave no hint of the size of this predominantly extracanalicular tumor. ~

ebellopontine angle were obtained after positioning of the contrast material under fluoroscopic television screening. Lumbar subarachnoid-injected Amipaque CT cisternography with 5-6 ml doses of 190 mg % iodine as performed at this center has been described elsewhere (2, 10, 11). Interpretation of Studies All studies were assessed in terms of (a) tumor detectability and (b) additional preoperative information considered useful by the otologic or neurosurgeon: size of tumor, anatomic relationships in the cerebellopontine angle, differential diagnostic characteristics from other cerebellopontine angle masses, and relationship to neighboring vessels. The nature of surgery often precluded accurate estimation of tumor size since exposure is limited and removal often piecemeal. Therefore, simple categorization into small « 1.5 ern), intermediate (1.5-3.0 ern) and large (> 3.0 cm) tumors was made, using the largest diameter of the tumor.

tomography (stratomatic or polytome) was performed in frontal, lateral and occasionally submentovertex .projections. When angiography was performed, the transfemoral route was used and radiography was by 2X direct magnification (0.2mm focal spot) and subtraction. CT was done with the EMI head scanner with a 160 X 160 matrix, and 8- or 13mm collimation. The posterior fossa was studied before and after intravenous contrast enhancement. Radlographtc positive contrast cisternography was performed with low volumes of Pantopaque (1-3 ml) introduced by lumbar puncture. Horizontal beam Chamberlain-Towne, transorbital, and submentovertex projections of the cer-

Fig. 2. Left acoustic neurilemmoma, approximately 3.5 em. A. Intravenous enhanced CT. A large, homogeneously enhancing mass in the left cerebellopontine angle is seen. Considerable edema surrounds the tumor. B. Unenhanced CT at midbrain level. Brain stem rotation and displacement away from the side of the tumor are evidenced by widening of the ambient and quadrigeminal cisterns on the side of the lesion.


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Plain Radiography and Tomography Detection: Twenty-seven of 31 tumors caused bony erosion detectable with specialized radiography (TABLE II). Tomography was omitted in many lesions with obvious bony erosion so that its accuracy (86 %) is artificially lowered compared to plain film radiography. Two patients with normal plain films had abnormal tomograms. Additional information: Bony erosion almost invariably indicates an extra-axial mass, but no differentiation from other cerebellopontine angle masses could be made with certainty. Tumor calcification was not demonstrable in any patient. Conclusion: Size of tumor could not be predicted with any success since small tumors sometimes caused much more bony erosion than large ones (Fig. 1). Predominantly extracanalicular tumors tended to cause less bony erosion than those with a large intracanalicular component. Pantopaque (/ophendylate) Cisternography Detection: Pantopaque cisternography was performed in 9 patients and detected an abnormality in all. One 3mm intracanalicular tumor was detected only by this modality. Additional information: The size of tumors removed at surgery is compared with estimated tumor size from Pantopaque cisternography in TABLE III. While good radiologic and surgical correlation was maintained for small and intermediate tumors, the size of large tumors was grossly underestimated by this technique. Conclusion: Extra-axial location of the tumor was demonstrated in all cases. Vascular anatomy was not displayed with the low volume technique employed. No features at Pantopaque cisternography distinguished

Fig. 5. Left acoustic neurilemmoma, approximately 1.5 cm. A. Intravenously enhanced CT (8mm cut) shows excellent visualization of basal vessels with no evidence of a cerebellopontine angle mass. B. Intrathecally enhanced CT using Amipaque (8mm cut). The left cerebellopontine angle and ambient cisterns are widened. The brain stem is slightly rotated clockwise. The fourth ventricle is accurately marked by this technique and subtle flattening of its left side is evident. C. Four-millimeter cut, measure mode. The cerebellopontine angle cistern is discontinuous due to a filling defect in the region of the porus acusticus (arrow). D. Eight-millimeter cut, measure mode (equivalent to Fig. 5, B). The widened ambient cistern (a), discontinuous cerebellopontine angle cistern, rotated brain stem, and flattened fourth ventricle are seen to greater advantage.






Pantopaque 1.0 1.0 0.5






6 24 26 30

2.0 3.0 1.5 2.0

4.0 4.0


Fig. 3. Meningioma. Unenhanced CT. A high attenuation coeHicient mass is seen in the left cerebellopontine angle. There was further increase in attenuation after intravenous contrast enhancement but the size of the mass was unchanged. Fig. 4. Epidermoid cyst. Intravenous enhanced CT. Low attenuation mass is seen in the left cerebellopontine angle. Appearance was identical at unenhanced CT. (Reproduced by permission of Australasian Radiology. )


1.0 1.0 1.0 0.5

1 2 9 13



6°1 -large



Neovascularity Vertebral Selected external carotid Selected internal carotid Common carotid Retrobrachial



6/15 2/6 0/12 1/1 1/1

(16 LESIONS) Vessel Displacement 13/15






January 1978

performed. No internal carotid feeding arteries were detected. Additional information: Course and variations of the anterior inferior cerebellar artery (AICA) and its relationship to the tumor were shown in all cases. Size of tumor could be accurately assessed in only 5 instances (31 %), i.e., those in which capsular vessels, as described by Takahashi (13) and Wilner (17), outlined the tumor periphery. Conclusion: Extra-axial location of the mass was evident in all cases by arterial and/or venous displacements. Hypervascular lesions could not reliably be distinguished from meningiomas, nor avascular lesions from epidermoids. Cranial Computed Tomography (Cn

Fig. 6. Left acoustic neurilemmoma, approximately 1.0 em in diameter. A. Unenhanced CT. No abnormality is visualized. B. After intravenous enhancement. A subtle oval mass becomes apparent immediately medial to the left porus acusticus (arrow). C. After intrathecal Amipaque enhancement, same level as 6, B. A localized defect (arrow) in the Amipaque-filled left cerebellopontine angle cistern precisely corresponds to the enhanced mass seen in Fig. 6, B. D. Measure mode of Fig. 6, C. Visualization of the filling defect in the left cerebellopontine angle cistern may be improved by this technique.

Detection: There was a low rate of tumor detection at CT without intravenous contrast enhancement (TABLE V). Following intravenous contrast injection, tumors greater than 1.5 cm in diameter were detected by CT, except for one large (6-cm diameter) neurilemmoma. This lesion, however, caused displacement of the brain stem to the contralateral side with resultant ipsilateral ambient cistern widening. Therefore, positive CT findings existed in all patients with large and intermediate tumors. Additional information: Extra-axial location was suggested by ambient cistern widening (Fig. 2) in 9 of 12 (75 %) large tumors in patients in whom the cistern was demonstrated (TABLE VI). Cerebellopontine angle cistern dilatation is less easy to detect because of computer-


0-1.5 cm

1.5-3.0 cm







Without enhancement With enhancement

acoustic neurilemmomas from other mass lesions of the cerebellopontine angle.


Angiography Detection: In 4 patients, angiography was the initial special procedure performed after tomograms showed erosion of the porus acusticus. In 2 of these, clinical signs of raised intracranial pressure were present and a large cerebellopontine angle tumor was shown in each. In 2 others with small tumors (1.0 and 1.5 ern), no abnormality was demonstrated despite direct magnification angiography, subtraction, and selective (vertebral, internal carotid, and external carotid) catheterizations. All tumors over 1.5 cm in diameter produced detectable mass effect and eight (50 %) showed neovascularity (TABLE IV). Selective external carotid injection showed neovascularity with feeding arteries from the ascending pharyngeal and other branches in only 2 patients and in both, neovascularity was also demonstrable on vertebral angiography. Superselective catheterization of external carotid tributaries was not



IV ventricle displaced Lateral & III ventricles enlarged Ipsilateral ambient cistern widened

Under 1.5 cm

1.5-3.0 cm

Over 3.0 cm










Denominator is the number of patients in whom the relevent structure was seen at CT.



Signs of Cerebellopontine Angle Mass 1. 2. 3. 4. 5.

Filling defect in cerebellopontine angle cistern. Widening of cerebellopontine angle cistern. Widening of ipsilateral ambient cistern. Shift of brain stem to the contralateral side. Displacement of fourth ventricle.


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Fig. 7. Acoustic neurilemmoma, approximately 3.0 cm in diameter. A. Intravenous enhanced CT. The enhancing mass is seen in the left cerebellopontine angle. B. Same level section after intrathecal Amipaque enhancement. A well-demarcated filling defect (asterisk), in the opacified widened left cerebellopontine angle cistern corresponds precisely to the tumor. The fourth ventricle is deformed. C. At a higher level, brain stem shift to the right is shown by opacification of the ambient, interpeduncular, crural and suprasellar cisterns. No other special procedures were performed prior to successful surgical removal.

generated overrange artifact due predominantly to the mastoid air cells and adjacent bony interfaces. Conclusion: Prediction of tumor size at CT correlated well with the surgeons' estimate of size at operation. Characteristic CT features often enabled separation from meningiomas and epidermoids (Figs. 3 and 4). Amipaque (Metrizamide) CT Cisternography Detection: Amipaque CT cisternography was positive in the 4 patients in whom it was performed. Two had small tumors (1.0 and 1.5 cm in diameter, respectively), one was undetected at CT (Fig. 5) and the other was suggested only in careful retrospective analysis (Fig. 6). Additional information: Extra-axial location of the tumors was indicated by coating the interface between the tumor and the brain stem or cerebellum. It clearly demonstrated derangement of the subarachnoid cisterns (Fig. 7) as classically described in pneumoencephalography (TABLE VII) and provided opacification of the opposite cistern for comparison. Fourth ventricle deformity could also be ascertained. Conclusion: More reliable and accurate visualization of subarachnoid spaces enabled detection of smaller tumors than those that could be demonstrated by conventional intravenous enhanced CT.

of size and contour of the internal auditory canals. However, to increase the tumor detection rate above that reported for specialized plain films one must often perform additional invasive procedures (e.g., Pantopaque cisternography) due to the high incidence of developmental asymmetry of the internal auditory canals. Valvassori (15) performed two normal Pantopaque studies for each tumor detected, while over the past 18 months our yield has been one positive for four negative studies. Higher ratios are experienced at some centers. Despite recent reports to the contrary (9), we believe complex motion tomography has a place in the evaluation of patients with unilateral sensorineural hearing loss. It is useful when plain radiographs are negative, since we have encountered subtle erosions of the internal auditory canal, congenital abnormalities of the labyrinth, advanced but unsuspected otosclerosis, other otodystrophies, and primary and metastatic tumors by this method in such patients. Even in patients with obvious bony destruction on





Plain film radiography with specialized projections, appropriate equipment and meticulous technique gives a yield approaching 90 % in the detection of acoustic neurilemmomas (4, 9). This inexpensive, low morbidity and rapid screening method should therefore be applied in all patients with suspected acoustic neurilemmoma. Pluridirectional tomography reveals minor abnormalities














plain radiographs, demonstration of the extent of intraosseous invasion and the relationship to the carotid canal and ninth through twelfth cranial nerves may be invaluable to the surgeon. In addition, calcification in a cerebellopontine angle mass is better delineated by tomography and suggests meningioma, epidermoid, chordoma, or cartilaginous tumor rather than acoustic neurilemmoma. Its absence, however, is of no differential diagnostic value. Few technetium-99m brain scans were performed in this series and no evaluation of this modality was made. In centers with CT facilities, there would appear to be few indications for this study in evaluation for acoustic neurilemmoma. Only one patient had pneumoencephalography. This modality combined with pluridirectional tomography provides exquisite demonstration of the extra-axial nature of extracanalicular tumors. Complete absorption of the negative contrast agent is a major advantage when compared with Pantopaque cisternography. However, small intracanalicular tumors cannot be reliably demonstrated (6) and larger extracanalicular tumors are demonstrated with less morbidity by CT or angiography. Pantopaque cisternography is the most effective special procedure in detection of acoustic neurilemmoma, and more importantly, in exclusion of small intracanalicular tumors. Sones et al. (12) and others have argued that this should be the first (and definitive) investigation in patients with unilateral sensorineural hearing loss. Nevertheless, this approach has several shortcomings. Underestimation of the size of large tumors (TABLE III) may significantly complicate the surgical procedure. Large tumors may cause clinically inapparent obstructive hydrocephalus, a relative contraindication to lumbar puncture. If a large volume of Pantopaque is introduced into the posterior fossa to outline the extent of a large tumor, the contrast often flows over the surface of the tumor into the dilated ambient cistern and through the tentorial hiatus to the middle fossa, whence it is permanently irretrievable. While harmful effects of intracranial Pantopaque have not been conclusively demonstrated in man, we have felt uneasy about the unnecessary and permanent introduction of this agent into the cranial subarachnoid space in view of the consistent demonstration of arachnoiditis in baboons (1) and pigs (8) with spinal intrathecal Pantopaque. When Pantopaque droplets remain in the posterior and middle fossa, subsequent CT and angiographic images are likely to be degraded limiting further evaluation including the size of larger tumors. Postoperative hemorrhage is also likely to accelerate arachnoiditis. The role of angiography in the detection of large tumors with raised intracranial pressure is vitiated since the advent of CT. Even with selective external carotid arterloqraphy-' we were unable to lower the threshold of detection below the 2-cm limit suggested by Takahashi with vertebral ar-

3 Our patients were not examined under general anesthesia with a large dose technique.

January 1978

teriography (13). Neurosurgeons were divided as to whether an arteriographic map to show the position of the anterior inferior cerebellar artery and its relationship to the tumor was helpful at surgery (retromastoid or suboccipital craniotomy). Otologists using a translabyrinthine approach did not use this arteriographic information. In differential diagnosis and in size estimation, angiography was inferior to CT. Extra-axial location of tumors was reliably shown, but such localization was indicated by bony erosion in all but one patient in whom angiography was performed. Detailed analyses of CT findings in acoustic neurilemmomas (3,5) have been reported elsewhere. Opacification of the subarachnoid cisterns with Amipaque (Amipaque CT cisternography) (2,10,11) adds a new dimension to CT, enabling detection of smaller tumors and accurately defining the extent, extra-axial location, and relationships of larger ones. We have not found any value in high dose Amipaque cisternography with poly tomography because of its attendant high morbidity (seizures, vomiting, vertigo). The purpose of a new substitute for pneumoencephalography or Pantopaque cisternography should be to provide similar depiction with reduced morbidity. The use of CT enables visualization of much lower doses of lumbar subarachnoid injected Amipaque (total dose of iodine approximately one gram) resulting in low morbidity and no need for premedication. A proposed protocol for evaluation of patients with clinical or radiographic evidence of acoustic neurilemmoma is shown in TABLE VIII. Since the degree of bony erosion gives no indication as to the size of the extracanalicular portion of the tumor, intravenous enhanced CT proves the best initial special procedure in assessing the presence of acoustic neurilemmoma. The high detection rate, accurate size estimation, differentiation from other common tumors of the region, and reliable detection of obstructive hydrocephalus make this relatively noninvasive procedure a substitute for angiography, pneumoencephalography, or Pantopaque cisternoqraphy in acoustic neurilemmomas greater than 1.5 cm in diameter. If CT (with intravenous enhancement) is positive, surgery may be performed without further special studies at the option of the operating surgeon. If an angiographic map is required by the surgeon, the relationship of pertinent vessels to the mass can be evaluated and its extra-axial location confirmed. No other useful diagnostic information would appear to be provided; however, there is the potential for embolization of the external carotid supply of the tumor. If no angiographic map is required, the location of the mass can be further assessed by Amipaque CT cisternography; this modality provides precise surface relationships of the tumor to regional structures, confirming its extra-axial location. In the presence of large tumors, it has the usual attendant risk of lumbar puncture. If CT is negative, exclusion of a small acoustic neurilemmoma is necessary. Intrathecal Amipaque appears safe in quite low doses and lowers the threshold of detection by CT of those tumors smaller than 1.5 cm. Intracanalicular tumors have not been evaluated by the low-


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dose Amipaque technique; newer generation scanners probably will detect their presence when the subarachnoid space occupies the internal auditory canals sufficiently. The disadvantages of Pantopaque cisternography become minimized when restricted to a low-volume technique in patients with small tumors, and this is currently the procedure of choice. With larger volumes and larger tumors, the likelihood of middle and anterior cranial fossa spillage is significantly increased. With improved CT technology, Amipaque CT cisternography is likely to become the preferred examination for detection or exclusion of small acoustic neurilemmomas just as intravenous enhanced CT has displaced other special procedures in evaluation of larger cerebellopontine angle tumors. ACKNOWLEDGEMENTS: Figure 4 is reproduced by permission of the editor of Australasian Radiology. We are grateful to Mrs. Janet Hanson for her painstaking efforts in preparing the manuscript.

Dr. Philip J. Dubois Department of Radiology University of Pittsburgh Health Center Presbyterian-University Hospital Pittsburgh, Pa. 15216

REFERENCES 1. Bergeron RT, Rumbaugh CL, Fang H, et al: Experimental Pantopaque arachnoiditis in the monkey. Radiology 99:95-101, Apr

1971 2. Drayer BP, Rosenbaum AE, Kennerdell JS, et al: Computed tomographic diagnosis of suprasellar masses by intrathecal enhancement. Radiology 123:339-344, May 1977 3. Dubois PJ, Drayer BP, Boehnke M, et al: Computed cranial



tomography in the investigation of acoustic neurilemmomas. Australas Radiol 20: 108-117, Jun 1976 4: Etter LE: Plain film demonstration of acoustic nerve tumors. Arch Otolaryngol 98:414-415, Dec 1973 5. Gyldensted C, Lester J, Thomsen J: Computer tomography in the diagnosis of cerebellopontine angle tumors. Neuroradiology 11:191-197,30 Jul 1976 6. Kieffer SA: Special procedures in the diagnosis of ear problems. Radiol Clin North Am 12:441-462, Dec 1974 7. Levine HL, Ferris'EJ, Spatz EL: External carotid blood supply to acoustic neurinomas. Report of two cases. J Neurosurg 38: 516-520, Apr 1973 8. Punto L, Suolanen J: Testing of myelographic contrast media using the pig as an experimental animal. Invest Radiol 11:331-334, Jul-Aug 1976 9. Reidy J, de Lacey GJ, Wignall BK, et al: The accuracy of plain radiographs in the diagnosis of acoustic neuromas. Neuroradiology 10:31-34, Feb 1971 10. Rosenbaum AE, Drayer BP, Bank WO, et al: Computer tomographic cistemography (CTC)using a water soluble agent (Amipaque) (abst). Neuroradiology 12:48-49, Oct 1976 11. Rosenbaum AE, Drayer BP, Dubois PJ, et al: Amipaque CT cisternographic visualization of acoustic neurilemmomas. Arch Otolaryngol. (Submitted for publication) 12. Sones PJ Jr, Cioffi CM, Hoffman JC Jr: A practical approach to the diagnosis of cerebellopontine angle tumors. Am J Roentgenol 122:554-559, Nov 1974 13. Takahashi M, Okudera T, Tomanaga M, et al: Angiographic diagnosis of acoustic neurinomas: analysis of 30 lesions. Neuroradiology 2:191-200, Sep 1971 14. Theron J, Lasjaunias P: Participation of the external and internal carotid arteries in the blood supply of acoustic neurinomas. Radiology 118:83-88, Jan 1976 15. Valvassori GE: The abnormal internal auditory canal: the diagnosis of acoustic neuroma. Radiology 92:449-459, Mar 1969 16. Wende S, U.idecke B: Technique and value of gas and Pantopaque cisternography in the diagnosis of cerebello-pontine angle tumours. Neuroradiology 2:24-29, Mar 1971 17. Wilner HI, Austin D: Magnification angiography. Identifying the capsular vasculature of acoustic neuromas. Am J Roentgenol 123:31-35, Jan 1975

An evaluation of current diagnostic radiologic modalities in the investigation of acoustic neurilemmomas.

Neuroradiology An Evaluation of Current Diagnostic Radiologic Modalities in the Investigation of Acoustic Neurilemmomas 1 Philip J. Dubois, M.B.B.S.,...
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