European Journal of Radiology, 13 ( 199 1) 103-l 06 0 1991 Elsevier Science Publishers B.V. All rights reserved.
EURRAD
0720-048X/91/$03.50
103
00193
Magnetic resonance of intracranial epidermoids John J. Wasenko ‘, Scott A. Rosenbloom ‘, Melinda Estes2, Charles F. Lanzieri Paul M. Duchesneau’ ‘Division of Radiology and 2Division of Pathology, The Cleveland Clinic Foundation,
(Received 4 February
Key words: Magnetic resonance
1991; accepted
imaging, brain; Computed
after revision
tomography,
’
and
Cleveland, OH, U.S.A.
16 April 199 1)
brain; Brain, neoplasm;
Brain, MRI; Brain, CT
Abstract The magnetic resonance images of seven patients with biopsy-proven epidermoids were evaluated. The epidermoids were hypointense on Tl-weighted images. Intermediate density images revealed the tumors to be heterogeneous in signal intensity consisting of areas of hypo- and isointensity. Signal intensity on TZ-weighted images was hyperintense and inhomogeneous in all but one case. CT performed in five patients demonstrated the tumors to be well-defined hypodense lesions without contrast enhancement.
Introduction Epidermoid tumors are rare congenital neoplasms comprising approximately 1 y0 of intracranial tumors [ 11. The CT appearance of epidermoid tumors has been well described in the literature. The tumor characteristically appears as a well-defined, hypodense, nonenhancing mass [2-61. This study was undertaken to characterize the MR appearance of these tumors and to correlate this appearance with computed tomography and pathology. Materials and Methods The magnetic resonance images of seven patients with pathologically-proven epidermoids were reviewed. The patients’ ages ranged from 12-69 years. Five were male and two were female. Patients were examined with 0.6T and 1.5T Technicare and l.OT Magnetom (Siemens) imaging systems utilizing spin echo pulse sequences. Sagittal Tl-weighted images were obtained with repetition times of 400 or 500 ms and echo times
Address for reprints: John J. Wasenko, M.D., Department of Radiology, SUNY Health Science Center at Syracuse, 750 E. Adams Street, Syracuse, New York 13210, U.S.A.
of 17 or 32 ms. Intermediate density images were performed in the axial plane with repetition times of 2000 or 2500 ms and echo times of 32 or 35 ms. Repetition times of 2000 or 2500 ms and echo times of 120 or 90 ms were utilized for axial T2-weighted images. In several cases, coronal Tl- and T2-weighted images were performed. Slice thickness was 6.5 mm with a 1.9-mm interslice gap. A matrix size of 192 )(: 192 was utilized with 2 excitations (NEX 2) and a field a view (FOV) of 25.6 cm. In one case (Case 2), sagittal and coronal Tl-weighted images only were performed. The slice thickness in this case was 5 mm with a l-mm interslice gap. The matrix size and the FOV were the same as for routine head examinations, but the NEX was 6. CT examinations were performed on a Picker 1200X scanner. Angiography was performed on one patient. The MR studies were correlated with CT examinations and pathology reports. Results The signal intensity of epidermoids was compared with that of CSF on all pulse sequences. On the Tlweighted images, the epidermoids were decreased in signal intensity compared to brain tissue, but slightly increased in signal compared with CSF. Intermediate density images revealed the tumors to be of hetero-
a
C
b
Fig. 1. (a) Fourth ventricular epidermoid with hypointense signal on Tl (500/32) image. (b) The lesion demonstrates iso- and hypointensity on intermediate density (2500/32) image. (c) T2 (2500/120) image shows heterogeneous signal intensity.
a
C
b
Fig. 2. (a) Tl (500/32) image shows hypointense signal of a right cerebellopontine angle epidermoid. (b) Iso- and hypointense on intermediate density (2500/32) image (c) Heterogeneous signal is present on T2 (2500/120) image.
geneous signal consisting of areas of hypo- and isointensity. (Figs. 1 and 2) The heterogeneous signal was slightly greater than CSF. In one case, a rim of hyperintensity was present around the periphery of the tumor. The tumors were hyperintense relative to brain on T2-weighted images. The signal intensity on T2weighted images was isointense with CSF, however, inhomogeneous in all but one case. The tumor margins were well-defined, being irregular in three and smooth in four cases. Mass effect on adjacent brain structures was present in all cases. Parenchymal edema was not present in any case. Contrast-enhanced CT was performed in five patients. The lesions were well-defined and decreased in attenuation similar to that of CSF without contrast
signal is seen
enhancement. Calcifications were not present in any case. All patients underwent tumor excision. Pathologic analysis revealed the tumors to consist of squamous epithelium, keratin, and cholesterol. Discussion Epidermoid tumors are rare congenital neoplasms that occur in several locations within the central nervous system. The neoplasm arises from inclusion of ectoderm in the neural tube at the time of closure, during the third to fifth week of gestation [ 11. The peak incidence occurs in the fifth decade, with males affected more commonly than females. The most frequent loca-
105
tion is the cerebellopontine angle, with the parasellar region and the middle cranial fossa being the next most common sites. The tumor may also occur within the cerebral hemispheres or the ventricular system, with the fourth ventricle most commonly involved. The tumor produces a variety of symptoms depending upon its location. The MR appearance of epidermoids has been noted in several reports which have described the tumors as hypointense on Tl- and hyperintense on T2-weighted images [7-191. The results observed in this series were similar to previously reported experiences. The tumors were hypointense on Tl-weighted images but not as hypointense as CSF. Intermediate density images demonstrated the tumors to be of heterogeneous signal intensity consisting of areas of hypo- and isointensity. This finding is similar to that observed by Tampieri [ 161, who noted heterogeneous signal in five of nine patients. Signal intensity on T2-weighted images was hyperintense compared with brain but inhomogeneous and similar to that noted by others [ 17-191. The signal intensity observed in these tumors reflects their contents. The desquamated debris of the squamous epithelial lining consists of cholesterol and keratin. Variable quantities of the tumor constituents may be responsible for the heterogeneous signal observed on intermediate density and T2-weighted images. Unfortunately, the percentages of cholesterol and keratin were not noted in the pathology reports. The state of hydration may also explain the signal intensity of the tumors, as variable water content alters rates of relaxation [ 181. The hypointense signal observed on Tl and the hyperintense signal observed on T2weighted images reflects increased water content. As water content increases, Tl and T2 relaxation rates decrease or become prolonged. This results in a lesion appearing decreased in signal intensity relative to brain on Tl-weighted images. The decreased relaxation rate results in increased signal intensity compared with brain on T2-weighted images. The pathologic reports did not mention the degree of water content of the tumors. Gadopentetate dimeglumine (Gd-DTPA) is a paramagnetic contrast material that behaves similar to iodinated contrast material in that it crosses a defective blood-brain-barrier. The mechanism of enhancement is however different in that Gd-DTPA produces local magnetic field inhomogeneities that result in increased T 1 and T2 relaxation rates. Gd-DTPA was not utilized in the evaluation of the epidermoid tumors because the tumors do not show enhancement with contrast enhanced CT and would therefore not show enhancement with Gd-DTPA. In addition, the signal intensity
of epidermoids on MR is characteristic and allows diagnosis without the use of contrast material. Several reports have noted the signal intensity of some epidermoids to be increased on Tl-weighted images [ 7,9,15]. This was not observed in our series or those of others [ 16-191. A possible explanation may be failure to differentiate epidermoid tumors from cholesterol granulomas [ 191. The cholesterol granuloma is hyperintense on Tl-weighted images due to the presence of blood degradation products. Cholesterol in epidermoids is in a solid crystalline, rather than liquid, state [ 121. This explains the hypointense signal observed on Tl-weighted images. Craniopharyngiomas and dermoids contain cholesterol in a liquid state, with resultant hyperintense signal on Tl-weighted images. Pathologically, these tumors are noted to be ‘motor oil-like’ in consistency rather than solid as in epidermoids. Lesions to be considered in the differential diagnosis of epidermoids include arachnoid cysts and dermoids. Arachnoid cysts are extra-axial masses that are most commonly thought to be congenital in origin. Common locations include the middle cranial fossa, cerebellopontine angle cisterns, and suprasellar region. The tumors are smoothly marginated and do not contain calcification or enhance with contrast material. The signal intensity of arachnoid cysts is identical to that of CSF on all pulse sequences [20,21]. Dermoids occur less frequently than epidermoids. They tend to occur near the midline in the posterior fossa and skull base with the most common site being the area of the vermis or fourth ventricle. The tumors are sometimes calcified. Dermoids are hyperintense on Tl-weighted and intermediate density images. Their signal intensity is hypointense on T2-weighted images. The full extent of the tumors was appreciated better with MR than with CT. This was particularly evident in the posterior fossa. The multiplanar imaging capability of MR and the lack of beam-hardening artifacts allowed determination of the full extent of the tumors as well as the relationship to adjacent brain structures. References 1 Rubenstein LJ. Tumors of the central nervous system. Second Series, Fasicle 6, Atlas of Tumor Pathology, Armed Forces Institute of Pathology, Washington, DC, 1985. 2 Zimmerman RD, Bilaniuk LT. Cranial computed tomography of epidermoid and congenital fatty tumors of maldevelopmental origin. J Comp Tomog 1979; 1: 40-47. 3 Mikhael MA, Mattar AG. Intracranial pearly tumors: the role of computer tomography, angiography and pneumoencephalography. J Comput Assist Tomogr 1978; 2: 421-429. 4 Fawcitt RA, Isherwood I. Radiodiagnosis of intracranial pearly
106
5
6
7
8
9
10
11
12
tumours with particular reference to the value of computed tomography. Neuroradiology 1976; 11: 234-242. Chambers AA, Lukin RR, Tomsick TA. Cranial epidermoid tumors: diagnosis by computed tomography. Neurosurgery 1977; 3: 276-280. Davis KR, Roberson GH, Taveras JM, New PFJ, Trevor R. Diagnosis of epidermoid tumor by computed tomography. Radiology 1984; 119: 347-353. Lee BCP, Kneeland JB, Deck MDF, Cahill PT. Posterior fossa lesions: magnetic resonance imaging. Radiology 1984; 153: 137-143. Sartor K, Karnaze MG, Winthrop JD, Gado M, Hodges FJ III. MR imaging in infra-, para-, and retrosellar mass lesions. Neuroradiology 1987; 29: 19-29. MacKay JM, Bydder GM, Young IR. MR imaging of central nervous system tumors that do not display increase in Tl or T2. J Comput Assist Tomogr 1985; 6: 1055-1061. Rinck PA, Meindl S, Higer HP, Bieler EU, Pfannenstiel P. Brain tumors: detection and typing by use of CPMG sequences and in-vivo T2 measurements. Radiology 1985; 104: 103-106. Brant-Zawadzki M, Badami JP, Mills CM, Norman D, Newton TH. Primary intracranial tumor imaging: a comparison of magnetic resonance and CT. Radiology 1984; 150: 435-440. Lee SHL, Rao KCVG. Cranial computed tomography, 2nd edn. New York: McGraw-Hill, 1987; 795-796.
13 Brant-Zawadzki M, Norman D. Magnetic resonance imaging of the central nervous system. New York: Raven Press, 1987; 179. 14 Davidson HD, Ouchi T, Steiner RE. NMR imaging of congenital intracranial germinal layer neoplasms. Neuroradiology 1987; 27: 301-303. 15 Latack JT, Kartush JM, Kermink JL, Graham MD, Knake JE. Epidermoidomas of the cerebellopontine angle and temporal bone: CT and MR aspects. Radiology 1985; 157: 361-366. 16 Tampieri D, Melanson D, Ethier R. MR imaging of epidermoid cysts. AJNR 1989; 10: 351-356. 17 Steffey DJ, DeFilipp GJ, Spera T, Gabrielsen TO. MR imaging of primary epidermoid tumors. J Comput Assist Tomogr 1988; 12: 438-440. 18 Vion-Dury J, Vincentelli F, Jiddane M, VanBunnen Y, Rumeau C, Grisoll F, Salamon G. MR of epidermoid cysts. Neuroradiology 1987; 29: 333-338. 19 Gentry LR, Jacoby CG, Turski PA, Houston LW, Strother CM, Sackett JF. Cerebellopontine angle petromastoid mass lesions: comparative study of diagnosis with MR imaging and CT. Radiology 1987; 162: 513-520. 20 Kjos BO, Brant-Zawadzki M, Kucharczyk W, Kelly WM, Norman D, Newton TH. Cystic intracranial lesions: magnetic resonance imaging. Radiology 1985; 155: 363-369. 21 Weiner SN, Pearlstein AE, Eiber A. MR imaging of intracranial arachnoid cysts. J Comput Assist Tomogr 1987; 11: 233-241.
EURRAD
0720-048X/91/$03.50
103
00193
Magnetic resonance of intracranial epidermoids John J. Wasenko ‘, Scott A. Rosenbloom ‘, Melinda Estes2, Charles F. Lanzieri Paul M. Duchesneau’ ‘Division of Radiology and 2Division of Pathology, The Cleveland Clinic Foundation,
(Received 4 February
Key words: Magnetic resonance
1991; accepted
imaging, brain; Computed
after revision
tomography,
’
and
Cleveland, OH, U.S.A.
16 April 199 1)
brain; Brain, neoplasm;
Brain, MRI; Brain, CT
Abstract The magnetic resonance images of seven patients with biopsy-proven epidermoids were evaluated. The epidermoids were hypointense on Tl-weighted images. Intermediate density images revealed the tumors to be heterogeneous in signal intensity consisting of areas of hypo- and isointensity. Signal intensity on TZ-weighted images was hyperintense and inhomogeneous in all but one case. CT performed in five patients demonstrated the tumors to be well-defined hypodense lesions without contrast enhancement.
Introduction Epidermoid tumors are rare congenital neoplasms comprising approximately 1 y0 of intracranial tumors [ 11. The CT appearance of epidermoid tumors has been well described in the literature. The tumor characteristically appears as a well-defined, hypodense, nonenhancing mass [2-61. This study was undertaken to characterize the MR appearance of these tumors and to correlate this appearance with computed tomography and pathology. Materials and Methods The magnetic resonance images of seven patients with pathologically-proven epidermoids were reviewed. The patients’ ages ranged from 12-69 years. Five were male and two were female. Patients were examined with 0.6T and 1.5T Technicare and l.OT Magnetom (Siemens) imaging systems utilizing spin echo pulse sequences. Sagittal Tl-weighted images were obtained with repetition times of 400 or 500 ms and echo times
Address for reprints: John J. Wasenko, M.D., Department of Radiology, SUNY Health Science Center at Syracuse, 750 E. Adams Street, Syracuse, New York 13210, U.S.A.
of 17 or 32 ms. Intermediate density images were performed in the axial plane with repetition times of 2000 or 2500 ms and echo times of 32 or 35 ms. Repetition times of 2000 or 2500 ms and echo times of 120 or 90 ms were utilized for axial T2-weighted images. In several cases, coronal Tl- and T2-weighted images were performed. Slice thickness was 6.5 mm with a 1.9-mm interslice gap. A matrix size of 192 )(: 192 was utilized with 2 excitations (NEX 2) and a field a view (FOV) of 25.6 cm. In one case (Case 2), sagittal and coronal Tl-weighted images only were performed. The slice thickness in this case was 5 mm with a l-mm interslice gap. The matrix size and the FOV were the same as for routine head examinations, but the NEX was 6. CT examinations were performed on a Picker 1200X scanner. Angiography was performed on one patient. The MR studies were correlated with CT examinations and pathology reports. Results The signal intensity of epidermoids was compared with that of CSF on all pulse sequences. On the Tlweighted images, the epidermoids were decreased in signal intensity compared to brain tissue, but slightly increased in signal compared with CSF. Intermediate density images revealed the tumors to be of hetero-
a
C
b
Fig. 1. (a) Fourth ventricular epidermoid with hypointense signal on Tl (500/32) image. (b) The lesion demonstrates iso- and hypointensity on intermediate density (2500/32) image. (c) T2 (2500/120) image shows heterogeneous signal intensity.
a
C
b
Fig. 2. (a) Tl (500/32) image shows hypointense signal of a right cerebellopontine angle epidermoid. (b) Iso- and hypointense on intermediate density (2500/32) image (c) Heterogeneous signal is present on T2 (2500/120) image.
geneous signal consisting of areas of hypo- and isointensity. (Figs. 1 and 2) The heterogeneous signal was slightly greater than CSF. In one case, a rim of hyperintensity was present around the periphery of the tumor. The tumors were hyperintense relative to brain on T2-weighted images. The signal intensity on T2weighted images was isointense with CSF, however, inhomogeneous in all but one case. The tumor margins were well-defined, being irregular in three and smooth in four cases. Mass effect on adjacent brain structures was present in all cases. Parenchymal edema was not present in any case. Contrast-enhanced CT was performed in five patients. The lesions were well-defined and decreased in attenuation similar to that of CSF without contrast
signal is seen
enhancement. Calcifications were not present in any case. All patients underwent tumor excision. Pathologic analysis revealed the tumors to consist of squamous epithelium, keratin, and cholesterol. Discussion Epidermoid tumors are rare congenital neoplasms that occur in several locations within the central nervous system. The neoplasm arises from inclusion of ectoderm in the neural tube at the time of closure, during the third to fifth week of gestation [ 11. The peak incidence occurs in the fifth decade, with males affected more commonly than females. The most frequent loca-
105
tion is the cerebellopontine angle, with the parasellar region and the middle cranial fossa being the next most common sites. The tumor may also occur within the cerebral hemispheres or the ventricular system, with the fourth ventricle most commonly involved. The tumor produces a variety of symptoms depending upon its location. The MR appearance of epidermoids has been noted in several reports which have described the tumors as hypointense on Tl- and hyperintense on T2-weighted images [7-191. The results observed in this series were similar to previously reported experiences. The tumors were hypointense on Tl-weighted images but not as hypointense as CSF. Intermediate density images demonstrated the tumors to be of heterogeneous signal intensity consisting of areas of hypo- and isointensity. This finding is similar to that observed by Tampieri [ 161, who noted heterogeneous signal in five of nine patients. Signal intensity on T2-weighted images was hyperintense compared with brain but inhomogeneous and similar to that noted by others [ 17-191. The signal intensity observed in these tumors reflects their contents. The desquamated debris of the squamous epithelial lining consists of cholesterol and keratin. Variable quantities of the tumor constituents may be responsible for the heterogeneous signal observed on intermediate density and T2-weighted images. Unfortunately, the percentages of cholesterol and keratin were not noted in the pathology reports. The state of hydration may also explain the signal intensity of the tumors, as variable water content alters rates of relaxation [ 181. The hypointense signal observed on Tl and the hyperintense signal observed on T2weighted images reflects increased water content. As water content increases, Tl and T2 relaxation rates decrease or become prolonged. This results in a lesion appearing decreased in signal intensity relative to brain on Tl-weighted images. The decreased relaxation rate results in increased signal intensity compared with brain on T2-weighted images. The pathologic reports did not mention the degree of water content of the tumors. Gadopentetate dimeglumine (Gd-DTPA) is a paramagnetic contrast material that behaves similar to iodinated contrast material in that it crosses a defective blood-brain-barrier. The mechanism of enhancement is however different in that Gd-DTPA produces local magnetic field inhomogeneities that result in increased T 1 and T2 relaxation rates. Gd-DTPA was not utilized in the evaluation of the epidermoid tumors because the tumors do not show enhancement with contrast enhanced CT and would therefore not show enhancement with Gd-DTPA. In addition, the signal intensity
of epidermoids on MR is characteristic and allows diagnosis without the use of contrast material. Several reports have noted the signal intensity of some epidermoids to be increased on Tl-weighted images [ 7,9,15]. This was not observed in our series or those of others [ 16-191. A possible explanation may be failure to differentiate epidermoid tumors from cholesterol granulomas [ 191. The cholesterol granuloma is hyperintense on Tl-weighted images due to the presence of blood degradation products. Cholesterol in epidermoids is in a solid crystalline, rather than liquid, state [ 121. This explains the hypointense signal observed on Tl-weighted images. Craniopharyngiomas and dermoids contain cholesterol in a liquid state, with resultant hyperintense signal on Tl-weighted images. Pathologically, these tumors are noted to be ‘motor oil-like’ in consistency rather than solid as in epidermoids. Lesions to be considered in the differential diagnosis of epidermoids include arachnoid cysts and dermoids. Arachnoid cysts are extra-axial masses that are most commonly thought to be congenital in origin. Common locations include the middle cranial fossa, cerebellopontine angle cisterns, and suprasellar region. The tumors are smoothly marginated and do not contain calcification or enhance with contrast material. The signal intensity of arachnoid cysts is identical to that of CSF on all pulse sequences [20,21]. Dermoids occur less frequently than epidermoids. They tend to occur near the midline in the posterior fossa and skull base with the most common site being the area of the vermis or fourth ventricle. The tumors are sometimes calcified. Dermoids are hyperintense on Tl-weighted and intermediate density images. Their signal intensity is hypointense on T2-weighted images. The full extent of the tumors was appreciated better with MR than with CT. This was particularly evident in the posterior fossa. The multiplanar imaging capability of MR and the lack of beam-hardening artifacts allowed determination of the full extent of the tumors as well as the relationship to adjacent brain structures. References 1 Rubenstein LJ. Tumors of the central nervous system. Second Series, Fasicle 6, Atlas of Tumor Pathology, Armed Forces Institute of Pathology, Washington, DC, 1985. 2 Zimmerman RD, Bilaniuk LT. Cranial computed tomography of epidermoid and congenital fatty tumors of maldevelopmental origin. J Comp Tomog 1979; 1: 40-47. 3 Mikhael MA, Mattar AG. Intracranial pearly tumors: the role of computer tomography, angiography and pneumoencephalography. J Comput Assist Tomogr 1978; 2: 421-429. 4 Fawcitt RA, Isherwood I. Radiodiagnosis of intracranial pearly
106
5
6
7
8
9
10
11
12
tumours with particular reference to the value of computed tomography. Neuroradiology 1976; 11: 234-242. Chambers AA, Lukin RR, Tomsick TA. Cranial epidermoid tumors: diagnosis by computed tomography. Neurosurgery 1977; 3: 276-280. Davis KR, Roberson GH, Taveras JM, New PFJ, Trevor R. Diagnosis of epidermoid tumor by computed tomography. Radiology 1984; 119: 347-353. Lee BCP, Kneeland JB, Deck MDF, Cahill PT. Posterior fossa lesions: magnetic resonance imaging. Radiology 1984; 153: 137-143. Sartor K, Karnaze MG, Winthrop JD, Gado M, Hodges FJ III. MR imaging in infra-, para-, and retrosellar mass lesions. Neuroradiology 1987; 29: 19-29. MacKay JM, Bydder GM, Young IR. MR imaging of central nervous system tumors that do not display increase in Tl or T2. J Comput Assist Tomogr 1985; 6: 1055-1061. Rinck PA, Meindl S, Higer HP, Bieler EU, Pfannenstiel P. Brain tumors: detection and typing by use of CPMG sequences and in-vivo T2 measurements. Radiology 1985; 104: 103-106. Brant-Zawadzki M, Badami JP, Mills CM, Norman D, Newton TH. Primary intracranial tumor imaging: a comparison of magnetic resonance and CT. Radiology 1984; 150: 435-440. Lee SHL, Rao KCVG. Cranial computed tomography, 2nd edn. New York: McGraw-Hill, 1987; 795-796.
13 Brant-Zawadzki M, Norman D. Magnetic resonance imaging of the central nervous system. New York: Raven Press, 1987; 179. 14 Davidson HD, Ouchi T, Steiner RE. NMR imaging of congenital intracranial germinal layer neoplasms. Neuroradiology 1987; 27: 301-303. 15 Latack JT, Kartush JM, Kermink JL, Graham MD, Knake JE. Epidermoidomas of the cerebellopontine angle and temporal bone: CT and MR aspects. Radiology 1985; 157: 361-366. 16 Tampieri D, Melanson D, Ethier R. MR imaging of epidermoid cysts. AJNR 1989; 10: 351-356. 17 Steffey DJ, DeFilipp GJ, Spera T, Gabrielsen TO. MR imaging of primary epidermoid tumors. J Comput Assist Tomogr 1988; 12: 438-440. 18 Vion-Dury J, Vincentelli F, Jiddane M, VanBunnen Y, Rumeau C, Grisoll F, Salamon G. MR of epidermoid cysts. Neuroradiology 1987; 29: 333-338. 19 Gentry LR, Jacoby CG, Turski PA, Houston LW, Strother CM, Sackett JF. Cerebellopontine angle petromastoid mass lesions: comparative study of diagnosis with MR imaging and CT. Radiology 1987; 162: 513-520. 20 Kjos BO, Brant-Zawadzki M, Kucharczyk W, Kelly WM, Norman D, Newton TH. Cystic intracranial lesions: magnetic resonance imaging. Radiology 1985; 155: 363-369. 21 Weiner SN, Pearlstein AE, Eiber A. MR imaging of intracranial arachnoid cysts. J Comput Assist Tomogr 1987; 11: 233-241.
