A 39-year-old male was brought to the emergency department after being hit by a car. There was an episode of loss conciousness followed by dizziness and left facial pain. Emergency compterized tomography (CT) demonstrates abnormal increased attenuation anterior to the pons and medulla.
A 39-year-old male was brought to the emergency department after
being hit by a car. There was an episode of loss of consciousness
followed by dizziness and left facial pain. Emergency computerized
tomography (CT) (figure 1) demonstrates abnor- mal increased
attenuation anterior to the pons and medulla. Magnetic resonance
(MR) imaging performed on a 1.5-Tesla unit revealed a well-defined
extraaxial lesion of homogeneous T1 (figures 2A and 2B)
hyperintensity anterior to the pons and medulla as well as the
upper cervical spine, extending to the C2-3 level. On T2-weighted
images (figure 3), the lesion demonstrates significant
hypointensity. There is mild mass effect on the medulla, and the
basilar artery is encased.
Epidermoids are divided into black epidermoids and white
epidermoids depending on their CT and MR characteristics. This case
represents a unique combination of imaging features for a
surgically proven epidermoid, which demonstrated high density on CT
and high signal on T1-weighted MRI.
Epidermoids are referred to as "pearly tumors" and occasionally
as congenital or primary cholesteatomas. Favored locations include
the cerebellopontine angle and suprasellar and prepontine cisterns.
When in the basal cisterns, they often surround or displace major
arteries. Other locations include sella turcica, middle cranial
fossa, intraventricular, cerebellar hemispheres, and diploic
space.1-4 They are of ectodermal origin resulting from inclusion of
epithelial elements at the time of neural tube closure.1,5
Epidermoids are composed of stratified squamous epithelium
surrounding cholesterol crystals and debris from desquamated
keratin from the epithelial cyst lining.2,4,6 Progressive
desquamation leads to slow growth.
There are two types of epidermoids, both of which are extraaxial
well-circumscribed lesions. The "black epidermoid" has CT
attenuation of water or fat with hypodensity secondary to cerebral
spinal fluid, lipid, and cholesterol content. Peripheral
calcifications are seen occasionally. Magnetic resonance imaging
typically demonstrates T1 and T2 signal intensity which
approximates that of cerebral spinal fluid; that is, there is
hypointensity on T1 and hyperintensity on T2 relative to adjacent
brain parenchyma. The low T1 signal may be secondary to the
presence of cholesterol in the solid rather than liquid state or
perhaps secondary to inclusion of cellular debris and cerebral
spinal fluid. Black epidermoids have a low lipid content and no
triglycerides or fatty acids.5-7 The "white epidermoids" have high
T1 signal on MR with negative Hounsfield units on CT secondary to
their high cholesterol, lipid, triglyceride, and fatty acid
content.4-6 Epidermoids rarely are hyperdense on noncontrast
Proposed theories for the high CT density include saponification
of debris to calcium soaps, calcification of keratin debris,
elevated protein content, fibrosis, deposition of ferrocalcium or
deposition of hemoglobin or hemosiderin secondary to a prior
Signal intensity on T2 typically is hyperintense for "black
epidermoids" and isointense for "white epidermoids." Hypointensity
on T2 is very rare and may be caused by calcification, low
hydration, viscous secretions, or the paramagnetic effect of iron
Our case demonstrates CT hyperdensity with T1 hyperintensity and
T2 hypointensity. We were able to find only 13 reported cases of
epidermoids that were hyperdense on noncontrast CT.1 However, we
were unable to find any reported cases of an epidermoid that
demonstrated high density on CT as well as high signal on
T1-weighted MRI. One theory for this unique constellation of
imaging findings would be recent hemorrhage into the epidermoid
with the presence of intracellular methemoglobin. This appearance
should not be mistaken for a subarachnoid hemorrhage.
1. Gao P, Osborn AG, Smirniotopoulos JG, et al: Epidermoid tumor
of the cerebellopontine angle. AJNR 13:863-872, 1992.
2. Gualdi GF, Dibiasi C, Trasimeni G, et al: Unusual MR and CT
appearance of an epidermoid tumor. AJNR 12:771-772, 1991.
3. Steffey DJ, DeFilipp GJ, Spera T, et al: MR imaging of
primary epidermoid tumors. JCAT 12(3):438-440, 1988.
4. Tampieri D, Melanson D, Ethier R: MR imaging of epidermoid
cysts. AJNR 10:351-356, 1989.
5. Horowitz BL, Chari MV, James R, et al: MR of intracranial
epidermoid tumors: Correlation of an vivo imaging with in vitro 13C
spectroscopy. AJNR 11:299-302, 1990.
6. Wagle WA, Jaufmann B, Mincy JE: Magnetic resonance imaging of
fourth ventricular epidermoid tumors. Arch Neurol 48:438-440,
7. Tekkok IH, Cataltepe O, Saglam S: Dense epidermoid of the
cerebellopontine angle. Neuroradiology 33:255-257, 1991.