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TITLE:   TEMPORAL BONE IMAGING
SOURCE:  Dept. of Otolaryngology, UTMB, Grand Rounds
DATE:    October 25, 1995
RESIDENT PHYSICIAN:     Ramtin Kassir, M.D.
FACULTY:                Jeffrey T. Vrabec, M.D.
SERIES EDITOR:          Francis B. Quinn, Jr., M.D.
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"This material was prepared by resident physicians in partial fulfillment 
of educational requirements established for the Postgraduate Training 
Program of the UTMB Department of Otolaryngology/Head and Neck Surgery 
and was not intended for clinical use in its present form.  It was 
prepared for the purpose of stimulating group discussion in a conference 
setting.  No warranties, either express or implied, are made with respect 
to its accuracy, completeness, or timeliness.  The material does not 
necessarily reflect the current or past opinions of members of the UTMB 
faculty and should not be used for purposes of diagnosis or treatment 
without consulting appropriate literature sources and informed professional 
opinion." 

INTRODUCTION:

Imaging studies most commonly used for the temporal bone are CT, MRI, and
arteriography.  Conventional radiography now used mainly for to assess
mastoid and petrous pneumatization or the placement of cochlear implant
electrodes.

Computed Tomography:
Modality of choice for intratemporal pathology.  Iodinated contrast
enhances vascular structures and tumors; it also is useful for extension
of disease intracranially or below the skull base.  Axial and coronal
images should always be obtained; imaging in one plane will miss parallel
pathology.  A 20 degree coronal oblique is used for assessment of the oval
window, promontory, and tympanic facial canal.  Sagittal reconstructions
are used for mastoid facial canal and vestibular aqueduct.

Magnetic Resonance Imaging:
Modality of choice for extratemporal, CPA, petrous apex lesions; also
useful for assessing processes affecting the central auditory vestibular
pathways.  Advantages are no ionizing radiation and imaging in any plane
without moving the patient's head.  The concentration of the hydrogen
nuclei and two magnetic relaxation times, T1 and T2, affects the strength
of the MR signal.  Fat and body fluids have large quantities of free
protons and produce signals of high intensity.  Cortical bone and fibrous
tissue contain few protons and emit low intensity or no signal.  The
normal mastoid, EAC, and ME are dark because air emits no signal.  Fluid
in the IAC and inner ear structures are the only signal emitting
structures in the petrous pyramid.  Fast blood flow shows a signal void,
slow flow emits a high signal.  

Angiography: 
Arteriography is used to identify feeding vessels of vascular lesions 
( i.e., glomus tumors) prior to embolization or ligation.  Because 
the dense temporal bone can obscure the mass, subtraction techniques must 
be used.  

Anatomic Variations: 
The degree of mastoid and petrous pneumatization varies.  A diploic 
type of petrous apex, because of its fatty bone marrow, will emit a 
high signal; a highly pneumatized petrous apex will appear as a signal 
void.  The position of the sigmoid sinus and the size of the jugular 
vein and bulb can be variable.  

Congenital Anomalies: 
Anomalies of the outer ear include stenosis and atresia of the EAC.
Stenosis may cause epithelial entrapment and acquired cholesteatoma of the
external canal.  Complete osseous atresia of the EAC consists of a bony
plate across the EAC, where the tympanic membrane is usually located.
This deformity is associated with fusion of the neck of the malleus to the
atresia plate.  Since the tympanic ring is significantly involved in all
anomalies of the outer ear, there may be alterations in the TMJ (deformity
of the mandibular condyle, absent glenoid fossa, abnormal position of the
TMJ).  Anomalies of the middle ear range from minimal involvement
(tympanic ring dysplasia) to moderate and severe (first and second arch
involvement).  Inner ear anomalies involve the membranous or osseous
labyrinth.  

Radiographic diagnosis of membranous anomalies ( Scheibe, Bing- 
Siebenmann) is not possible since the osseous labyrinth is normal.
Defects of the bony labyrinth involve the semicircular canals, vestibule
and cochlea.  Malformation of the lateral SCC is the most common inner ear
anomaly.  Anomalies of the vestibule frequenty are present with other
inner ear anomalies.  The spectrum of cochlear anomalies corresponds to
the arrested stage of development.  These include complete labyrinthine
aplasia (Michel's), common cavity, cochlear aplasia, cochlear hypoplasia,
and incomplete partition (Mondini's).  Finally, anomalies of the
vestibular aqueduct range from total obliteration to widely dilated forms.
CT is the modality of choice evaluation of the above anomalies.

Vascular Anomalies:
The internal carotid artery may be aberrant, partially absent, or
completely absent.  CT scanning of an aberrant carotid reveals a soft
tissue mass in the hypotympanum extending toward the oval window area,
indenting the promontory or displacing the TM laterally.  On carotid
angiography there is lateral deviation of the ICA past the vestibular
line; a persistent stapedial artery is a common finding.  CT and
angiography are also diagnostic in partial or complete absence of the ICA.

Jugular vein anomalies include a high jugular bulb, protruding jugular
bulb, or a jugular diverticulum.  CT and jugular venography are
diagnostic.  A high jugular bulb is one that is located above the level of
the bony annulus of the temporal bone; the hypotympanic plate is thin.
This is the most common vascular anomaly of the petrous bone.  A
protruding jugular bulb on CT is seen as a dehiscence of the bony floor of
the hypotympanum with a soft tissue mass in the middle ear.  A jugular
diverticulum is an irregular outpouching of the jugular bulb that rises
superiorly and medially in the petrous pyramid.  It is situated more
medially and posteriorly than a protruding jugular bulb; it does not
invade the middle ear and is not visible on otoscopic inspection.
Inflammatory disease, neoplasms, trauma and miscellaneous Mastoid

Inflammatory: 
CT is used to see homogenous clouding with air fluid levels
within the air cells in acute mastoiditis.  With progression of disease
the trabeculae are demineralized and then destroyed forming areas of
coalescent suppuration.  CT or MRI can be used if a resultant intracranial
abscess is suspected.  In chronic mastoiditis the trabeculae are thickened 
and there is nonhomogenous clouding of the air cells.  Some air cells 
become obliterated while others become filled with granulation tissue 
and fluid.  

MIDDLE EAR: 

Congenital and Trauma: 
Once again CT is the study of choice to evaluate the middle ear 
and ossicular chain.  It will show aplasia or agenesis of the 
middle ear space; ossicular abnormalities (fused or rudimentary 
malleus and incus, abnormal stapes superstructure) can also be 
assessed.  In traumatic injuries (mostly longitudinal fxs), coronal 
CT is excellent for evaluating ossicular disruption or a fx
through the attic.  

Inflammatory:  
As in the mastoid, AOM is evident as air fluid levels and homogenous 
opacification of the middle ear and mastoid air cells by fluid density.  
In chronic suppurative otitis media there is clouding of the middle ear 
space and due to granulation tissue, polyps and pus.  The long process 
of the incus may be eroded and there is some aeration of the middle ear 
since there is a TM perforation.  Tympanosclerosis can show up as plaques.  

Cholesteatoma: 
Typically the acquired cholesteatoma arises in Prussak's space and 
causes bone erosion and medial displacement of the ossicles.  The 
cholesteatoma may extend through the aditus to the antrum or there 
may be blockage of fluid drainage producing opacification of the 
antrum and mastoid air cells.  Erosion of bone (Korner's septum) 
can at times distinguish between cholesteatoma and blocked secretions.  
The cholesteatoma may also extend along the promontory and erode 
the lateral SCC or the facial nerve.

Otosclerosis:  
Coronal oblique (20 deg) CT can be useful post stapedectomy to assess
persistent or recurrent HL or immediate or delayed vertigo.  It can
demonstrate the prosthesis itself and whether it has been dislocated off
the oval window or incus; at times the prosthesis is seen protruding into
the vestibule.  Neoplasms Glomus tympanicum tumors are seen on CT as
enhancing soft tissue mass of variable size, usually in the hypotympanum.
If the lesion erodes into the jugular fossa, then it becomes
indistinguishable from a glomus jugulare.  Carcinomas or osteomas can
extend from the EAC into the middle ear causing bone erosion.  

INNER EAR:

Congenital: 
Defects in the otic capsule are evident on CT.  These have been mentioned
above.  A preop CT is usually obtained to evaluate the inner ear in
cochlear implant candidates.  

Trauma: 
The inner ear is usually affected by transverse temporal bone fractures, 
which cross the petrous pyramid at right angles to the longitudinal 
axis.  These fractures can course through the SCCs and the vestibule 
or through the cochlea and fundus of the IAC.  

Otosclerosis: 
CT findings include varying foci of demineralization of the otic capsule.  
As these foci enlarge, a band of demineralization separates the outer 
and inner ring within the thickness of the capsule; this is known as 
the "double ring" effect.  Petrous apex CT with contrast of glomus tumors 
shows enlargement of the jugular fossa by an enlarging soft tissue mass.  
As the tumor enlarges, it erodes the septum dividing the jugular fossa 
from the opening of the carotid canal. Large lesions protrude 
extradurally into the posterior cranial fossa and inferiorly below the 
base of the skull along the jugular vein.  MRI can better evaluate 
involvement of the carotid artery and the jugular vein; the tumor appears 
as a mass of medium signal intensity, containing several areas of 
signal void produced by blood vessels.  

Cholesterol granuloma: 
CT shows lesions that are nonenhancing with intravenous contrast, are 
sharply marginated, and are isodense with brain.  These characteristics 
exclude most lesions of the petrous apex other than primary epidermoids 
and mucoceles.  However, in both of these lesions, the density approximates
the density found in CSF rather than brain.  MRI characteristics of
cholesterol granulomas of the petrous apex include an increased signal
intensity on both the T1 and T2 weighted image.  These cysts often contain
areas of signal void produced by deposition of hemosiderin. Congenital
cholesteatomas have been described as having a low to medium intensity
signal on T1 weighted image and high intensity signal on T2 weighted
image.  

Hemangioma: 
These lesions characteristically produce a thickened, sclerotic, and 
pockmarked petrous bone, typically the tegmen.  They are not as extensive 
or homogenous as fibrous dysplasia and lack the multiple phases seen with 
Paget's disease.  Metastasis, primary malignant bone tumors, and 
histiocytosis X also affect the petrous bone.  CT demonstrates aggressive 
bone destruction in the petrous and peripetrous regions.  Chordoma may 
extend from the clivus to involve the petrous apex; some calcifications 
may be present but CT and MRI findings are nonspecific.  CPA masses 

Schwanomma:  
On CT, this lesion is typically isodense with the brainstem and enhances 
dramatically.  Tumors greater than 1 cm may have low density areas of 
central necrosis or cyst formation.  CT is excellent for extracanilicular 
tumors greater than 1 cm, but a purely intracanilicular tumor or one less 
than 1 cm may be missed even on thin section contrast CT.  The study of 
choice for schwanomma is MRI, performed before and after injection of 
paramagnetic agents.  In the plain study, the tumor appears brighter than 
CSF and isointense to gray matter on T1 images.  In T2 sections the tumors 
are brighter than brain but isointense to CSF.  Gadolinium enhanced MRI 
(T1) can demonstrate a markedly hyperintense mass within the IAC; lesions 
as small as 1mm can be diagnosed by this technique.  Larger tumors of the 
CPA may be less homogenous.

Meningioma: 
This tumor mimics schwanomma clinically and radiographically but differs 
in that it only rarely extends into the IAC.  On contrast CT and gad-MRI 
the tumor enhances intensely and homogenously.  

Arachnoid cysts: 
The very thin membrane between the cyst and surrounding CSF is usually 
not visible on CT and MRI; positive contrast CT cisternography is 
usually required to show the lesion.  
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                          BIBLIOGRAPHY

Valvassori GE. Imaging Studies of the Temporal Bone.  Head and Neck
Surgery - Otolaryngology, edited by Byron J. Bailey.  J.B. Lippincott
Company, Philadelphia, 1993.

Bergeron RT et al, The Temporal Bone, in Head and Neck Imaging, edited by
Som, PM and Bergeron RT. Mosby Year Book, Inc. St. Louis, 1991.   

Latchaw RE, Dreisbach JN.  Imaging the Petrous Bone and Associated
Intracranial Structures.  In Otolaryngology-Head and Neck Surgery, edited
by Cummings CW.  Mosby Year book, St. Louis, 1991.

Thedinger BA et al.  Radiographic Diagnosis, Surgical Treatment, and Long
Term Follow-up of Cholesterol Granulomas of the Petrous Apex.
Laryngoscope 99: September 1989, pp896-907.
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