Petrous Apex Lesion
Logan A. McLean, MD & H. Ric Harnsberger, MD
DIFFERENTIAL DIAGNOSIS
Common
Asymmetric Marrow, Petrous Apex
Trapped Fluid, Petrous Apex
Cholesterol Granuloma, Petrous Apex
Primary or Metastatic Disease
Less Common
Cephalocele, Petrous Apex
Meningioma, T-Bone
Fibrous Dysplasia, T-Bone
Schwannoma, Trigeminal, Skull Base
Cholesteatoma, Petrous Apex
Chondrosarcoma, Skull Base
Langerhans Histiocytosis, Skull Base
Apical Petrositis
Mucocele, Petrous Apex
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
Best imaging tool
CT and MR are complementary
Bone CT to evaluate petrous apex (PA) “bony” expansion or destruction
MR for lesion tissue analysis; characteristic signal per diagnosis
Beware confusing high T1 MR signal in trapped fluid with PA cholesterol granuloma
When protein content is high, T1 signal may be high
In absence of bony expansion on CT, lesion should be considered trapped fluid
Helpful Clues for Common Diagnoses
Asymmetric Marrow, Petrous Apex
Asymmetric pneumatization makes contralateral fatty marrow conspicuous
CT findings
Nonexpansile fat density PA
MR findings
High T1 normal fatty marrow
Trapped Fluid, Petrous Apex
Remote otomastoiditis leaves behind PA air cell fluid of variable protein content
CT findings
Opacified air cells, trabeculae present
MR findings
Low T1, high T2 signal
Cholesterol Granuloma, Petrous Apex
Chronic otitis media; pneumatized PA with recurrent hemorrhage
CT findings
Smooth, expansile margins
Larger lesions affect clivus, jugular tubercle, ICA
MR findings
High T1 and T2 signal in expanded PA
Primary or Metastatic Disease
Rhabdomyosarcoma (primary or mets), neuroblastoma
Metastasis occurs late in the disease process
Dx of primary disease is usually known
Radiographic appearance is variable
Helpful Clues for Less Common Diagnoses
Cephalocele, Petrous Apex
Incidental PA lesion where Meckel cave appears herniated into subjacent PA
CT findings
Expansile ovoid lesion
MR findings
Low T1, high T2 “pseudopod” from Meckel cave to PA; nonenhancing
Meningioma, T-Bone
CT findings
Permeative
Sclerotic or hyperostotic bony changes
MR findings
Dural-based mass invades PA with avid contrast-enhancement, dural tail
Fibrous Dysplasia, T-Bone
Bone disorder of younger women (< 30 years) with progressive replacement of normal marrow by mixture of fibrous tissue and disorganized trabeculae
Active phase: Cystic
Least active/burned out: Sclerotic
CT findings
Expansile bone lesion with mixed ground-glass/sclerotic and cystic components sparing otic capsule
MR findings
Low T1 and T2 signal, foci of enhancement common
Schwannoma, Trigeminal, Skull Base
Larger trigeminal nerve schwannoma involves preganglionic segment as it passes into Meckel cave
CT findings
Smooth PA remodeling of inferior porus trigeminus margin
MR findings
Homogeneously enhancing tubular mass
Intramural cysts when large
Cholesteatoma, Petrous Apex
Congenital or acquired PA cholesteatoma
CT findings
Smooth, expansile, low-density lesion
MR findings
Low T1, high T2
Nonenhancing PA lesion with restricted diffusion on DWI
Chondrosarcoma, Skull Base
Originates from petrooccipital fissure
CT findings
Characteristic chondroid matrix in 50%, invasive bony changes
MR findings
T2 high signal, mixed enhancement
Langerhans Histiocytosis, Skull Base
Langerhans histiocytes proliferation forming lytic sites in skull and skull base
Peds
Onset at 1 year; multifocal < 5 years
CT findings
Lytic lesion with beveled margins
MR findings
Avidly enhancing soft tissue mass
Apical Petrositis
Fever, retroorbital pain, diplopia, otorrhea
CT findings
Bony destructive changes
MR findings
Enhancing thick dura, PA pus does not enhance
Mucocele, Petrous Apex
Mimics cholesteatoma; no DWI restriction
CT findings
Expansile, smooth margined lesion
MR findings
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