|ECR 2019 / C-3474|
|Petrous apex lesions: pictorial review and algorithmic diagnostic approach|
Findings and procedure details
Several methods have been proposed for classifying lesions of the petrous apex. For instance, lesions may be classified as cystic or solid, as surgically treatable or not surgically treatable, as aggressive or nonaggressive, or on the basis of their specific cause or origin.
We prefer to categorize petrous apex abnormalities into four groups: those that primarily arise within the petrous apex (developed within the apical structure or its walls), those involving the ICA, the meningeal envelope and the adjacent nerves, a third group concerning areas adjacent to the petrous bone or the temporal bone in which pathological phenomena can spread secondarily to the region, and finally metastases.
First of all, one should also be familiar with anatomic variants or pseudolesions in the petrous apex that can be mistaken for pathologic conditions.
a) Petrous apex pneumatization:
Pneumatization can be highly variable involving a large portion of the petrous temporal bone or only a small posterolateral segment.
The air cells of the petrous apex are susceptible to similar pathologic processes that occur in the mastoid segment including obstruction, opacification, inflammation, and infection.
b) Asymmetric fatty marrow in the petrous apex:
Typically, normal marrow contains significant adipose tissue, and signal characteristics parallel those of scalp or orbital fat.
Fatty marrow is hyperintense on routine T1- and T2-weighted sequences. Confirmation is made by observing the complete loss of signal with fat-saturation techniques, such as a STIR sequence and frequency-selected fat-suppressed sequences.
II- Intrinsic lesions of petrous apex:
a) Cholesterol granuloma:
The most common lesions arising in the petrous apex are cholesterol granulomas.
It can extend in multiple directions from the petrous apex, most commonly posterolaterally into the mastoid segment, the internal auditory canal, or the middle ear.
Temporal bone CT reveals an expansile, sharply defined, and often rounded mass of the petrous apex with cortical thinning and trabecular breakdown.
The general appearance is that of a slowly progressive benign process.
There is central soft-tissue density without an internal matrix, a calcification, or residual septations. If the lesion is sufficiently enlarged, frank bony dehiscence is observed.
On MRI, cholesterol granulomas are typically hyperintense on both T1 and T2 sequences because of the accumulation of blood breakdown products and proteinaceous debris.(Fig. 3)
Small lesions may be relatively homogeneous, whereas large lesions show more heterogeneity. Often cholesterol granulomas have a distinct hypointense peripheral rim on T2-weighted images due to hemosiderin deposition.
After contrast administration, there may be subtle peripheral enhancement secondary to inflammatory response but no central enhancement that would indicate solid tissue.
Congenital cholesteatomas are slow-growing lesions and may be asymptomatic for years.
On temporal bone CT, there is a smooth expansile lesion of the petrous apex (Fig 4). The central portion of the lesion shows no calcification or bony matrix.
On MRI, cholesteatoma typically is hypointense on T1, hyperintense on T2, and intermediate in signal on FLAIR images. Diffusion restriction is reported to be characteristic of this lesion.
After contrast administration, there may be subtle peripheral rim enhancement.
c) Petrous apicitis:
Petrous apicitis is an infectious process caused by medial extension of acute otitis media into a pneumatized petrous apex. Subsequent obstruction of drainage from the petrous apex to the middle ear may result in formation of a purulent abscess. Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus are the most common causative organisms.
Patients with petrous apicitis usually present with an acute febrile illness and some or all of the symptoms of the classic Gradenigo triad (ear pain, palsy of the sixth cranial nerve, and facial pain).
Possible complications of petrous apicitis include meningitis, cerebral abscess formation, and venous sinus thrombosis.
The MR imaging findings of petrous apicitis are high signal intensity on T2-weighted images, low signal intensity on T1-weighted images, and contrast enhancement in a pneumatized anterior petrous apex. There may be associated enhancement of the adjacent dura mater and cranial nerves due to meningitis. Abscesses demonstrate ring enhancement and restricted diffusion on diffusion-weighted images.
CT demonstrates opacification of petrous air cells in the early stage of the disease and bone destruction in later stages.
d)- Bone tumors
Chondrosarcoma of the skull base is a rare neoplastic lesion with potentially lethal outcome.
CT and MR imaging play a crucial role in visualization.
CT of chondrosarcoma depicts a dense soft tissue structure with moderate enhancement after contrast material application. The number of calcifications which may appear in ring-like structures is variable.
MR imaging shows low signal intensity using T1-weighted sequences and high signal intensity on T2-weighted images. The tumour shows enhancement after gadolinium application.
Partly there can be depicted matrix mineralization causing signal heterogeneity.
Chordomas are rare locally invasive tumorsarising from embryonic remnants of primitive notochordal elements.
CT reveals an expansile, lytic mass of the clivus that variably invades the sella, sphenoid sinus, and cavernous sinus.
Most authors argue that chordomas are more typically midline in location compared with chondrosarcomas.
CT often reveals scattered hyperdense foci representing residual bony sequestra or calcification within chordomas.
MRI better characterizes the soft-tissue invasive features associated with chordoma.
On MRI, chordomas can have variable signal on T1-weighted images including localized areas of hyperintensity due to hemorrhage or mucoid material.
Like chondrosarcomas, chordomas are characteristically hyperintense on T2-weighted sequences and tend to have well-defined margins (Fig. 5)
T2 signal may be heterogeneous as well because of the presence of hemorrhage, calcification, residual bone fragments, or high proteinaceous mucus pool.
There is generally prominent but heterogeneous enhancement.
III- Vascular and neuromeningeal lesions
a) Petrous internal carotid artery aneurysm:
Aneurysms of the petrous segment of the ICA are rare, especially when compared with aneurysms of more distal intracranial segments.
Petrous ICA aneurysms are typically asymptomatic and may be incidentally discovered during workup for unrelated symptoms.
CT shows expansion of the carotid canal in the anterior petrous apex. The walls of the canal may be thin or even dehiscent, and the aneurysms may mimic cholesterol granulomas at unenhanced CT.
However, contrast-enhanced studies show marked enhancement in the lumen of a nonthrombosed aneurysm.
At MR imaging, mixed signal intensity or heterogeneous enhancement can be seen in the aneurysm lumen due to turbulent flow or mural thrombus. Endovascular interventions or surgical trapping procedures are considered for symptomatic patients.
Petroclival and cerebellopontine angle meningiomas are the most likely to involve the petrous apex.
Petroclival meningiomas originate from the medial aspect of the petrous apex and course over its wall or enter the Dorello canal. Cerebellopontine angle meningiomas arise from the dura mater along the posterior surface of the petrous apex and can extend into the IAC.
Meningiomas appear as dural-based masses that are typically slightly hyperattenuating to brain tissue on CT images, iso- to hypointense on T1-weighted images, and iso- to hyperintense on T2-weighted images.
They usually enhance avidly after contrast material administration. Meningiomas may cause hyperostosis of the petrous apex, a finding that is generally most evident at CT; at MR imaging, the hyperostotic bone has low signal intensity on T1- and T2-weighted images.
Petrous apex schwannomas usually originate from the fifth, seventh, or eighth cranial nerves. At CT, schwannomas are usually isoattenuating to brain tissue and enhance after contrast medium administration.
At MR imaging, they generally appear as well-circumscribed, smoothly expansile masses that are iso- to hypointense on T1-weighted images and hyperintense on T2-weighted images, with enhancement on gadolinium-enhanced images (Fig 6).
IV- Extrinsic diseases of adjacent regions:
All neighboring tumors can reach the petrous apex. As an example we will see paragangliomas and endolymphatic sinus tumours.
Paragangliomas may invade the petrous apex via preformed air cell tracts from their sites of origin in the jugular foramen or middle ear.
Glomus tympanicum tumours (also known as paragangliomas of the middle ear) are highly vascular tumours that arise from paraganglia In the middle ear.
On CT images produced with a bone algorithm, paragangliomas show typical moth-eaten or permeative bone changes around the jugular foramen as a result of tumor infiltration through the haversian canal system.
At MR imaging, large paragangliomas may have a characteristic salt-and-pepper appearance on T1-weighted images.
The hypointense "pepper" represents high-velocity flow voids of feeding arterial branches in the tumor (also evident on T2-weighted images), while the less commonly seen hyperintense "salt" represents underlying foci of hemorrhage. These tumors usually enhance intensely at both CT and MR imaging after contrast material administration (Fig 7).
b) Endolymphatic sinus tumour:
An endolymphatic sac tumour is a rare locally aggressive papillary adenomatous neoplasm that originates from the epithelium of the endolymphatic sac and duct. Although most endolymphatic sac tumors occur sporadically, there is a known association with von Hippel-Lindau syndrome.
Most commonly, it grows posteriorly to involve the cerebellopontine angle.
This tumor usually presents with symptoms of endolymphatic hydrops (gradual-onset, low-frequency hearing loss, tinnitus, fullness, and vertigo). Less commonly, patients present with acute hearing loss due to acute intralabyrinthine hemorrhage and associated inflammation.
On CT, an endolymphatic sac tumour presents as a soft mass with aggressive bone erosion in the retrolabyrinthine petrous area.
Internal amorphous calcifications are present in almost all cases.
Because of the high frequency of intratumoral hemorrhage, the lesion can show variable T1 hyperintensity.
On T2-weighted sequences, the lesion is generally hyperintense. On contrast-enhanced images, there is intense nodular enhancement.
Because of its hypervascularity, this tumour, particularly large (> 2 cm) tumors, is frequently associated with flow voids.
For the same reason, this tumour shows tumour blush on angiography.
paragangliome, carcinoma nasopharynx , endolyphatic tumeur
As part of the skeletal system, the skull base is susceptible to hematogenously spread metastatic disease. Within the temporal bone, the petrous apex is the most commonly affected site.
Adenocarcinoma is the most common cell type and primary breast malignancy is the most common source. Other metastases occur in patientswith lung, prostate, skin (melanoma), or kidney cancer.
If the lesion is solitary, differentiating the lesion from myeloma, chondrosarcoma, chordoma, invasive or intraosseous meningioma, or even petrous apicitis may be difficult.
CT findings in patients with hematogenous metastasis can be extremely variable. Metastatic disease can cause insidious infiltration of the marrow space with little, if any, change in the trabecular bone or cortex. More often, symptomatic tumor reveals a soft- tissue mass destroying cortical and cancellous bone of the apex, ranging in appearance from sclerotic to permeative to frankly lytic.
Unenhanced and contrast-enhanced fat-suppressed MR images reveal an infiltrating enhancing mass replacing normal marrow fat signal with or without extraosseous extension (Fig 8).
Thematically related posters
ECR 2019 / C-3083
Arrested pneumatization of the sphenoid sinus in four symptomatic patients: principal diagnostic elements, pitfalls and differential diagnosis.