When is a high‑resolution CT scan of the temporal bone indicated?

High-Resolution CT of the Temporal Bone: Clinical Indications

High-resolution CT (HRCT) of the temporal bone without IV contrast is the imaging modality of choice for evaluating osseous structures, bony abnormalities, and specific pathologies of the temporal bone, while MRI remains superior for soft tissue and neural evaluation. 1

Primary Indications for Temporal Bone CT

Evaluation of Pulsatile Tinnitus

• HRCT temporal bone without contrast detects bony causes of pulsatile tinnitus including otospongiosis, Paget disease, sigmoid sinus diverticulum and dehiscence, high-riding jugular bulb, and superior semicircular canal dehiscence (SSCD). 1
• IV contrast is not necessary for assessment of semicircular canal dehiscence, enlarged vestibular aqueduct, sigmoid sinus wall abnormalities, jugular bulb dehiscence, or glomus jugulare paraganglioma. 1
• When a vascular retrotympanic lesion is seen on otoscopy, CT temporal bone distinguishes between glomus tumors and vascular variants (aberrant/lateralized internal carotid artery, persistent stapedial artery, dehiscent jugular foramen) to avoid unnecessary biopsies with potentially devastating complications. 1

Hearing Loss and Vertigo Assessment

• CT temporal bone provides excellent delineation of the bony labyrinth and is highly sensitive for detecting temporal bone fractures in post-traumatic vertigo, superior semicircular canal dehiscence in patients with vertigo provoked by loud noises, and erosions in the bony labyrinth from inflammatory or iatrogenic causes. 1
• For mixed or conductive hearing loss, HRCT identifies ossicular erosion, fusion, or displacement, as well as changes of otospongiosis. 1, 2
• HRCT demonstrates ossicular chain abnormalities with 80.65% sensitivity despite surrounding soft tissue, making it valuable for diagnosing small ossicular breaks or fractures. 2, 3

Preoperative Cochlear Implant Planning

• High-spatial resolution CT provides preoperative delineation of underlying cochlear malformation in congenital hearing loss, detects otospongiosis changes, suggests round window occlusion, identifies labyrinthitis ossicans, congenital bony fusion of ossicles, and alerts surgeons to otomastoiditis or variant anatomy (such as facial nerve). 1
• CT delineates cochlear and vestibular aqueduct size, alerting surgeons to the possibility of intraoperative cerebrospinal fluid gusher. 1

Facial Nerve Palsy Evaluation

• High-resolution temporal bone CT is complementary to MRI by characterizing osseous integrity of the temporal bone through which the facial nerve courses. 1
• CT excels at imaging temporal bone fractures, presurgical osseous anatomy, bony facial nerve canal involvement with inflammatory middle ear disease, facial canal foraminal expansion, patterns of bone erosion, and intrinsic bone tumor matrices. 1
• Thin sections should be obtained to evaluate the course of cranial nerve VII, with contrast useful in settings of infection or suspected tumor. 1

Middle Ear Disease and Cholesteatoma

• HRCT shows high sensitivity (89.29%) for identifying cholesteatoma and provides detailed assessment of ossicular chain changes and lateral semicircular canal erosion. 3
• CT is instrumental in diagnosing middle ear disease and delineating most temporal bone fractures. 4

Temporal Bone Trauma

• HRCT is highly sensitive for detecting temporal bone fractures and is particularly valuable in post-traumatic hearing loss, demonstrating fractures extending across the otic capsule or involving the ossicular chain. 1, 2

Technical Specifications

Optimal Imaging Protocol

• Non-contrast high-resolution CT is recommended as IV contrast is not beneficial due to the density of surrounding temporal bone. 2
• Bone algorithm reconstructions maximize visualization of ossicular structures. 2
• Reconstructions along multiple planes (axial, coronal, and sometimes oblique) are necessary to fully assess the ossicular chain. 2
• Contiguous images at 1.0 mm thickness provide adequate detail for anatomical assessment. 5

Field of View Advantage

• CT temporal bone has a smaller field-of-view and higher resolution to evaluate the temporal bone compared with CTA, making it superior for detailed osseous evaluation. 1

Critical Pitfalls to Avoid

Inappropriate Imaging Choices

• Standard CT head lacks necessary resolution to adequately visualize small ossicular structures and should not be used for temporal bone evaluation. 1, 2
• CT head provides insufficient details of the inner ear to be useful in patients with peripheral vertigo, with low diagnostic yield. 1
• CTA head is not indicated for ossicular chain injuries or routine temporal bone evaluation. 2
• MRI alone is insufficient for evaluating ossicular fractures due to limited ability to delineate bony details. 2

Limitations of CT

• HRCT has major limitations in stapes and oval window areas, especially in cases of perilymph fistulas. 4
• CT has low sensitivity (33.33%) for facial canal dehiscence despite being excellent for other bony abnormalities. 3
• HRCT is less sensitive for soft tissue abnormalities that may accompany ossicular injuries. 2
• CT temporal bone without IV contrast does not show arterial abnormalities in the neck or intracranial vascular abnormalities such as dural arteriovenous fistulas, arteriovenous malformations, and transverse sinus stenosis. 1

When to Choose Alternative Imaging

• When intracranial or cervical vascular pathologies are suspected in pulsatile tinnitus, CTA head and neck with IV contrast is the imaging modality of choice as it shows both arterial and venous pathologies in addition to bony details. 1
• MRI with gadolinium contrast is the definitive first-line modality when evaluating for tumors, perineural spread, and intracranial extension, providing superior soft tissue contrast resolution. 6
• MRI provides complementary role to temporal bone CT in preoperative cochlear implant assessment, detecting cochlear malformations or cochlear nerve deficiency that directly impact surgical approach. 1