Imaging of the temporal bone following otologic and lateral skull base trauma

Introduction

Extent and severity of injury in the setting of otologic and lateral skull base trauma are readily detected noninvasively by imaging. Due to its speed and excellent spatial resolution, multidetector CT (MDCT) remains the primary modality of choice in the acute traumatic setting to delineate temporal bone trauma. Detection of subtle fractures and involvement of the otic capsule, carotid canal, and facial nerve canal can be readily assessed with MDCT. MDCT is also considered the primary imaging method of choice due to its ability to visualize the intracranial structures and exclude an acute intracranial hemorrhage or additional signs of intracranial trauma such as cerebral edema or infarct. Depending on the severity of the trauma or clinical suspicion of skull base involvement, CT angiogram (CTA) can also be acquired to assess the internal carotid artery or dural venous system.

In the nonacute setting, MDCT or cone beam CT (CBCT) can also be used to provide excellent delineation of the middle ear anatomy. Magnetic resonance imaging (MRI) may also be used to assess for subtle changes in the labyrinth, including intralabyrinthine hemorrhage or postinflammatory changes from labyrinthitis. In addition, MRI can help to characterize persistent soft tissue abnormalities in the temporal bone, in particular, to assess for a site of cerebrospinal fluid (CSF) leak or possible meningoencephalocele.

Multidetector CT

MDCT provides fast, high spatial resolution images (0.5–0.6 mm), which can be readily reconstructed in multiple planes. Reconstructed images of the temporal bone in an axial plane parallel to the lateral/horizontal semicircular canal, coronal images 90 degrees from the axial reformatted images, Pöschl reformats perpendicular to the long axis of the temporal bone, and Stenvers reformats parallel to the long axis of the temporal bone can help to assess for signs of trauma. In addition, MDCT images can provide soft tissue window images, which are critical to assess for signs of intracranial trauma, such as intraparenchymal, subarachnoid, subdural or epidural hemorrhage, cerebral edema, infarct, mass effect, or midline shift. When a patient arrives in an acute trauma or demonstrates unstable or concerning clinical symptoms, MDCT is the best modality to exclude more severe sequelae of trauma and is the safest modality due to its speed.

CT angiography (CTA) can be added to an MDCT temporal bone protocol to assess for injury to the internal carotid arteries or dural venous sinuses based on clinical indication or concern for significant skull base trauma. CTA can be timed in the early arterial and delayed venous phases to better delineate the type of vascular injury. With faster MDCT units becoming more readily available, these images can be acquired alongside dedicated chest, abdomen, and pelvis images to further assess the entire degree of bodily trauma.

Cone beam CT

In the nonacute setting, CBCT can provide excellent delineation of temporal bone anatomy. However, due to its slower speed of acquisition, it typically requires patient cooperation to perform without substantial motion artifact. As a result, this modality is best used in adult patients who are alert and cooperative. Some CBCT units require a sitting position and may be further limited due to height restrictions, limiting its use in some pediatric settings. CBCT also does not provide adequate soft tissue characterization and should be avoided when there is concern for intracranial disease or an extracranial soft tissue abnormality.

Magnetic resonance imaging

MRI should be reserved for the nonacute traumatic setting when the patient has stabilized and the patient has been carefully screened for any MRI safety considerations, such as implants or foreign bodies. MRI is preferred for better delineation of the soft tissue structures, intracranial anatomy, and fluid within the labyrinth. MRI can be acquired at 1.5 or 3 T magnetic field strengths. High-resolution (2–3 mm slice thickness), small field-of-view images of the temporal bone are necessary to provide adequate information. In addition, a cisternographic sequence that is heavily T2-weighted with 0.5–0.6 mm slice thickness is very helpful for assessing the fluid signal within the inner ear structures. Phase-corrected/sensitive inversion recovery MR images in the coronal and/or sagittal plane are also helpful additions to assess for sites of meningoencephalocele. Intravenous gadolinium can be considered particularly when assessing for labyrinthitis.