The EAC has two portions: the lateral portion (one third) is fibrocartilaginous and demonstrates an incomplete elastic cartilage ring measuring 8 mm in length. 2 The canal is widest in diameter at its lateral/superficial end adjacent to the pinna, where it flares out in a trumpet bell-like configuration. The resulting cross-sectional diameter has a roughly oblique oval configuration. The canal travels inferiorly and posteriorly to the TM in a slightly S-shaped course. The EAC is lined by squamous epithelium that is continuous with the skin from the pinna. The adult EAC is ∼ 25 mm in length and air-filled in its normal state. 1 Thus, atresia and stenosis of the EAC can result in a conductive hearing loss of up to 60 dB, manifested as an air–bone gap on audiological testing. The purpose of the pinna and EAC is presumably to collect and funnel sound to the TM as a resonating tube resulting in a 10 to 20 dB gain. The EAC extends from the meatus of the pinna to the TM, its medial boundary, which divides the external from the middle ear. For patients with facial paralysis, it is imperative that the imaging studies include the entire course of the facial nerves.Īt birth, the tympanic membrane (TM), ossicles, and otic capsule are already of adult size, but the EAC grows progressively until the age of 9 years, when it achieves its typical S-shaped course and adult size. Images using heavily T2-weighted techniques with high resolution (constructive interference in steady state CISS) may be performed if anatomical assessment of the structures in the inner ear or internal auditory canal is required. Additionally, 4 to 5 mm thick axial and coronal postcontrast T1-weighted images are obtained to include the entire brain. After gadolinium contrast administration, we obtain T1-weighted (3 mm thick) images in axial and coronal planes through the regions of interest with and without fat suppression techniques. Our MRI protocol includes sagittal T1-weighted images (4 mm thick), axial T1-weighted images (2 mm thick), axial T2-weighted images (2 mm thick), and axial FLAIR (fluid attenuated inversion recovery T2-weighted images, 4 mm thick), all obtained before contrast is administered. MRI is also sensitive for detection of bone marrow edema, and it can occasionally demonstrate bony involvement not suspected by CT in carcinoma cases. Subtle dural enhancement may be a first indication of intracranial involvement. Although lesions that spread through bone into the middle cranial fossa can be obvious on reconstructed coronal and sagittal images, MRI is generally better for the assessment of soft tissue, brain, and dural extension. MRI is usually used to assess the spread of tumor or inflammatory processes intracranially. For some inflammatory and neoplastic lesions, contrast material may be given and images processed with soft tissue window settings. For congenital lesions, noncontrast studies with bone windows will suffice for most patients. Although direct coronal imaging may be performed using a similar protocol, coronal and sagittal reconstructions can also be obtained at 0.75 mm intervals with limited radiation exposure, which is especially desirable in children. With the use of a multislice CT scanner, submillimeter (0.6 to 0.75 mm) axial images are routinely obtained and processed with bone and soft tissue algorithms. Intracranial complications arising from disorders of the pinna and EAC can also be evaluated by the radiologist.Īt our institution, we perform temporal bone computed tomography (CT) studies more frequently than magnetic resonance imaging (MRI) studies for examination of the EAC because of CT’s superiority for the evaluation of bone erosion. Despite the ease of visualization of these external structures by the otolaryngologist, the radiologist can assist with important information for surgical planning and aid in determining the type and extent of the pathology. This unique embryonic origin, in conjunction with the superficial location of the pinna and EAC, results in a distinct pathology spectrum when compared with that involving the middle and inner ears. The external ear, comprised of the pinna or auricle and the external auditory canal (EAC), has a different embryological origin than that of the middle and inner ears. Jewells, Mauricio Castillo, and Craig Buchman
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