This copy is for personal use only. To order printed copies, contact [email protected] 1161 TRAUMA/EMERGENCY RADIOLOGY Skull Base–related Lesions at Routine Head CT from the Emer- gency Department: Pearls, Pitfalls, and Lessons Learned Hernan R. Bello, MD Joseph A. Graves, MD Routine non–contrast material–enhanced head CT is one of the Saurabh Rohatgi, MD most frequently ordered studies in the emergency department. Mona Vakil, MD Skull base–related pathologic entities, often depicted on the first or Jennifer McCarty, MD last images of a routine head CT study, can be easily overlooked in Rudy L. Van Hemert, MD the emergency setting if not incorporated in the interpreting radi- Stephen Geppert, MD ologist’s search pattern, as the findings can be incompletely imaged. Ryan B. Peterson, MD Delayed diagnosis, misdiagnosis, or lack of recognition of skull base pathologic entities can negatively impact patient care. This article Abbreviations: ASB = anterior skull base, reviews and illustrates the essential skull base anatomy and com- CCJ = craniocervical junction, CPA = cerebello-­ pontine angle, CSB = central skull base, mon blind spots that are important to radiologists who interpret CSF = cerebrospinal fluid, IIH = idiopathic nonenhanced head CT images in the acute setting. The imaging intracranial hypertension, MCF = middle cra- characteristics of important “do not miss” lesions are emphasized nial fossa, PCF = posterior cranial fossa, PNS = paranasal sinuses, PPF = pterygopalatine fossa, and categorized by their cause and location within the skull base, SIH = spontaneous intracranial hypotension, and the potential differential diagnoses are discussed. An interpre- TMJ = temporomandibular joint tation checklist to improve diagnostic accuracy is provided. RadioGraphics 2019; 39:1161–1182 ©RSNA, 2019 • radiographics.rsna.org https://doi.org/10.1148/rg.2019180118 Content Codes: From the Department of Radiology and Im- aging Sciences, Emory University School of Introduction Medicine, Emory University Midtown Hos- pital, 550 Peachtree Rd, Atlanta, GA 30308 Non–contrast material–enhanced CT of the head is the primary (H.R.B., J.A.G., M.V., R.B.P.); Department of imaging modality used to evaluate patients with neurologic deficits Radiology, University of Massachusetts Medi- who present to the emergency department. Routine head CT often cal School, Worcester, Mass (S.R.); Department of Diagnostic and Interventional Imaging, Uni- includes images of portions of the skull base, extracranial spaces, and versity of Texas Health Science Center at Hous- upper cervical spine. Skull base anatomy is complex, and pathologic ton, Houston, Tex (J.M.); and Department of Radiology, University of Arkansas for Medi- conditions in this region can be subtle and easily overlooked. There- cal Sciences, Little Rock, Ark (R.V.H., S.G.). fore, all radiologists working in the acute care setting must have a Presented as an education exhibit at the 2017 thorough knowledge of normal skull base anatomy at CT and the RSNA Annual Meeting. Received March 30, 2018; revision requested June 21 and received wide spectrum of disease that may manifest in this area. July 17; accepted July 24. For this journal-based SA-CME activity, the authors, editor, and re- viewers have disclosed no relevant relationships. Address correspondence to J.A.G. (e-mail: [email protected]). ©RSNA, 2019 SA-CME LEARNING OBJECTIVES After completing this journal-based SA-CME activity, participants will be able to: ■■Discuss the complex anatomy of the skull base. ■■Characterize easily missed skull base lesions on nonenhanced head CT images in the acute setting. ■■Identify clinical or imaging red flags that may necessitate further workup. See rsna.org/learning-center-rg. 1162 July-August 2019 radiographics.rsna.org greater than 5 mm. Studies have shown the supe- TEACHING POINTS riority of depicting findings on thinly collimated ■■ If there is discordance between the imaging findings and the spiral CT images (2). Obtaining thin-section im- clinical history, a more detailed investigation of the electronic ages of 3 mm or less is recommended in the setting medical record or a direct conversation with the clinician or consultation service members should be performed. of trauma, as use of thick-section images has been ■■ Any time that facial pain or paresthesia (in a trigeminal nerve found to miss a significant number of linear frac- distribution) manifests, the radiologist should interrogate the tures and skull base lesions, particularly when the PPF for evidence of abnormal soft tissue replacing the normal images are obtained in the horizontal plane (3,4). fatty contents, which could indicate extraosseous extension In addition to obtaining directly acquired axial of metastatic disease and compression of the pterygopalatine images, high-resolution multiplanar reformatted ganglion. images, including those obtained in the coronal ■■ The frontal sinus has bony dehiscences, structural abnor- and sagittal planes, may be constructed from the malities that can channel the spread of infection. Patients can present with headaches, photophobia, seizures, or focal neu- axial dataset to increase sensitivity in the acute rologic signs. Complications include subdural and epidural setting (2,5). At our trauma center, in addition to empyema, cerebritis, brain abscess, and meningitis, which are obtaining CT images with 5-mm section thick- often neurosurgical emergencies. ness, we routinely obtain 0.625-mm thin-section ■■ If there is any suggestion of temporal bone injury, evaluating images by using a bone algorithm with sagittal nonenhanced lung-window CT images of the head may allow and coronal reformatted images for all trauma identification of extra-axial pneumocephalus, which is a find- ing highly suggestive of injury. head CT protocols, thereby enhancing the depic- tion of complex skull base anatomy. ■■ When one encounters mastoid effusions (especially a unilat- eral effusion), the soft tissues along the eustachian tube and within the nasopharynx should be thoroughly investigated to Anatomy Review exclude an underlying obstructive mass. The anatomy depicted on the first few images of a routine nonenhanced head CT study can be cat- egorized as being located in the skull base, cranio- cervical junction (CCJ), or extracranial soft tissues. In addition, using a systematic search pattern and having an overall awareness of the neurologic Skull Base manifestations of a skull base lesion can aid in The skull base separates the intracranial and increasing the diagnostic yield for the interpreting extracranial compartments. It is formed from radiologist. These neurologic manifestations are portions of the frontal, ethmoid, sphenoid, paired often referred to as red flags and include cranial temporal, paired parietal, and occipital bones nerve deficits, strokelike symptoms that are atypi- (Fig 1). Arteries, veins, and multiple cranial cal for a specific vascular territory, and progres- nerves traverse the skull base through a constel- sive and/or recurrent onset of symptoms. Delayed lation of canals and foramina. The bones and diagnosis, misdiagnosis, or lack of recognition foramina of the skull base can be segmented into of skull base conditions can negatively impact three main components: anterior, middle, and patient care, highlighting the importance of using posterior (Figs 2,3) (6–9). a mental checklist when interpreting studies of The ASB is formed by the ethmoid, frontal, and the skull base, especially the first few images of sphenoid bones. Laterally, the ASB is composed of the study. the orbital plates of the frontal bones and antero- In this article, we review and illustrate the es- medially by the cribriform plate, crista galli, and sential skull base anatomy and the common blind fovea ethmoidalis of the ethmoid bone. Posteri- spots important to radiologists interpreting a orly, the flat surface of the planum sphenoidale nonenhanced head CT study in the acute setting. separates the sphenoid sinus from the intracranial We review the imaging characteristics of impor- contents. The line dividing the anterior and middle tant “do not miss” lesions at the skull base on the skull base can be drawn along the posterior aspect basis of their anatomic location and manifesta- of the lesser sphenoid wing and along the posterior tions. An interpretation checklist to help improve aspect of the planum sphenoidale, which (depend- diagnostic accuracy is provided. ing on the source used) typically does not include the anterior clinoid processes (6–9). Imaging Parameters The CSB is formed predominantly by the According to the practice parameter for the per- sphenoid and temporal bones. Anteriorly, the formance of head CT developed by the American sphenoid tuberculum sellae, anterior clinoid College of Radiology, the American Society of processes, and greater sphenoid wings define the Neuroradiology, and the Society for Pediatric Ra- CSB. Laterally, the temporal bone squamosal diology (1), contiguous or overlapping axial images portion and parietal bone confine the CSB. The should be acquired with a section thickness of no petrous ridge of the petrous temporal bone seg- RG • Volume 39 Number 4 Bello et al 1163 Figure 1. Basic skull base anatomy. (a) Axial three- dimensional reconstructed CT image with color-coded overlay shows the skull base sections. Blue = central skull base (CSB), purple = posterior skull base, teal = anterior skull base (ASB). (b) Axial CT image with color-coded overlay shows the skull base bones. Blue