Skull Base Imaging

Vincent Chong

• Publisher: Elsevier
• Year: 2017
• ISBN:
  9780323496049
• Topic: Médecine
• Subtopic: Radiologie, radiothérapie et médecine nucléaire

Use today's latest technology and methods to optimize imaging of complex skull base anatomy. This practical reference offers expert guidance on accurate preoperative lesion localization and the evaluation of its relationship with adjacent neurovascular structures.

• Features a wealth of information for radiologists and surgeons on current CT and MR imaging as they relate to skull base anatomy.
• Covers localizing skull base lesions, reaching the appropriate differential diagnosis, and deciding which surgical approach is best.
• Consolidates today's available information and guidance in this challenging area into one convenient resource.

Chapter 1

Anterior Skull Base

David S.Y. Sia, MBBS, FRCR, MMED, Clement Yong, MBBS, FRANZCR, James T.P.D. Hallinan, MBChB, BSc, FRCR, and Vincent Chong, MD, MBA, MHPE

Abstract

The anterior skull base is an important anatomic structure that separates the cranial cavity above and the sinonasal cavity and the orbits below. This region is frequently breached by aggressive disease, such as malignancy and infection. Therefore, diseases in the sinonasal cavity can invade the cranium and vice versa. The resultant transcompartmental spread has significant impact on the prognosis and management. The primary role of imaging is to assess the extent of disease and direct treatment strategies. This chapter aims to provide a concise description of the regional anatomy, followed by key considerations when interpreting relevant imaging studies. Finally, several examples of neoplastic and nonneoplastic lesions involving the anterior skull base are included.

Keywords

Anterior skull base; Imaging

The anterior skull base is a region of interest because it is frequently breached by aggressive disease such as malignancy and infection or even chronic long-standing but progressive lesions such as mucoceles. This resultant transcompartmental spread has a significant impact in prognosis and management. Imaging plays a key role in the assessment of disease extent and management planning.

Most tumors that involve the anterior skull base originate in the sinonasal compartment. Malignant sinonasal tumors are relatively rare, comprising 3% of all head and neck malignancies.¹ These tumors (ranging from the common squamous cell carcinoma [SCC] to the rare sarcomas) are usually aggressive. They generally have a poor prognosis because of extensive disease at presentation and high local recurrence rate.² Benign sinonasal tumors are also common, and they include sinonasal polyps, juvenile angiofibroma, and inverted papilloma. These lesions are usually confined to their site of origin and rarely encroach or erode the skull base.

As most sinonasal tumors are amenable to biopsy, the primary role of imaging is in mapping the tumor extent. Information such as osseous erosion, meningeal and brain invasion, or orbital extension is crucial for management planning.³ Significant advancement in craniofacial surgical approach has enabled safe and reliable resection of sinonasal tumors.⁴ The craniofacial approach essentially allows for a wide exposure of the anterior craniofacial structures and can be modified to extend surgical accessibility (depending on the individual case and surgeon preference). The anterior craniofacial approach, as the name suggests, incorporates a combination of transfacial and transcranial procedures. It is important to note that the advances in craniofacial resection techniques are largely contributed by the wealth of information provided by preoperative CT and MRI.

Anatomy

The anterior skull base separates the cranial cavity above from the orbit and sinonasal compartments below. This boundary is formed by two bones: the cribriform plate centrally and the orbital plates of the frontal bone laterally (Fig. 1.1). The cribriform plate is the part of the ethmoid bone that consists of two parallel grooves on which the olfactory bulbs sit, separated by a midline triangular process called the crista galli (because of its resemblance to the comb of a rooster). The crista galli serves as an attachment site for the falx cerebri. The floor of each groove has multiple tiny perforations that allow passage of olfactory nerves to the nasal mucosa. The frontal sinuses are located anterior to the cribriform plate.

The thin cribriform plate is well demonstrated on coronal CT images. This structure is also visible on MRI as a linear hypointense signal on T1-weighted and T2-weighted sequences. It is frequently involved by tumors and fractured by trauma or surgery. The orbital plates of the frontal bone constitute a thick and robust barrier to disease spread. The lamina papyracea is the medial orbital wall that separates the orbit from the ethmoid sinus. It is thus named because of its paper-thin appearance and is easily fractured or eroded by tumors. This structure, like the cribriform plate, is easily evaluated on CT. The periorbita is the periosteum lining the walls of the bony orbit to which it is loosely connected. It is continuous with the periosteum of facial bones anteriorly and the dura mater posteriorly through the superior orbital fissure. This structure is indistinguishable from the adjacent orbital wall in normal patients but can be seen as a hypointense line on T1-weighted and T2-weighted MRI when lifted off the underlying bone by tumor, blood, or pus.

Key Imaging Considerations

Intracranial invasion is recognized as the most adverse prognostic factor in sinonasal tumors and is a crucial feature requiring careful imaging evaluation.⁵ Dural invasion alone significantly decreases survival rate from 68% to 25% in a series of adenocarcinomas.⁶ On the other hand, orbital invasion by sinonasal malignancy is an independent prognostic factor with a significant impact on survival rates.⁷ The incidence of visual involvement is estimated at 50%.⁸ This is related to its close proximity to the sinuses and the presence of multiple routes of spread via anatomic foramina, traversing vessels, and nerves, as well as thin bones such as the lamina papyracea. Imaging plays a critical role in determining orbital disease extension (Fig. 1.2).

Fig. 1.1 Anterior skull base anatomy. Coronal CT images of the anterior skull base show the thin cribriform plate centrally consisting of olfactory grooves (asterisks in A) separated by the crista galli in the midline (solid arrow in A). There are tiny perforations on the floor of each olfactory groove as seen on sagittal CT (arrowheads in B). The thick orbital plates of the frontal bone form lateral portions of anterior skull base (dashed arrows in A). The anterior margin of the anterior skull base is formed by the frontal sinuses centrally (hash in C). Photograph of a dry skull specimen (D) shows the anterior skull base seen f...