The Black Sombrero: A Helpful Landmark in Axial Imaging Evaluation of the Larynx and Hypopharynx

Abstract

Computed Tomograpy (CT) scan and Magnetic Resonance Imaging (MRI) evaluation of the larynx and hypopharynx can be tricky for the untrained eye. We discuss a peculiar pattern of the supraglottic airway at the level of the base of the epiglottis seen on axial CT and MR images, resembling a “black hat” or a “black sombrero.” The level of the base of the epiglottis also corresponds to that of the hyoid bone, where two valuable hypopharyngeal sub-sites are visible- the pyriform sinuses and the posterior hypophayngeal wall.

A hat has basically two parts, an apical convex crown and a basal horizontal brim. (Figure 1.A) Correlating the hat’s appearance on axial CT and MR imaging, the hat appears black due to the presence of air at the supraglottic level wherein the crown is positioned anteriorly and the brim positioned posteriorly. (Figure 1.B)

The top-most convex part of the crown corresponds to the base of the epiglottis.  Anterior to it is the pre-epiglottic fat plane and the body of the hyoid bone. The crown is also flanked by the paraglottic fat plane on both sides that appears hypodense (dark) on CT and hyperintense (bright) on T1 MRI. On the other hand, the posterior-most portion of the hat’s brim abuts the posterior hypopharyngeal wall - laterally outlining the pyriform sinuses on each side. The area where the crown joins the brim corresponds to the aryepiglotic folds that are delineated laterally by the medial walls of the pyriform sinuses while its medial aspects correspond to portions of the supraglottis. (Figure 2)

The size and configuration of the hat varies and can range from one with a big crown and a small brim (resembling a  “bowler” or “derby hat”) to one with a tall crown and a floppy brim (resembling a “Mexican sombrero”). (Figures 2, 3) Infiltrating tumors and inflammatory processes can distort the hat’s usual symmetric configuration, and can be useful in determining possible sites of inflammation or infection, tumor origin, and spread to hypopharyngeal and supraglottic structures, as the hat’s center represents a potential space at the supraglottic area devoid of any soft tissue structures. (Figure 4)

In radiology, “the bowler hat sign” and “Mexican hat sign” have been used to describe gastrointestinal polyps.^1,2,3 The former “is produced by the acute angle of attachment of the polyp to the mucosa” while the latter “consists of two concentric rings and is produced by visualizing a pedunculated polyp head-on.”¹ The “Mexican hat sign” has also been used to describe osmotic demyelination on MRI.⁴

To our knowledge, these terms have not been applied to the interpretation of  supraglottic and hypopharyngeal CT and MRI axial images. Recognition of the “Black Sombrero” pattern can be helpful in understanding the anatomic relationship of the supraglottic and hypopharynx structures on axial CT and MR imaging, particularly in the evaluation of supraglottic and hypopharyngeal tumor spread.