Techniques And Modes Of Imaging The Upper Airway

A wide array of options exists to image the upper airway. In the past, the upper airway was examined by measuring pressure at different levels to obtain data pertaining to airway compliance and collapsibility (1-4). Investigators have also examined electromyographic activity of various upper airway muscles such as genioglossus, levator palatini, alae nasi, tensor palatini, geniohyoid, sternohyoid, palatoglossus, and the pharyngeal constrictors (1,5-11). These studies have

Anatomic Radiology Oral Cavity

FIGURE 2 Retropalatal axial magnetic resonance imaging (MRI) in a normal subject. The tongue, soft palate, parapharyngeal fat pads, lateral parapharyngeal walls (muscles between the airway and lateral parapharyngeal fat pads), parotid, subcutaneous fat, teeth, and mandibular rami are all highlighted on this axial MRI. Source: From Ref. 23.

FIGURE 2 Retropalatal axial magnetic resonance imaging (MRI) in a normal subject. The tongue, soft palate, parapharyngeal fat pads, lateral parapharyngeal walls (muscles between the airway and lateral parapharyngeal fat pads), parotid, subcutaneous fat, teeth, and mandibular rami are all highlighted on this axial MRI. Source: From Ref. 23.

provided collectively important information pertaining to the maintenance of airway patency and pathogenesis of airway collapse. During wakefulness, pharyn-geal patency is maintained predominantly by activation of pharyngeal dilator muscles (11). During apnea, it has been demonstrated that genioglossus and tensor palatine activation is primarily influenced by negative pressure in the airway (7,11). The negative pressure milieu is amplified during apnea due to greater inspiratory effort and increased tonic muscular activity (11). Further evidence demonstrates that administration of nasal continuous positive airway pressure (CPAP) reduces the accentuated genioglossus response to apnea to normal levels (7). However, the accentuated genioglossus electromyographic activation in apneics does not necessarily indicate that the tongue is moving since electromyographic activity does not correlate with mechanical action. Newer imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI) have allowed for a greater depth of understanding of the mechanical behavior of the soft-tissue structures.

The list of upper airway imaging techniques includes: cephalography (cephalometric radiography), nasopharyngoscopy, acoustic reflection, conventional and electron beam CT, three-dimensional CT, MRI/volumetric MRI, and optical coherence tomography. Together, these imaging modalities have greatly improved our understanding of OSA; however, each has its limitations. Fluoroscopy, an older imaging technique, involves significant radiation exposure and is time-consuming; it has been replaced by newer imaging modalities that can also provide dynamic images with the capability to obtain more precise anatomic measurements. The ideal imaging technique would provide safe, accurate, and repetitive measurements in the supine position. Such a modality would allow for three-dimensional volumetric reconstructions of the upper airway and its surrounding tissues/craniofacial structures and allow for measurements during wakefulness and sleep. A critique of the different imaging modalities is provided below beginning with the most recent development.

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