New Methods of Bedside Airway Assessment: Cone Beam Computed Tomography, Ultrasound, and Craniofacial Phenotyping

Jacek Wojtczak

Bo Hu

Most anesthesiologists predict difficult intubation based on several bedside preoperative screening tests. The most popular is establishing the degree of visibility of oropharyngeal structures based on Mallampati classification,¹ measuring thyromental distance (TMD) and assessing neck movement and mouth opening.²,³,⁴ Unfortunately, all these tests have only modest sensitivity and specificity in predicting difficult intubation.²,³,⁴ Baker et al⁵ performed the meta-analysis of 24 studies where the accuracy of TMD measurements was assessed as predictor of difficult intubation. The test sensitivity was only 16% when fingerbreadths were used for assessment and increased to 48% when the ruler or calipers were used for measurements. It implies that the preoperative airway evaluation has to be much more quantitative in order to be predictive. In this chapter, we will review three modalities that potentially may improve preoperative airway evaluation.

MRI and CT are considered gold standards in the quantitative, three-dimensional evaluation of the airway. Both imaging techniques can perform complete volumetric airway analysis,⁶,⁷,⁸ but MRI has an obvious advantage over CT in improved visualization of soft tissues. Both techniques are expensive, hardly bedside, not feasible in patients with metal implants (MRI), require long examination time (MRI), and expose patients to radiation (CT). Recently introduced cone beam CT, however, is becoming very popular in the office setting due to its low cost, small size, very short examination time, and very low level of patient irradiation.

Ultrasound (US) of the airway is a bedside imaging technique that has been used for several years, but the quality of US scans of the airway has been generally poor. Recent advances in this technology, availability of high-frequency probes, and inexpensive portable US units revived the interest in this modality as a convenient bedside tool.⁹

Craniofacial phenotyping is the newest of the new methods of bedside airway assessment and requires three-dimensional (3-D) laser scanning with an automated 3-D rendering10,11 or two-dimensional (2-D) digital photography.¹²

CONE BEAM COMPUTED TOMOGRAPHY

Cone beam computed tomography (CBCT) is a recent advancement in CT imaging, facilitated by parallel advancements in flat panel detectors technology, improved computing power, and the relatively low power requirements of the X-ray tubes used.¹³,¹⁴ The imaging-source detector and the method of data acquisition distinguish CBCT from traditional CT imaging. Traditional CT uses a high-output rotating anode X-ray tube, whereas CBCT uses a low-power, medical fluoroscopy tube that provides continuous imaging throughout the scan. Traditional CT records data with a fan-shaped X-ray beam onto image detectors arranged in an arc around the patient, producing a single slice image per scan. Each slice must overlap slightly in order to properly reconstruct the images. The advanced CBCT technology uses a cone-shaped X-ray beam that transmits onto a solid-state area sensor for image capture, producing the complete volume image in a single rotation. The sensor contains an image intensifier and a CCD camera, or an amorphous silicon flat panel detector.

The single-turn motion image-capture used in CBCT is quicker than the traditional spiral motion and can be accomplished at a lower radiation dose as a result of no overlap of slices. Manufacturers are designing CBCT scanners with the physical space available in clinics and patients’ comfort in mind. Usually, upright seating is used in CBCT scanners with the X-ray tube and panel detector rotating around the patient’s head (Figs. 11-1 and 11-2).

FIGURE 11-1 Cone beam computed tomograph (i-CAT) used for imaging of dental implants (courtesy of Eastman Dental Center, University of Rochester, NY).

FIGURE 11-2 (A) Sagittal i-CAT CBCT scan: T- turbinates, HP- hard palate, H – hyoid bone, M – mandible, E – epiglottis. (B) Coronal i-CAT CBCT scan: T- turbinates, HP- hard palate, M – maxilla.

CBCT became a very popular modality in dentistry, especially implantology. Its value as a clinical tool is also studied in oral and maxillofacial surgery. As it provides not only skeletal but also soft tissue images with an option of 3-D reconstruction, it may become a very useful tool in upper airway examination in anesthesiology in patients known or suspected to be difficult to intubate. Osorio et al¹⁵ published a preliminary report on the applicability of CBCT for the purpose of the clinical airway management. They performed 3-D reconstructions of the airway as well as “virtual laryngoscopy” by generating “flying through” reconstructions. They found the resulting video clips to be of high quality, similar to fiberoptic imaging, but without the invasiveness. They concluded that virtual laryngoscopy may be a promising future technique to support clinical anesthesia practice. In their opinion, CBCT has the potential to emerge as a comprehensive and practical system to evaluate the upper airway and should become an excellent research and teaching tool for understanding the normal and abnormal airway.

ULTRASOUND IMAGING

US imaging of the upper airway offers several advantages compared with other imaging techniques. It is widely available, portable, repeatable, relatively inexpensive, pain-free, and safe.⁹,¹⁶,¹⁷,¹⁸,¹⁹

The curved array low-frequency (5 MHz) transducers (Fig. 11-3) are preferred for submandibular scans to visualize the tongue and the swallowing dynamics. Patients with long hyomental distances may require a standoff to enable an accurate measurement of intraoral distances (Fig. 11-4).

FIGURE 11-3 A: Midsagittal submandibular sonography using 5 MHz curved array transducer; B: US anatomy of the suprahyoid region. M, mandibular shadow; H, shadow of the hyoid bone; GH, geniohyoid muscle; MH, mylohyoid muscle; TS, tongue surface.

FIGURE 11-4 (A) Standoff gel pad attached to the curved 5 MHz ultrasound probe to enlarge the field of view and improve visualization of the near field areas (e.g. floor of the mouth). (B) Transverse submandibular scan with the standoff pad presenting as a hypoechoic space between the surface of the skin and the probe. M – mandible; GH – geniohyoid muscle.

The high-frequency linear probes are useful in imaging the superficial structures yielding high-resolution scans (Fig. 11-5); however, the US penetration is very poor (Fig. 11-6).

Only gold members can continue reading. Log In or Register to continue