Radiography
Findings
Conventional plain films (ie, AP and lateral projections and images obtained with selective high-kV filtration techniques) of the larynx provide preliminary or definitive information about foreign bodies, trauma, and other types of acute and chronic airway obstruction. These radiographs can demonstrate soft-tissue swelling, alterations of the cartilaginous framework (if it is sufficiently calcified), and the position of the air column.
Xeroradiography, with its capacity for edge enhancement, can be used to clarify intrinsic soft-tissue detail (eg, calcifications), delineate masses and stenoses, sometimes depict cartilage abnormalities (eg, fractures, erosions), and identify foreign bodies by their type and location. Unfortunately, the radiation exposure with this technique is 3-5 times that of conventional radiography, and xeroradiography is rarely used because of the high cost of leasing the equipment.
Using radiologic guidance and local anesthesia, Profili and colleagues evaluated endoscopic airway stenting in 16 patients with malignant tracheobronchial stricture.4 The authors reported good visualization of the stenotic tract and satisfactory control of the positioning stent before and during release. The procedure was also less invasive, more rapid, and more cost-effective compared with a combined endoscopic/fluoroscopic technique.
Degree of Confidence
The variability of calcification in the laryngeal cartilages can create a diagnostic problem.
Computed Tomography
Findings
CT scanning, sectional image data acquisition, and 3-dimensional (3-D) airway image reconstruction have become increasingly useful in head and neck surgery.
The acquired images provide detailed information regarding the tracheobronchial tree and its associated pathology. Moreover, 2-D and 3-D images that are generated by CT scan data provide additional information regarding airway pathology. A variety of computer-processing algorithms can be applied in acquired CT scan data, including multiplanar reformatting (MPR), shaded surface display (SSD), maximum or minimum intensity projection (MIP), and volume-rendering techniques (VRT), as well as a more recent technique, virtual endoscopy (VE).
Conventional coronal CT scanning allows visualization of the frontal view anatomy without a superimposed spine. This technique enables satisfactory analysis of the vertical extent of the tracheal stenosis or stricture, but conventional coronal CT scanning is only used occasionally because of its limited gray-scale ability to differentiate soft tissue. However, airway images, especially with the added sagittal projection, are excellent.
The best compromise among the combined factors of CT-scan airway measurement precision, patient breath-holding time, and total x-ray dose is the use of a 3-mm section thickness, a reconstruction interval of 1.5 mm, and a maximal pitch of 1.3-1.5, as well as the application of the edge-enhancing modus.
Inner-surface reconstructions that are calculated from helical CT scan data sets offer a diagnostic option for upper airway assessment. With special software, it is possible to create a virtual and continuous endoscopic overview of the inner surface of a hollow viscera on a monitor; these images are similar to endoscopic views and have been compared with the intraoperative findings in patients with laryngeal or tracheal stenoses. Excellent results have been reported and have led to the conclusion that virtual endoscopy offers a valuable overview for assessing the extent and location of the stenoses.
Xiong et al reported the sensitivity of CT scan-based virtual bronchoscopy (CTVB) in detecting central tumors as 93.3%, with an accuracy of 93.5%.5 A study by Hoppe et al resulted in a 90% sensitivity for detecting stenoses of the central airways with CTVB, a specificity of 96.6%, and an accuracy of 95.5%.6 Furthermore, Koletsis and colleagues demonstrated that detection of tracheal stenoses with VE was comparable to that of fiberoptic bronchoscopy, but VE had the added advantage of detecting additional stenoses beyond the areas the bronchoscope could not traverse.7 These findings possibly indicate that VE has high diagnostic yield in the setting of multiple stenotic lesions.
Disadvantages of CT scanning include its cost, radiation exposure, limitation to axial scans of the larynx and trachea, and a static image.
Degree of Confidence
Axial CT scan images can sufficiently evaluate the majority of airway abnormalities, but there are some limitations, including the following:
- Limited ability to detect subtle airway stenosis
- Underestimation of the craniocaudal extent of disease
- Difficulty displaying the relationships of the airway to the adjacent mediastinal structures
- Inadequate representation of the airways that are oriented obliquely to the axial plane
- Difficulty assessing the interfaces and surfaces of airways that lie parallel to the axial plane
- Generation of a large number of images for review
Despite the advantages of 2-D imaging and 3-D virtual endoscopy, both techniques also have limitations; these are related to the maximal spatial resolution of 1.5 mm, the lack of color, and the inability to depict the mucosa. The appearance of the cartilages on CT scans and MRIs also varies depending on the degree of ossification, which is not uniform and is frequently asymmetric.
Magnetic Resonance Imaging
Findings
MRI is rapidly becoming the definitive imaging modality for assessing tracheal and bronchial disorders in children. The advantages of MRI include noninvasive, high-resolution imaging with excellent soft-tissue contrast; the absence of ionizing radiation; and the identification of vascular structures without the necessity of administering iodinated contrast media. Unfortunately, standard MRI has a limited ability to depict dynamic organs, requires the use of long acquisition times, is very expensive, and is prone to motion artifacts in the images. Real-time, dynamic, cine MRI (CMRI) techniques, however, may serve as useful adjuncts for imaging moving structures.
Using CMRI, Faust and colleagues assessed airway patency and function in a study with 30 patients, equally divided among pediatric patients, adult patients, and volunteer controls.8 The patients who were enrolled for tracheal evaluation fell into 2 groups, those with intrinsic pathology, such as tracheomalacia, and those with extrinsic compression. Depending on the patient's clinical history, endoscopic findings, and static MRI findings, the authors were able to obtain CMRI axial, coronal, and sagittal images during the patients' quiet respiration, as well as during a variety of provocative maneuvers. The imaging findings were correlated with endoscopy when possible.
On CMRI, dynamic tracheomalacia was seen as functional tracheal narrowing or collapse with a dynamic component; the findings coincided with the patient's respiratory cycle in all cases. In 1 patient, CMRI detected a dynamic component to a tracheal stenosis that was not appreciated by either static MRI or endoscopic evaluation. Tracheal compression that had a dynamic component and was caused by tracheomalacia, mass lesions, or anomalous vasculature was similarly demonstrated on CMRI, whereas static MRI frequently either overestimated or underestimated the degree of airway compromise that was visible with CMRI and endoscopy.
Ultrasonography
Findings
Animal studies have confirmed that ultrasound morphometric measurements of the laryngeal lumen are reliable. The transverse diameter of the trachea in the neck can be visualized by ultrasound, but the AP diameter cannot be assessed because the acoustic shadow that is generated by the air column obscures the location of the posterior tracheal wall.
Degree of Confidence
Ultrasonography has significant limitations because the laryngeal and tracheal cartilages reflect most of the sound waves with this technique. However, in a 2007 study of 19 healthy volunteers, Lakhal and colleagues compared the transverse diameter of the cricoid lumen as assessed by ultrasonography and MRI and found a strong correlation between the 2 modalities.9
Nuclear Imaging
Findings
Nuclear medicine imaging is seldom useful for upper airway imaging. In addition, increased radionuclide uptake has been reported in association with inflammatory arthropathies and relapsing polychondritis.
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Further Reading
Keywords
upper airway stenosis, stenotic trachea, tracheal trauma, laryngotracheal stenosis, narrow airway
