Imaging Technique

Radiography

Anteroposterior and Lateral Films of the Neck Including Cervical Trachea. Routine radiologic investigation of the larynx and cervical trachea is composed of anteroposterior (AP) and lateral films of the neck (see Figures 4-1, 4-2) and oblique views of the trachea with the patient in a 45° to 60° rotation.12,13 The lateral view of the neck is obtained with the head slightly hyperextended to bring the larynx and upper trachea up from the retrosternal position. This lateral view provides useful information about the base of tongue, vallecular area, thyroid and cricoid cartilages, intralaryngeal structures (including the epiglottis, aryepiglottic folds, arytenoids, false cords, ventricles, true cords, and subglottic space), posterior pharyngeal wall, and precervical soft tissues. Diseases that arise or spread in the sagittal plane, including the anterior and posterior tracheal wall, are readily visible. The frontal AP view of the larynx and trachea is obtained by using a high-kilovoltage technique (120 kV) and placing a 1 mm copper filter in front of the x-ray tube.14 This view provides a survey of the entire airway from the hyoid bone to the tracheal bifurcation and main bronchi. This technique enhances the air-soft tissue interface by obscuring bone shadows. The frontal projection aids in lateralizing disease processes and supplements the lateral view. In cases of a suspected foreign body, a lateral view during swallowing is added to distinguish a foreign body from the calcified cartilaginous structures of the larynx, which move upward. This swallowing film also allows visualization of a foreign body in the upper esophagus that has been obscured by the soft tissue structures of the superimposed shoulders. One to 2 cm of the trachea and esophagus ascend out of the mediastinum during swallowing, depicting additional trachea obscured by soft tissue of the thoracic inlet. As much as 2 to 3 cm of the trachea can move above the suprasternal notch in hyperextension of the neck. This is age dependent and decreases progressively in older patients, especially if they suffer from chronic obstructive pulmonary disease.

Radiography of the Intrathoracic Trachea. The chest radiograph is the traditional screening study of the trachea. Posteroanterior (PA) and lateral views of the chest are routinely employed. The distal cervical and intrathoracic portions of the trachea are visible on both views; however, overlying mediastinal and bony structures often obscure intrathoracic tracheal abnormalities. Bilateral oblique chest radiographs rotate the spine and mediastinal structures so that the trachea and carina are less obscured (Figure 4-5). High-kilovoltage radiographic technique can improve the visualization of intrinsic airway lesions.6 Digital radiography can improve the visibility of tracheal walls and mediastinal reflections by virtue of edge enhancement techniques.7 Conventional tomography of the trachea is no longer routinely employed, having been replaced by multidetector computed tomography (CT) scanning.15

Fluoroscopy

Fluoroscopy of the Larynx and Cervical Trachea. In order to assess the dynamics of the larynx and cervical trachea, fluoroscopy in the sitting position is indicated.12 It supplements all other radiologic studies of the larynx and trachea, including CT scans. A thorough knowledge of the normal roentgenographic anatomy of the larynx and of functional changes encountered during different phonation maneuvers is a prerequisite. Assessment of vocal cord motion is important in the staging of malignant tumors of the larynx. Fixation of the vocal cords or paralysis of the cords from other causes (eg, thyroid carcinoma, lung cancer with mediastinal extension, aortic aneurysm, or iatrogenic trauma) can be assessed easily with phonation maneuvers such as "E" and inspiration. Opening of the ventricles can be accomplished by having the patient phonate "E" during inspiration (reversed "E"). The aryepiglottic folds, true and false cords, and ventricles are always symmetrical. The subglottic space is tubular in shape and is limited superiorly by the vocal cords, which form a right angle with the lateral wall of the subglottic space. The lumen of the subglottic

Oblique Chest Ray Views
figure 4-5 Tracheal adenoid cystic carcinoma. Posteroanterior (A) and oblique (B) views of the chest. A focal soft tissue mass is evident in the midtrachea, superimposed on the thoracic spine on the posteroanterior view and better visualized on the oblique view.

space is well marginated, and is oval to round in shape. Asymmetry of the pyriform sinuses is a common finding; however, the medial walls of the pyriform sinuses are usually symmetrical.

In the assessment of lesions of the larynx and cervical trachea, the following radiologic parameters should be determined: 1) the location, extent, size, and density of the lesion; 2) definition of the margin, presence of calcification, degree of airway compromise, cartilaginous abnormalities including destruction, and invasion of contiguous structures; 3) distensibility of the pyriform sinuses and ventricles; 4) mobility of the true and false cords; 5) displacement or tilt of the larynx; 6) extralaryngeal masses; 7) calcifications; and 8) presence of an air-filled sac.

In the fluoroscopic study of the larynx, the air-soft tissue interface can be increased easily by mounting a 1 mm copper filter on the tabletop in the field of the x-ray beam. This is especially useful in studying the infant larynx in the frontal view, for assessment of the vocal cords and subglottic space with suspected pathology in this region (eg, hemangioma, cyst, or subglottic stenosis). Simultaneously, spot films on the cervical trachea with different degrees of rotation are taken to free the trachea from superimposed normal anatomic structures at the thoracic inlet.

Fluoroscopy of the Trachea. The dynamic changes of the tracheal wall cannot be evaluated on static imaging.16

The nature and severity of tracheal caliber changes are best observed during real-time imaging, such as fluoroscopy. Tracheal compliance can be evaluated by the Valsalva, modified Valsalva, and Müller maneuvers and by coughing. During the Valsalva maneuver, the patient takes a deep inspiration and performs forced expiration against a closed glottis. The modified Valsalva maneuver simulates the action of blowing up a balloon. With both of these maneuvers, a weakness of the cervical trachea will manifest as a bulge (eg, laryngocele or pharyngocele). The Müller maneuver is a sniff test that consists of a forceful inspiration through the nose. With this maneuver, the pleural pressure becomes more negative than with normal inspirations, causing the intrathoracic trachea to widen more than with normal inspiration and the cervical trachea to collapse to a greater degree. When there is obstruction of the cervical trachea, inspira-tory collapse may occur during forced inspiration because the pressure around the cervical trachea exceeds the intratracheal pressure. The exact opposite will occur in the intrathoracic trachea. During coughing or forced expiration, the pressure outside the airways is greater than the pressure inside, resulting in a com-pressive force to the intrathoracic trachea. As a result, expiratory collapse is commonly found in tracheo-bronchomalacia and in peripheral obstructive airway diseases such as asthma, bronchitis, or bronchiolitis (Figure 4-6).

Barium Esophagogram

The barium esophagogram is an important component of the work-up of congenital and acquired lesions of the airway, due to the close anatomic association of the esophagus with the larynx, trachea, carina, and main bronchi. The esophagogram is invaluable in providing a clue to a malignant laryngeal or tracheal tumor and in identifying primary esophageal tumors that may secondarily invade the trachea. The presence, size, and course of a tracheoesophageal fistula can be established by an esophagogram.

Computed Tomography

CT scanning is the preferred technique for evaluating the larynx, trachea, and main bronchi.15,17,18 Helical CT scanning has dramatically improved the quality of CT imaging of the airways by acquiring a volumetric data set in a single breath-hold while using a short scanning time. In comparison, conventional CT scanning uses a long scanning time and obtains individual axial scans during separate individual breath-holds. As a result, a major advantage of helical scanning is reduced cardiac and respiratory motion. The quality of two-dimensional (2-D) and three-dimensional (3-D) reformatted images are thus markedly improved. The

Inspiration Expiration Scan

figure 4-6 Tracheomalacia. Lateral fluoroscopic images of the trachea during inspiration (A) and expiration (B), computed tomography (CT) of the midtrachea on inspiration (C) and expiration (D), and two-dimensional reformatted sagittal CT images of the trachea on inspiration (E) and expiration (F) demonstrate > 50% collapse in the anteroposterior diameter of the trachea, consistent with tracheomalacia.

newest generation multidetector helical CT scanners employ multidetector arrays, which increase the speed of scanning by factors of 4, 8, or 16, thereby decreasing motion artifacts and improving the image quality of reformatted images. In addition, with multidetector helical CT imaging, high quality reconstructed images can be obtained routinely without prospective planning or rescanning the patient. The ability to create 2-D and 3-D reformatted images of the central airways overcomes the inherent limitations of axial images, including the limited ability of detecting subtle stenoses, evaluating the accurate craniocaudad extent of disease, visualizing obliquely oriented airways, and displaying the complex relationship of adjacent mediastinal structures (Figure 4-7A). Reformatted 2-D and 3-D images do not offer any new information; rather, they provide a complementary way to view the same data sets.

Although the use of intravenous contrast is not necessary to assess the central airways, it is recommended to evaluate adjacent mediastinal extent of tumor and adenopathy or to assess adjacent mediasti-nal vascular structures or masses that may compress the airway. CT scanning of the airway is routinely obtained at end-inspiration during a breath-hold. When assessing tracheomalacia, it is helpful to obtain additional scan sequences at the same table levels during end-expiration.

Reconstruction methods that are used for airway imaging include 2-D multiplanar and 3-D internal and external rendering techniques. 2-D images are the easiest to obtain because they can be generated at the CT console, and 2-D reformations can be displayed in the coronal or sagittal planes, orthogonal to a reference point or curved along the axis of the airway. 3-D reformations require the transfer of data to a separate workstation, but 3-D images of the airway can be visible on external 3-D rendered images or on internal renderings that create virtual bronchoscopic images of the central airways.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is another modality being used for the radiologic evaluation of the larynx and trachea.19 A major advantage of MRI over CT is the acquisition of coronal, oblique, and sagittal sections that demonstrate long segments or the entire length of the trachea (Figure 4-7B). With MRI, it is possible to visualize the laryngeal and tracheal structures with great detail in transverse, sagittal, and coronal planes.3,20 Normal anatomic structures can be differentiated on the basis of different signal intensities on T1-weighted sequences. A bright signal is elicited by fat, hyaline cartilage, submucosal fascial planes

figure 4-7 Adenoid cystic carcinoma. A, Two-dimensional reformatted computed tomography image of the distal trachea and carina demonstrates a smooth intraluminal mass arising from the right lateral wall of the trachea. B, T1-weighted coronal magnetic resonance imaging through the trachea and carina in the same patient demonstrates a soft tissue mass in the distal trachea, consistent with adenoid cystic carcinoma.

figure 4-7 Adenoid cystic carcinoma. A, Two-dimensional reformatted computed tomography image of the distal trachea and carina demonstrates a smooth intraluminal mass arising from the right lateral wall of the trachea. B, T1-weighted coronal magnetic resonance imaging through the trachea and carina in the same patient demonstrates a soft tissue mass in the distal trachea, consistent with adenoid cystic carcinoma.

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