Introduction
Multidetector computed tomography (MDCT) has emerged as the primary imaging modality for the assessment of the central airways. Current generation MDCT scanners can provide high-spatial-resolution images of the entire central airways in just a matter of seconds, and exceptional quality two-dimensional (2D) multiplanar and three-dimensional (3D) reformation images can be generated simply in a few minutes. MDCT imaging is particularly useful for the evaluation of airway stenoses, endobronchial neoplasms, and complex congenital airway disorders. In addition to providing exquisite anatomic detail of the tracheobronchial tree, the use of dynamic expiratory MDCT imaging can provide important functional information about the airways, including the diagnosis of tracheobronchomalacia. Furthermore, MDCT has become a pivotal imaging tool for the bronchoscopist both pre-procedurally, by helping to plan and guide bronchoscopic procedures, and postprocedurally, by providing a noninvasive imaging method for follow-up after interventions.
Multidetector Computed Tomography Imaging: Axial, 2D Multiplanar, and 3D Reconstruction Images
MDCT-acquired, high-spatial-resolution axial images provide exquisitely detailed anatomic and pathologic information about the airways. The precise size and shape of the airway lumen (Figure 2.1), as well as the presence and distribution of airway wall thickening and/or calcification, can be clearly illustrated (Figure 2.2). Conventional axial images also help to define the relationship of the airways to adjacent structures and extraluminal abnormalities not visible on bronchoscopy.
Figure 2.1 Saber-sheath trachea. Axial, noncontrast MDCT image through the intrathoracic trachea demonstrates narrowing of the airway in the transverse dimension with inward bowing of the lateral walls and widening of the airway in the anteroposterior dimension. Saber-sheath trachea is commonly associated with chronic obstructive pulmonary disease.
Evaluation of the airways solely with axial MDCT images, however, can be limiting and challenging. On axial MDCT images, subtle airway stenoses can escape detection, craniocaudal extent of airway disease may be underestimated, and complex airway anatomy may not be clearly defined.
Two-dimensional multiplanar reformations and 3D reconstructions can overcome the limitations of axial MDCT imaging by displaying the airways in a more anatomically meaningful representation. Multiplanar reformation images are single-voxel-thick sections that can be displayed in the coronal, sagittal, or oblique plane. Curved reformations along the axis of the airway can also be obtained, thereby allowing for multiple contiguous airway segments to be simultaneously and completely imaged on a single section. Multiple adjacent thin slices can be added together to form a thick slab image or multiplanar volume reformation image, which typically varies from 5 mm to 10 mm in thickness. These multiplanar volume reformation images advantageously combine the high spatial resolution imaging of multiplanar reformation images with the anatomic display of thick slabs (Figure 2.3). Additionally, the use of minimal intensity projection can improve visualization of the airways within the lung parenchyma by emphasizing low-attenuation voxels. These 2D MDCT image reformation techniques allow for easier identification of airway stenosis, precise delineation of the extent of airway disease in multiple planes, and clarification of complex congenital airway abnormalities. Furthermore, because these images are more visually accessible than conventional axial MDCT images, communication among the radiologist, pulmonologist, and patient can be facilitated.
Figure 2.3 Normal 2D multiplanar volume reformations of the airways. Coronal (A) and sagittal (B) minimal intensity projection images demonstrating normal appearance of the central airways.
Three-dimensional reconstructed images, including external and internal rendering of the airways, play an important complementary role to conventional bronchoscopy by providing detailed bronchographic and bronchoscopic road map images of the airways. External 3D rendering of the airways, or CT bronchography, displays the external surface of the airways and its relationship to adjacent structures (Figure 2.4). These images improve conspicuity of subtle airway stenoses and enhance the display and characterization of complex congenital airway abnormalities. Internal 3D rendering of the airways, or virtual bronchoscopy, allows the viewer to navigate through the internal lumen of the airways, providing a prospective similar to conventional bronchoscopy (Figure 2.5). These virtual bronchoscopic images can provide important complementary information to bronchoscopy by assessing the airways distal to a high-grade stenosis, beyond which a conventional bronchoscope cannot pass, and by providing a unique, global, 360-degree assessment of endoluminal lesions.
Figure 2.4 Normal 3D external renderings of the airways with lung parenchyma (A) and without lung parenchyma (B). Exclusion of lung parenchyma highlights segmental bronchial anatomy.
Multidetector Computed Tomography Imaging of Central Airway Pathology
MDCT has become the initial diagnostic modality in the evaluation of many patients suspected of having a central airway abnormality. MDCT can quickly and noninvasively determine if the tracheobronchial tree is anatomically normal, or if an extraluminal, intraluminal, or even functional abnormality of the airway is present. In the following sections, the role of MDCT with 2D and 3D reformation imaging in the evaluation of airway stenoses, central airway neoplasms, and congenital airway abnormalities will be discussed, as well as the use of dynamic expiratory MDCT in the diagnosis of tracheobronchomalacia.
Airway Stenoses
MDCT-generated 2D multiplanar reformation images of the airways are crucial for accurate imaging evaluation of airway stenoses. Multiplanar volume reformation images, in particular, can highlight the presence of mild stenoses, precisely delineate the length of the airway stenoses, and identify the presence of intraluminal horizontal webs (Figure 2.6). These 2D reformation techniques not only provide important information for preprocedural planning prior to airway stent placement or surgery, but also allow for an accurate, noninvasive method of follow-up for these patients postprocedure.
Figure 2.6 Extrinsic airway compression from goiter. Coronal 2D reformation image displays the severity and craniocaudal extent of airway narrowing involving the intrathoracic and extrathoracic trachea due to a large goiter.
In addition to 2D image reformations, both external and internal 3D renderings of the airways have been shown to be important postprocessing techniques for the complete imaging assessment of airway stenosis. As shown in a study by Remy-Jardin and colleagues, external 3D reconstructions, when compared with axial CT imaging alone, can provide important additional information about airway stenoses and allow for more accurate depiction of the length, shape, and severity of airway narrowing (Figure 2.7). In recent years, virtual bronchoscopy has become a rapidly emerging technique for the evaluation of airway stenosis, as this method provides an intraluminal perspective of the airways distal to an area of high-grade luminal narrowing, beyond where a flexible bronchoscope can pass. Furthermore, virtual bronchoscopy can depict airway stenoses from a distal viewpoint, thereby allowing for a comprehensive depiction of the airway narrowing from multiple perspectives (Figure 2.8).
Figure 2.7 Airway stenosis caused by inflammatory bowel disease. (A) Axial, non-contrast MDCT image of the subglottic trachea demonstrates subtle narrowing of the airway (arrow) due to smooth soft tissue thickening along the right lateral and posterior aspects of the tracheal wall. (B) External 3D rendering of the central airways more clearly depicts the location and severity of subglottic tracheal stenosis (arrow).
Figure 2.8 Amyloidosis. Virtual bronchoscopic images demonstrate high-grade, irregular narrowing of the proximal trachea due to amyloidosis from both proximal (A) and distal (B) perspectives. (C) Virtual bronchoscopic image obtained at a level distal to the stenosis shows diffuse airway abnormality with patency of the distal trachea and carina.
MDCT imaging with 2D multiplanar and 3D reconstruction techniques has been shown to be a highly sensitive, specific, and accurate method for the detection and diagnosis of airway stenoses when compared with flexible bronchoscopy, with reported sensitivities ranging from 87 percent to 92 percent and specificities ranging from 86 percent to 95 percent. In addition, Hoppe and colleagues demonstrated that 2D sagittal reformatted images and virtual bronchoscopic images were more accurate than axial MDCT images alone in the detection of bronchoscopically confirmed central airway stenoses, with accuracies of 96.5, 98, and 96 percent, respectively. These findings underscore the added value of 2D multiplanar and 3D reconstructions in the MDCT imaging evaluation of airway narrowing.
Central Airway Neoplasms
MDCT is the imaging modality of choice for the diagnosis, staging, and preoperative planning of central airway neoplasms. The sensitivity of MDCT for identifying airway abnormalities, including neoplasms, is 97 percent. MDCT allows for accurate determination of the 3D size, including craniocaudal length, and precise location of the tumor within the airways. MDCT clearly maps the relationship of the airway neoplasm to surrounding airways and vasculature, providing critical preoperative information for the surgeon. The intraluminal and extraluminal extent of tumor and presence of postobstructive complications (including air trapping, mucous plugging, atelectasis, and pneumonia) are readily demonstrated on MDCT (Figure 2.9). By detecting the presence of lymphadenopathy and distant metastases, MDCT facilitates accurate staging of the disease and aids in guiding biopsy procedures.
MDCT imaging can determine the amenability of airway neoplasms for complete surgical resection. The approach, type, and extent of surgical resection, including the need for prosthetic airway reconstruction and adjuvant therapy, can be anticipated based on MDCT imaging findings. For those patients who are deemed to be nonsurgical candidates, MDCT can assess the patency of the airways distal to the neoplasm and determine suitability of the airways for airway stent placement.
Although MDCT is highly sensitive for detecting the presence of central airway neoplasms, only rarely do MDCT imaging features allow for an exact diagnosis. Certain MDCT imaging characteristics, however, do enable classification of a neoplasm as benign or malignant.
Benign neoplasms typically are small in size, measuring 2 cm or less in diameter, and appear as round, polypoid, or sessile, focal intraluminal masses. These lesions are well circumscribed and smoothly marginated without evidence of mediastinal invasion or extraluminal extension.
In contrast, malignant airway neoplasms are usually larger in size, often measuring 2–4 cm in diameter. These lesions are often flat or lobulated masses, and their margins are irregular or poorly defined (Figure 2.10). Extramural extension and mediastinal invasion may be present. In 10 percent of malignant airway neoplasms, circumferential airway wall thickening and associated luminal narrowing may be present, findings that are highly characteristic of malignancy. Lymphadenopathy is more commonly seen in cases of malignancy; however, reactive lymphadenopathy may be present in both benign and malignant tumors that are complicated by postobstruction pneumonia. Thus, biopsy of enlarged lymph nodes is necessary for accurate staging purposes.
Figure 2.10 Squamous cell carcinoma. (A) Axial, noncontrast MDCT image demonstrates a lobulated mass with ill-defined borders within the proximal right main stem bronchus causing near complete occlusion. (B) Coronal minimal intensity projection image more clearly displays the size and location of this endobronchial mass with extraluminal extension.
Although most airway neoplasms are soft tissue in attenuation (a nonspecific finding), in some cases, the MDCT density of an airway lesion can enable a specific diagnosis. MDCT is remarkably sensitive and specific in the detection of fat, and the identification of fat within an airway neoplasm indicates a benign etiology. The most common fat-containing airway tumors include lipomas and hamartomas. The presence of calcification within an airway neoplasm can be seen both in carcinoids and in cartilaginous tumors, including chondroma, chondroblastoma, and chondrosarcoma. Recognition of both fat and calcification within an airway tumor is virtually diagnostic for a hamartoma. MDCT enhancement characteristics may also further enable a diagnosis. For example, carcinoid tumors characteristically demonstrate marked homogenous enhancement after the administration of intravenous contrast (Figure 2.9).
Congenital Airway Disorders
MDCT imaging is an excellent, noninvasive technique for evaluating congenital airway abnormalities, particularly in young children in whom bronchoscopy is often avoided because of its invasive nature and need for general anesthesia. Recently, Heyer and colleagues described low-dose MDCT techniques that substantially limit the radiation dose to children but remain 87 percent sensitive for diagnosing central airway abnormalities with a positive predictive value of 97 percent. MDCT can easily elucidate the cause of congenital airway stenoses, which may be due to either an intrinsic cause such as complete tracheal rings, or an extrinsic cause, such as compression resulting from vascular rings, pulmonary slings, or other aberrant or abnormal mediastinal vasculature (Figure 2.11). Congenital airway anomalies, including tracheoesophageal fistulas, tracheal and bronchial hypoplasia or agenesis, supernumerary or accessory airways, and anomalous airway branching, can be clearly illustrated, particularly with multiplanar and 3D reformation techniques.
Figure 2.11 Right aortic arch with aberrant left subclavian artery. (A) Axial, contrast-enhanced MDCT image shows a right aortic arch (white arrow) with aberrant left subclavian artery (black arrow) coursing posteriorly to the trachea, causing airway luminal narrowing. (B) Coronal multiplanar reformatted image shows the origin of the aberrant left subclavian artery (arrow) causing extrinsic lateral compression on the trachea. External (C) and internal (D) 3D renderings of the trachea clearly depict extent and length of extrinsic airway compression and luminal narrowing (arrow).
Tracheobronchomalacia: Use of Dynamic Expiratory MDCT
Tracheobronchomalacia is a condition defined by excessive collapse of the central airways during expiration due to weakness of the airway walls and supporting cartilage. Tracheobronchomalacia is believed to be a widely underdiagnosed condition, as this disease escapes detection on routine end-inspiratory MDCT imaging.
Paired end-inspiratory, dynamic expiratory MDCT imaging of the central airways has been shown to be a highly sensitive method for detecting excessive airway collapse in patients with bronchoscopically proven tracheobronchomalacia. Historically, the diagnosis of airway malacia on MDCT is established when the cross-sectional area of the airway lumen collapses more than 50 percent on dynamic expiration when compared with end inspiration (Figure 2.12). Recently, however, studies of healthy volunteers with normal pulmonary function have reported mean levels of expiratory collapse in the trachea, right and left main bronchi of 54, 67, and 61 percent, respectively. Moreover, in a minority of healthy individuals, the bronchus intermedius may show complete expiratory collapse. Although future studies are necessary to precisely determine a more rigorous threshold for diagnosing tracheobronchomalacia, it is likely that such a threshold will be defined as at least >70 percent expiratory collapse. In the meantime, in order to avoid over diagnosis of this condition, the identification of excessive airway collapse on CT or bronchoscopy should be closely correlated with clinical symptoms, risk factors, and pulmonary function testing.
