Chapter 13 The Bronchi
The airways are categorized as purely conducting and gas-exchanging regions, with a transitional zone in between. Air reaches the gas-exchanging units of lungs from nasal and oral cavities through the conducting tracheobronchial airways. The trachea divides into two primary or main bronchi, which divide into the lobar bronchi, which branch further to become the segmental and intrapulmonary bronchi and bronchioles. There are about 23 divisions from the trachea to the alveoli. The bronchioles are seen after 6 to 20 divisions from the segmental bronchus. The bronchi contain cartilage in their walls, whereas the bronchioles lack cartilage. The last generation of purely conducting airways is the terminal bronchiole. The respiratory bronchioles are the transitional branches that lead to gas-exchanging alveolar ducts, alveolar sacs, and alveoli. An acinus is lung parenchyma distal to a terminal bronchiole, and it is composed of two to five generations of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. Airways that are less than 2 mm in diameter are called small airways.
The secondary pulmonary lobule is the smallest portion of the lung that is surrounded by connective tissue septa. It contains three to five acini, has a polyhedral shape, and is 10 to 25 mm in diameter. It is most visible in the subpleural portion of the lung on computed tomography (CT) and is best seen at the lung apices and lung bases. A lobular bronchiole and a pulmonary artery branch are found in the center of the lobule. Pulmonary veins and lymphatics are found in the peripheral interlobular septa. The limit of CT visibility for a small bronchus is 2 mm. Lobular bronchioles are beyond the limit of CT visibility because they are less than 1 mm in diameter and their walls are less than 0.1 mm thick. Lobular bronchioles branch into three or more terminal bronchioles. Each terminal bronchiole supplies one acinus.
The best CT technique for evaluating the central bronchi is to obtain 1- to 2.5-mm multiplanar images using multidetector CT. The isotropic data set obtained with multidetector CT has the same resolution in all imaging planes, and artifacts seen with image reconstruction using single-row detector CT can be avoided. Multiplanar images are important because they allow visualization of the full length of the bronchi in a single plane. If single-row detector CT is used, contiguous, overlapping, 2.5- to 5-mm scans through the hilar regions should be obtained. All of the normal lobar and segmental bronchi should be identified routinely by this technique. Occasionally, evaluation of small and obliquely oriented bronchi, such as the lingular bronchus, may necessitate scanning with angling of the gantry in the plane of the bronchus.
High-resolution CT (HRCT) is the preferred technique for the evaluation of bronchiectasis and bronchiolitis. For HRCT, 1- to 1.5-mm collimation, 10-mm intervals, and reconstruction with an edge-enhancing algorithm (i.e., bone) should be used. A window level 700 Hounsfield units (HU) and window width of 1000 to 1500 HU is recommended. Modern CT scanners allow retrospective reconstruction of HRCT images from data acquired for regular chest CT. Paired inspiratory and expiratory scans performed at identical table levels are used to evaluate bronchomalacia and air trapping in patients with small airways disease, such as obliterative bronchiolitis. Measurement of attenuation values is useful in identifying fat in hamartomas and calcification in endobronchial hamartomas, chondromas, carcinoid tumors, and broncholiths. CT pixel histogram analysis allows measurement of fat not seen macroscopically.
Magnetic resonance imaging (MRI) of the bronchi is not routinely performed because the spatial and temporal resolution is inferior to that of CT. The MR signal, which depends on the proton density, is also weak in the lungs due to the low density of air. The weak signal also decays rapidly because of magnetic susceptibility artifacts from air-tissue interfaces, and the lungs appear dark. However, MRI does not involve ionizing radiation and has better contrast resolution compared with CT. Interest has developed in functional imaging using hyperpolarized gas. Gases such as helium (He) are hyperpolarized using a process called optical pumping. This brings the nuclear spins of the gas atoms in to significant alignment, and when inhaled during scanning, the hyperpolarized gas causes a strong MR signal. Despite the low density of gas, the lungs appear bright. This method has shown ventilation defects in patients with obstructive airway disease even when pulmonary function tests were normal. These techniques are available in only few centers and continue to be evaluated.
Bronchial anomalies include abnormal origin, supernumerary bronchi, bronchial atresia, and bronchial isomerism. A cardiac bronchus is a supernumerary bronchus that is seen in 0.1% of population (Box 13-1). The bronchus arises from the medial aspect of the bronchus intermedius and extends medially toward the heart (Fig. 13-1). The bronchus may end in a blind stump or supply a segment of lung; this is mostly an incidental finding. However, the blind ending bronchus may be a potential reservoir of infection and lead to recurrent infections and hemoptysis. If the patient is symptomatic, surgical resection is the treatment. Other bronchial anomalies are discussed in Chapter 2.
Box 13-1 Cardiac Bronchus
Broncholithiasis is a condition in which calcified material within a bronchus or adjacent to a bronchus distorts the lumen. Broncholiths usually arise from calcified peribronchial lymph nodes that erode in to the adjacent bronchi. Most are caused by Histoplasma capsulatum infection. However, other fungal infections, tuberculosis, sarcoidosis, or silicosis may predispose the patient to broncholithiasis. Rarely, a neglected endobronchial foreign body may calcify.
Radiographically, the key finding is a calcified endobronchial lesion or peribronchial lymph node without associated soft tissue, which may be associated with findings of bronchial obstruction such as atelectasis, postobstructive pneumonia, and air trapping (Box 13-2 and Fig. 13-2).
Box 13-2 Findings for Broncholithiasis
Most aspirated foreign bodies occur in children younger than 10 years, and 50% occur in children younger than 3 years. Aspirated foreign bodies in adults are often seen in patients with altered mental status or poor dentition. Most are of vegetable origin, such as peanuts, or result from broken teeth or dental fixtures. Foreign bodies are usually aspirated into the lower lobe bronchi. Aspirations into the right side are more common because of the more direct angle of the right main bronchus with the trachea.
Air trapping is the most common finding after acute aspiration of a foreign body because of a check-valve mechanism of bronchial obstruction. Air trapping may not be apparent on inspiration, but it is apparent on expiratory radiographs, expiratory CT scans, and decubitus chest radiographs. On expiration, the lung becomes more opaque. Similarly, on decubitus radiographs, the dependent lung is hypoinflated and therefore more opaque. If air trapping is present, the obstructed lobe or lung will remain lucent and not decrease in volume on expiration, or if the affected lung is in a more dependent position, the mediastinum will shift away from the side of air trapping (Fig. 13-3). Hypoxic vasoconstriction may develop within an obstructed lobe or lung; it manifests radiographically as a hyperlucency of the lung and attenuation of the pulmonary vessels (Fig. 13-4). In adults, aspirated foreign bodies may remain clinically silent. In cases of chronic obstruction, atelectasis, recurrent pneumonia, intrabronchial mass, hemoptysis, or bronchiectasis may develop (Box 13-3). CT can identify the endobronchial site of an aspirated foreign body and demonstrate air trapping. A high-density foreign body suggests bone, metallic foreign body, or a tooth (Fig. 13-5). Vegetable material has soft tissue density and occasionally has fat density (Fig. 13-6). MRI may be helpful in identifying aspirated peanuts, which characteristically have a high-intensity signal on T1-weighted images.
Figure 13-3 Air trapping in a case of left main bronchial carcinoid. A, The inspiratory chest radiograph reveals an oval lesion obstructing the left main bronchus (arrow), hyperlucency of the left lung, and attenuation of the left pulmonary vasculature due to hypoxic vasoconstriction. There is slight volume loss in the left lung, with a shift of the mediastinum to the left. B, The expiratory chest radiograph demonstrates a normal decrease in right lung volume, elevation of the right diaphragm, shift of the mediastinum to the right, and crowding of right pulmonary vessels. There is hyperlucency and diffuse air trapping in the left lung without a change in lung volume.
Figure 13-4 Air trapping in a case of carcinoid tumor. A, The inspiratory CT scan demonstrates an obstructing carcinoid tumor in the right upper lobe bronchus (arrow). The right lung is more lucent than the left lung due to vascular attenuation from hypoxic vasoconstriction. B, The expiratory CT scan demonstrates a normal decrease in the left lung volume and increased attenuation of the left lung. The right lung remains inflated and lucent.
Box 13-3 Findings for Bronchial Obstruction
Figure 13-5 Aspirated tooth in a patient who was in a motor vehicular accident. Axial CT (A) and coronal CT (B) scans show the tooth (thin arrow) in the right middle lobe bronchus that is causing middle lobe atelectasis (thick arrow).
Hamartoma is the most common benign tumor of the lung, accounting for 77% of tumors in a series by Arrigoni. Approximately 3% to 10% of these tumors arise within the bronchi. Bronchial hamartomas are slow-growing tumors seen within large bronchi. They contain normal components of bronchi, such as cartilage, fat, fibrous tissue, and epithelium, arranged in a disorganized fashion. The appearance of fat or characteristic popcorn calcification on CT suggests the diagnosis. Calcification is present in approximately 25% of hamartomas and is more common in larger lesions. Signs of bronchial obstruction such as collapse or recurrent pneumonia may be seen (Box 13-4 and Fig, 13-7).
Box 13-4 Bronchial Hamartomas
Carcinoid tumors originate from bronchial Kulchitsky (argentaffin) cells and are classified as neuroendocrine tumors of the lung. The Kulchitsky cell contains neurosecretory granules capable of producing serotonin, adrenocorticotropic hormone, bradykinin, and others substances. Based on the aggressiveness, these tumors are classified as typical (classic) carcinoid, atypical carcinoid, and small cell carcinoma.
The main clinical differences among typical carcinoid, atypical carcinoid, and small cell carcinoma lie in the prevalence of metastases and in the prognosis (Box 13-5). Typical carcinoids manifest in patients at an earlier age, usually 40 to 50 years. They are usually smooth, round, well-defined, small (<2.5 cm) masses that arise in the central bronchi (Fig. 13-8). There is a predilection for women, and they are not associated with smoking. Most carcinoids arise with the main, lobar, or segmental bronchi. Only 10% of carcinoids are seen in the periphery of the lung (Fig. 13-9). These tumors have an excellent prognosis, with 5-year survival rates of 90% to 95%. Atypical carcinoids are diagnosed in patients between 50 and 60 years old and are more common in men and smokers. They tend to be larger at diagnosis, may be centrally or peripherally located, and have a tendency to metastasize to hilar or mediastinal lymph nodes (Fig. 13-10). The 5-year survival rate is between 50% and 70%. Small cell carcinoma manifests in patients between 60 and 70 years old and is four times more common in men. The tumors commonly metastasize and have a strong association with smoking. These extremely malignant tumors are most often associated with bulky hilar and mediastinal adenopathy. A small, peripheral nodule representing the primary tumor is often found on CT. These tumors carry a poor prognosis.
Box 13-5 Neuroendocrine Tumors of the Lung
Figure 13-8 Right middle lobe typical carcinoid tumor. A, The radiograph shows a round, well-defined, right hilar mass. B, On contrast-enhanced CT, the mass obstructs the right middle lobe bronchus (m). The tumor demonstrates heterogeneous enhancement and calcification (arrows).
Figure 13-9 Peripheral carcinoid tumor with contrast enhancement. A, Nonenhanced CT at the level of the right lung base demonstrates a soft tissue nodule (open arrow) lateral to a small lower lobe pulmonary artery (small arrow).B, After contrast administration, there is dense, uniform enhancement of the carcinoid tumor.
Figure 13-10 Atypical carcinoid tumor. A contrast-enhanced CT scan demonstrates an irregular, peripheral mass (M) in the right upper lobe and abutting the pleura. There is a partially enhancing metastatic right hilar lymph node mass (N).
Imaging studies may identify several distinguishing features of carcinoid tumors (Box 13-6). They are well-defined endobronchial tumors that may have extrabronchial extension. Calcification is seen in up to 40% of these tumors on CT (see Fig. 13-8). These highly vascular tumors demonstrate marked contrast enhancement with iodinated contrast on CT (see Fig. 13-9) and with gadolinium on MRI. Somatostatin receptors have been identified in a wide variety of human tumors with neuroendocrine characteristics, including carcinoid tumors. 123I-Tyr3-octreotide and 111In-octreotide are radionuclide-coupled somatostatin analogues that can be used to visualize somatostatin receptor–bearing tumors (Fig. 13-11). Known tumor sites have been visualized in 86% of patients in whom histologically confirmed carcinoids were present.
Box 13-6 Imaging Features of Carcinoid Tumors
Figure 13-11 Octreotide uptake within a carcinoid tumor. A, CT demonstrates a 6-mm, left upper lobe nodule (arrow). B, Octreotide uptake occurs within the carcinoid tumor in the left upper lobe (arrow).
Endobronchial carcinoids often manifest with distal pneumonia, atelectasis, or bronchiectasis. Hemoptysis also is a common finding because of the increased vascularity of these tumors. Several clinical syndromes are associated with secreted neuroendocrine substances. The carcinoid and Cushing syndromes are most common, but they still are seen in only a minority of patients, accounting for about 5% of tumors. Other conditions such as Zollinger-Ellison syndrome and acromegaly are rare.
Mucoepidermoid tumors are uncommon, representing 0.2% of all lung tumors and 1% to 5% of bronchial tumors. These tumors resemble salivary gland tumors, and as the name implies, they contain squamous and mucus-secreting columnar cells. The mean age of patients at presentation is 36 years. There is no sex predilection, and smoking is not a risk factor. Mucoepidermoid tumors of the central airways may be high- or low-grade malignancies. The radiograph usually shows a focal endobronchial mass within a large central airway, and it may be associated with bronchial obstruction (Fig. 13-12).
Figure 13-12 Mucoepidermoid tumor in a 37-year-old woman. A, Axial CT shows an endobronchial lesion (arrow) that partially obstructs the lumen. B, Coronal CT shows a nodular lesion that obstructs the left lower lobe bronchus (arrow), distal obstructive pneumonia, and atelectasis. Notice the elevation of the left hemidiaphragm and downward displacement of the major fissure.
Fibrosing mediastinitis is a rare disorder of the mediastinum characterized by an exuberant proliferation of fibrous tissue that replaces mediastinal fat. It is most commonly a complication of a granulomatous mediastinitis resulting from infection by H. capsulatum or Mycobacterium tuberculosis. It is also associated with the use of drugs such as methysergide, with autoimmune disorders, and with idiopathic conditions. The mediastinal structures, such as the trachea, main bronchi, esophagus, superior vena cava, and pulmonary veins and arteries, are encased, invaded, and narrowed by the fibrous tissue. Structures with thin walls and long mediastinal courses are affected the most. Complications may include tracheobronchial stenosis, superior vena caval obstruction, esophageal obstruction, pulmonary artery occlusion, and obstruction of the pulmonary veins and thoracic duct.
Patients with fibrosing mediastinitis most often have cough, dyspnea, and hemoptysis. The most common radiologic finding is widening of the mediastinum and replacement of mediastinal fat by soft tissue density (Box 13-7). Calcification or calcified nodes may be identified within the mediastinal fibrosis, particularly on CT (Fig. 13-13). The mediastinal fibrosis typically encases and narrows affected mediastinal and hilar structures, including the bronchi. Contrast-enhanced CT or MRI is necessary to assess the vascular patency of the superior vena cava and the pulmonary veins and arteries.
Figure 13-13 Fibrosing mediastinitis due to histoplasmosis in two patients. A, Calcified right paratracheal and subcarinal mediastinal tissue encases the right bronchus intermedius (i) and esophagus (e). B, Calcified right hilar mass encases and narrows the right lower lobe bronchus.
Esophagotracheobronchial fistulas may be congenital or acquired (Box 13-8). Congenital esophagotracheobronchial fistulas are discussed in Chapter 2. Most acquired fistulas result from tumors of the esophagus, tracheobronchial tree, thyroid gland, and mediastinal nodes. Infection, trauma, and radiation are the chief causes of nonmalignant esophagotracheobronchial fistulas. Infections include histoplasmosis, tuberculosis, actinomycosis, and syphilis. Esophageal Crohn’s disease is also a cause of fistula. Erosion of calcareous particles from peribronchial lymph nodes in which the infectious process is no longer active can also lead a fistula, particularly in cases of histoplasmosis.
Box 13-8 Esophagotracheobronchial Fistula
The typical complaint of these patients is a strangulating sensation occurring immediately after ingestion of solids or liquids. Routine chest radiography does not disclose the presence of the fistula, but may reveal pneumonia or bronchiectasis related to recurrent aspiration pneumonias, particularly in the lower lobes. Occasionally, a nasogastric tube entering the tracheobronchial tree through the esophageal orifice is seen. Early diagnosis can be established with the judicious use of a contrast esophagogram (Fig. 13-14). High-osmolar contrast should be avoided because of the risk of pulmonary edema if contrast reaches the lungs. CT scan is useful in demonstrating the fistula and in evaluating causative pathology and associated findings and complications (Fig. 13-15