The Bronchi

Chapter 13 The Bronchi




ANATOMY AND TERMINOLOGY


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.



EVALUATION



Computed Tomography Technique


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 image 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.





ENDOBRONCHIAL LESIONS




Aspiration of a Solid Foreign Body


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.








Bronchial Neoplasms




Carcinoid Tumors


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.






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.




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.




FIBROSING MEDIASTINITIS


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.




If tracheobronchial obstruction is localized, surgical resection and reconstruction may be curative. In most cases, the obstruction is diffuse, and there is no effective treatment.



ESOPHAGOTRACHEOBRONCHIAL FISTULAS


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.



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

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Feb 28, 2016 | Posted by in RESPIRATORY IMAGING | Comments Off on The Bronchi
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