Radiology of the Thorax

Javier H. Campos, Archit Sharma

Abstract

Radiologic images play an important role during evaluation of patients undergoing thoracic surgery. Radiologic studies must be reviewed, including a posterior-anterior chest radiograph and computed tomography (CT) scan of the chest. There should be a special emphasis on reviewing CT scans for the presence of a mediastinal mass that compromises the airway or great vessels. Multidetector CT imaging and tracheobronchial reconstruction are more specific studies that allow measurements of the airway. Magnetic resonance imaging (MRI) provides greater contrast resolution than CT scans and offers the potential for tissue characterization. MRI is indicated in cases with a mediastinal mass with invasion of the superior vena cava.

Keywords

trachea bifurcation; chest radiograph; ultrasonography; mediastinal mass; computed tomography scan

Introduction

Radiologic images play an important role in the preoperative, intraoperative, and postoperative evaluation, and diagnosis of patients undergoing thoracic surgery. During the preoperative visit for evaluation of the thoracic surgical patient, the clinician must have an understanding of the disease and also become familiar with radiologic studies to be able to identify abnormal airway anatomy or compromises to the airway or to use caliper measurements in the tracheobronchial tree if necessary when selecting lung isolation devices. An important component of a successful preoperative evaluation is an understanding of normal tracheobronchial anatomy.¹ This chapter will be focused on reviewing normal tracheobronchial anatomy, radiologic images, and thoracic ultrasound with special interest for anesthesiologists involved in the care of the thoracic surgical patient.

Normal Tracheobronchial Anatomy

The trachea is a cartilaginous and fibromuscular structure that extends from the inferior aspect of the cricoid cartilage to the level of the carina.² The adult trachea is, on average, 15 cm long. The trachea is composed of 16 to 22 C-shaped rings of cartilage that form the anterior and lateral walls and are connected posteriorly by the membranous wall of the trachea, which lacks cartilage and is supported by the trachealis muscle.

The average diameter of a normal trachea is 22 mm in men and 19 mm in women. In men, the coronal diameter ranges from 13 to 22 mm and the sagittal diameter ranges from 13 to 27 mm. In women, the average coronal diameter is 10 to 21 mm and the sagittal is 10 to 23 mm.³ The tracheal wall is about 3 mm in thickness in both men and women, with a tracheal lumen that is often ovoid shape.

The trachea is located in the midline position, but often can be deviated to the right at the level of the aortic arch, with a greater degree of displacement in the setting of an atherosclerotic aorta, advanced age, or in the presence of severe chronic obstructive pulmonary disease (COPD). With COPD or aging, the lateral diameter of the trachea may decrease, with a corresponding increase in the anteroposterior diameter. Conversely, COPD may also lead to softening of the tracheal rings with a decrease in the anteroposterior diameter of the trachea.⁴ The cricoid cartilage is the narrowest part of the trachea, with an average diameter of 17 mm in men and 13 mm in women.

The trachea bifurcates at the carina into the right and left mainstem bronchi. An important fact is that the tracheal lumen narrows slightly as it progresses toward the carina. The tracheal bifurcation is located at the level of the sternal angle anteriorly and the fifth thoracic vertebra posteriorly. The right mainstem bronchus continues as the bronchus intermedius after the branching point from the right upper lobe bronchus. In men, the average distance from the tracheal carina to the branching point from the right upper lobe bronchus is 2.0 cm, whereas it is approximately 1.5 cm in women. One in every 250 individuals from the general population may have an abnormal branching point from the right upper lobe bronchus emerging from above the tracheal carina on the right side.⁵ The diameter of the right mainstem bronchus is an average of 17.5 mm in men and 14.0 mm in women. The trifurcation of the right upper lobe bronchus consists of the apical, anterior, and posterior divisions. The average distance from the tracheal carina to the bifurcation of the left upper and left lower lobes is approximately 5.0 cm in men and 4.5 cm in women. The left mainstem bronchus is longer than the right mainstem bronchus, and it divides into the left upper and the left lower lobe bronchus. The left upper lobe bronchus has a superior and inferior division.⁶

Chest Radiographs

The most common study in the patient undergoing thoracic, esophageal, or cardiac surgery is the chest x-radiograph (x-ray). The standard routine for chest radiography consists of an erect radiograph in the posterior-anterior (PA) projection and a left lateral radiograph, both obtained at full inspiration. The normal chest cavity contains four radiographic densities that are easily identified: air, fat, water, and calcium and other metals, including bones, granulomas, and vascular calcification. The lungs, which are mostly air and contain some water, blood vessels, bronchi, nerves, lymphatics, alveolar walls, and interstitial tissues, provide a natural contrast that is the basis of chest radiology. Evaluating a chest x-ray involves checking for any changes in these densities.

Radiologists generally consider two features in chest x-radiographs: the silhouette sign and summation. Depending on the part or parts of the lungs that are involved, a change in the absorption of x-rays is seen by the effect on the normal surface of the hemidiaphragm, for example. If the descending aorta cannot be seen, then this is an indication that an unusual amount of aerated lung no longer touches the anatomic part. This occurs in part because the alveolar spaces are wholly or partially filled with fluid, usually blood, pus, or water, or in part because the lung has collapsed and decreased the normal ratio of air and soft tissue. The end result in the latter case would be an increase in opacity which is what the x-ray beam reveals. The heart border and the diaphragm are normally seen because of their interface with aerated lung. When the adjoining lung is not aerated, the opaque tissue of the affected lung blends visually with the soft-tissue opacity of the heart, and the heart border is no longer visible. This is what is termed the silhouette sign. Summation by definition is the result of superimposition of many layers of lung tissue, so that the final visual effect is that of a greater amount of tissue in the path of a particular part of the x-ray beam. This is observed when the opacity of fluid in the pleural space, interstitial space, or even lung parenchyma has an additive effect on the normal structure of the lung.⁷ The most important skill for evaluating a chest x-ray is knowledge of normal anatomic variants, specific patterns with pathologic changes, and the common signs of abnormal states. Lateral decubitus radiographs are commonly used to determine the presence or mobility of pleural effusions. These views can also be obtained to detect small pneumothoraces, particularly in patients who are confined to bed and unable to sit or stand erect. A new generation of digital x-ray systems based on flat panel detectors is now emerging, and these provide good image quality and very rapid direct access to digital images.

Chest Radiographs and Pulmonary Disease

The basic underlying change in a chest film that allows the detection and diagnosis of abnormalities is usually an alteration in lung opacity. This can be caused by technical factors, physiologic variation, or pathologic mechanisms.⁸ Diseases that affect the chest can be thought of as either those that make the film lighter (increase opacity) or those that make it darker (increase lucency).

For chest disease with increased opacity on radiographs, conditions that are marked by a focal or global increase in the opacity of the film are first examined. These conditions include atelectasis, pulmonary edema, acute respiratory distress syndrome, and so on.

Lung Mass and Chest X-Rays

Pulmonary neoplasia presents as opacity on films, although usually as a more focal area with varying contours. However, smaller lesions may be very difficult to visualize, particularly when there are overlying bones or other soft tissue. Fig. 3.1A shows a PA chest radiograph of a 68-year-old male with a lung neoplasm on the right lower lobe. The mass is clearly visible between the eighth and ninth ribs on the right hemithorax. Fig. 3.1B shows a lateral chest radiograph with a round mass on the right hemithorax.

• Fig. 3.1 (A) Posteroanterior chest radiograph of a 68-year-old male with a lung neoplasm (*arrow*) in the right lower lobe. The mass is visible between the eighth and ninth ribs on the right hemithorax. (B) Lateral chest radiograph showing a round mass on the right hemithorax.

Mediastinal Mass and Chest X-Rays

To understand mediastinal masses and radiologic images, it is important to be familiar with the anatomy of the mediastinum. The mediastinum is situated between the two pleural cavities (see Chapter 53). It extends superiorly from the root of the neck and the thoracic inlet to the hemidiaphragm inferiorly. It is divided into the superior and inferior mediastinum by the transverse thoracic plane, which is an imaginary plane extending horizontally from the sternal angle anteriorly to the border of the fourth thoracic vertebra posteriorly. The inferior mediastinum is subdivided into anterior, middle, and posterior compartments. The anterior mediastinum contains the thymus, trachea, esophagus, vessels, and arteries, as well as lymph nodes; any abnormal growth in this region will affect the adjacent area. A mass in this area may compress the tracheobronchial tree and/or major vessels (superior vena cava and pulmonary vessels). The middle mediastinum is the space occupied by the heart and pericardium.⁹^,¹⁰ Fig. 3.2A shows a schematic representation of the mediastinal anatomy and Fig. 3.2B a lateral normal radiograph of the chest showing the potential location of a mediastinal mass. A variety of neoplasms and other lesions present with anterior mediastinal involvement. Thymoma is the most common primary neoplasm of the anterior mediastinum.¹¹

• Fig. 3.2 (A) Schematic representation of the mediastinal anatomy and (B) a lateral normal radiograph of the chest showing the potential location of a mediastinal mass. The arrows to the left indicate potential areas of mediastinal mass growth. Arrows to the right indicate potential areas of posterior mediastinal mass growth. (From Campos JH. Managing the patient with an anterior mediastinal mass. In: Cohen NH, ed. *Medically Challenging Patients Undergoing Cardiothoracic Surgery*. Baltimore: Lippincott Williams & Wilkins; 2009:285–302. With permission.)

Regarding radiologic studies in patients with a suspected anterior mediastinal mass, the initial study generally is a standard biplane chest radiograph, which will identify up to 97% of mediastinal tumors. The chest x-ray also provides important information regarding the size and the location of the mass.¹²

In addition, in this patient group in particular, special attention must be paid to lateral radiographs of the chest to determine the overall extent of the mass and potential involvement of adjacent structures in this patient group. A barium contrast esophagogram may help to determine whether there is esophageal or tracheobronchial involvement. Fig. 3.3A shows an anterior mediastinal mass in the left hemithorax of the PA chest radiograph. Fig. 3.3B shows a lateral radiograph with esophageal contrast where there is a mediastinal mass without compromise to the tracheobronchial tree. Fig. 3.4A shows a posterior mediastinal mass in the right hemithorax of the PA chest radiograph in a female with severe kyphoscoliosis; Fig. 3.4B shows the corresponding lateral radiograph and Fig. 3.4C a portable PA chest radiograph after removal of the tumor.

• Fig. 3.3 (A) An anterior mediastinal mass (*arrow*) in the left hemithorax on a posteroanterior chest radiograph. (B) Lateral radiograph with esophageal contrast revealing a mediastinal mass (*arrow*) without compromise to the tracheobronchial tree.

• Fig. 3.4 (A) A posterior mediastinal mass in the right hemithorax on a posterior-anterior chest radiograph of a female with severe kyphoscoliosis. (B) A lateral radiograph showing the posterior mediastinal mass. (C) A portable posterior-anterior chest radiograph taken in the postoperative acute care unit after resection of the posterior mediastinal mass. The *yellow dots* indicate the contour of the thoracic column showing severe kyphoscoliosis.

Bullae

Emphysema is characterized by a permanent increase in air spaces distal to the terminal bronchiole beyond the normal size. There is destruction of tissue, leading to a loss of alveolar surface available to participate in air exchange and, sometimes, severe displacement of the adjacent normal lung. Many cases of emphysema seen in adult patients are strongly associated with cigarette smoking. The most striking image in a patient with advanced emphysema is marked hyperinflation with an increase in the anteroposterior diameter of the chest, flattening of the diaphragmatic surfaces, and a general increase in the blackness of the film. There is also a change in the vascular pattern, with attenuated vessels thinned and spread apart.

Often bullous areas are noted as large, thin-walled air cysts, especially the apices. A bulla is defined as an air-filled space of 1 cm or greater in diameter within the lung parenchyma that forms as a result of a destructive process. Rarely, one or more bullae enlarge to such a degree that they occupy more than one-third of the hemithorax. The term giant bullae is then applied. These easily distensible reservoirs are preferentially filled during inspiration, causing the collapse of adjacent, more normal lung parenchyma.¹³^,¹⁴ Fig. 3.5A shows a patient with emphysema, and Fig. 3.5B shows a lateral chest radiograph of the same patient. Fig. 3.6 shows a PA chest x-ray and lateral radiograph of a patient with bullae. Fig. 3.7 displays the chest x-ray showing a giant bulla occupying more than two-thirds of the right hemithorax and compressing the underlying lung.

• Fig. 3.5 (A) Posterior-anterior chest x-ray of a patient with emphysema. There is marked hyperinflation with an increase in the anterior-posterior diameter of the chest, as well as flattening of the diaphragm bilaterally and a general increase in the blackness of the film. (B) Lateral chest x-ray of the same patient.

• Fig. 3.6 (A) Posterior-anterior chest x-radiograph of a patient with a bulla in the left hemithorax. (B) Lateral radiograph of the same patient.

• Fig. 3.7 Chest x-ray showing a giant bulla occupying more than two-thirds of the right hemithorax and compressing the underlying lung.

Pneumothorax

Pneumothorax is the presence of air in the pleural space between the lung and the chest wall. Primary pneumothoraces arise in otherwise healthy people without any lung disease. Secondary pneumothoraces arise in subjects with underlying lung disease. Despite the absence of underlying pulmonary disease in patients with primary pneumothorax, subpleural blebs and bullae are likely to play a role in the pathogenesis. It is frequently the result of trauma, although sometimes the source of the air leak cannot be readily detected.

The radiographic diagnosis of pneumothorax is usually straightforward. A visceral pleural line is seen without distal lung markings. On standard lateral views, a visceral pleural line may be seen in the retrosternal position or overlying the vertebrae, parallel to the chest wall.¹⁵^,¹⁶

Pneumothoraces can be seen on lateral chest radiographs; although the value of expiratory views is controversial, many clinicians still find them useful in the detection of small pneumothoraces when clinical suspicion is high and an inspiratory radiograph appears normal. The British Thoracic Society guidelines divide pneumothoraces into small and large based on the distance from visceral pleural surface (lung edge) to the chest wall: less than 2 cm is classified as small and more than 2 cm as large.¹⁷ A small rim of air around the lung actually translates into a relatively large loss of lung volume, with a 2-cm-deep pneumothorax occupying about 50% of the hemithorax.

In the supine position, air in the pleural space will usually be most readily visible at the lung bases in the cardiophrenic recess and may enlarge the costophrenic angle. Fig. 3.8 shows a chest radiograph of a patient with right-sided pneumothorax.

• Fig. 3.8 Chest radiograph of a patient with a right-sided pneumothorax and bilateral chest tubes.

Several well-known artifactual appearances can mimic the presence of a pneumothorax and should always be borne in mind during evaluation of chest radiography. The medial border of the scapula can imitate a lung edge, but once considered can be traced in continuity with the rest of the bone, revealing its true nature. Skin folds overlying the chest wall can simulate a visceral pleural line and, with the relative lack of lung markings in the upper zones, can lead to erroneous diagnosis. Once considered, however, the true nature of the image is readily apparent. Skin folds are usually seen to pass outside the chest cavity, are straight or only minimally curved, and do not run parallel to the chest wall as with a true visceral pleural line. Skin folds also form a dense line that is sharp on one side and blurred on the other, in contrast to the less dense visceral pleural line. Also radioopaque lines are often seen accompanying the inferior margins of ribs, which may simulate a visceral pleural line. These are often called companion shadows, although some restrict this term to densities accompanying the first and second ribs. They are caused by protruding extrapleural fat or the subcostal grove.

Tension pneumothorax occurs when the intrapleural pressure exceeds the atmospheric pressure throughout inspiration, as well as expiration. It is thought to result from the operation of a one-way valve system, drawing air into the pleural space during inspiration and not allowing it out during expiration. The development of tension pneumothorax is often, but not always, heralded by a sudden deterioration in the cardiopulmonary status of the patient related to impaired venous return, reduced cardiac output, and hypoxemia. The development of tension in a pneumothorax is not dependent on the size of the pneumothorax and the clinical scenario of tension pneumothorax may correlate poorly with chest radiographic findings.¹⁸^,¹⁹ In extreme cases of tension pneumothorax, the air leak leads to significant displacement of the mediastinum and contralateral lung into the opposite hemithorax, causing significant hemodynamic instability.

Pleural Effusion

Pleural effusion does not involve the lung parenchyma; it can be the source of a significant amount of absorbing material in the path of the x-ray beam, causing opacity on the films. Large pleural effusions are easy to spot on a chest radiograph, but small pleural effusions can be overlooked easily, especially on portable films because the patients are almost always supine or semierect, and the fluid can collect either in a subpulmonic location or deep in the posterior costophrenic sulcus where it is hidden from view. Most often smaller effusions are seen as a veil-like opacity that is most pronounced at the base and tapers toward the lung apex. Fig. 3.9 shows a male patient with a large pleural effusion of the left hemithorax occupying two-thirds of the chest cavity on the left side.