Normal Chest Radiography and Computed Tomography

Radiography

Technique

Projections

The standard radiographic views for evaluation of the chest are the posteroanterior (PA) and lateral projections with the patient standing; such projections provide the essential requirement for proper three-dimensional (3D) assessment ( Fig. 1.1 ). In patients who are unable to stand, anteroposterior (AP) upright or supine projections offer alternative but considerably less satisfactory views. The AP projection is of inferior quality because of the shorter focal-detector distance, the greater magnification of the heart, and the restricted ability of many such patients to suspend respiration or achieve full inspiration.

Fig. 1.1, Normal chest radiograph. (A) Posteroanterior projection. (B) Lateral projection.

Basic Radiographic Techniques

Diagnostic accuracy in thoracic pathology is related partly to the quality of the radiographic images themselves, and multiple variables require attention.

Patient Positioning and Respiration

Images are ideally acquired at full inspiration to avoid magnification of the cardiomediastinal silhouette and vascular crowding, which decrease diagnostic capability.

A well-centered x-ray beam, lack of rotation, and exclusion of the scapulae from the field of view are essential for adequate positioning. The distance between the medial ends of the clavicles (anterior structures) and the spinous processes of the thoracic vertebrae (posterior structures) are reliable landmarks to assess position. In a properly centered radiograph the medial ends of the right and left clavicles are equidistant from the spinous processes ( Fig. 1.2 ). The American College of Radiology standards for performance of standard chest radiography in adults specify the use of at least a 72-inch (1.8-m) tube-detector distance for PA radiographs and a minimum of 40 inches in portable radiographs.

Fig. 1.2, Properly centered chest radiograph. A view from a frontal radiograph shows that the medial ends of the right and left clavicles (highlighted in black) are equidistant to the spinous process (arrow) of the vertebra at the same level, thus indicating that the radiograph is properly centered.

Exposure

Exposure factors should be such that there is faint visualization of the thoracic spine and the intervertebral disks on the PA radiograph so that lung markings behind the heart are clearly visible.

Kilovoltage

A high-kilovoltage technique (115–150 kVp) should be used for PA and lateral chest radiographs. The high kVp allows better penetration of the mediastinum and shorter exposure times, thus minimizing cardiac motion artifacts and providing sharper outline of the pulmonary and mediastinal structures.

Image Acquisition

Conventional screen-film radiography has a number of limitations and therefore has been replaced by computed radiography (CR) and digital radiography (DR).

The process of acquisition, transmission, display, and storage of digital images has many advantages over conventional screen-film systems, and it is widely used as the primary technology in conventional radiographic studies. A potential disadvantage of CR and DR is that patients may receive unnecessarily high radiation doses, which may not be detected because they do not result in perceivable alterations in image quality. The wider latitude of digital systems allows them to be used under a much broader range of exposure conditions and makes them an ideal choice for applications in which exposure is highly variable or difficult to control, such as bedside radiography. This results in a reproducible quality and considerable decrease in the repeat rate for bedside chest radiographs when using CR, compared with screen-film radiography.

Two main types of digital systems are available commercially: systems based on photostimulable storage phosphor image receptors, known as CR, and systems based on flat-panel x-ray detectors or array of detectors that directly capture the radiographic image, known as DR.

Computed Radiography

CR (storage phosphor radiography) involves the use of a reusable photostimulable phosphor plate rather than film to record the image. Plates coated with the phosphor are loaded into special cassettes that are outwardly similar to screen-film cassettes. During exposure the receptor stores the x-ray energy and is then scanned by a laser beam, which results in the creation of visible or infrared radiation, the intensity of which corresponds to the absorbed x-ray energy. The resultant luminescence is measured and recorded digitally.

Digital Radiography

DR uses a flat-panel x-ray detector or an array of detectors to directly capture the radiographic image numerically, thus eliminating the step of reading out the detector. The detectors in DR are selenium based and provide the additional advantage of having considerably greater quantum efficiency than conventional screen-film systems and photostimulable phosphor detectors do, which results in an image quality superior to that of screen-film systems and existing storage phosphor detectors at a comparable or lower radiation dose.

Normal Anatomy of the Chest

Airways

Trachea and Bronchi

The trachea is a midline structure, and the tracheal walls are parallel except on the left side just above the bifurcation, where the aorta commonly causes a smooth indentation ( Fig. 1.3 ). The trachea measures 10 to 12 cm in length and has 16 to 20 C-shaped cartilage rings on its lateral and anterior aspects and a fibromuscular posterior margin. Calcification of the cartilage rings is a common normal finding in patients older than 40 years, particularly women, but it is seldom evident on radiographs ( Fig. 1.4 ). The upper limits of normal for coronal and sagittal diameters of the trachea in men are 25 and 27 mm, respectively; in women, they are 21 and 23 mm, respectively. The lower limit of normal for both dimensions is 13 mm in men and 10 mm in women.

Fig. 1.3, Normal trachea and main bronchi. (A) Frontal radiograph shows that the tracheal air column is fairly straight and located in the midline except at the level of the aortic arch (AA), where the trachea is indented and may be slightly deviated to the right. The trachea divides into a short right main bronchus and a longer and more horizontal left main bronchus. (B) Coronal reformatted image from a CT scan demonstrating the normal anatomy of the trachea, the slight indentation at the level of the aortic arch (AA), and the main bronchi. Approximately 2 cm after its origin, the right main bronchus branches into the right upper lobe bronchus and the bronchus intermedius.

Fig. 1.4, Tracheal and bronchial wall calcification. A magnified view from a posteroanterior chest radiograph (A) in an elderly patient shows calcification of the tracheal and bronchial walls. Coronal (B) and sagittal (C) reformatted images from a CT scan show the extent of tracheal and bronchial calcification. Airway wall calcification is a normal finding in elderly patients.

The trachea divides into the left and right main bronchi at the carina, at approximately the level of the fifth thoracic vertebra. The subcarinal angle ranges widely from 35 to 90 degrees (mean, 61 degrees) ; thus routine measurement of this angle has no diagnostic value.

The right main bronchus measures approximately 2 cm in length and has a more vertical course than the left main bronchus does (see Fig. 1.3 ). It divides into the right upper lobe bronchus and the bronchus intermedius. The bronchus intermedius continues distally for 3 to 4 cm from the takeoff of the right upper lobe bronchus and then bifurcates into bronchi to the middle and lower lobes. The left main bronchus is approximately 5 cm in length and divides into the left upper and lower lobe bronchi.

The lobar bronchi branch into segmental bronchi. Segmental bronchi are visible on chest radiographs only when they are seen end-on as ring shadows or when they are abnormally thickened. The most commonly seen segmental bronchi on a chest radiograph are the anterior segmental bronchi of the upper lobes; seen as ring shadows adjacent to the accompanying segmental pulmonary artery ( Fig. 1.5 ).

Key Points: Trachea

Normal diameter in men is 13–27 mm.
Normal diameter in women is 10–23 mm.
The tracheal cartilages are C shaped and spare the posterior tracheal wall, which is membranous.

Fig. 1.5, Anterior segmental bronchus of the right upper lobe seen end-on. A view from a frontal radiograph shows a ring shadow (curved arrow) corresponding to the anterior segmental bronchus of the right upper lobe seen end-on and the adjacent anterior segmental pulmonary artery (straight arrow). The outer diameter of the bronchus on the upright chest radiograph is slightly larger than that of the adjacent artery.

Pulmonary Arterial and Venous Circulation

Pulmonary Arteries

The pulmonary trunk or main pulmonary artery originates in the mediastinum at the pulmonary valve and extends cranially and slightly to the left for 4 to 5 cm before bifurcating within the pericardium into the shorter left and longer right pulmonary arteries ( Fig. 1.6 ). The left pulmonary artery continues until it reaches the hilum, where it arches over the left main bronchus and gives off the left upper lobe and interlobar arteries, from which segmental and subsegmental branches arise. The left interlobar artery lies posterolateral to the upper lobe bronchus.

The right pulmonary artery courses behind the ascending aorta before dividing in front of the right main bronchus into ascending (truncus anterior) and descending (interlobar) branches.

Measurements of pulmonary arterial diameter can be helpful in the assessment of pulmonary vascular disease, although in conventional radiography, it is limited to the measurement of the right interlobar artery. The upper limit of normal of the transverse diameter of the right interlobar artery (measured from its lateral aspect to the air column of the bronchus intermedius) is 16 mm in men and 15 mm in women ( Fig. 1.7 ). Dilation of the interlobar pulmonary artery may result from increased pressure (e.g., pulmonary arterial hypertension), increased flow (e.g., left-to-right shunts), or aneurysm formation (e.g., Behçet disease) ( Fig. 1.8 ).

Key Points: Pulmonary Arteries

The pulmonary trunk or main pulmonary artery is not normally visualized on radiography in adults.
Normal diameter of the right interlobar pulmonary artery is <16 mm.

Fig. 1.7, Normal right interlobar artery visualized on a frontal chest radiograph. The upper limit of normal of the transverse diameter of the interlobar artery measured from its lateral aspect to the air column of the bronchus intermedius (black bar) is 16 mm in men and 15 mm in women.

Fig. 1.8, Enlarged central pulmonary arteries from severe pulmonary arterial hypertension. A frontal radiograph shows markedly enlarged central pulmonary arteries.

Pulmonary Veins

The pulmonary veins arise from venules that drain the alveolar capillaries and the capillary network of the pleura. In contrast to the pulmonary arteries, they are not associated with the airways. Although their final course is variable, there are usually two main superior and two main inferior vessels, the former draining the middle and upper lobes on the right and the upper lobe on the left and the latter draining the lower lobes ( Fig. 1.9 ).

Pulmonary Hila

The anatomic structures rendering the hila visible on radiographs are primarily the pulmonary arteries and veins, with lesser contributions from the bronchial walls, surrounding connective tissue, and lymph nodes ( Fig. 1.10 ).

Fig. 1.10, Normal hila. A view from a frontal radiograph shows that the main shadow of the right hilum is formed by the vertically oriented interlobar artery (arrowhead) and that of the left hilum by the distal left pulmonary artery (right arrow) and descending left pulmonary artery. Because the left pulmonary artery arches above the left main and left upper lobe bronchi, the left hilum is normally 1–2 cm higher than the right hilum.

On a PA radiograph the main shadow of the right hilum is formed by the vertically oriented interlobar artery. Right hilar structures immediately cephalad to the interlobar artery include the ascending pulmonary artery (truncus anterior) and superior pulmonary vein ( Fig. 1.11 ). The end-on opacity and radiolucency of the contiguous anterior (and occasionally posterior) segmental artery and bronchus can be identified in approximately 80% of normal individuals (see Fig. 1.5 ).

Fig. 1.11, Normal hilar anatomy. A coronal image from a CT scan demonstrates the hilar anatomy as seen on a radiograph. The right pulmonary artery (RPA) is in the mediastinum and therefore not visible on the frontal radiograph. On a frontal radiograph the main shadow of the right hilum is formed by the vertically oriented interlobar artery (RI). Right hilar structures immediately cephalad to the interlobar artery include the ascending pulmonary artery (truncus anterior) (curved arrow) and the right superior pulmonary vein (straight arrow). Most of the left hilum is formed by the distal left pulmonary artery (LPA) and left descending pulmonary artery. LA, Left atrium.

On the left the upper hilar opacity is formed by the distal left pulmonary artery, the proximal portion of the left interlobar artery and its segmental arterial branches, and the left superior pulmonary vein and its major tributaries. Because the left pulmonary artery arches over the left main and left upper lobe bronchi, the left hilum is typically 1 to 2 cm higher than the highest point of the right hilum (right interlobar artery) (see Fig. 1.10 ).

Surrounding the hilar vessels and bronchi are small amounts of fat and small lymph nodes, which cannot normally be distinguished from other structures on the radiograph. Enlargement of the hilar lymph nodes leads to increased size and opacity of the hilum, a lobulated contour, and obscuration of the interlobar artery. Hilar fullness from lymphadenopathy can be differentiated from hilar fullness from pulmonary arterial enlargement by carefully examining the adjacent hilar vessels. In lymphadenopathy the vessels will be visible through the region of hilar fullness ( Fig. 1.12 ), whereas the vessels will converge on the margins of the hilar fullness in the setting of pulmonary arterial enlargement (see Fig. 1.8 ). This finding is known as the hilar convergence sign (see Fig. 1.12 ).

Fig. 1.12, Enlarged bilateral hilar lymph nodes from sarcoidosis. A frontal chest radiograph shows a lobulated contour of the hila and increased size and opacity, findings characteristic of hilar lymph node enlargement. Note that the hilar vessels course through the convex hilar opacities (absence of the hilar convergence sign), indicative of the nonpulmonary artery origin of the hilar opacities.

The radiographic anatomy of the hila on a lateral projection is complex because the right and left hilar components are to a large degree superimposed ( Fig. 1.13 ). The trachea is usually well seen down to the level of the carina, where the air column can be seen to taper. At the level of the carina the left pulmonary artery can be seen as a curved structure as it courses above the left main bronchus and continues as the descending left pulmonary artery; consequently, the left upper lobe bronchus is seen in about 75% because it is completely surrounded by vessels (the left pulmonary artery above, the descending artery behind, and the mediastinal component of the left superior pulmonary vein in front). The air column of the normally more cephalic right upper lobe bronchus can be identified end-on as a rounded radiolucency in 50% of subjects because it is devoid of vascular envelopment on its posterior aspect such that aerated upper or lower lobe parenchyma normally abuts its wall. Consequently, clear identification of the right upper lobe bronchial lumen en face constitutes highly suggestive evidence that the airway is completely surrounded by soft tissue, most likely enlarged lymph nodes.

Fig. 1.13, Normal hilar anatomy on a lateral chest radiograph. A magnified view of the hila from a lateral radiograph shows the trachea (TR) tapering off at the level of the carina immediately above the level of the left pulmonary artery (LPA), which is seen as a curved structure as it courses above the left main bronchus (LMB) and continues as the descending left pulmonary artery. The air column of the right upper lobe bronchus (RB) is seen at the level of the carina. The posterior wall of the trachea is continuous with the posterior wall of the right main and bronchus intermedius, which form the intermediate stem line (arrowheads). The right hilum (RH) is located anterior to the right bronchus intermedius.

Below the level of the carina, a line continuous with the posterior tracheal wall represents the posterior wall of the right main and intermediate bronchi and is known as the intermediate stem line (see Fig. 1.13 ). The posterior wall of the right upper lobe and bronchus intermedius is rendered visible by air in their lumina in front and by aerated lung parenchyma in the azygoesophageal recess behind. On a well-centered lateral projection the intermediate stem line transects the middle or posterior third of the circular, radiolucent left upper lobe bronchus. The intermediate stem line is visible in 95% of individuals and normally measures 1 to 3 mm in thickness. The intermediate stem line can become abnormally thickened in patients with interstitial edema or in the presence of right hilar disease, most commonly right hilar lymphadenopathy.

On a lateral view the right hilar opacity is composed of the superior pulmonary vein anteriorly, the ascending and descending right pulmonary arteries posteriorly, and surrounding connective tissue and lymph nodes. The right pulmonary artery is a mediastinal vessel enveloped by other vessels or soft tissue elements and is therefore not seen on the lateral radiograph. The major portion of the left hilar vasculature is visible behind the intermediate stem line. The top of the left pulmonary artery is seen in 95% of subjects, usually as a sharply marginated opacity above and behind the radiolucency of the left upper lobe bronchus (see Fig. 1.13 ).

The right and left inferior pulmonary veins are commonly imaged end-on as a result of their horizontal orientation, with a nodular opacity created below and behind the lower portion of the hila. Fortunately, vessels can generally be identified as they converge toward the opacity, thereby permitting their distinction from a true parenchymal mass.

The shadows of the right hilum in front of the main bronchi and left pulmonary artery, as it courses over the left main and upper lobe bronchi, result in an inverted U appearance that delineates the radiolucent “inferior hilar window” (see Fig. 1.13 ). Normally, there are no large vessels coursing immediately below the middle and lower lobe bronchi on the right and the upper and lower lobe bronchi on the left. Therefore any rounded opacity greater than 1 cm in the inferior hilar window on a lateral view is likely to be an enlarged node or a mass ( Fig. 1.14 ).

Fig. 1.14, Enlarged hilar lymph nodes on a lateral projection. A magnified view of a lateral chest radiograph in a patient with sarcoidosis shows a lobulated contour and increased opacity in the hila (arrows), characteristic of hilar lymph node enlargement.

Lung Parenchyma

Secondary Lobule

The secondary lobule is the smallest discrete portion of the lung that is surrounded by connective tissue septa. The normal secondary lobule cannot be identified on a chest radiograph. Only when the interlobular septa are rendered visible as septal lines as a result of thickening by fluid or tissue, such as edema or carcinoma, can the lung between two lines be recognized as a secondary lobule. Septal lines (Kerley lines) are identified most commonly in the anterior and lateral aspects of the lungs, where they are best developed ( Fig. 1.15 ).

Fig. 1.15, Thickening of the interlobular septa in interstitial pulmonary edema. (A) A frontal chest radiograph in a patient with left heart failure and interstitial pulmonary edema shows septal (Kerley B) lines (black arrow). Also noted is bronchial cuffing as a result of edema (straight arrow) and increased size of the anterior segmental left upper lobe pulmonary artery (curved arrow), which has a greater diameter than the adjacent bronchus does. Also noted are small bilateral pleural effusions. (B) A magnified view of the right lower lung region better demonstrates the septal lines (arrows).

Segments and Lobes

The normal anatomy of the pulmonary segments is variable, complex, and difficult to appreciate on radiographs ( Fig. 1.16 ); it can, however, be readily assessed on computed tomography (CT). An approximate location of an abnormality within a given segment can often be determined by careful analysis of its location on frontal and lateral radiographs.

Fig. 1.16, (A) Pulmonary lobes and segments: anterior view. A schematic drawing superimposed on a three-dimensional surface reformatted image obtained with a multidetector CT scanner shows the location of the pulmonary segments and lobes on the anterior surface of the right and left lungs. The segments that form the anterior surface of the right lung include the apical (S1) and anterior (S3) segments of the right upper lobe, the lateral (S4) and medial (S5) segments of the right middle lobe, and the anterior basal (S8) segment of the right lower lobe. The anterior view of the left lung shows the location of the apicoposterior segment (S1 + 2), anterior (S3) segment, and superior (S4) and inferior (S5) lingular segments of the left upper lobe, as well as the anteromedial basal (S7 + S8) segment. Because the heart was removed from the original image, the lateral basal (S9) and posterior basal (S10) segments of the left lower lobe are also demonstrated on this image. (B) Pulmonary lobes and segments: posterior view. The segments that form the posterior surface of the right lung include the apical (S1), posterior (S2), and anterior (S3) segments of the right upper lobe and the superior (S6), lateral basal (S9), posterior basal (S10), and (adjacent to the heart, which is not shown) medial ( S7) basal segments of the right lower lobe. The segments that form the posterior surface of the left lung include the apicoposterior (S1 + S2) segments of the left upper lobe and the superior (S6), anteromedial basal (S7 + S8), lateral basal (S9), and posterior basal (S10) segments of the left lower lobe.

Radiographic Density

The radiopacity of the lungs is a result of the absorptive power of each of its components: gas, blood, and tissue. Nonaerated lung in vivo consists of approximately half blood and half tissue,

On a properly positioned radiograph the radiographic density of the right and left lungs is symmetric. If the patient is rotated, the lung closer to the detector is more uniformly radiopaque (whiter) than the other lung; conversely, the lung that is farthest away from the detector is uniformly less radiopaque (more black). Provided that the patient is not rotated and the x-ray beam is centered properly, any discrepancy in the density of the two lungs must be interpreted as being abnormal. The cause varies from such benign conditions as scoliosis and congenital absence of the pectoral muscles to more significant disorders such as Swyer-James-McLeod syndrome.

Pulmonary Markings

Correct interpretation of the chest radiograph requires a thorough knowledge of the pattern of linear markings throughout the normal lung. These markings are created by the pulmonary arteries, bronchi, veins, and accompanying interstitial tissue. The pulmonary arteries fan outward from both hila and gradually taper as they proceed distally, up to about 1 to 2 cm from the visceral pleural surface.

The PA chest radiograph of a normal erect individual invariably shows some discrepancy in the size of the pulmonary vessels in the upper lung zones compared with the lower zones as a result of the apicobasal pressure gradient. In a recumbent individual a decrease in the influence of gravity renders this discrepancy in vascular size minimal.

Pleura

The pleural space is enclosed by the visceral pleura, which is tightly attached to the lungs, and by the parietal pleura, which lines the chest wall, diaphragm, and mediastinum. The two join at the hila. The visceral pleura consists of mesothelial cells overlying two layers of elastic tissue that separate a small amount of connective tissue and lymphatic vessels. Because the combined thickness of the normal parietal and visceral pleural layers is approximately 0.2 mm, the pleura over the convexity of the lungs and over the diaphragmatic and mediastinal surfaces is not normally visible on CR or CT. Each pleural space normally contains 8.4 ± 4.3 mL of fluid. Control of the volume and composition of pleural liquid is affected by a number of mechanisms, including Starling forces (providing filtration through the parietal and absorption through the visceral mesothelium), lymphatic drainage, and the activity of mesothelial cells. Fluid is usually produced in the parietal pleura and drained at the visceral pleura, and it tends to be drawn into the space because recoil of the lung and chest wall acts as an opposing force over most of the range of respiratory volumes.

Interlobar Fissures

Fissures are invaginations of the pleura that extend from the outer surface of the lung into its substance. They are traditionally considered in two groups: those that separate the lungs into the three right-sided and two left-sided lobes ( normal fissures) and those that occur within one of the lobes themselves ( accessory fissures). The normal fissures are the minor fissure (located between the right middle and upper lobes), the right major fissure (between the combined right upper and middle lobes and the right lower lobe), and the left major fissure (between the left upper and lower lobes) ( Fig. 1.17 ). On a lateral chest radiograph the major fissures can be seen as lines that extend obliquely downward and forward from the level of the fifth thoracic vertebra and end at the diaphragm a few centimeters behind the anterior pleural gutter ( Fig. 1.18 ). In the majority of cases the interlobar fissures are incomplete, and the incidence of accessory fissures is variable.

Fig. 1.18, Normal major and minor fissures. Lateral radiograph showing the right major (arrows) and minor (arrowheads) fissures. The left major fissure normally projects slightly posterior to the right major fissure and is not well seen on this radiograph.

Accessory Fissures

Azygos Fissure.

The azygos fissure is one of the most recognized accessory fissures, created by downward invagination of the azygos vein through the apical portion of the right upper lobe. It is manifested radiographically as a curvilinear shadow that extends obliquely across the upper portion of the right lung and terminates at a variable distance above the right hilum in a teardrop shadow caused by the azygos vein itself ( Fig. 1.19 ). Because the vein runs outside the parietal pleura, four pleural layers (two parietal and two visceral) form the fissure.

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