1 Examination Technique and Normal Findings

Describe and Discuss: If a pulmonary abnormality is detected, it may be assigned to one of the radiographic patterns in Chapter 13.

Differential diagnosis: Good results have been achieved with the “gamut approach” (Reeder and Felson 2003). Just as a musician deliberately strikes each note while practicing scales on a piano, every possible interpretation of a pattern should be considered when reading an image. At the very least, the reader should consider the various main disease categories. These include congenital malformations, inflammatory disorders, chronic obstructive pulmonary disease (COPD), inhalational diseases and pneumoconioses, neoplasia, vascular disorders, and trauma.

Decide: The ultimate goal of imaging is to make a definitive diagnosis. The clinical data frequently are important in making a definitive diagnosis. Sometimes, however, this is not possible and the radiologist can do no more than offer a differential diagnosis in which diagnoses are listed in order of probability. In this case cytology/histology may be required to make a definitive diagnosis.

Radiographic Examination

Frontal Chest Radiograph

The upright patient positions his anterior chest against the film cassette (Fig. 1.1a, b). The dorsal aspects of the hands are placed on the hips, and the shoulders are rolled forward to project the scapulae outside the lungs. For better support, weak patients may place their arms around the cassette stand.

Fig. 1.1a–e Chest radiographs in PA and AP projections. a, b Standing PA radiograph. c, d Sitting AP radiograph. e Supine AP radiograph.

The patient wears a short lead apron. The upper border of the cassette is at the level of C7, and 35 × 35 to 40 × 40 cm size film is used, depending on patient size. The roentgen beam is collimated laterally on the skin surface over the lower ribs and is centered on the fourth thoracic vertebra (T4).

The radiograph is taken at full inspiration using, for example, a film-focus distance (FFD) of 185 cm and exposure parameters of approximately 125 kV, 5 mA, or automatic exposure control.

The AP projection is useful for imaging sick patients (Fig. 1.1c, d). Bedridden patients are filmed in the supine position, and internal rotation of the shoulders aids in separating the scapulae (Fig. 1.1e).

The technical parameters recommended for chest radiographs are listed in Table 1.1.

Table 1.1 Technical parameters for chest radiographs (Zimmer and Zimmer-Brossy 1992)

Lateral Chest Radiograph

The patient, wearing a lead apron, stands sideways against the cassette with arms raised. The roentgen beam is centered approximately 10 cm caudal to the axilla (Fig. 1.2a, b).

The radiograph is taken at full inspiration using film size 30 × 40 or 35 × 35 cm. A FFD of 185 cm and settings of 125 kV, approximately 8 mA, or automatic exposure control are appropriate.

Debilitated patients are radiographed in the sitting position (Fig. 1.2c, d) or, if necessary, in the lateral decubitus position with the head supported, the arms drawn forward and upward, and the legs slightly flexed to improve stability (Fig. 1.2e).

Fig. 1.2a–e Lateral chest radiographs. a, b Standing lateral radiograph. c, d Sitting lateral radiograph. e Supine lateral radiograph.

Oblique Views

The patient is rotated in the frontal plane to approximately 45° with either the right (right anterior oblique or “fencer” position; Fig. 1.3a) or left anterior chest wall (left anterior oblique or “boxer” position; Fig. 1.3b) in contact with the cassette. The depth of inspiration, film size, FFD, and exposure parameters are the same as for the PA view.

Fig. 1.3a, b Oblique chest radiographs. a Right anterior oblique (fencer) position, b left anterior oblique (boxer) position.

Apical Lordotic View

The patient stands about 4 cm from the chest stand and arches the upper body backward until the shoulders touch the cassette. A FFD of 100 cm and 35–45° of cephalad tube angulation are appropriate. The beam is centered on the manubrium sterni (Fig. 1.4).

A technically acceptable image shows the lung apices projected clear of the scapulae.

Fig. 1.4a, b Apical lordotic views. a Standing, b supine.

Fluoroscopy

Scatter-Reduction Grids

Focused moving grids, which are used on a Bucky table. The strips in the grid are aligned to match the divergence of the primary beam (Fig. 1.5a). Because the grid moves during the exposure, grid lines do not appear in the final image. There are two main sources of exposure errors with this type of grid system:

Fig. 1.5a–c Scatter-reduction grids. The strips of the grid converge toward the roentgen focus (a). A defocused grid (b) leads to underexposure of the lateral areas. An off-center grid (c) leads to asymmetrical exposure (modified from Laubenberger 1980).

Fig. 1.6 Asymmetrical exposure due to a tilted grid. Note the decreased lucency of the lung and “haziness” of the bones and soft tissues on the left side.

Fig. 1.7 Stationary grid for bedside radiography. Vertical grid lines are visible on the image.

• Defocused grid. If the necessary focusing distance is not maintained between the grid and X-ray tube, the lateral portions of the image will be underexposed (Fig. 1.5b).
  
• Off-center grid. If the central ray is not aimed at the center of the grid (due to a poorly adjusted collimator, for example), the film will be asymmetrically exposed (Figs. 1.5c, 1.6).

Stationary grids with parallel strips are used for bedside radiography. Often it is difficult to position the patient as desired, and it is not unusual to work with a tilted grid. This also causes asymmetrical film exposure because the strips at one edge are aligned with the beam divergence while the strips at the other edge are not (Fig. 1.7). When an image made with a stationary grid is examined closely (e. g., with a magnifying lens), grid lines always can be seen.

The Normal Chest Radiograph

The chest radiograph requires a systematic evaluation (Figs. 1.8 and 1.9) in which the

• chest wall,
  
• diaphragm,
  
• pleura,
  
• lungs, and
  
• mediastinum

are identified, analyzed, and described. To reinforce this approach, these regions will be considered in the above order.

Structures of the Chest Wall

On the lateral view, the posterior rib segments project behind the vertebral bodies while the lateral portions appear as faint bands running obliquely forward and downward.

The soft tissues form the contour of the chest wall. They also may project over the intrathoracic organs as opacities and interface lines (Figs. 1.10 and 1.11).

Skin Folds

Skin folds are especially prominent on supine radiographs of cachectic patients. They may form linear opacities and should not be mistaken for a pneumothorax.

Breast Shadow

The breast shadows decrease the lucency of the lower lung zones. Nipple shadows may mimic small pulmonary nodules and may be quite asymmetrical in appearance.

Fig. 1.10 Axillary folds: posterior axillary fold (white arrow), anterior axillary fold (arrowhead), apex of axilla (black arrow).

Fig. 1.11 Soft-tissue shadows of the chest wall.

Axillary Folds

This narrow soft-tissue stripe along the upper border of the clavicle represents soft tissue that is tangential to the x-ray beam (Fig. 1.12).

Fig. 1.12 Companion shadow of the clavicle (tangential projection of the skin over the clavicle) and the vertical shadow of the sternocleidomastoid muscle.

Sternocleidomastoid Muscle

The sternocleidomastoid muscle appears as an almost vertical soft-tissue opacity in the neck. It has a well-defined lateral border, and it merges inferiorly with the companion shadow of the clavicle.

Supraclavicular Fossa

Visible only in thin patients, the supraclavicular fossa appears as a thin horizontal line projected above or below the clavicle. It extends laterally beyond the lung, thus distinguishing it from an apical pneumothorax (see Fig. 1.11).

Jugular Fossa

In thin patients, both sternocleidomastoid muscles have sharp medial borders and their contours unite inferiorly in a U-shaped configuration. This may sometimes mimic widening of the trachea.

Upper Arms

The lateral radiograph projects the soft tissues of the upper arm over the anteroapical lung. Usually the left and right upper arms can both be seen and their contours are continuous with the posterior axillary fold.

Diaphragm

Several features can be used to differentiate the left and right hemidiaphragms. On the lateral chest radiograph, the left hemidiaphragm is usually lower anteriorly and higher posteriorly with the gastric bubble lying inferiorly. Its contour extends anteriorly only to the posterior border of the cardiac silhouette. The right hemidiaphragm is usually higher anteriorly and lower posteriorly. Its contour may be traced throughout its posteroanterior course from the posterior costophrenic angle forward to the sternum (Fig. 1.13).

Pleura

Apposition of the pleural leaves is made possible by a negative pressure within the pleural cavity (approximately -5 cm H₂O); this counteracts the elastic recoil of the pulmonary interstitium. Because of the intrinsic weight of the lung, this partial vacuum is greater in the apical region than at the base (Paré and Fraser 1983).

Negative pressure is important in keeping the lungs inflated. Serum inflow would tend to equalize this pressure were it not for the low protein content of the pleural fluid and the oncotic “suction” of the adjacent capillary blood, which tends to draw fluid from the pleural space. Negative pressure is maintained even though approximately 100 mL of fluid flows from the parietal pleura to the visceral pleura each day. This is possible because of the unequal hydrostatic pressures in the chest wall and pulmonary capillaries (Fig. 1.14).

Fig. 1.13 Radiographic appearance of the diaphragm.

Fig. 1.14 Hydrostatic and oncotic pressure in the pleural layers (from Müller and Fraser 2001).

Radiographic Anatomy

The peripulmonary pleura invests the surface of the lung, and the interlobar pleura lines the fissures between the lobes. Radiographically, the peripulmonary pleura appears only as an interface between the lung and soft tissues. The interlobar pleura which comprises the fissures, on the other hand, often is directly visible as a hairline shadow.

Peripulmonary Pleura

A dense stripe 0.5 mm in width may occasionally be seen in the area where the lung abuts the chest wall, diaphragm, and mediastinum. This stripe does not represent the pleura; it is a visual artifact caused by the collateral inhibition or stimulation of retinal receptors (Mach effect; Fig. 1.15). This is easily confirmed by covering the lung with a piece of paper next to the stripe and noting that the stripe is gone (Fig. 1.16).

The peripulmonary pleura has several radiographically distinct components (Coussement and Butori 1978; Fig. 1.17).

Companion Shadow of the Second Rib

The apical pleura appears in profile as a stripe, 2 mm in width, at the inferior border of the posterior aspect of the second rib (Fig. 1.18).

Companion Shadow of the Lateral Chest Wall

The pleural line, if seen tangentially, usually is smooth but occasionally bulges into the intercostal spaces, creating a scalloped contour. In obese patients an extrapleural fat stripe, several millimeters in width, may be visible between the inner surface of the ribs and the lung edge; this should not be confused with pleural thickening or a small pleural effusion. Occasionally the subcostal muscle fibers elevate the pleura, producing a wavy contour (see Fig. 1.16).

Fig. 1.15a–c The Mach effect. The contour of the aorta on a chest radiograph (a) appears to be accompanied by a dark stripe (arrow), and the paravertebral space appears to be bounded by a bright stripe (arrowhead). These are optical illusions due to accentuation of contrast in the human retina. When two homogeneous areas of different brightness are apposed (1 in c), the actual densitometric profile (top row in c) is not perceived. Instead, the bright area appears even brighter while the dark area appears somewhat darker (bottom row in c). If the bright area darkens toward the boundary, the contrast appears to be accentuated only in the dark area (2 in b and c). If the dark area gradually lightens toward the boundary, a bright edge will be perceived (3 in b and c).

Basal Pleural Reflection

A line may be seen passing medially and horizontally from the deepest point of the lateral costophrenic angle. This is seen particularly well on overexposed radiographs. It corresponds to the pleural reflection at the posterior costophrenic angle.

Apical Pleural Cap

Anteriorly the lung extends to the level of the clavicle, but posteriorly it projects more superiorly. On the lateral view, the soft-tissue shadow of the upper arm usually is superimposed usually on the lung apex. However, if the shoulders are lowered, the lung apex with its accompanying pleural shadow may be seen.

Posterior Pleural Stripe

On the lateral view, the posterior pleural stripe is visible in profile along the posterior ribs. The pleural stripe extends inferiorly to the posterior costophrenic angle and the diaphragmatic reflection. On the lateral view, the pleural line can help in identifying right and left diaphragmatic leaflets (see Fig. 1.13).

Fig. 1.16 An apparent bright stripe is perceived at the boundary between the lung and soft tissue as a result of the Mach effect (see Fig. 1.15).

Fig. 1.17a–e Pleural reflections (from Heitzman 1993, Meschan 1981).

Fig. 1.18 The companion shadow of the second rib corresponds to the lung apex or the boundary between air and soft tissue. The catheter position defines the course of the subclavian vein.

Retrosternal Stripe

On the lateral view, the retrosternal stripe is relatively wide superiorly behind the manubrium. Here, the brachiocephalic vein is interposed between the lung and sternum. The stripe is narrower in its midportion, where the right and left anterior lung borders may be differentiated. The right pleura protrudes into the intercostal spaces and thus has a wavy contour; it is frequently projected over the body of the sternum. The left anterior pleural border is smoother because of the interposed heart. In the basal part of the retrosternal stripe, epicardial fat frequently displaces the left anterior lung border from the chest wall, producing a triangular retrosternal opacity (cardiac incisure).

Posterior Superior Pleural Junction Line

The pleura sweeps medially from both sides as a continuation of the companion shadow of the 2nd rib, unites at the level of the T3/4 intervertebral space, then passes as a vertical stripe to the aortic arch. This creates a Y-shaped shadow that corresponds to the posterior pleural reflection (see Figs. 1.17d, 1.19). It is composed of four layers of pleura and corresponds to the course of the superior intercostal veins.

Anterior Superior Pleural Junction Line

This pleural reflection corresponds to the course of the innominate and subclavian veins. It forms a Y-shaped configuration, the limbs of which originate from the inferior margins of the sternoclavicular joints. They course caudally, medially, and, on uniting, slant to the left. The anterior junction line complex extends inferiorly to the heart and encompasses the potential space of the anterior mediastinum (see Figs. 1.17e, 1.19).

Azygoesophageal Stripe

The right lung extends in front of the vertebral column and forms the azygoesophageal recess. It borders the azygos vein and the esophagus and forms a mediastinal pleural stripe in a paramedian location, paralleling the course of the esophagus (see Figs. 1.17e, 1.20).

Paravertebral Stripe

The posterior parts of the lungs are contiguous to the lateral aspects of the vertebral bodies. Pleura seen in tangent to the roentgen beam in this region forms a visible stripe on the left side. On the right side, the stripe is seen only if it is widened by aging or pathologic processes such as degenerative osteophyte formation, hematoma, or tumor (see Figs. 1.15a, 1.17e). On the left side, the paravertebral stripe can reach a width equal to half the diameter of the descending aorta.

Para-aortic Stripe

This accompanies the descending aorta on the left side. Comparison of the paravertebral and para-aortic stripes reveals an interesting visual phenomenon: The aortic border to the lung is a convex arch; this produces a negative Mach effect, resulting in an apparently darker stripe paralleling the descending aorta. The paravertebral soft tissues have a concave boundary with the lung; this results in a positive Mach effect creating the impression of a white border stripe (Figs. 1.15a, 1.17e, 1.20).

Right Paratracheal Stripe

This corresponds to the mediastinal pleural reflection on the tracheal wall. The inferior widening of the stripe is due to the horizontally orientated arch of the azygos vein. The width of the paratracheal stripe should not exceed 4mm. It frequently is superimposed on the superior vena cava. A well-exposed view, conventional or computed tomography (CT) will demonstrate the paratracheal stripe, azygos vein, and superior vena cava and will determine the true width of this stripe (see Fig. 1.17e).

Retrotracheal Stripe

On the lateral radiograph, this stripe blends with the posterior border of the trachea and should not exceed 3 mm in width (see Fig. 1.17d).

Fig. 1.19 The posterior pleural reflection (= posterior superior pleural junction line, arrows) and anterior pleural reflection (= anterior superior pleural junction line, arrowheads).

Fig. 1.20

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