5 курс / Пульмонология и фтизиатрия / Clinical_Manifestations_and_Assessment_of_Respiratory

2For a complete review of common heart arrhythmias, see Des Jardins, T. (2019). Cardiopulmonary anatomy and physiology: essentials of respiratory care (7th ed.). Clifton Park, NY: Delmar/Cengage Learning.

3Thermodilution is a method of cardiac output determination. A bolus of a solution of known volume and temperature is injected into the right atrium, and the resultant change in blood temperature is detected by a thermistor previously placed in the pulmonary artery with a catheter.

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C H A P T E R 8

Radiologic Examination of the Chest

CHAPTER OUTLINE

Fundamentals of Radiography

Standard Positions and Techniques of Chest Radiography

Posteroanterior Radiograph

Anteroposterior Radiograph

Lateral Radiograph

Lateral Decubitus Radiograph

Inspecting the Chest Radiograph

Technical Quality of the Radiograph

Sequence of Examination

Computed Tomography

Positron Emission Tomography

Positron Emission Tomography and Computed Tomography Scan

Magnetic Resonance Imaging

Pulmonary Angiography

Ventilation-Perfusion Lung Scan

Fluoroscopy

Bronchography

Ultrasound

Echocardiogram

Radiation Safety

Radiation Safety Techniques

Self-Assessment Questions

CHAPTER OBJECTIVES

After reading this chapter, you will be able to:

•Describe the fundamentals of radiography.

•Differentiate among the standard positions and techniques of chest radiography.

•Define the radiologic terms commonly used during inspection of the chest radiograph.

•Describe the three steps to evaluate technical quality of the radiograph.

•Describe a logical, systematic sequence of radiograph examination.

•Describe the push-pull notion of malposition of thoracic structures.

•Describe the application and diagnostic value of the following radiologic procedures:

•Computed tomography (CT)

•Positron emission tomography (PET)

•Positron emission tomography and computed tomography scan (PET/CT scan)

•Magnetic resonance imaging (MRI)

•Pulmonary angiography

•Ventilation-perfusion scan

•Fluoroscopy

•Bronchography

•Ultrasound Imaging

•Echocardiography

•Describe the principles and techniques of radiation safety.

•Describe the respiratory therapist's role in chest radiology.

•Define key terms and complete self-assessment questions at the end of the chapter and on Evolve.

KEY TERMS

Air Cyst

Anteroposterior Radiograph

Bleb

Bronchogram

Bronchography Bullae Cardiothoracic Ratio Cavity

Color Doppler

Computed Tomography (CT)

Computed Tomography Pulmonary Angiogram (CTPA) Consolidation

Doppler Echocardiogram Echocardiogram

Fetal Echocardiogram Fluoroscopy

High-Resolution CT (HRCT) Scans Hilar Displacement Homogeneous Density Honeycombing

Hot Spots in Chest PET Scan Infiltrates

Interstitial Density

Lateral Decubitus Radiograph Lateral Radiograph

Lesion

Lung Tissue Markings

Lung Window CT Scan

Magnetic Resonance Imaging (MRI) Mediastinal Window CT Scan Opacity

Pleural Density

Pleural Nodule

Positron Emission Tomography (PET)

Positron Emission Tomography and Computed Tomography Scan (PET/CT Scan) Posteroanterior Radiograph

Power Doppler Pulmonary Angiography Pulmonary Mass

“Push-Pull” Abnormal Positioning of Chest Structures Radiodensity

Radiolucency

Rib Series (Radiograph) Spectral Doppler

Spiral Computed Tomography Pulmonary Angiogram Scan Stress Echocardiogram

Subcutaneous Emphysema Tracheal Shift

Transesophageal Echocardiogram (TEE) Translucency

Transthoracic Echocardiogram (TTE) Ultrasound Imagining

Ventilation-Perfusion Lung Scan

Radiography is the making of a photographic image of the internal structures of the body by passing x-rays through the body to an x-ray film, or radiograph. In patients with respiratory disease, radiography plays an important role in the diagnosis of lung disorders, the assessment of the extent and location of the disease, and the evaluation of the subsequent progress of the disease.

Fundamentals of Radiography

X-rays are created when fast-moving electrons with sufficient energy collide with matter in any form. Clinically, x-rays are produced by an electronic device called an x-ray tube.

The x-ray tube is a vacuum-sealed glass tube that contains a cathode and a rotating anode. A tungsten plate approximately a half-inch square is fixed to the end of the rotating anode at the center of the tube. This tungsten plate is called the target. Tungsten is an effective target metal because of its high melting point, which can withstand the extreme

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heat to which it is subjected, and because of its high atomic number, which makes it more effective in the production of x- rays.

When the cathode is heated, electrons “boil off.” When a high voltage (70 to 150 kV) is applied to the x-ray tube, the electrons are driven to the rotating anode, where they strike the tungsten target with tremendous energy. The sudden deceleration of the electrons at the tungsten plate converts energy to x-rays. Although most of the electron energy is converted to heat, a small amount (less than 1%) is transformed to x-rays and allowed to escape from the tube through a set of lead shutters called a collimator. From the collimator the x-rays travel through the patient to the x-ray film.

The ability of the x-rays to penetrate matter depends on the density of the matter. For chest radiographs the x-rays must also pass through bone, air, soft tissue, and fat. Dense objects such as bone absorb more x-rays (preventing penetration) than objects that are not as dense, such as blood and the air-filled lungs.

After passing through the patient, the x-rays strike the x-ray film. X-rays that pass through low-density objects strike the film at full force and produce a black image on the film. X-rays that are absorbed by high-density objects (such as bone) either do not reach the film at all or strike the film with less force. Relative to the density of the object, these objects appear as light gray to white on the film.

Standard Positions and Techniques of Chest Radiography

Clinically, the standard radiograph of the chest includes two views: a posteroanterior radiograph and a lateral projection (either a left or right lateral radiograph) with the patient in the standing position. When the patient is seriously ill or immobilized, an upright radiograph may not be possible. In such cases, a supine anteroposterior radiograph is obtained at the patient's bedside. A lateral radiograph is rarely obtainable under such circumstances.

Posteroanterior Radiograph

The standard PA chest radiograph is obtained by having the patient stand (or sit) in the upright position. The anterior aspect of the patient's chest is pressed against a film cassette holder, with the shoulders rotated forward to move the scapulae away (aside) from the lung fields. The distance between the x-ray tube and the film is 6 feet. The x-ray beam travels from the x-ray tube, through the patient from back to front, and to the x-ray film.

The x-ray examination is usually performed with the patient's lungs in full inspiration to show the lung fields and related structures to their greatest possible extent. At full inspiration, the diaphragm is lowered to approximately the level of the ninth to eleventh ribs posteriorly (Fig. 8.1). For certain clinical conditions, radiographs are sometimes taken at the end of both inspiration and expiration. For example, in patients with obstructive lung disease an expiratory radiograph also may be made to evaluate diaphragmatic excursion and the symmetry or asymmetry of such excursion (Fig. 8.2).

FIGURE 8.1 Standard posteroanterior chest radiograph with the patient's lungs in full inspiration.

FIGURE 8.2 A posteroanterior chest radiograph of the same patient shown in Fig. 8.1 during expiration.

Anteroposterior Radiograph

A supine AP radiograph may be taken in patients who are debilitated, immobilized, or too young to tolerate the PA procedure. The AP radiograph is usually taken with a portable x-ray unit at the patient's bedside. The film is placed behind the patient's back, with the x-ray unit positioned in front of the patient, approximately 48 inches from the film.

Compared with the PA radiograph, the AP radiograph has disadvantages. For example, the heart and superior portion of the mediastinum are significantly magnified in the AP radiograph. This is because the heart is positioned in front of the thorax as the x-ray beams pass through the chest in the anterior-to-posterior direction, causing the image of the heart to be enlarged (Fig. 8.3).

FIGURE 8.3 Compared with the posteroanterior (PA) chest radiograph, the heart is significantly magnified in the anteroposterior (AP) chest radiograph. In the PA radiograph, the ratio of the width of the heart to the thorax is normally less than 1 : 2. The reason the heart appears larger in the AP radiograph is that it is positioned in front of the thorax as the x-ray beams pass through the chest in the anterior-to-posterior direction. This allows more space for the heart shadow to “fan out” before it reaches the x-ray film.

The AP radiograph frequently has less resolution and more distortion. Because the patient is often unable to sustain a maximal inspiration, the lower lung lobes frequently appear hazy, erroneously suggesting pulmonary congestion or pleural effusion. Finally, because the AP radiograph is commonly taken in the intensive care unit, extraneous shadows, such as those produced by ventilator tubing and indwelling lines, are often present (Fig. 8.4). The position in which the chest x-ray is taken will be at the discretion of the radiology technician, unless specifically ordered to do otherwise.

FIGURE 8.4 Anteroposterior (AP) chest radiograph. The diaphragms are elevated, the lower lung lobes appear hazy, the ratio of the width of the heart to the thorax is greater than 2 : 1 (i.e., the width of the heart is greater than 50% of the width of the thorax), and an extraneous object is apparent outside the patient's left lateral chest (probably an electrocardiograph lead). X-ray examinations using portable devices are frequently performed on patients too ill to be transported to the radiology department. These films, in the best of circumstances, are of poorer quality than erect films taken with standard x-ray apparatus. The films are usually AP projections taken with the x-ray unit in front of and the film plate behind the patient. Overexposure, underexposure, malpositioning, marginal cutoffs, and motion artifacts are often present. In this setting, major events such as partial pneumothoraces, pleural effusions, and infiltrates in dependent parts of the lung may go unrecognized. Therefore careful clinical correlation with the patient's pathophysiology and symptomatology is imperative.

Lateral Radiograph

The lateral radiograph is obtained to complement the PA radiograph. It is taken with the side of the patient's chest compressed against the cassette. The patient's arms are raised, with the forearms resting on the head.

To view the right lung and heart, the patient's right side is placed against the cassette. To view the left lung and heart, the patient's left side is placed against the cassette. Therefore a right lateral radiograph would be selected to view a density or lesion that is known to be in the right lung. If neither lung is of particular interest, a left lateral radiograph is usually selected to reduce the magnification of the heart. The lateral radiograph provides a view of the structures behind the heart and left diaphragmatic dome. It is combined with the PA radiograph to give a three-dimensional view of the cardiac and pulmonary structures or of any abnormal densities (Fig. 8.5).

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FIGURE 8.5 Lateral chest radiograph. The patient has an overexpanded lung and chest wall (barrel chest deformity) consistent with his known emphysema.

Lateral Decubitus Radiograph

The lateral decubitus radiograph is obtained by having the patient lie on the left or right side rather than standing or sitting in the upright position. The naming of the decubitus radiograph is determined by the side on which the patient lies; thus a right lateral decubitus radiograph means the patient's right side is down.

The lateral decubitus radiograph is useful in the diagnosis of a suspected or known fluid accumulation in the pleural space (e.g., a pleural effusion; see Chapter 24, Pleural Effusion and Empyema) that is not easily seen in the PA radiograph. A pleural effusion, which is usually more thinly spread out over the diaphragm in the upright position, collects in the gravity-dependent areas while the patient is in the lateral decubitus position, allowing the fluid to be more readily seen (Fig. 8.6).

FIGURE 8.6 Right lateral decubitus view. Subpulmonic pleural effusion. Subdiaphragmatic fluid has run up the lateral chest wall, producing a band of soft tissue density. The medial curvilinear shadow (arrows) indicates fluid in the lips of the major fissure.

Inspecting the Chest Radiograph

Before the respiratory therapist can effectively identify abnormalities on a chest radiograph, he or she must be able to recognize the normal anatomic structures. Fig. 8.7 represents a normal PA chest radiograph with identification of important anatomic landmarks. Fig. 8.8 labels the anatomic structures seen on a lateral chest radiograph.

FIGURE 8.7 Normal posteroanterior chest radiograph. 1, Trachea (note vertebral column in middle of trachea in this correctly centered and exposed film); 2, carina; 3, right mainstem bronchus; 4, left mainstem bronchus; 5, right atrium; 6, left ventricle; 7, hilar vasculature; 8, aortic knob; 9, diaphragm; 10, costophrenic angles; 11, breast shadows; 12, gastric air bubble; 13, clavicle; 14, rib.

FIGURE 8.8 Normal lateral chest radiograph. 1, Manubrium; 2, sternum; 3, cardiac shadow; 4, retrosternal air space in the lung; 5, trachea; 6, bronchus, on end; 7, aortic arch (ascending and descending); 8, scapulae; 9, vertebral column; 10, diaphragm; 11, breast shadow.

Table 8.1 lists some of the more important radiologic terms used to describe abnormal chest x-ray findings.

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TABLE 8.1

Common Radiologic Terms

Term	Definition
Air cyst	A thin-walled radiolucent area surrounded by normal lung tissue.
Bleb	A superficial air cyst protruding into the pleura; also called bullae.
Bronchogram	An outline of air-containing bronchi beyond the normal point of visibility. An air bronchogram
	develops as a result of an infiltration or consolidation that surrounds the bronchi, producing a
	contrasting air column on the radiograph—that is, the bronchi appear as dark tubes surrounded
	by a white area produced by the infiltration or consolidation.
Bullae	A large, thin-walled radiolucent area surrounded by normal lung tissue.
Cavity	A radiolucent (dark) area surrounded by dense tissue (white). A cavity is the hallmark of a lung
	abscess. A fluid level may be seen inside a cavity.
Consolidation	The act of becoming solid; commonly used to describe the solidification of the lung caused by a
	pathologic engorgement of the alveoli, as occurs in acute pneumonia.
Homogeneous	Refers to a uniformly dense lesion (white area); commonly used to describe solid tumors, fluid-
density	containing cavities, or fluid in the pleural space.
Honeycombing	A coarse reticular (netlike) density commonly seen in pneumoconiosis.
Infiltrate	Any poorly defined radiodensity (white area); commonly used to describe an inflammatory lesion.
Interstitial	A density caused by interstitial thickening.
density
Lesion	Any pathologic or traumatic alteration of tissue or loss of function of a part.
Opacity	State of being opaque (white); an opaque area or spot; impervious to light rays or, by extension, x-
	rays; opposite of translucent or radiolucent.
Pleural density	A radiodensity caused by fluid, tumor, inflammation, or scarring.
Pulmonary mass	A lesion in the lung that is 6 cm or more in diameter; commonly used to describe a pulmonary tumor.
Pulmonary	A lesion in the lung that is less than 6 cm in diameter and composed of dense tissue; also called a
nodule	solitary pulmonary nodule or “coin” lesion because of its rounded, coinlike appearance.
Radiodensity	Dense areas that appear white on the radiograph; the opposite of radiolucency.
Radiolucency	The state of being radiolucent; the property of being partly or wholly permeable to x-rays; commonly
	used to describe darker areas on a radiograph such as an emphysematous lung or a
	pneumothorax.
Translucent	Permitting the passage of light (or in this case, x-rays); commonly used to describe darker areas of
(translucency)	the radiograph.

Technical Quality of the Radiograph

The first step in examining a chest radiograph is to evaluate its technical quality. Was the patient in the correct position when the radiograph was taken? To verify the proper position, check the relationship of the medial ends of the clavicles to the vertebral column. For the PA radiograph the vertebral column should be precisely in the center between the medial ends of the clavicles and the distance between the right and left costophrenic angles and the spine should be equal. Even a small degree of patient rotation relative to the film can create a false image, erroneously suggesting tracheal deviation, cardiac displacement, or cardiac enlargement.

Second, the exposure quality of the radiograph should be evaluated. Normal exposure is verified by determining whether the spinal processes of the vertebrae are visible to the fifth or sixth thoracic level (T-5 to T-6). X-ray equipment is now available that allows the vertebrae to be seen down to the level of the cardiac shadow. The degree of exposure can be evaluated further by comparing the relative densities of the heart and lungs. For example, because the heart has a greater density than the air-filled lungs, the heart appears whiter than the lung fields. The heart and lungs become more radiolucent (darker) with greater exposure of the radiograph. A radiograph that has been overexposed is said to be “heavily penetrated” or “burned out.” Conversely, the heart and lungs on an underexposed radiograph may appear denser and whiter. The lungs may erroneously appear to have infiltrates, and there may be little or no visibility of the thoracic vertebrae.

Third, the level of inspiration at the moment the radiograph was taken should be evaluated. At full inspiration, the diaphragmatic domes should be at the level of the ninth to eleventh ribs posteriorly. On radiographs taken during expiration, the lungs appear denser, the diaphragm is elevated, and the heart appears wider and enlarged (see Fig. 8.2).

Sequence of Examination

Although the precise sequence in examining a chest radiograph is not important, the inspection should be done in a systematic manner to avoid errors of omission. A common preference is an “inside-out” approach to inspecting the chest radiograph, which entails beginning with the mediastinum and proceeding outward to the peripheral extrathoracic soft tissue. Some practitioners prefer the reverse. The following sequence reflects an “inside-out” method.

Mediastinum

The mediastinum should be inspected for width, contour, and shifts from the midline. The respiratory therapist should inspect the anatomy of the mediastinum, including the trachea, carina, cardiac borders, aortic arch, and superior vena cava (see Fig. 8.7).

Trachea

On the PA projection, the trachea should appear as a translucent column overlying the vertebral column. The diameter of the right and left bronchi progressively tapers for a short distance beyond the carina, which then disappears (see Fig. 8.7). A number of clinical conditions can cause the trachea to shift from its normal position. For example, fluid or gas accumulation in the pleural space causes tracheal shift away from the affected area. Atelectasis or fibrosis usually causes the trachea to shift toward the affected area. The trachea also may be displaced by tumors of the upper lung regions.

Anatomic structures in the chest (e.g., the trachea) move out of their normal position because they are either “pushed or pulled” in a given direction. In other words, they may be moved up or down or from side to side by lesions “pushing or

pulling” in that direction. Table 8.2 lists examples of factors that push or pull the trachea and other anatomic structures out of their normal position in the chest radiograph.

TABLE 8.2

Examples of Factors That “Push or Pull” Anatomic Structures Out of Their Normal Position in the Chest Radiograph

	Structure	Examples of Abnormal	Lesion
		Position
	Mediastinum and hilar	Leftward shift	Pulled left by left upper lobe tuberculosis, atelectasis, or
	region		fibrosis
	Trachea		Pushed left by right upper lobe emphysematous bullae, fluid,
	Carina		gas, or tumor
	Heart
	Major vessels
	Left diaphragm	Upward shift	Pulled up by left lower lobe atelectasis or fibrosis
			Pushed up by distended gastric air bubble
	Horizontal fissure	Downward shift	Pulled down by right middle lobe or right lower lobe
			atelectasis
			Pushed down by right upper lobe neoplasm
	Left lung	Rightward shift	Pulled right by right lung collapse, atelectasis, or fibrosis
			Pushed right by left-sided tension pneumothorax or
			hemothorax

Heart

On the PA projection, the ratio of the width of the heart to the thorax (the cardiothoracic ratio) is normally less than 1 : 2. In other words, the width of the heart should be less than 50% of the width of the thorax. A small portion of the heart should be visible on the right side of the vertebral column. Two bulges should be seen on the right border of the heart. The upper bulge is the superior vena cava; the lower bulge is the right atrium. Three bulges are normally seen on the left side of the heart. The superior bulge is the aorta, the middle bulge is the main pulmonary artery, and the inferior bulge is the left ventricle (see Fig. 8.7). See Table 8.2 for examples of factors that push or pull the heart out of its normal position in the chest radiograph.

Hilum

The right and left hilar regions should be evaluated for change in size or position (hilar displacement). Normally, the left hilum is about 2 cm higher than the right (see Fig. 8.7). An increased density of the hilar region may indicate engorgement of hilar vessels caused by pulmonary hypertension. Vertical displacement of the hilum suggests volume loss from one or more upper lobes of the lung on the affected side. In infectious lung disorders such as histoplasmosis or tuberculosis, the lymph nodes around the hilar region are often enlarged, calcified, or both. Malignant pulmonary lesions, including hilar malignant lymphadenopathy, also may be seen. See Table 8.2 for additional factors that push or pull structures in the hilar region out of their normal position in the chest radiograph.

Lung Parenchyma (Tissue)

The lung parenchyma should be systematically examined from top to bottom, one lung compared with the other. Normally, lung tissue markings can be seen throughout the film (see Fig. 8.7). The absence of tissue markings may suggest a pneumothorax, recent pneumonectomy, or chronic obstructive lung disease (e.g., emphysema) or may be the result of an overexposed radiograph. An excessive amount of tissue markings may indicate fibrosis, interstitial or alveolar edema, lung compression, or an underexposed radiograph. The periphery of the lung fields should be inspected for abnormalities that obscure the interface of the lung with the pleural space, mediastinum, or diaphragm. Abnormal lung tissue markings are call infiltrates. See Table 8.2 for additional examples of factors that push or pull the lung tissue out of its normal position in the chest radiograph.

Pleura

The peripheral borders of the lungs should be examined for pleural thickening, presence of fluid (pleural effusion) or air (pneumothorax) in the pleural space, or mass lesions (see Fig. 8.7). The costophrenic angles should be inspected. Blunting of the costophrenic angle suggests the presence of fluid. A lateral decubitus radiograph may be required to confirm the presence of fluid (see Fig. 8.6).

Diaphragms

Both the right and left hemidiaphragms should have an upwardly convex, dome-shaped contour. The right and left costophrenic angles should be clear. Normally, the right diaphragm is about 2 cm higher than the left because of the liver below it (see Fig. 8.7). Chronic obstructive pulmonary diseases (e.g., emphysema) and diseases that cause gas or fluid to accumulate in the pleural space (e.g., pneumothorax or pleural effusion) flatten and depress the normal curvature of the diaphragm. Abnormal elevation of one diaphragm may result from excessive gas in the stomach, collapse of the middle or lower lobe on the affected side, pulmonary infection at the lung bases, phrenic nerve damage, or spinal curvature. See Table 8.2 for additional examples of factors that push or pull the diaphragm out of its normal position in the chest radiograph.

Gastric Air Bubble

The area below the diaphragm should be inspected. A stomach air bubble is commonly seen under the left hemidiaphragm (see Fig. 8.7). Free air may appear under either diaphragm after abdominal surgery or in patients with peritoneal abscess.

Bony Thorax

The ribs, vertebrae, clavicles, sternum, and scapulae should be inspected. The intercostal spaces should be symmetric and equal over each lung field (see Fig. 8.7). Intercostal spaces that are too close together suggest a loss of muscle tone, commonly seen in patients with paralysis involving one side of the chest. In chronic obstructive pulmonary disease, the intercostal spaces are generally far apart because of alveolar hyperinflation. Finally, the ribs should be inspected for deformities or fractures. If a rib fracture is suspected but not seen on the standard chest radiograph, a special rib series (radiographs that focus on the ribs) may be necessary.

Extrathoracic Soft Tissues

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The soft tissue external to the bony thorax should be closely inspected. If the patient is a female (or an obese male), the outer boundaries of the breast shadows may be seen (see Fig. 8.7). If the patient has undergone a mastectomy, there will be a relative hyperlucency on the side of the mastectomy. Large breasts can create a significant amount of haziness over the lower lung fields, giving the false appearance of pneumonia or pulmonary congestion. Although nipple shadows are easily identified when they are bilaterally symmetric, one may become less visible when the patient is slightly rotated. The other nipple then appears abnormally opaque and may be mistaken for a pulmonary nodule. Fatty tissue in the chest wall in obese patients also may be seen. After a tracheostomy or pneumothorax, subcutaneous air bubbles (called subcutaneous emphysema) often form in the soft tissue, especially if the patient is on a positive-pressure ventilator.

Computed Tomography

The same basic principles used in film radiography apply to computed tomography (CT) scanning—that is the absorption of x-rays by tissues that contain anatomic structures and organs of different atomic number. A CT scan provides a series of cross-sectional (transverse) pictures (called tomograms) of the structures within the body at numerous levels. The procedure is painless and noninvasive and requires no special preparation. The patient simply lies on the examination table, and this moves the patient through the opening of the CT scanner. The major components of a CT scanner are (1) an x-ray tube, which rotates in a continuous 360-degree motion around the patient to image the body in cross-sectional slices;

(2) an array of x-ray detectors opposite the x-ray tube, which record the x-rays that pass through the body; and (3) a computer, which converts the different x-ray absorption levels to cross-sectional images based on the density of the structures being scanned (Fig. 8.9). This cross-sectional slice is called an axial view or computerized axial tomogram.

FIGURE 8.9 The principle of spiral computed tomography. The patient moves into the scanner with the x-ray tube continuously rotating and the detectors acquiring information. The rapidity of data acquisition allows a complete examination of the thorax to be performed in a single breath hold. (From Spiro, S. G., Silvestri, F. A., & Agusti, A. [2012]. Clinical respiratory medicine [4th ed.]. St. Louis, MO:

Elsevier.)

Up to 250 images, about 1 mm apart, can be generated on a chest CT scan. These “cuts” are often called highresolution CT (HRCT) scans (also called spiral, volume, or helical scans). In essence, each CT scan provides an image of what a “slice” through the body looks like at specific points—similar to cutting a piece of fruit in half and viewing the crosssection of the structures inside the fruit. Dense structures, such as bone, appear white on the tomogram, whereas structures with a relatively low density, such as the lungs, appear dark or black. Therefore a dense tumor in the lungs would appear as a white object surrounded by dark lung parenchyma.

The resolution of a CT scan can be adjusted to primarily view (1) lung tissue—commonly called a lung window CT scan —or (2) bone and mediastinal structures—commonly called a mediastinal window CT scan. In a mediastinal window CT scan, the lung tissue is overexposed and appears mostly black; the bones and mediastinal organs appear mostly white. Fig. 8.10 provides an overview of a normal lung window CT scan. Fig. 8.11 shows a close-up of one “slice” of a normal lung window CT scan. Fig. 8.12 provides a close-up view of one slice of a normal mediastinal window CT scan.