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 -inch square is fixed to the end of the rotating anode at the center of the tube. This tungsten block is called the target. Tungsten is an effective target metal because of its high melting point, which can withstand the extreme 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 may 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 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 (PA) projection 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 (AP) 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 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, 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 (Figure 7-1). 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 may be made to evaluate diaphragmatic excursion and the symmetry or asymmetry of such excursion (Figure 7-2).

Anteroposterior Radiograph

The 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 machine at the patient’s bedside. The film is placed behind the patient’s back, with the x-ray machine positioned in front of the patient approximately 48 inches from the film.

Compared with the PA radiograph, the AP radiograph has a number of 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 (Figure 7-3).

The AP radiograph also often 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 often taken in the intensive care unit, extraneous shadows, such as those produced by ventilator tubing and indwelling lines, are often present (Figure 7-4).

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 diaphragmatic dome. It also can be combined with the PA radiograph to give the respiratory care provider a three-dimensional view of the structures or of any abnormal densities (Figure 7-5).

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 that 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 (pleural effusion) 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 (Figure 7-6).

Inspecting the Chest Radiograph

Before the respiratory care practitioner can effectively identify abnormalities on a chest radiograph, he or she must be able to recognize the normal anatomic structures. Figure 7-7 represents a normal PA chest radiograph with identification of important anatomic landmarks. Figure 7-8 labels the anatomic structures seen on a lateral chest radiograph.

Table 7-1 lists some of the more important radiologic terms used to describe abnormal lung findings.

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. 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 Figure 7-7). See Table 7-2 for examples of factors that push or pull the heart out of its normal position in the chest radiograph.

Hilar Region

The right and left hilar regions should be evaluated for change in size or position. Normally the left hilum is about 2 cm higher than the right (see Figure 7-7). An increased density of the hilar region may indicate engorgement of hilar vessels caused by increased pulmonary vascular resistance. 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 7-2 for additional factors that push or pull the hilar region out of its normal position in the chest radiograph.

Lung Parenchyma (Tissue)

The lungs parenchyma should be examined systematically from top to bottom, one lung compared with the other. Normally, tissue markings can be seen throughout the lungs (see Figure 7-7). 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 lung’s interface with the pleural space, mediastinum, or diaphragm. See Table 7-2 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 Figure 7-7). 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 Figure 7-6).

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 Figure 7-7). Chronic obstructive pulmonary diseases (e.g., emphysema) and diseases that cause gas or fluid to accumulate in the pleural space 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 7-2 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 Figure 7-7). 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 Figure 7-7). 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

The soft tissue external to the bony thorax should be closely inspected. If the patient is a female, the outer boundaries of the breast shadows should be identified (see Figure 7-7). 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. 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—namely 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. This cross-sectional slice is called an axial view, or computerized axial tomogram (Figure 7-9).

Up to 250 images, approximately 1 mm apart, can be generated on a chest CT scan. These “cuts” are often called high resolution 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 cross-section 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 lungs.

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. Figure 7-10 provides an overview of a normal lung window CT scan. Figure 7-11 shows a close-up of one “slice” of a normal lung window CT scan. Figure 7-12 provides a close-up view of one slice of a normal mediastinal window CT scan.

Finally, for poorly defined lesions evident on the standard radiograph, the CT scan is a useful supplement in determining the precise location, size, and shape of the lesion. The CT scan is especially helpful in confirming the presence of a mediastinal mass, small pulmonary nodules, small lesions of the bronchi, pulmonary cavities, a small pneumothorax, pleural effusion, and small tumors (as small as 0.3 to 0.5 cm). The CT scan can be done with contrast material in the vessels to delineate vascular structures.

Positron Emission Tomography

The positron emission tomography (PET) scan shows both the anatomic structures and the metabolic activity of the tissues and organs scanned. Used in conjunction with a chest x-ray and CT scan for comparison, the PET scan is an excellent diagnostic tool for early detection of cancerous lesions. The unique aspect of the PET scan is its ability to evaluate the metabolic rate of certain tissue cells that may be cancerous. In other words, the PET scan is able to detect cancerous cells in the tissues of the body before changes develop in the anatomic shape of the organ.

Before undergoing the scan, the patient is injected intravenously (IV) with a solution of glucose that has been tagged with a radioactive chemical isotope (generally fluorine-18 fluorodeoxyglucose, or F18-FDG compound). Cancer cells metabolize glucose at extremely high rates. The PET scan measures the way cells burn glucose. When present, the cancer cells rapidly consume the tagged glucose. As the glucose molecules break down, end products that emit positrons are produced. The positrons collide with electrons that give off gamma rays. The gamma rays are converted to dark spots on the PET scan image. These dark spots are commonly referred to as “hot spots.” The presence of a hot spot on a PET scan likely confirms a rapidly growing tumor.

Clinically, a PET scan is an excellent tool to rule out suspicious findings (i.e., a possible cancerous area) that are identified on either the chest radiograph or CT scan. For example, Figure 7-13 shows a chest radiograph that identifies two suspicious findings—one small nodule in the right upper lung lobe and a larger density in the left lower lung lobe, just behind the heart. Figure 7-14 shows two CT scans that also identify the two suspicious findings and their precise location. Figures 7-15, 7-16 and 7-17 show PET scans that all confirm a hot spot (likely cancer) in the lower left lobe. However, the PET scan shown in Figure 7-18 confirms that the nodule in the right upper lobe is benign (i.e., no hot spot noted).

Although the PET scan is relatively painless (i.e., tantamount to intravenous insertion), it is lengthy. It may take up to 90 minutes to complete the scan. After the injection, the patient quietly rests in a reclining chair for 30 to 60 minutes before the scan is performed. This allows time for the body to absorb the compound. This step may be difficult or impossible for patients who are unable to remain motionless for long periods of time. PET scans are very expensive to perform, compared with CT or magnetic resonance imaging (MRI) studies.

Positron Emission Tomography and Computed Tomography Scan

As described in the preceding sections, PET and CT are both standard imaging tools used by the radiologist to pinpoint the location of cancer or infection within the body before developing a treatment strategy. Individually, however, each scan has its own benefits and limitations. For example, the PET scan detects the metabolic activity of growing cancer cells in the body, and the CT scan provides a detailed picture of the internal anatomy that shows the precise location, size, and shape of a tumor or mass. On the other hand, because the PET scan and CT scan are done at different times and locations, variations in the patient’s body position often make the interpretation of the two images difficult.

Technology has now been developed that allows both the PET scan and the CT scan to be merged together and performed at the same time. The image produced is called a positron emission tomography and computed tomography scan (PET/CT scan) scan (also known as a PET/CT fusion). The PET/CT scan provides an image far superior to that afforded by either technology independently. When combined, the CT scan provides the anatomic detail regarding the precise size, shape, and location of the tumor, and the PET scan provides the metabolic activity of the tumor or mass. The PET/CT image provides excellent image quality and high sensitivity and specificity in detecting malignant lesions in the chest. Figure 7-19 shows a PET/CT scan alongside a CT scan and a PET scan; all the images show the same malignant nodule in the right upper lung lobe.

The benefits of a combined PET/CT scan include earlier diagnosis, accurate staging and localization, and precise treatment and monitoring. With the high quality and accuracy of the PET/CT image, the patient has a better chance for a favorable outcome, without the need for unnecessary procedures. In addition, the PET/CT scan provides early detection of the recurrence or metastasis of cancer, revealing tumors that might otherwise be obscured by scars from surgery and radiation therapy. This is because the combined PET/CT scan provides the radiologist with a more complete overview of what is occurring in the patient’s body, both anatomically and metabolically at the same time.

Magnetic Resonance Imaging

MRI uses magnetic resonance as its source of energy to take cross-sectional (transverse, sagittal, or coronal) images of the body. It uses no ionizing radiation. The patient is placed in the cylindric imager, and the body part in question is exposed to a magnetic field and radiowave transmission. The MRI produces a high-contrast image that can detect subtle lesions (Figure 7-20).

MRI is superior to CT scanning in identifying complex congenital heart disorders, bone marrow diseases, adenopathy, and lesions of the chest wall. MRI is an excellent supplement to CT scanning for study of the mediastinum and hilar region. For most abnormalities of the chest, however, CT scanning is generally better than MRI for motion (patient motion causes loss of resolution), spatial resolution, and cost reasons.

Because the magnetic resonance imager generates an intense magnetic field, objects made of ferromagnetic material are strongly attracted to it. Therefore patients with ferromagnetic cerebral aneurysm clips or ferromagnetic prosthetic cardiac valves should not undergo MRI because the magnetic force of the imager can cause these devices to heat, shift and harm the patient. The magnetic force of the imager also can interfere with the normal function of cardiac pacemakers and most ventilators.

Pulmonary Angiography

Pulmonary angiography is useful in identifying pulmonary emboli or arteriovenous malformations. It involves the injection of a radiopaque contrast medium through a catheter that has been passed through the right side of the heart and into the pulmonary artery. The injection of the contrast material into the pulmonary circulation is followed by rapid serial pulmonary angiograms. The pulmonary vessels are filled with radiopaque contrast material and therefore appear white. Figure 7-21 shows an abnormal angiogram in which the major blood vessels appear absent distal to pulmonary emboli in the left lung.

Ventilation-Perfusion Scan

A ventilation-perfusion scan is useful in determining the presence of a pulmonary embolism. The perfusion scan is obtained by injecting small particles of albumin, called macroaggregates, tagged with a radioactive material such as iodine-131 or technetium-99m. After injection the radioactive particles are carried in the blood to the right side of the heart, from which they are distributed throughout the lungs by the blood flow in the pulmonary arteries. The radioactive particles that travel through unobstructed arteries become trapped in the pulmonary capillaries because they are 20 to 50 µm in diameter and the diameter of the average pulmonary capillary is approximately 8 to 10 µm.

The lungs are then scanned with a gamma camera that produces a picture of the radioactive distribution throughout the pulmonary circulation. The dark areas show good blood flow, and the white or light areas represent decreased or complete absence of blood flow. The macroaggregates eventually break down, pass through the pulmonary circulation, and are excreted by the liver. The injection of these radioactive particles has no significant effect on the patient’s hemodynamics because the patent pulmonary capillaries far outnumber those “embolized” by the radioactive particles. In addition to pulmonary emboli, a perfusion scan defect (white or light areas) may be caused by a lung abscess, lung compression, loss of the pulmonary vascular system (e.g., emphysema), atelectasis, or alveolar consolidation.

The perfusion scan is supplemented with a ventilation scan. During the ventilation scan the patient breathes a radioactive gas such as xenon-133 from a closed-circuit spirometer. A gamma camera is used to create a picture of the gas distribution throughout the lungs. A normal ventilation scan shows a uniform distribution of the gas, with the dark areas reflecting the presence of the radioactive gas and therefore good ventilation. White or light areas represent decreased or complete absence of ventilation. See Figure 7-22 for an abnormal perfusion scan and a normal ventilation scan of a patient with a severe pulmonary embolism. An abnormal ventilation scan also may be caused by airway obstruction (e.g., mucous plug or bronchospasm), loss of alveolar elasticity (e.g., emphysema), alveolar consolidation, or pulmonary edema.

Fluoroscopy

Fluoroscopy is a technique by which x-ray motion pictures of the chest are taken. Fluoroscopy subjects the patient to a larger dose of x-rays than does standard radiography. Therefore it is used only in selected cases, as in the assessment of abnormal diaphragmatic movement (e.g., unilateral paralysis) or for localization of lesions to be biopsied during fiberoptic bronchoscopy.

Bronchography

Bronchography entails the instillation of a radiopaque material into the lumen of the tracheobronchial tree. A chest radiograph is then taken, providing a film called a bronchogram. The contrast material provides a clear outline of the trachea, carina, right and left main stem bronchi, and segmental bronchi. Bronchography is occasionally used to diagnose bronchogenic carcinoma and determine the presence or extent of bronchiectasis (Figure 7-23). CT of the chest has largely replaced this technique.