Exposure

Making a properly exposed chest x-ray is much more difficult than making x-rays of other parts of the body because the chest contains tissues with a great range of contrast. The range stretches from small vessels in air-filled lungs to dense bony structures located behind the heart. A correctly exposed image should allow visualization of vessels to at least the peripheral one-third of the lung and, at the same time, allow visualization of the paraspinous margins and the left hemidiaphragm behind the heart.

Overexposure causes the image to be dark. Under these circumstances, the thoracic spine, mediastinal structures, retrocardiac areas, and nasogastric and endotracheal tubes are well seen, but small nodules and the fine structures in the lung cannot be seen ( Fig. 3.1A ). If the image was obtained by using either digital or computed radiography, the image can be “windowed” lower on the computer, resulting in an interpretable image.

Effect of Overexposure and Underexposure of a Chest X-Ray.

(A) Overexposure makes it easy to see behind the heart and the regions of the clavicles and thoracic spine, but the pulmonary vessels peripherally are impossible to see. (B) Underexposure accentuates the pulmonary vascularity, but you cannot see behind the heart or behind the hemidiaphragms.

Underexposure causes the image to be quite white. This is a major problem for adequate interpretation. It will make the small pulmonary blood vessels appear prominent and may lead to thinking that there are generalized infiltrates when none are really present. Underexposure also makes it impossible to see the detail of the mediastinal, retrocardiac, or spinal anatomy (see Fig. 3.1B ). Even with digital or computed radiography, nothing can be done to an underexposed image to improve the image.

Male Versus Female Chest

The major difference between male and female chest x-rays is caused by differences in the amount of breast tissue. This is generally relevant only in interpretation of a posteroanterior (PA) or anteroposterior (AP) projection and not of the lateral projection. Breast tissue absorbs some of the x-ray beam, essentially causing underexposure of the tissues in the path. This results in the lung behind the breast appearing whiter and the pulmonary vascular pattern in the same area appearing more prominent. If the breasts are large, bilateral basilar lung infiltrates may appear to be present on the PA or AP projection.

One common problem is encountered in the woman who has had a unilateral mastectomy. In this circumstance, the lung density will be asymmetric. The lung on the side of the mastectomy will appear darker than the lung on the normal side. In this circumstance, recognition of the mastectomy will prevent you from making an erroneous diagnosis of an infiltrate or effusion based on the relatively increased density on the side with the remaining breast ( Fig. 3.2 ).

Left Mastectomy.

The right breast, which remains, causes the pulmonary vessels at the base of the right lung to be accentuated, and this can be mistaken for a right lower lobe infiltrate. In contrast, the left lung appears darker than the right, and you might mistakenly think there is hyperinflation of the left lung.

Visualization on a PA or AP chest x-ray of a single well-defined nodule in the lower lung zone should raise the suspicion that you are seeing a nipple shadow and not a real pulmonary nodule. Nipple shadows are common in both men and women. First, look at the opposite lung to see if a comparable nodule appears there. If one does, usually you can stop worrying ( Fig. 3.3 ), but before you completely stop worrying, also look at the lateral image and make sure that the nodule is not seen projecting within the lung. If only one nodule is found projecting over a lung in the PA projection and no nodule is seen on the lateral view, a small metallic BB can be taped over the nipple and the single PA view repeated to see whether it was the nipple that was being visualized.

Nipple Shadows.

Prominent nipple shadows can be seen in men and women and are seen in the midclavicular line over the lower half of both the right and left lung (arrows) . These should be bilateral and sometimes can be seen in the anterior soft tissue of the chest on the lateral view.

Posteroanterior Versus Anteroposterior Chest X-Rays

Chest x-rays of ambulatory patients are usually done with the subject’s chest up against the detector plate. The x-ray tube is behind the patient, and the x-ray beam passes in from the back and exits the front of the chest. This is referred to as a PA (posterior to anterior) projection . If the patient is lying down, it is standard practice to take the image with the x-ray beam entering the front of the chest and have the detector plate behind the patient. This is called an AP (or anterior to posterior) chest x-ray .

For interpretive purposes, the main difference is that the heart will be more magnified on the AP projection ( Fig. 3.4 ). This is because in the AP projection, the heart is farther from the detector plate and the x-ray beam diverges as it goes farther from the tube. Thus, the shadow of the heart appears larger on an AP chest x-ray than on a PA view. Simply remember to make sure that you are looking at a PA view before you interpret an image as showing mild or moderate cardiomegaly. Usually the technician will have written the projection on the x-ray requisition, and occasionally it may be marked on the image.

Effect of Position on the Chest X-Ray.

(A) A posteroanterior (PA) upright view allows fuller inspiration than does a supine view. The small round objects over the left lower chest are snaps. (B) In an anteroposterior supine view, the abdominal contents are pushing the hemidiaphragms up, and the chest appears hypoinflated. This projection also magnifies the heart relative to a PA view.

AP , Anteroposterior.

Upright Versus Supine Chest X-Rays

As you can imagine, patients who are able to stand or sit up usually have their chest x-rays done in that position for a number of reasons. The amount of inspiration is greater in each of these positions, spreading the pulmonary vessels and allowing clearer visualization. It is obviously easier to see a bird in a tree if the branches can be spread out instead of being squashed together. Another reason for preferring an upright examination is that small pleural effusions tend to run down into the normally sharp costophrenic angles, allowing relatively small effusions to be identified. Small pneumothoraces tend to go to the lung apex and can be relatively easy to see on an upright chest x-ray.

Now let us think about a patient lying in bed (supine). The typical chest x-ray will be done with a detector cassette under the patient. No lateral view is done. In this circumstance, the patient cannot take a full inspiration; the liver and abdominal contents push up on the lungs and heart, and the result is that the pulmonary vessels are crowded. In the supine position, the blood flow to the upper lungs essentially equals that in the lower lobes, and this will mimic congestive failure. On a supine image, the standard AP projection combined with the cephalic push of the abdominal contents will make a normal heart appear large. In addition, with the patient in a supine position, small pleural effusions will layer in the posterior pleural space, whereas small pneumothoraces will go to the anterior pleural surface, and both will easily be missed. As a result, you must be much more conservative and careful when interpreting the image of a portable supine examination.

Inspiration and Expiration Chest X-Rays

The degree of inspiration is important not only for assessing the quality and limitations of the examination but also for diagnosing different diseases. When standing, most adults can easily take an inspiration that brings the domes of the hemidiaphragms down to the level of the 10th posterior ribs. When the patient is sitting or lying down, often the level is between the eighth and tenth ribs. If the image has the domes of the diaphragms at the seventh posterior ribs, the chest should be considered hypoinflated, and you must be very careful before diagnosing basilar pneumonia or cardiomegaly ( Fig. 3.5 ). You should be cognizant of the major differences in the appearance of a chest x-ray as a result of combining all the factors mentioned earlier. Unless you are aware of these issues, you will diagnose cardiomegaly, lung infiltrates, and congestive heart failure (CHF) in a patient who is in fact normal ( Fig. 3.6 ).

Effect of Respiration.

(A) Good inspiration on a chest x-ray makes the hemidiaphragms come down to about the level of the posterior 10th or 11th ribs. Breast shadows are clearly seen on both sides, and this overlying soft tissue accentuates the pulmonary markings behind them. (B) On an expiration view, the hemidiaphragms are higher, making the heart appear larger and crowding the basilar pulmonary vessels. The breast shadows overlap the hemidiaphragms, and these findings together may make you think that bilateral basilar lung infiltrates are present, when this is actually a normal chest.

Summation of the Effect of Position, Projection, and Respiration.

(A) A normal posteroanterior upright chest x-ray with full inspiration. (B) Another x-ray was taken of this perfectly healthy college student 1 minute later; it was done in an anteroposterior projection while he was lying supine and during expiration. The wide cardiac shadow and prominent pulmonary vascularity could easily trick you into thinking that this individual was in congestive heart failure.

AP , Anteroposterior; PA , posteroanterior.

Expiration images do have occasional constructive uses. If a small pneumothorax is present, an expiration view makes the lung smaller and denser and, at the same time, makes the pneumothorax relatively larger and easier to see. Thus, if your prime interest is in identification of a small pneumothorax, order an upright expiration image. In the case of a foreign body (such as a peanut) lodged in a major bronchus, both inspiration and expiration examinations should be ordered. Either postobstructive atelectasis or a ball valve phenomenon may be seen. In the latter case, air can get in past the object during inspiration, but during expiration (as the bronchus gets smaller), air cannot get out around the object. As a result, on the expiration image, air trapping will occur in the affected lung, with a shift of the mediastinum toward the normal side.

Before hyperinflation is diagnosed on a chest x-ray, the lateral image should be examined. With hyperinflation, the diaphragms should be flattened on the lateral view. Many young adults can normally take a very deep inspiration, but on the lateral view they will not have an increased AP diameter or truly flattened hemidiaphragms. In long-standing chronic obstructive pulmonary disease (COPD), additional findings may appear, such as an increased AP diameter and an increase in the clear space between the sternum and the ascending aorta.

A typical chest x-ray is performed by using a level of energy that is a compromise for visualizing lung markings, soft tissues, and bones at the same time. Bones can be well seen by using relatively low-voltage x-rays, but then the pulmonary markings are hard to see ( Fig. 3.7 ).

Chest Versus Rib Technique.

(A) A normal chest x-ray is taken at a relatively high voltage, allowing you to see the heart, pulmonary vessels, and skeletal structures. (B) By lowering the voltage of the x-ray beam, the pulmonary vessels become much harder to see and the bones become easier to see.

Hila and Lungs

The hila are made up of the main pulmonary arteries and major bronchi. The right hilum is usually somewhat lower than the left; it should not be at the same level or higher. The pulmonary veins usually are more difficult to see than the arteries. They converge on the atria at a level 1 to 3 inches below the pulmonary arteries. Lymph nodes are not normally seen on a chest x-ray, either in the hilar regions or in the mediastinum.

The lungs are composed mostly of air, so normally not much is seen other than blood vessels. These should be distinct and remain that way as they are traced back to the hila. If you cannot see blood vessels clearly near the hila, a perihilar infiltrate or fluid may be present (such as from CHF). Normal hila are sometimes indistinct on portable x-rays because the exposure takes longer and the vessels are blurred by motion.

The blood vessels in the lung are usually clearly seen out to within 2 to 3 cm of the chest wall. Some people say that visualization of vessels in the outer third of the lung is abnormal, but this is not true. It depends on the quality of the image and how hard you look. Lines located within 2 cm of the chest wall are abnormal and probably represent edema, fibrosis, or metastatic disease. Secondary bronchi are not normally visualized except near the hilum, where they can sometimes be seen end-on. The walls of the visualized bronchi normally should not be thicker than a fine pencil point.

A normal variant called an azygos lobe can occasionally be seen in the right upper lung. This is seen on the PA view as a fine curved line extending from the right lung apex down toward the mediastinum ( Fig. 3.11 ). It has a teardrop shape at its lower edge. This is caused embryologically by the azygos vein migrating inferiorly from the lung apex while trapping some of the lung medially.

Azygos Pseudolobe.

A thin curvilinear line extends from the right lung apex down and medially toward the azygos vein (arrows) . The line has a teardrop bottom end. This normal variant is seen only on the right side.

Remember that on a PA or AP chest x-ray, the lungs go behind the heart, behind and below the dome of the hemidiaphragms, and behind and in front of mediastinal structures. Forty percent of the lung area and 25% of the lung volume will be obscured by these other structures. If you do not look carefully at these regions, you will miss a significant amount of pulmonary pathology.

Diaphragms

The diaphragms are typically dome-shaped, although many persons have polyarcuate diaphragms that look like several domes rather than one. This is an important normal variant and should not be mistaken for a pleural or diaphragmatic tumor; it should not be called an eventration ( Fig. 3.12 ). The right hemidiaphragm is usually higher than the left, and most people believe that this is because the liver is pushing up the right hemidiaphragm. This is nonsense, because the liver, which weighs many pounds, cannot push up into the lungs while the person is standing. The diaphragms are at different levels because the heart is pushing the left hemidiaphragm down. The edges of both hemidiaphragms form acute angles with the chest wall, and blunting of these angles should raise the suspicion of pleural fluid.

Polyarcuate Diaphragm.

This is a common normal variant in which the diaphragm has several small domes (arrows) instead of one large one.

Most people have trouble telling the right from the left hemidiaphragm on the lateral view, but several ways exist to tell them apart. The right hemidiaphragm is usually higher than the left and can be seen extending from the anterior chest wall to the posterior ribs. The left side usually can be seen only from the posterior aspect of the heart to the posterior ribs. It also is the hemidiaphragm that is most likely to have a gas bubble (stomach or colon) immediately beneath it.

Bony Structures

Skeletal structures of interest on a chest x-ray include the ribs, sternum, spine, and shoulder girdle. Twelve ribs should appear, but only the upper ones are completely seen on a PA chest x-ray. Ribs are very difficult to evaluate on the lateral view owing to superimposition of the right and left ribs and the many soft tissue structures. Evaluation should include searches for cervical ribs ( Fig. 3.13 ), fractures, deformity, missing ribs (from surgery), and lytic (destructive) lesions. The upper margin of the ribs is usually well seen, because the rib is rounded here. The lower edge of the ribs is usually very thin, and the inferior cortical margin can be difficult to appreciate. Look for symmetry between the right and left ribs at the same level. If they are symmetric, they are usually normal. At the anterior ends of the ribs, cartilage connects to the sternum. In older individuals, significant calcification of this cartilage may occur; this is a normal finding ( Fig. 3.14 ).

Cervical Ribs.

A congenital abnormality in which small ribs project off the lateral aspect of C7 (arrows) . Occasionally, these can be symptomatic.

Costochondral Calcification.

Calcification between the anterior ends of the ribs and sternum is quite common, particularly in older persons, and can be quite striking (arrows) . A Dobbhoff feeding tube is noted in this patient as well.

In the case of minor blunt trauma confined to the ribs, a chest x-ray is the initial imaging examination and the addition of dedicated rib views is often not very helpful. Most rib fractures affect the fifth to ninth ribs, likely because the upper ribs are protected by the shoulder girdle and the lower ribs are mobile. Rib fractures after cardiopulmonary resuscitation (CPR) are more commonly anterior and on the left. A PA chest x-ray is specific but not very sensitive for rib fractures, and an AP chest is even less so. Finding a rib fracture is not a big deal but rather should alert you to look for the more significant complications of flail chest, pneumothorax, hemothorax, pulmonary contusion, cardiovascular injury, and potential injury to abdominal organs.

The sternum is well seen only on the lateral view of the chest. On this view, look for pectus deformity, fractures, and lytic lesions. A pectus deformity can cause apparent cardiomegaly, because the sternum is depressed and squashes the heart against the spine, making the heart look wider than normal on the PA chest view ( Fig. 3.15 ). Occasionally, overexposed oblique views of the chest can show the sternum well. If this does not work, you may need to resort to a CT scan with bone windows.

Pectus Excavatum.

A congenital abnormality in which the sternum is depressed. Because the heart is squashed between the sternum and spine, it appears big in the frontal view (A) of the chest, and the right heart border is indistinct, suggesting a right middle lobe infiltrate. A lateral view (B) clearly shows the depressed position of the sternum (arrows) .

The clavicles and shoulders also should be routinely examined. Often the inferior and medial portions of the clavicle have a scalloped appearance. This is called a rhomboid fossa , and it is bilateral. It should not be mistaken for a pathologic bone lesion ( Fig. 3.16 ). The medial aspect of the scapula projects over the upper lateral aspect of the lungs and sometimes can be mistaken for a pathologic line, such as a pneumothorax. When you think that you see a pneumothorax, make sure that it is not the scapular border. Note that the medial scapular border is usually straight rather than curved, and traces the outline of the scapula.

Rhomboid Fossa.

A normal finding in which an indentation appears along the medial and inferior aspects of the clavicles (arrows) . This should be bilateral and is of no clinical significance.

The thoracic spine is seen only incompletely on a standard chest x-ray because, on the frontal view, it is obscured by the heart and mediastinal structures. In older people, substantial degenerative changes or bone spurs may extend laterally from the vertebral bodies. These can often be seen on the PA view ( Fig. 3.17 ) and, on the lateral view, the spurs can look like pulmonary nodules. A key to differentiating bony spurs from nodules is that spurs project over the vertebral disks on the lateral view and do not look like round nodules on the frontal chest x-ray.

Degenerative Spurs or Osteophytes.

These projections occur at the level of the disks and can cause an unusual appearance along the lateral aspect of the thoracic spine (arrows) . On the lateral chest x-ray, these bony spurs can simulate nodules projecting near or over the thoracic spine.

Soft Tissues

The soft tissues also should be examined. We have already seen the problems in interpretation that can arise as a result of a mastectomy or nipple shadows, but other soft tissues also are important. It is important to look for asymmetry of soft tissues and for air or calcium in them. Calcification may be seen in the carotid arteries or great vessels in older persons ( Fig. 3.18 ). A common confusing artifact can be caused by hair (especially braids). If the hair is greasy and braided, strange artifacts that may be mistaken for apical lung infiltrates can be seen ( Fig. 3.19 ).

Carotid Calcification.

In older patients, as a result of atherosclerosis, calcification of the aortic arch and great vessels often is seen. In this case, calcification is seen in the carotid arteries (arrows) .

Braid Artifacts.

Tightly woven or greasy hair can cause streaky artifacts, which may resemble an upper lobe infiltrate. A key finding is that these artifacts can be seen extending above the apex of the lung and projecting over the cervical soft tissue region (arrows) .

Normally, not much soft tissue or water density should exist between the peripheral aerated lung and the ribs. The pleura are not normally seen at the lung margins. In some adults, a collection of fat appears along the chest wall between the lung and the ribs. This is extrapleural fat, which is usually seen only on the PA view of the chest and almost always in the upper outer portion of the thoracic cavity ( Fig. 3.20 ). The biggest pitfall is mistaking this for bilateral pleural effusions. If no other sign of effusion (such as costophrenic angle blunting) is noted, and if the finding is bilateral, is seen near the upper lateral lung zones, and does not exceed 3 to 4 mm in thickness, it is almost certainly extrapleural fat rather than pleural fluid.

Extrapleural Fat.

This normal finding in the upper and lateral hemithorax (arrows) is symmetric between the right and left and should not be mistaken for a pleural effusion.

Computed Tomography Anatomy

The cross-sectional anatomy of the chest is important to understand, because computed tomography (CT) is commonly used for the evaluation of thoracic pathology. CT scanning of the chest may be done with or without intravenous (IV) contrast. Most standard CT scanning techniques provide CT slice images that are 5 mm thick. To evaluate a lung nodule, use thinner cuts, and IV contrast is not needed. For evaluation of a potential dissecting aortic aneurysm, a bolus of IV contrast is essential. Consult a radiologist if you are in doubt about what to order. The radiologist usually will use the correct technique, provided that you have supplied complete clinical information. With current spiral multidetector CT technology, the entire chest can be scanned in several seconds while the patient holds his or her breath. After the scan is done, the data can be manipulated on almost any computer to show bone, mediastinal windows, or pulmonary parenchymal windows. This affords a good look at the pulmonary parenchyma and still allows differentiation of mediastinal structures ( Fig. 3.21 ).

Normal Anatomy of the Chest on Transverse (Axial) Computed Tomography Scans.

Identical anatomy has been imaged using pulmonary parenchymal windows and soft tissue windows (A–L).

ANT , Anterior.

In some special circumstances, you will want to look at the fine details of the lung. In these circumstances, high-resolution computed tomography (HRCT) can be performed. The slices that are obtained are 1 to 2 mm thick. This usually is not done for the whole lung because it would involve too many images, and it is not necessary for making most diagnoses. For this reason, a regular CT scan is often done with thin cuts at selected levels ( Fig. 3.22 ). Repeated chest CT scans for follow-up of pulmonary nodules should be performed using a low-dose protocol.

Normal and High-Resolution Computed Tomography (CT) of the Lungs.

(A) The normal slice thickness of a CT scan of the chest is 0.5 cm. (B) A high-resolution slice taken at exactly the same level is 1.0 mm in thickness and shows much greater detail of the vessels and bronchi.

Routine Chest Radiography

The literature over the past four decades does not show evidence for the benefit of a broad utilization of routine chest x-rays, and generally most abnormalities can be predicted with a good history and physical examination. Preoperative examinations in apparently healthy patients have a very low yield. There is increased value when the patient is older than 70 years, when there is a clinical suspicion of cardiopulmonary disease, unreliable history, or confusing physical examination, or the surgery is high risk.

Portable Chest Radiography in the Intensive Care Unit

Much has been written about the utility or overuse of chest x-rays in intensive care units (ICUs). In general, standing or routine orders for a chest x-ray should be avoided. However, by definition, patients in ICUs are very sick, are usually lying supine all day, and are not ventilating normally. Almost all of these patients have supporting tubes or lines that are changed or repositioned frequently. Daily routine chest x-rays are not indicated. Placement of an endotracheal (ET) tube or nasogastric (NG) tube, Swan-Ganz catheter, central venous catheter, or any other life-support item is an indication for a chest x-ray. Change in clinical condition is also an indication. A chest x-ray after chest tube removal is not recommended unless there is a change in clinical condition. Abnormalities to look for on a postsurgical or posttraumatic chest x-ray are shown in Box 3.3 .