I. Introduction
The chest radiograph ("CXR") is one of the most common diagnostic imaging studies. Chest radiographs are obtained for screening (preventive) purposes in many circumstances, and are also the primary study used for the initial detection of many forms of thoracic disease, including pneumonitis, haemo- and pneumothorax, congestive heart failure ("CHF"), intrathoracic neoplasia, for the evaluation of chest trauma, and many other conditions.
In the following, I will sketch the systematic approach to reading a chest film, and discuss some of the more common findings.
II. Initial Considerations
Before beginning to interpret a CXR, one must first determine what one is dealing with. Does the name on the film match the patient's chart? Is the film adequate? Does it cover all the relevant anatomy? Is it overpenetrated (i.e. x-ray so strong that it rendered important structures transparent) or underpenetrated (structures that ought to be transparent or translucent are actually opaque)? Is the patient properly centred? How is the level of inspiration (inhalation)? What about the projection: is the film lateral, anteroposterior (front-to-back), or posteroanterior (back-to-front)?
A. Exposure
If the film is over- or underexposed, it will be all but useless. If the film is properly exposed, you should be able to see the lungs, bony structures in the chest, some of the upper abdominal organs, as well as the vascular structures of the lungs, and the disc spaces of the mid-thoracic spine. If these structures cannot be clearly seen on the film, it will need to be redone.
B. Patient Positioning
If the projection is PA or AP, the film is only really useful if the patient is properly centred. Rotation of the patient can distort important anatomical features such as the heart, the diaphragm (or, to use the common radiologist's misnomer: "the hemidiaphragms"), and the mediastinum. If there is little or no rotation of the patient, the right and left sides of the film will be roughly symmetrical. In order to determine whether the patient is rotated, find the clavicles and the processi spinosi of the cervical spine. An imaginary line drawn through the spinous processes should be perpendicular to an imaginary line drawn between the two clavicular heads, with the point of intersection between the two lines equidistant from both clavicular heads. Another method of assessing for patient rotation is to determine whether the claviculae themselves are symmetrical. In an unrotated patient, the claviculae will describe a symmetrical, if incomplete, V shape.
Equal in importance to patient rotation is whether the patient is in an upright or supine position in the film. Usually, this is marked on the film; however, if there is doubt as to the patient's position, it is generally possible to figure out based on the film itself whether the patient is upright or supine. This is because air and fluid redistribute depending on the patient's position. Fluid will always gravitate toward the most dependent (lowest) part of its container, whereas air will move toward the highest point. Because of this general principle, air will always be "on top" of fluids as shown in an upright film. Fluid is more "dense" on conventional radiography than air, and thus shows up lighter. When air is directly on top of fluid, as in an upright chest film, there will be a slightly lighter area directly below an entirely black area. This is known as an "air-fluid level." Air-fluid levels will generally only be visible in the upright patient.
Because of this tendency of air and fluid to redistribute themselves depending on patient positioning, many structures can only be adequately visualised on an upright film. Pleural effusions (fluid in the chest cavity) and pneumothoraces, in particular, may be easily visualised on an upright film but virtually imperceptible on a supine film. It is also more difficult to get an accurate estimate of the size of the heart on a supine film, as the cardiac silhouette often appears enlarged on a supine view. Similarly, the pulmonary vasculature may appear enlarged on a supine film, simulating CHF or pulmonary hypertension, a highly dangerous misdiagnosis to make.
C. Projection
Chest radiographs are usually taken either posteroanteriorly (PA) and laterally or anteroposteriorly (AP). Ideally, one PA and one lateral view will be obtained of a patient in a single study; however, when dealing with patients who are too ill to stand or sit upright, only a portable AP film will be possible.
Despite the sometimes bothersome reversal of left and right on PA films, they are generally preferable to AP radiographs. For one thing, since the patient is facing the x-ray film when taking a PA film, magnification of the heart is reduced, allowing for a more reliable estimate of cardiac size.
Lateral films are often underrated. Many clinicians erroneously believe that everything they need to know shows up on the PA/AP projection; the annoying, somewhat confusing layout of the relevant anatomy on a lateral film — on which the lungs are superimposed upon each other — probably also contributes to the widely-held bias against this projection. However, the lateral projection is essential to proper diagnosis for several reasons. First, it is important to remember that x-rays are two-dimensional representations of three-dimensional objects; thus, having views in two perpendicular projections allows the clinician to locate relevant lesions in three dimensions. Second, a substantial minority of pulmonary pathology — 10% — is either better seen on lateral projections or can only be seen on lateral films. Moreover, lateral views can provide confirmation of lesions merely suspected on a frontal (AP/PA) view.
The aforementioned ten per cent includes pathology located in so-called blind spots, where over- or underlying bony or soft-tissue structures obscure lung tissue. One particularly notorious blind spot is the left lower lobe, which is substantially masked by the left ventricle. The sternum is also difficult to evaluate on PA/AP projections only, as it overlies the mediastinum and the vertebrae. In fact, for the evaluation of the sternum, lateral plain films are second only to CT of the chest.
D. Inspiration
"Inspiration" refers to the degree of the patient's inhalation. Most of the time, it is preferable for the patient's lungs to be as full of air as possible, as this allows for the best estimation of cardiac size and the clearest visualisation of the pulmonary vascularity, lung fields, and other structures. An expiratory chest film — taken when the patient is in a state of exhalation — will show magnification of the heart and pulmonary arteries, mimicking congestive heart failure. It will also reduce the visible area of the lung fields, making it harder to evaluate for abnormalities.
There are, however, exceptions to the general rule that inspiratory films are preferred. For example, a pneumothorax (free air in the chest cavity) can be more easily seen at the end of expiration because the volume of the free air is expanded (making it easier to spot) and because the lung becomes more opaque with less air in it, allowing for better contrast.
II. Evaluation
One the film has been determined to be technically adequate, the process of evaluating the film for specific findings begins. This begins with an inventory of any tubes or catheters or other devices in or on the patient. The tube-line-catheter inventory is followed by an examination of the cardiovascular structures shown on the film: principally the heart, aorta, and pulmonary vascularity. Then, the lungs, chest wall, and bony structures (ribs, sternum, etc.) are examined.
A. Tube Inventory
The presence of extraneous plastic and metal is as good a means as any of determining quickly whether a chest film depicts a relatively healthy patient or one who is in less than perfect health. Moreover, since anything inserted into a patient — central lines, endotracheal tubes, pacemakers — is associated with its own list of potential complications, the clinician should be aware of the presence of hardware in order to properly attribute any abnormalities that are likely related to it. Another reason to check for lines and tubes before proceeding to evaluate the rest of the film is that hardware has occasionally been mistaken for pathology.
B. Cardiovascular Structures
1. The Heart
The first thing is to evaluate for cardiomegaly (excessive heart size). If the heart, at its widest, is greater than 50% of the transverse diameter of the chest1, the patient may be in congestive heart failure (inability of the heart to pump enough blood through the body, resulting in excessive blood in the heart and pulmonary vasculature). Another useful rule of thumb is to examine whether the cardiac silhouette touches either of the lateral chest walls — contact between the cardiac silhouette and the chest wall on either side indicates probable cardiomegaly.
However, a number of conditions and other factors can mimic cardiomegaly, creating the impression of an overlarge heart (NB: including chest wall contact!) in the absence of real cardiomegaly.
Portable AP film: The heart is farther away from the film on an AP projection, which inherently leads to magnification on the film; in addition, patients who get AP films rather than the standard PA and lateral combination are often supine and less able to take a deep breath.
Obesity, Pregnancy, Ascites Obese and pregnant patients, as well as patients with ascites, may not be able to provide a deep enough inspiratory effort, leading to magnification of the heart.
Straight Back Syndrome: Loss of the normal curvature (kyphosis) of the thoracic spine may compress the heart, making the cardiac silhouette appear wider.
Pectus Excavatum: If the inferior sternum is angled inwards ("pectus excavatum"), this may put pressure on the heart similarly to Straight Back Syndrome.
Poor Inspiratory Effort: Failure to inhale sufficiently will result in magnification of the cardiac silhouette on radiography. To determine whether inspiratory effort is adequate, look for 8 - 9 posterior ribs. Any less than that, and the inspiration is likely inadequate.
After examining the size of the heart, the next step is to have a look at its contours. There are five "bumps" on the heart of importance in assessing cardiovascular health on a frontal chest film. The first to look for is the bump produced by the aorta ascendens, which is low density (grey fading into black) and almost straight. It can be seen to the right of the trachea, and actually represents the vena cava superior and the vena brachiocephalica, but is used as an indicator of the size of the ascending aorta. In normal patients, the ascending aorta should not project beyond the right heart border. A small ascending aortic shadow can be indicative of an atrial septal defect ("ASD"); if enlarged, the differential diagnosis includes aortic stenosis, aortic regurgitation, and hypertensive cardiovascular disease.
The "double density of left atrial enlargement" is another important sign to look for when evaluating for cardiac abnormalities. Normally, the left atrium does not constitute a border of the heart on a frontal film; however, when it enlarges toward the right, it overlaps with the right atrium, eliminating the normal indentation produced by the meeting of the ascending aorta and the right atrium. When the left atrium is enlarged, this will be replaced by increased density (i.e. lightness) of the normal right atrium due to the overlap. This also produces straightening of the left heart border.
On the left side, the uppermost and most easily noticeable contour is the "aortic knob," which represents the aortic arch and a portion of the descending aorta. Its normal size is 35.0 mm. Enlargement of the aortic knob may be secondary to increased pressure, flow, or changes in the elasticity of the wall such as cystic medial necrosis, arteriosclerotic cardiovascular disease, or aortic dissection.
Next, look for the main pulmonary artery segment. This is the first contour below the aortic knob, and is adjacent to the tangle of vessels that is the left pulmonary artery. To measure the MPA, draw a tangent line from the apex of the left ventricle to the aortic knob, and use a line perpendicular to that line to determine the distance from the MPA to the tangent line. In normal patients, this will be between 0.0 and 15.0 mm. If the MPA projects beyond the tangent line, this may be indicative of increased pressure or flow in the pulmonary vasculature. If it projects more than 15.0 mm away from the tangent line, a congenital abnormality2 (e.g. Tetralogy of Fallot) may be present. More likely, however, is hypoxic coronary vasodilatation ("HCVD" - dilatation of the coronary vasculature due to inadequate O2) or atherosclerotic disease, both of which increase the distance between the tangent line and the MPA by enlarging the left ventricle and aortic knob.
Below the MPA, in the space between the MPA and the left ventricle, there is an indentation in which the left atrium will appear when enlarged. If the left atrium enlarges leftward, this indentation will be straightened out. This may be seen in diseases of the mitral valve or congenital heart abnormalities such as ventricular septal defect ("VSD"), patent ductus arteriosus ("PDA"), chronic CHF, dysfunction of papillary muscles, and, in some rare cases, left atrial myxoma.
The left ventricle, like the right atrium, is not a particularly good source of information. In order to determine what ventricle is enlarged in a patient with cardiomegaly, focus on the respective outflow tracts. If cardiomegaly is seen coexisting with enlargement of the MPA (beyond the tangent line), there is some enlargement of the right ventricle. A prominent aorta, on the other hand, indicates left ventricular involvement.
2. The Pulmonary Vascularity
The pulmonary arteries and veins are another important source of information on a patient's cardiovascular health. In a normal erect patient, the pulmonary vessels should be largest at the lung bases, as gravity causes blood to pool at the lowest point. Thus, the relative diameter of the apical and basilar pulmonary vessels must be examined. If the apical vessels are equal or greater in diameter than the basilar vessels ("cephalisation" of flow), this may indicate the presence of pulmonary venous hypertension. Second, look at the gradual narrowing of the vessels themselves as they move from central to peripheral. A sudden drop-off in diameter may be a sign of pulmonary arterial hypertension.
C. The Lungs
Normal, well-aerated lungs should generally be practically transparent on CXR. Increased opacity (whiteness) is characteristic of most pulmonary abnormalities. First, examine areas in which the lungs border on other areas (heart, diaphragm, chest wall). Normally, these should show clear boundaries, contrasting between the low-to-zero-density lungs and the mid-to-high density bony and soft-tissue structures. Increased density in these areas will obliterate the normal contrast, and indicates the presence of fluid or infiltrate3 in or near the lung. In addition to the loss of normally present contrast, look for the presence of contrast where there normally is none. This is often the result of fluid or other material present adjacent to air-filled lung (e.g. air bronchograms, air alveolograms). If an area of the lung is of increased (but not fluid) density, this can be indicative of atelectasis (inadequate aeration). More regular, solid-appearing shadows may be nodules (< 3.0 cm) or masses (> 3.0 cm).
1This is known as the cardiothoracic ratio, and is measured from the inside rib margin at the widest point above the costophrenic angles (angles formed by the diaphragm and ribs) on the PA projection.
2NB: this aetiology is exceedingly rare in adult patients.
3If opacities are found, it is important to rule out an extrapulmonary cause (e.g. skin folds, breast tissue, nipples. Often, the lateral film will help in ruling these causes out.