a
Respiratory system
Projections
 Posteroanterior (PA): it the standard projection, means that the x-ray fired from
behind the patient and the film placed in front of the patient, while Anteropsterior
(AP) is the reverse, sometimes very helpful in deciding whether a small questionable
pulmonary opacity on the PA view is genuine, by altering its relationship to the overlying
rib and unscrambling overlapping vascular shadows.
 Lateral CXR: the affected side put near the film, which may be needed to make with
frontal projection 3D display to the lesion (to if it is anterior?, posterior?....).
 Lordotic films: are excellent ways of confirming middle-lobe collapse and are
sometimes useful for demonstrating a questionable apical opacity otherwise partly
obscured by the clavicle.
 Oblique projections: are invaluable for demonstrating hyaline pleural plaques and
other pleural and chest wall lesions by providing a tangential view of them. Lower-lobe
collapse is also well shown on oblique projections.
 Lateral decubitus: views may be required for the demonstration of pleural fluid. The
differentiation between a subpulmonary effusion and a high diaphragm is readily made
by this means. Lateral decubitus views with the affected side dependent provide a
sensitive means of detecting small quantities of pleural fluid (50–100 ml).
 Films exposed in expiration are invaluable in the investigation of air trapping. They are
mandatory in any patient suspected of having inhaled a foreign body. An expiratory film
is also often required for the demonstration of a small pneumothorax.
The interpretation of a chest radiograph involves five basic steps that are concerned with the
identification of the radiograph, technical considerations, the detection and description of
abnormalities, the generic diagnosis, and the specific diagnosis. These steps form a logical
sequence and must be taken in order.
A. Identification of the Radiograph: Name, date, and age must be read from the
radiograph label and checked.
B. Technical Considerations: The interpretation of many radiographic findings depends
critically on the conditions under which the radiograph was taken. Of particular importance
are:
1. Side Marker: The position of the side marker allows the radiograph to be orientated
correctly for viewing.
2. Projection (PA vs. AP): The anteriorly placed heart is more magnified on an AP than on a
PA radiograph. This, coupled with the reduced and variable tube–film distance used in AP
projections, makes the assessment of cardiac size from AP radiographs unreliable. The
judgment as to whether a radiograph was taken AP or PA is made on the following evidence, in
decreasing order of reliability: a. the position of the label, the convention varying from
department to department; b. a specific record as to projection made on the radiograph by the
radiographer; c. the relationship of the scapulae to the lung margins. The scapulae lie
posterolaterally and the diverging X-ray beam usually, but not necessarily, projects them clear
of the lungs on PA views. In the AP projection the lungs and scapulae invariably overlap; d. the
b
appearance of the cervicodorsal vertebrae. Vertebral end-plates in this region slope downwards
and forwards and are usually tangential to the diverging AP beam, making them clearly visible
in this projection. The laminae slope in the opposite direction and are therefore seen more
clearly in PA projections.
3. Posture (Erect vs. Supine): Erect radiographs are preferred to supine radiographs because
the lungs are more expanded in the former, making pulmonary and mediastinal structures easier
to assess. Additional advantages of the erect radiograph are: a. the relative size of pulmonary
vessels in the upper and lower zones can be used to assess pulmonary venous pressure; b. air–
fluid levels are detectable; c. pleural pathology (pneumothorax, pleural effusion) is easier to
detect than in supine radiographs; and d. the tube–film distance is optimal. The decision as to
whether a patient was examined in the supine or erect position is most reliably based on a
record made on the radiograph by the radiographer. In the absence of this, the position of the
gastric air bubble is used (fundal if erect, antral if supine). A PA radiograph can be assumed to
have been taken erect.
4. Rotation: The patient may be rotated around one or more of three axes. The commonest
rotation, around the long axis of the body, distorts mediastinal and hilar structures and makes
the radio-opacity of the right and left hemithoraces unequal. In practice axial rotation is the
commonest cause of unilateral transradiancy. The degree of rotation can be gauged by
observing the relationship of the medial ends of the clavicles to the margins of the vertebral
body at the same level.
5. Tube–Film Distance: A change in this will alter the apparent heart size.
6. Degree of Inspiration: A shallow inspiration may result from lack of effort by the patient or
may be related to a pathological process. Normally the midpoint of the right hemidiaphragm
lies between the anterior ends of the fifth to seventh ribs and deviations from this are likely to
be abnormal. Underinflation reduces the difference in size between upper- and lower-zone
vessels and affects the contour and size of the mediastinal contents, particularly the heart.
7. Penetration: look at the lower part of cardiac shadow, the vertebral bodies should be just
visible through cardiac shadow. If they are too clearly visible, then the film is overpenetrated
and you may miss low density lesions e.g. miliary shadows, but if you can not see the vertebral
bodies at all, then the film is underpenetrated and lung fields will appear falsely white.
C. Detection and Description of Abnormalities: Radiographs should be inspected on a
viewing box under quiet conditions with low ambient lighting. Previous radiographs for
comparison and a bright light facility should be available.
Three types of abnormality may be detected on the radiograph during this examination:
1.
A changed appearance of a normally visualized structure;
2.
A radio-opacity;
3.
A transradiancy.
Silhouette Sign: Interfaces between the air in the lung and opacities of soft tissue density will
have clear, sharp margins on a chest radiograph provided such interfaces are reasonably smooth
and tangential to the X-ray beam. If the air in the lung at the interface is removed either by
alveolar collapse (e.g. obstructive atelectasis) or substitution (e.g. consolidation), an opacity
replaces the air transradiancy and the radiographic boundary will disappear because of the loss
of contrast, so 'silhouette sign' to indicate loss of the radiographic border. A common and
striking example of this is seen on the right lateral chest radiograph, where the heart obliterates
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the anterior border of the contiguous left hemidiaphragm. The silhouette sign is used in three
main ways: 1. to localize and therefore identify normal structures (as with the hemidiaphragm
above); 2. to localize obvious abnormal opacities; and 3. to detect subtle lesions of low radioopacity. An example of this last application is seen with right middle-lobe collapse, in which
effacement of the right heart border may be the only definite sign on a PA radiograph.
1. Alterations in a Normally Visualized Structure: The detection of this type of
abnormality necessitates a detailed knowledge of normal appearances and variations.
a. Transradiancy: The transradiancy of both lungs in corresponding zones should be equal.
In the lateral view the dorsal spine becomes increasingly transradiant inferiorly, and in
adults the high retrosternal region is transradiant, though to a variable degree.
b. Soft Tissue and Bone: The scrutiny of these must include the breasts and a detailed
examination of the posterior, axillary, and anterior aspects of the ribs, looking particularly
for evidence of surgery, fractures, notching, and changes in architecture.
c. Diaphragm: The following features should be assessed:
(i) The costo- and cardiophrenic angles. The lateral and posterior costophrenic angles should
be sharp and acute. The appearances of the cardiophrenic angles are very variable owing to
the presence of fat pads.
(ii) The absolute height of the diaphragm. Normally the margin of the right hemidiaphragm
in the mid-lung line lies between the anterior ends of the 5th and 7th ribs. Deviations from
this are considered to represent over- or underinflation.
(iii) The relative height of the hemidiaphragms. In more than 90% of normal subjects the
right hemidiaphragm is higher than the left by up to 3–4 cm and this difference may be
exaggerated in the supine position, especially in the elderly.
(iv) Clarity and margin. The lung/diaphragm interface should be sharp and, given adequate
exposure, effacement suggests local pleural or pulmonary pathology.
(v) Contour. In general, the hemidiaphragm is smoothly arcuate with its highest point lying
medial to the mid-lung line, particularly on the right. Lateral peaking should raise the
possibility of a subpulmonary effusion. On the left the highest point tends to be more
laterally placed than on the right, because of the heart. The contour of the diaphragm,
especially on the right, may be complicated by lobulations, the commonest of which is
situated anteromedially ('dromedary hump').
(vi) Curvature. The curvature of a hemidiaphragm is assessed by drawing a line between its
cardio- and costophrenic angles and dropping the maximum perpendicular onto this line
from the silhouette of the diaphragm. A measurement of less than 1.5 cm is taken to indicate
flattening.
In the lateral view: both hemidiaphragms should be visible. Effacement of the left
hemidiaphragm anteriorly by the heart is an almost universal finding on a right lateral
radiograph but is less common with a left lateral. Basal lung pathology may efface part of a
hemidiaphragm in the lateral view (silhouette sign) and yet be invisible on a PA radiograph.
Under these circumstances it is important to be able to identify which hemidiaphragm is
which in the lateral view. This distinction is made on the following findings, most of which
result from beam divergence:
1. On a lateral radiograph posterior ribs are seen as though in cross-section as two parallel
rows of oval opacities that make contact with the ipsilateral hemidiaphragm in the posterior
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costophrenic angle. The hemidiaphragm associated with the larger (i.e. more magnified) ribs
is that furthest from the film. Application of this finding is complicated by rotation .
2. The hemidiaphragm that passes through the heart shadow to the anterior chest wall is the
right, while the left ends at the posterior heart border. As noted above, this rule breaks down
sometimes on a left lateral projection and in the case of dextrocardia;
3. Gastric gas lies under the left hemidiaphragm. In the absence of a sub-phrenic abscess,
this allows unequivocal identification if fundal gas is projected between the hemidiaphragms
but if it lies below both it may be unhelpful unless its contour exactly matches that of one of
the hemidiaphragms.
d. Root of Neck and Trachea: The upper trachea is central, but inferiorly it may deviate
slightly to the right because of the aorta, and this effect becomes more marked with age. On
a well penetrated or high-kV radiograph the right paratracheal line is nearly always visible,
extending upwards for 4–5 cm from the azygos vein 'tear drop'. The line should be less than
5 mm thick and loss of its outer interface implies adjacent pathology, commonly nodal
enlargement.
e. The Mediastinum and Heart: The mediastinum should be central and its silhouette, in
general, sharp. Short segments in which the silhouette is ill defined are sometimes seen near
the cardiophrenic angles, at the apices and near the right hilum. Other segments may be ill
defined but should only be considered normal once local pathology has been excluded. The
mediastinal margin should be inspected for any abnormality of shape; and the visible
mediastinal lines and interfaces for their integrity, position and contour.
f. Hila: Hilar shadows are made up of pulmonary arteries and veins with 2 minor
contributions from other components (airways, nodes). In general the hila are equally dense
and approximately the same size. The detection of abnormalities in their size and contour
requires considerable experience and familiarity with the normal anatomy.Such changes
may be produced by an alteration in size (usually enlargement) of one or more of the normal
components; by a re-orientation of normal components (e.g. secondary to lobar collapse); or
by the development of abnormal tissue (e.g. bronchial carcinoma). A spurious hilar opacity
may result from an overlapping lesion, particularly in the apical segment of the lower lobe.
This should be suspected when hilar borders remain visible within the opacity (negative
silhouette sign).
g. Fissures and Vessels: Apart from vessels, the only shadows visible in normal lungs are
those due to fissures and end-on airways. Fissures should be checked for their position,
configuration, and thickness. Normal fissures are of hairline thickness. The oblique fissures
are usually only seen on lateral radiographs, they extend downwards and forwards from
D4/5, reaching the diaphragm a few centimeters behind the sternum. The left fissure is the
more vertical of the two and ends inferiorly just behind the right. The minor fissure is seen
in both frontal and lateral views. In the frontal view it extends laterally to the chest wall
from the margin of the basal pulmonary artery.
Abnormal Opacities: Opacities are also called shadows, densities, or infiltrates, though the
last two terms are not recommended. Opacities must be described and analysed in detail,
noting in particular their size, number, position (distribution), shape, contour, marginal
clarity, and structure. Their structure may be homogeneous or non-homogeneous, and in the
latter instance this is usually due to the presence of an air bronchogram, focal transradiancy,
or calcification. If a series of radiographs has been taken, it is good practice to look at
selected previous examinations in addition to the current and immediately preceding
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radiographs, because some changes are undetectable over the short term. The changes in
opacity with time and therapy often provide valuable extra information that is useful in
identifying its aetiology, e.g. in distinguishing between infective and oedematous
consolidation.
a. Consolidation: Consolidation is caused by the replacement of air in distal airways and
alveoli by fluid (transudate, exudate including eosinophilic exudate, blood) or, rarely, tissue.
(i) Ill-defined Margins: This results from piecemeal alveolar involvement at the edge of a
consolidated area though once the process comes up against a pleural surface the
margination becomes sharp.
(ii) Nonsegmental Distribution: In general consolidation does not respect segmental
boundaries as these do not constitute a physical barrier to the spread of fluid.
(iii) Irregular Shape: Apart from the effect of gravity, alveolar fluid spreads in a rather
arbitrary fashion, and regular shapes such as spheres are rarely produced. They can
occasionally occur, however, particularly when the consolidation is caused by tissue rather
than fluid. Conditions giving spherical lesions containing an air bronchogram include
alveolar carcinoma, lymphoma, pneumonia (due to Strep. pneumoniae, or in children), and
sarcoidosis.
(iv) Tendency for Coalescence
(v) No Volume Change: Usually there is a one-to-one replacement of air by fluid, and
consolidation per se is an isovolumetric change. In some disorders, however, (e.g. infarction,
pneumonia), additional volume loss is sometimes seen, and on other occasions the volume
increases. Expansive consolidation is most commonly seen with pneumococcal and
Klebsiella pneumonia, tuberculosis, abscess formation, and when the consolidation harbours
a neoplasm.
(vi) Air Bronchogram: Normal intrapulmonary airways are invisible unless end-on to the
X-ray beam. If they pass through a zone of increased radio-opacity secondary to the
reduction or replacement of surrounding alveolar air, however, then the airways become
visible as linear, branching transradiances. This radiographic appearance is called an air
bronchogram. It is a reliable sign of an intrapulmonary lesion, since the airways have to be
intimately surrounded by the opacifying process for it to be produced.
(vii) Vascular Changes: Blood vessels are normally seen because their soft tissue density is
contrasted against air-containing lung. With consolidation they become obscured (silhouette
sign) and this is a reliable sign of an intrapulmonary opacifying process.
(viii) Acinar Nodules: Ill-defined nodular shadows 5–10 mm in diameter are sometimes
seen in consolidation, and represent consolidated acini contrasted against adjacent air-filled
ones.
(ix) Ground-Glass Opacity: This is a finely granular, veil-like shadowing that is seen with
both consolidation and interstitial processes. In clinical practice it is most commonly seen
with diffuse pulmonary haemorrhage, extrinsic allergic alveolitis, Pneumocystis pneumonia,
and fibrosing alveolitis.
b. Collapse (Atelectasis)
c. Nodular Opacities:
Nodular opacities are essentially spherical. They may be single or multiple and range in size
from massive (15 cm or more) to pinpoint (less than 1 mm — micronodules). Nodules more
than 3 cm in diameter are called masses and multiple small nodules about 2 mm in diameter
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are called miliary nodules. An isolated nodule must be assessed for its position, size, shape,
contour (smooth, umbilicated, lobulated) and marginal clarity (sharp, ill-defined,
spiculated). Lack of homogeneity is usually due to either calcification or cavitation. When a
nodule abuts a normal structure, the resultant silhouette sign often helps to localize the
lesion. When nodules are multiple, several additional features must be evaluated, and these
include their approximate number, distribution, size range, and density.
d. Ring Opacities: Ring shadows are annular opacities with a central transradiancy and a
number are due to cavitation of pre-existing nodules, masses, or consolidations.
Neoplasms tend to have, at least in part, thick walls (16 mm or more) and irregular, nodular
inner margins. Ring opacities with walls 4 mm or less in thickness are commonly benign.
Cysts appear as evenly thin-walled (1 mm) rings, 1 cm or more in diameter, which form a
useful subset with a limited number of common causes(bulla, bleb, laceration,
pneumatocele, bronchogenic cyst and bronchiectasis).
e. Linear Opacities: A linear opacity is an elongated opacity which is approximately even
in width. It is useful to distinguish band shadows (5 mm or more wide) from line shadows that
are thinner.
Some abnormal line shadows have characteristic features that allow their identification.
(i) Septal (Kerley B): lines are short, straight, horizontal, peripheral lines that end at right
angles against the pleura and are generally 1–2 mm thick and less than 20 mm long. With raised
pulmonary venous pressure they are found mainly in the costophrenic angles but when due to
other conditions they may appear elsewhere.
(ii) Septal (Kerley A): lines represent deeper interlobular septa made visible by the same
mechanisms that produce B lines. Septal A lines occur in any zone, start away from the lung
edge, and point towards the hila. They are straight or slightly angular and are 20–30 mm long
and 1–3 mm wide.