Flashcards in Twenty Three Deck (19):
What are 6 radiographic densities in a chest x-ray? What will give a good image of an intrathoracic structure? What things are valuable clues to underlying pathological changes?
Normal anatomical features of the respiratory system, as
well as pathologies of the lungs and cardiothoracic system are
visualized by the interplay among seven different radiographic
densities (Fig 15.1): air, fat, soft tissue (eg, muscle vs blood or
fl uid), calcium, x-ray contrast media, and metal (≅ dense bone).
Discrimination among these features is accomplished because
of differential absorption of radiation by various normal or
diseased tissues that ultimately results in the creation of radiographic
images. Of note, an intra-thoracic structure is best rendered
visible by the juxtaposition of two different radiographic
densities. Moreover, it is necessary for the x-ray beam to tangentially
strike the interface between tissues of different density in
order to appear as a well-defi ned boundary line on chest radiographs.
Because diseases of the respiratory system often result in
differential x-ray absorption, both the absence of a normal radiographic
interface and the presence of an unexpected interface
are valuable clues to underlying pathological changes.
How is a normal chest x-ray taken? What are the advantages and disadvantages to a PA x-ray view and an AP x-ray view? Why is a lateral chest x-ray view necessary?
By convention, the routine frontal chest x-ray view is taken
in the radiological suite with the patient upright and during full inspiration. In this view the x-ray beam is horizontal to
the patient, and the x-ray tube is ~2 m from the fi lm detector
as the beam traverses the patient from a posterior to anterior
(PA) direction or view. This distance used in acquiring the PA
view reduces unnecessary magnifi cation and enhances sharpness
as does the close apposition of the chest and the fi lm. In
contrast, it is frequently necessary in hospitalized and critically
ill patients to obtain portable chest x-ray images at the
bedside, which are performed using an anteroposterior (AP)
direction or view. Such AP fi lms involve less powerful radiation
energy and a shorter x-ray tube-to-fi lm distance, resulting
in greater magnifi cation but reduced anatomical resolution.
Using only the PA view, it is often diffi cult to detect chest
lesions located behind the heart, near the mediastinum, or near
the hemidiaphragms because these are radiographically denser
structures with poor interfaces. For this reason, the lateral
chest x-ray view is also performed to pinpoint such lesions
three-dimensionally and localize them within the lungs. The
lateral view is routinely taken with the left side of the patient
against the fi lm cassette.
Name 14 landmarks that should be spotted on a frontal chest radiograph and where they are seen.
1. The aortic arch (AA) normally dominates the left
superior cardiomediastinal contour of the patient,
although it may form part of the right cardiomediastinal
contour in older individuals.
2. The left lateral wall of the descending aorta (DA) is
usually visible as it courses inferiorly through the
3. The proximal left pulmonary artery (PLPA) is
visible in the left hilar region just inferior to the aortic
arch. When visualized, the left interlobar pulmonary
artery (LIPA) is noted to be inferior and lateral to
4. The aorto-pulmonary window is the concavity created
by the overlap of the aortic arch and the left pulmonary
5. The left mainstem bronchus (LMB) is often seen on
frontal views just below the main pulmonary artery segment
and the left pulmonary artery.
6. The appendage of the left atrium (LA) projects slightly
inferior to the left mainstem bronchus and along the
left cardiomediastinal contour. The left ventricle (LV)
completes the rest of the left cardiomediastinal
7. The superior vena cava (SVC) is seen in the most
superior portion of the patient’s right cardiomediastinal
8. The right paratracheal stripe (RPS) is the soft tissue
stripe created by the interface of the right lateral tracheal
wall and right upper lobe. Near the inferior RPS in the right
tracheobronchial angle the azygos vein (AV) may be seen.
9. The right interlobar pulmonary artery (RIPA) exits
through the right hilum inferiorly and laterally and is the
predominant shadow in this region.
10. The right atrium (RA) forms the right cardiac
border, and occasionally coursing through the
cardiophrenic angle between the heart and the right
diaphragm is the shadow representing the inferior
vena cava (IVC).
11. The trachea (TRA) is usually easily seen on PA or AP
12. The right diaphragm (RD) and left diaphragm (LD)
contours are clearly visible.
13. The costophrenic angle (CPA) is visible in the lower left
portion of the thorax.
14. The anterior junction line (AJL) may be seen as an
obliquely oriented line overlying the mediastinum
representing points of contact between the two lungs.
What are 12 landmarks located on a lateral wall radiograph and where are they located?
1. The retrosternal space (RS) is the clear region anterior
to, and just beneath, the sternum. This space is decreased
with enlargement of the right ventricle during conditions
that result in pulmonary hypertension (PHT).
2. The TRA is again easily visualized as in the frontal
3. The bronchial orifi ce of the right upper lobe (RUL)
appears as a circular lucency that projects over the
continuation of the tracheal air column.
4. The posterior wall of the bronchus intermedius is
represented by the soft tissue stripe just below the orifi ce
of the right upper lobe bronchus.
5. The left pulmonary artery (LPA) appears as a structure
having soft tissue density that courses over the bronchus
of the left upper lobe (LUL).
6. The right pulmonary artery (RPA) is visible as a
rounded soft tissue density. It is anterior and inferior to
the orifi ce of the RUL bronchus (#3 above).
7. The infrahilar window (IH, at the
What views can be useful to visualize pleural effusions and pneumothoraces? What do they tell you?
Additional radiographic projections may be used in the
chest x-ray evaluation of patients with diseases of the
respiratory system. For example, lateral decubitus views
obtained with the patients lying on their right side, left
side, or both sides in sequence are helpful in distinguishing
pleural eff usions from underlying lung consolidation. If
eff usions are present, such decubitus views help determine
whether they are loculated or free-fl owing, and thereby
amenable to thoracentesis (Chaps. 19 and 29). Lateral
decubitus views are also useful in evaluating the possible
presence of a pneumothorax, as are frontal fi lms taken
in full expiration. Classically
What is the silhouette sign? What causes it and why is it important?
Knowledge of lobar and segmental lung anatomy is indispensible
for understanding patterns of lung disease and parenchymal collapse
(atelectasis). Application of this knowledge during review
of a chest radiograph is furthermore important in planning additional
diagnostic and therapeutic procedures, such as bronchoscopy,
surgery, or radiation therapy. In this context, one of the
most commonly recognized and helpful radiological signs during
review of chest x-rays is the silhouette sign (loss of normal
interface). Under normal conditions a distinct boundary is seen
when aerated lung contacts a structure of different radiographic
density such as the heart, mediastinum, or diaphragm through
the creation of an interface as defi ned above (Fig. 15.4).
However, diseases of the respiratory system often feature
replacement of air in the alveolar spaces with infl ammatory cells,
pus, blood, or edema fl uid that singly or combined, result in consolidated
lung tissue. When such consolidated lung is adjacent
to soft tissues that have similar soft tissue/water density such as
that of the heart or mediastinum, the normal interface created by
aerated lung is lost. This loss of normal air-water radiographic
density interface is termed the silhouette sign (Fig.15.5).
How is a pneumothorax detected with radiographs?
Abnormal presence of air in the pleural space, or pneumothorax,
is generally manifested radiologically as unilateral, darker
homogeneous shadowing outside the zone of vascular lung markings on a frontal chest x-ray (Fig. 15.6). Pneumothorax
can occur spontaneously after rupture of a visceral pleural
bleb or following invasive procedures such as thoracentesis
that puncture the visceral pleura and cause an air leak (Chaps.
19 and 37). Placing the patient in an erect position to obtain
PA or AP frontal views is ideal to diagnose a pneumothorax,
although this condition can also be detected by a lateral decubitus
fi lm when taken with the presumed affected side facing
WHen is it critical to promptly recognize a pneumothorax? Why?
It is critical to promptly recognize a pneumothorax on
chest x-rays of hospitalized patients receiving positivepressure
ventilation, or in acutely traumatized patients
requiring emergent operation for their injuries who
are about to receive mechanical ventilatory assistance.
Pneumothorax under these conditions of superimposed
positive-pressure ventilation can rapidly progress to
life-threatening tension pneumothorax, characterized
by elevations in intrapleural pressure that progressively
impede systemic venous return, reduce cardiac output,
leading to arterial hypotension and cardiovascular
collapse. Defi nitive treatment of pneumothorax consists of
needle decompression followed by tube thoracostomy.
What are air space opacities? What do they tell you and when would they occur? What are air bronchograms?
1. Air-space opacities: These appear as confl uent, ill-defi ned
opacities that obliterate the normal shadows created by
pulmonary blood vessels, and often display a tendency
to extend to the pleural surfaces. Air-space opacifi cation
occurs with the replacement of the air in the alveolar spaces
of the lung parenchyma by an alternate substance such as
infl ammatory cells in acute respiratory distress syndrome
(ARDS), pus in conditions of pneumonia, blood from
pulmonary hemorrhage, water representing cardiogenic
or noncardiogenic pulmonary edema fl uid, or tumor cells.
An air bronchogram is the characteristic manifestation
of air-space opacity and may be seen when the alveoli
surrounding a patent air-fi lled bronchus are rendered airless
(Fig. 15.8). An air bronchogram is therefore a fundamental
sign of consolidation and enables confi dent localization of
the opacity within the lung parenchyma.
What are interstitial opacities? What do they tell you and when do they occur?
Interstitial opacities: These are described as linear or
reticular, or as septal lines, peribronchovascular thickening,
nodules, or as a generalized miliary pattern consisting of
innumerable small opacities (miliary is defi ned medically
as multiple small foci that resemble small millet seeds
on an x-ray) (Fig. 15.9). Collectively, these radiographic
fi ndings suggest disease processes that are localized to the
pulmonary interstitium; depending on the patient’s clinical
history, they may suggest specifi c diagnoses.
What are nodules and masses? What do they tell you and when do they occur? How are they classified?
Nodules and masses: A nodule is a discrete opacity
on a chest x-ray measuring 3 cm on
chest x-rays are referred to as masses (Fig. 15.10b).
How are lympadenopathies best viewed in radiographs? How do they look? Where are they found?
Lymphadenopathy: Abnormal contouring of the
mediastinal shadows in frontal PA or AP views may
represent lymph node enlargement (Fig.15.11) or a
mass. Characteristic locations of commonly observed
intrathoracic lymphadenopathy on PA x-rays include
the right paratracheal area, the hilar regions, the
aortopulmonary window, the subcarinal region, and the
superior mediastinum. Lymphadenopathy in the retrosternal
area is best visualized using lateral radiographs, since
lymph node enlargement fi lls the normally clear infrahilar
window with an unexpected contour.
What are cysts and cavities? What causes them? What do they look like? How are they classified?
Cysts and cavities: These abnormalities include areas
of pulmonary parenchymal space that normally contain
lung tissue but instead are fi lled with air, fl uid, or both.
Cysts usually have thin walls that may be composed of
cellular elements. Cavities are usually created by tissue
necrosis within a lung nodule or mass, and they evolve
to become air-fi lled when the internal necrotic elements are expelled into the tracheobronchial tree (Fig. 15.12).
Pulmonary cysts and cavities are characterized on chest
fi lms by noting their distribution, number, any special
characteristics of their inner lining, the thickness of their
walls, and the nature of their contents.
What pleural abnormalities are there? How are they best seen? What do they look like?
Pleural abnormalities: Pleural disease has several
manifestations, the commonest being pleural effusions,
which when free-fl owing characteristically blunt the
costophrenic angle to form a meniscoid opacity. Large
effusion volumes may show blunting or opacify an entire
hemithorax (Fig. 15.13). In contrast to free-fl owing
effusions, pleural thickening reveals its nondependence
on gravity and a nonlayered appearance on lateral
decubitus x-rays. Nodular pleural thickening may suggest malignancy. Pleural calcifi cations are common
in asbestos-related pleural disease, or as the sequelae of a prior hemothorax or tuberculous empyema.
What are the advantages and disadvantages to a chest CT? When is it particularly useful? What variation of it is used for pulmonary embolism?
By itself, a chest x-ray often cannot clearly defi ne disease
processes, due to inherent limitations of resolution, imprecise
segmental lung localization, and two-dimensional representation.
In contrast, a chest CT scan provides three-dimensional
visualization of intrathoracic anatomical structures including
the lung parenchyma, making diagnostic interpretation easier
and more accurate albeit at increased cost and radiation dose.
For example, the radiation dose to the patient from a single
chest x-ray is 10 mrem versus ~580 mrem for a standard
chest CT scan, but the chest CT increases contrast resolution
by a factor of 200. Such resolution is particularly useful for
defi ning the segmental localization of parenchymal opacities,
assessing mediastinal structures including lymphadenopathy,
and evaluating pleural diseases. Importantly, CT angiography
of the chest that is performed with intravenous injections
of contrast media has become the primary method for detecting
pulmonary embolism (Fig. 15.14) (Chap. 27).
What are HRCT? What structures are they useful for viewing? What are the different parts of that structure?
High-resolution CT scans (HRCT) are most useful in evaluating
patients with suspected interstitial lung disease, since their
history and physical exam are frequently of limited diagnostic
value. Optimal utilization of HRCT fi ndings requires appreciation
of the secondary pulmonary lobule as the lung’s basic
structural unit (Chap. 2) whose several components are normally
visible on thin-section HRCTs of the lung. The interlobular
septal region consists of peripheral interstitium containing
interlobular septa and subpleural septa, pulmonary veins, and
lymphatics. The centrilobular region contains the axial interstitium
with its peribronchovascular structures including bronchiolar,
pulmonary arterial, and lymphatic branches. The lobular parenchyma region contains alveoli and their pulmonary capillaries
as well as intralobular septal fi bers (Fig. 15.15).
What pathological alterations are visible with HRCT? What are 4 HRCT Radiological patterns of diffuse parenchymal lung disease?
Recognizing abnormal structures of the secondary pulmonary
lobule is fundamental to interpreting HRCT scans (Fig. 15.16).
Pathological alterations visible on thin-section CT scans
include interlobular septal thickening, diseases with peripheral
lobular distribution, centrilobular abnormalities, and panlobular
abnormalities. To simplify the approach with HRCT, the
signs of diffuse lung disease can be grouped into four general
patterns: (1) increased attenuation, referred to as “ground glass
opacity” or consolidation; (2) reticulation with parenchymal
distortion, typifi ed by pulmonary fi brosis; (3) nodules, whether
large or small, singular or multiple; and (4) mosaic patterns
and cysts. These patterns of disease are combined with the distribution
of disease to formulate a differential diagnosis.
other hand, using MRI for lung imaging is currently limited by
the longer time required to completion versus a CT scan, the
greater cooperation required of a patient, and its unsuitability
for very large or claustrophobic individuals. The proximity of
an implant or any other metallic object in the vicinity of the
chest may be a relative or absolute contraindication for MRI
What are the advantages and disadvantages of an MRI?
Magnetic resonance imaging (MRI) is a non-invasive diagnostic
scanning technique to identify the distribution of water
and other hydrogen-rich molecules in the body through the
use of a powerful and highly uniform, static magnetic fi eld
but no ionizing radiation. MRI provides a comprehensive
but non-invasive evaluation of the morphology and function
of intrathoracic vessels, including the aorta and its branches,
the pulmonary vasculature, and the central veins. This imaging modality has several advantages including multiplanar images,
intrinsic contrast between blood pool and vessel wall, and a
wide range of soft tissue contrasts that delineate vascular and
perivascular structures while avoiding the use of iodinated
contrast media or radioisotopes.