Chest x rays made easy
In the third of a five part series, Elizabeth Dick looks at abnormalities of the lung fields
The basics of looking at a chest x ray (recap):
- First look at the mediastinal contours - run
your eye down the left side of the patient
and then up the right.
- The trachea should be central. The aortic
arch is the first structure on the left,
followed by the left pulmonary artery;
notice how you can trace the pulmonary
artery branches fanning out through the
lung (see figure 1).
- Two thirds of the heart lies on the left side
of the chest, with one third on the right.
The heart should take up no more than
half of the thoracic cavity. The left border
of the heart is made up by the left atrium
and left ventricle.
- The right border is made up by the right
atrium alone. Above the right heart border
lies the edge of the superior vena cava.
- The pulmonary arteries and main bronchi
arise at the left and right hila. Enlarged
lymph nodes can also occur here, as can
primary tumours. These make the hilum
seem bulky - note the normal size of the
hila on this film.
- Now look at the lungs. Apart from the
pulmonary vessels (arteries and veins), they
should be black (because they are full of
air). Scan both lungs, starting at the apices
and working down, comparing left with
right at the same level, just as you would
when listening to the chest with your
stethoscope. The lungs extend behind the
heart, so look here too. Force your eye to
look at the periphery of the lungs-you
should not see many lung markings here; if
you do then there may be disease of the air
spaces or interstitium. Don't forget to look
for a pneumothorax.
- Make sure you can see the surface of the
hemidiaphragms curving downwards, and
that the costophrenic and cardiophrenic
angles are not blunted - suggesting an
effusion. Check there is no free air under
the hemidiaphragm.
- Finally, look at the soft tissues and bones.
Are both breast shadows present? Is there
a rib fracture? This would make you look
even harder for a pneumothorax. Are the
bones destroyed or sclerotic?
Fig 1 Right pneumothorax. The right side of the lung is blacker, and the lung edge is seen (arrow). There is no mediastinal shift and therefore no tension
Pneumothorax
It is important to view around the periphery of the lungs to look for a pneumothorax (air in the pleural space with associated collapsed lung). It is very easy to miss
a pneumothorax. Watch out for the
following signs:
- One half of the lung may seem
blacker - that is, more radiolucent -
than the other, which will be more
radio-opaque or whiter. In particular,
the area beyond the collapsed lung
will be very radiolucent because there
are no pulmonary vessel markings.
- You should be able to identify the
edge of the collapsed lung (see fig 1).
Having identified a pneumothorax you
need to look for several more associated
abnormalities:
- Most importantly - this is a pass or fail
observation - is there evidence of a
tension pneumothorax? This occurs
when air can enter the pleural space
(via a hole in the lung surface or the
chest wall) but, because of a ball-valve
effect, air cannot leave by the same
route. So more and more air
accumulates in the pleural space.
This pushes the mediastinum over to
the opposite (normal) side and
eventually compresses the normal
lung so that less inspiration occurs
on the normal side, with compression
on the heart and decreased venous
return until finally the patient arrests
(see fig 2). Always look for this and
say: "There is no shift of the
mediastinum and therefore no
tension pneumothorax" or "There is
shift of the mediastinum away from
the side of the pneumothorax
indicating a (right/left) tension
pneumothorax. This is a medical
emergency which I would treat
immediately by inserting a large bore
cannula into the (right/left) pleural
space."
Fig 2 Left tension pneumothorax with shift of the mediastinum to the right. The lung edge is arrowed
- The cause of the pneumothorax may
be apparent - for example, fracture of
the ribs.
- There may be associated surgical
emphysema - that is, air in the soft
tissues - and air in the mediastinum
(see fig 3).
Fig 3 Surgical emphysema (arrow) and pneumomediastinum (arrowhead)
There is extra shadowing in the lungs
It may be difficult to work out what is causing extra shadowing in the lungs, especially near the mediastinum where normal
structures may overlay the extra shadowing. It is useful to look at the periphery of
the lungs because normally the outermost
edge of the lungs should be fairly black
with a few tapering blood vessels. If you do
see more shadowing in the periphery then
there may be either interstitial or air space
disease. As examiners often show films
with one of these two types of shadowing,
understanding the difference between
these two is worth while because it will help
you to interpret what you see and lead you
to the correct differential diagnosis.
The lung is made up of bronchi, which
branch, at the end of which are alveoli. The
interstitial space (or potential space) surrounds the alveoli. The whole of the lung
from bronchi to alveoli is the air space - that is, it normally contains air. But the
air spaces can fill up - with fluid (such as in
severe pulmonary oedema), with pus (as
in infection), with blood (as in rare diseases
such as Goodpasture's syndrome, associated with renal failure), or with tumour cells (alveolar carcinoma).
Fluid and pus are more common than
the second two. When the air spaces fill up,
the alveoli fill first, with the bronchi being
relatively spared. Therefore the bronchi,
which are still air filled, stand out against
the alveoli, which are filled with pus or
fluid. This is called an air bronchogram
and is simply a sign that there is air space
disease. Consolidation is another term for
air space shadowing (see figs 4 and 5). If
there is air space disease then you need to
work out which part of the lungs it is affecting. A quick way is to use the word "zone"
to describe which part of the lung is affected. Say something like "There is shadowing in the air spaces of the right mid and lower zone." You can then take your time
to work out which lobe is affected. You can find out more about lobar anatomy in the
later section on collapse and consolidation.
Fig 4 Left and right lower lobe air space shadowing in an ITU patient
Fig 5 Right middle and lower zone consolidation/air space shadowing. Note air bronchogram (arrow). There is no loss of volume, which is a key feature of consolidation
Let's turn to the interstitial space. This
surrounds bronchi, vessels, and groups of
alveoli. When there is disease in the
interstitium it manifests itself by reticulonodular shadowing (criss cross lines or
tiny nodules or both). The main two
processes affecting the interstitium are
accumulation of fluid (occurring in pulmonary oedema or in lymphangitis carci-
nomatosa) and inflammation leading to fibrosis (occurring in industrial lung dis-
ease, inflammatory arthritides such as
rheumatoid arthritis, inflammation of
unknown cause such as cryptogenic fibrosing alveolitis and sarcoidosis). If you see
criss cross lines or tiny nodules or both say:
"There is reticulo-nodular shadowing within the lower zones." (See figure 6.)
Fig 6 Recticular-nodular shadowing caused by lung fibrosis (circled). Note how the heart has lost its normal smooth outline and seems "shaggy"
Use the table to work out whether the extra shadowing you can see is air space or interstitial.
Next month: we will look at collapse, consolidation, and pleural effusions.
I would like to thank Dr Anju Sahdev, Dr Brian Holloway, and Dr Robert Dick for contributing some of the films which are illustrated.
Elizabeth Dick, specialist registrar in radiology, North Thames Deanery
studentBMJ 2000;08:395-434 November ISSN 0966-6494
