CHEST X-RAY
The plain CXR is the most commonly performed imaging exam because:
– Cardio-pulmonary disease is common
– The exam is quick, easy to do, cheap, with low radiation exposure (a PA CXR gives
only about 3-days-worth of radiation exposure we get anyway from natural sources)
– Most importantly the contrast elements involved allow us to see the common
pathologies we’re looking for
It’s all about the contrast: For any imaging exam to be useful, there must
be contrast (signal difference) between lesion and surrounding tissue
There are 4 tissues that have densities that can be distinguished from each
other (have contrast) on plain X-ray:
–
–
–
–
Calcium (bone)
Soft tissue and fluid (not distinguishable on plain X-rays)
Fat
Gas/air
The natural contrast agent of air in the lungs allows us to see the common
soft tissue/fluid pathologies (pneumonia, lung CA, pleural effusion, Kerley
lines, etc.)
The difficulty in reading a CXR is that it’s a 2-D
representation of a 3-D object, with everything
front to back (the z-axis) projected into a single x,y
planar image
The task of reading a CXR requires sorting out
each important individual piece of anatomy from
the overlapped jumble
This requires a “system” for methodically checking
each of the individual pieces the same way each
time so that important findings aren’t missed
Any system is just a crutch to help you remember
everything you’re supposed to check
Any system that works for you is fine
Suggested system:
– White things (bones, man-made things like
tubes, pacer wires, clips)
– Gray things (soft tissues of neck/chest
wall/under diaphragm, pleural surfaces,
mediastinum)
– Black things (lungs, including lung seen
through heart and diaphragm)
Following image is normal PA CXR
Note that X-ray consists of overlapping areas
of varying opacity which are often separated
by edge shadows (e.g., edge of heart with
lung, edge of rib with soft tissue)
What creates an edge shadow? Two criteria
need to be met:
– 2 different tissue opacities (calcium, soft
tissue/fluid, fat, air) next to each other
– X-ray beam contacting the interface between the
2 opacities tangentially
Following image is normal lateral CXR
Principle of plain X-ray is to try to get at
least 2 projections (to overcome the z-axis
overlap problem)
Note that there are 2 hemidiaphragm and
2 costophrenic angle edge shadows.
Note retrocardiac lucency (the spine
should get darker as it is followed down to
the diaphragm)
On a lateral CXR what would cause loss of the retrocardiac lucency
and loss of the edge shadow of one hemidiaphragm, and why?
There must be violation of one of the 2 criteria that allow you to see
the edge shadow in the first place. Because the diaphragm is
usually dome-shaped, criterion #2 (tangential X-ray) won’t usually
be violated, so it’s probably that criterion #1 is no longer met (there
has been equalization of density at the point where the X-ray is
tangential to diaphragm)
This could be caused by something of same density as diaphragm
displacing the air-containing lung away from contact with diaphragm
(pleural effusion)
Or it could be caused by a process within the lung casing it to
become same density as diaphragm (consolidation such as
pneumonia, atelectasis)
Vast majority of basilar opacities on CXR are due to pleural effusion,
consolidation, atelectasis, or a combination of these
How does one distinguish among pleural
effusion, consolidation, and atelectasis? (the
causes and treatments are different)
Classic sign of pleural effusion on CXR is a
meniscus (a sharp edge between the fluid
and adjacent lung). In general, a sharp edge
between the lung and anything suggests that
a pleural surface is being crossed.
Classic sign of atelectasis is evidence of
volume loss (shift of fissures or mediastinum,
diaphragm elevation)
Following 2-view CXR shows RML pneumonia
Note opacity that obscures right heart border on
PA (because RML opacity causes equalization of
density at point where X-ray is tangential to right
heart)
On lateral, note sharp inferior edge shadow of the
pneumonia (this is major or oblique fissure pleural
edge with X-ray tangential to 2 different densities)
Note hazy, fuzzy superior edge to the pneumonia,
because it doesn’t involve the whole RML up to
minor (horizontal) fissure. If it had, it would have
had a sharp top edge shadow also.
Following PA CXR shows left base opacity behind heart
(retrocardiac lucency is absent), and expected edge shadows
of left hemidiaphragm, descending aorta, and lower lobe
pulmonary vessels are all absent
Of the big 3 basilar opacity diagnoses, this is atelectasis
because there are associated signs of volume loss
(mediastinum shifted left, left hemidiaphragm elevated)
Leftward shift of mediastinum is real and not because of
rotation (spinous processes are midline between medial
heads of clavicles)
This post-operative patient had a mucous plug occluding the
left lower lobe bronchus (note the post-op free air under the
right hemidiaphragm)
Atelectasis and lung collapse mean the same
thing (airlessness and loss of volume of a piece of
lung)
Atelectasis (collapse) may involve a subsegment
of lung, a whole lobe, or even a whole lung)
Atelectasis (collapse) is different from
pneumothorax, an example of which is on the
following CXR
Although lay language may call a pneumothorax a
collapsed lung, medically atelectasis (collapse)
does not imply any air in the pleural space
On the following lateral CXR, not the
posterior retrocardiac basilar opacity which
obscures one hemidiaphragm edge shadow
The obscured hemidiaphragm is the left
(identified because of the bowel under it)
This is left lower lobe atelectasis (collapse)
because of the associated volume loss
(elevated left hemidiaphragm)
This is the lateral CXR that goes with the PA
CXR of LLL collapse already shown
The 2 following images are a normal 2view CXR
Note the normal basilar/retrocardiac
lucency and normal hemidiaphragm edge
shadows
Following 2-view CXR is of same patient
who had the preceding normal CXR, but at
a later time
Note the left base retrocardiac opacity,
loss of LLL edge shadows, and volume
loss on left
This is LLL collapse due to mucous plug in
asthmatic
Following 2-view CXR is same asthmatic
patient on a different ED visit
Note right base retrocardiac opacity, loss
of RLL edge shadows, and volume loss on
right
This is RLL collapse
Following CXR shows opacity medially at
apex of right chest
Note sharp lateral edge of the opacity
suggesting a pleural surface tangential to
X-ray, an elevated minor fissure
There is associated elevation of right
hemidiaphragm
This is case of RUL collapse
Following 2-view CXR show an opacity
adjacent to right heart with obscuration of
right heart border
This is RML collapse because lateral shows
two sharp pleural edge shadows (major and
minor fissures) which have moved close to
each other as the RML between them has
collapsed
The wide mediastinum in this case is just due
to a tortuous aorta in an elderly patient
Following 2-view CXR shows a left chest
opacity
Left heart border is obscured and
retrocardiac lucency is preserved, indicating
that opacity is anterior in location of LUL
Note sharp edge shadow of left major fissure
on lateral
Leftward shift of mediastinum indicates LUL
collapse (due to lung CA, see narrowing of
trachea and left mainstem bronchus due to
adjacent adenopathy)
Following case shows complete homogeneous
opacity of left hemithorax (no, patient did not have
a pneumonectomy)
Differential diagnosis is between a massive pleural
effusion (so large that it compresses underlying
lung) and a completely collapsed left lung
Volume loss on left (mediastinal shift, elevation of
left hemidiaphragm) indicates complete collapse of
left lung
Massive pleural effusion takes up space and
would shift mediastinum to right.
Following CXR shows complete opacity of
the lower 2/3 of right chest
The opacity forms a sharp edge shadow with
the RUL and extends lateral to the RUL
The sharp edge shadow indicates a pleural
surface, placing the opacity outside lung, in
pleural space
This is a large pleural effusion, showing what
is essentially a high meniscus.
Following CXR is same patient as preceding,
but following a thoracentesis
Right effusion is much smaller, but not gone,
and there is now pneumothorax as well, a
hydropneumthorax (note the air-fluid levels)
In spite of drainage of most of the effusion,
there is still nodular thickening of the right
pleural surfaces, secondary to tumor implants
in this patient with malignant mesothelioma,
secondary to prior asbestos exposure
Another CXR on same patient shows
progression of mesothelioma encasing
entire right lung (over a year later)
Following CXR on 70-year-old female
patient who complains of shortness of
breath climbing one flight of stairs,
worsening over last couple weeks. Had
been smoker until 5 years ago when she
had small MI
History suggests CHF
Findings of CHF on CXR in general
– Cardiomegaly (width of heart greater than 50% of width of lungs
at widest point, on standard 6-foot upright PA CXR with good
inspiration and not rotated)
This is actually assessment of cardiac silhouette, so remember
possibility of pericardial effusion
Don’t apply 50% rule without allowing for any non-standard factors
– Pleural effusions
– Pulmonary vessel enlargement (especially upper lobe vessels on
upright CXR)
– Pulmonary edema
Interstitial edema (Kerley lines, peribronchial cuffing, fuzzy vessels)
Alveolar edema (symmetrical air-space infiltrates, diffuse or perihilar/bat
wing)
On this patient’s CXR (standard upright PA),
cardiac silhouette size is borderline (50%)
She has no visible pleural effusion
No visible pulmonary edema (not surprising
since she is only symptomatic with exercise)
However, she does have upper lobe vessel
enlargement (compare to following CXR
which patient had done 2 months before she
became symptomatic)
Following image is magnification of upper
lobe vessels when patient was
asymptomatic
Note typically thin upper lobe vessels seen
on upright CXR
Following image is magnification of upper
lobe vessels when patient was
symptomatic
The same upper lobe vessels are now
much more dilated
Patient has mild CHF, although not
pulmonary edema
Following CXR and 2 magnified views shows patient with
CHF and interstitial pulmonary edema
Be careful about calling a large cardiac silhouette on this AP
supine CXR, but allowing for non-standard factors,
considering that patient is thin and has taken a very good
inspiration, the silhouette is large
The images show a good example of Kerley lines
– Kerley B lines are lines measuring no more than about a cm,
oriented perpendicular to the pleural surface at the edge of the
lung
– The lines are most visible inferiorly (hydrostatic pressure)
– Kerley B lines are thickened interlobular septa, separating the
secondary lobules of lung (small ~1cm subunits of lung)
– The septa are thickened because of edema, and dilatation of
lymphatics and pulmonary venules which run in the septa