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Med-Challenger EM Career
Comprehensive Review and Assessment for
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Issue: November 15, 2006
Case: Homogeneous Densities
Taken from
Challenger Acute Care
Radiology
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Question:
This adolescent chest x-ray (see the illustration at
right) presents a rather
dramatic appearance. We can see what appears to be a significant
density of the left hemithorax when compared to the right. The
right costophrenic angle has been omitted, but this is not
relevant to this illustration. The point that is being discussed
here is: is the right side pathologically lucent from air
trapping or bullous lung disease? Or is the left side
pathologically dense from some cause of uniform density (e.g.,
partial atelectasis with obstruction of the left main stem
bronchus)?
However, before we embark upon this diagnostic search, does this
x-ray show any abnormality? If so, is it indicative of true
pathology, positioning of the patient, or both? (More than one
answer may be correct.)
Answer Options:
- There is evidence that this asymmetry is due to a
positioning/technique-only error.
- This x-ray is normal.
- There is evidence on this film of intrathoracic
pathology.
- There is a positioning/technique error on this film.
Answer:
C, D
Remediation:
If we look at the
extrathoracic soft tissue structures around the left shoulder we
see a density extending well beyond the thoracic cavity. The
axilla and the shoulder appear to be quite dense on the left
side when compared to the right. What we are seeing here is an
example of a phenomenon referred to as "grid cutoff."
In other words, if the central beam of the x-ray is not directed
at the mid-portion of the sternum and is off to the right or
left, these x-ray beams will penetrate one side of the chest
directly and more obliquely on the other side. The side where
the x-ray beams penetrate directly will appear more lucent, as
we see on this patient's right. The more oblique x-rays will be
partially blocked by the x-ray grid, causing the appearance of
additional density on the left. When x-rayed again with proper
technique to avoid the grid cutoff phenomenon, this child had
symmetrically penetrated right and left hemithoraces. When in
doubt regarding the source of asymmetry on a chest x-ray, a
repeat film can often resolve the issue.
Before we sign off on this film, let's try to determine if any
of the lucency-density discrepancy might be real -- and not just
artifacts from grid cutoff.
If this is an upright film, the density on the left hemithorax
could be atelectasis of the left lung. The atelectasis would be
partial and possibly from some partial endobronchial obstruction
of the left main-stem bronchus. There is subtle elevation of the
left hemidiaphragm but no mediastinal shift.
About the Image(s):
Image 1:
Asymmetric Penetration Due to
the Grid Cutoff Phenomenon Versus Atelectasis
This x-ray demonstrates asymmetric penetration of the
two hemithoraces due to the grid cutoff phenomenon. If the
central beam of the x-ray is not centered on the patient, the
x-ray beams will penetrate one side of the chest directly and
more obliquely on the other side. The more oblique x-rays will
be partially blocked by the x-ray grid, causing the appearance
of additional density. When x-rayed again with proper technique
to avoid the grid cutoff phenomenon, this patient's asymmetry
disappeared.
Before we sign off on this film, let's try to determine if any
of the lucency-density discrepancy might be real -- and not just
artifacts from grid cutoff.
If this is an upright film, the density on the left hemithorax
could be atelectasis of the left lung. The atelectasis would be
partial and possibly from some partial endobronchial obstruction
of the left main-stem bronchus. There is subtle elevation of the
left hemidiaphragm but no mediastinal shift.
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