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Chest X-ray quality
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Tutorial introduction
Before interpreting a chest X-ray it is important to assess the quality
of the image. Without this step you may diagnose disease that is not
genuine or you may be wrongly reassured. This tutorial covers the principles of chest X-ray quality and discusses
the limitations of sub-optimal images. Anatomical inclusion,
projection, rotation, inspiration/lung volume, penetration and
artifact all contribute to image quality. Each are discussed in turn
Discarding/repeating images
If the image is not of best quality but the clinical question can still be
answered, a chest X-ray need not be repeated. If you are not sure if
a repeat image will be of use then discuss the case with a
radiographer or radiologist. Do not discard a chest X-ray because it is not perfect. Even sub-
optimal images demonstrate life-threatening abnormalities, which
may require your immediate attention.
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Tutorial Key Points
Image quality influences interpretation
Check the image for - Inclusion, Projection,Rotation, Inspiration, Penetration and Artifact
Quality is influenced by radiographic techniqueand patient factors
Does a poor quality X-ray answer the clinicalquestion?
Does a poor quality X-ray demonstrate a lifethreatening abnormality?
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Inclusion
Key points Is all necessary anatomy
included?
Can the clinicalquestion still be
answered?
Image quality - anatomy
inclusion
First ribs?
Costophrenic angles?
Lateral edges of ribs?
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Inclusion
A chest X-ray should include the entire
thoracic cage. Occasionally, important
anatomical structures are not included. If the
clinical question can still be answered then
acquiring another image is not always
necessary.
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Inclusion
Image quality - anatomy
inclusion
First ribs?
Costophrenic angles?
Lateral edges of ribs?
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Projection
Key points
Posterior-Anterior (PA) is the standard projection
PA projection is not always possible
Both PA and AP views are viewed as if looking at the
patient from the front
PA views are of higher quality and more accurately
assess heart size than AP images
If an AP projection is performed, ask yourself if theclinical question can still be answered
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Posterior-Anterior (PA) projection
The standard chest radiograph is acquiredwith the patient standing up, and with the X-
ray beam passing through the patient fromPosterior to Anterior (PA).
The chest X-ray image produced is viewed as if
looking at the patient from the front, face-to-face. The heart is on the right side of theimage as you look at it.
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Projection
PA projection
X-rays pass from the posterior to the anterior ofthe patient - hence Posterior-Anterior (PA)
projection. The image is viewed as if looking at the
patient face-to-face.
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Projection
AP projection
X-rays pass from the anterior to the posterior of the
patient - hence Anterior-Posterior (AP) projection. The
image is still viewed as if looking at the patient face-to-face.
This is usually because the patient is too unwell to stand
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Projection
v eart s ze
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v - eart s ze
The heart, being an anterior structure within the chest, is magnified by
an AP view. Magnification is exaggerated further by the shorter distance
between the X-ray source and the patient, often required when
acquiring an AP image. This leads to a more divergent beam to coverthe same anatomical field.
As a rule of thumb, you should never consider the heart size to be
enlarged if the projection used is AP. If however the heart size is normal
on an AP view, then you cansay it is notenlarged
AP v PA projection
The upper diagram shows an AP projection. Heart size is exaggerated
because the heart is relatively farther from the detector, and also
because the X-ray beam is more divergent as the source is nearer the
patient.
The lower diagram shows a conventional PA projection. The apparent
heart size is nearer to the real size, as the heart is relatively nearer the
detector. Magnification of the heart is also minimised by use of a
narrower beam, produced by the increased distance between thesource and the atient.
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Projection
AP PA S l d
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AP v PA - Scapular edges
Radiographers will often label a chest X-ray as either PA or AP. If the image is
not labelled, it is usually fair to assume it is a standard PA view. If, however, you
are not sure, then look at the medial edges of each scapula.
AP projection - example AP projection images are of lower quality than PA images. Compare this image
with the PA view below.
The image has been acquired by a mobile X-ray unit in the resuscitation room.
Note the AP SITTING label.
The scapulae are not retracted laterally and they remain projected over each
lung.
Heart size is exaggerated (cardiothoracic ratio approximately 50%). If seen on a
PA image this would be at the borderline for cardiac enlargement.
The radiograph was repeated - see below.
PA projection - example
This PA X-ray is of the same patient as the image above.
The edges of the scapulae are retracted laterally with only a small portion
projected over each lung. The lungs are therefore more easily seen.
The cardiothoracic ratio is clearly well within the normal limit of 50%.
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Rotation
Key points Check for rotation
If there is rotation ask -
does it matter?
Rotation may lead to
misinterpretation of heart
contours, tracheal position
and lung appearances
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Rotation
If the patient is very rotated and you do not recognise this,certain appearances may become misleading.
Principles of rotation The spinous processes of the thoracic vertebrae are in the
midline at the back of the chest. They should form a verticalline that lies equidistant from the medial ends of theclavicles, which are at the front of the chest. Rotation of the
patient will lead to off-setting of the spinous processes sothey lie nearer one clavicle than the other.
Does rotation matter ?
If the patient is rotated then interpretation may becomedifficult. Firstly, it may be difficult to know if the trachea is
deviated to one side by a disease process. It also becomesdifficult to comment accurately on the heart size. Changes inlung density due to asymmetry of overlying soft-tissue maybe incorrectly interpreted as lung disease.
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Rotation
Well centred PA chest X-ray Find the medial ends of the
clavicles
Find the vertebral spinous
processes
The spinous processes
should lie half way
between the medial endsof the clavicles
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Inspiration and lung volume
Key points
Always assess inspiration and lung volumes
Incomplete inspiration can lead to exaggeration of lung
markings and heart size
Lung hyperexpansion is a sign of obstructive lung diseaseAssessing inspiration
To assess the degree of inspiration it is conventional to
count ribs down to the diaphragm. The diaphragm should
be intersected by the 5th to 7th anterior ribs in the mid-clavicular line. Less is a sign of incomplete inspiration.
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Inspiration and lung volume
Chest X-rays are conventionally acquired in the
inspiratory phase of the respiratory cycle. Theradiographer asks the patient to, 'breathe in andhold your breath!' Patients who are short ofbreath, or those who are unable to follow the
instructions may find this difficult. When interpreting a chest X-ray it is important to
recognise if there has been incompleteinspiration. If the image is acquired in the
expiratory phase, the lungs are relatively airlessand their density is increased. Also, the raisedposition of the diaphragm leads to exaggerationof heart size, and obscuration of the lung bases.
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Inspiration and lung volume
Expiration
Anteriorly only the third rib intersects the diaphragm at the
mid-clavicular line
The lung bases are white - Is there consolidation?
How big is the heart?
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Inspiration and lung volume
InspirationAnteriorly the sixth rib intersects the diaphragm at the
midclavicular line
The lungs are not consolidated
The heart size is clearly normal
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Assessing for hyperexpansion
Normal expansion
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Assessing for hyperexpansion
While checking for adequate inspiration you may notice that a
patient's lungs are hyperexpanded (>7th anterior rib intersecting
the diaphragm at the mid-clavicular line). This is a sign ofobstructive airways disease.
It is possible to assess for hyperexpansion by counting ribs, or by
checking for flattening of the hemidiaphragms.
Normal expansion
This patient has taken a good breath in such that the diaphragm is
intersected by the 6th rib in the mid-clavicular line.
The hover over image shows an imaginary line (dotted) between
the costophrenic and cardiophrenic angles. The distance betweenthis line and the diaphragm (green line) should be greater than
1.5cm(asterisk) in normal individuals. In practice this is rarely
measured and a quick assessment of diaphragm shape is all that
is necessary.
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Assessing for hyperexpansion
Hyperexpansion
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Hyperexpansion
It is often quicker to assess for hyperexpansion
by looking at the hemidiaphragms. These are
clearly flattened (red line) in this patient.
The ribs are difficult to count as they have lost
density. This is due to long term steroid
treatment for the patient's emphysema.
There is also consolidation of the lung bases
due to pneumonia.
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Penetration
Penetration is the degree to which X-rays have passed
through the body
Digital correction may compensate for an incorrectly
penetrated X-ray
Always check the structures behind the heart
A well penetrated chest X-ray is one where the
vertebrae are just visible behind the heart
The left hemidiaphragm should be visible to the edgeof the spine
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Penetration
Penetration is the degree to which X-rays have passed through
the body. Assessment of penetration is traditionally a standard
part of assuring chest X-ray quality. With modern digital systemsover or under penetrated/exposed images are rarely a problem.
Image data can be 'windowed' to optimise visibility of anatomical
structures. This is often performed by radiographers after they
have acquired the image or can be performed using web-basedimaging software on the wards.
A well penetrated chest X-ray is one where the vertebrae are just
visible behind the heart. Although X-rays are still occasionally
over or under exposed, a discussion of penetration now best
serves as a reminder to check behind the heart. The left
hemidiaphragm should be visible to the edge of the spine. Loss of
the hemidiaphragm contour or of the paravertebral tissue lines
may be due to lung or mediastinal pathology.
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Penetration
Under penetration
The left hemidiaphragm
is not visible to the spine
Lung tissue behind theheart cannot be assessed
Re-windowing the image
using digital software can
compensate
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Penetration
Re-windowing The diaphragm (long
arrows) is visible to the
spine.
The left paravertebral
soft tissues are visible
(short arrows) , and the
right side of the spine is
clear (arrowheads).
There is no abnormality
of lung tissue behind the
heart.
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Artifact
Key points
Some artifacts are unavoidable
Kind of artifact : Radiographic artifact, Patient
artifact, Medical/surgical artifact
A chest X-ray may be obtained to assess
position of medical devices
Ask yourself if artifact limits image
interpretation
Can the question clinical question still be
answered?
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if
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Artifact
Neck surgical emphysema?/