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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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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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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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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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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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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?/

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