The chest x-ray is the most common radiological investigation in the emergency department ¹. The PA view is frequently used to aid in diagnosing a range of acute and chronic conditions involving all organs of the thoracic cavity. Additionally, it serves as the most sensitive plain radiograph for the detection of free intraperitoneal gas or pneumoperitoneum in patients with acute abdominal pain.

Patient position

patient is erect facing the upright image receptor, the superior aspect of the receptor is 5 cm above the shoulder joints
the chin is raised as to be out of the image field
shoulders are rotated anteriorly to allow the scapulae to move laterally off the lung fields, and this can be achieved by either:
- hands placed on the posterior aspect of the hips, elbows partially flexed rolling anterior or
- hands are placed around the image receptor in a hugging motion with a focus on the lateral movement of the scapulae
shoulders are depressed to move the clavicles below the lung apices

Technical factors

posteroanterior projection
suspended inspiration
centering point
- the level of the 7^th thoracic vertebra, approximately the inferior angle of the scapulae
collimation
- superiorly 5 cm above the shoulder joint to allow proper visualization of the upper airways
- inferior to the inferior border of the 12^thrib
- lateral to the level of the acromioclavicular joints
orientation
- portrait or landscape
detector size
- 35 cm x 43 cm or 43 cm x 35 cm
exposure
- 100-110 kVp
- 4-8 mAs
SID
- 180 cm
grid
- yes

Image technical evaluation

The entire lung fields should be visible from the apices down to the lateral costophrenic angles.

the chin should not be superimposing any structures
arms are not superimposed over lateral chest wall (this can mimic pleural thickening)
minimal to no superimposition of the scapulae borders on the lung fields
sternoclavicular joints are equidistant from the spinous process
the clavicle is in the same horizontal plane
a maximum of ten posterior ribs are visualized above the diaphragm
The 5^th-7^thanterior ribs should intersect the diaphragm at midclavicular line
the ribs and thoracic cage are seen only faintly over the heart
clear vascular markings of the lungs should be visible

Practical points

The phase of respiration has a profound effect on the appearance of several structures on the chest radiograph (see Case 2 for inspiration and expiration images in the same patient). A poor-inspiratory PA radiograph can mimic pathology. Structures that can appear different on expiration include:

heart size
mediastinal contours and width
lung inflation
diaphragm contours

Rotation of a chest radiograph can simulate common pathological processes and make it hard to produce an appropriate diagnosis.

The PA view is used to investigate a plethora of conditions and it is the radiographer's responsibility to ensure high-quality diagnostic images are achieved consistently.

The sternoclavicular joints are a sound indicator for positional rotation, if one sternoclavicular joint is notably wider than the other, that respected side needs to be rotated toward the image receptor to correct rotation.

Patients with a longstanding history of emphysema or COPD will have abnormally long lungs compared to the general population, remember this when collimating superior to inferior.

Side marker placement is imperative; patients can have congenital conditions that mimic a mirrored image ².

Remember to explain to your patient what you are about to do; that is ask them to take a breath in and hold it. Many times this gives the patient time to prepare and results in a better breath hold and therefore a higher quality radiograph.

Always remember to tell your patient to breathe again!