Chest (AP erect view)

The erect anteroposterior chest view is performed with the x-ray tube anteriorly, firing photons through the patient to form the image on a detector positioned behind the patient. A detector can be positioned behind a relatively immobile patient. 

Indications

The erect anteroposterior chest view is an alternative to the PA view when the patient is too unwell to tolerate standing or leaving the bed ¹. The AP view examines the lungs, bony thoracic cavity, mediastinum, and great vessels. This particular projection is often used frequently to aid diagnosis of acute and chronic conditions in intensive care units and wards. The AP view is of lesser quality than the PA view for many reasons, yet sometimes it is the only imaging available to that patient.

It is important to note that the AP projection will produce a magnified mediastinal shadow due to the increased distance of the heart from the image receptor and beam divergence (see figure 3 AP supine and figure 4 PA projection of the same patient).

Patient position

• patient is upright as possible with their back against the image receptor
• the chin is raised as to be out of the image field
• if possible, the hands are placed by the patient's side
• shoulders are depressed to move the clavicles below the lung apices

Technical factors

• anteroposterior projection
• suspended inspiration  
• centring point
  • the level of the 7th thoracic vertebra, approximately 7 cm below the jugular notch of the sternum
  • the central ray is angled to be perpendicular to the long axis of the patient's sternum generally resulting in a caudal angle
• collimation
  • superiorly 5 cm above the shoulder joint to allow proper visualisation of the upper airways
  • inferior to the inferior border of the 12th rib
  • 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 (this may be departmentally dependent)

Image technical evaluation

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

• three posterior ribs should be seen above the superior aspect of the clavicle
• the chin should not be superimposing any structures
• sternoclavicular joints are equal distant apart
• the clavicle is in the same horizontal plane
• a minimum of eight posterior ribs are visualised above the diaphragm
• the ribs and thoracic cage are seen only faintly over the heart
• clear vascular markings of the lungs should be visible

Practical points

This projection can be very challenging in emergency situations; clear communication is the key to ensuring your patient gets the best image possible under the situation at hand.

The AP view, although a supplementary projection for the PA comes with a broad range of technically challenging factors and is hence inferior.

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

Positioning

By and large, AP erect patients are quite unwell and may not have the ability to aid the radiographer in positioning. The projection can be done in the wards using a mobile machine or in the general rooms using a portable image receptor.

When sitting the patient up for the x-ray, it is important to explain not only to the patient but the staff around; that you will be sitting that patient quite upright for their radiograph, often this will require aid.

There will be occasions where the patient cannot hold themselves upright, in these situations, it is not uncommon to wedge pillows in between the patient and the bedside to prop them up or ultimately resort to a supine chest projection.

Rotation of a chest radiograph can simulate common pathology processes and make it hard to produce a pertinent diagnosis.

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 away from the image receptor to correct rotation.  

Angulation and collimation

As a general rule, there will be a rough 10-15° caudal angle although in heavily kyphotic patients no angle may be required.

If the clavicles are projected overly inferior less angle is required, if the clavicles are projected above the apices more angle should be applied to obtain a diagnostic image.

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

Phase of respiration

The phase of respiration has a profound effect on the appearance of several structures on the chest radiograph. A poor-inspiratory AP radiograph can mimic pathology. Structures that can appear different on expiration include:

• heart size
• mediastinal contours and width
• lung inflation
• diaphragm contours

The AP 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.

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!