The difference between Chest Posterior Anterior (PA) and Anterior Posterior (AP) radiographs.

5. The chest radiograph assessement

The chest radiograph is assessed to determine if it is a visually sharp reproduction of the anatomy with limited / no distortion of:

  • Lungs
    • Lateral margins, apex and diaphragm
    • Trachea and proximal bronchi
    • Vascular pattern in the whole lung particularly the peripheral vessels
    • Costophrenic angles
    • Retrocardiac lung
  • Heart, mediastinum and aorta
    • Borders of the heart, mediastinum and aorta
  • Bone margins and cortex/trabecular patterns

The X-ray beam is projected onto the detector with the patient in the beam to produce a radiograph

Three things improve the quality of the image in CXR’s

  • The patient needs to be straight (not twisted or leaning)
  • The patient needs to be as close to the detector as possible
  • The X-ray tube needs to be a long way from the patient and detector (it is usually 180 to 200 cms for a CXR

Figures 2 and 3 demonstrates a PA erect projection and diagrams to show a normal projection and how the image may be distorted


Figure 2 The position for a PA radiograph and a diagram showing limited distortion and magnification if the patient is parallel to the detector and X-ray beam is at right angles

We get distortion when the patient is not parallel to the detector or the beam is angled


Figure 3 Distortion of the image with angulation of the patient or X-ray beam

It is easy to get confused by left and right because we view all the images as if the patient is standing facing us as if in the anatomical position. Figure 4


Figure 4 The anatomical position

Figure 5 Radiographs which demonstrates the anatomy which can be recognised on a PA projection


Figure 5 Anatomy labelled on a PA erect Radiographs

We evaluate the image using the 10 point plan (see section in e-learning)

Note. The image should not be blurred

  • No blurring because the patient has breathed or moved during the exposure
  • Use the fine focus (an option on the X-ray equipment to reduce Unsharpness)

The image should not be distorted

  • The PA is distorted because the patient is learning forward 10 degrees and the heart is not parallel to the detector
    • But it is what we are used to looking at so we see this as normal

Patient positioning

  • Patient should not be rotated / angled in relation to the detector
    • Rotated around any plane
    • Angled to the detector
    • Patient as close to the detector as possible and X-ray tube a far away as possible / reasonable

Common positioning problems with AP erect mobile CXR’s


Patient leaning backwards or the tube angled incorrectly (also no anatomical marker)


Patient rotated