Chest Radiography: AP vs. PA Projections

1. The Two Standard Projections

Posteroanterior (PA) - The Standard View

• Focal spot-to-film distance: 72 inches (183 cm) - this long distance minimizes magnification
• kVp: ~120-130 kVp (higher energy reduces motion blur and improves vascular detail)
• Position: Patient erect, full inspiration, breath held
• Scapulae: Rotated forward and out of the lung fields by placing hands on hips and rolling shoulders forward

• The heart and anterior mediastinal structures are close to the detector, so they are minimally magnified
• Accurate cardiothoracic ratio (CTR) assessment is only reliable on an erect PA film
• Normal CTR on PA: less than 0.5 (heart width < half of thoracic width)

Anteroposterior (AP) - The Portable/Supine View

• Focal spot-to-film distance: typically 40 inches (portable units)
• kVp: ~100 kVp (lower energy, shorter exposure distance)
• Position: Supine or semi-recumbent (erect AP if possible)

2. Assessing Radiographic Quality (RIPE)

R - Rotation

• The medial ends of the clavicles should be equidistant from the spinous processes of the vertebrae
• Rotation causes apparent mediastinal widening and asymmetric lung density

I - Inspiration

• Adequate inspiration: the anterior end of the 6th rib (or posterior end of the 10th rib) should intersect the right hemidiaphragm at the midclavicular line
• Poor inspiration elevates the diaphragm, crowding the lung bases and exaggerating cardiomegaly

P - Projection

• Identify whether AP or PA - an AP label is usually present; if not, look for magnification of the heart or scapular overlap over the lungs

E - Exposure

• On a correctly exposed film, the thoracic vertebrae should be faintly visible through the cardiac shadow
• Overexposure: lung fields appear too dark; interstitial markings may be lost
• Underexposure: lungs appear too white; fat pads at the cardiophrenic angles may be misread as cardiomegaly

3. Systematic Interpretation - Structures to Evaluate

Trachea and Mediastinum

• Trachea: midline or slightly right of center above the carina
• Tracheal deviation away from pathology: tension pneumothorax, large pleural effusion
• Tracheal deviation toward pathology: lung collapse, post-pneumonectomy fibrosis
• Carina angle: normally <70 degrees; widening suggests left atrial enlargement

Heart and Cardiac Borders

• Right heart border: right atrium (lower), superior vena cava (upper)
• Left heart border: aortic knuckle (top), pulmonary trunk, left atrial appendage, left ventricle (lower)
• Cardiothoracic ratio: reliably assessed only on erect PA; ratio >0.5 suggests cardiomegaly (but see AP caveats above)

Hila

• Left hilum is normally 0.5-1.5 cm higher than the right
• Hilar enlargement: lymphadenopathy, pulmonary artery dilation
• Hilar depression: ipsilateral lower lobe collapse

Lung Fields

• Consolidation: airspace opacification that respects lobar boundaries; air bronchograms may be visible
• Collapse: loss of volume + shift of fissures, mediastinum, or diaphragm
• Nodules/masses
• Pneumothorax: absence of lung markings peripheral to a visible pleural line

Pleural Spaces

• Free pleural fluid: on an upright PA, 200-250 mL minimum is needed to see blunting of the lateral costophrenic angle
• A lateral radiograph may detect as little as 50-75 mL
• Classic meniscus: concave upper border, higher laterally than medially

Diaphragm

• Right hemidiaphragm is normally higher than the left by ~15 mm (up to 30 mm is acceptable)
• Normal level: anterior end of the 5th or 6th rib interspace at the midlung field
• Flattening: emphysema, acute severe asthma
• Unilateral elevation: phrenic nerve palsy, adjacent pulmonary/pleural/subphrenic disease
• Bilateral elevation: supine position, poor inspiration, obesity, pregnancy, ascites

Bones and Soft Tissues

• Ribs: count carefully for fractures; first and second rib fractures suggest major force and raise concern for aortic injury
• Clavicles, scapulae, vertebrae
• Soft tissues: surgical emphysema, masses, breast shadows

4. AP-Specific Pitfalls in Trauma and the ICU

• Mediastinal widening on AP/supine does not necessarily indicate aortic injury - portable AP magnification and patient positioning routinely produce a wider-appearing mediastinum. Always try to get an erect PA before declaring the mediastinum abnormal.
• Haemothorax on supine AP appears as a homogeneous increase in opacity of the hemitorax - be careful not to confuse the haemothorax (less radiolucent) with a pneumothorax (more radiolucent); look carefully for lung markings.
• Pneumothorax is best seen on an erect PA or lateral decubitus; on a supine AP, air rises anteriorly and may be invisible at the apex - look for deep sulcus sign instead.
• Up to 300 mL of blood may pool behind the diaphragm and may not be visible even on an erect view.

5. Summary Comparison