X-ray projections

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 24 October 2026 · Level: Beginner

A projection is the direction the X-ray beam travels through the patient, together with the patient's position. The same chest can look strikingly different on a PA erect film and an AP supine film: the heart changes size, air and fluid move to different places, and the scapulae may cover the lungs. Before you interpret any radiograph you must know which projection you are looking at, because it changes what counts as normal. This page explains how views are named, what each standard projection is for, and the traps each one sets.

1. How projections are named

The convention is simple: name the surface where the beam enters, then the surface where it leaves (and meets the detector).

• PA (posteroanterior): the beam enters through the back and exits through the front. The patient faces the detector.
• AP (anteroposterior): the beam enters through the front. The detector is behind the patient, which is the usual arrangement for a patient in bed.
• Lateral: the beam passes from side to side. A left lateral has the left side against the detector.
• Oblique: the patient or beam is angled between frontal and lateral.

Position is described separately: erect (standing or sitting upright), semi-erect, supine (lying on the back), prone, or decubitus (lying on one side). A horizontal beam means the beam travels parallel to the floor, which is what lets you see a fluid level.

Always check the label. Digital systems usually print "AP", "supine", "erect" or "mobile" on the image, but labels are sometimes missing or wrong, so learn to recognise the projection from the anatomy too.

2. PA versus AP chest

Feature	PA erect	AP (often mobile, semi-erect or supine)
Typical patient	Outpatient or ED patient who can stand	Unwell, ICU, ward or trauma patient
Scapulae	Rotated forward and projected clear of the lungs	Often overlap the upper lungs
Clavicles	Fairly horizontal, projected over the apices	Project higher, often above the apices, and may look more horizontal or tilted
Heart size	Minimal magnification: the cardiothoracic ratio can be used	Magnified: cardiothoracic ratio is not reliable
Distance	Standard source–image distance of about 180 cm	Usually shorter, adding magnification
Inspiration	Usually good	Often poor, which crowds the lung bases and widens the mediastinum

The magnification on an AP film happens because the heart lies in the front of the chest. On a PA film it is close to the detector; on an AP film it is further away and its shadow spreads out. The physics page explains the geometry. The practical rule is: a big heart on an AP film is uncertain, but a normal-sized heart on an AP film is reassuring. Some texts use a higher cut-off of about 0.55 for AP films, but this is not a validated standard.

3. Erect versus supine: where air and fluid go

Gas rises and fluid falls. Knowing where the "top" of the patient was when the film was taken tells you where to look.

Erect

• Pneumothorax: air collects at the apex and laterally. Look for the visceral pleural line at the top of the lung. See pneumothorax.
• Pleural effusion: fluid collects at the base, blunting the costophrenic angle with a meniscus that rises laterally. See pleural effusion.
• Free intraperitoneal gas: collects under the diaphragm. The patient should ideally sit upright for about 10 minutes first so the gas can rise. See pneumoperitoneum.
• Fluid levels are visible because the beam is horizontal, for example in a hydropneumothorax, an abscess or obstructed bowel.

Supine

• Pneumothorax: air rises to the front of the chest and collects anteriorly and inferiorly. The classic apical line may be absent. Look instead for a deep, lucent costophrenic angle (the deep sulcus sign), an unusually sharp heart border or hemidiaphragm, or a lucent upper abdomen. Up to about 30% of pneumothoraces in supine trauma patients are not visible on the chest film [VARIES between studies]; CT and ultrasound are more sensitive.
• Pleural effusion: fluid layers along the back of the chest, producing a hazy veil over the whole hemithorax with vessels still visible through it. There is no meniscus, and a moderate effusion can be easy to miss.
• No fluid levels are seen, because the beam is vertical.

The abdominal X-ray in the UK is taken supine as standard. An erect abdominal film is no longer routine. When perforation is suspected, the right plain film is an erect chest X-ray, and CT is more sensitive. See how to read an abdominal X-ray.

4. The lateral view

A lateral chest film is usually taken as a left lateral (left side against the detector). It is less often requested than it once was, because CT answers most questions, but it still has real value:

• Localising an opacity seen on the PA film to a lobe or compartment.
• Showing the retrosternal and retrocardiac spaces, which are hidden on the frontal view.
• Detecting small effusions: fluid in the posterior costophrenic angle can be seen with smaller volumes than on a PA film (roughly 50–75 mL compared with about 200–300 mL on an erect PA, though quoted figures vary).
• Showing both oblique fissures and lower-lobe disease.

On a normal lateral, the thoracic vertebral bodies get darker towards the bottom, because there is more air-filled lung in front of the lower spine. If the lower vertebrae look whiter instead (the spine sign), suspect disease in a lower lobe or the posterior costophrenic recess. Details are in normal chest X-ray anatomy.

5. Decubitus, horizontal-beam and oblique views

View	How it's taken	Classic use
Lateral decubitus	Patient lies on one side; horizontal beam	Effusion: affected side down, so fluid layers along the dependent chest wall (very small volumes, about 5–10 mL, can be shown). Pneumothorax: affected side up. Largely replaced by ultrasound and CT.
Left lateral decubitus abdomen	Left side down, horizontal beam, after about 10 minutes in position	Free gas rises over the liver when an erect film is impossible. Also used in neonates.
Horizontal-beam (cross-table) lateral	Patient supine; beam passes side to side	Knee lipohaemarthrosis (a fat–fluid level that signals an intra-articular fracture), C-spine in trauma, and free gas in neonates.
Oblique	Patient or beam rotated	Hands and feet, the scaphoid series, and the pars interarticularis of the lumbar spine.
Expiratory chest	Taken in full expiration	Was used to accentuate a pneumothorax. Not routinely recommended by the BTS. May be used for a suspected inhaled foreign body in children (air trapping).

6. Two views at right angles: MSK and spine

A single radiograph hides depth. For bones and joints the standard is at least two orthogonal views (at 90° to each other), usually AP and lateral. This is the first of the "rule of twos" in fracture description: two views, two joints, two sides when needed, two occasions and two opinions.

• Shoulder: an AP alone is never enough. A Y-view (scapular lateral) or an axial view is needed, because a posterior dislocation can look almost normal on the AP.
• Long bones: include the joint above and below, especially in the forearm and lower leg, where a fracture of one bone is often paired with a dislocation or fracture elsewhere (see Monteggia and Galeazzi).
• Scaphoid: a dedicated series with extra angled views, because a scaphoid fracture may be invisible on standard wrist views.
• Cervical spine: lateral, AP and open-mouth peg views, and a swimmer's view if C7/T1 is not shown. In UK adults who meet imaging criteria, NICE recommends CT first. See cervical spine X-ray.

7. Common pitfalls

• Assuming PA when it's AP. Check the label and the scapulae before commenting on heart size or mediastinal width.
• Looking at the apex for a pneumothorax on a supine film. On a supine patient, look at the bases, the heart border and the upper abdomen.
• Calling a supine effusion "consolidation" or missing it. A uniform haze with visible vessels and no air bronchograms suggests layering fluid.
• Accepting one view of a joint. Many fractures and dislocations are seen only on the second view.
• Mixing up the side. The side marker is placed by the radiographer; confirm it against the anatomy (cardiac apex and gastric bubble on the left in most people).

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	Tell PA from AP, explain why the heart looks bigger on AP, and know where air and fluid go in erect and supine patients.
Nurse	Know why a patient may need to sit upright before an erect chest film for free air, and why a mobile film in bed looks different.
Junior doctor	Request the right view for the question (erect CXR for perforation, two views for a joint) and recognise supine signs of pneumothorax.
Radiographer	Correct projection labelling, centring, horizontal-beam technique and when an extra view (Y-view, swimmer's, cross-table lateral) changes management.
Advanced / FRCR	Lateral chest anatomy and the subtle supine pneumothorax signs, including the double diaphragm sign.

Next: check quality with RIPE, then use the system in how to read a chest X-ray.

Practise identifying projections →

References

1. Radiology Masterclass. Chest X-ray quality: projection. radiologymasterclass.co.uk
2. Roberts ME, et al. British Thoracic Society Guideline for pleural disease. Thorax 2023;78(Suppl 3):s1–s42. thorax.bmj.com
3. Radiology Masterclass. Abdominal X-ray introduction. radiologymasterclass.co.uk
4. Radiology Masterclass. Pneumoperitoneum. radiologymasterclass.co.uk
5. NICE. Head injury: assessment and early management (NG232). nice.org.uk
6. Royal College of Radiologists. iRefer. irefer.org.uk

For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.