Basics of Reading Chest X ray

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Xray is a type of radiography and most widely used investigation. It first appears too complicated to read the chest xrays because we barely know what lies where and what to make out of it. But the basics of Chest Xray here will guide you through various aspects, including Counting ribs, PA vs AP view, Inspiratory vs Expiratory Xray, Erect vs Supine, Lucency and Opacity and some common terms like Consolidation and Pleural Effusion.

Part 1: Basics First

Best Chest Xray

PA vs AP view

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AP vs PA view of Chest Xray

  • AP or Anteroposterior view- The view is from front to back.
  • PA or Posteroanterior view- The view is from back to front

Difference between PA vs AP view Chest Xray

PA vs AP view Chest Xray

Features PA view AP view
Position of clavicle Oblique Horizontal
Scapula Away from lung field Over the lung field
Spirolamina angle Inverted ‘V’ Not significant

PA is most common X-Ray done where AP is usually done when patient cannot stand and XRay machine is brought to him on bed and view taken from anterior to posterior.

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The point to add is that there is apparent Cardiomegaly in AP view as compared to PA view because there is slight magnification of heart since heart is away from view capturing film.

This can be well understood by the following:-

The approach to cardiomegaly on Chest Xray is as follows:

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Chest xray cardiomegaly

  • A/B x 100 = cardio ratio
  • In PA view, Cardiomegaly when ratio is more than 50%
  • In AP view, Cardiomegaly when ratio is more than 60%

Erect vs Supine position

erect vs supine cxrThere is fundal view in erect position because all the air in stomach comes in fundus when the patient is standing.

Inspiratory vs Expiratory

inspiratory vs expiratory chest xrayinspiratory vs expiratory chest xray 2If anterior end of 6th or 7th rib reaches mid-clavicular line of diaphragm, it is Inspiratory Xray.

Counting Ribs in Chest Xray

Counting ribs CXRTwo points can just help you quickly count ribs from top to bottom:

  • The front opaque appearing side of ribs is actually it’s posterior side.
  • Ribs are counted from anterior sides.
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Counting ribs chest xrayBefore we proceed, let us see what structures lie in a normal Chest Xray:

Best Chest XrayThe Chest Xray is usually divided into three zones as:

Zones in Chest xray

  • Upto 2nd rib- First zone
  • 2nd to 4th rib- Second zone
  • 4th to 6th rib- Third zone

Now let’s proceed to start studying the Xray.

Lucency and Opacity in Chest Xray


Anything that appears dark or black on chest xray is said to be lucent.

  • This is because of less density.
  • Black color appears because of AIR.


Anything that appears light or white on chest xray is said to be Opaque.

  • This is because of high density.
  • White color appears because of Bones and soft tissues.

Therefore, we can conclude the following easily:-

Increase in lucency:

  • Increase in air
  • Decrease in soft tissues or absence of bone

Increase in Opacity:

  • Increase in soft tissue or abnormal bone
  • Decrease in air

The basic approach when seeing a chest xray always sequentially as:-

  1. Define whether xray is normal or abnormal
  2. If xray is abnormal, where is this abnormality
  3. Extent of abnormality
  4. What is the final diagnosis

Before we proceed to pathological approaches to Chest X-Rays, let’s see what layers the xrays hit when they enter the body. Note this strengthens further basics:-

Muscle> Ribs> Pleura> Lung

Talking about when Hyperlucency (increase in blackness) or Hyperopacity (increase in whiteness) occurs:-

Unilateral Lung Hyperopacity

Hyperopaque lung

  1. Consolidation- Replacement of air by something abnormal
  2. Atelectasis- Collapse of lung resulting in loss of air

Also seen in Plethora, i.e, increase i vascularity.

The differential diagnosis of three important causes if unilateral (one side) opaque thorax are:-

1. Atelectasis- collapse of lung

  • Displacement of interlobar fissure: because the lobes of lung collapse, the fissures in between the lobes move up or down because of hyperinflation of normal lobe against collapsed lobe. This is the most reliable direct sign of Collapse.
  • Mediastinal shift: The structures on mediastinum shift to side of collapsed lung
  • Crowding of ribs
  • Elevation of hemidiaphragm
  • Sharp defined margins of opacity

2. Consolidation- replacement of air


  • No mediastinal shift
  • Ill defined margins of opacity
  • Airbronchogram sign: visualization of air in bronchus sorrunded by alveolar opacity
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Positive Airbronchogram sign is seen in:

  • All except interstitial (viral) pneumonia
  • Pulmonary oedema (water replace air)
  • ARDS (Acute respiratory distress syndrome)
  • Goodpasture syndrome (blood)
  • HMD (Hyaline membrane disease)
  • Pulmonary alveolar proteinosis (macrophages congested in alveoli making crazy paving pattern)

Airbronchogram sign is NOT seen in:

  • Lung abscess
  • All except bronchoalveolar carcinoma

3. Pleural effusion-accumulation of fluid

Normally, there is no air in pleura. But effusion in pleura can occur.

  • Mediastinal shift: which is on opposite side, i.e, structures shift to opposite side of pleural effusion.

Note: Pleural effusion and Haemothorax cannot be differentiated because soft tissue cannot be differentiated on Chest Xray.

Unilateral Lung Hyperlucency

hyperlucent lung

  • Rotation: apparent increase in air gap
  • Scoliosis
  • Masectomy
  • Poland syndrome (absent pectoralis major muscle)
  • Airway obstruction
  • Large pulmonary embolus
  • Pneumothorax

A small mnemonic to quickly grab the names:


  • P- Poland syndrome/Pneumothorax
  • O- Oligemia/Obstruction (like Pulmonary embolism)
  • E- Emphysema
  • M- Mastectomy/Mucous plug
  • S- Swyer’s james syndrome

Enjoyed reading? In our upcoming blog of Chest xray, we will be explaining Silhoutte sign (where exactly the abnormality is in the lung relating with intervening border of an organ or its part) and some pathological diseases observed on chest xrays like pulmonary embolism, left ventricular failure, bronchogenic carcinoma, bronchiectasis, and lots more. Stay tuned.

Part 2: ABCDEF Approach to Reading Chest X-Ray

Before Analyzing

Before Analyzing Chest Xray, ABCDEF approach for preliminary check can be used as:

A: AP or PA view

B: Body Position

C: Confirm Name of patient on film

D: Date of Xray

E: Exposure adequate?

F: Films for comparison

Analysis of Chest XRay

Again, an ABCDEFGHI approach can be used to recall the steps of Analysis of Chest Xray. It can be related as:

A: Airways- Initially check tracheal deviation, and then check for hilar adenopathy or enlargement

B: Bones / Breast Shadows- Check for fracture of ribs, scapula or clavicle)

C: Cardiac Silhouette- Cardiac Enlargement/ Costophrenic Angles- Check for sharp angles, if blunt could be pleural effusion

D: Diaphragm- Check for free air, could mean perforation peritonitis/ Digestive tract

E: Extra-thoracic tissues / Edges- Apices for fibrosis, pneumothorax, pleural thickening or plaques)

F: Fields- Check for alveolar filling in lung fields / Failure- Alveolar air space disease with prominent vascularity with or without pleural effusions

G: Gastric Bubble- Visible at left upper abdomen

H: Hilum- Check for lymph node enlargement

I: Insertion / Artefacts: Check for any external object appearing in xray film

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