How to read chest x-rays

by  Ozlem Koksal

Introduction

Chest X-ray interpretation is one of the fundamental skills of every doctor. Emergency physicians are particularly exposed to various chest x-rays during a regular shift. Therefore, knowing the basics and pathologies in the ED setting is very important. This chapter will summarize the basics of chest x-ray interpretation and give some pathologic examples.

There are 3 types of chest films;

  • AnteroPosterior (AP)
  • PosteroAnterior (PA)
  • Lateral

The ideal timing can be defined as the end of inspiration, and the patient should hold his breath at that time. Meanwhile, the X-ray tube should be 180 cm away. Unfortunately, the majority of the patients may not fit the ideal situation because of their acute problems. Emergency physicians interpret many portable (bedside) anteroposterior chest x-rays with poor quality, without lateral views to make the diagnosis. The image quality is one of the most important things in image interpretation.

Assessing The Image Quality, “RIPE” mnemonic is used; Rotation, Inspiration, Position, Exposure(Penetration).

  • Rotation: The clavicles should appear symmetrical and be seen as equal length. The distance between the thoracic spinal process and clavicular heads should be equal (Figure-1). If there is a rotation, mediastinum may look abnormal.

Figure-1: The clavicular heads and spinous process alignment. The x-ray shows minimal rotation. Compare X and Y.

336.1 - PA chest x-ray - Rotation

 

  • Inspiration: On good inspiration, the diaphragm should be seen at the level of the 8th – 10th posterior rib or 5th – 6th anterior rib.

Figure 2 – The chest x-ray shows adequate inspiration.

336.2 - normal PA chest x-ray - inspiration till 11th posterior rib

 

  • Position: PA, AP, or lateral view? The standard chest X-Rays consists of a PA and lateral chest X-Ray.

The normal lateral chest x-ray view is obtained with the left chest against the cassette. If the x-ray is a true lateral, the right ribs are larger due to magnification and usually projected posteriorly to the left ribs (Figure-3).

Figure-3: The right ribs (red arrows) and left ribs (green arrows) on the lateral chest X-Ray.

337.3 - normal lateral chest x-ray showing right and left ribs

On the AP film, the chest has a different appearance. The heart and mediastinal shadow are magnified because of anterior structures, mainly sternum. This view is taken mostly at the bedside as portable. Some patients are at semi-erect or supine position. Therefore, mediastinal structures are widened because of gravity.

The pulmonary vasculature is altered when patients are examined in the supine position. The size of the pulmonary vasculature is more homogeneous throughout the upper and the lower lobes. (Figure-4 and 5). Supine views are less useful and should be reserved for critical patients who cannot stand erect position.

Figure-4: The normal X-Ray film.

336 - normal PA chest x-ray

Figure-5: The AP X-Ray shows magnification of the heart and widening of the mediastinum.

375 - pneumonia other

 

  • Exposure / Penetration: Ideally, you should be able to see the heart, the blood vessels, and the intervertebral spaces. Exposure should be adequate if you are able to see approximately T4 vertebra and spinal process. If the film is underexposed, you will not be able to see them (Figure-6). If the film is overexposed, details of bone structures will be lost (Figure-7).

Figure-6: Underexposed PA X-Ray film.You can not appreciate thoracic vertebras.

71.1 - AD1

Figure-7: Overexposed PA X-Ray film. You are able to see all vertebral bodies with obvious intervertebral spaces.

337.4 - PA chest x-ray overexposure

Interpretation

The interpretation of a chest X-Ray should be approached systematically. For chest X-Rays, there is a classic schematic: “ABCDEF.” You should first check the patient’s name and date of the film. You should also check the side marker, and the film position (PA or AP). Finally, you should check patient’s position such as supine, erect or semi-erect.

The analysis is ABCDEF:

  • Airways
  • Bones
  • Cardiac
  • Diaphragm
  • Extrathoracic tissues
  • Fields and Fissures

A – AIRWAY

The trachea, carina and both main bronchi are called the upper airway and should all be visible on an AP view (Figure-8). Look for if there is any deviation of the trachea away from the midline. Introduction of air into one side of the chest cavity will cause that side of the lung to collapse. The collapsed lung will push the trachea to the opposite side and resulting in a deviation that will show up on chest X-Ray.

Figure-8: Airway structures on the chest X-Ray. (Red Arrows: trachea, Green Arrow: carina, Pink Arrows: left and right main bronchus)

336.3 - normal PA chest x-ray AIRWAY STRUCTURES

 

B – BONES

A chest X-Ray provides a good view to look for ribs and clavicle fractures. Clavicular fractures are usually at the middle 3rd of the clavicle, which is easy to see in chest X-Rays. Rib fractures, however, can sometimes be hard to see. Each rib should be followed across its length to look for fracture lines or step-offs that could indicate a fracture.

Hyperinflated lungs are seen as the result of chronic obstructive pulmonary disease where the patient is unable to fully expel the air that is inhaled with every breath. Because of this, overinflation will result in a greater number of ribs that can be visible on the chest X-Rays. Normally, 8-10 ribs are expected to be seen on the chest X-Ray (Figure-9).

Figure-9: Bone structures on the PA chest X-Ray. (Numbers: ribs, red dashed line and arrows: clavicle, yellow dashed line and arrows: medial border of scapula, green dashed line and arrows: 3rd rib, pink dashed line: vertebras)

336.4 - normal PA chest x-ray - BONY STRUCTURES

C –  CARDIAC

This part involves the heart and surrounding structures. The silhouette of the heart should be identified, and the heart borders should be clear. As a general rule, the heart base should not be wider than 1/2 the total width of the diaphragm. If the heart base is 1/2 the width of the diaphragm on the chest X-Ray, it refers to cardiomegaly or pericardial effusion.

The aortic arch and the left pulmonary artery should be visible as two semi-circles above the left atrium. There is a space called the “Aortopulmonary Window” that has the following borders: ascending aortic arch (anterior), descending aortic arch (posterior), left pulmonary artery (inferior), inferior border of aortic arch (superior). The window should be “concave” in the lateral border (Figure-10). If it is not, mediastinal lymphadenopathy and aorta/pulmonary artery aneurysms are possible. The left hilar point is slightly higher than the right hilar point. The hilar point should be at the level of the lateral extent of the right 6th rib. The inferior vena cava lies end of the right cardiophrenic angle.
The structures should be visible behind the heart especially the spine, paraspinal region and azygoesophageal line.

In ideal circumstances, mediastinum is maximum 6 cm in a PA chest x-ray, and further investigation is considered if it is more than 8 cm.

Figure-10: Heart borders on the AP chest X-Ray. (Pink dashed lines and arrows: heart borders, Yellow dashed line and arrow: Aortic Arch, Blue circle, and arrow: Aortopulmonary Window)

336.5 - normal PA chest x-ray HEART BORDERS

D – DIAPHRAGM

The outline of the diaphragm should be clear and smooth. Right hemidiaphragm should be higher than the left (Figure-11). It has 3 major characteristics that can be found on chest X-Ray:

  1. The gastric air bubble on the left.
  2. The diaphragmatic contour looks like a “dome” shape, and the right side located little higher than the left.
  3. The costophrenic angle is the lateral point of attachment for the diaphragm, and it should be a clear, sharp, and a triangle-shaped at either end. If the angle is closer to 90 degrees, then the lungs could be hyperexpanded (e.g., COPD) and be pushing the diaphragm down into the abdomen. If the costophrenic angle is blunting, that usually is indicative of pleural effusion.

Figure-11: The view of the diaphragm on the AP chest X-Ray. (Yellow dashed lines and arrows: diaphragm, red arrow: gastric air bubble, pink dashed lines: costophrenic angles)

336.6 - normal PA chest x-ray DIAPHRAGMA STRUCTURES

 

E – EXTRATHORACIC TISSUES

Mostly this means as the lung parenchyma. Lung fields can be divided into zones: upper, middle, and lower zones (Figure-12);

  • Upper zone: from the apex to 2nd costal cartilage.
  • Middle zone: between 2nd and 4th costal cartilage.
  • Lower zone: between 4th and 6th costal cartilage.

So you should compare the lung parenchyma left to right in the upper, middle and lower zones and see whether there is a difference.

Look for equal radiolucency between the left and the right lungs zones. The horizontal fissure on the right divides the upper and middle lobes; from the hilum to the 6th rib at the axillary line.

You should also check soft tissues outside the thorax for subcutaneous air, foreign body, bizarre density, etc.

Figure-12: Radiological lung zones.

336.7 - normal PA chest x-ray - ZONES

 

F – FIELDS AND FISSURES

You should check lung fields for infiltrates. Identify the location of infiltrates and identify the pattern of infiltration (interstitial or alveolar pattern). Look for air bronchograms, nodules, Kerley B lines. Pay attention to the apices. You should also check for masses, consolidation, pneumothorax and vascular markings. Vessels should be almost invisible at the lung periphery. Finally, you should evaluate the major and minor fissures for fluid collection (Figure-13).

Figure-13: Minor (A) and major (B) fissures of the lung.

337.5 - normal PA chest x-ray FISSURES 1 Minor (A) and Major (B) 337.6 - normal lateral chest x-ray FISSURES 2 Minor (A) and Major (B)

 

Please visit our Flickr channel to see various chest x-ray pathologies.

 

References and Further Reading