Radiological zones—an introduction to landmarks

Learn the radiological zones of the chest in order to identify the various anatomical landmarks within them. By the end of this lesson, you will know how to identify the major anatomical regions on a chest x-ray and what exactly you're looking at.

Want to master chest x-ray interpretation? Take our Chest X-ray Essentials course and learn how to interpret chest x-rays like a pro. Your instructor, Dr Julian Dobrowski—an award-winning author and Professor of diagnostic imaging at McMaster University in Ontario, Canada—will guide you through the essentials of chest x-rays.

[00:00:00] What will I gain from this video? After watching this video, you will know how to define and locate the six radiological zones on the PA and lateral chest x-rays. The concept of radiological zones is introduced to facilitate discussion about anatomy in general terms, while you're learning how to accurately localize pathology, to specific anatomical structures. The radiological zones are used to describe anatomical areas.

[00:00:30] As you develop a better understanding of the radiological anatomy of the chest, you will gradually move away from the description of radiological zones, to description of the specific anatomy. Six zones are described on the PA and lateral chest x-rays. There's the mediastinal zone, the hilar zone, cardiac zone, the lung zone, the pleural zone, and the peripheral zone. The mediastinal zone on the PA x-ray is the whiter area, between the medial

[00:01:00] aspects of both lungs. The margins of these zones are clearly seen on the PA x-ray because a significant part of the mediastinum abuts the adjacent lungs and forms interfaces. On the lateral x-ray, the mediastinal zone covers the same area as the pleura and lung zones, from the front of the thorax to the back, and from the top to the bottom. The hilar zones are two medial areas of the thorax that correspond to the location of the proximal pulmonary arteries. On the PA

[00:01:30] x-ray, they appear as distinct white extension, from the central part of the mediastinal zone. On the PA x-ray, the shape of the hilar zone roughly resembles the letter H. On the lateral x-ray, the hilar zone lies centrally within the thorax and contributes to the whiteness that we see adjacent to the lower trachea and the carina. The cardiac zone lies in the anterior and inferior part of the mediastinal zone. On both the PA and lateral x-rays, the outer

[00:02:00] edge of the zones are formed by the heart, pericardium and mediastinal fat. The specific heart chambers, that contribute to the visible edges, will be discussed in the future presentation on the heart. The cardiac zone on the lateral x-ray is identified anteriorly and inferiorly. The lung zone is the area in the thorax that contains the lungs and corresponds to the air-filled region between the ribs and the mediastinum medially. On the PA

[00:02:30] x-ray, the lung zone will have lateral, medial, superior, and inferior borders. On the lateral x-ray, the lung zone has anterior, posterior, superior, and inferior borders. The lung zone occupies a large percentage of the thorax. Because of the close relationship of the pleura to the lungs, the pleural zone roughly corresponds to the lung zone, on the PA and on the lateral x-ray. The peripheral zone is

[00:03:00] identified here, on the PA film, as everything that lies lateral to the pleural zone, between the two red areas. Similarly, on the lateral view, the peripheral zone represents all of the structures that lie lateral to the pleural edge. The reason the concept of zones is being discussed is because of the overlap of the zones anatomically.

[00:03:30] When we think that an abnormality may lie within the lungs, it may actually lie within the pleura or may lie within the chest wall. It is important to remember this overlap. On the lateral view, the overlap is even more complex because the mediastinal, the pleural, and lung zones overlap completely. One of the keys to identification of specific anatomical

[00:04:00] structures, in the chest, is through the recognition of important landmarks. A landmark should be easy to distinguish on a grayscale image. It should be easy to identify and should have consistent features. An anatomical structure, that meets these criteria, is the trachea. Other useful landmarks that we will discuss in further presentations are the 4th thoracic vertebral body, the carina, the domes of the diaphragms, and the top

[00:04:30] of the aortic arch. In summary, using the concept of radiological zones helps in localization of pathology. Always consider the path of the x-ray beam through the chest and all the structures the x-ray beam encounters. Don't jump to conclusions regarding localization.