# Getting into the chest x-ray (part 3)—the importance of shapes and key landmarks

In this video from our Chest X-ray Essentials course, you will learn how shapes and structures are formed on a chest x-ray.

Julian Dobranowski, MD FRCPC FCAR
16th Oct 2015 • 4m read

In this video from our Chest X-ray Essentials course, you will learn how shapes and structures are formed on a chest x-ray. Key landmarks are structures such as the trachea, the aorta, and the heart. These landmarks exhibit very characteristic shapes that vary between the PA image and the lateral image. Knowing about these basic principles is important if you want to master the interpretation of the chest x-ray.

This video is part 3 in a 4-part series on chest x-ray problems:

If you want to take your chest x-ray skills to the next level, then check out our Chest X-ray Essentials course, which was specifically designed for non-radiologists who want to use the chest x-ray in order to make better clinical decisions.

## Video Transcript

[00:00:00] What will I again from this video? After watching this video, you will know how to identify anatomical structures based on their radiological shape. When a light beam shines on an object, the object will cast a shadow. By understanding the shape of the object and the direction of the light beam, we can predict the shape of the shadow. There are analogies to this also in the x-ray world. We take a solid cylinder in standard upright.

[00:00:30] If we shine a light beam on the cylinders, so that the beam direction is perpendicular to the cylinder, the shape of the shadow is a rectangle. If the cylinder is positioned parallel to the x-ray beam, the shadow produced is a circle. If the cylinder is hollow and vertical, the shadow remains a rectangle. If the cylinder is hollow and horizontal, the shadow is a circle. If the cylinder is curved the shadow will be

[00:01:00] an arch. If the object is a sphere, the shadow will always be a circle. If we take this hollow cylinder and now bring it closer to the light beam, we get magnification of the rectangle. If the cylinder is now further away from the light beam, there is less magnification. The thoracic anatomical structures are three dimensional and course in various directions. Depending on their anatomical

[00:01:30] course in geometry and a direction of the x-ray beam, they will form shapes that are predictable. This forms the basis of object recognition on chest x-ray examination. Shapes that can be identified on x-ray are rectangles, both black and white, circles, both black and white and white arches. The trachea is a hollow cylinder. If the x-ray beam direction is perpendicular to the trachea, we can predict

[00:02:00] that the shape of the trachea on the x-ray will be a rectangle. And this is both on the PA and on the lateral projections. Air is black. So the shape that we can expect on a PA x-ray is a black rectangle. This also holds true for the lateral x-ray, the trachea will look like a black rectangle. Blood vessels have the shape of cylinders and course in various directions. If the course of the blood

[00:02:30] vessel is vertical, then we can predict that the radiological shape will be a rectangle. On this x-ray examination, adjacent to the right heart border, we see vertical white rectangles, which represent branches of the right pulmonary artery. Because the aorta arches, we can predict that its shape will be that of a white arch or white candy cane. Know that even though we do not see the medial border of the aorta, we can still

[00:03:00] perceive its shape. The reason we don't see the medial border is because the medial border of the aorta is silhouetted with other anatomical structures within the mediastinum. The aortic arch can also be seen on the lateral x-ray as a white arch coursing from front to back. Because of its shape and greyscale, the trachea can be easily identified on the chest x-ray. We can now

[00:03:30] use the trachea as a landmark to learn about and to identify other anatomical structures such as the carina, the superior vena cava, the right paratracheal stripe, the aortic arch, and the left pulmonary artery. All anatomical structures that we will be discussing in future videos. So in summary, recognition of normal and abnormal anatomical structures on a chest x-ray depends on identifying the

[00:04:00] radiological shapes formed by these structures. An anatomical shape will be recognizable only if interfaces outline the structure. Remember, an interface will only occur if there is enough density difference between structures that are adjacent to each other. In the example of the aortic arch, we could not identify the medial border because the medial border of the aorta has the same radiodensity as other mediastinal

[00:04:30] structures. Changes in position of the patient or direction of the x-ray beam can change the expected shape of an anatomical structure.