How to read a normal chest x ray: a step by step approach

X-ray of the chest (also known as a chest radiograph) is a commonly used imaging study, and is the most frequently performed imaging study in the United States. It is almost always the first imaging study ordered to evaluate for pathologies of the thorax, although further diagnostic imaging, laboratory tests, and additional physical examinations may be necessary to help confirm the diagnosis.

X-rays are emitted by a machine, travel through the patient, and are picked up by a receptor on the other side of the patient. Some of these rays are absorbed more than others depending on the tissues through which they travel. This differential absorption allows for the creation of an image on radiographic film. In this image, air is black, bones are white, and the rest of the tissues fall on a spectrum in between.

1. Step 1: Determine the view
2. Step 2: Determining image quality
   1. Rotation
   2. Inspiration
   3. Penetration
   1. Air, airway, apices
   2. Bones
   3. Cardiac shadow, cardiovascular system
   4. Diaphragm
   5. Edges, effusions, extrathoracic soft tissues
   6. Foreign bodies
   7. Gastric bubble, great vessels
   8. Hilum
   9. Impression
   1. Aortic dissection
   2. Emphysema
   3. Sarcoidosis
   4. Coin lesion
   5. Pancoast tumor
   6. Acute respiratory distress syndrome
   7. Tuberculosis
   8. Clinical case

   Step 1: Determine the view

   When presented with a chest X-ray, the first thing one should do is try to determine the view, that is, the positions of the patient and machine and thus the trajectory of the rays relative to the patient. Chest X-ray can be:

   • Posteroanterior (PA)
   • Lateral
   • Anteroposterior (AP)

   These terms refer to the patient’s position and therefore tell you the direction that the X-ray beam travels through the body to the receptor. PA films are the standard: the patient stands or sits upright approximately 6 feet in front of the beam source and faces the receptor on the other side, with the X-ray taken while the patient is maximally inspiring (i.e. the lungs are filled with as much air as the patient is capable of inhaling). To take left lateral films, the patient stands or sits upright with his or her arms raised and turns 90 degrees so that the left side faces the receptor; this allows the X-ray beams to travel from the emitter through the patient from right to left to the receptor on the other side.

   Posteroanterior X-ray (border of left atrium in green)

   If a patient is not able to stand or sit upright, an image can be taken with the patient lying down on top of the receptor, such that the X-ray beam travels through the patient from front to back (i.e. as an AP study). These can be taken in the X-ray department, but they are more commonly taken at the patient’s bedside as portable studies. AP studies do not tend to be as diagnostically revealing as PA studies for a number of reasons. For example, bed-bound patients may struggle with achieving maximal inspiration, limiting the potential for full lung expansion and therefore the view of the lungs on the film. Another notable feature of AP studies is the enlarged appearance the heart and mediastinum take on AP views.

   Lateral chest X-ray

   Let’s take a second to try to understand why it is that the heart appears bigger than normal on AP studies. Imagine you are holding a flashlight, pointing it so that the circle of light appears against a white wall a foot away from you. Now imagine holding a pen in front of that light, so that the pen casts a shadow onto the wall. If you hold the pen directly in front of the flashlight, is the shadow big or small? In this case, the shadow formed by the pen is large, and much larger than the pen’s true size. Contrarily, what happens if you hold the pen as far as you can away from the flashlight, so that it sits directly in front of the wall? In this case, the shadow is much smaller, and is a much more accurate representation of the pen’s true size.

   Do you see now the relevance of this example? The same thing that happens to the pen’s shadow when you hold it closer to the flashlight and farther from the wall happens to the picture of the heart created by the rays in an AP view. Anatomically, the heart is located in the anterior thoracic cavity; so when a person faces forward toward an X-ray machine, the heart is closer to the X-ray machine and farther from the film behind it. If a person faces away from the X-ray machine, however, the heart is farther from the X-ray machine and closer to the film on the other side. If the light from the flashlight is a metaphor for the beams emitted by the X-ray machine and the pen is a metaphor for the heart, it makes sense that the shadow cast by the heart on the film will be larger if the X-ray beams travel through the patient’s body from front to back (AP), overestimating the true size of the heart and mediastinum.

   Maybe you’re feeling a little overwhelmed by all this; besides, how can you tell if the view is PA or AP just by looking at it? What if the patient just has cardiomegaly, how could you tell the difference at first glance between this and just a big shadow? Good news: in the real world of the hospitals, the X-ray is usually labeled in some way, either on the image itself or in the report, especially if it was taken via the portable AP technique.

   Step 2: Determining image quality

   In assessing a chest X-ray, there’s a lot to consider, and a lot to remember to look for. This is where mnemonics become extremely useful. To evaluate the quality of an image, you can use the mnemonic R.I.P., which stands for rotation, inspiration, and penetration.

   Rotation

   To assess whether the patient is rotated, first direct your attention towards the medial aspects of the clavicles and compare the positions of the left and right medial clavicular joints to the spinous processes in the more central aspect of the image. If the patient was positioned straight-on and not significantly rotated, the distances between the medial margins of the left and right clavicles and the central spinous process should appear to be approximately equal.

   If a patient is rotated more to the right, the distance between the medial margin of the right clavicle and the spinous process will be greater than the distance between the medial margin of the left clavicle and the spinous process. If a patient is rotated more to the left, the distance between the left clavicle and the spinous process will be greater.

   Clavicle and clavicular joint X-rays (Anterior view)

   Inspiration

   In a good diagnostic study, the image will have been taken when the patient is maximally inspiring (i.e. the lungs are at their fullest). This can be evaluated by counting the posterior ribs visible in the lung fields. If the patient exhibited good inspiratory effort (i.e. he or she was willing and able to take a nice deep breath and hold it for the image), you should be able to count approximately 10 ribs before you get to the diaphragm marking the lower border of the thoracic cavity.

   Penetration

   Penetration describes the extent to which X-rays pass through the body, allowing for the creation of an image with clearly discernible features. In an X-ray with good penetration, the vertebrae behind the heart are barely visible, and the diaphragm can be traced up until reaching the edge of the spine.