Fundamentals Of X-Ray Studies

 OVERVIEW

Interpreting a chest X-ray is a very valuable skill given how commonly it is ordered. The following guide helps walk you through some of the basic elements of X-ray imaging that are important to understand in order to interpret these studies effectively.

HOW DOES IT WORK?

This imaging modality works by literally penetrating the human body with X-rays (and measuring X-rays that can not penetrate the medium in question). If the X-rays penetrate a structure, they will appear black on the film. If they can’t penetrate the structure (i.e. are reflected) they appear white on the film.

This image helps demonstrate the basics of how X-ray technology is used clinically (source)
This image helps demonstrate the basics of how X-ray technology is used clinically (source)

There are 4 major radiological/physical mediums that appear different visually given their relative densities. They are (listed from least to most dense):

  • Air: this will appear black because the X-rays can easily penetrate air
  • Fat: fat will not be completely penetrated and will appear faintly on film
  • Water (soft tissues): this medium will appear more white on X-rays (compared to fat) given its increased density.
  • Bone (calcium): bony/calcified structures show up the most clearly on X-rays (bright white!) because the X-rays have difficulty penetrating these structures.
TYPE OF X-RAY MACHINE

When looking at an X-ray image, arguably the first detail you should notice is if the image was taken with a portable or non-portable device. Portable devices are advantageous because they can taken to a bed ridden patient, however non-portable X-rays are preferred (when a patient is able to ambulate) because of their superior image quality. It is important to be wary of comparing portable to non-portable X-rays simply because there are differences between the two (a patient’s pneumonia may look worse on a portable X-ray compared to a baseline non-portable image). We will discuss quality measures for X-rays more below.

Example of a portable X-ray machine
Example of a portable X-ray machine (source)

***Keeping this type of X-ray machine in mind will contextualize your analysis of the image further on.***

PENETRATION OF THE FILM

The penetration of the film refers to how well the X-rays have penetrated the body of the patient. The more penetration the more X-rays will interact with the film placed on the other side of the patient. We must appreciate that their is a “sweet spot” with regards to how well penetrated the film is (to maximize the clinical information we can gain from the study). Below are some examples from a chest X-ray study to help demonstrate this concept.

Is the film under-penetrated? If film is under penetrated then there will be an excess of white present.

This image shows a clearly underpenetrated chest X-ray. It is very difficult to interpret such a film (source)
This image shows a clearly under-penetrated chest X-ray. It is very difficult to interpret such a film (source)

Is the film over-penetrated? Alternatively, if the X-rays over penetrate the patient, there will be an excess of black present on the film.

This film is over-penetrated, made evident by the fact that the ribs can not be fully visualized (source)
This film is over-penetrated, made evident by the fact that the ribs can not be fully visualized (source)

***PROPER PENETRATION IS CRITICAL TO INTERPRETING AN X-RAY!***

Magnification: How far away is the structure from the film when the image was taken? This can be a difficult concept to retain, however the further away a structure is from the X-ray film, the more it will MAGNIFY as the X-ray film is developed. It is for this reason that a standard PA (with the heart closer to the film) shows a more accurate size of the heart while an AP film (which increases the distance between the film and the heart) will show enlarged cardiac structures.

Comparison of PA vs. AP views of chest X-rays (source)
Comparison of PA vs. AP views of chest X-rays. Note the larger appearing heart on the AP view. Learning about the concept of magnification helps explain why this happens, and why PA film are generally preferred (source)
 
Page Updated: 08.24.2016