X-ray machines are imaging devices used to reveal the internal structure of patients. They make use of the fact that x-rays can penetrate a patient, and that this behavior varies in degree with the makeup of the patient’s body.

What are x-rays?

X-rays are a type of electromagnetic radiation—basically, they are a higher-energy form of light. Because of this, x-rays are able to penetrate most ordinary materials. However, the larger or denser an object is, the more it will or absorb x-rays. We can get information about what is inside an object by generating an image from the x-rays that pass completely through it.

What is projectional radiography?

When people refer to “x-rays” they usually have in mind the images produced using a technique called “projectional radiography.” In this approach, a machine irradiates a patient from one side with x-rays, and a flat detector (either film or digital) on the other side records x-rays that pass through the patient. The irradiation takes place in a fraction of a second, and the resulting two-dimensional image, or “radiograph,” provides information about the patient’s insides. The familiar color scheme of x-ray images—think white bones on a black background—comes from the properties of the silver-halide films used in medicine for decades. When a film is developed, the areas exposed to x-rays turn black while unexposed areas (for example, where x-rays were mostly blocked by bones) remain white. Most modern-day x-ray imaging is performed using digital detectors rather than films, but the shades of gray between black and white (known as “grayscale”) are still used.

X-ray images are generally used for diagnostic purposes—that is, to look inside the body noninvasively for evidence of damage or disease. This method of imaging is attractive because it is fast, inexpensive, and yields a relatively low radiation dose to the patient. However, the technique does have two significant limitations: information about the patient’s three-dimensional body is overlaid on to two dimensions, and x-rays are not effective at imaging soft tissues such as muscle and organs. Projection radiography is therefore most useful for quickly identifying gross damage or disease, such as broken bones or fluid in the lungs.

What is fluoroscopy?

X-ray machines are also sometimes used to provide images in real time, via a technique known as fluoroscopy. This imaging method is similar to projectional radiography except that x-ray images are taken repeatedly in order to obtain a “live movie” of the patient’s internal anatomy. To keep dose to the patient low, each image uses significantly fewer x-rays than in standard projectional radiography, which also results in poorer image quality. Fluoroscopy is nevertheless very useful in the field of interventional radiology, where it is used to guide a wide range of minimally invasive procedures, such as the insertion of catheters or stents in blood vessels, or the taking of biopsies.

What about CT imaging?

CT machines also use x-rays to image the insides of patients noninvasively, but in a different, more complicated way. Such scanners produce three-dimensional (3D) images of patients, and this enhancement comes at a greater cost in time, complexity, expense, and dose.

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