Foto: argus/ Maik Schröder mit freundlicher Genehmigung des Instituts für Mikrotechnik Mainz GmbH Quelle: DLR Göttingen Quelle: DLR Göttingen
Physik im Kontext
What are X-Rays
Röntgen's Discovery
History of Radiology
X-Ray Production
X-rays & Matter
Detection Issues
Detection Methods
Background: Fluorescence

Physics & the Detection of Medical X-Rays

In the early days of x-ray diagnosis the film used in the creation of x-rays pictures was standard silver halide film. This is similar to photographic film used in cameras. The difficulty in using this film for high energy x-rays is related to their penetrating power. The x-rays must penetrate human tissue and thus will also pass through most types of matter. Thus, most x-ray photons will go through the film and not be absorbed by the silver halide molecules. In most cases the standard film will absorb about 1% of the x-ray photons that strike it. The other 99% will penetrate passed through the film. Thus, to create an image that would be useful for medical diagnosis the patient needed to receive a very large dose of potential dangerous electromagnetic radiation.

The solution to this dilemma is to direct the x-rays to a material which converts them to visible light. The photons of visible light then strike the film and expose it. Because an x-ray photon’s energy is much higher than that of a photon of visible light, one x-ray photon can create several light photons. Further, dense materials which absorb a large fraction of the x-rays can be used. Thus, the x-ray dose that the patient receives is much smaller than it would be when the x-rays are absorbed directly by the film. An x-ray image that required 10-15 minutes to take during much of the early 20th Century can now be completed in a fraction of a second.

Fluorescent materials are the key to making this process work. The x-rays are absorbed be a fluorescent layer. As described in the background section, the atoms and molecules in fluorescent material emit new photons in a serious of steps. In the case of converting ultraviolet light to visible light one visible and one infrared phtom are the usual result. For x-rays, several visible photons can appear. For practical reasons related to the sensitive fo the film, photons in the blue region of the spectrum are preferred.

Today’s images are captured in a film cassette which is a "sandwich" with the photographic film in the middle and a layer of fluorescent material on each side. These cassettes are frequently called image intensifiers because of the many photons which are emitted as a result of a single incoming x-ray.


Intensifying Screen
A short encyclopedia entry that describes the components and processes of the intensify screen in a film cassette.

X-Ray Imaging Techniques
A set of PowerPoint slides in a PDF file.  The discussion of film starts on page 8.

The Production and Medical Use of X-Ray
A general discussion of the technical side of medical x-rays.  It includes a discussion of film.  On this page search for "Recording system" to go directly to the section on film.

Kasei Optonix
Not much information here, but at the bottom of the screen is a comparison of a picture with intensification and one without.

Der Röntgenfilm und seine Verarbeitung
The first part of this site describes the the physical process by which image intensifiers work.  the secdon part is a discussion of film processing.

Filmaufbau, Filmverarbeitung, Konstanzprüfung
The first section of this site describes the construction of image intensifying film.

Röntgendiagnostik II
A set of PowerPoint slides in PDF format.  The discussion of using film to record the images begins on page 14.  Even those who do not read German should find the drawings informative.

Bildgebung mit Röntgenstrahlen: Detektoren
A set of PowerPoint slides in PDF format. Film is discussed at the beginning of this presentations.  Again, the graphics are useful for everyone.  Some of them are identical to the previous link's.

History of Medical Diagnosis and Diagnostic Imaging
A very brief history of the various forms of detecting x-rays and using other imaging techniques

X-Ray Diagnositcs and Imaging
A set of PowerPoint lsodes which emphaisze image intensification.  This set inlcudes mathematics invovled in the process.

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In cooperation with
Modern Miracle Medical Machines
Physics Education Research Group
Kansas State University