Imaging Equipment

William J. Callaway

Outline

Key terms

direct digital radiography

Imaging the human body is the major thrust of radiography. In-depth knowledge of the equipment is essential for making proper exposures. Such studies are covered in detail in physics and equipment courses. This chapter summarizes the types of equipment used in imaging to help you understand what you will be seeing early in your educational program.

X-ray tube

X-rays are not stored, nor do they come from radioactive materials. The radiographer manufactures x-rays for each exposure using technical factors manipulated on the x-ray control panel. X-rays are produced by a series of energy conversions. The primary items needed for the production of x-rays are: (1) a source of electrons, (2) a means to accelerate the electrons, and (3) a way to bring the electrons to a sudden stop. All of these items are accomplished in the x-ray tube. (Details concerning the process of imaging are presented in Chapter 10.)

The x-ray tube is an evacuated glass bulb with positive (anode) and negative (cathode) electrodes (Fig. 8-1). The anode is an electrode toward which negatively charged electrons migrate. The cathode is a filament that gives off electrons when heated (i.e., the source of electrons). As several thousand volts of electricity are applied to the tube, these electrons are driven across a short distance at a very high speed (i.e., the means to accelerate the electrons) and strike the anode with high kinetic energy (i.e., the way to bring the electrons to a sudden stop). Because energy can be neither created nor destroyed, an energy conversion takes place; this energy conversion is the result of the sudden deceleration of the electrons at the anode. Heat is the primary by-product (>99%) of this energy conversion. However, x-rays are also produced (<1%), and they emanate from the tube in all directions. The x-rays exit the tube housing through a device consisting of open lead shutters called a collimator.

FIGURE 8-1 A typical x-ray tube shows cathode (top) and anode (bottom) within the glass envelope.

Digital imaging

Digital imaging equipment enhances images of the body. The primary advantages of digital equipment include the ability to post-process images in a variety of ways to provide multiple views of the anatomy. In digital radiography, the density and contrast of the image can be altered any time after the completion of the study without re-exposing the patient. The images are stored in a computer and can be transferred to multiple locations on a network or sent via e-mail. In general, two types of digital imaging equipment are in routine use.

In computed radiography (CR), the x-rays exit the patient and strike a cassette containing an imaging place (IP) (Fig. 8-2). The IP is coated with a substance known as a photostimulable phosphor. This phosphor becomes excited as a result of the deposit of x-ray energy, and it remains so until the cassette is placed in a reader. Once inside the reader, the IP is scanned with a laser beam, releasing the x-ray energy that is then converted to a visible image by the computer. The resulting image is viewed on a high-resolution monitor.