a. The first step provides a means of controlling the quantity of x- radiation by
control of filament temperature. As a higher mA station is selected on the x-ray
generator, the filament temperature of the x-ray tube is raised, which in turn increases
electron emission. (Because the filament reaches extreme temperatures, it is usually
made of tungsten, which has a melting point of 3,370C.) If a lower mA station is
selected, the filament temperature and the rate of emission will decrease.
Consequently, by varying the mA station, the quantity of x-radiation can be varied. The
last statement applies only to a set period. Naturally, the elapsed time of x-ray
production will also affect quantity.
b. The second step (acceleration of the electrons) controls the quality of the x-
rays. When high voltage is applied across the tube, the anode becomes positively
charged with respect to the cathode and the electrons are repelled by the cathode and
attracted by the anode, which causes higher voltage and greater speed of the electrons.
The higher speed increases the energy range of the photons. In other words, by
varying the kilovoltage (kVp), we can vary the quality of the x-rays.
c. The process by which x-radiation is produced when electrons strike the target
(step 3) is described earlier in the subcourse and will not be repeated here. However,
the target plays an important role in other areas as will be explained later in this lesson.
DEVELOPMENT OF X-RAY TUBES
a. The Hittorf-Crookes tube, used by Roentgen in his discovery, had no target
as such. The x-radiation was produced in the glass wall of the tube when it was
bombarded by the cathode rays (accelerated electrons), as seen in figure 3-7. The
glass wall was not practical as an x-ray source because it could not withstand the heat
generated by the electron bombardment and because it was too large to produce
detailed radiographs. Consequently, one major step in the development of x-ray tubes
was to place a metallic target inside the tube and direct the electrons to it instead of
allowing them to interact with the glass wall. Not only did the metallic target better
withstand the heat, but it also reduced the size of the x-ray source and enhanced
b. Although the addition of the metallic target was a significant improvement, the
early tubes still presented some serious problems. One of the problems stemmed from
the method by which the electrons were obtained. Rarefied gas in the tube was partially
ionized, and the positive ions in the gas were attracted by the negatively charged
cathode, striking it with sufficient energy to release electrons from its surface, attracted
the positive ions in the gas. The electrons thus released were accelerated by the
electric field between the cathode and the anode. A major problem with these tubes
was lack of control over both the quantity and quality of the x-radiation because of
variations in gas pressure as the tube warmed up.