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Logarithmic graph paper

RPO THK-basic
transmission through plaster for different values of the tube voltage

The figure above shows the transmission of X-radiation through gypsum (= plaster) for tube voltages of 50 kV, 70 kV and 90 kV. Gypsum is calcium sulphate, a substance that is chemically closely related to calcium phosphate, the main constituent of bone and enamel. The bitewing recording is of a hypothetical molar tooth of plaster with a diameter of 10 mm.

The difference in the density of light and dark parts of an X-ray image is called contrast. With extremely small contrast, the image is evenly gray and with extremely large contrast only the shades of white and black occur. Most blackening occurs when the transmission is maximum (at thickness = 0 mm) and the least blackening when the transmission is minimal (at thickness = 10 mm).

It can clearly be seen in the figure that the transmission curve is getting steeper and steeper - and with that the difference between maximum and minimum transmission becomes increasingly larger - when the tube voltage decreases. The photoshopped pictures illustrate the increase in contrast in the bitewing recording that results from this.

The teeths of a child are about 30% smaller than those of an adult. If you want the same contrast, i.e. the same difference between maximum and minimum transmission, you have to choose a smaller tube voltage for children. The figure above shows that at 50 kV the optimal recording of a 7 mm thick object is obtained.

Last modified:07 February 2019 1.54 p.m.
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