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The amount of darkness in the radiographic image reflects the radiographic density. It is called "transmitted density" in conventional film radiography because it measures the light transmitted through the film. In digital imaging, it refers to the shift of the histogram to the lower grey levels.3,6-8
There are common factors that affect the radiographic density of conventional film and digital receptors or plates, and other factors depend on the nature of the receptor. We will only discuss common factors related to the exposure and the subject. We will consider one variable at a time, keeping the other variables fixed.
Change in mA and in Exposure Time
When the mA or exposure time increases, the number of x-ray photons generated at the anode increases linearly without increasing beam energy. The result will be a higher number of photons reaching the receptor, and this leads to an overall increase in the density of the radiographic image (Figure 2).2,4,6,9
Change of image density related to change of mA and/or exposure time: Image A is used as reference. Image B shows a brighter image due to decrease in mA and/or exposure time. Image C shows a darker image due to increase in mA and/or exposure time.
Change in kVp
When the kVp increases, the number, and energy of x-ray photons generated at the anode increase, this will result in a higher number of photons with higher energies reaching the receptor, and this leads to an overall increase in density of the radiographic image at a larger scale when increasing mA or exposure time (Figure 3).2-4,8
Change of image contrast related to change of kVp: Image A is used as a reference. Image B shows a higher contrast image due to a decrease in kVp. Image C shows a lower contrast image due to an increase in kVp.
Change in Source to Object Distance
When the source to object distance increases, the intensity of the x-ray beam decreases following the inverse square law, this will result in a decrease in the intensity of the beam reaching the object, and this results in a decrease in image density (Figure 4).5-7,10,11
Change of image density related to change of source to detector distance: Image A is used as reference. Image B shows a brighter image due to increased source to detector distance.
Thickness of the Absorber
When the thickness of the absorber increases, the number of photons absorbed increases, leading to fewer photons reaching the receptor, which will result in a decrease in the image density (Figure 5).3,7,10,11
Change of image density related to change of absorber thickness: Image A is used as a reference. Image B shows a brighter image due to an increase in absorber thickness.