Radiographic Density

Radiographic density is reflected by radiographic image darkness. In conventional film radiography, it is called "transmitted density" because it is a measure of the light transmitted through the film. In digital imaging, it refers to how much the overall histogram of the image is shifted towards the lower grey levels.3,6-8

There are common factors that affect the radiographic density of conventional film and digital receptor or plate, and there are other factors that depend on the nature of the receptor. We will only discuss common factors that relate to the exposure and to the subject and 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. This will result in 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

Figure 2.
Change of image density
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 of density of the radiographic image at a greater scale when increasing mA or exposure time (Figure 3).2-4,8

Figure 3.
Change of image contrast
Change of image contrast related to change of kVp: Image A is used as reference. Image B shows a higher contrast image due to decrease in kVp. Image C shows a lower contrast image due to 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 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

Figure 4.
Change of image density
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 increase of source to detector distance.

Thickness of the Absorber

When the thickness of the absorber increases, the number of photons absorbed increases, leading to less photons reaching the receptor. This will result in a decrease in the image density (Figure 5).3,710,11

Figure 5.
Change of image density
Change of image density related to change of absorber thickness: Image A is used as reference. Image B shows a brighter image due to increase of absorber thickness.