The inverse square law describes that the intensity of a radiation field is inversely proportional to the square of the distance from the source.

(mA): 10^-3 ampere. In radiography, the current flow from the cathode to the anode in the x-ray tube is measured in milliampere. This current regulates the radiation intensity emitted by the x-ray tube.

See X-Ray Tube.

The photoelectric effect describes the following interaction of electromagnetic radiation with a metallic surface: a photon with an energy (frequency) above the binding energy of an electron gets absorbed and the electron is emitted. The positive energy difference is transferred to the electrons kinetic energy. If the photons energy is not high enough for the electron to overcome its binding forces, the photon will be re-emitted. It is not the intensity of a photon beam (amount of photons) which allows the photoelectric effect; it is the energy (frequency) of a single photon which will allow the emission of a single photoelectron.
The discovery and study of the photoelectric effect leads to a new quantized understanding in physics. Albert Einstein was awarded the Noble prize for physics in 1921 'for his services to theoretical physics and especially for his discovery of the law of the photoelectric effect'.
The photoelectric effect is the most important effect in medical radiography. E.g. it is photoelectric absorption that is responsible for most of the absorption in a mammogram which creates the contrast in the image.

See also Photon, Electron.

Digital images can be manipulated for evaluation in various ways. Post-processing includes: Subtraction, addition, rotation, inversion, multiplanar reconstruction (MPR), maximum intensity projection (MIP), etc.
Subtraction is particularly useful in contrast enhanced examinations. The pre contrast images are subtracted from the images after an injection of contrast agents (sometimes also called dye) for better tumor detection. See also Contrast Enhanced Computed Tomography, Digital Subtraction Angiography and Active Zone.

The international system for units.
Le Systeme international d'Unites officially came into being in October 1960 and has been adopted by nearly all countries, though the amount of actual usage varies considerably.
It is based upon 7 principal units:

From these basic units many other units are derived and named.