The numbers of nuclei or electrons in different energy levels. At thermal equilibrium, the relative populations of the energy levels will be given by the Boltzmann distribution.

See also Boltzmann Distribution.

The collective effective dose is the product of the mean effective dose for a population and the number of individuals; or the sum of all individual effective doses in the population of concern.

When a group of spins is placed in a magnetic field, each spin aligns in one of the two possible orientations. The relative numbers of spins with different alignments will be given by the Boltzmann distribution.
Definition: if a system of particles, which are able to exchange energy in collisions is in thermal equilibrium, then the relative number (population) of particles, N1 and N2, in two particular energy levels with corresponding energies, E1 and E2, is given by N1/N2 = exp [-(E1 - E2)/kT ] where k is the Boltzmann constant and T is the absolute temperature. For example, in NMR of protons at room temperature in a magnetic field of 0.25 tesla, the difference in relative numbers of spins aligned with the magnetic field and against the field is about one part in a million; the small excess of nuclei in the lower energy state is the basis of the net magnetization and the resonance phenomenon.

The collective dose is the sum of individual doses. Collective doses are received in a given period of time by a specific population and a specific radiation exposure.

The digital mammography is an electronic imaging procedure of the breast. The number of breast imaging facilities equipped with digital mammography (also called computed radiography mammogram (CRM), CR mammogram) is growing due to a number of advantages.
Digital images can be stored directly in a picture archiving and communication system (PACS) and allows the printing, enhancement, magnification, or brightness and contrast manipulation for further evaluation. The sensitivity of digital mammography compared to film mammography is better in women with dense breasts, a population at higher risk for breast cancer, due to these post processing possibilities.
'The American College of Radiology's (ACR) Imaging Network found that digital mammography detected up to 28 percent more cancers than film-screen mammography in women age 50 and younger, premenopausal and perimenopausal women, and women with dense breasts, as reported in October 2005 in the New England Journal of Medicine.'

Advantages of digital mammography:
Existing mammography equipment can be converted to 'digital' operation, which allows cost savings compared to integrated digital mammography systems.

See also Breast MRI.