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.

Electron relaxation is the release of a short time stored discrete energy by an orbital electron which will then falls back to a lower energy level. The energy difference is emitted by the electron as a photon or given to another particle.

See also Electron Excitation.

X-rays contain a range of energies (polychromatic photons), the higher energies pass through the patient, the lower energies are absorbed or scattered by the body. Ideally, the x-ray beam should be monochromatic or composed of photons having the same energy. Strong filtration of the beam results in more uniformity. The more uniform the beam, the more accurate the attenuation values or CT numbers are for the scanned anatomical region.
There are two types of filtration utilized in CT:
Inherent tube filtration and filters made of aluminum or Teflon are utilized to shape the beam intensity by filtering out the undesirable x-rays with low energy. Filtration of the x-ray beam is usually done by the manufacturer prior to installation. The half value layer provides information about the energy characteristics of the x-ray beam. Too much filtration produces a loss of contrast in the x-ray image.
A mathematical filter such as a bone or soft tissue algorithm is included into the CT reconstruction process to enhance resolution of a particular anatomical region of interest.

The half value layer (also half value thickness) is the amount of absorbing material, which is needed to reduce the x-ray intensity by one half.
The half value layer provides important information about the energy characteristics of the radiation. A low (or thin) half value layer for a given x-ray beam indicates that the x-ray beam contains more low energy and less penetrating radiation. An x-ray beam containing high energy or highly penetrating radiation has a high (or thick) half value thickness.

An imaging plate is used in computed radiography (CR) instead of a conventional film cassette.
The imaging plate is coated with photostimulable phosphors. The phosphor layer is doped with special substances to alter the crystalline structure and physical properties. After radiation, the enhanced phosphor material absorbs and stores x-ray energy in gaps of the crystal structure, building a latent image.
Usually, the storage phosphors are stimulated with a low-energy laser to release visible light at each point of x-ray absorption. To read-out the image, the plate is inserted into a computed radiography scanner. The scanning laser beam causes the electrons to relax to lower energy levels, emitting light that is captured by a photo-multiplier tube and converted into an electrical signal. The electronic signal is then converted to digital data and can be displayed on laser-printed films, workstations, transmitted to remote systems, and stored digitally.
The CR units automatically erase the image plate after the complete scan. Phosphor imaging plates, like film, are stored in cassette format and can be re-used very often if they are handled carefully. Existing conventional x-ray equipment, from generators to x-ray tubes and examination systems, can be used with imaging plates.