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.

[Also: Half-Life Time, Radioactive Half-Life] The half-life is the time in which half the atoms (always a fraction, not a number) of a given radionuclide disintegrate from the amount of atoms present when measurement starts. From 200 atoms of a radionuclide with a half-life of one minute will 100 atoms disintegrate in the first minute, 50 in the second minute, etc. The half-life is a characteristic property of radioactive isotopes. The effective half-life includes all processes of elimination, including radioactive decay.
Different half-life terms:
- Physical Radioactive Half-Life
- Biological Radioactive Half-Life
- Effective Radioactive Half-Life.

See also Decay Constant, Decay.

(HDD) [Hard Disk Drive] Computer equipment to store large amounts of data on rapidly rotating platters with magnetic surfaces.

[Computer Hardware] Usually describes the physical components needed for data processing, e.g. monitor, computer keyboard, recording disk, central processing unit, etc.

See also Software.

Nuclear cardiology has a wide range of techniques that permit the accurate assessment of perfusion, metabolism, sympathetic innervation, and mechanical function of the heart.

Scintigraphic techniques of the heart include:
See also Echocardiography and Cardiovascular Imaging.