An irradiating apparatus is capable to produce ionizing radiation or to accelerate atomic particles.

Ischemia refers to a lack of adequate blood flow to an organ, such as the heart or the brain, which may be the result of a blood clot, blood vessel constriction or a hemorrhage.

See also Regional Cerebral Blood Flow, Blood Brain Barrier, Cerebral Blood Pool Imaging and Hematoma.

Isobars are atoms with equal mass number but different atomic number.

The isocenter in x-ray imaging is the space through which the central ray of the radiation beams passes.
The CT isocenter is defined by intersection of the gantry axis of rotation. In case of a cylindrical CT type, the isocenter is at the center of the gantry bore.
In radiation oncology the isocenter is defined by the beam centerline of a medical accelerator or cobalt unit. Typically, the isocenter is located 1 m from the radiation source.

Isomers are nuclides with the same number of neutrons and protons of a given element in different states of excitation. Most of these unstable isomers decay very quickly (~ 10^-12 seconds).
They are characterized in the chemical formula by an m (e.g. Tc-99m).

See also Isomeric Transition.