An element consists of atoms having the same structure and the same chemically reaction.

The resolution element has the size of the smallest spatially resolved region in an image. It may be anisotropic, e.g. with an asymmetric acquisition matrix or slice thickness, and may be larger than the pixel or voxel.

Actinides are elements in the periodic table with an equal or higher atomic number than actinium, which has an atomic number of 89 and up to 103 lawrencium, including plutonium, curium, and californium). Actinides are also called 'rare earth metals'.
Actinides include most of the well-known elements found in nuclear reactions with atomic numbers higher than 92. These elements do not occur naturally but are produced by bombarding other elements with particles in an accelerator.

(NAA) Neutron activation analysis is a very sensitive analytical technique to determine even very low concentration of chemical elements, trace elements for example, in small biological samples.
NAA becomes commercial available in the USA in 1960.
In the activation process stable nuclides in the sample, which is placed in a neutron beam (neutron flux, 90-95% are thermal neutron with low energy levels under 0.5 eV), will change to radioactive nuclides through neutron capture (artificial radioactivity). These radioactive nuclides decay by emitting alpha-, beta-particles and gamma-rays with a unique half-life. Qualitative and quantitative analysis of the sample is done with a high-resolution gamma-ray spectrometer.
NAA is subdivided into the following techniques:

Isotopes of a given element have the same number of protons but a different number of neutrons in the core. Isotopes have the same electronic structure as the given element and thus have a nearly identical chemical behavior. Larger elements tend to have more stable isotopes than smaller ones.
For example, I123, I125 and I131 are isotopes of iodine; they have a different mass number.