A proton is a basic particle located in the nucleus of an atom. The number of protons in the nucleus governs the chemical properties of that element. The positive electrical charge is equal to the negative charge of an electron and the mass is similar to that of a neutron (see also baryon). The proton belongs to the group of hadrons and is composed out of three quarks.
A hydrogen (H1) nucleus is build of one proton.
Proton beam therapy is the preferred treatment for cancers near the eye and the spinal cord.

A real number is a component of a complex number. In CT, complex data are used for example in the Fourier transforms.