(mA s) The product of the x-ray tube operating current and the exposure time in minutes.
See Milliampere and X-Ray Tube.

A vacuum is a volume of space that is substantively empty of matter so that gaseous pressure is much less than standard atmospheric pressure.
A vacuum is used in the x-ray tube because otherwise the electrons would be stopped by air or any other gas. Unlike in closed (or sealed) x-ray tubes, in which the vacuum is permanent, open tube systems have to be outfitted with a vacuum pump for the purpose of creating vacuum during tube warm-up.

An anion is a negative charged ion; the atom has more electrons than protons. Anions are attracted from the positively charged anode. This is a basic part for producing radiation in the x-ray tube.

The anode is the positive terminal of an x-ray tube, usually consisting of a tungsten block embedded in a copper stem. Electrons flow from the cathode toward the anode and the anode emits x-rays from the focal spot.

(B) The Bucky factor is the ratio of x-rays arriving at the grid (incident radiation), and those being transmitted through the grid. The Bucky factor describes how much the output of the x-ray tube must be increased in order to compensate for the removal of incident radiation by a grid.
B = incident radiation / transmitted radiation
For example, if for every two x-rays impinging the grid, only one will pass through, then the grid has a Bucky factor of two and the intensity of the x-ray beam must be doubled.