We will be shifting gears a little from the x-ray machine to radiation protection. We will also learn what specific dose limits may be from a regulatory aspect. The whole purpose of radiation protection is to minimize exposure to the patient, the occupational worker, and to the public. Unfortunately, there are several ways of expressing exposure and dose depending on what you are describing. On top of that, there is also traditional units and the International System (SI) to know. The United States has always been slow to adopt international standards. Examples include using miles instead of kilometers, inches instead of centimeters, and gallons instead of liters. We are just now starting to use the SI units for exposure and dose.
Exposure vs. Dose
Exposure - The term exposure has multiple uses. Making an exposure is the act of pushing the exposure button and creating x-rays. Exposure to the patient is the administration of x-rays to the patient. Exposure to the image receptor is when x-rays penetrate the body part and exit to strike the film cassette, CR cassette, or digital sensors. The term exposure is also used to describe the measurement of ionization of air as x-rays interact with it. We can measure this by putting a special meter in the air in the x-ray beam path. Remember that x-rays are a form of ionizing radiation. Ionization occurs when x-rays interact with the electrons in an atom and causing one or more to eject. This causes a neutral charged atom to become electrically charged, which is called an ion. It is this electric charge that can be measured.
Dose - The term dose is used to describe the amount of x-rays a body, or body part, aborbs. There are 2 types of dose, absorbed dose and effective dose. Absorbed dose is the amount of radiation absorbed by the body. Effective dose takes into account the type of radiation you are being exposed to. Remember that there are other types of ionizing radiation. There are x-rays, gamma rays, beta particles, and alpha particles. X-rays and gamma rays are both electromagnetic energies that only differ in its origination. Beta particles are high speed electrons. Alpha particles are composed of 2 protons and 2 neutrons. Alpha particles have much more mass than beta particles, x-rays, or gamma rays. When something of mass strikes tissue, it will cause more damage. Which would hurt you more, getting hit by a wad of paper (x-rays) or a bowling ball (alpha particle)? To quantify these differences, we use a radiation weighting factor. Beta particles are just electrons, which do not have much mass to speak of, so it is given a weighting factor of 1 (just like x-rays and gamma rays). Alpha particles are given a weighting factor of 20.
So remember that exposure is a measurement in air, and dose is a measurement in tissue.