Answer to Question #1506 Submitted to "Ask the Experts"

In a modern imaging department, the half-value layer is considered in two important applications. The first is the half-value layer of the primary x-ray beam used in patient diagnosis. A diagnostic x-ray beam produces a range of energies, although only the maximum energy is quoted, that is, the x-ray beam is set for 90 kVp during the examination. The half-value layer provides important information about the energy characteristics of the x-ray beam. Basically, if the half-value layer for a given x-ray beam is low or thin, then the x-ray beam contains more low energy and less penetrating radiation. If the half-value layer is high or thick, the x-ray beam contains more high energy or highly penetrating radiation. This is important because x rays used for diagnosis must have enough energy to penetrate the body part of interest and expose the film sufficiently. This is because lower-energy radiation is absorbed or scattered by the body and may not reach the film. Adding more filtration to the beam, which is typically done by the manufacturer of the unit prior to installation, will remove the undesirable low-energy x rays while allowing the desirable higher-energy x rays to pass through the patient to the film. On the other hand, if there is too much filtration in the beam, the x-ray beam energy becomes too high, and there is a loss of contrast in the x-ray image. This is why it is important to have a physicist evaluate all x-ray equipment on a regular basis, typically once a year, and measure half-value layer as part of that testing. A second application of half-value layer in an imaging department has to do with room shielding. Rooms that contain fixed x-ray equipment are typically shielded with lead-lined walls to reduce the radiation exposure to workers and the public from the use of x rays within the department. When designing the shielding for a room, the physicist will perform calculations based on the half-value layer of the x-ray beam. In general, the design will call for enough half-value layers of shielding to reduce radiation exposure outside the room to acceptable levels. For example, if the exposure rate at the wall inside the room is 16 (arbitrary units), and we want the exposure rate just outside the wall to be 2, we would use 3 half-value layers of shielding. The first half-value layer reduces the exposure rate to 8, the second half-value layer to 4, and the third to 2. In practice, there are some other factors that must be taken into account, but this provides the basic idea. Curry TS, et al. Christensen's introduction to the physics of diagnostic radiology. 3rd ed. Philadelphia: Lea and Febiger; 1984: 63, 410-411.

Ken "Duke" Lovins, MS, CHP