The AAPM/RSNA physics tutorial for residents - X-ray attenuation

Attenuation is the reduction of the intensity of an x-ray beam as it traverses matter. The reduction may be caused by absorption or by deflection (scatter) of photons from the beam and can be affected by different factors such as beam energy and atomic number of the absorber. An attenuation coefficient is a measure of the quantity of radiation attenuation by a given thickness of absorber. Linear and mass attenuation coefficients are the coefficients used most often. The equation I = I(o)e(-mu x) expresses the exponential relationship between incident primary photons and transmitted photons for a monoenergetic beam with respect to the thickness of the absorber and thus may be used to calculate the attenuation by any thickness of material. The quality or penetrating ability of an x-ray beam is usually described by stating its half-value layer (HVL). Another parameter used to describe the penetrating ability of a beam is the homogeneity coefficient. Among other things, use of added filtration reduces the intensity of the x-ray beam, increases the HVL, decreases patient exposure, and improves image quality for a given radiation dose.