Radiographic techniques, contrast, and noise in x-ray imaging.

Abstract

AJR:204, February 2015 helpful or informative to describe any radiographic examination as being performed at a given milliampere-second value. The intensity of an x-ray beam is quantified by an air kerma, which relates to the kinetic energy released per unit mass when xrays interact with air [12]. Air kerma is the energy transferred to electrons when normalized by the mass of air (energy/mass) and is measured using grays or milligrays. Air kerma can be thought of as the number of x-rays per unit area, with the photon energies of minimal concern. The air kerma in any x-ray beam is directly proportional to both the tube current and the corresponding exposure time. Because air kerma is well defined and universally understood, it is the metric of choice for specifying x-ray beam intensity. The higher the air kerma, the higher the x-ray beam’s intensity, and the more photons will be incident on the patient. When air kerma is multiplied by the corresponding beam area, one obtains the kermaarea product in grays times square centimeters (Gy = cm2), which is often referred to as the dose-area product [12]. The kerma-area product is the total amount of radiation incident on the patient and will affect the energy deposited into the patient. Air kerma is used to estimate the entrance skin dose and thereby the likelihood of a deterministic (skin) radiation risk. Kerma-area product quantifies the total amount of radiation incident on the patient and is most closely related to the total stochastic patient risk. For most patients, the stochastic risk can be taken as the carcinogenic risk. Table 1 shows representative values of kerma-area product for complete examinations in radiologic imaging [13].