Principles of radiology

This article discusses the principles of conventional radiography. The production of X-rays is described along with the different techniques available to image them. The different types and applications of contrast agents are discussed along with adverse side effects and how to avoid or minimize them. Standard radiographic projections are described and methods used to improve image quality. X-rays cause tissue ionization. The radiation dose for a number of radiographic procedures is listed along with regulations governing their administration. Methods of reducing dose are discussed. X-rays are high-energy electromagnetic photons produced by bombarding an anode with high-energy electrons. X-rays are used in diagnostic imaging, both in conventional radiography and CT. Different tissues absorb X-rays according to their density. Conventional radiographs allow the resolution of four densities (air, fat, soft tissue and calcification). Imaging can also be performed fluoroscopically or digitally. Exogenous contrast agents can be used to artificially enhance tissue visualization when there is no natural contrast; these have a wide spectrum of applications. Contrast agents may be classified as ‘positive’ (e.g. barium) or ‘negative’ (e.g. air) according to whether they are of high or low radiodensity. X-ray exposure should be performed only when it produces a net benefit to the patient and should be kept as low as reasonably achievable (the ALARA principle). The Ionizing Radiation (Medical Exposure) Regulations (IR(ME)R) require by law that all medical exposures to ionizing radiation are clinically justified and authorized.