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For almost half a century following the discovery of x-rays by Roentgen in 1895, radiologic imaging was mainly based on plain and contrast-enhanced radiography. Those images were created by exposing film to an x-ray beam attenuated after penetrating the body. The production of x-rays and radiographic images is described in the next chapter. In the recent half century, diagnostic radiology has undergone dramatic changes and developments. Conventional angiography, nuclear medicine, ultrasonography, and computed tomography (CT) were developed between 1950 and 1970. Magnetic resonance (MR) imaging, interventional radiology, and positron emission tomography (PET) were developed later. Conventional radiology, including contrast-enhanced radiography and CT, uses ionizing radiation created from x-ray equipment. Nuclear medicine uses ionizing radiation that is emitted from injected or ingested radioactive pharmaceuticals in various parts of the body. Ultrasonography and MR imaging modalities use sound waves and magnetism, respectively, rather than ionizing radiation.
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Radiologic subspecialties have been developed based on organ systems, modalities, and specific fields. Organ-oriented subspecialties of radiology include musculoskeletal, breast, neurologic, abdominal, thoracic, cardiac, gastrointestinal, and genitourinary imaging. Modality-oriented subspecialties comprise nuclear medicine, interventional, ultrasonography, and MR imaging. Specific field subspecialties include pediatric and women's imaging. Functional and metabolic imaging methods are now being used clinically, with genetic and molecular marker imaging expected in the future.
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This chapter is intended to provide an overview of a variety of modalities in diagnostic radiology and basic knowledge regarding radiologic image-based diagnosis. Specific modality settings in each field and diagnostic interpretation for the use of these modalities in evaluating various organ systems are described in subsequent chapters.
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Conventional radiography refers to plain radiographs that are generated when x-ray film is exposed to ionizing radiation and developed by photochemical process. During development, the metallic silver on the x-ray film is precipitated, rendering the latent image black. The amount of blackening on the film is proportional to the amount of x-ray radiation exposure. Plain radiography relies on natural and physical contrast based on the density of material through which the x-ray radiation must pass. Thus, gas, fat, soft tissue, and bone produce black, gray-black, gray, and white radiographic images, respectively, on film (Figure 1-1).
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Although other image modalities such as CT, ultrasonography, and MR imaging are being used with increasing frequency to replace plain radiographs, conventional radiography remains a major modality in the evaluation of chest, breast, bone, and abdominal diseases.
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Computed radiography (CR) or digital radiography is presently replacing conventional screen-film combination techniques. The most common CR technique, photostimulable phosphor computed radiography (PPCR), uses a phosphor-coated plate to replace the film-screen combination. When a cassette containing the phosphor-coated plate is exposed to x-rays, the phosphor stores the absorbed x-ray energy. The ...