O Contrast Agents

Radiography and Computed Tomography

A photographic film containing a radiographic image is properly called a radiograph, although it is commonly referred to as an x-ray or a film. The relative difference between the light and dark areas on a radiographic image reflects what is called radiographic contrast. Traditional radiographic images of the body, such as skeletal, abdominal, and chest x-rays, have four radiographic densities: air, fat, water (organs, muscle, soft tissue), and bone/metal. Organs and tissues are made visible according to how well they attenuate x-rays. The attenuation of x-rays by tissues is a complex process that depends on many factors, including, but not limited to, the energy of the x-ray beam and the density of the tissue. Bone has an average density of about 1.16 g/cm3, which accounts for its ability to absorb most of the radiation it encounters.

Whereas traditional radiological "film" studies have been used since 1895 and continue to be a mainstay of diagnostic medical imaging, they have their limitations. Many organs and tissues of the body are not easily discriminated from each other on traditional radiographic images. For example, the liver, spleen, kidneys, intestines, bladder, and abdominal musculature all have very similar radiographic densities and are difficult, if not impossible, to distinguish from each other.

To produce diagnostic x-rays, a very high voltage (20,000-150,000 volts) is applied to a glass vacuum tube that contains a cathode and a rotating anode (Fig. 11.6). The cathode is a filament that is heated to a very high temperature, which provides a copious source of electrons. These electrons are accelerated toward the positively charged anode. When the accelerated electrons strike the anode (called the target), x-rays are produced. The distribution of x-rays is as a continuous spectrum. Lower-energy x-rays are absorbed by an aluminum filter, while higher-energy x-rays travel through the patient, exposing the x-ray film, and producing a traditional radiograph. Digital radiography is the process of producing a digital radiographic image where a specialized phosphor plate is used in place of traditional film. Such digital images can be readily viewed on a monitor, stored, and transferred much like any other type of digital information.

Rotating Anode

Tube

Filament Electrons

Filter (3mm Al) Lead Collimator

Rotating Anode

Tube

Filament Electrons

Filter (3mm Al) Lead Collimator

Grid

X-ray Film

Screens

Figure 11.6 • Schematic diagram of an x-ray tube for production of x-ray films.

Grid

X-ray Film

Screens

Figure 11.6 • Schematic diagram of an x-ray tube for production of x-ray films.

video camera aperature image intensifier video camera aperature patient

image intensifier anti-scatter grid table collimation filters x-ray tube

Figure 11.7 • Schematic diagram of a fluoroscopic x-ray system for real-time imaging during medical procedures. (Reprinted with permission from Bushberg, J. T., et al.: The Essential Physics of Medical Imaging, 2nd ed. Philadelphia: Lippincott Williams and Wilkins, 2002.)

anti-scatter grid patient table collimation filters x-ray tube

Figure 11.7 • Schematic diagram of a fluoroscopic x-ray system for real-time imaging during medical procedures. (Reprinted with permission from Bushberg, J. T., et al.: The Essential Physics of Medical Imaging, 2nd ed. Philadelphia: Lippincott Williams and Wilkins, 2002.)

Computer

X-ray Tube

Original Image Reconstruction

Radiation Detectors

Figure 11.8 • Schematic diagram of a computerized axial tomography (CAT) system for producing cross-sectional images of the body.

Original Image Reconstruction

Computer

X-ray Tube

Radiation Detectors

Figure 11.8 • Schematic diagram of a computerized axial tomography (CAT) system for producing cross-sectional images of the body.

An invaluable modification of the x-ray system is fluoroscopy (Fig. 11.7). This modality allows one to visualize organs in motion, position the patient for spot film exposures, instill contrast media into hollow cavities, and insert catheters into arteries.

CT (sometimes called CAT, Fig. 11.8) uses ordinary x-ray energies for imaging and applies complex mathematical reconstructions to produce multiple images of the body in the axial and other planes. This technique typically provides more detailed information about the anatomical region of interest because the images obtained are numerous thin slices of the imaged area. With this process, the reconstructed images increase the visibility of small differences in the radiographic densities between tissues to a far greater extent than traditional radiographic film.

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