Section III. CONTRAST MEDIA
a. Photo Absorption. It would be difficult, if not impossible, to produce
diagnostic radiographs of arteries, veins, cavities, or passages without special contrast
media. Radiographic contrast is made possible by the selective absorption of x-ray
photons. However, photon absorption by a structure that is surrounded by structures of
equal or similar densities is not sufficiently selective to produce adequate radiographic
contrast. In cases where structure densities are similar, contrast media are used to
alter the photon absorption and, therefore, produce the necessary radiographic contrast.
The radiographs in figure 1-9 A, B and C, illustrate how a contrast medium alters photon
absorption to allow visualization of the stomach. Radiograph 1-9 A is a plain film of the
abdomen. The stomach is not seen because its density is similar to the surrounding
structures. Radiograph 1-9 B shows that the radiographic contrast between the
stomach and surrounding structures has been enhanced because the stomach has
been filled with a contrast medium that increased the photon absorption. Radiograph
1-9 C shows the stomach filled with air. In this case, contrast has been improved over
that of A because photon absorption has been decreased.
b. Radiopaque and Radiolucent Contrast Media. Contrast media that
increase photon absorption are termed radiopaque (positive) contrast media and are
made from substances of high atomic numbers, such as iodine and barium. Media that
decrease photon absorption are classified as radiolucent (negative) contrast media and
are substances with low atomic numbers, like gases. These media are sometimes
referred to as positive and negative media; however, in this subcourse, they will be
termed radiopaque and radiolucent.
c. Toxicity. It should be noted that regardless of atomic number, a contrast
medium must not be excessively toxic to the patient. If, for example, pure iodine or pure
barium were used as a medium, the patient would become violently ill. This is because
iodine and barium in their natural states are poisons. So the two requirements for a
good contrast medium are that it must change photon absorption and that it must be
relatively nontoxic. In the case of iodine and barium, low toxicity is obtained by
chemically combining them with other elements.
d. Grouping of Contrast Media. In this subcourse, we will consider contrast
media grouped by chemical makeup and usage. Media that are chemically similar and
those used for the same examination are grouped together. Three examples of such
groups are oral, injectable, and noninjectable contrast media. These terms are rather
general, so more specific groupings will also be discussed.