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A 50-year-old man presented to the ED with abdominal pain. He had end-stage renal disease that was being managed with peritoneal dialysis.

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His abdominal radiograph is shown in Figure 1.

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To better localize the radiopaque object, a lateral view of the abdomen was added by the radiologist.

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  • What is the radiopaque object seen in the mid-abdomen?

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There are several factors that contribute to the radiopacity of an object. We usually think that the intrinsic radiopacity of the material that makes up an object is the major determinant of its radiographic appearance. However, the shape of the object can play a major role in determining its radiographic appearance. In this case, the apparently radiopaque object is actually composed of relatively radiolucent material.

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The intrinsic radiopacity of a substance depends, in part, on the atomic numbers of its constituent atoms. A substance is more radiopaque if it contains atoms of high atomic number such as calcium, iodine, barium, or lead.

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X-ray radiation is absorbed when it ejects an electron from one of the inner orbitals of the atom—the photoelectric effect (Figure 2). More x-ray energy is absorbed when the electron is tightly bound to its orbital. The force binding an electron to the orbital is determined by the electrical charge of the nucleus, i.e., the number of protons in the nucleus, which is equivalent to the atomic number of the element. This is why bone, which contains calcium (atomic number 20), is more radiopaque than soft tissue, which is made up mostly of carbon (atomic number 6), hydrogen (atomic number 1), and oxygen (atomic number 8). Iodine (atomic number 53) is the key constituent of radiocontrast material and lead (atomic number 82) is an effective barrier to x-rays.

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In this patient, however, the object’s shape (i.e., the patients abdomen) rather than its intrinsic radiopacity is more important in determining its radiographic appearance. The round radiopaque mass seen in the middle of the abdomen on the AP view cannot represent an object within the abdominal cavity, e.g., a solid ball that was swallowed or an intra-abdominal tumor, because such an object would be visible within the abdomen on the lateral view (Figure 1). Alternatively, a thin flat object can be difficult to see when viewed from the side. However, a thin flat disc of sufficient radiopacity to have this appearance on the AP view would appear as a very radiopaque line when viewed from the side on a lateral view.

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