A 50-year-old man presented to the ED with abdominal pain. He
had end-stage renal disease that was being managed with peritoneal
His abdominal radiograph is shown in Figure 1.
To better localize the radiopaque object, a lateral view of the
abdomen was added by the radiologist.
- What is the radiopaque object seen in the mid-abdomen?
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
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.
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.
The photoelectric effect.
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