We studied the size,
the depth,
the US characteristics,
the related complications and the US pitfalls of foreign bodies.
We classified the different foreign bodies depending on composition: metal,
glass and vegetal (including wood,
thorn,
cactus).
It has been observed that different materials have characteristic US patterns that are useful in their diagnostic approach.
Most of foreign bodies are linear in shape and they should be visualized in both long and short axis.
If there are doubts about their presence,
comparison with the contralateral side at the same location is often helpful.
All foreign bodies demonstrate to be hiperechogenic compared to surrounding soft tissue.
Some of them show posterior shadowing,
reverberation and surrounding hypoechoic rim.
US artefacts can aid in the foreign body identification:
- The surface characteristics of the object influence the type of artefacts produced,
particularly “clean” versus “dirty” shadowing.
- Objects with a rough and irregular surface result in clean shadowing.
- Objects with a smooth and flat surface result in dirty shadowing and reverberation artefacts.
The flat surface has to be perpendicular to the ultrasound beam to produce this artefact.
- Surrounding hypoechoic rim.
- An inflammatory process usually develops within 24 hours of the presence of a retained foreign body,
as a hypoechoic halo around the foreign body.
- Hypoechoic halos are due to oedema,
abscess,
or granulation tissue.
- This hypoechoic rim improves the sensitivity and specificity of US foreign body detection.
US FOREIGN BODY EVALUATION
METAL
- Detection with radiography: Metal foreign bodies can be seen on radiographs (Fig.
2).
However,
the exact location of a radiopaque foreign bodies,
its relationship to surrounding structures,
and the degree of associated soft-tissue injurie are not always visible on radiography and can be further defined with US.
- Ultrasound characteristics: metal foreign bodies are brightly hyperechoic with a comet tail or reverberation artefact due to their smooth surface (Fig.
3).
GLASS
- Detection with radiography: Glass foreign bodies are visible on radiographs.
Glass does not have to contain lead to be radiopaque; all glass material is radiopaque.
Common glass products such as drinking glasses,
bottle glasses and light bulbs,
contain no lead,
but are radiopaque (Fig.
4).
- Ultrasound characteristics: glass appears hyperechoic with posterior shadowing intermixed with a small amount of reverberation artefact (Fig.
5).
VEGETAL
- Detection with radiography: splinters,
thorn and cactus are not visible on radiographs (Fig.
6).
- Ultrasound characteristics: splinters and thorns are generally slightly hyperechoic with acoustic shadowing of variable depth and density (Fig.
7) Wood and other organic material foreign bodies lose echogenicity over time.
The inflammatory process,
seen as a surrounding hypoechoic rim and an increase of Doppler signal,
is more evident and accelerated with organic material like wood or contaminated metal.
Other foreign bodies:
- Plastic: hyperechoic with acoustic shadowing.
- Sand and pebbles (Fig.
8): produce a strong acoustic shadow (like a gallstone).
ASSOCIATED COMPLICATIONS
US allows an excellent examination of the surrounding muscles,
tendons,
ligaments,
and neurovascular structures and the assessment of associated soft tissue injurie.
The main complications are: cellulitis,
abscess formation,
tenosynovitis,
tendon rupture,
septic arthritis,
bone erosion and migration.
The associated complications depend on the depth and localization of the foreign body.
Soft-tissue infection is,
by far,
the most common complication of a penetrating foreign body.
Infection can result in the development of an abscess,
which will appear as an anechoic collection on ultrasound.
Gas bubbles within the tissues or abscess,
can be detected in cases of infection by gas-forming organisms.
Some materials are more likely to produce some specific complications:
- The most significant soft tissue inflammation and infections are present when the foireign body is organic (wood and contaminated metal): cellulitis (Fig.
9),
abscess formation (Fig.
10),
tenosynovitis (Fig.
11) and septic arthritis (Fig.
12).
- To originate bone erosion is necessary a tough material like metal (Fig.
13).
- Tendon rupture is more frequently associated to metal and glass foreign bodies (Fig.
14).
- Migration,
an uncommon phenomenon,
can be present in metallic and glass materials (Fig.
15).
There are some cases of migration in literature of sternal hardware,
metal wire and glass.
The following table summarizes foreign bodies radiographic,
US characteristics and associated complications (Table 2).
The ideal treatment window for retained foreign bodies is within 24 hours,
which allows for visualization of the entry and exit wounds and decreased inflammation,
induration,
and scarring.
Delayed treatment may lead to complications including infection,
delayed wound healing,
inflammation,
and loss of function.
Attempts of removal can also result in further tissue damage,
foreign body migration or retention,
infection,
and neurological damage.
US FOREIGN BODY PITFALLS
Various normal and pathological structures can also present as foreign bodies,
resulting in false positives:
- Calcifications (Fig.16).
- Sesamoid bones.
- Scar tissues.
- Intermuscular septums (Fig.
17).
- Hyperechoic muscle fibres.
- Fresh hematomas.
- Air trapped in the soft tissues.
Surgical exploration can introduce air into the soft tissues that can limit visualization of the foreign body.
Calcifications typically appear as rounded echogenic foci with strong posterior shadowing and are radiopaque.
Scar tissue and hematomas are solid mass lesions and usually would not be confused with a linear echogenic foreign body.
Hematomas can have variable echogenicity depending on the presence of fluid components.
Finally,
the soft tissues in smaller and more complex parts of the body,
such as the hands and feet,
contain various structures that can complicate the localization of the foreign body.