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Rajesh Geria, M.D., RDMS

V.  Foreign Body Localization

I.  Introduction and Indications
The patient presenting with a wound should always raise concern for a possible retained foreign body.  Despite meticulous irrigation and Xray imaging, some foreign bodies are still missed.  Although radiopaque foreign bodies will often be seen on standard Xrays, radiolucent objects such as wood are often not visualized.  Ultrasound has proven to be a valuable tool for localizing foreign bodies in soft tissue, especially those that are radiolucent. (1,2)

Indications

• Detection of radiolucent foreign body
• Assistance and verification of foreign body removal

II.  Anatomy

The area of interest will usually be the soft tissue, most often of the feet or hands.  Very superficial objects may be very difficult to find since sound is not transmitted nor reflected well in the area immediately near a transducer footprint. 

III.  Scanning Technique and Sonographic Findings

Ultrasound Technique:
A high frequency linear transducer (7.5 to 10 MHz) is placed on the structure of interest with or without the use of a standoff pad.  Using a ‘stand-off pad’ can elevate the transducer several millimeters above the structures of interest.  This allows better sound transmission and an improved view of the underlying soft tissues.  Filling a glove with ultrasound gel can make a standoff pad.  Foreign bodies will usually appear hyperechoic to the surrounding soft tissue.  Material such as wood or plastic tends to produce shadowing (Figure 1).  Metal objects tend to produce reverberation or comet tail artifact (Figure 2). The body part can also be placed in a water bath to enhance visualization of the structure of interest. The area is scanned throughout its entirety in search for a hyperechoic object in both the sagittal and transverse planes.  Once found, the depth down from the skin can be measured as well as the size of the object.  Survey the area surrounding the object for vessels.  Vessels in close proximity to the object may prompt the practitioner to avoid probing the area in the emergency department.

Figure 1:  Superficial located foreign body causing shadowing.

Figure 2:  Small metal foreign body in the neck causing a significant ‘comet tail’ artifact. (image courtesy of B. Hoffmann, M.D.)

 

Foreign Body Removal:
Center the transducer over the foreign body and mark the skin to identify the optimal incision site. After making a lateral incision, image the foreign body in the long axis and insert forceps or hemostat under ultrasound vision and guide towards the object. If you are unable to remove the foreign body under long axis view, attempt in a short axis view. In general, the technique works better for linear shaped objects. Lidocaine should be injected prior to the start of the procedure.

Video clip 1:  Foreign body located in soft tissue.

IV.  Pathology
Unsuccessful location and manipulation of the foreign body can lead to increased tissue injury, increased infection risk and wound healing problems. 

V.  Pearls and Pitfalls Inadequate knowledge of regional anatomy.  Remember, bone and articular surfaces may appear hyperechoic and cast shadows. Failure to realize that scar tissue may appear hyperechoic. Not looking for vascular structures in close proximity to the foreign body – this can lead to potential injuring .

VI.  References

1. Lyon M, Brannam L, Johnson D, Blaivas M, Duggal S.
   Detection of soft tissue foreign bodies in the presence of soft tissue gas. J Ultrasound Med.
   2004;23:677-81.
   
   
2. Crawford R, Matheson AB.
   Clinical value of ultrasonography in the detection and removal of radiolucent foreign bodies.
   Injury.1989;20:341-3.