Methods of Localization

Accurate localization of a foreign body prior to removal is important because blind searching is time-consuming and can cause further injury. However, it is usually easier to detect the presence of a foreign body than to locate its exact position. If a foreign body is radiopaque, one can estimate its location and depth by taping radiopaque skin markers such as lead circles or paper clips on the skin at the wound entrance or directly over the object. With multiple projections, the object can be seen in relation to the markers. Hypodermic needles can be used as skin markers. Two or three needles are inserted into the skin near the object at approximately 90° to each other to provide a frame of reference around the object. Plain films taken in multiple projections allow the physician to gauge the distance of the object from the closest needle or its distance between two needles.

The limitations of this technique are that it does not provide a true three-dimensional image and that images on radiographs are distorted by divergence and parallax. Tendons and other structures may block the most accessible path to the foreign body. Alternatively, the site of injury can be rotated under fluoroscopy to visualize the object between the markers.19 Radiation exposure should be minimized by brief, intermittent imaging and appropriate shielding. An incision is made between needles or markers, or dissection is carried along the path of the closest needle.

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