Imaging studies should be ordered in most cases in which a retained foreign body is suspected but not found during wound exploration or when exploration of the entire wound is technically impossible.3,17 Imaging is also useful after initial removal of multiple foreign bodies to determine if all the pieces were found.
Four imaging modalities are available: plain radiography, US, CT, and MRI.18 The sensitivity and specificity of each imaging modality depend on the object's size, shape, density, and orientation relative to the imaging beam (Table 45-1).3,18 Materials that are the same density as surrounding soft tissue are difficult to see with any type of radiographic or sonographic technique.
TABLE 45-1Imaging Modalities for Detection of Soft Tissue Foreign Bodies20,21,22,23 ||Download (.pdf) TABLE 45-1 Imaging Modalities for Detection of Soft Tissue Foreign Bodies20,21,22,23
|Material ||Plain Radiographs ||High-Resolution US ||CT ||MRI |
|Wood ||Poor ||Good ||Moderate to good ||Moderate |
|Metal ||Excellent ||Good ||Excellent ||Poor |
|Glass ||Excellent ||Good ||Excellent ||Good |
|Organic (most plant thorns and cactus spines) ||Poor ||Good ||Good ||Good |
|Plastic ||Moderate ||Moderate to good ||Good ||Good |
|Palm thorn ||Poor ||Moderate ||Good ||Good |
Accurate localization of a foreign body before 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, paper clips, or a grid, on the skin at the wound entrance or directly over the object.19 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 degrees 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 (Figures 45-3 and 45-4).
Hypodermic needles used as skin markers.
Needle markers are used to triangulate the location of a radiopaque foreign body. A. Plain radiograph, anteroposterior view. B. Oblique view.
The limitations of this technique are that it does not provide a true three-dimensional view and that images on radiographs are distorted by divergence and parallax. Tendons and other structures may block the most direct path to the foreign body.
Unless a foreign body is embedded at a relatively superficial level or lies within the cavity of a fresh wound, it will not be easily detected or located by physical examination. If a foreign body is suspected based on the mechanism of injury but not found during exploration of a wound, a radiograph should be ordered first, because plain radiography will detect as many as 80% to 90% of all foreign bodies. It is also prudent to order films if a patient believes there is a retained object.11,15,17 If the wound was caused by metal, glass, or gravel and no foreign body was found on plain films or wound exploration, the physician can end the search. For objects not routinely visible (or not found) on plain radiography, like wood, sonography is the modality of choice, with CT or MRI as alternatives.18 The bottom line is that no single imaging modality is ideal for all types of foreign bodies.
Most foreign bodies that can be missed during the initial clinical evaluation can be seen on plain radiographs, but the images must be inspected carefully to detect small and faint objects.3 Metal, mammalian bone, some types of fish bones (cod, haddock, grey mullet, red snapper, and sole), teeth, pencil graphite, certain plastics, glass, gravel, sand, and aluminum are visible on plain radiographs. Almost all glass is visible on radiographs if it is 2 mm or larger, and glass does not have to contain lead to be visible on plain films (Figure 45-5).24 A radiopaque fragment is more easily seen if its long axis is positioned parallel to the central ray of the x-ray beam, increasing its apparent density; thus a foreign body may be evident on one radiographic view but not another.
A. The marker points to a glass fragment in the plantar surface of this patient's foot on the lateral view of this radiograph. B. Oblique view.
Obtain plain film radiographs using an underpenetrated soft tissue technique, producing a lighter image that increases the contrast between the foreign body and surrounding tissue. If the radiograph is displayed on a digital imaging system, the contrast and brightness of the image can be adjusted to achieve the same effect as an underpenetrated film. Digital edge enhancement adjustments may make the foreign object stand out from the background. Radiographs should be taken in multiple projections to separate the shadow of the foreign body from underlying bone and to help gauge the depth of the object in the tissue (Figure 45-6). Chronic inflammatory changes may create secondary bony changes, such as osteolytic and osteoblastic lesions, pseudotumor formation, and periosteal reaction, revealing the object's location.
A. The identity of this metallic foreign body is not apparent on the anteroposterior view of this radiograph. B. The shape and depth of the blade are best seen on the lateral view. C. Oblique view.
Many common or highly reactive materials, such as wood, thorns, cactus spines, some fish bones, other organic matter, and most plastics, are not visible on plain radiographs.3,18,25 Sometimes, there is indirect evidence of their presence. A radiolucent filling defect may occur when the object is less dense than surrounding tissue. However, even radiopaque foreign bodies may be invisible on plain films if they are obscured by, or impacted in, bone.
CT is capable of detecting more types of foreign materials than plain film radiography because it is 100 times more sensitive in differentiating densities (Figure 45-7).18 Subtle density differences can be distinguished with a narrow radiographic density window adjustment, particularly if a computer workstation is used to vary the gain and contrast settings. Thorns, spines, wood splinters and toothpicks, fish bones, and plastic foreign bodies can be identified with CT.3 In dry wood, the interstices between the fibers are filled with air, imparting reduced radiodensity and making dry wood less visible on CT.18 Water-rich wood, either fresh from a tree or as occurs when dry wood has been imbedded in tissue long enough to absorb serohematic fluid, is more visible as a slightly hyperdense object on CT.26
This patient sustained forehead lacerations when he struck his head on a car windshield. Glass foreign bodies were not identified before wound closure and were not visible on plain films, but were evident on CT.
CT will often detect the inflammatory response to an object that has been in place long enough to elicit the reaction. CT may detect objects embedded in bone, and isodense objects may be outlined by surrounding air within the wound. Digital edge enhancement can further improve the visibility of these objects. CT images can be created in multiple planes and can demonstrate the relationship of a foreign object to important anatomic structures. The principal disadvantages of CT are its cost, higher radiation dose, and the fact that wood and other organic material have radiographic density close to water, making them difficult to distinguish from surrounding tissue. Another pitfall of CT is that wood foreign bodies may initially mimic air bubbles on CT images.27
US is useful for directing exploration and foreign body removal.28,29,30 US can identify a wide variety of soft tissue foreign bodies such as wood, fish bones, sea urchin spines, other organic material, fiber, and plastic, with >90% sensitivity for foreign bodies >4 to 5 mm (Figure 45-8).31,32,33,34,35,36 Detection rates may be less due to the size and sonographic nature of the foreign body, the number of foreign bodies, the presence of confounding factors (e.g., bone, blood, purulence, scars, old sutures) associated with the foreign body, and operator skill and experience. Foreign bodies appear as hyperechoic foci, usually with acoustic shadowing extending distally.37 A hyperechoic rim, or halo sign, indicates an abscess or granuloma around the object. Sonography can estimate the depth of a foreign body below the skin surface.34
Sonographic images of (A) beer bottle glass (arrow), (B) sewing needle with long axis parallel to the scan plane (arrow), and (C) cactus spine (arrow), in a tissue model. All objects appear as bright, hyperechoic foci.
With experience, sonography can be applied to body areas previously difficult to image for soft tissue foreign bodies, such as the hands and feet.38,39,40,41 Soft tissue gas does not appear to reduce the ability to detect a foreign body but does decrease the ability to discriminate between metal and wood.42
An important technical aspect of US for soft tissue foreign body detection is the transducer frequency.43 Higher frequencies have a reduced effective depth of penetration for the US wave. A 3.5-MHz transducer will locate foreign bodies that are as deep as 10 cm, a 5-MHz transducer at depths of approximately 7 cm, a 7.5-MHz transducer at depths of 5 cm, and a 12.5-MHz transducer at depths of 2.0 to 0.2 cm. Conversely, the resolution of the image—ability to distinguish two adjacent objects and detect small objects—is greatest with higher frequencies. Thus, low frequencies can detect larger objects at greater depth but may miss smaller objects and may not be able to discriminate multiple objects. Higher frequencies will detect smaller and multiple objects, but only at shallower depths, and may miss deeper objects. Use both low and high transducer frequencies for best advantage.
US has a unique set of limitations. Areas with many echogenic structures, such as calcifications, sesamoid bones, and tendons, may hide foreign bodies within their acoustic shadows, so these areas must be scanned slowly to detect foreign bodies that are small or oriented perpendicular to the skin surface. Some areas of the body that are prone to foreign body penetration, such as the web spaces of the hands or toes, may not accommodate a US probe using standard gel. In this circumstance, a water-bath interface between the probe and body part is useful.38,39,40,43,44 False-positive findings result in an unnecessary surgical dissection. False-positive rates for sonography vary from 3% to 15% in clinical case series, depending on the material being studied.34,36,45,46
Once a foreign body is confirmed by plain films or CT studies, US can be used in place of fluoroscopy to guide an instrument to the object during retrieval.28,29,30,38,39 The scanning beam should be oriented parallel to the long axis of a hemostat, which can be directed toward the long axis of the foreign body (Figure 45-9). Transverse and longitudinal scans provide views in multiple planes. A 7.5-MHz linear-array transducer can be used to find objects that are small and superficial (up to 5 cm deep), and a 5.0-MHz transducer can be used for larger and deeper objects. The linear scan is preferred for localization, and the sector scan for retrieval.28 The primary advantage of sonography is the avoidance of radiation exposure.
A. Sonographic image of the long axis of a hemostat grasping a piece of glass (arrow). B. Sonographic image of the tips of a hemostat surrounding a piece of glass (arrow).
MRI can detect nonmetallic radiolucent foreign bodies and, in comparison studies, is more accurate (less sensitive than US but fewer false-positive interpretations) than any other modality in identifying wood, plastic, spines, and thorns.3,18,47 MRI should not be used with gravel or metal-containing foreign bodies because ferromagnetic streaks obscure visualization. MRI may provide more information than CT about the position of a foreign body relative to (and its effects on) nearby structures, such as tendons, neurovascular bundles, joints, and muscles.
Bedside fluoroscopy accurately detects radiopaque (metal, gravel, glass, and pencil graphite) foreign bodies as small as 3 mm.48,49 Using fluoroscopy to accurately detect glass foreign bodies can be accomplished in a brief training session.50,51 Advantages are convenience, reduced cost, shortened ED time, and, if done with brief intermittent imaging and appropriate shielding, less radiation exposure than CT. An important limitation is that the body part must be able to fit within the fluoroscopic beam path and be thin enough that a viewable image can be obtained. For most adult patients, this limits fluoroscopy to the limbs. Fluoroscopy can be used to assist foreign body removal by helping guide the instrument, as the body part (usually extremity) can be rotated during fluoroscopic imaging to provide a real-time, three-dimensional view of the object relative to the physician's instruments, skin surface, or skin markers.48,49 An incision is made between needles or markers, or dissection is carried along the path of the closest needle.