Radiographic imaging for FBs has been used for many years with some degree of success. Because radiography relies on the density of the FB, it has varied sensitivities for imaging the different types of FBs. Radiography detects 98% of radiopaque objects such as gravel, most glass, and metal¹; however, radiography detects only 15% or less of radiolucent FBs such as wood, plastic, some glass, and cactus spine.² Despite the poor sensitivity of radiography to demonstrate many organic objects and some inorganic objects, conventional radiography remains the most commonly used imaging study for the evaluation of FBs in the emergency department.³ Consequently, many FBs are missed on initial evaluation. In fact, missed FBs are the second leading cause of malpractice lawsuits against emergency medicine physicians.^4,5 Wound care litigation constitutes 5% to 20% of lawsuit claims against emergency medicine physicians and results in 3% to 11% of monetary rewards.^6,7

Sonography performs well in demonstrating the FBs traditionally considered the most difficult to image on radiographic studies. In a retrospective study where radiography failed to demonstrate a STFB, sonography had a sensitivity of 95% and a specificity of 89%.^8,9 In a prospective study using cadaver feet and wooden FBs, Jacobson et al.¹⁰ reported that sonography can reveal wooden FBs as small as 2.5 mm in length with 86.7% sensitivity and 96.7% specificity. At the time of this study, the resolution of the sonography equipment either makes visualizing FBs smaller than 2.5 mm more difficult or limits adequate visualization. The equipment used today are capable of resolving objects within ±1 mm. Most recently, Mecado and Hayre¹¹ compared direct digital radiography (DDR) with sonography and concluded that sonography remains superior in identifying and detecting smaller FBs.

Whenever possible, a 7-to-18-MHz high-resolution, linear array transducer should be used to obtain the images.¹² A large-footprint transducer is generally preferable for initial FB imaging and localization, but a small-footprint transducer may be used when indicated. Maximizing image size by using a large, high-resolution monitor also facilitates FB identification. It may be difficult to identify small FBs on smaller monitors seen on some portable equipment. The visualization of superficial FBs may be facilitated by use of a water bath (Fig. 27-1). When it is necessary to increase the near field length for better visualization of the skin
surface and to place a superficial FB in the focal zone, the sonographer can use a standoff gel pad¹³, ¹⁴ and ¹⁵ or something as simple as a thicker layer of scanning gel¹⁰ (Fig. 27-2).

Shadowing and reverberation artifacts are helpful in both identifying and locating an FB. Newer technology does not provide any apparent improvement when imaging FBs and actually may make detection more difficult. Two of these instrumentation selections are tissue harmonics imaging (THI) and instrumentation that uses transmit beam-steering to create multiple imaging angles and then combines these microimages from different viewing angles into a single compounded image. The use of multiple imaging angles reduces shadow artifact, which is an important secondary sign in the detection of FBs (Fig. 27-3).
The same is true for THI, where shadowing is diminished or eliminated completely. Only the speckle reduction imaging (SRI) feature has helped improve image quality over normal imaging parameters (Fig. 27-4).

Color and power Doppler imaging may further increase sensitivity because the normal inflammatory reaction materializes in a hypoechoic ring around the FB as a result of hyperemic flow.¹⁶ Color Doppler parameters should be set to visualize slow flow (low velocity) to help determine if reactive hyperemia is present.¹⁶ Varying the transducer angle in relation to the FB will ensure the best color flow image is obtained with the highest Doppler shift.

The organic FB is the most difficult to locate and is rarely demonstrated on radiography; however, organic FBs are easier to locate and demonstrate on sonography. The echogenic pattern of an organic FB varies with its indwelling age. Some inorganic FBs can be difficult to image using radiography but present little challenge for sonography. Metallic FBs can be seen on radiography and sonography. Metals and most glass are radiopaque (with metals being more radiopaque than glass) and relatively easily seen with radiography (Fig. 27-5).

When locating an FB, radiographs should be use if available to provide further information regarding the location, depth, and composition of the FB. When used, radiographs should be obtained in two perpendicular projections (imaging planes) in order to triangulate the location. The FB will only be radiographically visualized if its density is greater than the surrounding soft tissue.

Radiographs may assist in targeting the location for initial insonation. This is especially helpful when external signs such as swelling or redness are not present. A working knowledge of the normal soft-tissue structures can assist in separating normal structures from that of the FB (Fig. 27-6A). By documenting these anatomic structures and their relationship to the FB (veins, arteries, tendons, etc.), the physician can plan the best approach for its removal to diminish postoperative complications¹⁴ (Fig. 27-6B).

The smaller the FB, the more difficult it is to visualize.¹⁷ The depth of an FB will determine the optimal frequency of the transducer. Ideally, the highest frequency possible that allows adequate depth of penetration should be utilized. Every attempt should be made to visualize the FB perpendicular to the transducer to minimize the potential errors in location and position within the soft tissue¹⁴ (Fig. 27-7). When the FB is visualized, its location should be marked on the overlying skin for the physician to use during sonography-guided FB removal.