FOREIGN BODY ULTRASOUND
By Dr. Ammar Ismail
Accurately diagnosing and reliably managing retained soft tissue foreign bodies (FBs) can be challenging, especially when the foreign body are radiolucent. The complications associated with retained missed FBs are often delayed and mainly related to infections which can be difficult to predict as the patient may have no obvious symptoms initially or is distracted by other injuries. Retained foreign bodies can be overlooked in up to 38% at the initial examination.1 Ultrasound can be very useful for FBs that are not easily visible on plain films, as the sensitivity of radiography can be as low as 15% for wooden FBs for example.1 The amount of ionizing radiation of computed tomography (CT) and high cost of magnetic resonance imaging (MRI) make ultrasound the study of choice in most cases. Using ultrasound can also precisely localize radiopaque FBs visualized on plain radiography.
How accurate is it?
The following table summarizes multiple studies looking at radiopaque and radiolucent FBs of different sizes and shapes, in various settings (such as cadaveric or live patients).
A high-frequency linear transducer should be used (7.5 MHz or higher) taking advantage of the improved near-field resolution since most FB will be found within 2 cm under the skin surface. Scanning the area of interest allows detection of a foreign body by finding its associated posterior acoustic shadowing or reverberation. Determination of the precise location, its size and shape can be achieved by scanning in different orientations. The surrounding tissue is examined to determine presence of fluid collections and associated injury to tendons and neurovascular structures.
A variety of soft-tissue foreign bodies can be detected by ultrasound, including wooden splinters (figures 1 and 2), glass fragments (figure 3), shards (figure 4), metal shrapnel (figure 5), needles (figures 6 and 7), and metal projectile BB (figure 8).
Ultrasound versus other imaging modalities:
For detection of superficial, non-radiopaque FBs, US has been shown to be more effective than CT, 6 provided the familiarity of the operator with appearance of FBs, with an average examination time of approximately 10 minutes. 7 CT also has a higher cost, involves radiation and can require sedation in pediatric patients. Magnetic resonance imaging (MRI) has even higher cost and more limited availability than US.
Anderson MA , Newmeyer WL and Kilgore ES. Diagnosis and treatment of retained foreign bodies in the hand. Am J Surg. 1982;144(1): 63-67.
Jacobson, Jon A., et al. “Wooden Foreign Bodies in Soft Tissue: Detection at Ultrasound." Radiology. 1998;206(1): 45-48.
Blyme PJH, et al. Ultrasonographic detection of foreign bodies in soft tissue. Arch Orthop Trauma Surg. 1990;110(1): 24-25.
Bray PW, Mahoney JL, and Campbell JP. Sensitivity and specificity of ultrasound in the diagnosis of foreign bodies in the hand. J Hand Surg. 1995;20(4): 661-666.
Gilbert FJ, Campbell RSD, and Bayliss AP. The role of ultrasound in the detection of non-radiopaque foreign bodies. Clin Radiol. 1990;41(2): 109-112.
Mizel, MS, Steinmetz ND, and Trepman E. Detection of wooden foreign bodies in muscle tissue: experimental comparison of computed tomography, magnetic resonance imaging, and ultrasonography. Foot Ankle Int. 1994;15(8): 437-443.
Crawford R, and Matheson AB. Clinical value of ultrasonography in the detection and removal of radiolucent foreign bodies. 1989;20(6): 341-3.