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Pearls & Pitfalls
for Soft Tissue Ultrasonography
1 Probe Choice: Use a high frequency (7.5 to 10 MHz) linear array transducer.
2 Optimize Imaging: Superficial structures are difficult to visualize due to echo reverberations from the transducer. Even with high-frequency transducers, better visualization might be achieved with a standoff pad or liquid interface to provide a better acoustic window. You can try water submersion or a 500ml bag of saline or saline-filled glove sandwiched between two layers of acoustic gel. With water submersion, float the ultrasound probe 1 cm above the skin.
3 Know Normal Structures: It helps to know what normal looks like in order to properly identify abnormal findings. Skeletal muscle will appear hypoechoic with interwoven echogenic striations and hyperechoic fascial planes. Tendons will appear brightly hyperechoic with visible linear fibers on long-axis scanning. Bones will have a very hyperechoic cortex and posterior shadowing.
4 Know Abnormal Structures: Foreign bodies will have different appearance depending on the material. For wood, ultrasound is 90-95% sensitive. A splinter will appear echogenic and produce an acoustic shadow farfield. You can also easily visualize other material such as glass, plastic, and metal on bedside ultrasound. Look for sonographic artifacts such as acoustic shadowing or reverberation artifact coming off the foreign body. If an inflammatory reaction occurs, which usually requires 1-2 days, a black rim of fluid may surround the foreign body and aid visualization.
5 Compare: Use the contralateral side for a comparison view when imaging findings are equivocal
6 Removal Tricks: A short angiocath “finder needle” can be inserted during localization with ultrasound and left in place during to help pinpoint the offending foreign body during removal attempts.
7 Practice Makes Perfect: With bedside ultrasound, there is no substitute for experience. The more scans you do, the better you will be able to differentiate abnormal from normal, even when you may not be sure exactly what the abnormality is. You can find many ultrasound images in the ultrasound section of epmonthly.com.
1 EP attendings are able to identify foreign bodies with 98% sensitivity and 70% specificity while trainees had 86 and 83% sensitivity and specificity, respectively. Neither attendings nor trainees were able to accurately identify the number of foreign bodies present in a wound. (Nienaber 2010).
2 Manthey et al., demonstrated far less accuracy with Radiologists evaluating chicken bones, with the sensitivity of ultrasound in detecting gravel: 40%, metal: 45%, glass: 50%, cactus spine: 30%, wood: 50%, and plastic: 40%.
3 Radiography detected foreign bodies generally considered radiopaque (gravel, glass, metal) 98% of the time, but it never detected bodies considered radiolucent (wood, plastic, cactus spine). (Manthey et al.)
4 Turkcer et al., also assessed chicken bones, but found better diagnostic performance with the overall sensitivity, specificity, as well as positive predictive and negative predictive values of plain radiography for both nonradiopaque foreign bodies were 5%, 90%, 33%, and 48%, respectively; those of soft-tissue radiography for both nonradiopaque foreign bodies were 5%, 90%, 33%, and 48%, respectively; and those of ultrasonography for both nonradiopaque foreign bodies were 90%, 80%, 81%, and 89%, respectively.
5 Hill et al., demonstrated intermediate accuracy. Ultrasound detected 44 of 53 foreign bodies (83% sensitivity). Wood foreign bodies were detected 25 of 27 times (93%) and plastic foreign bodies 19 of 26 times (73%). Overall, there were 11 false-positive readings (59% specificity).