Larry Mellick, MD, MS, FAAP, FACEP
Professor of Emergency Medicine and Pediatrics
Medical College of Georgia
Pearl: When using ultrasound to locate a foreign body, the problem of near field acoustic dead space can be overcome by using interposed liquids in latex gloves, intravenous bags, or a water bath.
Case 1: A 7-year-old boy had evidence of an infection beginning in his leg following a tree branch puncture wound several days before. The operator was convinced that there was a foreign body in his anterior leg, but couldn't initially find it with the ultrasound. However, persistence paid off, and the foreign body eventually was located using the ultrasound.(Figure 1) When the skin was incised, the foreign body was located easily and removed from the patient's leg. (Figure 2)
Case 2: A 6-year-old boy presented for evaluation of a hand wound that had been present for three weeks. The wound had begun to drain purulent material, according to his mother. (Figure 3) The child fell in the backyard and, because the wound was relatively small, the mother did not seek medical care for him. A plain film of the hand demonstrated the presence of a glass foreign body. Additionally, the ultrasound operator eliminated the near field acoustic dead space with a water-filled examination glove, and the foreign body was identified easily on ultrasound. (Figure 4) After specific identification of its location, the glass fragment was removed easily. (Figure 5)
Case 3:A 26-year-old woman felt a sharp pain in her left foot while walking barefoot on her carpet. After a brief search, she and a friend found part of a sewing needle. A radiograph demonstrated a broken sewing needle under the skin. Again, near field acoustic dead space hampered localizing the needle under the skin. However, when a water bath was used (Figure 6), the ultrasound image of the needle was unmistakable, and the orientation and location of the needle were established. (Figure 7) The needle was removed without difficulty. (Figure 8)
Discussion The initial difficulty in finding the foreign body described in the first case was caused by near field acoustic dead space. This term describes the ultrasound image immediately adjacent to the transducer. The near field image quality is known to be poor and, consequently, the identification of very superficial foreign bodies often is difficult. In addition to using a higher frequency (7.5 MHZ or higher) linear array transducer,1 the near field acoustic dead space can be overcome by interposing a stand-off. There are commercially available stand-off pads made of low acoustic impedance material, which elevate the transducer from the near field acoustic dead space. However, water-filled latex gloves, smaller bags of intravenous fluids, or water baths can be used to fix the dead space problem.2,3 The water bath technique eliminates the need for ultrasound gel between the transducer and the patient's skin. In summary, when foreign bodies are suspected but not easily identified by ultrasound, the use of stand-off pads, interposed liquids in latex gloves, intravenous bags, or simply a water bath can be used to overcome the near field acoustic dead space.