Written by Chris Thom
Spoon Feed
There are several good options for POCUS coupling interfaces, but saline bags were shown to be inferior in this prospective study.
Sound waves just need a way to travel
This was a prospective study of ultrasound imaging interfaces in the detection of foreign bodies using porcine feet. Eight emergency medicine (EM) interns, 8 EM senior residents, and 2 EM ultrasound fellows evaluated four different imaging interfaces. Three types of foreign bodies (FB) were used (wood, plastic, and metal). Interfaces studied included normal gel, standoff saline bag, water bath, and a novel patients belonging bag (PBB) method. The PBB method involved placing the porcine foot into the PBB, filling it with water, and then sealing it tightly with a zip tie.
Results demonstrated similar accuracy for FB detection using PBB, water bath, and gel (76%, 68%, and 65% respectively). However, the accuracy was significantly lower for the saline bag (44%, p=0.012 compared to gel). Time to FB detection was quicker among fellows compared to residents. There was no difference in accuracy based on the type of foreign body evaluated.
How will this change my practice?
Ultrasound waves need a medium to travel. We often use gel, but there are scenarios where gel application and inadequate probe pressure can make the task difficult. An infected hand digit is a good example of a situation where getting the probe footprint onto the object of interest can be challenging. Water baths and “stand off” pads can be employed as coupling interfaces in cases where gel is thought to be a more difficult option (see below water bath image). The current study reinforces my own observations – saline bags do not work well. This is perhaps because we create an additional interface (e.g. bag to skin and probe to bag) and both require gel for adequate transmission. The PBB is a new trick which may have a limited role in specific limb imaging scenarios.
POCUS pro-tips and clips:
Definitely try out the water bath for hand and finger imaging. It provides terrific images with no contact on the skin. Diagnosing flexor tenosynovitis in a water bath can be very rewarding and pain free for the patient. I’ve also had success with saline or water spray instead of gel for more localized and brief skin imaging that does not require extensive time or surface area (e.g. sterile saline for US IV placement). And I’m going to keep in mind the PBB, perhaps for a FB in the plantar surface of the foot evaluation. One could conceivably do the PBB and image the entire plantar surface of the foot with minimal pain and minimal probe pressure.
Source
POCUS Standoff: Comparing Ultrasound Interfaces for Soft Tissue Foreign Body Imaging. Pediatr Emerg Care. 2025 Feb 25. doi: 10.1097/PEC.0000000000003359. Epub ahead of print. PMID: 39995269
