Retained Unexploded Ordnance: A Safety Analysis of Diagnostic Ultrasound on Piezoelectric Fuzes

Retained unexploded ordnance (UXO) represents a rare, high‑stakes clinical entity in military trauma. Current Joint Trauma System Clinical Practice Guidelines explicitly advise against the use of diagnostic ultrasonography for UXO evaluation because of the theoretical risk of actuating piezoelectric fuzes via acoustoelectric coupling. This prohibition forces surgeons to rely on plain radiography, sacrificing real-time assessment of soft tissue and neurovascular structures. This study evaluates the validity of this theoretical risk by subjecting functional piezoelectric fuze components to diagnostic ultrasound energy under clinically extreme but realistic exposure conditions.

In collaboration with the Naval Explosive Ordnance Disposal (EOD) Technology Division, we assessed the effect of ultrasound energy on seven configurations involving five electrically functional piezoelectric fuze assemblies (PZ‑11, VP‑7M, M69, M79, and VP‑16), including intact, damaged, and isolated crystal conditions. To detect potential actuation, fuzes were wired to a continuous monitoring oscilloscope with a preset safety threshold of 4 V, which is significantly lower than the standard 400 V firing requirement for military-grade fuzes. Imaging was performed using a Philips EPIQ 7 system equipped with C5-1 (1-5 MHz) and L12-5 (5-12 MHz) transducers. Testing conditions simulated maximum acoustic coupling: immersion in ultrasound transmission gel, direct probe contact with exposed piezoelectric elements, and a dynamic "hand-hold" model to introduce tissue harmonics and boney refraction. The system was operated at maximum gain settings across B-mode, color Doppler, and harmonic imaging modes.

Across all tested frequencies (1-12 MHz) and standoff distances, no voltage generation exceeding the 4 V safety threshold was observed. The maximum recorded output remained consistently below the detection baseline (<1 V) for all fuze configurations, including unshielded piezoelectric elements. Although mechanical handling artifacts were observed, they were negligible compared to firing thresholds. Positive control testing via mechanical impact verified fuze functionality and oscilloscope sensitivity between all iterations.

Diagnostic ultrasound did not induce sufficient voltage to actuate the internal piezoelectric crystal of the tested fuzes, likely because of significant acoustic impedance mismatch at the munition casing and mechanical damping of the internal crystal. Although current guidelines recommend only plain radiography, these findings suggest that the risk of acoustoelectric detonation is minimal. Diagnostic ultrasound may be considered a safe adjunct for surgical planning in retained UXO scenarios when applied with appropriate standoff techniques.