Size-Dependent Ultrasonic Characterization of Cylindrical Surrogate Targets Using Through-Transmission Ultrasound

Background: Thrombus formation remains a major complication during extracorporeal membrane oxygenation (ECMO), potentially resulting in circuit failure and thromboembolic events. Although several techniques have been proposed for thrombus detection, the relationship between measured ultrasonic features and target size has not been systematically characterized. This study evaluated size-dependent ultrasonic signal changes using standardized cylindrical surrogate targets under controlled conditions. Methods: A through-transmission ultrasonic system incorporating paired point-focused transducers (5 and 10 MHz), a water chamber, an ultrasonic pulser/receiver, a digital storage oscilloscope, and a manual translation stage was constructed. Cylindrical targets made of acrylic, acrylonitrile butadiene styrene (ABS), and SUS304 stainless steel, with diameters ranging from 0.1 to 5.0 mm, were scanned at 0.5 mm intervals over a 30 mm range. Baseline-subtracted waveforms were used to calculate peak height, pulse area, and squared-amplitude sum. Results: Exploratory regression analyses demonstrated positive diameter-dependent associations for all evaluated features. Pulse area showed strong linear associations across the six material–frequency conditions (R2 = 0.904–0.997), although no single feature consistently demonstrated the highest goodness of fit. Strong associations were observed at both frequencies; however, the different receiver gains precluded direct inference regarding relative frequency sensitivity. Under the 10 MHz condition, the 0.3 mm SUS304 wire target was the smallest tested surrogate target meeting the study-specific operational detection-index criterion (DI > 3). Conclusions: Through-transmission ultrasound demonstrated size-dependent changes in signals obtained from standardized cylindrical surrogate targets. Pulse area was identified as a practical candidate feature because of its consistently strong associations and computational simplicity, but neither statistical superiority nor a predictive sizing model was established. The 0.3 mm result applies to a high-acoustic-contrast SUS304 target under the specified experimental conditions and does not represent a biological thrombus detection limit.