Ultrasound acoustic shadow width is an accurate predictor of kidney stone size

Previous studies have shown overestimation of kidney stone size in ultrasound images. We explored measuring the stone's acoustic shadow as a predictor of stone size. Forty-five calcium oxalate monohydrate (COM) kidney stones ranging from 1 to 10 mm were imaged in a water bath using a research-based ultrasound system and C5-2 transducer. Stones were imaged at depths of 6, 10, and 14 cm. The widths across the stone image and across acoustic shadow distal to the stone image were measured by the operator and through an automated algorithm. Measuring size across the stone image consistently overestimated: overestimation was 0.9±0.8 mm, 1.5±1.0 mm, 2.0±1.2 mm (manual) and 0.5±1.7 mm, 0.4±1.5 mm, 0.8±1.1 mm (automated) at 6, 10, and 14 cm depths. Measurement of the acoustic shadow width more accurately estimated stone size: 0.0±0.4 mm, 0.0±0.6 mm, and-0.2±0.8 mm (manual) and 0.2±0.5 mm, 0.1±0.8 mm, and 0.1±1.0 mm (automated) at 6, 10, and 14 cm depths. Measurement from the shadow reduced misclassification of passable stones <5 mm to requiring surgery >5 mm from 25% to 7%. The results have implications for directing treatment of asymptomatic stones based on ultrasound images. [Work supported by NIH DK043881, DK092197, and NSBRI through NASA NCC 9-58.]