Interlaboratory Comparison of Backscatter Coefficient Estimates for Tissue-Mimicking Phantoms

Abstract

Utrasonic backscatter is useful for characterizing tissues and several groups have reported methods for estimating backscattering properties. Previous interlaboratory comparisons have been made to test the ability to accurately estimate the backscatter coefficient (BSC) by different laboratories around the world. Results of these comparisons showed variability in BSC estimates but were acquired only for a relatively narrow frequency range, and, most importantly, lacked reference to any independent predictions from scattering theory. The goal of this study was to compare Faran-scattering-theory predictions with cooperatively-measured backscatter coefficients for low-attenuating and tissue-like attenuating phantoms containing glass sphere scatterers of different sizes for which BSCs can independently be predicted. Ultrasonic backscatter measurements were made for frequencies from 1 to 12 MHz. Backscatter coefficients were estimated using two different planar-reflector techniques at two laboratories for two groups of phantoms. Excellent agreement was observed between BSC estimates from both laboratories. In addition, good agreement with the predictions of Faran’s theory was obtained, with average fractional (bias) errors ranging from 8–14%. This interlaboratory comparison demonstrates the ability to accurately estimate parameters derived from the BSC, including an effective scatterer size and the acoustic concentration, both of which may prove useful for diagnostic applications of ultrasound tissue characterization.

Cross-imaging system comparison of backscatter coefficient estimates from a tissue-mimicking material

“…The laboratory results are from combined measurements using four single-element transducers as previously presented. 8 From Fig. 2, it can be observed that BSC estimates from all systems are in very good agreement with values from Faran's theory as well as the laboratory measurements.…”

“…[7][8][9] These studies have enabled researchers to uncover sources of errors in measurements that, once eliminated, resulted in inter-laboratory agreement among BSC estimates on identical samples. 8,9 The studies by Wear et al, 7 Anderson et al, 8 and King et al 9 focused on laboratory-based systems, measurement, and data processing techniques. However, to apply QUS in a clinical setting, it is necessary to also demonstrate system and operator independence of BSC estimates using arraybased ultrasound imaging systems.…”

“…However, to apply QUS in a clinical setting, it is necessary to also demonstrate system and operator independence of BSC estimates using arraybased ultrasound imaging systems. Normalizing data using echo signals from planar reflectors, as performed in the preceding studies, [7][8][9] is complicated in clinical machines because of dynamic focusing of the received beam and use of internal TGC. These systems generally exhibit greater variability in transducer geometry and beamforming functions than simple, single-element transducer systems, and this makes calculation of pulse-echo beam properties as used for BSC data reduction more challenging.…”

“…10 RF echo data were acquired from a sample used previously to verify performance accuracy of laboratory BSC measurement systems. 8 BSC estimates from data acquired by the different clinical imaging systems were compared with these laboratory measurements as well as with results from a theoretical model.…”

“…8 Fit parameters are also presented in Table I. The backscatter measurements will be presented in Sec.…”