Phantoms for Quantitative Ultrasound.

Tissue-mimicking materials and phantoms have an important role in quantitative ultrasound. These materials allow for investigation of new techniques with the ability to design materials with properties that are stable over time and available for repeated measurements to refine techniques and analysis algorithms. This chapter presents an overview of the history of phantoms, methods of creation of materials with a variety of acoustic properties, and methods of measurement of those properties. It includes a section addressing the measurement of variance in those techniques using interlaboratory comparisons. There is a wide range of existing tissue-mimicking materials that exhibit properties similar to those of most soft tissues. Ongoing work is part of the expansion of QUS as materials are developed to better mimic specific tissues, geometries, or pathologies.

Pulse-Echo Quantitative US Biomarkers for Liver Steatosis: Toward Technical Standardization.

Excessive liver fat (steatosis) is now the most common cause of chronic liver disease worldwide and is an independent risk factor for cirrhosis and associated complications. Accurate and clinically useful diagnosis, risk stratification, prognostication, and therapy monitoring require accurate and reliable biomarker measurement at acceptable cost. This article describes a joint effort by the American Institute of Ultrasound in Medicine (AIUM) and the RSNA Quantitative Imaging Biomarkers Alliance (QIBA) to develop standards for clinical and technical validation of quantitative biomarkers for liver steatosis. The AIUM Liver Fat Quantification Task Force provides clinical guidance, while the RSNA QIBA Pulse-Echo Quantitative Ultrasound Biomarker Committee develops methods to measure biomarkers and reduce biomarker variability. In this article, the authors present the clinical need for quantitative imaging biomarkers of liver steatosis, review the current state of various imaging modalities, and describe the technical state of the art for three key liver steatosis pulse-echo quantitative US biomarkers: attenuation coefficient, backscatter coefficient, and speed of sound. Lastly, a perspective on current challenges and recommendations for clinical translation for each biomarker is offered.

An exposimetry system using tissue-mimicking liquid.

Acoustic output measurements of diagnostic ultrasound scanners are currently performed in water and derated to approximate in situ values. The derating scheme ignores nonlinear propagation of sound waves and has been shown in previous numerical and experimental studies to tend to underestimate relevant pressure and intensity values in tissue mimicking media. This work describes an alternative method, which uses a tissue-mimicking liquid with attenuation coefficient slope of 0.3 dB/cm/MHz, speed of sound of 1,540 m/s and nonlinearity parameter B/A of 7.5. The acoustic properties of this liquid are stable for at least 2 y after production. Initial results using a single M-mode configuration are presented. These results confirm that derating can significantly underestimate the pulse intensity integral and peak rarefactional pressure.

Tissue-mimicking liquid for use in exposimetry.

OBJECTIVE: Current determinations of diagnostic ultrasound exposure parameters (eg, peak rarefactional pressure and pulse intensity integral) are intended to correspond to propagation through soft tissue with a propagation speed of 1540 m/s and attenuation of 0.3 dB x cm(-1) x MHz(-1). These current measurements are made in water, which has very little attenuation, and a linear derating factor is applied to approximate 0.3 dB x cm(-1) x MHz(-1) attenuation. The fact that propagation through water as well as through soft tissue involves nonlinear propagation is not directly addressed. A better way to determine exposure parameters would be to use a liquid that has the desired tissue-mimicking properties, including a value of the nonlinearity parameter B/A representative of soft tissue. To be of practical use in the laboratory, the ultrasonic properties of this liquid must remain stable and spatially uniform for many months or years without need for periodic mixing by the user. METHODS: Fifty-two samples of fat-free milk that was concentrated to one third of its original volume by ultrafiltration were created. Each sample was preserved by a different method. The speed of sound, attenuation, and nonlinearity parameter B/A of each sample were periodically monitored by narrowband through-transmission techniques. RESULTS: Six of the 52 samples remained liquid and retained acceptably stable acoustic properties over 22 months of storage at room temperature. CONCLUSIONS: Fat-free milk, concentrated via ultrafiltration and preserved in 1 of 6 different methods, has been found to be a stable tissue-mimicking liquid with acoustic properties appropriate for use in exposimetry.

Interlaboratory comparison of ultrasonic backscatter coefficient measurements from 2 to 9 MHz.

OBJECTIVE: As are the attenuation coefficient and sound speed, the backscatter coefficient is a fundamental ultrasonic property that has been used to characterize many tissues. Unfortunately, there is currently far less standardization for the ultrasonic backscatter measurement than for the other two, as evidenced by a previous American Institute of Ultrasound in Medicine (AIUM)-sponsored interlaboratory comparison of ultrasonic backscatter, attenuation, and speed measurements (J Ultrasound Med 1999; 18:615-631). To explore reasons for these disparities, the AIUM Endowment for Education and Research recently supported this second interlaboratory comparison, which extends the upper limit of the frequency range from 7 to 9 MHz. METHODS: Eleven laboratories were provided with standard test objects designed and manufactured at the University of Wisconsin (Madison, WI). Each laboratory was asked to perform ultrasonic measurements of sound speed, attenuation coefficients, and backscatter coefficients. Each laboratory was blinded to the values of the ultrasonic properties of the test objects at the time the measurements were performed. RESULTS: Eight of the 11 laboratories submitted results. The range of variation of absolute magnitude of backscatter coefficient measurements was about 2 orders of magnitude. If the results of 1 outlier laboratory are excluded, then the range is reduced to about 1 order of magnitude. Agreement regarding frequency dependence of backscatter was better than reported in the previous interlaboratory comparison. For example, when scatterers were small compared with the ultrasonic wavelength, experimental frequency-dependent backscatter coefficient data obtained by the participating laboratories were usually consistent with the expected Rayleigh scattering behavior (proportional to frequency to the fourth power). CONCLUSIONS: Greater standardization of backscatter measurement methods is needed. Measurements of frequency dependence of backscatter are more consistent than measurements of absolute magnitude.