Low-echo sphere phantoms and methods for assessing imaging performance of medical ultrasound scanners.

Tissue-mimicking phantoms and software for quantifying the ability of human observers to detect small low-echo spheres as a function of depth have been developed. Detectability is related to the imager's ability to delineate the boundary of a 3-D object such as a spiculated tumor. The phantoms accommodate a broad range of transducer shapes and sizes. Three phantoms are described: one with 2-mm-diameter spheres (for higher frequencies), one with 3.2-mm-diameter spheres (for lower frequencies) and one with 4-mm-diameter spheres (for lower frequencies). The spheres are randomly distributed in each phantom. The attenuation coefficients of spheres and surroundings are nearly identical; thus, compromising shadowing or enhancement distal to spheres does not occur. Reproducibility results are given for pairs of independent data sets involving eight different combinations of scanner, transducer and console settings. The following comparison results are also reported: (i) only the selected frequency differs; (ii) transducers and scan parameters are nearly the same but manufacturers differ; (iii) ordinary B-scanning, spatial compounding and tissue harmonic imaging are addressed. The phantoms and software promise to be valuable tools for scanning system and setup comparisons and for acceptance testing.

A nonlinear elasticity phantom containing spherical inclusions.

The strain image contrast of some in vivo breast lesions changes with increasing applied load. This change is attributed to differences in the nonlinear elastic properties of the constituent tissues suggesting some potential to help classify breast diseases by their nonlinear elastic properties. A phantom with inclusions and long-term stability is desired to serve as a test bed for nonlinear elasticity imaging method development, testing, etc. This study reports a phantom designed to investigate nonlinear elastic properties with ultrasound elastographic techniques. The phantom contains four spherical inclusions and was manufactured from a mixture of gelatin, agar and oil. The phantom background and each of the inclusions have distinct Young's modulus and nonlinear mechanical behavior. This phantom was subjected to large deformations (up to 20%) while scanning with ultrasound, and changes in strain image contrast and contrast-to-noise ratio between inclusion and background, as a function of applied deformation, were investigated. The changes in contrast over a large deformation range predicted by the finite element analysis (FEA) were consistent with those experimentally observed. Therefore, the paper reports a procedure for making phantoms with predictable nonlinear behavior, based on independent measurements of the constituent materials, and shows that the resulting strain images (e.g., strain contrast) agree with that predicted with nonlinear FEA.

Nonlinear elastic behavior of phantom materials for elastography.

The development of phantom materials for elasticity imaging is reported in this paper. These materials were specifically designed to provide nonlinear stress/strain relationship that can be controlled independently of the small strain shear modulus of the material. The materials are mixtures of agar and gelatin gels. Oil droplet dispersions in these materials provide further control of the small strain shear modulus and the nonlinear parameter of the material. Since these materials are mostly water, they are assumed to be incompressible under typical experimental conditions in elasticity imaging. The Veronda-Westman model for strain energy density provided a good fit to all materials used in this study. Materials with a constant gelatin concentration (3.0% dry weight) but varying agar concentration (0.6-2.8% dry weight) demonstrated the same power law relationship between elastic modulus and agar concentration found for pure agar (1.89 +/- 0.02), consistent with percolation theory, and provided a consistent nonlinearity parameter of 4.5 +/- 0.3. The insights provided by this study will form the basis for stable elastography phantoms with stiffness and nonlinear stress/strain relationships in the background that differ from those in the target.

Evaluation of a cardiac ultrasound segmentation algorithm using a phantom.

This paper evaluates the performance of a level set algorithm for segmenting the endocardium in short-axis ultrasound images. The evaluation is carried out using an anthropomorphic ultrasound phantom. Details of the phantom design, including comparison of the ultrasound parameters with in-vitro measurements, are included. In addition to measuring segmentation accuracy, the effectiveness of the energy minimization scheme is also determined. It is argued that using the phantom along with global minimization algorithms (simulated annealing and random search) makes is possible to assess the minimization strategy.

Anthropomorphic phantoms for assessment of strain imaging methods involving saline-infused sonohysterography.

Two anthropomorphic uterine phantoms were developed that allow assessment and comparison of strain imaging systems adapted for use with saline-infused sonohysterography (SIS). Tissue-mimicking (TM) materials consist of dispersions of safflower oil in gelatin. TM fibroids are stiffer than the TM myometrium/cervix, and TM polyps are softer. The first uterine phantom has 3-mm-diameter TM fibroids distributed randomly in TM myometrium. The second uterine phantom has a 5-mm and 8-mm spherical TM fibroid, in addition to a 5-mm spherical and a 12.5-mm-long (medicine capsule-shaped) TM endometrial polyp protruding into the endometrial cavity; also, a 10-mm spherical TM fibroid projects from the serosal surface. Strain images using the first phantom show the stiffer 3-mm TM fibroids in the myometrium. Results from the second uterine phantom show that, as expected, parts of inclusions projecting into the uterine cavity will appear very stiff, whether they are stiff or soft. Results from both phantoms show that although there is a five-fold difference in the Young's moduli values, there is not a significant difference in the strain in the transition from the TM myometrium to the TM fat. These phantoms allow for realistic comparison and evolution of SIS strain imaging techniques and can aid clinical personnel to develop skills for SIS strain imaging.

Estimating the breast surface using UWB microwave monostatic backscatter measurements.

This paper presents an algorithm for estimating the location of the breast surface from scattered ultrawideband (UWB) microwave signals recorded across an antenna array. Knowing the location of the breast surface can improve imaging performance if incorporated as a priori information into recently proposed microwave imaging algorithms. These techniques transmit low-power microwaves into the breast using an antenna array, which in turn measures the scattered microwave signals for the purpose of detecting anomalies or changes in the dielectric properties of breast tissue. Our proposed surface identification algorithm consists of three procedures, the first of which estimates M points on the breast surface given M channels of measured microwave backscatter data. The second procedure applies interpolation and extrapolation to these M points to generate N > M points that are approximately uniformly distributed over the breast surface, while the third procedure uses these N points to generate a 3-D estimated breast surface. Numerical as well as experimental tests indicate that the maximum absolute error in the estimated surface generated by the algorithm is on the order of several millimeters. An error analysis conducted for a basic microwave radar imaging algorithm (least-squares narrowband beamforming) indicates that this level of error is acceptable. A key advantage of the algorithm is that it uses the same measured signals that are used for UWB microwave imaging, thereby minimizing patient scan time and avoiding the need for additional hardware.

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.

Anthropomorphic breast phantoms for testing elastography systems.

Two equivalent anthropomorphic breast phantoms were constructed, one for use in ultrasound elastography and the other in magnetic resonance (MR) elastography. A complete description of the manufacturing methods is provided. The materials used were oil-in-gelatin dispersions, where the volume percent oil differentiates the materials, primarily according to Young's moduli. Values of Young's moduli are in agreement with in vitro ranges for the corresponding normal and abnormal breast tissues. Ultrasound and nuclear magnetic resonance (NMR) properties are reasonably well represented. Phantoms of the type described promise to aid researchers who are developing hardware and software for elastography. Examples of ultrasound and MR elastograms of the phantoms are included to demonstrate the utility of the phantoms. Also, the level of stability of elastic properties of the component materials is quantified over a 15-month period. Such phantoms can serve as performance-assessing intermediaries between simple phantoms (consisting, for example, of homogeneous cylindrical inclusions in a homogeneous background) and a full-scale clinical trial. Thus, premature clinical trials may be avoided.

Anthropomorphic breast phantoms for qualification of Investigators for ACRIN Protocol 6666.

The purpose of this study was to evaluate various ultrasonic properties of breast phantoms developed for use in qualifying investigators for participation in the American College of Radiology Imaging Network (ACRIN) protocol 6666, "Screening Breast Ultrasound in High-Risk Women." Specifically, a tool was sought to consistently measure the performance of radiology personnel in detecting and characterizing lesions similar to those expected with screening breast ultrasonography (US). The phantoms are equivalent to one another except for the randomization of positions of 14 of the 17 simulated lesions. The lesions differ in depth and ultrasonic properties. Representative values of propagation speed, attenuation, relative echogenicity, and mass density are reported for all tissue-mimicking components. Beam refraction occurs at the interface between the subcutaneous fat layer and the glandular parenchyma and can result in beam distortion artifacts similar to those encountered in clinical breast US.

Stability of heterogeneous elastography phantoms made from oil dispersions in aqueous gels.

A set of five tissue-mimicking phantoms with cylindrical inclusions were produced for assessing long-term stability of geometry and elastic properties and assessing accuracy of determination of elastic properties. The base aqueous materials were either gelatin or a mixture of agar and gelatin. Stiffness was controlled by selection of the volume percent consisting of microscopic safflower oil droplets. Cylinder diameters remained unchanged within 1% or 2% over many months. Strain ratios from elastograms of the phantoms were stable over many months, implying that elastic contrasts were also stable. Test samples, called production samples, for measurement of Young's moduli were made at the time of manufacture of each phantom and were stored separately from one another. Each production sample was homogeneous and consisted of either inclusion material or background material. For all five phantoms, it was found that the elastic contrast computed using Young's modulus values determined using the production samples accurately represented the true elastic contrasts in the corresponding phantom. This finding was established by the fact that the (true) elastic contrasts determined using samples excised from the phantoms themselves agreed with the elastic contrasts obtained using the homogeneous production samples.

Spherical lesion phantoms for testing the performance of elastography systems.

A set of three cubic one-litre phantoms containing spherical simulated lesions was produced for use in comparing lesion detection performance of different elastography systems. The materials employed are known to be stable in heterogeneous configurations regarding geometry and elastic contrast identical with (storage modulus of lesion material) / (storage modulus of background material), and regarding ultrasound and NMR properties. The materials mimic soft tissues in terms of elastic, ultrasound and NMR properties. Each phantom has only one value of elastic contrast (3.3, 4.6 or 5.5) and contains arrays of 1.6 mm, 2 mm, 3 mm and 4 mm diameter spherical simulated lesions. All the spheres of a given diameter are arranged in a regular array with coplanar centres. Elastograms of an array made with ultrasound allow determination of the depth range over which lesions of that diameter and elastic contrast can be detected. Two phantoms are made from agar-plus-gelatin-based materials, and one is made from oil-in-gelatin dispersions. The methods for producing the phantoms are described in detail. Lesion detection performances for two ultrasound systems, both operating at about 7.5 MHz and focused at about 5 cm, were quantified with distinctions between the two systems demonstrated. Neither system was capable of detecting any of the 1.6 mm lesions. Phantoms such as these should be useful in research labs that are refining hardware and/or software for elastography.

Tissue-mimicking agar/gelatin materials for use in heterogeneous elastography phantoms.

Five 9 cm x 9 cm x 9 cm phantoms, each with a 2-cm-diameter cylindrical inclusion, were produced with various dry-weight concentrations of agar and gelatin. Elastic contrasts ranged from 1.5 to 4.6, and values of the storage modulus (real part of the complex Young's modulus) were all in the soft tissue range. Additives assured immunity from bacterial invasion and can produce tissue-mimicking ultrasound and NMR properties. Monitoring of strain ratios over a 7 to 10 month period indicated that the mechanical properties of the phantoms were stable, allowing about 1 month for the phantom to reach chemical equilibrium. The only dependable method for determining the storage moduli of the inclusions is to make measurements on samples excised from the phantoms. If it is desired to produce and accurately characterize a phantom with small inclusions with other shapes, such as an array of small spheres, an auxiliary phantom with the geometry of the cylindrical inclusion phantoms or the equivalent should be made at the same time using the same materials. The elastic contrast can then be determined using samples excised from the auxiliary phantom. A small increase of about 10% in volume of the cylindrical inclusions occurred-a tolerable increase. Interestingly, the smallest increase (about 5%) occurred in the phantom with the largest elastic contrast.

Tissue-mimicking phantom materials for narrowband and ultrawideband microwave applications.

We propose and characterize oil-in-gelatin dispersions that approximate the dispersive dielectric properties of a variety of human soft tissues over the microwave frequency range from 500 MHz to 20 GHz. Different tissues are mimicked by selection of an appropriate concentration of oil. The materials possess long-term stability and can be employed in heterogeneous configurations without change in geometry or dielectric properties due to osmotic effects. Thus, these materials can be used to construct heterogeneous phantoms, including anthropomorphic types, for narrowband and ultrawideband microwave technologies, such as breast cancer detection and imaging systems.

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.

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.