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

A key step toward implementing quantitative ultrasound techniques in a clinical setting is demonstrating that parameters such as the ultrasonic backscatter coefficient (BSC) can be accurately estimated independent of the clinical imaging system used. In previous studies, agreement in BSC estimates for well characterized phantoms was demonstrated across different laboratory systems. The goal of this study was to compare the BSC estimates of a tissue mimicking sample measured using four clinical scanners, each providing RF echo data in the 1-15 MHz frequency range. The sample was previously described and characterized with single-element transducer systems. Using a reference phantom for analysis, excellent quantitative agreement was observed across the four array-based imaging systems for BSC estimates. Additionally, the estimates from data acquired with the clinical systems agreed with theoretical predictions and with estimates from laboratory measurements using single-element transducers.

Analysis of shear strain imaging for classifying breast masses: finite element and phantom results.

PURPOSE: Features extracted from axial-shear strain images of breast masses have been previously utilized to differentiate and classify benign from malignant breast masses. In this paper, we compare shear strain patterns exhibited by both the full-shear (axial and lateral component) versus only the axial-shear strain component for differentiating between bound masses (malignant) when compared to unbound masses (benign). METHODS: We examine different breast mass characteristics such as mass shape, asymmetric location of masses, stiffness variations, and mass bonding characteristics to background tissue to assess their impact on shear strain patterns generated due to a uniaxial applied deformation. Two-dimensional finite element simulations of both circular and elliptical inclusions embedded within a uniform background were utilized. Different degrees of bonding were characterized using friction coefficient values ranging from 0.01 to 100 denoting loosely bound to firmly bound masses. Single-inclusion tissue-mimicking phantoms mimicking firmly bound and loosely bound ellipsoidal masses oriented at four different angles to the applied deformation were studied to corroborate the mass differentiation performance. RESULTS: Our results indicate that the normalized axial-shear strain and full-shear strain area features are larger for bound when compared to unbound masses. A higher stiffness ratio or contrast between the inclusion and background also improves differentiation. Larger applied deformations reduce the discrimination performance for masses with friction coefficients lower than 0.4, due to increased mass slippage with applied deformations. Potential errors with the use of these features would occur for unbound inclusions at larger applied deformations and for asymmetric mass positions within the background normal tissue. CONCLUSIONS: Finite element and tissue-mimicking phantom results demonstrate the feasibility of utilizing both the normalized axial-shear and full-shear strain area features to classify breast masses. Differentiation between bound or unbound masses was not affected by the mass size or shape for masses where the applied deformation is normal to the mass surface. Shear strain patterns vary significantly especially within unbound masses, when the mass surface is not normal to the applied deformation. Discrimination performance for unbound masses was improved by utilizing only the normalized shear strain area patterns located outside the mass as illustrated in this paper.

Absolute backscatter coefficient estimates of tissue-mimicking phantoms in the 5-50 MHz frequency range.

Absolute backscatter coefficients in tissue-mimicking phantoms were experimentally determined in the 5-50 MHz frequency range using a broadband technique. A focused broadband transducer from a commercial research system, the VisualSonics Vevo 770, was used with two tissue-mimicking phantoms. The phantoms differed regarding the thin layers covering their surfaces to prevent desiccation and regarding glass bead concentrations and diameter distributions. Ultrasound scanning of these phantoms was performed through the thin layer. To avoid signal saturation, the power spectra obtained from the backscattered radio frequency signals were calibrated by using the signal from a liquid planar reflector, a water-brominated hydrocarbon interface with acoustic impedance close to that of water. Experimental values of absolute backscatter coefficients were compared with those predicted by the Faran scattering model over the frequency range 5-50 MHz. The mean percent difference and standard deviation was 54% ± 45% for the phantom with a mean glass bead diameter of 5.40 µm and was 47% ± 28% for the phantom with 5.16 µm mean diameter beads.

Ultrasonic backscatter coefficients for weakly scattering, agar spheres in agar phantoms.

Applicability of ultrasound phantoms to biological tissue has been limited because most phantoms have generally used strong scatterers. The objective was to develop very weakly scattering phantoms, whose acoustic scattering properties are likely closer to those of tissues and then compare theoretical simulations and experimental backscatter coefficient (BSC) results. The phantoms consisted of agar spheres of various diameters (nominally between 90 and 212 microm), containing ultrafiltered milk, suspended in an agar background. BSC estimates were performed at two institutions over the frequency range 1-13 MHz, and compared to three models. Excellent agreement was shown between the two laboratory results as well as with the three models.

Shear strain imaging using shear deformations.

In this article we investigate the generation of shear strain elastograms induced using a lateral shear deformation. Ultrasound simulation and experimental results demonstrate that the shear strain elastograms obtained under shear deformation exhibit significant differences between bound and unbound inclusions in phantoms, when compared to shear strain images induced upon an axial compression. A theoretical model that estimates the decorrelation between pre- and postdeformation radio frequency signals, as a function of extent of shear deformation, is also developed. Signal-to-noise ratios of shear strain elastograms obtained at different shear angles are investigated theoretically and verified using ultrasound simulations on a uniformly elastic phantom. For the simulation and experiment, a two-dimensinal block-matching-based algorithm is used to estimate the axial and lateral displacement. Shear strains are obtained from the displacement vectors using a least-squares strain estimator. Our results indicate that the signal-to-noise ratio (SNR) of shear strain images increases to reach a maximum and saturates, and then decreases with increasing shear angle. Using typical system parameters, the maximum achievable SNR for shear strain elastography is around 8 (18 dB), which is comparable to conventional axial strain elastography induced by axial compression. Shear strain elastograms obtained experimentally using single inclusion tissue-mimicking phantoms with both bound and unbound inclusions (mimicking cancerous masses and benign fibroadenomas, respectively) demonstrate the characteristic differences in the depiction of these inclusions on the shear strain elastograms.

Radio-frequency ablation electrode displacement elastography: a phantom study.

This article describes the evaluation of a novel method of tissue displacement for use in the elastographic visualization of radio-frequency (rf) ablation-induced lesions. The method involves use of the radio-frequency ablation electrode as a displacement device, which provides localized compression in the region of interest. This displacement mechanism offers the advantage of easy in vivo implementation since problems such as excessive lateral and elevational displacements present when using external compression are reduced with this approach. The method was tested on a single-inclusion tissue-mimicking phantom containing a radio-frequency ablation electrode rigidly attached to the inclusion center. Full-frame rf echo signals were acquired from the phantom before and after electrode displacements ranging from 0.05 to 0.2 mm. One-dimensional cross-correlation analysis between pre- and postcompression signals was used to measure tissue displacements, and strains were determined by computing the gradient of the displacement. The strain contrast, contrast-to-noise ratio, and signal-to-noise ratio were estimated from the resulting strain images. Comparisons are drawn between the elastographically measured dimensions and those known a priori for the single-inclusion phantom. Electrode displacement elastography was found to slightly underestimate the inclusion dimensions. The method was also tested on a second tissue-mimicking phantom and on in vitro rf-ablated lesions in canine liver tissue. The results validate previous in vivo findings that electrode displacement elastography is an effective method for monitoring rf ablation.

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.

Three-dimensional electrode displacement elastography using the Siemens C7F2 fourSight four-dimensional ultrasound transducer.

Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods may enable clinicians for the first time to have readily available, cost-effective, real-time guidance to identify the location and boundaries of thermal lesions. Electrode displacement elastography is a method of strain imaging tailored specifically to ultrasound-guided electrode-based ablative therapies (e.g., radio-frequency ablation). Here tissue deformation is achieved by applying minute perturbations to the unconstrained end of the treatment electrode, resulting in localized motion around the end of the electrode embedded in tissue. In this article, we present a method for three-dimensional (3D) elastographic reconstruction from volumetric data acquired using the C7F2 fourSight four-dimensional ultrasound transducer, provided by Siemens Medical Solutions USA, Inc. (Issaquah, WA, USA). Lesion reconstruction is demonstrated for a spherical inclusion centered in a tissue-mimicking phantom, which simulates a thermal lesion embedded in a normal tissue background. Elastographic reconstruction is also performed for a thermal lesion created in vitro in canine liver using radio-frequency ablation. Postprocessing is done on the acquired raw radio-frequency data to form surface-rendered 3D elastograms of the inclusion. Elastographic volume estimates of the inclusion compare reasonably well with the actual known inclusion volume, with 3D electrode displacement elastography slightly underestimating the true inclusion volume.

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.

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.

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.

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.

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.

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.

Association of automated and human observer lesion detecting ability using phantoms.

A set of tissue-mimicking phantoms containing spherical negative contrast simulated lesions was employed to associate an automated method for determining detectability with human observers. Six alternative methods for computing the lesion signal-to-noise ratio (LSNR) were employed for quantifying automated detecting ability. The six methods differ regarding effective lesion area and whether or not gradients in local mean background echo levels were accounted for. The two-alternative-forced-choice (TAFC) technique was used to associate detecting ability of human observers with LSNR values. Although the six methods gave similar results, one method exhibited the least dependency on lesion diameter and is recommended; that method accounts for gradients in local mean background echo levels and employs an effective sphere area of 2/pi times the projected sphere area. A reasonable LSNR detection threshold value of -2.0 was found to apply for nominal transducer frequencies from 4 through 6 MHz and for lesion diameters from 2 through 5 mm. This result allows rapid human-observer-calibrated automated determination of the depth range of detectability as a function of sphere diameter and contrast.

Imaging the shear modulus of the heel fat pads.

BACKGROUND: Steady state, dynamic MR elastography provides quantitative images of the shear modulus of tissues in vivo. MR elastography was evaluated for its ability to characterize the mechanical properties of the weight bearing plantar soft tissues in vivo. METHODS: MR elastography was used to image the heel fat pad and surrounding soft tissues when the subject applied a low pressure on the foot and again when the subject applied high pressure. The placement of the foot was identical for both sets of images. FINDINGS: The results agree well with expected trends. The shear modulus of the tissue under the calcaneus increased from 8 kPa to 12 kPa with increasing pressure while that of peripheral tissues remained constant at 8 kPa which is similar to the shear modulus of fat in breast tissue. INTERPRETATION: Preliminary results from the steady state MR elastography methods being developed to measure the shear modulus of plantar soft tissues are promising. MR elastography is sufficiently accurate to observe the change in shear modulus with changes in applied pressure and is capable of characterizing the mechanical properties of the plantar soft tissues. Detailed anatomic information can be combined with co-registered mechanical properties. MR elastography could play a significant role in understanding the weight bearing functions of the plantar soft tissues and in evaluating those structures for improved diagnosis and assessment of disease progression.

Noise reduction using spatial-angular compounding for elastography.

Ultrasound elastography has developed into an imaging modality suitable for detection and diagnosis of cancers in the breast, prostate, and thyroid and for monitoring ablative therapies in the liver, kidneys, and other sites. In this article, a new approach is described that enables the reduction of noise artifacts in elastography without a significant reduction in either the contrast or spatial resolution. The technique uses angular-weighted compounding of local angular strains estimated from echo signals scanned at different insonification angles. Strain estimated along angular insonification directions can be separated into strain tensor components along the axial (direction of compression) and lateral directions. The mechanical stimulus is applied only along one direction. Angular-weighting factors are derived from the relationship between the axial and lateral strains under the assumption of tissue incompressibility. Experimental results using a uniformly elastic, tissue-mimicking phantom demonstrate the improvement in the signal-to-noise ratio obtained with angular-weighted compounding. Variation in the signal-to-noise ratio obtained using different angular increments also is investigated. Elastograms obtained from an inclusion phantom also demonstrate the improvement in contrast detail resolution obtained using spatial-angular compounding.

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.

Evaluation of the impact of backscatter intensity variations on ultrasound attenuation estimation.

PURPOSE: Quantitative ultrasound based approaches such as attenuation slope estimation can be used to determine underlying tissue properties and eventually used as a supplemental diagnostic technique to B-mode imaging. The authors investigate the impact of backscatter intensity and frequency dependence variations on the attenuation slope estimation accuracy. METHODS: The authors compare three frequency domain based attenuation slope estimation algorithms, namely, a spectral difference method, the reference phantom method, and two spectral shift methods: a hybrid method and centroid downshift method. Both the reference phantom and hybrid method use a tissue-mimicking phantom with well-defined acoustic properties to reduce system dependencies and diffraction effects. The normalized power spectral ratio obtained is then filtered by a Gaussian filter centered at the transmit center frequency in the hybrid method. A spectral shift method is then used to estimate the attenuation coefficient from the normalized and filtered spectrum. The centroid downshift method utilizes the shift in power spectrum toward lower frequencies with depth. Numerical phantoms that incorporate variations in the backscatter intensity from -3 to 3 dB, by varying the scatterer number density and variations in the scatterer diameters ranging from 10 to 100 µm are simulated. Experimental tissue mimicking phantoms with three different scatterer diameter ranges (5-40, 75-90, and 125-150 µm) are also used to evaluate the accuracy of the estimation methods. RESULTS: The reference phantom method provided accurate results when the acoustical properties of the reference and the sample are well matched. Underestimation occurs when the reference phantom possessed a higher sound speed than the sample, and overestimation occurs when the reference phantom had a lower sound speed than the sample. The centroid downshift method depends significantly on the bandwidth of the power spectrum, which in turn depends on the frequency dependence of the backscattering. The hybrid method was the least susceptible to changes in the sample's acoustic properties and provided the lowest standard deviation in the numerical simulations and experimental evaluations. CONCLUSIONS: No significant variations in the estimation accuracy of the attenuation coefficient were observed with an increase in the scatterer number density in the simulated numerical phantoms for the three methods. Changes in the scatterer diameters, which result in different frequency dependence of backscatter, do not significantly affect attenuation slope estimation with the reference phantom and hybrid approaches. The centroid method is sensitive to variations in the scatterer diameter due to the frequency shift introduced in the power spectrum.