Urban, Matthew W

AbstractEvaluating mechanical properties of biological soft tissues and viscous mucus is challenging because of complicated dynamic behaviors. Soft condensed matter models have been successfully used to explain a number of dynamical behaviors. Here, we reported that optical coherence elastography (OCE) is capable of quantifying mechanical properties of soft condensed matters, micellar fluids. A 7.5 MHz focused transducer was utilized to generate acoustic radiation force exerted on the surface of soft condensed matters in order to produce Rayleigh waves. The waves were recorded by optical coherence tomography (OCT). The Kelvin‐Voigt model was adopted to evaluate shear modulus and loss modulus of soft condensed matters. The results reported that various concentrations of micellar fluids can provide reasonable ranges of elasticity from 65.71 to 428.78 Pa and viscosity from 0.035 to 0.283 Pa·s, which are close to ranges for actual biological samples, like mucus. OCE might be a promising tool to differentiate pathologic mucus samples from healthy cases as advanced applications in the future.

Introduction: Ultrasound radiation force-based methods can quantitatively evaluate tissue viscoelastic material properties. A limitation of current methods includes neglecting the inherent anisotropy nature of tissue. To explore this phenomenon, we created a phantom incorporating fibrous material that has preferential orientations. Methods: Two phantoms were made in a cube-shaped mold using a fibrous material arranged in multiple layers and embedded in porcine gelatin using two different concentrations of the gelatin (8%, 14%). Shear wave measurements were made in the phantoms at different angles by rotating the phantom, where 0° and 180° were defined along the fibers, and 90° and 270° across the fibers. Measurements were performed using a Verasonics ultrasound system equipped with a linear array transducer. Results/Discussion: The mean shear wave speeds and mean standard deviations for 8% and 14% gelatin along the fibers (0°) were (3.60 ± 0.03 and 4.10 ± 0.11 m/s) and across the fibers (90°) were (3.18 ± 0.12 and 3.90 ± 0.02 m/s), respectively. Conclusion: The fibrous gelatin-based phantoms exhibited anisotropy that could be measured using quantitative shear waves speed measurements. Increasing the gelatin percentage increases the shear wave speed and anisotropic moduli. [This study was supported by NIH grant DK092255.]

Thyroid cancer is the fastest growing age and gender adjusted cancer in 2011 according the American Cancer Society. The majority of the clinically diagnosed thyroid nodules are benign while less than 5% represent intrathyroidal cancers. Currently, the clinical gold-standard procedure for assessing the thyroid nodules is needle biopsy, a procedure that is associated with significant financial burden as well as pain and risk for patients. Therefore, a noninvasive, affordable, and potentially widely available method to differentiate between benign and malignant thyroid nodules can play an important role in reducing the number of unnecessary biopsies. In this study, we investigate the feasibility of two acoustic radiation force elastography techniques, shear wave dispersion ultrasound vibrometry (SDUV) and comb-push ultrasound shear wave elastography (CUSE imaging), in identifying thyroid nodules (imaging) and differentiating between benign and malignant pathologies based on their elasticity and viscosity (SDUV measurements). Our preliminary results show that the measured shear elasticity and shear viscosity parameters depend on tissue type; hence, these measurements may be utilized to differentiate between healthy normal thyroid tissue, benign nodules, and malignant nodules. Further studies on a large population of patients is required to better evaluate the role of the combination of elasticity and viscosity properties of tissue in differentiating various thyroid nodules.

Diagnostic ultrasound images suffer from degradation due to tissues with sound speed inhomogeneities causing phase shifts of propagating waves. These phase shifts defocus the ultrasound beam, reducing spatial resolution and image contrast in the resulting image. We describe a phase aberration correction method that uses dynamic ultrasound radiation force to harmonically excite a medium using amplitude-modulated continuous wave ultrasound created by summing two ultrasound frequencies at f0=3.0 MHz and f0+Δf=3.0005 MHz. The phase of each element of a linear array transducer is sequentially adjusted to maximize the radiation force and obtain optimal focus of the ultrasound beam. The optimization is performed by monitoring the harmonic amplitude of the scatterer velocity in the desired focal region using Doppler techniques. Simulation results show the ability to regain a 3.0-MHz focused field after applying a phase screen with an rms time delay of 95.4 ns. The radiation force magnitude increased by 22 dB and the resolution of the field was regained. Simulation results show that the focus of the beam can be qualitatively and quantitatively improved with this method. [This study was supported in part by Grants EB002640 and EB002167 from the NIH.]

Vibro-acoustography image is a function of the dynamic characteristics of the object at the vibration (difference) frequency (df). The dynamic characteristic of tissue is closely related to pathology. Therefore, it is important to evaluate image features versus df. Here, the influence of df on breast vibro-acoustography images is studied by scanning human breast at various df values ranging from 20 to 90 kHz. The subjects were chosen from a group of volunteers with different breast abnormalities. Images were compared subjectively to study image features and the appearances of breast lesions versus df. It is demonstrated that having a collection of images of the same tissue at different df values generally provides a better perception of the tissue structure and improves lesion identification. In most cases, higher df resulted in a higher signal-to-noise ratio and thus a higher image quality. Finally, a frequency-compounded images was obtained by calculating the weighted sum of images at different df values. It is demonstrated that image compounding normally improves visualization of breast tissue and abnormalities. [Work supported by NIH Grant EB-00535 and Grant BCTR0504550 from the Susan G. Komen Breast Cancer Foundation. Disclosure: Parts of the techniques used here are patented by MF and JFG.]

Shear wave dispersion ultrasound vibrometry (SDUV) has been introduced to use the dispersive nature of shear wave speed to locally estimate the material properties or tissue. Shear waves were created using a mechanical shaker and pulsed radiation force. Using the information from the phase one can estimate the shear velocity. A theoretical Voigt model allows us to find, at different frequencies (100–400 Hz), the viscosity and shear elasticity of the polymer gel dosimeter. The aim of this work was to perform the SDUV and mechanical vibrometry techniques to study the viscoelastic properties of absorbed dose phantoms. The experiments were performed using five phantoms of different absorbed doses varying 10 up to 50 Gy, while a nonirradiated phantom was kept as a control. The SDUV technique was performed using a pulsed radiation force to vibrate the gel provide by a confocal transducer with a focal depth of 70 mm, nominal frequency of 3 MHz. For the mechanical vibrometry and SDUV experiments, shear wave motion measurements were made with a commercial ultrasound machine with a research using a 10 MHz linear array transducer. The quantitative elasticity and viscosity results for these two experiments were compared.

The objective of this study was to analyze and optimize the ultrasound radiation force source for creating shear waves for quantitative elasticity and viscosity measurements. A simulation model based on work by Bercoff et al., [IEEE Trans. Ultrason. Ferroelectr. Freq. Contr. 51(11), 1523–34 (2004)] was utilized to examine the shear wave propagation from a spatiotemporal impulsive force and the simulated radiation force generated from a linear array transducer. Parameters such as aperture size, F-number of the transducer, and medium shear elasticity and viscosity were varied to analyze how the shear wave amplitude and shape changes. The wave attenuation and group and phase velocities were evaluated. The aperture size and the F-number of the transducer affected the spatial distribution of the propagating shear wave significantly. The influence of the coupling term between the bulk and shear waves was analyzed spatially and temporally by evaluating parameters such as shear wave amplitude, wave attenuation, and the group and phase velocities. This simulation model provides insight for making shear wave propagation measurements from the perspective of optimizing the spatial and temporal characteristics of induced shear wave motion from an applied radiation force. [This work was supported in part by Grant EB002167 from NIH.]

The objective of this study was to assess the potential for noninvasively identifying viscoelastic material properties in arterial vessels using ultrasound radiation force and modern computational inverse problem techniques. A rubber tube embedded in gelatin was excited at different points along the axis using impulsive ultrasound radiation force. The resulting mechanical waves were measured using a laser vibrometer and pulse-echo ultrasound. The inverse problem was cast as an optimization problem, in which the discrepancy between the measured dynamic response and the computed finite element model response was minimized with respect to viscoelastic material parameters. The model was very sensitive to the tube’s viscoelastic parameters and relatively insensitive to those of the gelatin. The viscoelastic properties of the tube obtained from the inverse problem were compared to the results obtained from a dynamic mechanical analyzer (DMA). For the overlapping range of frequencies (130–190 Hz), the results of the inverse problem gave an equivalent modulus that range from 7.3–8.5 MPa, while the DMA values range from 6.0–9.2 MPa. These results have important implications for using ultrasound radiation force in noninvasive characterization of the viscoelastic properties of arteries in vivo. [This work was supported in part by Grant No. EB002640 from NIH.]

Vibro-acoustography is a method that uses ultrasound radiation force to investigate the mechanical response of an object [Fatemi and Greenleaf, ‘‘Ultrasound-stimulated vibro-acoustic spectrography,’’ Science, 280, 82–85 (1998)]. The dynamic radiation force causes the object to vibrate within the focal region of the transducer, which creates a sound field called acoustic emission measured by a hydrophone. To gain a better understanding of the objects investigated with vibro-acoustography, a method is proposed that measures the displacement of the local vibration. A pulse-echo ultrasound method is implemented with the same transducer used to create the radiation force to measure the displacement of the object. A Kalman filter approach is used to process the pulse-echo data to obtain estimates of the local displacement [Zheng et al., ‘‘Kalman filter motion detection for vibroacoustography using pulse-echo ultrasond,’’ 2003 IEEE Ultrasonics Symposium, pp. 1812–1815 (2003)]. Results using this ultrasound method to measure vibration of a metal beam compare well with laser vibrometer results. This method can reliably measure displacements with amplitudes of 120–2000 nm with error less than 9%. A pulse-echo method is described that provides displacement detection with simultaneous radiation force generation for vibro-acoustography applications using a single transducer. [Work supported by Grants EB002640 and EB002167 from NIH.]