Kinnick, Randall R.

This study concerns the development and preliminary experimental verification of a topological sensitivity–based platform for the material characterization of tissue anomalies exposed by vibroacoustography-type imaging techniques. Vibroacoustography (VA) is a high-resolution imaging method that has been applied to the detection of pathological changes in soft tissues. Although the data provided by this method is related to the mechanical properties of tissue, the viscoelastic parameters of the object cannot be estimated by this imaging method itself. Topological sensitivity (TS) method is a data processing methodology that can be used to estimate the viscoelastic parameters of an object from vibration data. In this study, the concept of topological sensitivity is applied to interpret the vibroacoustography measurements for the purpose of lesion characterization. In the proposed approach, the topological sensitivity function, which signifies the variation of a given cost functional when an infinitesimal inclusion with trial material parameters is placed at the location of a point force, is formulated in terms of the adjoint field. The effectiveness of the resulting formula as a material indicator for lesion characterization is demonstrated by estimating the relative elastic parameters of a well-controlled neoprene sphere embedded in a tissue-mimicking phantom specimen.

The intersection of two ultrasound beams with slightly different frequencies results in generation of a localized radiation force and stimulates emission of audio signals from targeted objects. Vibro-acoustography uses this phenomenon to probe elastic properties of objects. Vibro-acoustography of contrast microbubbles in degassed water produced quantitative flow measurements from analysis of their acoustic emission. We used a dual-beam transducer generating bursts of 40-kHz vibrations. The vibrations resulted from interference of 3.48-MHz and 3.52-MHz confocal beams intersecting at the center of a thin plastic conduit. We tested flows of 13, 48, 85, and 120 mL/min of contrast microbubbles at concentrations from 1.2 times 10 5 to 6 times 10 5 bubbles/mL. The amplitude of the acoustic emission was linear with microbubble concentrations up to a value of 3.6 times 10 5 bubbles/mL. A replenishment method for microbubble contrast and flow rate analysis was used with radiation force bursts deployed at 0.05, 0.1, 0.2, 0.5, 1, and 2-second pulsing intervals. The relation between the pulsing intervals and the peak amplitude was fitted by an exponential curve and a rate constant calculated for each tested flow rate. The rate constant values were linearly correlated with the tested flows. The vibro-acoustography method provides objective, quantitative, and highly-localized assessment of flow using contrast microbubbles.

Acoustoelasticity is a technique that allows quantification of the elastic nonlinearity coefficients (the so-called third-order moduli) of a material by measuring the variation of the sound speed in different directions (and/or polarizations) versus the applied uniaxial static stress. When dealing with nominally isotropic solids, the variation of the shear wave speed in two orthogonal directions with the applied stress permits the computation of two out of three independent nonlinearity coefficients. To generate the shear wave in an experimental setting, the acoustic radiation force pulse was applied to uniaxially deformed phantoms with strains of up to 30%. The results demonstrate that the compressional-shear coefficient C, which governs the variation of the (linear) shear modulus with hydrostatic pressure was found to vary substantially from one phantom to another, with the highest value observed in an agarose-based phantom. The importance of this nonlinearity parameter resides in the fact that the magnitude of the acoustic radiation force (ARF) in soft solids is proportional to C-1. For consistency, the values of C obtained from the acoustoelasticity experiment are compared to those deduced from the shear wave amplitude via its relationship to the magnitude of the ARF used to generate the shear wave.

Pulse wave velocity (PWV) is widely used for estimating the stiffness of an artery. PWV is measured by the time of travel of the “foot” of the pressure wave over a known distance. This technique has a low time resolution and is an average measurement of artery stiffness between the two measuring positions. In this paper an arterial wave is generated non-invasively in the vessel wall by the radiation force of ultrasound. The wave velocity in the vessel is measured non-invasively by ultrasound with high time resolution over a short distance of a few millimeters.

The mechanical response of tissues to external forces has gained considerable interest in medical diagnosis. Typically, a force is needed to produce displacement in the tissue for estimation of the tissue’s elasticity. One approach is to apply an external force on the body’s surface. Another emerging technique is to generate a localized force inside the tissue with ultrasound. Shear waves can be used in bulky tissues for estimating the tissue’s elasticity. The bending wave can be generated and measured in vessels for noninvasive estimation of a vessel’s elasticity. We have found that the Rayleigh surface wave can be generated on the tissue’s surface, [X. Zhang et al., Proc. International Congress on Ultrasonics, Vienna, April 9–13, 2007], [X. Zhang et al., IEEE Trans. Medical Imaging, 26, 843–852 (2007)]. In our method, a localized radiation force of ultrasound is remotely and noninvasively applied inside the tissue. This force can generate the shear wave as well as compression wave inside the tissue. However, only the surface wave can exist on the body’ surface. The surface wave can be used to estimate and image the stiffness of tissue. Some results of using the surface wave are presented.

Thyroid imaging with conventional ultrasound scanners does not always lead to conclusive results. Furthermore, current ultrasound imaging technology has difficulty in detecting important microcalcifications. Ambiguities in ultrasound and other thyroid imaging methods lead to a large number of unnecessary biopsies. For these reasons, alternative imaging methods capable of detecting both microcalcifications and lesions in thyroid are of great interest. This paper describes recent results on imaging human thyroids with vibro-acoustography. Experiments were conducted on eight excised human thyroids from autopsy. The specimens were each imbedded in a block of gel to simulate the surrounding tissue and to facilitate scanning. Three types of images were acquired from each specimen: X-ray, B-mode ultrasound, and vibro-acoustography. Anatomical details seen in the images were correlated and image quality was compared. X-ray images displayed calcifications with high contrast, but did not show anatomical details of soft tissue. B-mode displayed some soft tissue structures; however, the speckle masked the details and reduced the contrast. Vibro-acoustography displayed calcifications, anatomical details, and some nodules where they existed. Furthermore, vibro-acoustography images displayed tissue structures with high contrast and free from speckle. It is concluded that vibro-acoustography may be a suitable technique for thyroid imaging.

Considerable interest in the elastic properties of soft tissue has increased in medicine due to their clinical relevance for monitoring various diseases as well as a biomarker for cancer. A recent emerging field is the use of acoustic radiation force for imaging and estimating the elastic properties of tissue. Newly developed imaging modalities include shear wave elasticity imaging, supersonic shear wave imaging, acoustic radiation force impulse imaging, and vibro-acoustography. In these new methods, the shear wave is used to estimate the elasticity of tissue. We have developed a novel surface wave method for noninvasively estimating the elasticity of tissue [X. Zhang et al., J. Acoust. Soc. Am., 122, 2522-2525 (2007)]. In this method, a localized ultrasound radiation force is remotely and noninvasively applied inside the tissue. The surface wave speed is used to estimate the elasticity of tissue. This method has been further developed and applied to estimate the viscoelasticity of animal myocardium tissue. In this abstract, we report our recent results of in vitro studies on animal myocardium tissue. Both the surface wave and shear wave are measured and analyzed. Noninvasive characterization of viscoelastic properties of myocardium tissue may be a very important tool for assessing heart function.

Detection of calcifications in breast is an important problem in the diagnosis of breast cancer. Vibro-acoustography is a recently developed method that uses the radiation force of ultrasound to create images of the mechanical response of an object at a low frequency using the magnitude or phase of the response. Small spheres are used to explore the use of the phase of vibration as a contrast modality for use in detection and identification of calcifications in breast tissue. An experiment is presented to measure the magnitude and phase of vibration at different frequencies. The theoretical and experimental results are compared for spheres of two different sizes. Phase images are shown in which five spheres of different density can be clearly distinguished from each other. With phase measurements and images, it is demonstrated that predictable image contrast exists for spheres of different density embedded in a viscoelastic medium.

Coupled vibration of arterial tubes is analyzed with the wave propagation approach and first-order shear deformation theory. Both the interior and exterior fluids are considered as compressible so that acoustic waves can be generated and propagated in the fluids. Results obtained using the theory have been evaluated against those available in the literature and the agreement has been found to be good. The theory can be used for future research on the vibration and acoustics of arterial walls. Vibration experiments were carried out on a silicone rubber tube in a water tank with a novel ultrasound stimulated optical vibrometry system. This system uses the radiation force of ultrasound to vibrate the tube at low frequency and records the resulting response by a laser vibrometer. Both the excitation and measurement are remote and noncontact. The silicone rubber tube was chosen because it has mechanical properties close to those of arteries. The fundamental frequency is well excited by the radiation force and measured with the laser. The measured fundamental frequency is in good agreement with the present theory.