Silva, Glauber T.

This work introduces a new method based on acoustic radiation force and magnetic measurement for interrogating the mechanical properties of fluid and biological tissues. In this method, ultrasound radiation is used to exert a low-frequency (in kHz range) force on a rigid magnetized target immersed in a viscoelastic medium. In response, the target vibrates in a pattern determined by viscoelastic properties of medium. We report the relation between the magnetic field signal and the incident ultrasonic pressure field in terms of the mechanical parameters of the medium. Simulations were conducted to demonstrate a simple approach based on using amplitude-modulated ultrasound to generate a dynamic acoustic radiation force on a magnetic target. The magnetic field generated by vibration of this target is then obtained and used to estimate the radiation-force-induced displacement as a function of time. It was observed that the intensity of the dynamic component of the magnetic field caused by the acoustic excitation is high enough to be registered by a conventional magnetic sensor. This proposed methodology presents a powerful tool for evaluation of acoustic radiation force as well as the mechanical properties of soft material.

Vibro-acoustography is a technique that images the vibro-acoustic response of an object to the harmonic ultrasound radiation force. The system point-spread function is associated to the radiation force exerted on a point-target by a dual-frequency ultrasound beam. So far the radiation force was calculated by assuming a dual-frequency beam composed by two plane waves. This reduces the radiation force to a one-dimensional quantity neglecting transverse components. Here, we model the radiation force as a three-dimensional vector. In this model, the scattering of a dual-frequency beam with any spatial distribution by a point-target is solved. The incident and scattered fields are used to calculate the radiation force on a sphere whose radius approaches to zero. The force is proportional to the gradient of the product of the incident wave amplitudes. Evaluation of the radiation force produced by a two-element co-focused transducer shows that the transverse component of the force is about −20 dB smaller than the corresponding axial component. Effects of the transverse force on image formation are still under investigation. In conclusion, the presented model describes vibro-acoustography systems with dual-frequency beams of any spatial distribution. [Work partially supported by Grant No. DCR2003.013-FAPEAL/CNPq.]

In this work, we study linear array beamforming for vibro-acoustography systems. The goal of this research is to assess the feasibility of vibro-acoustography imaging systems based on linear arrays for clinical applications. The vibro-acoustography transducer is designed with two arrays driven by two continuous-wave or tone-burst signals at slightly different frequencies. The system beamforming is modeled in a homogeneous and lossless fluid. In this model, the point-spread function (psf) of the system is obtained as the product of the spatial impulse function of each linear array. Three linear array configurations are analyzed for vibro-acoustography systems. Simulations considering 64 elements in each array are performed. Aspects related to clinical applications of vibro-acoustography, such as system spatial resolution, sidelobes, grating lobes and image frame-rate are discussed. It is concluded that linear array transducers can produce acceptable spatial resolution for clinical applications.

This work presents a theory for the acoustic radiation force exerted on a solid sphere by an arbitrary spatially distributed beam. The theory is developed for an sphere suspended in an ideal fluid. We assume that the acoustic beam can be decomposed in a set of plane waves with same frequency, propagating in different directions. The sphere radius is considered to be much smaller than the wavelength of the beam. Bulk properties of the sphere such as shear and compressional sound speed are taken into account. The radiation force is obtained by solving the linear acoustic scattering problem for the sphere. Theoretically, the radiation force depends on the sphere cross section area, the radiation force function, and the vector energy flux upon the sphere. The radiation force function is related to the sphere scattering properties. We apply the developed theory to study the radiation force produced by an spherical concave transducer. The generated radiation force can be decomposed into two components, namely, axial and transverse with respect to the wave propagation direction. The ratio between the transverse and axial components of the force depends on the transducer F-number and wave frequency. Results show that this ratio for a 2 MHz transducer with 3.5 F-number on the focal plane is less than 5%.

High-resolution ultrasound imaging systems for inspection of defects and flaws in materials are of great demand in many industries. Among these systems, Vibro-acoustography (VA) has shown excellent capabilities as a non-contact method for non-destructive high-resolution imaging applications. This method consists of mixing two confocal ultrasound beams, slightly shifted in frequency, to produce an acoustic emission field at the difference frequency of the primary incident ultrasound beams. In addition to the difference frequency signal, there exists another signal at the sum frequency, formed in the intersection region of the two primary beams. The goal of this study is to investigate the formation of high-resolution images using the sum frequency of ultrasound waves in VA while concurrently forming the conventional difference-frequency VA image, thereby increasing the amount of information acquired during a single scan. A theoretical model describing the sum frequency wave propagation, including beam forming and image formation in the confocal configuration is developed and verified experimentally. Moreover, sample experiments are performed on a flawed fiber-reinforced ceramic composite plate. Images at both the difference and sum frequencies are compared and discussed. Results show that the sum frequency image produces a high-resolution C-scan of the plate by which the flaws and structural details of the plate can be detected.