Hermes A. S. Kamimura

Pressure transducer for measuring acoustic radiation force based on a magnetic sensor

This work presents a pressure transducer based on a magnetic sensor to measure acoustic radiation force (ARF) and small displacements. The methodology presented in this paper allowed this transducer to be calibrated for use as an acoustic pressure and intensity meter. It can control the acoustic intensity emitted by ultrasound used, for example, in ARF impulse imaging, vibro-acoustography and high-intensity focused ultrasound techniques. The device comprises a magnet, a membrane, a magnetoresistive sensor and a coil to cancel the external magnetic field. When ARF is applied to the membrane, the magnetic field on the sensor changes due to the magnetic target displacement. The variation of the output signal from the magnetic transducer is proportional to the acoustic pressure applied to the membrane. A focused ultrasound transducer with a central frequency of 3 MHz was used to apply a continuous ARF. The sensitivities of the magnetic transducer as an acoustic pressure and intensity meter, evaluated in water, were respectively 0.597 µV MPa−1 and 0.073 µV (W cm−2)−1/2, while those of the needle hydrophone (Onda model HNP-0400) used in the magnetic transducer calibration were respectively, 0.5024 mV MPa−1 and 6.153 mV (W cm−2)−1/2. The transducer resolution to displacement is 5 nm and 6 dB of signal attenuation occurs for 7° of misalignment. The transducer responded well to acoustic pressure in water above 200 kPa.

Piezoelectret-based hydrophone: an alternative device for vibro-acoustography

Piezoelectric polymers are highly desirable for ultrasound applications since their low acoustic impedance is much closer to the impedances of air and water than those of traditional piezoceramics. However, the piezoelectric effect observed in these poled polymers is limited to a few dozen picocoulombs per newton and requires very low-noise amplification. A different class of polymeric material known as piezoelectrets, presents piezoelectric effect in the same order of magnitude as those found on piezoceramics. This new class of materials has been explored in a wide range of applications such as flexible keyboards and airborne ultrasound transducers. Based on these polymers, we present here a new transducer for vibro-acoustography (VA), which is an ultrasonic image technique employed in medical diagnosis or material analysis based on ultrasound scattered by a target object. A calibration was carried out using a standard hydrophone, which is normally employed in VA. The average sensitivity of the transducer in the continuous wave mode was 1.712 mV Pa−1(−182.7 dB re 1 V Pa−1) with a maximum sensitivity of 18.25 mV Pa−1 (−154.8 dB re 1 V Pa−1) at its resonance frequency around 40 kHz. Subsequently, measurements in the burst mode and of directional sensitivity were taken and are presented here together with VA images obtained from a chicken bone and a metal sphere.

Preliminary results of vibro-acoustography evaluation of bone surface and bone fracture

Background Vibro-acoustography (VA) uses two co-focused ultrasound beams with slightly different frequencies. The beams interact and generate a low-frequency focus to excite an object. Methods A two-element confocal ultrasound transducer with central frequency at 3.2 MHz was used to generate the low-frequency excitation (30 kHz) and the response of the bone to that excitation was acquired by a dedicated hydrophone. The face of the confocal transducer was positioned parallel to the surface of the bone at a focal length of 7 cm. The hydrophone was fixed to the side of the transducer, out of the path of the ultrasonic beam. Results The resulting image clearly showed the bone fracture with resolution of 0.25 mm and high contrast with well-defined borders. Conclusions In this paper, we present preliminary results of VA imaging of bone surface and of bone fracture using an experimental set-up. Our results encourage future studies using VA to evaluate bone fractures.

Nonlinear mixing of two ultrasonic beams for transcranial sonothrombolysis

Evidences of ultrasound enhancement of fibrinolysis - called sonothrombolysis - using acoustic beams in the kHz range pointed out acceleration of enzymatic processes and improvement in the tissue penetration of sound waves in comparison with MHz range trials. However, trials using kHz range increased bleeding rates in strokes cases due to the formation of standing waves in the brain. This paper describes the use of two cofocused ultrasonic beams for transcranial sonothrombolysis. A confocal ultrasound transducer of 3.2 MHz of central frequency generated two independent ultrasonic beams with 20 kHz of difference frequency. The low-frequency acoustic field was mapped using a laser Doppler vibrometer based on the acousto-optic phenomenon. The nonlinear mixing of two cofocused ultrasonic beams generated a 20-kHz acoustic field with lower standing wave formation and higher focusing of the beam in comparison with the focal beam with AM modulation. The findings presented here will assist in the improvement of transcranial sonothrombolysis by focusing the site of excitation on the blood clot to avoid hemorrhage in the brain parenchyma.

Vibroacoustography for the assessment of total hip arthroplasty

OBJECTIVES: This paper proposes imaging with 3-dimensional vibroacoustography for postoperatively assessing the uncovered cup area after total hip arthroplasty as a quantitative criterion to evaluate implant fixation. METHODS: A phantom with a bone-like structure covered by a tissue-mimicking material was used to simulate a total hip arthroplasty case. Vibroacoustography images of the uncovered cup region were generated using a two-element confocal ultrasound transducer and a hydrophone inside a water tank. Topological correction based on the geometry of the implant was performed to generate a 3-dimensional representation of the vibroacoustography image and to accurately evaluate the surface. The 3-dimensional area obtained by the vibroacoustography approach was compared to the area evaluated by a 3-dimensional motion capture system. RESULTS: The vibroacoustography technique provided high-resolution, high-contrast, and speckle-free images with less sensitivity to the beam incidence. Using a 3-dimensional-topology correction of the image, we accurately estimated the uncovered area of the implant with a relative error of 8.1% in comparison with the motion capture system measurements. CONCLUSION: Measurement of the cup coverage after total hip arthroplasty has not been well established; however, the covered surface area of the acetabular component is one of the most important prognostic factors. The preliminary results of this study show that vibroacoustography is a 3-dimensional approach that can be used to postoperatively evaluate total hip arthroplasty. The favorable results also provide an impetus for exploring vibroacoustography in other bone or implant surface imaging applications.

Vibro-acoustography and B-mode integration for 3D imaging

Tridimensional representation of vibro-acoustography images based on the topology acquired by B-mode acquisitions is proposed for the evaluation of bone and implant surfaces. A tridimensional evaluation of the implant coverage used in a total hip arthroplasty procedure was performed to show the feasibility of this approach. A vibro-acoustography image of the uncovered area of the implant was acquired and represented in planar representation. However, tridimensional representation of the exposed surface area is necessary for proper evaluation of the stability of the implant. Hence, the topologies of the implant and the bone region around it were determined by acquiring 280 B-mode images. The B-scan images were processed in order to reconstruct the tridimensional surface of the objects. Finally, the vibro-acoustography image and the B-mode-based surface were aligned for the tridimensional visualization. The B-mode tridimensional representation of the bone and implant was improved by the enhancement of contrast and resolution provided by the vibro-acoustography image. The final tridimensional image presented a resolution of 0.25 mm. The topological correction based on B-mode slices allowed an accurate evaluation of the surface area.

Vibro-acoustography beam formation with reconfigurable arrays

Vibro-acoustography (VA) is an imaging modality that measures the acoustic response from stimulation produced by the interaction of two ultrasound beams at different frequencies. In this work, we present a numerical study of the use of reconfigurable arrays (RCA) for VA beam formation. A parametric study of the aperture selection, number of channels, number of elements, focal distance, and steering parameters is presented in order to show the feasibility and evaluate the performance of VA imaging based on RCA. Furthermore, an optimization for beam steering based on the channel assignment is proposed for balancing the contribution of the two waves in the steered focus. The point-spread function is calculated based on angular spectrum methods using the Fresnel approximation for rectangular sources. Simulations considering arrays with 50 × 50 to 200 × 200 elements with the number of channels varying in the range of 32 to 128 are evaluated to identify the best configuration for VA. We concluded that RCA transducers can produce spatial resolution similar to confocal transducers, steering is possible in elevation and azimuthal planes, and effective setting parameters including number of elements, number of channels, maximum steering, and focal distance are suggested for VA clinical imaging.

A new apparatus for analysis of viscoelastic fluids by ultrasound radiation force

The acoustic radiation force has been used as the method to examine the physical properties of materials in several areas. Vibro-acoustography is an acoustic radiation force technique that is being used to perform analysis of mechanical properties of materials. In this application a focused acoustic modulated force excites target which vibrates at the frequency of modulation. The emitted sound is characteristic of the medium mechanical impedance and it is measured using a dedicated hydrophone. In this paper, we propose a modification of the vibro-acoustography (VAG) technique and apply a technique called vibro-acustomagnetography (VAMG) by replacing the hydrophone by a magnetic sensor with high sensitivity. In this case, the modulated acoustic radiation will be applied on a magnetized target immersed in the fluid under study. With this procedure, static and dynamic displacement of the magnetic target (ball) will be measured when acoustically excited. In this study, we used a magnetoresistive sensor with resolution of about nT for mounting the transducer to detect displacement of the magnetic target. The vibration of the target was induced by a non-contact force, using an ultrasonic beam modulated by two concentric beams generated by confocal piezoelectric elements with equal area and common focus to 7 cm. The target used was a magnetic sphere of NdFeB with a radius of 2.36 mm. The apparatus was evaluated through of measurements in water and oil. Viscoelastic parameters were estimated fitting the nonlinear response of the magnetic transducer function of frequency modulation.