Citations

Vibro-acoustography with rectilinear raster scanning using confocal or linear array transducers have been reported before. Endo-cavity imaging requires a different type of scanning due to space limitation. This paper describes 3-D endo-cavity vibro-acoustography by a rotating linear array that scans the volume of interest. In this configuration a linear array is used to steer the beam electronically, which is combined with the rotational motion of the transducer to scan the beam in a cylindrical volume. This particular geometry produces special artifacts in the image. One such artifact is non-uniformity in signal power due to angular variations with respect to the hydrophone. Another artifact is related to the geometrical distortion in the image. In this paper, we will present examples of vibro-acoustography images with linear and rotational scanning and describe image characteristics and methods to reduce the artifacts. [Work supported by NIH Grant P50 CA91956P2.]

Vibro-acoustography is an ultrasound-based imaging modality that maps the acoustic response of an object induced by acoustic radiation force. The method employs two ultrasound beams with frequencies separated by a small difference, typically in the kilohertz range. Using a 1.75D array transducer, which has multiple rows of elements, offers an advantage over linear array transducers because it allows for focusing in the elevation direction. We have performed simulations of different arrangements for the apertures assigned to the two ultrasound frequencies and evaluated resolution and sidelobe levels for different arrangements. However, the large size of the elements produces grating lobes in the transmitted fields. We will describe metrics for evaluating grating lobe levels compared to the main lobe of the transmitted field as a quantitative approach to optimize aperture design. We present experimental measurements of the transmitted field and compare with simulation results. Finally, we present imaging results from phantom experiments. This beamforming study is directed toward improving vibro-acoustography image formation using a General Electric Vivid 7 scanner for breast and thyroid imaging. [Mayo Clinic and two of the authors have potential financial interest related to devices or technology referenced above. Work supported by NIH Grant No. R21CA121579 and AIUM Award No. AIUM-EER#1.]

Vibro-acoustography (VA) is an imaging method based on ultrasound radiation force. In this method, a two dimensional image is formed by scanning two co-focused ultrasound beams with slightly different frequencies over the object (biological tissue) in a C-scan format and recording resulting sound that is emitted from the focal region at the difference frequency. As in other ultrasound-based imaging modalities, it is important to study the thermal safety of the technique. This paper addresses tissue heating due to a VA scan using a concentric confocal transducer. The three dimensional ultrasound intensity field calculated by Field II is used with the bio-heat equation to estimate tissue heating. Results calculated with thermal conduction and with blood perfusion, with conduction and without perfusion, and without conduction and without perfusion are compared. Maximum temperature rise occurs in the transducer near field during VA scans. Heating during one VA scan is below 0.05 °C for soft tissue with attenuation coefficient of 0.3, 0.5, or 0.7 dB/cm MHz. It is concluded that temperature rise in tissue is not significant. [Work supported by NIH Grants CA 91956, CA 127235, and CA121579. Some of the techniques are patented by Mayo Clinic and the some of the authors (M.F. and J.F.G.).]

Vibro-acoustography (VA) is an imaging modality that uses the radiation force of ultrasound to vibrate tissue at a low frequency and records the resulting acoustic field to produce an image that is represents object characteristics at both ultrasound as well as low (audio) frequencies. Currently, B-mode ultrasound is used for prostate imaging. This imaging method has some limitations, including the speckle noise, which limit its ability in detection of lesions and calcifications. The purpose of this study is to explore the potentials of VA for prostate imaging. VA, B-mode ultrasound, and radiography were performed on 12 excised human prostate samples and the resulting images were compared. VA system parameters were, center frequency 3 MHz, Δf=50 kHz, image area 5×5 cm, pixel size 0.2×0.2 mm. Images evaluated in terms of visibility of anatomical and pathological structures and contrast. Results showed that prostate anatomical structures, mass lesions, and calcifications were visible in the VA images taken at different depths, and VA image quality was superior to B-mode. VA images exhibited significant contrast between the central and peripheral zones with enhanced borders. It is concluded that, with further development, VA may become suitable modality for in prostate imaging.

Background. The vibrational characteristics of any object are directly dependent on the physical properties of that object. Therefore, changing the physical properties of an object will cause the object to adopt changed natural frequencies. A fracture in a bone results in the loss of mechanical stability of the bone. This change in mechanical properties of a bone should result in a change of the resonant frequencies of that bone. A vibrational method for bone evaluation has been introduced. Method of approach. This method uses the radiation force of focused amplitude-modulated ultrasound to exert a vibrating force directly, and remotely, on a bone. The vibration frequency is varied in the range of interest to induce resonances in the bone. The resulting bone motion is recorded and the resonance frequencies are determined. Experiments are conducted on excised rat femurs and resonance frequencies of intact, fractured, and bonded (simulating healed) bones are measured. Results. The experiments demonstrate that changes in the resonance frequency are indicative of bone fracture and healing, i.e., the fractured bone exhibits a lower resonance frequency than the intact bone, and the resonance frequency of the bonded bone approaches that of the intact bone. Conclusion. It is concluded that the proposed radiation force method may be used as a remote and noninvasive tool for monitoring bone fracture and healing process, and the use of focused ultrasound enables one to selectively evaluate individual bones.

Medical ultrasound scanners use high-energy pulses to probe the human body. The radiation force resulting from the impact of such pulses on an object can vibrate the object, producing a localized high-intensity sound in the audible range. Here, a theoretical model for the audio sound generated by ultrasound scanners is presented. This model describes the temporal and spectral characteristics of the sound. It has been shown that the sound has rich frequency components at the pulse repetition frequency and its harmonics. Experiments have been conducted in a water tank to measure the sound generated by a clinical ultrasound scanner in various operational modes. Results are in general agreement with the theory. It is shown that a typical ultrasound scanner with a typical spatial-peak pulse-average intensity value at 2 MHz may generate a localized sound-pressure level close to 100 dB relative to 20 μPa in the audible (<20 kHz) range under laboratory conditions. These findings suggest that fetuses may become exposed to a high-intensity audio sound during maternal ultrasound examinations. Therefore, contrary to common beliefs, ultrasound may not be considered a passive tool in fetal imaging.