Greenleaf, James F.

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.]

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.]

Measurement of surface roughness irregularities is an important indicator of product quality for many nondestructive testing industries. Many techniques exist; however, because of their qualitative, time-consuming and direct-contact modes, it is of some importance to work out new experimental methods and efficient tools for quantitative estimation of surface roughness. Continuous-wave ultrasound reflectometry (CWUR) is presented here as a novel nondestructive modality for imaging and measuring surface roughness in a noncontact mode. In CWUR, voltage variations due to phase shifts in the reflected ultrasound waves are recorded and processed to form an image of surface roughness. An acrylic test block with surface irregularities ranging from 4.22 to 19.05 μm as measured by a coordinate measuring machine (CMM) is scanned by an ultrasound transducer having a diameter of 45 mm, a focal distance of 70 mm, and a central frequency of 3 MHz. It is shown that CWUR technique gives very good agreement with the results obtained through CMM inasmuch as the maximum average percent error is around 11.5%.

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.]

The mechanical response of tissues to external forces has gained considerable interest in medical diagnosis. One approach of imaging the tissue elastic properties is to apply an external force on the surface of the body. Another approach is to generate a localized force inside the tissue with the radiation force of ultrasound. In this paper, a noninvasive method for imaging the vibration mode of an object is investigated. The radiation force of ultrasound is used to scan the object at a resonant frequency of the object. The vibration of the object is measured by laser and the resulting acoustic emission from the object is measured by a hydrophone. Experimental studies are carried out on a mechanical heart valve and arterial phantoms. The mode images on the valve are made by the hydrophone measurement and confirmed by FEM simulations. Compared with conventional B-scan imaging on arterial phantoms, the mode imaging can show not only the interface of the artery and the gelatin, but also the vibration modes of the artery. The images taken on the phantom surface suggest that an image of an interior artery can be made by vibration measurements on the surface of the body.

Vibro-acoustography is an imaging method based on the sound emitted due to the harmonic motion of tissue induced by the radiation force of focused ultrasound. This paper presents recent progress in vibro-acoustography of human breast. Matching vibro-acoustography and mammography images are acquired using a combined mammography/vibro-acoustography machine. Images of various types of breast masses are investigated and their characteristics features are compared. The study includes malignant masses, benign masses such as fibroadenomas, cysts, calcified lesions, and surgical scars. Image features, such as lesion borders, interior texture, and speculation pattern, are studied. Vibro-acoustography images demonstrate that calcified masses normally show sharp borders, benign noncalcified masses have smooth texture, and, in some cases, malignant masses are characterized with spiculation. These results, which are based on the data collected from an ongoing study, indicate that the above-mentioned image features may be used for differentiation of breast masses. It is noted that more data from a larger population are needed to establish the results in a statistical sense. [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.]

Dynamic ultrasound radiation force can induce local tissue vibration. To understand the tissue response, we developed a method to ultrasonically measure the induced motion. We present a model for motion detection in a vibrating scattering medium. We developed a harmonic pulsed excitation method using tonebursts repeated at frequency fr. This excitation method creates a multifrequency force with components at harmonics of fr. We use pulse-echo ultrasound and narrowband Kalman filtering to perform multifrequency motion detection. In an experiment, a 3.0 MHz air-backed transducer was used to create the radiation force, and with the same transducer performed pulse-echo motion detection at 9.0 MHz. Bovine muscle was used for radiation force excitation and motion detection. We present simulation results for a parameterized model of motion detection in a scattering medium. We show experimental measurements of vibration displacement and phase for motion with amplitude of 0.1–10 μm. Our simulation model provides a platform to optimize detection of small harmonic motion in a scattering medium. Multifrequency harmonic pulsed excitation and detection methods are presented that induce and measure very small amplitude harmonic motion.

The ultrasound radiation force can be used as a noncontact method for exciting vibrational modes. In one implementation, an audio-range modulation signal is combined with a 40-kHz carrier to form a double-sideband suppressed-carrier (DSSC) waveform that is emitted from an ultrasound transducer. The frequency of the ultrasound radiation force is the difference between the frequencies of the sidebands, therefore at twice the modulation frequency. We describe a method for selective mode excitation using a transducer pair, each emitting a DSSC signal. The modulation signal for one transducer was phase shifted relative to the other transducer. When the two modulation signals were in phase, the radiation stress field was uniform. For devices studied, this excited transverse modes while suppressing torsional modes. Similarly, if the modulation signals were out of phase, torsional modes were excited while suppressing transverse modes. This technique has been demonstrated for devices including an MEMS mirror, hard-drive suspension, and brass cantilever. For a cantilever, the ratio of the amplitudes of 180-Hz transverse over 1.3-kHz torsional modes was varied by over two orders of magnitude by shifting the phase difference between the two beams over a range of 90 deg. This noncontact technique allows control over the types of vibration excited.

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.]

Low-intensity pulsed ultrasound is a commonly prescribed therapy for delayed unions and nonunions after fractures. Several prospective double-blind clinical trials have shown that a 1.5-MHz ultrasound signal at a 200-s tone burst repeating at 1 kHz shortens the time to clinically evaluated healing by 30%. We have shown in vitro that pulsed ultrasound increases aggrecan gene expression and aggrecan production in chondrocytes, a key cell in the fracture healing process. Because the square wave pulsed 1.5-MHz signal produces radiation force vibration at 1 kHz, we tested the hypothesis that dynamic radiation force, not ultrasound, is responsible for the biological effect of the signal. Experiments showed that a 1-kHz acoustic square wave, simulating the radiation force caused by the clinically used signal, induced chondrogenesis similarly to pulsed ultrasound treatment in ATDC5 cells. These results may have implications for remote ultrasound or acoustic stimulation of different types of strain-sensitive cells in addition to those responsible for fracture healing.