Fatemi, Mostafa

Vibro-acoustography is an imaging modality that uses ultrasound as the source of energy to explore objects at two ends of the spectrum. Employing two ultrasound beams at slightly different frequencies, this technique produces a highly localized radiation stress field that vibrates the object at the difference frequency. In response, the object produces a secondary acoustic field. This sound, which contains information about the dynamics of the object at the low difference frequency as well as objects characteristics at the ultrasound range, is used to produce an image of the object. This combination makes vibro-acoustography a unique tool to study biological tissues at low and high frequencies simultaneously. Utility of this method has already been demonstrated for a wide range of applications in vitro. In vivo applications of vibro-acoustography have been demonstrated in a recent study on human breast. It has been shown that this technique can produce high quality in vivo images at ultrasound intensities within the recommended FDA limits. Vibro-acoustography may also be used for imaging solids, such as metallic implants. This paper describes recent achievements in vibro-acoustography as well as possible future role of this technology in medicine. [This abstract includes materials patented by MF and JFG.]

We have proposed generating a bending wave in the arterial wall using ultrasound radiation force and measuring the wave velocity along the arterial wall [Zhang et al., IEEE Trans. Ultrason. Ferroelectr. Freq. Control 52, 642–652 (2005)]. Here, we report the results of in vivo studies on pigs. The pig was anesthetized, and a micromanometer tip catheter was inserted into the femoral artery to measure luminal pressure. A water bath was created on the animal’s groin to allow unimpeded access of the ultrasound beams to the femoral artery. The femoral artery was first located using a 13-MHz linear-array transducer. Then, a vibro-acoustography image was obtained to ensure precise positioning of the excitation force relative to the artery. The artery was excited by the force transducer and the resulting vibration of the arterial wall was measured by a sensing Doppler transceiver. Measured wave velocity was 3.1 m/s at 300 Hz. With this new method wave velocity over a distance of 5 mm, and therefore stiffness of arteries, can be measured locally and non-invasively. Measurement time is short in a few tens of milliseconds, which allows pressure dependence and pharmacological effect on the wall properties to be measured at different cardiac times.

The head-gimbal assembly suspension is a cantilever-like structure that holds the heads on a hard drive. We will discuss a noncontact method for modal testing, of suspensions in air, that utilizes the radiation force at the difference frequency generated by two intersecting ultrasound beams. The resulting low-frequency excitations were measured using a scanning vibrometer. This excitation technique has been demonstrated for MEMS and other small devices. There are several unique advantages of the ultrasound radiation force relative to mechanical shakers. Since the ultrasound radiation force is noncontact, a specialized test fixture was not needed; the technique was relatively insensitive to distracting resonances of fixtures and support structures. Another advantage is broadband excitation; a 550-kHz confocal ultrasound transducer excited suspension resonance frequencies from under 1 kHz to over 50 kHz. Other advantages include the ability to selectively excite different modes. For example, the amplitude of the suspension’s 2.28-kHz transverse mode was suppressed by an order of magnitude by shifting the modulation phase between the two ultrasound beams by 90. In another test, the amplitude of the 6.01-kHz torsional mode was doubled by moving the ultrasound focus point from near the center to near the edge of the suspension.

Vibro-acoustography is based on audio-frequency harmonic vibrations induced in the object by the radiation force of focused ultrasound. The resulting object vibration produces a hydrophone detectable acoustic emission that is a function of regional dynamic properties of the object at the vibration frequency. The amplitude of the hydrophone signal is mapped into an image that represents the object’s vibro-acoustic response to the radiation force by scanning the focused ultrasound across the object. Contrast in Vibro-acoustography images represent several properties of the object, including the dynamic characteristics at the audio frequency and scattering and absorption properties at the ultrasound frequency. A Vibro-acoustography system has been combined with a stereo-tactic x-ray mammography system for in vivo breast imaging and has been tested on a number of volunteers. Resulting images show soft tissue structures and microcalcifications within breast with high contrast, high resolution, and no speckles. The results have been verified using x-ray mammograms of the breast. The encouraging results from in vitro and in vivo experiments suggest further development of vibro-acoustography may lead to a new clinical breast imaging modality. [Work supported in part by a grant from the Susan G. Komen Breast Cancer Foundation. Vibro-acoustography is patented by MF and JFG.]

With the advent of MEMS, modal analysis of small structures is increasingly important. However, conventional excitation techniques normally require contact, which may not be feasible for small objects. We present a non-contact method that uses interference of ultrasound frequencies in air to produce low-frequency excitation of structures. Objects studied included hard-drive HGA suspensions and MEMS devices. The vibration induced by the ultrasound radiation force was varied in a wide range from 0 Hz to 50 kHz. Object motion was detected using a laser vibrometer; measured frequencies agreed with expected values. Also demonstrated was the unique capability to selectively enhance or suppress modes independently. For example, the ratio of the vibrational amplitudes of the 175 Hz first-bending and 1.33 kHz torsional modes of a small cantilever could be changed from in excess of 10:1 to less than 1:10 by shifting the ultrasound modulation phase 90 degrees. Similar changes were obtained for a 3 mm square MEMS mirror in the ratios of vibration amplitude around its two separate axes. Torsional modes of a hard-drive suspension could be selectively enhanced by over a factor of two by moving the ultrasound focus point from near the center to near the edge of the suspension.

The use of contrast agents is becoming a routine practice in diagnostic ultrasound imaging. Microbubble contrast agents are proving to be a safe and practical way of enhancing conventional ultrasound images. This paper discusses the concept of using microbubble contrast agents in the vibro-acoustography (VA) technique. A brief background of microbubbles and their clinical applications is given as well as a summary of the different attempts at modeling microbubble contrast agents in ultrasound fields. A general Keller–Herring equation is modified to mathematically model the interaction of encapsulated microbubbles in blood with VA ultrasound. The frequency response and scattered pressure of microbubbles with initial radius of 1 and 3 m are presented. This method is accurate for pressure amplitudes up to 500 kPa. Understanding this interaction is important for the correct interpretation of clinical investigations when using VA.

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.]

Arterial wall stiffness can be associated with various diseases. The stiffness of an artery can be assessed by measurement of the pulse wave velocity (PWV). PWV is directly related to the Youngs modulus by the well-known Moens–Korteweg equation. Usually, PWV is estimated using the foot-to-foot method. However, the foot of the pressure wave is not very clear due to reflected waves. Also, the pressure wave is normally at a low frequency, hence, the time resolution is low. PWV is an average indicator of artery stiffness between the two measuring points, therefore it is not easy to identify local stiffness. We propose producing a very short pulse wave in the arterial wall using ultrasound radiation force and measuring its propagation speed along the artery by laser. The temporal resolution of this method is in the range of microseconds, which allows PWV to be measured accurately over a few millimeters. Experiments were carried out on a silicone tube in gelatin. PWV was measured by two scanning methods: (1) fixed source and scanning detector, (2) scanning source and fixed detector. Results: PWV was measured at 2 mm/40 μs by both methods. The Doppler technique was also tested which is potentially suitable for clinical applications.

Vibro-acoustography is an imaging technique that maps the acoustic response of an object to a localized harmonic radiation force. This force is generated by two interfering continuous-wave ultrasound beams at slightly different frequencies f1 and f2. The system point-spread function (PSF) is related to the radiation force on a point-target. Imaging artifacts depend on the PSF sidelobes, which can be reduced by mismatching the sidelobes of each ultrasound beam. Here, we propose a beamforming approach based on an 8-element sector transducer with consecutive elements alternately driven at f1 and f2. The transducer generates two ultrasound beams skewed by 22.5° with respect to each other. The system PSF is analytically derived. The theory is validated by experiments using a small steel sphere (radius =0.2 mm) as a point-target. A laser vibrometer is used to measure the vibration of the sphere and evaluate the PSF of the system. Theoretically, the PSF sidelobes are under −15.8 dB in eight spots circularly distributed and separated by 22.5°, which agrees with the experimental results. Simulation shows that with 16 elements sidelobes are under −35.2 dB. In conclusion, sector transducers for vibro-acoustography may have lower sidelobes as the number of array elements is increased. [Work supported by Grant Nos. EB00535-01, EB2640, and IMG0100744.]

Vibro-acoustography is a technique that uses the radiation force of amplitude-modulated ultrasound to evaluate the dynamic response of an object at the modulation frequency. One potential application of this technique is to assess the reaction of the host tissue to the implant material by evaluating the mechanical properties of the tissue surrounding the implant. To do this, we use vibro-acoustography to measure the frequency response of the implant, and use this information to evaluate the mechanical parameters of the surrounding. Two theoretical models are developed to study the dynamic response of an implant in tissue. The first model is for an implant (1×25-mm steel beam) fully embedded in a viscoelastic medium (tissue). The second model is for a partially embedded implant with one part exposed to the open environment. The exposed end is rigidly connected to a base, and the other end is freely embedded in the viscoelastic medium. At the interface between the two media matching of boundary conditions is achieved to determine the force response. For both models eigenvalues and eigenfunctions are determined and transfer functions are evaluated. The first two natural frequencies compare well with available experimental and finite-element data.