Urban, Matthew W.

The mechanical response of tissues to external forces has gained considerable interest in medical diagnosis. Typically, a force is needed to produce displacement in the tissue for estimation of the tissue’s elasticity. One approach is to apply an external force on the body’s surface. Another emerging technique is to generate a localized force inside the tissue with ultrasound. Shear waves can be used in bulky tissues for estimating the tissue’s elasticity. The bending wave can be generated and measured in vessels for noninvasive estimation of a vessel’s elasticity. We have found that the Rayleigh surface wave can be generated on the tissue’s surface, [X. Zhang et al., Proc. International Congress on Ultrasonics, Vienna, April 9–13, 2007], [X. Zhang et al., IEEE Trans. Medical Imaging, 26, 843–852 (2007)]. In our method, a localized radiation force of ultrasound is remotely and noninvasively applied inside the tissue. This force can generate the shear wave as well as compression wave inside the tissue. However, only the surface wave can exist on the body’ surface. The surface wave can be used to estimate and image the stiffness of tissue. Some results of using the surface wave are presented.

Modulating ultrasound can create dynamic ultrasound radiation force to induce local tissue vibration. Using different types of modulating signals produces radiation force with weighted multifrequency components. Modulation signals with frequency fr produce force with components at multiples of fr. Both amplitude modulation (AM) and double sideband suppressed carrier (DSB-SC) amplitude modulation were explored. Different waveforms were investigated such as sine, square, triangle, and sawtooth for modulating continuous wave ultrasound. A 3.0 MHz transducer produced ultrasound which was measured with a needle hydrophone and used to derive and analyze the spectrum of the radiation force function. The modulated ultrasound was used to vibrate a steel sphere in a gelatin phantom, and the motion was measured using a laser vibrometer. The measured spectra were used to find the frequency response of the sphere. Different modulating signals were used to produce multifrequency radiation force. The sphere frequency response was recovered using different modulating signals. Weighted multifrequency radiation force was produced by modulating ultrasound with different signals. Application of multifrequency radiation force can be used to measure the frequency response of objects or tissue. [This work was supported in part by grants EB002167, EB002640, and CA091956 from NIH.]

Considerable interest in the elastic properties of soft tissue has increased in medicine due to their clinical relevance for monitoring various diseases as well as a biomarker for cancer. A recent emerging field is the use of acoustic radiation force for imaging and estimating the elastic properties of tissue. Newly developed imaging modalities include shear wave elasticity imaging, supersonic shear wave imaging, acoustic radiation force impulse imaging, and vibro-acoustography. In these new methods, the shear wave is used to estimate the elasticity of tissue. We have developed a novel surface wave method for noninvasively estimating the elasticity of tissue [X. Zhang et al., J. Acoust. Soc. Am., 122, 2522-2525 (2007)]. In this method, a localized ultrasound radiation force is remotely and noninvasively applied inside the tissue. The surface wave speed is used to estimate the elasticity of tissue. This method has been further developed and applied to estimate the viscoelasticity of animal myocardium tissue. In this abstract, we report our recent results of in vitro studies on animal myocardium tissue. Both the surface wave and shear wave are measured and analyzed. Noninvasive characterization of viscoelastic properties of myocardium tissue may be a very important tool for assessing heart function.

Elasticity imaging is a medical imaging modality that creates images based on the mechanical response of tissue. In some techniques, ultrasound radiation force has been used to deform tissue and ultrasonic or other techniques detect the tissue displacement. The focus of this dissertation was development of methods utilizing multiple ultrasound frequencies simultaneously to produce multifrequency harmonic radiation force and development of harmonic motion detection techniques adapted for vibrometry with simultaneous radiation force excitation. Beamforming for multifrequency radiation force was developed analytically, numerically, and experimentally. Sample applications in the fields of vibrometry and vibro-acoustic breast imaging are shown. Numerical models for harmonic motion detection of a vibrating reflective target and a vibrating scattering medium were developed. Parametric analyses were performed to extensively analyze the error in the vibration amplitude and phase measurements. An experimental method using multifrequency radiation force and harmonic motion detection with a single transducer is described and experimental results for a reflective target and scattering medium are shown. This work produced techniques for excitation and measurement of small amplitude harmonic motion for performing vibro-acoustography and vibrometry in a multiplexed manner. Models of vibration responses were developed to further understanding of current and future applications in vibro-acoustography and vibrometry.

Elasticity imaging is an emerging medical imaging modality. Methods involving acoustic radiation force excitation and pulse-echo ultrasound motion detection have been investigated to assess the mechanical response of tissue. In this work new methods for dynamic radiation force excitation and motion detection are presented. The theory and model for harmonic motion detection of a vibrating reflective target are presented. The model incorporates processing of radio frequency data acquired using pulse-echo ultrasound to measure harmonic motion with amplitudes ranging from 100to10,000nm. A numerical study was performed to assess the effects of different parameters on the accuracy and precision of displacement amplitude and phase estimation and showed how estimation errors could be minimized. Harmonic pulsed excitation is introduced as a multifrequency radiation force excitation method that utilizes ultrasound tonebursts repeated at a rate fr. The radiation force, consisting of frequency components at multiples of fr, is generated using 3.0MHz ultrasound, and motion detection is performed simultaneously with 9.0MHz pulse-echo ultrasound. A parameterized experimental analysis showed that displacement can be measured with small errors for motion with amplitudes as low as 100nm. The parameterized numerical and experimental analyses provide insight into how to optimize acquisition parameters to minimize measurement errors.

Detection of calcifications in breast is an important problem in the diagnosis of breast cancer. Vibro-acoustography is a recently developed method that uses the radiation force of ultrasound to create images of the mechanical response of an object at a low frequency using the magnitude or phase of the response. Small spheres are used to explore the use of the phase of vibration as a contrast modality for use in detection and identification of calcifications in breast tissue. An experiment is presented to measure the magnitude and phase of vibration at different frequencies. The theoretical and experimental results are compared for spheres of two different sizes. Phase images are shown in which five spheres of different density can be clearly distinguished from each other. With phase measurements and images, it is demonstrated that predictable image contrast exists for spheres of different density embedded in a viscoelastic medium.