Fatemi, Mostafa

Dynamic radiation force has been used in several types of applications, and is performed by modulating ultrasound with different methods. By modulating ultrasound, energy can be transmitted to tissue, in this case a dynamic force to elicit a low frequency cyclic displacement to inspect the material properties of the tissue. In this paper, different types of modulation are explored including amplitude modulation (AM), double sideband suppressed carrier amplitude modulation AM, linear frequency modulation, and frequency-shift keying. Generalized theory is presented for computing the radiation force through the short-term time average of the energy density for these various types of modulation. Examples of modulation with different types of signals including sine waves, square waves, and triangle waves are shown. Using different modulating signals, multifrequency radiation force with different numbers of frequency components can be created, and can be used to characterize tissue mimicking materials and soft tissue. Results for characterization of gelatin phantoms using a method of vibrating an embedded sphere are presented. Different degrees of accuracy were achieved using different modulation techniques and modulating signals. Modulating ultrasound is a very flexible technique to produce radiation force with multiple frequency components that can be used for various applications.

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.).]

Thyroid imaging with conventional ultrasound scanners does not always lead to conclusive results. Furthermore, current ultrasound imaging technology has difficulty in detecting important microcalcifications. Ambiguities in ultrasound and other thyroid imaging methods lead to a large number of unnecessary biopsies. For these reasons, alternative imaging methods capable of detecting both microcalcifications and lesions in thyroid are of great interest. This paper describes recent results on imaging human thyroids with vibro-acoustography. Experiments were conducted on eight excised human thyroids from autopsy. The specimens were each imbedded in a block of gel to simulate the surrounding tissue and to facilitate scanning. Three types of images were acquired from each specimen: X-ray, B-mode ultrasound, and vibro-acoustography. Anatomical details seen in the images were correlated and image quality was compared. X-ray images displayed calcifications with high contrast, but did not show anatomical details of soft tissue. B-mode displayed some soft tissue structures; however, the speckle masked the details and reduced the contrast. Vibro-acoustography displayed calcifications, anatomical details, and some nodules where they existed. Furthermore, vibro-acoustography images displayed tissue structures with high contrast and free from speckle. It is concluded that vibro-acoustography may be a suitable technique for thyroid imaging.

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.]

This work presents a theory for the acoustic radiation force exerted on a solid sphere by an arbitrary spatially distributed beam. The theory is developed for an sphere suspended in an ideal fluid. We assume that the acoustic beam can be decomposed in a set of plane waves with same frequency, propagating in different directions. The sphere radius is considered to be much smaller than the wavelength of the beam. Bulk properties of the sphere such as shear and compressional sound speed are taken into account. The radiation force is obtained by solving the linear acoustic scattering problem for the sphere. Theoretically, the radiation force depends on the sphere cross section area, the radiation force function, and the vector energy flux upon the sphere. The radiation force function is related to the sphere scattering properties. We apply the developed theory to study the radiation force produced by an spherical concave transducer. The generated radiation force can be decomposed into two components, namely, axial and transverse with respect to the wave propagation direction. The ratio between the transverse and axial components of the force depends on the transducer F-number and wave frequency. Results show that this ratio for a 2 MHz transducer with 3.5 F-number on the focal plane is less than 5%.

A new method for imaging and detecting modal shapes of vessels is introduced. Theory is developed that predicts the measured velocity is proportional to the value of the mode shape at the focal point of the ultrasound beam. Experimental a cylindrical gel phantom of large radius. This model simulates approximately a large artery and the surrounding body. The fundamental frequency was measured 83 Hz for the tube-phantom system. At this frequency the ultrasound transducer was scanned across the vessel plane with velocity measurement at one single point on the vessel and on the phantom by laser. The images obtained show clearly the interior tube and the modal shape of the tube.

Vibro-acoustography is a new noncontact imaging method based on the radiation force of ultrasound. We extend this new technique for tissue characterization of arterial vessels by vibration techniques. In this paper a theoretical model for vibration analysis of arterial vessel with tissue is developed. Experimental studies were carried out on a silicone rubber tube embedded in a cylindrical gelatin phantom of larger radius, which simulates a large artery and the tissue body. The fundamental mode is well excited by the radiation force of ultrasound. The fundamental frequency was measured 81.8 Hz for a tube-phantom structure that is quite close to our theoretical prediction of 83.3 Hz.

Vibro-acoustography (VA) is based on conversion of ultrasound energy from high frequency to a low frequency. This conversion, which results from the nonlinear nature of wave equation, gives VA some key advantages over linear ultrasound imaging such as B-mode. Two key features of VA are analyzed: lack of speckle noise, and angle-independency, which is the ability of imaging specular reflectors regardless of their orientation. Both the speckle noise and angle dependency are major artifacts in B-mode. Lack of speckle noise results from the fact that VA images are constructed from the low frequency acoustic signal. Using an acoustical model, it is shown that the random scatterers within soft tissue do not promote speckle noise in VA as they do in B-mode; thus VA generally produces high contrast images. Angle-independency results from the broad beam pattern of the low-frequency acoustic field. A model is constructed describing the amplitude of the acoustic signal versus object orientation angle. It is shown that the signal amplitude is relatively insensitive to object orientation; thus VA may be suitable for imaging specular objects such as implants. It is concluded that VA may be advantageous in applications where speckle noise and angle dependency are of concern.

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.

High-resolution ultrasound imaging systems for inspection of defects and flaws in materials are of great demand in many industries. Among these systems, Vibro-acoustography (VA) has shown excellent capabilities as a non-contact method for non-destructive high-resolution imaging applications. This method consists of mixing two confocal ultrasound beams, slightly shifted in frequency, to produce an acoustic emission field at the difference frequency of the primary incident ultrasound beams. In addition to the difference frequency signal, there exists another signal at the sum frequency, formed in the intersection region of the two primary beams. The goal of this study is to investigate the formation of high-resolution images using the sum frequency of ultrasound waves in VA while concurrently forming the conventional difference-frequency VA image, thereby increasing the amount of information acquired during a single scan. A theoretical model describing the sum frequency wave propagation, including beam forming and image formation in the confocal configuration is developed and verified experimentally. Moreover, sample experiments are performed on a flawed fiber-reinforced ceramic composite plate. Images at both the difference and sum frequencies are compared and discussed. Results show that the sum frequency image produces a high-resolution C-scan of the plate by which the flaws and structural details of the plate can be detected.