Simulations of lesion detection using a combined phased array LHMI-technique

Ultrasound based elasticity imaging techniques have been developed during the past decades. Some of these techniques are based on an internal radiation force stimulation in which a transient or dynamic radiation force is produced by using a single or dual-frequency sonication. In addition, sonication and data acquisition can be implemented using combined or separate transducers. In this simulation study of lesion detection using localized harmonic motion imaging (LHMI), we used a combined phased array designed for simultaneous thermal ablation and lesion detection. In the sonication mode, a focused single-frequency amplitude-modulated sonication is used to induce harmonic motion and in the tracking mode, some of the array elements are used for pulse-echo tracking of the induced displacements. The results showed that the size of the lesion affected the induced displacement around the sonication point. The displacement tracking simulations demonstrated that these changes in the displacement distributions can be detected using only a few of the array elements in the tracking mode but the exact size of the lesion can not be detected accurately. The simulations also showed that two lesions having the radius of 2.5mm can be distinguished if distance between these lesions is at least 2.5mm.     

Estimating localized oscillatory tissue motion for assessment of the underlying mechanical modulus

The technique of harmonic motion imaging (HMI) uses the localized stimulus of the oscillatory ultrasonic radiation force as produced by two overlapping beams of distinct frequencies, and estimates the resulting harmonic displacement in the tissue in order to assess its underlying mechanical properties. In this paper, we studied the relationship between measured displacement and stiffness in gels and tissues in vitro. Two focused ultrasound transducers with a 100 mm focal length were used at frequencies of 3.7500 MHz and either 3.7502 (or 3.7508 MHz), respectively, in order to produce an oscillatory motion at 200 Hz in the gel or tissue. A 1.1 MHz diagnostic transducer (Imasonics, Inc.) was also focused at 100 mm and acquired 5 ms RF signals (pulse repetition frequency (PRF)=3.5 kHz) at 100 MHz sampling frequency during radiation force application. First, three 50x50 mm(2) acrylamide gels were prepared at concentrations of 4%, 8% and 16%. The resulting displacement was estimated using crosscorrelation techniques between successively acquired RF signals with a 2 mm window and 80% window overlap at 1260 W/cm(2). A normal 1-D indentation instrument (TeMPeST) applied oscillatory loads at 0.1-200 Hz with a 5 mm-diameter flat indenter. Then, 12 displacement measurements in 6 porcine muscle specimens (two measurements/case, as above) were made in vitro, before and after ablation which was performed for 10 s at 1260 W/cm(2). In all gel cases, the harmonic displacement was found to linearly increase with intensity and exponentially decrease with gel concentration. The TeMPeST measurements showed that the elastic moduli for the 4%, 8% and 16% gels equaled 3.93+/-0.06, 17.1+/-0.2 and 75+/-2 kPa, respectively, demonstrating that the HMI displacement estimate depends directly on the gel stiffness. Finally, in the tissues samples, the mean displacement amplitude showed a twofold decrease between non-ablated and ablated tissue, demonstrating a correspondence between the HMI response and an increase in stiffness measured with the TeMPeST instrument.     

A continuous-wave ultrasound system for displacement amplitude and phase measurement

A noninvasive, continuous-wave ultrasonic technique was developed to measure the displacement amplitude and phase of mechanical structures. The measurement system was based on a method developed by Rogers and Hastings ["Noninvasive vibration measurement system and method for measuring amplitude of vibration of tissue in an object being investigated," U.S. Patent No. 4,819,643 (1989)] and expanded to include phase measurement. A low-frequency sound source was used to generate harmonic vibrations in a target of interest. The target was simultaneously insonified by a low-power, continuous-wave ultrasonic source. Reflected ultrasound was phase modulated by the target motion and detected with a separate ultrasonic transducer. The target displacement amplitude was obtained directly from the received ultrasound frequency spectrum by comparing the carrier and sideband amplitudes. Phase information was obtained by demodulating the received signal using a double-balanced mixer and low-pass filter. A theoretical model for the ultrasonic receiver field is also presented. This model coupled existing models for focused piston radiators and for pulse-echo ultrasonic fields. Experimental measurements of the resulting receiver fields compared favorably with theoretical predictions.     

Internal deformation of a uniform elastic solid by acoustic radiation force

Tissue elasticity estimation is a growing area of ultrasound research. One proposed approach would apply acoustic radiation force to displace tissue and use ultrasonic motion tracking techniques to measure the resultant displacement. Such a technique might allow noninvasive imaging of tissue elastic properties. The potential of this method will be limited by the magnitude of displacements which can be generated at reasonable acoustic intensity levels. This paper presents methods for estimating the internal displacements induced in an elastic solid by acoustic radiation force. These methods predict displacements on the order of 400 microns in the human vitreous body, 0.008 micron in human breast, and 0.020 micron in human liver at an acoustic intensity of 1.0 W/cm2 (in water) and an operating frequency of 10 MHz. While the displacement generated in the vitreous should be readily detectable using ultrasonic methods, the displacements generated in the breast and liver will be much more difficult to detect. Methods are also developed for predicting the time dependent temperature increases associated with attenuated acoustic fields in the absence of perfusion. These results indicate promise for radiation force imaging in the vitreous, but potential difficulties in applying these techniques in other parts of the body.     

Optimization of acoustic scattering from dual-frequency driven microbubbles at the difference frequency

The second harmonic radiation of acoustically driven bubbles is a useful discriminant for their presence in clinical ultrasound applications. It is useful because the scatter from a bubble at a frequency different from the driving can have a contrast-to-tissue ratio better than at the drive frequency. In this work a technique is developed to optimize the scattering from a microbubble at a frequency different from the driving. This is accomplished by adjusting the relative phase and amplitudes of the components of a dual-frequency incident ultrasound wave form. The investigation is focused primarily on the example of dual-mode driving at frequencies of 1 MHz and 3 MHz, with the scattering optimized at 2 MHz. Bubble radii of primary interest are 0.5 to 2 microm and driving amplitudes to 0.5 atm. Bubbles in this size range are sensitive to modulation of driving. It is shown that an optimal forcing scheme can increase the target response eightfold or more. This suggests new applications in imaging and in bubble detection.     

Intravascular ultrasound tissue harmonic imaging: a simulation study

Recently, the in vivo feasibility of tissue harmonic imaging (THI) with a mechanically-rotated intravascular ultrasound (IVUS) catheter was experimentally demonstrated. To isolate the second harmonic signal content, both pulse inversion (PI) and analog filtering were used. In the present paper, we report the development of a simulation tool to investigate nonlinear IVUS beams and the influence of rotation on the efficiency of PI signal processing. Nonlinear 20 MHz beams were simulated in a homogeneous tissue-mimicking medium, resulting in second harmonic pressure fields at 40 MHz. The acoustic response from tissue was simulated by summing radio-frequency (RF) pulse-echo responses from many point-scatterers. When the transducer was rotated with respect to the point-scatterers, the fundamental frequency suppression using PI degraded rapidly with increasing inter-pulse angles. The results of this study will aid in the optimization of harmonic IVUS imaging systems.     

In vitro comparative study of vibro-acoustography versus pulse-echo ultrasound in imaging permanent prostate brachytherapy seeds

Background

Permanent prostate brachytherapy (PPB) is a common treatment for early stage prostate cancer. While the modern approach using trans-rectal ultrasound guidance has demonstrated excellent outcome, the efficacy of PPB depends on achieving complete radiation dose coverage of the prostate by obtaining a proper radiation source (seed) distribution. Currently, brachytherapy seed placement is guided by trans-rectal ultrasound imaging and fluoroscopy. A significant percentage of seeds are not detected by trans-rectal ultrasound because certain seed orientations are invisible making accurate intra-operative feedback of radiation dosimetry very difficult, if not impossible. Therefore, intra-operative correction of suboptimal seed distributions cannot easily be done with current methods. Vibro-acoustography (VA) is an imaging modality that is capable of imaging solids at any orientation, and the resulting images are speckle free.

Objective and methods

The purpose of this study is to compare the capabilities of VA and pulse-echo ultrasound in imaging PPB seeds at various angles and show the sensitivity of detection to seed orientation. In the VA experiment, two intersecting ultrasound beams driven at f₁ = 3.00 MHz and f₂ = 3.020 MHz respectively were focused on the seeds attached to a latex membrane while the amplitude of the acoustic emission produced at the difference frequency 20 kHz was detected by a low frequency hydrophone.

Results

Finite element simulations and results of experiments conducted under well-controlled conditions in a water tank on a series of seeds indicate that the seeds can be detected at any orientation with VA, whereas pulse-echo ultrasound is very sensitive to the seed orientation.

Conclusion

It is concluded that vibro-acoustography is superior to pulse-echo ultrasound for detection of PPB seeds.     

Estimation of mechanical properties of a viscoelastic medium using a laser-induced microbubble interrogated by an acoustic radiation force

An approach to assess the mechanical properties of a viscoelastic medium using laser-induced microbubbles is presented. To measure mechanical properties of the medium, dynamics of a laser-induced cavitation microbubble in viscoelastic medium under acoustic radiation force was investigated. An objective lens with a 1.13 numerical aperture and an 8.0 mm working distance was designed to focus a 532 nm wavelength nanosecond pulsed laser beam and to create a microbubble at the desired location. A 3.5 MHz ultrasound transducer was used to generate acoustic radiation force to excite a laser-induced microbubble. Motion of the microbubble was tracked using a 25 MHz imaging transducer. Agreement between a theoretical model of bubble motion in a viscoelastic medium and experimental measurements was demonstrated. Young's modulii reconstructed using the laser-induced microbubble approach were compared with those measured using a direct uniaxial method over the range from 0.8 to 13 kPa. The results indicate good agreement between methods. Thus, the proposed approach can be used to assess the mechanical properties of a viscoelastic medium.     

调频超声脉冲驱动微气泡运动偏移的研究

提出调频超声辐射力技术驱动微泡群,以加强微泡的吸附效率.基于改进的RP方程及粒子轨迹方程研究了微泡群整体的运动位移与调频信号的中心频率、调频范围、信号声压,以及微泡半径分布关系.研究结果表明调频信号在驱动半径具有宽泛分布的气泡群,以及半径分布远离谐振半径的气泡群时,作用效果好于传统正弦波信号.例如中心频率1 MHz、调频范围0.75 MHz的调频脉冲作用高斯分布（平均半径3.5 μm、均方差为1）的微泡群200 μs,可比同等声压的正弦波多约12%的微气泡产生位移30 μm. 关键词： 超声辐射力 调频波 高斯分布     

Study of inspection on metal sheet with subsurface defects using linear frequency modulated ultrasound excitation thermal-wave imaging (LFM-UTWI)

The linear frequency modulated ultrasound excitation thermal wave imaging (LFM-UTWI) was investigated on detection of subsurface defects of metal sheet. A numerical finite element analysis is carried out to calculate thermal wave signal dependence of time by linear frequency modulated ultrasonic wave excitation. Cross-correlation operation in time domain and frequency domain are used to extract the main peak value and the corresponding delay time, respectively. Fourier transform (FT) is applied to calculate the amplitude and phase angle of harmonic component of thermal wave. Experimental results show that various deep subsurface defects are readily detected using LFM-UTWI with once excitation, and LFM-UTWI has an advantage of better defect detectability compared to ultrasound lock-in thermography (ULIT).     

Calibration of high-frequency hydrophone up to 40 MHz by heterodyne interferometer

A calibration technique for high-frequency hydrophone utilizing a heterodyne interferometer is presented in this article. The calibration system is mainly composed of optical and signal processing modules. In the displacement measurement, a pellicle is mounted at the surface of water to avoid acousto-optical interaction. The phase modulated carrier signal is digitized and transferred to the computer, then processed by digital phase demodulation. A phase unwrapping algorithm is employed to remove ambiguity of the arctangent function and has proven effective in large displacement measurements. Pellicle displacement and voltage output of the hydrophone in focused ultrasonic field are processed by DFT to determine the amplitudes of the fundamental and harmonic components. Experiments show that the heterodyne technique can provide hydrophone calibration up to 40 MHz, with a slightly smaller sensitivity compared with the National Physical Laboratory (NPL) calibration results for most frequency ranges. Since the heterodyne technique is independent on assumptions about the geometry of the ultrasonic field and the performance of the transducer, it can be easily extended to high frequency and high power ultrasound measurement applications.     

Dynamic analysis of a multi-span uniform beam carrying a number of various concentrated elements

This paper employs the numerical assembly method (NAM) to determine the “exact” frequency–response amplitudes of a multiple-span beam carrying a number of various concentrated elements and subjected to a harmonic force, and the exact natural frequencies and mode shapes of the beam for the case of zero harmonic force. First, the coefficient matrices for the intermediate concentrated elements, pinned support, applied force, left-end support and right-end support of a beam are derived. Next, the overall coefficient matrix for the whole vibrating system is obtained using the numerical assembly technique of the conventional finite element method (FEM). Finally, the exact dynamic response amplitude of the forced vibrating system corresponding to each specified exciting frequency of the harmonic force is determined by solving the simultaneous equations associated with the last overall coefficient matrix. The graph of dynamic response amplitudes versus various exciting frequencies gives the frequency–response curve for any point of a multiple-span beam carrying a number of various concentrated elements. For the case of zero harmonic force, the above-mentioned simultaneous equations reduce to an eigenvalue problem so that natural frequencies and mode shapes of the beam can also be obtained.     

Acoustic characterization of echogenic liposomes: frequency-dependent attenuation and backscatter

Ultrasound contrast agents (UCAs) are used clinically to aid detection and diagnosis of abnormal blood flow or perfusion. Characterization of UCAs can aid in the optimization of ultrasound parameters for enhanced image contrast. In this study echogenic liposomes (ELIPs) were characterized acoustically by measuring the frequency-dependent attenuation and backscatter coefficients at frequencies between 3 and 30 MHz using a broadband pulse-echo technique. The experimental methods were initially validated by comparing the attenuation and backscatter coefficients measured from 50-μm and 100-μm polystyrene microspheres with theoretical values. The size distribution of the ELIPs was measured and found to be polydisperse, ranging in size from 40 nm to 6 μm in diameter, with the highest number observed at 65 nm. The ELIP attenuation coefficients ranged from 3.7 ± 1.0 to 8.0 ± 3.3 dB/cm between 3 and 25 MHz. The backscatter coefficients were 0.011 ± 0.006 (cm str)(-1) between 6 and 9 MHz and 0.023?±?0.006 (cm str)(-1) between 13 and 30 MHz. The measured scattering-to-attenuation ratio ranged from 8% to 22% between 6 and 25 MHz. Thus ELIPs can provide enhanced contrast over a broad range of frequencies and the scattering properties are suitable for various ultrasound imaging applications including diagnostic and intravascular ultrasound.     

A simulation study on tissue harmonic imaging with a single-element intravascular ultrasound catheter

Recently, in vivo feasibility of tissue harmonic imaging with a mechanically rotated intravascular ultrasound (IVUS) catheter was experimentally demonstrated. To isolate the second harmonic signal content, a combination of pulse inversion and analog filtering was used. In this paper the development of a simulation tool to investigate nonlinear IVUS beams is reported, and the influence of transducer rotation and axial catheter-to-tissue motion on the efficiency of PI signal processing is evaluated. Nonlinear beams were simulated in homogeneous tissue-mimicking media at a transmit frequency of 20 MHz, which resulted in second harmonic pressure fields at 40 MHz. The competing effects of averaging and decorrelation between neighboring rf lines on the signal-to-noise ratio (SNR) were studied for a single point scatterer. An optimal SNR was achieved when lines were combined over 3 degrees - 3.75 degrees. When the transducer was rotated with respect to point scatterers, simulating the acoustic response of tissue, the fundamental frequency suppression using PI degraded rapidly with increasing interpulse angles. The effect of axial catheter-to-tissue motion on the efficiency of pulse inversion seemed to be of less influence for realistic motion values. The results of this study will aid in the optimization of harmonic IVUS imaging systems.     

Sound radiation from fluid loaded orthogonally stiffened plates

The radiation of sound from infinite fluid loaded plates is examined when the plates are reinforced with two sets of orthogonal line stiffeners. The stiffeners are assumed to be equally spaced and exert only forces on the plate. The response to a convected harmonic pressure is found by using Fourier transforms and is given in terms of the harmonic amplitudes of the stiffener forces. These forces satisfy an infinite set of simultaneous equations to which a numerical solution must be found. An expression for the response to a general excitation is derived and from this the acoustic pressure in the far field is determined with particular reference to point force excitation.     

Determination of object resonances by vibro-acoustography and their associated modes

Vibro-acoustography technique known by its noncontact excitation was used to detect resonance frequencies of objects in water. Two intersecting ultrasound beams generated by a 40 mm-diameter annular array transducer, focused at 35 mm and driven at f1=2.2 MHz and f2=2.22 MHz respectively, were targeted inside the object under test to produce a radiation force beating at the difference frequency f2-f1. This low frequency radiation force was used to excite the resonance vibration modes of the object by sweeping the frequency f2 between 2.22 and 2.275 MHz. The amplitude of the acoustic emission produced by the vibrations of the object was detected by a low frequency hydrophone (BW=60 kHz). By this approach, it was possible to detect resonance frequencies through amplitude variations of the measured acoustic emission. Experiments were conducted in a water tank for objects of different shapes and sizes. With a chalk sphere (15 mm-diameter) two resonance frequencies were detected at 45.75 and 68.75 kHz, and with a cylinder (10.38 mm-diameter and 32.20 mm-length) four principal resonance frequencies were identified in the 60 kHz-bandwidth of the hydrophone. It was shown with finite element calculations performed with Ansys, in which both solid and fluid parts were modelled, that the measured resonance frequencies corresponded to compressional or dilatation vibration modes of the object. It was verified that shear waves generated by torsional vibration modes were not propagated in water, as it is well known. The use of this technique to characterize heterogeneities in different media seems to be relatively more advantageous to other ultrasonic methods.     

Difference-frequency ultrasound generation from microbubbles under dual-frequency excitation

The difference-frequency (DF) ultrasound generated by using parametric effect promises to improve detection depth owing to its low attenuation, which is beneficial for deep tissue imaging. With ultrasound contrast agents infusion, the harmonic components scattered from the microbubbles, including DF, can be generated due to the nonlinear vibration. A theoretical study on the DF generation from microbubbles under the dual-frequency excitation is proposed in formula based on the solution of the RPNNP equation. The optimisation of the DF generation is discussed associated with the applied acoustic pressure, frequency, and the microbubble size. Experiments are performed to validate the theoretical predictions by using a dual-frequency signal to excite microbubbles. Both the numerical and experimental results demonstrate that the optimised DF ultrasound can be achieved as the difference frequency is close to the resonance frequency of the microbubble and improve the contrast-to-tissue ratio in imaging.     

A preliminary in vitro assessment of polymer-shelled microbubbles in contrast-enhanced ultrasound imaging

This paper focuses on the use of poly (vinyl alcohol)-shelled microbubbles as a contrast agent in ultrasound medical imaging. The objective was an in vitro assessment of the different working conditions and signal processing methods for the visual detection (especially in small vessels) of such microbubbles, while avoiding their destruction. Polymer-shelled microbubbles have recently been proposed as ultrasound contrast agents with some important advantages. The major drawback is a shell that is less elastic than that of the traditional lipidic microbubbles. Weaker echoes are expected, and their detection at low concentrations may be critical. In vitro experiments were performed with a commercial ultrasound scanner equipped with a dedicated acquisition board. A concentration of 100 bubbles/mm³, excitation pressure amplitudes from 120 kPa to 320 kPa, and a central frequency of 3 MHz or 4.5 MHz were used. Three multi-pulse techniques (i.e., pulse inversion, contrast pulse sequence based on three transmitted signals, and contrast pulse sequence in combination with the chirp pulse) were compared. The results confirmed that these microbubbles produce a weaker ultrasound response than lipidic bubbles with a reduced second-order nonlinear component. Nevertheless, these microbubbles can be detected by the contrast pulse sequence technique, especially when the chirp pulse is adopted. The best value of the contrast-to-tissue ratio was obtained at an excitation pressure amplitude of 230 kPa: although this pressure amplitude is higher than what is typically used for lipidic microbubbles, it does not cause the rupture of the polymeric contrast agent.     

Motion of a solid sphere in a viscoelastic medium in response to applied acoustic radiation force: Theoretical analysis and experimental verification

The motion of a rigid sphere in a viscoelastic medium in response to an acoustic radiation force of short duration was investigated. Theoretical and numerical studies were carried out first. To verify the developed model, experiments were performed using rigid spheres of various diameters and densities embedded into tissue-like, gel-based phantoms of varying mechanical properties. A 1.5 MHz, single-element, focused transducer was used to apply the desired radiation force. Another single-element, focused transducer operating at 25 MHz was used to track the displacements of the sphere. The results of this study demonstrate good agreement between theoretical predictions and experimental measurements. The developed theoretical model accurately describes the displacement of the solid spheres in a viscoelastic medium in response to the acoustic radiation force.     

Image features in medical vibro-acoustography: in vitro and in vivo results

Vibro-acoustography is an imaging method based on audio-frequency harmonic vibrations induced in the object by the radiation force of focused ultrasound. The purpose of this study is to investigate features of vibro-acoustography images and manifestation of various tissue structures and calcifications in such images. Our motivation for this study is to pave the way for further in vitro and in vivo applications of vibro-acoustography. Here, vibro-acoustography images of excised prostate and in vivo breast are presented and compared with images obtained with other modalities. Resulting vibro-acoustography images obtained with a 3 MHz ultrasound transducer and at a vibration frequency of 50-60 kHz show soft tissue structures, tissue borders, and microcalcifications with high contrast, high resolution, and no speckle. It is concluded that vibro-acoustography offers features that may be valuable for diagnostic purposes.