Elasticity imaging using conventional and high-frame rate ultrasound imaging: experimental study

High-frame rate ultrasound imaging is necessary to track fast deformation in ultrasound elasticity imaging, but the image quality may be degraded. Previously, we investigated the performance of strain imaging using numerical models of conventional and ultrafast ultrasound imaging techniques. In this paper, we performed experimental studies to quantitatively evaluate the strain images and elasticity maps obtained using conventional and high frame rate ultrasound imaging methods. The experiments were carried out using point target and tissue mimicking phantoms. The experimental results were compared with the results of numerical simulation. Our experimental studies confirm that the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and axial/lateral resolution of the displacement and strain images acquired using high-frame rate ultrasound imaging are slightly lower but comparable with those obtained using conventional imaging. Furthermore, the quality of elasticity images also exhibits similar trends. Thus, high-frame rate ultrasound imaging can be used reliably for static elasticity imaging to capture the internal tissue motion if the frame rate is critical.     

A novel envelope detector for high-frame rate, high-frequency ultrasound imaging

This paper proposes a novel design of envelope detectors capable of supporting a small animal cardiac imaging system requiring a temporal resolution of more than 150 frames per second. The proposed envelope detector adopts the quadrature demodulation and the lookup table (LUT) method to compute the magnitude of the complex baseband components of received echo signals. Because the direct use of the LUT method for a square root function is not feasible due to a large memory size, this paper presents a new LUT strategy dramatically reducing its size by using binary logarithmic number system (BLNS). Due to the nature of BLNS, the proposed design does not require an individual LOG-compression functional block. In the implementation using a field programmable gate array (FPGA), a total of 166.56 Kbytes memories were used for computing the magnitude of 16-bit in-phase and quadrature components instead of 4 Gbytes in the case of the direct use of the LUT method. The experimental results show that the proposed envelope detector is capable of generating LOG-compressed envelope data at every clock cycle after 32 clock cycle latency, and its maximum error is less than 0.5 (i.e., within the rounding error), compared with the arithmetic results of square root function and LOG compression.     

A high-frame rate high-frequency ultrasonic system for cardiac imaging in mice

We report the development of a high-frequency (30-50 MHz), real-time ultrasonic imaging system for cardiac imaging in mice. This system is capable of producing images at 130 frames per second (fps) with a spatial resolution of less than 50 microm. A novel mechanical sector probe was developed that utilizes a magnetic drive mechanism and custom-built servo controller for high speed and accuracy. Additionally, a very light-weight (< 0.28 g), single-element transducer was constructed and used to reduce the mass load on the motor. The imaging electronics were triggered according to the angular position of the transducer in order to compensate for the varying speed of the sector motor. This strategy ensured the production of equally spaced scan lines with minimal jitter. Wire phantom testing showed that the system axial and lateral resolutions were 48 microm and 72 microm, respectively. In vivo experiments showed that high-frequency ultrasonic imaging at 130 fps is capable of showing a detailed depiction of a beating mouse heart.     

Experimental study of high frame rate imaging with limited diffraction beams

Limited diffraction beams have a large depth of field and have many potential applications. Recently, a new method (Fourier method) was developed with limited diffraction beams for image construction. With the method and a single plane wave transmission, both 2D (two-dimensional) and 3D (three-dimensional) images of a very high frame rate (up to 3750 frames/s for a depth of 200 mm in biological soft tissues) and a high signal-to-noise ratio (SNR) can be constructed with relatively simple and inexpensive hardware. If limited diffraction beams of different parameters are used in both transmission and reception and transducer aperture is shaded with a cosine function, high-resolution and low-sidelobe images can be constructed with the new method without montage of multiple frames of images [the image quality is comparable to that obtained with a transmit-receive (two-way) dynamically focused imaging system]. In this paper, the Fourier method was studied with both experiment and computer simulation for 2D B-mode imaging. In the experiment, two commercial broadband 1D array transducers (48 and 64 elements) of different aperture sizes (18.288 and 38.4 mm) and center frequencies (2.25 and 2.5 MHz) were used to construct images of different viewing sizes. An ATS539 tissue-equivalent phantom of an average frequency-dependent attenuation of 0.5 dB/MHz/cm was used as a test object. To obtain high frame rate images, a single plane wave pulse (broadband) was transmitted with the arrays. Echoes received with the arrays were processed with both the Fourier and conventional dynamic focusing (delay-and-sum) methods to construct 2D B-mode images. Results show that the quality (resolution and contrast) of constructed images is virtually identical for both methods, except that the Fourier method is simpler to implement. Both methods have also a similar sensitivity to phase aberration distortions. Excellent agreement among theory, simulation, and experiment was obtained.     

Dark-field imaging with cylindrical-vector beams

Dark-field illumination provides an imaging mode that rejects specular light, thereby highlighting edge features. We analyze dark-field imaging by using cylindrical vector beam illumination with a confocal microscope equipped with a microstructure fiber mode filter. A numerical model based on rigorous coupled-wave analysis has been used to analyze the method. We acquired images of separated edges features to investigate the edge separation resolution of the method. A through-focus comparison of azimuthal and radial polarization shows a measurable dependence of edge separation on polarization.     

Capon beamforming in medical ultrasound imaging with focused beams

Medical ultrasound imaging is conventionally done by insonifying the imaged medium with focused beams. The backscattered echoes are beamformed using delay-and-sum operations that cannot completely eliminate the contribution of signals backscattered by structures off the imaging beam to the beamsum. It leads to images with limited resolution and contrast. This paper presents an adaptation of the Capon beamformer algorithm to ultrasound medical imaging with focused beams. The strategy is to apply data-dependent weight functions to the imaging aperture. These weights act as lateral spatial filters that filter out off-axis signals. The weights are computed for each point in the imaged medium, from the statistical analysis of the signals backscattered by that point to the different elements of the imaging probe when insonifying it with different focused beams. Phantom and in vivo images are presented to illustrate the benefits of the Capon algorithm over the conventional delay and-sum approach. On heart sector images, the clutter in the heart chambers is decreased. The endocardium border is better defined. On abdominal linear array images, significant contrast and resolution enhancement are observed.     

Extended focus diffractive optical element for Gaussian laser beams

The depth of focus of the Gaussian beam is extended by introducing a wavefront phase correction with properly designed diffractive optical elements. Results of the computer simulations show that, compared with other methods, the presented method demonstrates a reduced focal spot size and low sidelobes in a focal domain, within a considerable range of defocusing distances. Experimental results for the visible range diffractive optical element with a focus of 40 mm and a depth of focus that extends to 1 mm agree with the theory.     

Second-harmonic generation imaging of metal nano-objects with cylindrical vector beams

We introduce an imaging technique based on second-harmonic generation with cylindrical vector beams that is extremely sensitive to three-dimensional orientation and nanoscale morphology of metal nano-objects. Our experiments and second-harmonic field calculations based on frequency-domain boundary element method are in very good agreement. The technique provides contrast for structural features that cannot be resolved by linear techniques or conventional states of polarization and shows great potential for simple and cost-effective far-field optical imaging in plasmonics.     

Fast B-flow imaging: a method for improving frame rate in Golay coded B-flow imaging

A technique for Golay coded B-flow imaging, called fast B-flow imaging, has been developed. This technique improves the frame rate of Golay coded B-flow imaging. In this technique, three instead of four input pulses are used to produce each scan line. A standard Golay pulse-pair is used as two of the three inputs, and pulse compression is performed upon receive returning the echoes from stationary (tissue) objects in the image. The third input is a repetition of one of the first two inputs. Upon receive, this pulse is cross correlated with an inverted copy of its input pulse. Addition of the cross-correlated signals produced from the identical input pulses results in the cancellation of the strong tissue echoes, and enables visualization of the weaker/moving blood echoes. Combining a small fraction of the tissue echoes with the weaker blood echoes allows both to be visualized in the same gray scale image. By using three instead of four input pulses, this technique can achieve a frame rate improvement of 33% compared with standard Golay coded B-flow imaging, with some loss in signal-to-noise ratio. The impact of axial and lateral motion on these techniques is examined. A quantitative comparison of both techniques is presented.     

Piezoelectric beams and vibrating angular rate sensors

A tuning fork angular rate sensor made out of a single piece of quartz has been studied. The piezoelectric effect is used both to excite a reference vibration in the plane of the tuning fork and to detect a vibration normal to this plane. The amplitude of the second vibration is directly proportional to the applied angular velocity. The structure is made rigid in order for it to survive in a harsh environment. This implies that the only vibrationally active areas are the tines of the tuning fork. The performance of the sensor is predicted with the help of a phenomenological piezoelectric beam theory. This theory shows that it suffices to study the two-dimensional (2-D) dielectric field in the cross-sections of the beams in order to obtain the values of the piezoelectric equivalent components. Estimates of these values can be obtained without the use of special computer programs. The predictions are shown to be in agreement with measurements.     

Extended numerical integration method for triangular surfaces

The paper includes a technique for numerical integration over the surface of a triangle. The mathematical transformation which is described, overcomes the difficulties associated with the derivation of new quadrature coefficients and yet leads to results which are accurate. An application to an axisymmetric finite element method with triangular elements is also included.     

带温度补偿结构的高灵敏度光读出红外成像阵列器件设计

本文提出了一种光读出红外成像阵列器件的结构设计,该设计提高了器件的热-机械灵敏度,同时能有效降低周围环境温度变化所带来的影响.理论计算表明,该阵列器件的关键性能指标热-机械灵敏度和噪声等效温差分别为2.39×10-3和2.42,此外,ANSYS模拟仿真的结果验证了该设计能抑制周围环境温度波动对器件红外目标探测的影响.     

Novel method for analyzing the behavior of composite beams with non-smooth interfaces

International Journal of Mechanics and Materials in Design - An almost exact solution is derived for the forced vibration of a composite beam with periodically varying non-smooth interface through...     

Extended focused imaging for digital holograms of macroscopic three-dimensional objects

When a digital hologram is reconstructed, only points located at the reconstruction distance are in focus. We have developed a novel technique for creating an in-focus image of the macroscopic objects encoded in a digital hologram. This extended focused image is created by combining numerical reconstructions with depth information extracted by using our depth-from-focus algorithm. To our knowledge, this is the first technique that creates extended focused images of digital holograms encoding macroscopic objects. We present results for digital holograms containing low- and high-contrast macroscopic objects.     

Extended finite element method for cohesive crack growth

The extended finite element method allows one to model displacement discontinuities which do not conform to interelement surfaces. This method is applied to modeling growth of arbitrary cohesive cracks. The growth of the cohesive zone is governed by requiring the stress intensity factors at the tip of the cohesive zone to vanish. This energetic approach avoids the evaluation of stresses at the mathematical tip of the crack. The effectiveness of the proposed approach is demonstrated by simulations of cohesive crack growth in concrete.