Conventional Ultrasound Imaging Systems Research Articles

The Spectrum-Beamformer for Conventional B-Mode Ultrasound Imaging System: Principle, Validation, and Robustness.

Fast and efficient imaging techniques are important for real-time ultrasound imaging. The delay and sum (DAS) beamformer is the most widely-used strategy in focused ultrasound imaging (FUI) modality. However, calculating the time delays and coherently summing the amplitude response in DAS is computationally expensive and generally require a high-performance processor to realize real-time processing. In this study, an efficient spectrum beamformer, namely full-matrix capture (FMC)-stolt, is proposed in FUI system with a linear phased array. The imaging performance of FMC-stolt was validated with the point-scatter simulation and in vitro point and cyst phantoms, and then compared with that of five beamformers, that is, Multiline acquisition (MLA), retrospective transmit beamforming (RTB) in the FUI modality, as well as DAS, Garcia's frequency-wavenumber (f-k), Lu's f-k in the coherent plane wave compounding imaging (CPWCI) modality, under specific conditions. We show that the imaging performance of FMC-stolt is better than MLA-DAS in non-transmit-focal regions, and comparable with RTB-DAS at all imaging depths. FMC-stolt also shows better discontinuity alleviation than MLA and RTB. In addition, FMC-stolt has similar imaging characteristics (e.g., off-axis resolution, computational cost) as the f-k beamformers. The computational complexity and actual computational time indicate that FMC-stolt is comparable to Garcia's f-k, Lu's f-k, and faster than RTB and CPWCI-DAS if the transmitting numbers are close for FUI and CPWCI. The study demonstrates that the proposed FMC-stolt could achieve good reconstruction speed while preserving high-quality images and thus provide a choice for software beamforming for conventional B-mode ultrasound imaging, especially for hand-held devices with limited performance processors.

Gas-Stabilizing Sub-100 nm Mesoporous Silica Nanoparticles for Ultrasound Theranostics.

Recent studies have demonstrated that gas-stabilizing particles can generate cavitating micron-sized bubbles when exposed to ultrasound, offering excellent application potential, including ultrasound imaging, drug delivery, and tumor ablation. However, the majority of the reported gas-stabilizing particles are relatively large (>200 nm), and smaller particles require high acoustic pressures to promote cavitation. Here, this paper reports the preparation of sub-100 nm gas-stabilizing nanoparticles (GSNs) that can initiate cavitation at low acoustic intensities, which can be delivered using a conventional medical ultrasound imaging system. The highly echogenic GSNs (F127-hMSN) were prepared by carefully engineering the surfaces of ∼50 nm mesoporous silica nanoparticles. It was demonstrated that the F127-hMSNs could be continuously imaged with ultrasound in buffer or biological solutions or agarose phantoms for up to 20 min. Also, the F127-hMSN can be stored in phosphate-buffered saline for at least a month with no loss in ultrasound responsiveness. The particles significantly degraded when diluted in simulated body fluids, indicating possible biodegradation of the F127-hMSNs in vivo. Furthermore, at ultrasound imaging conditions, F127-hMSNs did not cause detectable cell death, supporting the potential safety of these particles. Finally, strong cavitation activity generation by the F127-hMSNs under high-intensity focused ultrasound insonation was demonstrated and applied to effectively ablate cancer cells.

Reliable and accurate needle localization in curvilinear ultrasound images using signature-based analysis of ultrasound beamformed radio frequency signals.

Ultrasound imaging is used in many minimally invasive needle insertion procedures to track the advancing needle, but localizing the needle in ultrasound images can be challenging, particularly at steep insertion angles. Previous methods have been introduced to localize the needle in ultrasound images, but the majority of these methods are based on ultrasound B-mode image analysis that is affected by the needle visibility. To address this limitation, we propose a two-phase, signature-based method to achieve reliable and accurate needle localization in curvilinear ultrasound images based on the beamformed radio frequency (RF) signals that are acquired using conventional ultrasound imaging systems. In the first phase of our proposed method, the beamformed RF signals are divided into overlapping segments and these segments are processed to extract needle-specific features to identify the needle echoes. The features are analyzed using a support vector machine classifier to synthesize a quantitative image that highlights the needle. The quantitative image is processed using the Radon transform to achieve a reliable and accurate signature-based estimation of the needle axis. In the second phase, the accuracy of the needle axis estimation is improved by processing the RF samples located around the signature-based estimation of the needle axis using local phase analysis combined with the Radon transform. Moreover, a probabilistic approach is employed to identify the needle tip. The proposed method is used to localize needles with two different sizes inserted in ex vivo animal tissue specimens at various insertion angles. Our proposed method achieved reliable and accurate needle localization for an extended range of needle insertion angles with failure rates of 0% and mean angle, axis, and tip errors smaller than or equal to , 0.6mm, and 0.7mm, respectively. Moreover, our proposed method outperformed a recently introduced needle localization method that is based on B-mode image analysis. These results suggest the potential of employing our signature-based method to achieve reliable and accurate needle localization during ultrasound-guided needle insertion procedures.

Towards clinical photoacoustic and ultrasound imaging: Probe improvement and real-time graphical user interface.

Photoacoustic imaging is a non-invasive and non-ionizing biomedical technique that has been investigated widely for various clinical applications. By taking the advantages of conventional ultrasound imaging, hand-held operation with a linear array transducer should be favorable for successful clinical translation of photoacoustic imaging. In this paper, we present new key updates contributed to the previously developed real-time clinical photoacoustic and ultrasound imaging system for improving the clinical usability of the system. We developed a seamless image optimization platform, designed a real-time parameter control software with a user-friendly graphical user interface, performed Monte Carlo simulation of the optical fluence in the imaging plane, and optimized the geometry of the imaging probe. The updated system allows optimizing of all imaging parameters while continuously acquiring the photoacoustic and ultrasound images in real-time. The updated system has great potential to be used in a variety of clinical applications such as assessing the malignancy of thyroid cancer, breast cancer, and melanoma. Impact statement Photoacoustic imaging is a promising biomedical imaging modality that can visualize both structural and functional information of biological tissue. Because of its easiness to be integrated with conventional ultrasound imaging systems, numerous studies have been conducted to develop and apply clinical photoacoustic imaging systems. However, most of the systems were not suitable for general-purpose clinical applications due to one of the following reasons: target specific design, immobility, inaccessible operation sequence, and lack of hand-held operation. This study demonstrates a real-time clinical photoacoustic and ultrasound imaging system, which can overcome the limitations of the previous systems for successful clinical translation.

Robust Waveform Design of Ultrasound Arrays for Medical Imaging.

Sound speed is an effective parameter in designing an optimal beamformer. In conventional ultrasound imaging systems, the beamformer is designed assuming a fixed value of speed, whereas the speed in a tissue is not known precisely and also may fluctuate by a great value. The errors in estimating sound speed may lead to a severe degradation in the reconstructed image, as mainlobe width and sidelobe level of the beampattern are sensitive to the speed variations. In this paper, we consider the design of a transmit beamformer, which is robust to the speed variations. The problem is formulated as a convex optimization problem versus the covariance matrix of the excitation waveforms to obtain a beampattern with predefined mainlobe width and a minimum sidelobe level for all possible variations of speed. Then, by eigen-analysis of the obtained covariance matrix, a set of nonidentical single-carrier short-pulses for the excitation waveforms were designed. Various simulations indicate that the proposed method can yield a robust beampattern whose mainlobe width and sidelobe level almost remain constant by 10% speed variations. In contrast, the beampatterns obtained by nonrobust methods suffer extensive changes.

Optical Ultrasound Generation and Detection for Intravascular Imaging: A Review.

Combined ultrasound and photoacoustic imaging has attracted significant interests for intravascular imaging such as atheromatous plaque detection, with ultrasound imaging providing spatial location and morphology and photoacoustic imaging highlighting molecular composition of the plaque. Conventional ultrasound imaging systems utilize piezoelectric ultrasound transducers, which suffer from limited frequency bandwidths and reduced sensitivity with miniature transducer elements. Recent advances on optical methods for both ultrasound generation and detection have shown great promise, as they provide efficient and ultrabroadband ultrasound generation and sensitive and ultrabroadband ultrasound detection. As such, all-optical ultrasound imaging has a great potential to become a next generation ultrasound imaging method. In this paper, we review recent developments on optical ultrasound transmitters, detectors, and all-optical ultrasound imaging systems, with a particular focus on fiber-based probes for intravascular imaging. We further discuss our thoughts on future directions on developing combined all-optical photoacoustic and ultrasound imaging systems for intravascular imaging.

Blinking Phase-Change Nanocapsules Enable Background-Free Ultrasound Imaging.

Microbubbles are widely used as contrast agents to improve the diagnostic capability of conventional, highly speckled, low-contrast ultrasound imaging. However, while microbubbles can be used for molecular imaging, these agents are limited to the vascular space due to their large size (> 1 μm). Smaller microbubbles are desired but their ultrasound visualization is limited due to lower echogenicity or higher resonant frequencies. Here we present nanometer scale, phase changing, blinking nanocapsules (BLInCs), which can be repeatedly optically triggered to provide transient contrast and enable background-free ultrasound imaging. In response to irradiation by near-infrared laser pulses, the BLInCs undergo cycles of rapid vaporization followed by recondensation into their native liquid state at body temperature. High frame rate ultrasound imaging measures the dynamic echogenicity changes associated with these controllable, periodic phase transitions. Using a newly developed image processing algorithm, the blinking particles are distinguished from tissue, providing a background-free image of the BLInCs while the underlying B-mode ultrasound image is used as an anatomical reference of the tissue. We demonstrate the function of BLInCs and the associated imaging technique in a tissue-mimicking phantom and in vivo for the identification of the sentinel lymph node. Our studies indicate that BLInCs may become a powerful tool to identify biological targets using a conventional ultrasound imaging system.

A method to reduce the influence of reflected waves on shear velocity measurements using B-mode scanning time delay

A method to reduce the influence of reflected shear wave (SW) on ultrasound measurement of shear velocity of a soft tissue specimen was proposed in this study. A backward SW reflected from tissue boundaries may interfere with a forward SW induced, resulting in shear velocity measurement error. In the B-mode scan of a conventional ultrasound imaging system there is a scanning time delay among the sequential A-mode measurements, which causes an artificial spatial frequency shift in the forward and backward SWs. The amount of the frequency shift is dependent on the propagation direction of the SWs with respect to the B-mode scanning direction. Therefore, the forward and backward SWs can be separated using a spatial frequency filter. The proposed method was verified using a soft tissue mimicking phantom. In the experimental results, the artificial frequency shifts were clearly observed for induced and reflected SW. Shear velocities of the phantoms were successfully measured from the filtered SW by the proposed method.

METHOD FOR MANUFACTURING AN ULTRASOUND IMAGING TRANSDUCER ASSEMBLY

An ultrasound transducer includes an array of PZT elements mounted on a non-recessed distal surface of a backing block. Between each element and the backing block is a conductive region formed as a portion of a metallic layer sputtered onto the distal surface. Traces on a longitudinally extending circuit board—preferably, a substantially rigid printed circuit board, which may be embedded within the block—connect the conductive region, and thus the PZT element, with any conventional external ultrasound imaging system. A substantially “T” or “inverted-L” shaped electrode is thereby formed for each element, with no need for soldering. At least one longitudinally extending metallic member mounted on a respective lateral surface of the backing block forms a heat sink and a common electrical ground. A thermally and electrically conductive layer, such as of foil, transfers heat from at least one matching layer mounted on the elements to the metallic member.

MICROWAVE-INDUCED THERMO-ACOUSTIC TOMOGRAPHY SYSTEM USING TRM-PSTD TECHNIQUE

Time reversal imaging method based on full wave numerical technique for likely breast tumors biological tissue in the Microwave-Induced Thermo-Acoustic Tomography (MITAT) system is discussed. In this paper, the mechanism of microwave-induced thermo-acoustic is strictly described based on thermodynamics and thermo-difiusion principles; the equivalent relationship between the absorbed microwave energy distribution of the biological tissue and the induced thermo-acoustic source distribution is used as the basis of the imaging algorithm. Due to its unique noise suppression feature and the stability of the algorithm, the Time Reversal Method (TRM) based on the Pseudospectral Time-Domain (PSTD) technique is applied to image heterogeneous phantom tissues from low Signal-to-Noise-Ratio (SNR) thermo-acoustic signals. Thereafter, an integrated MITAT prototype system is presented to obtain the thermo-acoustic signals from some biologic tissue with millimeter scale. The proposed TRM method is based on PSTD technique produced two-dimensional images, presented to study the performances of the MITAT in terms of contrast and resolution. These images prove predominant advantages in both contrast and resolution compared with conventional microwave and ultrasound imaging systems for malignant tumor detection. Based on the current results, our TRM-PSTD MITAT system provides evidence to predict breast tumor in an early stage and millimeter scale.

The Computational Study of Microwave-Induced Thermo-Acoustic Tomography for Biologic Tissue Imaging Based on Pseudo-Spectrum Time Domain and Time Reversal Mirror Technique

Microwave-Induced Thermo-Acoustic Tomography (MITAT) own much concerns in recent years in biomedical imaging field. High contrast and resolution compared with conventional microwave or ultrasound imaging system especially for malignant tumors are outstanding characters of it. In this paper, the induced thermo-acoustic wave propagating in a mimic biologic tissue is simulated by numeric method Pseudo-Spectrum Time Domain (PSTD). Due to the excellent performance in noise-depress and the stability for the fluctuation of the model parameters, Time Reversal Mirror (TRM) imaging technique is studied computationally for the simulative received thermo-acoustic signals. Some thermo-acoustic objects with different initial pressure distribution are designed and imaged by TRM technique to represent the complex biologic tissue case in a random media. The quality of images generated by TRM technique based on PSTD method hints the potential of the MITAT technique.

The effects of transducer geometry on artifacts common to diagnostic bone imaging with conventional medical ultrasound

The portability, low cost, and non-ionizing radiation associated with medical ultrasound suggest that it has potential as a superior alternative to X-ray for bone imaging. However, when conventional ultrasound imaging systems are used for bone imaging, clinical acceptance is frequently limited by artifacts derived from reflections occurring away from the main axis of the acoustic beam. In this paper, the physical source of off-axis artifacts and the effect of transducer geometry on these artifacts are investigated in simulation and experimental studies. In agreement with diffraction theory, the sampled linear-array geometry possessed increased off-axis energy compared with single-element piston geometry, and therefore, exhibited greater levels of artifact signal. Simulation and experimental results demonstrated that the linear-array geometry exhibited increased artifact signal when the center frequency increased, when energy off-axis to the main acoustic beam (i.e., grating lobes) was perpendicularly incident upon off-axis surfaces, and when off-axis surfaces were specular rather than diffusive. The simulation model used to simulate specular reflections was validated experimentally and a correlation coefficient of 0.97 between experimental and simulated peak reflection contrast was observed. In ex vivo experiments, the piston geometry yielded 4 and 6.2 dB average contrast improvement compared with the linear array when imaging the spinous process and interlaminar space of an animal spine, respectively. This work indicates that off-axis reflections are a major source of ultrasound image artifacts, particularly in environments comprising specular reflecting (i.e., bone or bone-like) objects. Transducer geometries with reduced sensitivity to off-axis surface reflections, such as a piston transducer geometry, yield significant reductions in image artifact.

Interference-free ultrasound imaging during hifu therapy, using software tools

Disclosed herein is a method for obtaining a composite interference-free ultrasound image when non-imaging ultrasound waves would otherwise interfere with ultrasound imaging. A conventional ultrasound imaging system is used to collect frames of ultrasound image data in the presence of non-imaging ultrasound waves, such as high-intensity focused ultrasound (HIFU). The frames are directed to a processor that analyzes the frames to identify portions of the frame that are interference-free. Interference-free portions of a plurality of different ultrasound image frames are combined to generate a single composite interference-free ultrasound image that is displayed to a user. In this approach, a frequency of the non-imaging ultrasound waves is offset relative to a frequency of the ultrasound imaging waves, such that the interference introduced by the non-imaging ultrasound waves appears in a different portion of the frames.

Ultrasound Tomography-Guide TRM Technique for Breast Tumor Detecting in MITAT System

Microwave-Induced Thermo-Acoustic Tomography (MITAT) is a promising technique in biomedical imaging field. It has predominant advantages in both contrast and resolution compared with conventional microwave or ultrasound imaging system for malignant tumors. In this paper, Time Reversal Mirror (TRM) imaging technique based on numerical method Pseudo-Spectrum Time Domain (PSTD) for MITAT imaging is studied computationally. To obtain the precise position of the tumors, the ultrasound tomography-guide TRM routine is presented to decrease the effects from the indetermination of the ultrasound speed distribution parameters. The validity of this method is testified by some numerical simulations.

Development of microwave-induced thermo-acoustic tomography prototype system

Due to its advantages in both contrast and resolution compared with conventional microwave or ultrasound imaging system, microwave-induced thermo-acoustic tomography (MITAT) has received more and more concerns in biologic tissue image field. In this paper, an integrated prototype of MITAT system is presented. Using this system, some basic experiments for biologic tissue objects with high water-content imbedded in fatty material have been done. In the MITAT experiments, the signals generated by two porcine muscle tissue strips with millimeter order scale in cross-section were collected. Images with both good contrast and resolution were obtained. In order to demonstrate the advantages of MITAT, some ultrasonic experiments were studied using a commercial ultrasonic linear array system. The comparisons of the results of both systems show the good performance of the MITAT prototype system for the detection of high water content targets, which is generally the same as the tumor in biologic tissue.

Computational Study of Time Reversal Mirror Technique for Microwave-Induced Thermo-Acoustic Tomography

Microwave-Induced Thermo-Acoustic Tomography (MITAT) is a promising technique in biomedical imaging. It has predominant advantages in both contrast and resolution compared with conventional microwave or ultrasound imaging system for malignant tumors. In this paper, Time Reversal Mirror (TRM) imaging technique based on numeric method Pseudo-Spectrum Time Domain (PSTD) for MITAT imaging is studied computationally. The proportion relationship between the deposited microwave energy and the induced thermo-acoustic initial source is deduced by analytic method. Due to the globe higher order character and the efficiency, the PSTD method is employed to implement the forward core of the TRM and to model the complex biologic tissue. In order to illustrate the performance of the TRM technique applied in MITAT, some thermo-acoustic objects with different initial pressure distributions are simulated, then designed and imaged by this technique to represent the complex biologic tissue case in a random media. Under the consideration of noise and the velocity parameter change of the model, the contrast and resolution of the images generated by using TRM technique based on PSTD method are also studied.

The Prototype of Microwave-Induced Thermo-Acoustic Tomography Imaging by Time Reversal Mirror

Microwave-Induced Thermo-Acoustic Tomography (MITAT) is a promising technique in biologic tissue imaging. It has predominant advantages in both contrast and resolution compared with conventional microwave or ultrasound imaging system for malignant tumors. In this paper, an integrated prototype MITAT system is briefly introduced. And Time Reversal Mirror (TRM) imaging technique based on Pseudo-Spectrum Time Domain (PSTD) is also first time applied to MITAT system. Two tissue strips of porcine muscle are used as targets in the MITAT experiment to demonstrate the imaging potential of the system. TRM and Filtered Back-Projection (FBP) imaging results are both presented to compare the performance of the TRM and other imaging techniques in contrast and resolution for MITAT.

Ultrasonic Axial Strain Measurement for Lateral Tissue Deformation

An axial strain measurement technique and a one-dimensional (1D) shear modulus reconstruction technique were developed previously by using the multidimensional radio-frequency (RF) echo phase matching method and an axial strain ratio, respectively. In this study, these techniques have been applied using a conventional ultrasound (US) imaging system with the constraint that the tissue deforms predominantly in the lateral direction, orthogonal to the ultrasound beam axis. Conventionally, axial strain measurement and axial strain ratio are used when the tissue deforms predominantly in the axial direction by extracorporeally applied pressure or vibration. In any case, an axial strain can be accurately measured using the multidimensional RF-echo phase matching method; such a measurement will be useful and will enable a simple reconstruction in a lateral deformation case. The usefulness of these techniques is demonstrated by agar phantom experiments; several problems related to their use in cases such as accuracy, bias error and detectability of inhomogeneity (contrast-to-noise ratio) are addressed. These techniques will be effective for deep regions-of-interest (ROIs) such as liver tissues, which are inaccessible from the body surface and normally deformed by heart motion or pulsation. Moreover, such techniques will enable the evaluation of the elasticity of various tissues under normal motion such as the arm and leg muscles during exercise. (E-mail: c-sumi@sophia.ac.jp)

Sonic activation of molecularly-targeted nanoparticles accelerates transmembrane lipid delivery to cancer cells through contact-mediated mechanisms: Implications for enhanced local drug delivery

Liquid perfluorocarbon nanoparticles serve as sensitive and specific targeted contrast and drug delivery vehicles by binding to specific cell surface markers. We hypothesized that application of acoustic energy at diagnostic power levels could promote nanoparticle-associated drug delivery by stimulating increased interaction between the nanoparticle's lipid layer and the targeted cell's plasma membrane. Ultrasound (mechanical index = 1.9) applied with a conventional ultrasound imaging system to nanoparticles targeted to alpha(v)beta3-integrins on C32 melanoma cancer cells in vitro produced no untoward effects. Within 5 min, lipid delivery from nanoparticles into cell cytoplasm was dramatically augmented. We also demonstrate the operation of a potential physical mechanism for this effect, the acoustic radiation force on the nanoparticles, which may contribute to the enhanced lipid delivery. Accordingly, we propose that local delivery of lipophilic substances (e.g., drugs) from targeted nanoparticles directly into cell cytoplasm can be augmented rapidly and safely with conventional ultrasound imaging devices through nondestructive mechanisms.

Frequency decomposition and compounding of ultrasound medical images with wavelet packets.

Ultrasound beams propagating in biological tissues undergo distortions due to local inhomogeneities of the acoustic parameters and the nonlinearity of the medium. The spectral analysis of the radio-frequency (RF) backscattered signals may yield important clinical information in the field of tissue characterization, as well as enhancing the detectability of tissue parenchymal diseases. In this paper, we propose a new tissue spectral imaging technique based on the wavelet packets (WP) decomposition. In a conventional ultrasound imaging system, the received echo-signals are generally decimated to generate a medical image, with a loss of information. With the proposed approach, all the RF data are processed to generate a set of frequency subband images. The ultrasound echo signals are simultaneously frequency decomposed and decimated, by using two quadrature mirror filters, followed by a dyadic subsampling. In addition, to enhance the lesion detectability and the image quality, we apply a nonlinear filter to reduce noise in each subband image. The proposed method requires simple additional signal processing and it can be implemented on any real-time imaging system. The frequency subband images, which are available simultaneously, can be either used in a multispectral display or summed up together to reduce speckle noise. To localize the different frequency response in the tissues, we propose a multifrequency display method where three different subband images, chosen among those available, are encoded as red, green, and blue intensities (RGB) to create a false-colored RGB image. According to the clinical application, different choices can evidence different spectral proprieties in the biological tissue under investigation. To enhance the lesion contrast in a grey-level image, one of the possible methods is the summation of the images obtained from narrow frequency subbands, according to the frequency compounding technique. We show that by adding the denoised subband images created with the WP decomposition, the contrast-to-noise ratio in two phantom images is largely increased.