Biomedical ultrasound beam forming

Frequency-Dependent F-Number Suppresses Grating Lobes and Improves the Lateral Resolution in Coherent Plane-Wave Compounding

Ultrafast imaging modes, such as coherent plane-wave compounding (CPWC), increase image uniformity and reduce grating lobe artifacts by dynamic receive apertures. The focal length and the desired aperture width maintain a given ratio, which is called the F -number. Fixed F -numbers, however, exclude useful low-frequency components from the focusing and reduce the lateral resolution. Herein, this reduction is avoided by a frequency-dependent F -number. This F -number derives from the far-field directivity pattern of a focused aperture and can be expressed in closed form. The F -number, at low frequencies, widens the aperture to improve the lateral resolution. The F -number, at high frequencies, narrows the aperture to avoid lobe overlaps and suppress grating lobes. Phantom and in vivo experiments with a Fourier-domain beamforming algorithm validated the proposed F -number in CPWC. The lateral resolution, which was measured by the median lateral full-widths at half-maximum of wires, improved by up to 46.8% and 14.9% in a wire and a tissue phantom, respectively, in comparison to fixed F -numbers. Grating lobe artifacts, which were measured by the median peak signal-to-noise ratios of wires, reduced by up to 9.9 dB in comparison to the full aperture. The proposed F -number thus outperformed F -numbers that were recently derived from the directivity of the array elements.

A robust cascaded deep neural network for image reconstruction of single plane wave ultrasound RF data

Reconstruction of ultrasound data from single plane wave Radio Frequency (RF) data is a challenging task. The traditional Delay and Sum (DAS) method produces an image with low resolution and contrast, if employed with RF data from only a single plane wave. A Coherent Compounding (CC) method that reconstructs the image by coherently summing the individual DAS images was proposed to enhance the image quality. However, CC relies on a large number of plane waves to accurately sum the individual DAS images, hence it produces high quality images but with low frame rate that may not be suitable for time-demanding applications. Therefore, there is a need for a method that can create a high quality image with higher frame rates. Furthermore, the method needs to be robust against the input transmission angle of the plane wave. To reduce the method's dependence on the input angle, we propose to unify the RF data at different angles by learning a linear data transformation from different angled data to a common, 0° data. We further propose a cascade of two independent neural networks to reconstruct an image, similar in quality to CC, by making use of a single plane wave. The first network, denoted as "PixelNet", is a fully Convolutional Neural Network (CNN) which takes in the transformed time-delayed RF data as input. PixelNet learns optimal pixel weights that get element-wise multiplied with the single angle DAS image. The second network is a conditional Generative Adversarial Network (cGAN) which is used to further enhance the image quality. Our networks were trained on the publicly available PICMUS and CPWC datasets and evaluated on a completely separate, CUBDL dataset obtained from different acquisition settings than the training dataset. The results thus obtained on the testing dataset, demonstrate the networks' ability to generalize well on unseen data, with frame rates better than the CC method. This paves the way for applications that require high-quality images reconstructed at higher frame rates.

Ultrasound Imaging with Flexible Array Transducer for Pancreatic Cancer Radiation Therapy

Pancreatic cancer with less than 10% 3-year survival rate is one of deadliest cancer types and greatly benefits from enhanced radiotherapy. Organ motion monitoring helps spare the normal tissue from high radiation and, in turn, enables the dose escalation to the target that has been shown to improve the effectiveness of RT by doubling and tripling post-RT survival rate. The flexible array transducer is a novel and promising solution to address the limitation of conventional US probes. We proposed a novel shape estimation for flexible array transducer using two sequential algorithms: (i) an optical tracking-based system that uses the optical markers coordinates attached to the probe at specific positions to estimate the array shape in real-time and (ii) a fully automatic shape optimization algorithm that automatically searches for the optimal array shape that results in the highest quality reconstructed image. We conducted phantom and in vivo experiments to evaluate the estimated array shapes and the accuracy of reconstructed US images. The proposed method reconstructed US images with low full-width-at-half-maximum (FWHM) of the point scatters, correct aspect ratio of the cyst, and high-matching score with the ground truth. Our results demonstrated that the proposed methods reconstruct high-quality ultrasound images with significantly less defocusing and distortion compared with those without any correction. Specifically, the automatic optimization method reduced the array shape estimation error to less than half-wavelength of transmitted wave, resulting in a high-quality reconstructed image.

Adaptive enhancement of acoustic resolution photoacoustic microscopy imaging via deep CNN prior

Acoustic resolution photoacoustic microscopy (AR-PAM) is a promising medical imaging modality that can be employed for deep bio-tissue imaging. However, its relatively low imaging resolution has greatly hindered its wide applications. Previous model-based or learning-based PAM enhancement algorithms either require design of complex handcrafted prior to achieve good performance or lack the interpretability and flexibility that can adapt to different degradation models. However, the degradation model of AR-PAM imaging is subject to both imaging depth and center frequency of ultrasound transducer, which varies in different imaging conditions and cannot be handled by a single neural network model. To address this limitation, an algorithm integrating both learning-based and model-based method is proposed here so that a single framework can deal with various distortion functions adaptively. The vasculature image statistics is implicitly learned by a deep convolutional neural network, which served as plug and play (PnP) prior. The trained network can be directly plugged into the model-based optimization framework for iterative AR-PAM image enhancement, which fitted for different degradation mechanisms. Based on physical model, the point spread function (PSF) kernels for various AR-PAM imaging situations are derived and used for the enhancement of simulation and in vivo AR-PAM images, which collectively proved the effectiveness of proposed method. Quantitatively, the PSNR and SSIM values have all achieve best performance with the proposed algorithm in all three simulation scenarios; The SNR and CNR values have also significantly raised from 6.34 and 5.79 to 35.37 and 29.66 respectively in an in vivo testing result with the proposed algorithm.

Lung Ultrasound Artifacts Interpreted as Pathology Footprints

Background: The original observation that lung ultrasound provides information regarding the physical state of the organ, rather than the anatomical details related to the disease, has reinforced the idea that the observed acoustic signs represent artifacts. However, the definition of artifact does not appear adequate since pulmonary ultrasound signs have shown valuable diagnostic accuracy, which has been usefully exploited by physicians in numerous pathologies. Method: A specific method has been used over the years to analyze lung ultrasound data and to convert artefactual information into anatomical information. Results: A physical explanation of the genesis of the acoustic signs is provided, and the relationship between their visual characteristics and the surface histopathology of the lung is illustrated. Two important sources of potential signal alteration are also highlighted. Conclusions: The acoustic signs are generated by acoustic traps that progressively release previously trapped energy. Consequently, the acoustic signs highlight the presence of acoustic traps and quantitatively describe their distribution on the lung surface; they are not artifacts, but pathology footprints and anatomical information. Moreover, the impact of the dynamic focusing algorithms and the impact of different probes on the visual aspect of the acoustic signs should not be neglected.

High frame-rate contrast enhanced ultrasound (HIFR-CEUS) in the characterization of small hepatic lesions in cirrhotic patients

BACKGROUND

To show the effectiveness of plane wave HighFrame-Rate CEUS (HiFR-CEUS) compared with "conventional" (plane wave) CEUS (C-CEUS) in the characterization of small (< 2 cm) focal liver lesions (FLLs) not easily detected by CT in cirrhotic patients. HiFR-CEUS exploit an ultra-wideband nonlinear process to combine fundamental, second and higher-order harmonic signals generated by ultrasound contrast agents to increase the frame rate. C-CEUS is limited by the transmission principle, and its frame-rate is around 10 FPS. With HiFR-CEUS (Shenzhen Mindray Bio-Medical Electronics Co., China), the frame-rate reached 60 FPS.

MATERIAL AND METHODS

Ultrasound detected small FLLs (< 2 cm) in 63 cirrhotic patients during follow-up (June 2019-February 2020); (7 nodules < 1 cm and were not evaluable by spiral CT). Final diagnosis was obtained with MRI (47) or fine needle aspiration (16 cases) C-CEUS was performed and HiFR-CEUS was repeated after 5 min; 0.8-1.2 ml of contrast media (SonoVue, Bracco, Italy) was used. 57 nodules were better evaluable with HiFR-CEUS; 6 nodules were equally evaluable by both techniques; final diagnosis was: 44 benign lesions (29 hemangiomas, 1 amartoma, 2 hepatic cysts; 2 focal nodular hyperplasias, 3 regenerative macronodules, 3 AV-shunts, 3 hepatic sparing areas and 1 focal steatosis) and 19 malignant one (17 HCCs, 1 cholangioca, 1 metastasis); statistical evaluation for better diagnosis with X2 test (SPSS vers. 26); we used LI-RADS classification for evaluating sensitivity, specificity PPV, NPV and diagnostic accuracy of C- and HFR-CEUS. Corrispective AU-ROC were calculated.

RESULTS

C-CEUS and HiFR-CEUS reached the same diagnosis in 29 nodules (13 nodules > 1 < 1.5 cm; 16 nodules > 1.5 < 2 cm); HiFR-CEUS reached a correct diagnosis in 32 nodules where C-CEUS was not diagnostic (6 nodules < 1 cm; 17 nodules > 1 < 1.5 cm; 9 nodules > 1.5 < 2 cm); C-CEUS was better in 2 nodules (1 < 1 cm and 1 > 1 < 1.5 cm). Some patient's (sex, BMI, age) and nodule's characteristics (liver segment, type of diagnosis, nodule's dimensions (p = 0.65)) were not correlated with better diagnosis (p ns); only better visualization (p 0.004) was correlated; C-CEUS obtained the following LI-RADS: type-1: 18 Nodules, type-2: 21; type-3: 7, type-4: 7; type-5: 8; type-M: 2; HiFR-CEUS: type-1: 38 Nodules, type-2: 2; type-3:4, type-4: 2; type-5: 15; type-M: 2; In comparison with final diagnosis: C-CEUS: TP: 17; TN: 39; FP: 5; FN:2; HIFR-CEUS: TP: 18; TN: 41; FP: 3; FN:1; C-CEUS: sens: 89.5%; Spec: 88.6%, PPV: 77.3%; NPV: 95.1%; Diagn Acc: 88.6% (AU-ROC: 0.994 ± SEAUC: 0.127; CI: 0.969-1.019); HiHFR CEUS: sens: 94.7%; Spec: 93.2%, PPV: 85.7%; NPV: 97.6%; Diagn Acc: 93.2% (AU-ROC: 0.9958 ± SEAUC: 0.106; CI: 0.975-1.017) FLL vascularization in the arterial phase was more visible with HiFR-CEUS than with C-CEUS, capturing the perfusion details in the arterial phase due to a better temporal resolution. With a better temporal resolution, the late phase could be evaluated longer with HiFR-CEUS (4 min C-CEUS vs. 5 min HiFR-CEUS).

CONCLUSION

Both C-CEUS and HIFR-CEUS are good non invasive imaging system for the characterization of small lesions detected during follow up of cirrhotic patients. HiFR-CEUS allowed better FLL characterization in cirrhotic patients with better temporal and spatial resolution capturing the perfusion details that cannot be easily observed with C-CEUS.

Deep and Domain Transfer Learning Aided Photoacoustic Microscopy: Acoustic Resolution to Optical Resolution

Acoustic resolution photoacoustic micros- copy (AR-PAM) can achieve deeper imaging depth in biological tissue, with the sacrifice of imaging resolution compared with optical resolution photoacoustic microscopy (OR-PAM). Here we aim to enhance the AR-PAM image quality towards OR-PAM image, which specifically includes the enhancement of imaging resolution, restoration of micro-vasculatures, and reduction of artifacts. To address this issue, a network (MultiResU-Net) is first trained as generative model with simulated AR-OR image pairs, which are synthesized with physical transducer model. Moderate enhancement results can already be obtained when applying this model to in vivo AR imaging data. Nevertheless, the perceptual quality is unsatisfactory due to domain shift. Further, domain transfer learning technique under generative adversarial network (GAN) framework is proposed to drive the enhanced image's manifold towards that of real OR image. In this way, perceptually convincing AR to OR enhancement result is obtained, which can also be supported by quantitative analysis. Peak Signal to Noise Ratio (PSNR) and Structural Similarity Index (SSIM) values are significantly increased from 14.74 dB to 19.01 dB and from 0.1974 to 0.2937, respectively, validating the improvement of reconstruction correctness and overall perceptual quality. The proposed algorithm has also been validated across different imaging depths with experiments conducted in both shallow and deep tissue. The above AR to OR domain transfer learning with GAN (AODTL-GAN) framework has enabled the enhancement target with limited amount of matched in vivo AR-OR imaging data.

Optimized Simultaneous Axial Multifocal Imaging via Frequency Multiplexed Focusing

Simultaneous axial multifocal imaging (SAMI) using a single acoustical transmission was developed to enhance the depth of field. This technique transmits a superposition of axial multifoci waveforms in a single transmission, thus increasing the frame rate. However, since all the waveforms are transmitted at a constant center frequency, there is a tradeoff between attenuation and lateral resolution when choosing a constant frequency for all the axial depths. In this work, we developed an optimized SAMI method by adding frequency dependence to each axial multifocus. By gradually increasing the frequency as a function of the focal depth, this method makes it possible to compensate for the gradually increasing F-number in order to achieve constant lateral resolution across the entire field of view. Alternatively, by gradually decreasing the axial multifoci frequencies as a function of depth, enhanced penetration depth and contrast are obtained. This method, termed frequency multiplexed SAMI (FM-SAMI), is described analytically and validated by resolution and contrast experiments performed on resolution targets, tissue-mimicking phantoms, and ex vivo biological samples. This is the first real-time implementation of a frequency multiplexing approach for axial multifoci imaging that facilitates high-quality imaging at an increased frame rate.

A deep learning method to predict ankle joint moment during walking at different speeds with ultrasound imaging: A framework for assistive devices control

Robotic assistive or rehabilitative devices are promising aids for people with neurological disorders as they help regain normative functions for both upper and lower limbs. However, it remains challenging to accurately estimate human intent or residual efforts non-invasively when using these robotic devices. In this article, we propose a deep learning approach that uses a brightness mode, that is, B-mode, of ultrasound (US) imaging from skeletal muscles to predict the ankle joint net plantarflexion moment while walking. The designed structure of customized deep convolutional neural networks (CNNs) guarantees the convergence and robustness of the deep learning approach. We investigated the influence of the US imaging's region of interest (ROI) on the net plantarflexion moment prediction performance. We also compared the CNN-based moment prediction performance utilizing B-mode US and sEMG spectrum imaging with the same ROI size. Experimental results from eight young participants walking on a treadmill at multiple speeds verified an improved accuracy by using the proposed US imaging + deep learning approach for net joint moment prediction. With the same CNN structure, compared to the prediction performance by using sEMG spectrum imaging, US imaging significantly reduced the normalized prediction root mean square error by 37.55% ( < .001) and increased the prediction coefficient of determination by 20.13% ( < .001). The findings show that the US imaging + deep learning approach personalizes the assessment of human joint voluntary effort, which can be incorporated with assistive or rehabilitative devices to improve clinical performance based on the assist-as-needed control strategy.

Spatiotemporal Deconvolution of Hydrophone Response for Linear and Nonlinear Beams-Part I: Theory, Spatial-Averaging Correction Formulas, and Criteria for Sensitive Element Size

This article reports spatiotemporal deconvolution methods and simple empirical formulas to correct pressure and beamwidth measurements for spatial averaging across a hydrophone sensitive element. Readers who are uninterested in hydrophone theory may proceed directly to Appendix A for an easy method to estimate spatial-averaging correction factors. Hydrophones were modeled as angular spectrum filters. Simulations modeled nine circular transducers (1-10 MHz; F/1.4-F/3.2) driven at six power levels and measured with eight hydrophones (432 beam/hydrophone combinations). For example, the model predicts that if a 200- [Formula: see text] membrane hydrophone measures a moderately nonlinear 5-MHz beam from an F/1 transducer, spatial-averaging correction factors are 33% (peak compressional pressure or p_c ), 18% (peak rarefactional pressure or p ), and 18% (full width half maximum or FWHM). Theoretical and experimental estimates of spatial-averaging correction factors to were in good agreement (within 5%) for linear and moderately nonlinear signals. Criteria for maximum appropriate hydrophone sensitive element size as functions of experimental parameters were derived. Unlike the oft-cited International Electrotechnical Commission (IEC) criterion, the new criteria were derived for focusing rather than planar transducers and can accommodate nonlinear signals in addition to linear signals. Responsible reporting of hydrophone-based pressure and beamwidth measurements should always acknowledge spatial-averaging considerations.

Spatiotemporal Deconvolution of Hydrophone Response for Linear and Nonlinear Beams-Part II: Experimental Validation

This article reports experimental validation for spatiotemporal deconvolution methods and simple empirical formulas to correct pressure and beamwidth measurements for spatial averaging across a hydrophone sensitive element. The method was validated using linear and nonlinear beams transmitted by seven single-element spherically focusing transducers (2-10 MHz; F /#: 1-3) and measured with five hydrophones (sensitive element diameters d_g : 85-1000 [Formula: see text]), resulting in 35 transducer/hydrophone combinations. Exponential functions, exp( -αx ), where x = d_g /( λ₁F /#) and λ₁ is the fundamental wavelength, were used to model focal pressure ratios p'/p (where p' is the measured value subjected to spatial averaging and p is the true axial value that would be obtained with a hypothetical point hydrophone). Spatiotemporal deconvolution reduced α (followed by root mean squared difference between data and fit) from 0.29-0.30 (7%) to 0.01 (8%) (linear signals) and from 0.29-0.40 (8%) to 0.04 (14%) (nonlinear signals), indicating successful spatial averaging correction. Linear functions, Cx + 1, were used to model FWHM'/FWHM, where FWHM is full-width half-maximum. Spatiotemporal deconvolution reduced C from 9% (4%) to -0.6% (1%) (linear signals) and from 30% (10%) to 6% (5%) (nonlinear signals), indicating successful spatial averaging correction. Spatiotemporal deconvolution resulted in significant improvement in accuracy even when the hydrophone geometrical sensitive element diameter exceeded the beam FWHM. Responsible reporting of hydrophone-based pressure measurements should always acknowledge spatial averaging considerations.

Imaging the Local Nonlinear Viscoelastic Properties of Soft Tissues: Initial Validation and Expected Benefits

Imaging tissue mechanical properties has shown promise in noninvasive assessment of numerous pathologies. Researchers have successfully measured many linear tissue mechanical properties in laboratory and clinical settings. Currently, multiple complex mechanical effects such as frequency-dependence, anisotropy, and nonlinearity are being investigated separately. However, a concurrent assessment of these complex effects may enable more complete characterization of tissue biomechanics and offer improved diagnostic sensitivity. In this work, we report for the first time a method to map the frequency-dependent nonlinear parameters of soft tissues on a local scale. We recently developed a nonlinear elastography model that combines strain measurements from arbitrary tissue compression with radiation-force-based broadband shear wave speed (WS) measurements. Here, we extended this model to incorporate local measurements of frequency-dependent shear modulus. This combined approach provides a local frequency-dependent nonlinear parameter that can be obtained with arbitrary, clinically realizable tissue compression. Initial assessments using simulations and phantoms validate the accuracy of this approach. We also observed improved contrast in nonlinearity parameter at higher frequencies. Results from ex-vivo liver experiments show 32, 25, 34, and 38 dB higher contrast in elastograms than traditional linear elasticity, elastic nonlinearity, viscosity, and strain imaging methods, respectively. A lesion, artificially created by injection of glutaraldehyde into a liver specimen, showed a 59% increase in the frequency-dependent nonlinear parameter and a 17% increase in contrast ratio.

Quasi-Bessel Acoustic-Vortex Beams Constructed by the Line-Focused Phase Modulation for a Ring Array of Sectorial Planar Transducers

Acoustic Bessel beams are commonly used as ideal sources to study the characteristics for acoustic-vortex (AV) beams, exhibiting prosperous perspectives in contactless object manipulations and acoustic communications. However, accurate Bessel beams are difficult to construct using 2-D arrays in practical applications. By integrating active phase control and passive phase modulation to a ring array of sectorial planar transducers, quasi-Bessel AV (QB-AV) beams of arbitrary order are built by the line focus of AV fields in the current study. Based on Snell's refraction law, a circular sawtooth lens of phase modulation is designed to converge incident waves toward the beam axis at the same deflection angle. QB-AV beams constructed by the main lobes of the sectorial sources are demonstrated by theoretical derivations, numerical simulations, and quality evaluations, while those created by the sidelobes are neglected to avoid the pressure fluctuations in the near field. Experimental measurements for AV beams of different orders coincide basically with the simulations, demonstrating that line-focused QB-AV beams can be generated along the beam axis across the pressure peak. With the increase of the topological charge, the peak pressure of the beam decreases accordingly with a reduced effective axial range. The favorable results prove that, as a special kind of diffraction sources, the adjustable QB-AV beams may enable more important biomedical applications where Bessel beams are necessary, especially for the line-focused manipulation of biological and drug particles.

Shear Wave Elasticity Imaging Using Nondiffractive Bessel Apodized Acoustic Radiation Force

The acoustic radiation force impulse (ARFI) has been widely used in transient shear wave elasticity imaging (SWEI). For SWEI based on focused ARFI, the highest image quality exists inside the focal zone due to the limitation of the depth of focus and diffraction. Consequently, the areas outside the focal zone and in the near field present poor image quality. To address the limitations of the focused beam, we introduce Bessel apodized ARFI that enhances image quality and improves the depth of focus. The objective of this study is to evaluate the feasibility of SWEI based on Bessel ARF in simulation and experiment. We report measurements of elastogram image quality and depth of field in tissue-mimicking phantoms and ex vivo liver tissue. Our results demonstrate improved depth of field, image quality, and shear wave speed (SWS) estimation accuracy using Bessel push beams. As a result, Bessel ARF enlarges the field of view of elastograms. The signal-to-noise ratio (SNR) of Bessel SWEI is improved 26% compared with focused SWEI in homogeneous phantom. The estimated SWS by Bessel SWEI is closer to the measured SWS from a clinical scanner with an error of 0.3% compared to 2.4% with a focused beam. In heterogeneous phantoms, the contrast-to-noise ratios (CNRs) of shallow and deep inclusions are improved by 8.79 and 3.33 dB, respectively, under Bessel ARF. We also compare the results between Bessel SWEI and supersonic shear imaging (SSI), and the SNR of Bessel SWEI is improved by 8.1%. Compared with SSI, Bessel SWEI shows more accurate SWS estimates in high stiffness inclusions. Finally, Bessel SWEI can generate higher quality elastograms with less energy than conventional SSI.

Spiral sound-diffusing metasurfaces based on holographic vortices

In this work, we show that scattered acoustic vortices generated by metasurfaces with chiral symmetry present broadband unusual properties in the far-field. These metasurfaces are designed to encode the holographic field of an acoustical vortex, resulting in structures with spiral geometry. In the near field, phase dislocations with tuned topological charge emerge when the scattered waves interference destructively along the axis of the spiral metasurface. In the far field, metasurfaces based on holographic vortices inhibit specular reflections because all scattered waves also interfere destructively in the normal direction. In addition, the scattering function in the far field is unusually uniform because the reflected waves diverge spherically from the holographic focal point. In this way, by triggering vorticity, energy can be evenly reflected in all directions except to the normal. As a consequence, the designed metasurface presents a mean correlation-scattering coefficient of 0.99 (0.98 in experiments) and a mean normalized diffusion coefficient of 0.73 (0.76 in experiments) over a 4 octave frequency band. The singular features of the resulting metasurfaces with chiral geometry allow the simultaneous generation of broadband, diffuse and non-specular scattering. These three exceptional features make spiral metasurfaces extraordinary candidates for controlling acoustic scattering and generating diffuse sound reflections in several applications and branches of wave physics as underwater acoustics, biomedical ultrasound, particle manipulation devices or room acoustics.

Synergistic agents for tumor-specific therapy mediated by focused ultrasound treatment

This minireview highlights the recent advances in the therapeutic agents that aim to provide synergistic enhancements of focused ultrasound treatment of tumors. Even though focused ultrasound therapy itself can bring therapeutic effects in cancers, many biochemical agents have been reported in the literature to enhance the treatment efficacy significantly. Until now, many mechanisms have been researched to advance the therapy, such as sonodynamic-plus-chemo-therapy, microbubble-aided therapy, localized release or delivery of nanomaterials, and multimodal image-guided therapy. Here, the novel materials adopted in each mechanism are briefly reviewed to provide a trend in the field and encourage future research towards the successful deployment of focused ultrasound therapy in real clinical environments.

Sub-wavelength lateral detection of tissue-approximating masses using an ultrasonic metamaterial lens

Practically applied techniques for ultrasonic biomedical imaging employ delay-and-sum (DAS) beamforming which can resolve two objects down to 2.1λ within the acoustic Fresnel zone. Here, we demonstrate a phononic metamaterial lens (ML) for detection of laterally subwavelength object features in tissue-like phantoms beyond the phononic crystal evanescent zone and Fresnel zone of the emitter. The ML produces metamaterial collimation that spreads 8x less than the emitting transducer. Utilizing collimation, 3.6x greater lateral resolution beyond the Fresnel zone limit was achieved. Both hard objects and tissue approximating masses were examined in gelatin tissue phantoms near the Fresnel zone limit. Lateral dimensions and separation were resolved down to 0.50λ for hard objects, with tissue approximating masses slightly higher at 0.73λ. The work represents the application of a metamaterial for spatial characterization, and subwavelength resolution in a biosystem beyond the Fresnel zone limit.

Traditional methods for ultrasound detection in biomedical application suffer from limited lateral resolution. Here, the authors show that a phononic metamaterial lens can be used for spatial characterisation of subwavelength objects, even beyond the Fresnel zone of the emitting transducer.

A Review of Acoustic Impedance Matching Techniques for Piezoelectric Sensors and Transducers

The coupling of waves between the piezoelectric generators, detectors, and propagating media is challenging due to mismatch in the acoustic properties. The mismatch leads to the reverberation of waves within the transducer, heating, low signal-to-noise ratio, and signal distortion. Acoustic impedance matching increases the coupling largely. This article presents standard methods to match the acoustic impedance of the piezoelectric sensors, actuators, and transducers with the surrounding wave propagation media. Acoustic matching methods utilizing active and passive materials have been discussed. Special materials such as nanocomposites, metamaterials, and metasurfaces as emerging materials have been presented. Emphasis is placed throughout the article to differentiate the difference between electric and acoustic impedance matching and the relation between the two. Comparison of various techniques is made with the discussion on capabilities, advantages, and disadvantages. Acoustic impedance matching for specific and uncommon applications has also been covered.

The convolutional interpretation of registration-based plane wave steered pulse-echo local sound speed estimators

Pulse-echo reconstruction of sound speed has long been considered a difficult problem within the domain of quantitative biomedical ultrasound. However, recent results (Jaeger 2015 Ultrasound Med. Biol. 41 235-50; Jaeger and Frenz 2015 Ultrasonics 62 299-304; Jaeger et al 2015 Phys. Med. Biol. 60 4497-515) have demonstrated that pulse-echo reconstructions of sound speed are achievable by exploiting correlations in post-beamformed data from steered, plane-wave excitations in the presence of diffuse scatterers. Despite these recent advances, a coherent theoretical imaging framework for describing the approach and results is lacking in the literature. In this work, the problem of sound speed reconstruction using steered, plane-wave excitations is reformulated as a truncated convolutional problem, and the theoretical implications of this reformulation are explored. Additionally, a matrix-free algorithm is proposed that leverages the computational and storage advantages of the fast Fourier transform (FFT) while simultaneously avoiding FFT wraparound artifacts. In particular, the storage constraints of the approach are reduced down from [Formula: see text] to [Formula: see text] over full matrix reconstruction, making this approach a better candidate for large reconstructions on clinical machines. This algorithm was then tested in the open source simulation package k-Wave to assess its robustness to modeling error and resolution reduction was demonstrated under full-wave propagation conditions relative to ideal straight-ray simulations. The method was also validated in a phantom experiment.

Generating Bessel beams with broad depth-of-field by using phase-only acoustic holograms

We report zero-th and high-order acoustic Bessel beams with broad depth-of-field generated using acoustic holograms. While the transverse field distribution of Bessel beams generated using traditional passive methods is correctly described by a Bessel function, these methods present a common drawback: the axial distribution of the field is not constant, as required for ideal Bessel beams. In this work, we experimentally, numerically and theoretically report acoustic truncated Bessel beams of flat-intensity along their axis in the ultrasound regime using phase-only holograms. In particular, the beams present a uniform field distribution showing an elongated focal length of about 40 wavelengths, while the transverse width of the beam remains smaller than 0.7 wavelengths. The proposed acoustic holograms were compared with 3D-printed fraxicons, a blazed version of axicons. The performance of both phase-only holograms and fraxicons is studied and we found that both lenses produce Bessel beams in a wide range of frequencies. In addition, high-order Bessel beam were generated. We report first order Bessel beams that show a clear phase dislocation along their axis and a vortex with single topological charge. The proposed method may have potential applications in ultrasonic imaging, biomedical ultrasound and particle manipulation applications using passive lenses.

Improved Target Specificity of Transcranial Focused Ultrasound Stimulation (TFUS) using Double-Crossed Ultrasound Transducers

Ultrasound neuromodulation is a promising stimulation modality because of its non-invasiveness, focusing and steering capability, and relatively high spatial resolution compared to the other stimulation modalities. However, despite the high lateral resolution, the ultrasound beam in the axial direction is relatively long, especially when compared to the small size of the mouse brain. Here, we report a new ultrasound focusing technique for small animal in vivo experiments where a high spatial resolution in both lateral and axial directions is achieved by crossing two ultrasound beams. The focal volume of a full width half maximum (FWHM) of our proposed system is only 0.161 mm3 and the focal diameter in the axial direction is about 1 mm, which is ten times smaller than the previously reported ultrasound neuromodulation system. Thus, the proposed system enables targeting a sub-region of a mouse brain using ultrasound for the first time. We also demonstrate successful stimulation of the motor cortex through in vivo mice experiments where the movement of forepaw of the mouse was observed using the double-crossed ultrasound transducers. Moreover, by sweeping the focal point in the z-axis and measuring the success rate of stimulated movements, we show that our double-transducer system targeted a region with 2 mmresolution in the dorsal-ventral (DV) coordinates. The success rate of the double-crossed ultrasound stimulation was quantified by recording the electromyography (EMG) signals during the stimulation. Our results show that the double-crossed ultrasound transducer system with a ten times higher spatial resolution enables highly specific and noninvasive stimulation of small animals and thus enables versatile in vivo experiments to study functional connectivities of brain circuits.

Ultrasonic sculpting of virtual optical waveguides in tissue

Optical imaging and stimulation are widely used to study biological events. However, scattering processes limit the depth to which externally focused light can penetrate tissue. Optical fibers and waveguides are commonly inserted into tissue when delivering light deeper than a few millimeters. This approach, however, introduces complications arising from tissue damage. In addition, it makes it difficult to steer light. Here, we demonstrate that ultrasound can be used to define and steer the trajectory of light within scattering media by exploiting local pressure differences created by acoustic waves that result in refractive index contrasts. We show that virtual light pipes can be created deep into the tissue (>18 scattering mean free paths). We demonstrate the application of this technology in confining light through mouse brain tissue. This technology is likely extendable to form arbitrary light patterns within tissue, extending both the reach and the flexibility of light-based methods.

Generation and Analysis of Ultrasound Images Using Plane Wave and Sparse Arrays Techniques

Ultrasonic imaging is one of the most important techniques to help medical diagnosis. However, obtaining high quality images requires the acquisition, processing, and storage of a large amount of data. In this work, we evaluated a new ultrasound imaging technique based on plane wave and sparse arrays to increase the scan rate and reduce the amount of data amount to be stored. The performance of the proposed method was tested using simulated echo data (from Field II) and phantom data acquired using a Verasonics system equipped with a L11-4v linear array transducer. The tests were done using 128 elements for transmission and 128, 65, 44, and 23 elements sparsely distributed for reception. The simulated data were compared with images obtained with the Delay and Sum (DAS) method and the experimental data were compared with those acquired from Verasonics. The obtained results using the Full Width at Half Maximum (FWHM) criteria at -6 dB showed that the images generated by the proposed method were similar in terms of resolutions (axial and lateral) and contrast to the simulated and the Verasonics commercial ones, indicating that the sparse reception proposed method is suitable for ultrasound imaging.

The nonlinear ultrasound needle pulse

Recent work has established an analytical formulation of broadband fields which extend in the axial direction and converge to a narrow concentrated line. Those unique (needle) fields have their origins in an angular spectrum configuration in which the forward propagating wavenumber of the field ( kz ) is constant across any z plane for all of the propagated frequencies. A 3 MHz-based, finite amplitude distorted simulation of such a field is considered here in a water path scenario relevant to medical imaging. That nonlinear simulation had its focal features compared to those of a comparable Gaussian beam. The results suggest that the unique convergence of the needle pulse to a narrow but extended axial line in linear propagation is also inherited by higher harmonics in nonlinear propagation. Furthermore, the linear needle field's relatively short duration focal pulses, and the asymptotic declines of its radial profiles, also hold for the associated higher harmonics. Comparisons with the Gaussian field highlight some unique and potentially productive features of needle fields.

The Ultrasound Needle Pulse

Recent theoretical developments have defined the conditions for forming a broadband beam that is narrow in axial extent but extended in range. The formulation is based on the classical angular spectrum decomposition and the requirement for arranging all spatial and temporal frequencies such that all components have constant phase propagation between planes. Analytic formulations are given for a 1-D source and a 2-D axisymmetric source, and comparisons are made against conventional focused Gaussian beams under similar conditions relevant to medical imaging systems.

The feasibility of implementation of ultrasound equipment standards set by United Kingdom professional bodies

INTRODUCTION

The Royal College of Radiologists and the Society and College of Radiographers in the United Kingdom published 'Standards for the provision of an ultrasound service', including application-specific limiting values for resolution and penetration. No measurement methods were detailed. We aimed to explore a possible theoretical basis for the standards and to develop a measurement protocol.

METHODS

Since application-specific standards fail to account for probes of different frequencies used for similar applications and no evidence for the standards was provided, we developed generic standards based on theoretical considerations. In a trial implementation of the published standards, automated measurements were made on four recently purchased scanners with a total of eight probes, results being assessed against the standards. Measurements were made on 15 modern probes and used to develop our generic standards.

RESULTS

Automated measurements showed less inter- and intra-observer variability than manual/visual measurements. Four new ultrasound scanners with a total of eight probes all failed to meet the published axial and lateral resolution standards; three failed to meet the penetration standard. Our generic standards were tested on 15 probes, four probes failing to meet the revised standards.

CONCLUSIONS

Automated methods are essential for measurements against standards. New generic standards with a theoretical basis have been proposed. Further work is required to refine standards and methods and to determine the appropriate contributions of objective and subjective equipment selection methods.

Quality assurance testing of transoesophageal echocardiography probes

INTRODUCTION

In response to an ultrasound imaging issue with transoesophageal echocardiography probes, a testing protocol was developed to check features pertinent to the operation of these probes. The imaging problem was detected in multiple probes of the same make and model.

METHODS

Over a two-year period, a series of 26 probes of this model were tested at acceptance, then three to six months later before being replaced due to a defect. A range of visual, mechanical and electrical tests were performed. Image tests comprised low-contrast penetration measurements and a comparison of phantom images at regular intervals to highlight artefacts in both B-mode and colour Doppler imaging.

RESULTS

Of the 26 defective probes replaced, 7 suffered mechanical/electrical problems, 5 of which prevented imaging results being obtained. Low-contrast penetration reduction of greater than 5% occurred in 14 probes. B-mode artefacts were observed on 12 probes and Doppler noise artefacts on 6 probes. No faults were found on five probes. The manufacturer addressed the imaging problem identified and of the seven subsequent probes supplied, only one suffered an imaging fault.

CONCLUSIONS

The implementation of a quality assurance protocol for transoesophageal echocardiography probes resulted in cost savings on replacements/repairs. When provided with the evidence gathered, the manufacturer supplied 23 probes under warranty or as loan equipment. The regular testing of the probes substantially reduced the impact of downtime and poor diagnosis from this equipment on the clinical service.

Fact or Artifact in Two-Dimensional Echocardiography: Avoiding Misdiagnosis and Missed Diagnosis

Two-dimensional transthoracic echocardiography is the most widely used noninvasive imaging modality for the evaluation and diagnosis of cardiac pathology. However, because of the physical properties of ultrasound waves and specifics in ultrasound image reconstruction, cardiologists are often confronted with ultrasound image artifacts. It is particularly important to recognize such artifacts in order to avoid misdiagnosis of conditions ranging from aortic dissection to thrombosis and endocarditis. This overview article summarizes the most common image artifacts encountered in routine clinical practice, along with explanations of their physical mechanisms and guidance in avoiding their misinterpretation.

Time domain compressive beam forming of ultrasound signals

Ultrasound imaging is a wide spread technique used in medical imaging as well as in non-destructive testing. The technique offers many advantages such as real-time imaging, good resolution, prompt acquisition, ease of use, and low cost compared to other techniques such as x-ray imaging. However, the maximum frame rate achievable is limited as several beams must be emitted to compute a single image. For each emitted beam, one must wait for the wave to propagate back and forth, thus imposing a limit to the frame rate. Several attempts have been made to use less beams while maintaining image quality. Although efficiently increasing the frame rate, these techniques still use several transmit beams. Compressive Sensing (CS), a universal data completion scheme based on convex optimization, has been successfully applied to a number of imaging modalities over the past few years. Using a priori knowledge of the signal, it can compute an image using less data allowing for shorter acquisition times. In this paper, it is shown that a valid CS framework can be derived from ultrasound propagation theory, and that this framework can be used to compute images of scatterers using only one plane wave as a transmit beam.

Comb-push ultrasound shear elastography (CUSE) for evaluation of thyroid nodules: preliminary in vivo results

In clinical practice, an overwhelming majority of biopsied thyroid nodules are benign. Therefore, there is a need for a complementary and noninvasive imaging tool to provide clinically relevant diagnostic information about thyroid nodules to reduce the rate of unnecessary biopsies. The goal of this study was to evaluate the feasibility of utilizing comb-push ultrasound shear elastography (CUSE) to measure the mechanical properties (i.e., stiffness) of thyroid nodules and use this information to help classify nodules as benign or malignant. CUSE is a fast and robust 2-D shear elastography technique in which multiple laterally distributed acoustic radiation force beams are utilized simultaneously to produce shear waves. Unlike other shear elasticity imaging modalities, CUSE does not suffer from limited field of view (FOV) due to shear wave attenuation and can provide a large FOV at high frame rates. To evaluate the utility of CUSE in thyroid imaging, a preliminary study was performed on a group of five healthy volunteers and 10 patients with ultrasound-detected thyroid nodules prior to fine needle aspiration biopsy. The measured shear wave speeds in normal thyroid tissue and thyroid nodules were converted to Young's modulus (E), indicating a measure of tissue stiffness. Our results indicate an increase in E for thyroid nodules compared to normal thyroid tissue. This increase was significantly higher in malignant nodules compared to benign. The Young's modulus in normal thyroid tissue, benign and malignant nodules were found to be 23.2 ±8.29 kPa, 91.2±34.8 kPa, and 173.0±17.1 kPa, respectively. Results of this study suggest the utility of CUSE in differentiating between benign and malignant thyroid nodules.

Sonoporation-enhanced chemotherapy significantly reduces primary tumour burden in an orthotopic pancreatic cancer xenograft

PURPOSE

Adenocarcinoma of the pancreas remains one of the most lethal human cancers. The high mortality rates associated with this form of cancer are subsequent to late-stage clinical presentation and diagnosis, when surgery is rarely possible and of modest chemotherapeutic impact. Survival rates following diagnosis with advanced pancreatic cancer are very low; typical mortality rates of 50% are expected within 3 months of diagnosis. However, adjuvant chemotherapy improves the prognosis of patients even after palliative surgery, and successful newer neoadjuvant chemotherapeutical modalities have recently been reported. For patients whose tumours appear unresectable, chemotherapy remains the only option. During the past two decades, the nucleoside analogue gemcitabine has become the first-line chemotherapy for pancreatic adenocarcinoma. In this study, we aim to increase the delivery of gemcitabine to pancreatic tumours by exploring the effect of sonoporation for localised drug delivery of gemcitabine in an orthotopic xenograft mouse model of pancreatic cancer.

EXPERIMENTAL DESIGN

An orthotopic xenograft mouse model of luciferase expressing MIA PaCa-2 cells was developed, exhibiting disease development similar to human pancreatic adenocarcinoma. Subsequently, two groups of mice were treated with gemcitabine alone and gemcitabine combined with sonoporation; saline-treated mice were used as a control group. A custom-made focused ultrasound transducer using clinically safe acoustic conditions in combination with SonoVue® ultrasound contrast agent was used to induce sonoporation in the localised region of the primary tumour only. Whole-body disease development was measured using bioluminescence imaging, and primary tumour development was measured using 3D ultrasound.

RESULTS

Following just two treatments combining sonoporation and gemcitabine, primary tumour volumes were significantly lower than control groups. Additional therapy dramatically inhibited primary tumour growth throughout the course of the disease, with median survival increases of up to 10% demonstrated in comparison to the control groups.

CONCLUSION

Combined sonoporation and gemcitabine therapy significantly impedes primary tumour development in an orthotopic xenograft model of human pancreatic cancer, suggesting additional clinical benefits for patients treated with gemcitabine in combination with sonoporation.

Producing acoustic 'Frozen Waves': simulated experiments with diffraction/attenuation resistant beams in lossy media

The so-called Localized Waves (LW), and the "Frozen Waves" (FW), have raised significant attention in the areas of Optics and Ultrasound, because of their surprising energy localization properties. The LWs resist the effects of diffraction for large distances, and possess an interesting self-reconstruction -self-healing- property (after obstacles with size smaller than the antenna's); while the FWs, a sub-class of LWs, offer the possibility of arbitrarily modeling the longitudinal field intensity pattern inside a prefixed interval, for instance 0⩽z⩽L, of the wave propagation axis. More specifically, the FWs are localized fields "at rest", that is, with a static envelope (within which only the carrier wave propagates), and can be endowed moreover with a high transverse localization. In this paper we investigate, by simulated experiments, various cases of generation of ultrasonic FW fields, with the frequency of f0=1 MHz in a water-like medium, taking account of the effects of attenuation. We present results of FWs for distances up to L=80 mm, in attenuating media with absorption coefficient α in the range 70⩽α⩽170 dB/m. Such simulated FW fields are constructed by using a procedure developed by us, via appropriate finite superpositions of monochromatic ultrasonic Bessel beams. We pay due attention to the selection of the FW parameters, constrained by the rather tight restrictions imposed by experimental Acoustics, as well as to some practical implications of the transducer design. The energy localization properties of the Frozen Waves can find application even in many medical apparatus, such as bistouries or acoustic tweezers, as well as for treatment of diseased tissues (in particular, for the destruction of tumor cells, without affecting the surrounding tissues; also for kidney stone shuttering, etc.).

Analysis and measurement of the modulation transfer function of harmonic shear wave induced phase encoding imaging

Shear wave induced phase encoding (SWIPE) imaging generates ultrasound backscatter images of tissue-like elastic materials by using traveling shear waves to encode the lateral position of the scatters in the phase of the received echo. In contrast to conventional ultrasound B-scan imaging, SWIPE offers the potential advantages of image formation without beam focusing or steering from a single transducer element, lateral resolution independent of aperture size, and the potential to achieve relatively high lateral resolution with low frequency ultrasound. Here a Fourier series description of the phase modulated echo signal is developed, demonstrating that echo harmonics at multiples of the shear wave frequency reveal target k-space data at identical multiples of the shear wavenumber. Modulation transfer functions of SWIPE imaging systems are calculated for maximum shear wave acceleration and maximum shear constraints, and compared with a conventionally focused aperture. The relative signal-to-noise ratio of the SWIPE method versus a conventionally focused aperture is found through these calculations. Reconstructions of wire targets in a gelatin phantom using 1 and 3.5 MHz ultrasound and a cylindrical shear wave source are presented, generated from the fundamental and second harmonic of the shear wave modulation frequency, demonstrating weak dependence of lateral resolution with ultrasound frequency.

Producing acoustic frozen waves: simulated experiments

In this paper, we show how appropriate superpositions of Bessel beams can be successfully used to obtain arbitrary longitudinal intensity patterns of nondiffracting ultrasonic wave fields with very high transverse localization. More precisely, the method here described allows generation of longitudinal acoustic pressure fields whose longitudinal intensity patterns can assume, in principle, any desired shape within a freely chosen interval 0 ≤ z ≤ L of the propagation axis, and that can be endowed in particular with a static envelope (within which only the carrier wave propagates). Indeed, it is here demonstrated by computer evaluations that these very special beams of nonattenuated ultrasonic field can be generated in water-like media by means of annular transducers. Such fields at rest have been called by us acoustic frozen waves (FWs). The paper presents various cases of FWs in water, and investigates their aperture characteristics, such as minimum required size and ring dimensioning, as well as the influence they have on the proper generation of the desired FW patterns. The FWs are particular localized solutions to the wave equation that can be used in many applications, such as new kinds of devices, e.g., acoustic tweezers or scalpels, and especially in various ultrasound medical apparatus.

Apodization and windowing functions

In beamforming, a critical design issue is the choice of the window or apodization function. Because the transverse beam pattern at focal depth is related to the Fourier transform of the apodization function, designers must evaluate the properties of the function and its transform, jointly, to optimize the system. This paper illustrates systematic approaches to designing useful functions.

Fast simulation of second harmonic ultrasound field using a quasi-linear method

Nonlinear propagation of sound has been exploited in the last 15 years in medical ultrasound imaging through tissue harmonic imaging (THI). THI creates an image by filtering the received ultrasound echo around the second harmonic frequency band. This technique produces images of enhanced quality due to reduced body wall reverberation, lower perturbations from off-axis echoes, and multiple scattering of reduced amplitude. In order to optimize the image quality it is essential to be able to predict the amplitude level and spatial distribution of the propagating ultrasound pulse. A method based on the quasi-linear approximation has been developed to quickly provide an estimate of the ultrasound pulse. This method does not need to propagate the pulse stepwise from the source plane to the desired depth; it directly computes a transverse profile at any depth from the definitions of the transducer and the pulse. The computation handles three spatial dimensions which allows for any transducer geometry. A comparison of pulse forms, transverse profiles, as well as axial profiles obtained by this method and state-of-the-art simulators, the KZKTexas code, and Abersim, shows a satisfactory match. The computation time for the quasi-linear method is also smaller than the time required by the other methods.

Effects of phase cancellation and receiver aperture size on broadband ultrasonic attenuation for trabecular bone in vitro

Phase cancellation in ultrasound due to large receiver size has been proposed as a contributing factor to the inaccuracy of estimating broadband ultrasound attenuation (BUA), which is used to characterize bone quality. Transducers with aperture size ranging from 2 to 5 mm have been used in previous attempts to study the effect of phase cancellation. However, these receivers themselves are susceptible to phase cancellation because aperture size is close to one center wavelength (about 3 mm at 500 KHz in water). This study uses an ultra small receiver (aperture size: 0.2 mm) in conjunction with a newly developed two-dimensional (2-D) synthetic array system to investigate the effects of phase cancellation and receiver aperture size on BUA estimations of bone tissue. In vitro ultrasound measurements were conducted on 54 trabecular bone samples (harvested from sheep femurs) in a confocal configuration with a focused transmitter and synthesized focused receivers of different aperture sizes. Phase sensitive (PS) and phase insensitive (PI) detections were performed. The results show that phase cancellation does have a significant effect on BUA. The normalized BUA (nBUA) with PS is 8.1% higher than PI nBUA while PI BUA is well correlated with PS BUA. Receiver aperture size also influences the BUA reading for both PI and PS detection and smaller receiver aperture tends to result in higher BUA readings. The results also indicate that the receiver aperture size used in the confocal configuration with PI detection should at least equal the aperture of the transmitter to capture most of the energy redistributed by the interference and diffraction from the trabecular bone.

Site-specific sonoporation of human melanoma cells at the cellular level using high lateral-resolution ultrasonic micro-transducer arrays

We developed a new instrumental method by which human melanoma cells (LU1205) are sonoporated via radiation pressures exerted by highly-confined ultrasonic waves produced by high lateral-resolution ultrasonic micro-transducer arrays (UMTAs). The method enables cellular-level site-specific sonoporation within the cell monolayer due to UMTAs and can be applicable in the delivery of drugs and gene products in cellular assays. In this method, cells are seeded on the biochip that employs UMTAs for high spatial resolution and specificity. UMTAs are driven by 30-MHz sinusoidal signals and the resulting radiation pressures induce sonoporation in the targeted cells. The sonoporation degree and the effective lateral resolution of UMTAs are determined by performing fluorescent microscopy and analysis of carboxylic-acid-derivatized CdSe/ZnS quantum dots passively transported into the cells. Models representing the transducer-generated ultrasound radiation pressure, the ultrasound-inflicted cell membrane wound, and the transmembrane transport through the wound are developed to determine the ultrasound-pressure-dependent wound size and enhanced cellular uptake of nanoparticles. Model-based calculations show that the effective wound size and cellular uptake of nanoparticles increase linearly with increasing ultrasound pressure (i.e., at applied radiation pressures of 0.21, 0.29, and 0.40 MPa, the ultrasound-induced initial effective wound radii are 150, 460, and 650 nm, respectively, and the post-sonoporation intracellular quantum-dot concentrations are 7.8, 22.8, and 29.9 nM, respectively) and the threshold pressure required to induce sonoporation in LU1205 cells is ∼0.12 MPa.

Alignment and calibration of dual ultrasound transducers using a wedge phantom

We present a novel method of aligning two orthogonal ultrasound transducers into a coincident scan plane. A wedge phantom design provides visual feedback to the user to facilitate alignment. Calibration provides the transformation from one transducer to the other as well as a measure of the residual error in alignment. Mean alignment error is shown to be under 1° in the rotation axes and 1 mm in translation after repeated manual alignments. The repeatability of wedge based calibration has similar results compared with N-fiducial based calibration. The accuracy of the calibration for mapping points from one transducer to the other is found to have a mean error of 1.6 mm. The dual transducer system is well suited to imaging anatomy such as the breast and may be used for spatial compounding for improving B-mode images and motion estimation compounding for improving elastography results.

Ultrasonic backscatter imaging by shear-wave-induced echo phase encoding of target locations

We present a novel method for ultrasound backscatter image formation wherein lateral resolution of the target is obtained by using traveling shear waves to encode the lateral position of targets in the phase of the received echo. We demonstrate that the phase modulation as a function of shear wavenumber can be expressed in terms of a Fourier transform of the lateral component of the target echogenicity. The inverse transform, obtained by measurements of the phase modulation over a range of shear wave spatial frequencies, yields the lateral scatterer distribution. Range data are recovered from time of flight as in conventional ultrasound, yielding a B-mode-like image. In contrast to conventional ultrasound imaging, where mechanical or electronic focusing is used and lateral resolution is determined by aperture size and wavelength, we demonstrate that lateral resolution using the proposed method is independent of the properties of the aperture. Lateral resolution of the target is achieved using a stationary, unfocused, single-element transducer. We present simulated images of targets of uniform and non-uniform shear modulus. Compounding for speckle reduction is demonstrated. Finally, we demonstrate image formation with an unfocused transducer in gelatin phantoms of uniform shear modulus.

Ultrasonic array of thick film transducers for biological tissue characterization

The initial motivation for this work was to accomplish an easy way to manufacture different geometries of ultrasonic transducers and arrays using a PZT powder, combined with a standard process to have repetitive series of them. The piezoelectric thick film was obtained using a PZT paste and applying it by screen printing on an alumina substrate. Then, the film was drying and sintered with a temperature-time profile determined by the paste characteristics. Each transducer is composed by three layers, one by PZT and two acting as electrodes. The active element of the paste is a PZT powder which is dispersed in a commercial vehicle to obtain rheological properties suitable for use the screen printing process. The connection between PZT particles is improved by adding a lead borosilicate frit glass that also helps to attach the film to the substrate due to the relatively low temperature of sintered that has been used in this process. The PZT film has low density that is generated by internal porosity, so its acoustic impedance is lower than for a bulk ceramic transducer and so is well adapted to testing human tissues. At the same time the thick film technology is well suited to make medium size transducers and also arrays performed with tiny ultrasonic transducers.

Hybrid beamforming and steering with reconfigurable arrays

Reconfigurable arrays offer an advantage over traditional ultrasound arrays because of their flexibility in channel selection. To improve ultrasound beamforming and coverage through beam steering, we propose a hybrid beamforming technique to elongate the depth of focus of transmit beams and a method of element selection that improves steering capabilities that take advantage of array reconfigurability using annular rings. A local minimization technique to optimize the hybrid aperture is discussed in this paper. The chosen hybrid apertures covering four focal zones result in improved range in depth of focus when compared with pure spherical beams via point spread functions (PSF) and lesion signal-to-noise ratio (LSNR) calculations. Improvements were statistically significant at focal depth extremes. Our method of beam steering utilizing a quantized phase delay selection to minimize delay errors indicated better performance by removing an artifact present with traditional ringed element selection.

Design and acoustic characterization of limited diffraction ultrasonic devices

Limited diffraction ultrasonic transducers are devices that have a large depth of acoustic field without important effects of diffraction, which make them optimal in applications of medical images, among others. This report details how this special type of piezoelectric device was designed by means of a simple technology using three electrodes in the form of concentric rings in both faces of a ferroelectric ceramic disk, which were used to apply a profile of non-homogeneous polarization. Once designed, the radiation fields emitted by these resonators were characterized experimentally by electro-acoustic and acousto-optic techniques and were compared with those emitted by conventional devices. As shown in the experimental characterizations, ultrasonic transducers with optimal properties for use in medical applications such as good collimation of the ultrasound beam, high lateral resolution, as well as little effects of diffraction were obtained.

Cherenkov radiation versus X-shaped localized waves

Localized waves (LW) are nondiffracting ("soliton-like") solutions to the wave equations and are known to exist with subluminal, luminal, and superluminal peak velocities V. For mathematical and experimental reasons, those that have attracted more attention are the "X-shaped" superluminal waves. Such waves are associated with a cone, so that one may be tempted-let us confine ourselves to electromagnetism-to look [Phys. Rev. Lett.99, 244802 (2007)] for links between them and the Cherenkov radiation. However, the X-shaped waves belong to a very different realm: For instance, they can be shown to exist, independently of any media, even in vacuum, as localized non-diffracting pulses propagating rigidly with a peak-velocity V>c [Hernández et al., eds., Localized Waves (Wiley, 2008)]. We dissect the whole question on the basis of a rigorous formalism and clear physical considerations.

Interaction of a high-order Bessel beam with a submerged spherical ultrasound contrast agent shell - Scattering theory

BACKGROUND AND OBJECTIVE

Acoustic scattering properties of ultrasound contrast agents are useful in extending existing or developing new techniques for biomedical imaging applications. A useful first step in this direction is to investigate the acoustic scattering of a new class of acoustic beams, known as helicoidal high-order Bessel beams, to improve the understanding of their scattering characteristics by an ultrasound contrast agent, which at present is very limited.

METHOD

The transverse acoustic scattering of a commercially available albuminoidal ultrasound contrast agent shell filled with air or a denser gas such as perfluoropropane and placed in a helicoidal Bessel beam of any order is examined numerically. The shell is assumed to possess an outer radius a=3.5 microns and a thickness of approximately 105 nm. Moduli of the total and resonance transverse acoustic scattering form functions are numerically evaluated in the bandwidth 0<ka 3, which corresponds to a frequency bandwidth of 0-205 MHz that covers a wide range of applications for imaging with contrast agents. Particular attention is paid to the shell's material, the content of its interior hollow region and the fluid surrounding its exterior. The contrast agent shell is assumed to be immersed in an ideal compressible fluid so the viscous corrections are not considered. Analytical equations are derived and numerical calculations of the total and resonance form functions are performed with particular emphasis on the effect of varying the half-cone angle, the order of the helicoidal Bessel beam as well as the fluid that fills the interior hollow space.

RESULTS AND CONCLUSION

It is shown that shell wave resonance modes can be excited on an encapsulated micro-bubble. The forward and backscattering vanish for a helicoidal high-order Bessel beam. Additionally, the fluid filling the inner core affects the shell's response significantly. Moreover, there is no monopole contribution to the axial scattering of a helicoidal Bessel beam of order m1 so that the dynamics of contrast agents would be significantly altered. The main finding of the present theory is the suppression or enhancement for a particular resonance that may be used to advantage in imaging with ultrasound contrast agents for clinical applications.

Interaction of a nondiffracting high-order Bessel (vortex) beam of fractional type alpha and integer order m with a rigid sphere: linear acoustic scattering and net instantaneous axial force

Closed-form analytical solutions for the acoustic scattering and net axial force of a new class of Bessel beams, termed nondiffracting Bessel vortex beams of fractional type alpha, are derived. This new class of Bessel beams preserves the same nondiffracting feature of conventional high-order Bessel beams of integer order. The far-field acoustic scattering field by a rigid sphere centered on the beam axis is expressed as a partial wave series involving the real number alpha, the scattering angle relative to the beam axis theta, and the half-conical angle beta of the wave number components of the beam. Unlike the acoustic scattering properties of conventional high-order Bessel beams, the acoustic forward scattering (theta = 0 degrees) and backscattering (theta = 180 degrees) of Bessel vortex beams of fractional type alpha by a rigid sphere do not vanish unless alpha becomes an integer number. Furthermore, an expression for the net instantaneous axial force is derived for the case of progressive, stationary, and quasi-stationary waves. These results provide new insights into the acoustic scattering theory in the context of nondiffracting beams. The properties of nondiffracting Bessel vortex beams of fractional type alpha may lead to the development of an "acoustic blender" with possible applications in particle rotation, mixing, and manipulation. Imaging and other related applications may also benefit from this new type of acoustic beams.