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The 3-D ultrasound imaging provides distinct advantages over its 2-D counterpart leading to a more accurate analysis of tumors and cysts. However, the front end of a 3-D system must receive and process data at prodigious rates, making it impractical for power-constrained portable systems. Synthetic aperture sequential beamforming (SASB) is an ultrasound beamforming technique that splits the computation into two stages, such that the computation in Stage 1 can be completed in the power-constrained front end while the remaining computation can be done elsewhere. In this article, we present several algorithmic and architectural techniques to enable efficient computation of Stage 1 processing without compromising imaging quality. Specifically, we present algorithmic techniques that reduce the computational complexity in Stage 1 by $17\times $ through a systematic reduction in the number of apodization coefficients. We propose a 3-D die stacked architecture where the signals received by 961 active transducers are digitized, routed by a network-on-chip, and processed in parallel. This architecture does not require the explicit storage of incoming data samples. We synthesize the architecture using TSMC 28-nm technology node. The front-end power consumption is around 1.5 W, making it suitable for portable applications. [ABSTRACT FROM AUTHOR]

High volume acquisition rates are imperative for certain medical ultrasound imaging applications, such as 3D elastography and 3D vector flow imaging. As ultrasound imaging transitions from 2D to 3D, the massive data bandwidth and billions of trigonometric operations required to reconstruct each volume leaves conventional computer architectures falling short. Despite recent algorithmic improvements, high-volume-rate ultrasound imaging remains computationally infeasible on known platforms. In this article, we expand our previous work on Tetris, a novel hardware accelerator for separable ultrasound beamforming that enables volume acquisition rates up to the physics limits of acoustic propagation delay. Through algorithmic and hardware optimizations, we enable an image reconstruction system design outclassing previously proposed accelerators in performance while lowering hardware complexity, storage, and power requirements. Tetris operates in a streaming fashion—without requiring on-chip storage of the entire receive signal—reconstructing volumes in real-time. For a representative imaging task, our proposed system generates physics-limited 13,000 volumes per second in a 2 watt power budget. The Tetris beamformer has an unprecedented power efficiency of 2.03 tera-beamforming operations per watt—an increase in efficiency of nearly 3× compared to the prior work. [ABSTRACT FROM AUTHOR]

Blood volume flow (VF) estimation is becoming an integral part of quantitative medical imaging. Three-dimensional color flow can be used to measure volumetric flow, but partial volume correction (PVC) is essential due to finite beamwidths and lumen diameters. Color flow power was previously assumed to be directly proportional to the perfused fractional color flow beam area (voxel). We investigate the relationship between color flow power and fractionally perfused voxels. We simulate 3-D color flow imaging using Field II based on a 3.75-MHz mechanically swept linear array. A 16-mm-diameter tube with laminar flow was embedded into soft tissue. We investigated two study scenarios where soft tissue backscatter is 1) 40 dB higher and 2) 40 dB lower, relative to blood. Velocity and power were computed from color flow packets ($n = 16$) using autocorrelation. Study 1 employed a convolution-based wall filter. Study 2 did not employ a wall filter. VF was computed from the resulting color flow data, as published previously. Partial volume voxels in Study 1 show lesser power than those in Study 2, likely due to wall filter effects. An “S”-shaped relationship was found between color flow power and fractionally perfused voxel area in Study 2, which could be due to an asymmetric lateral–elevational point spread function. Flow computation is biased low by 7.3% and 7.9% in Study 1 and Study 2, respectively. Uncorrected simulation estimates are biased high by 41.5% and 12.5% in Study 1 and Study 2, respectively. Our findings show that PVC improves 3-D VF estimation and that wall filter processing alters the proportionality between color flow power and fractionally perfused voxel area. [ABSTRACT FROM AUTHOR]

Three-dimensional (3-D) ultrasound imaging is a promising modality for many medical applications. Unfortunately, it generates voluminous data in the front end, making it unattractive for high-volume-rate portable medical applications. We apply synthetic aperture sequential beamforming (SASB) to greatly compress the front-end receive data. Baseline 3-D SASB has a low volume rate, because subapertures fire one by one. In this paper, we propose to increase the volume rate of 3-D SASB without degrading imaging quality through: 1) transmitting and receiving simultaneously with four subapertures and 2) using linear chirps as the excitation waveform to reduce interference. We design four linear chirps that operate on two overlapped frequency bands with chirp pairs in each band having opposite chirp rates. Direct implementation of this firing scheme results in grating lobes. Therefore, we design a sparse array that mitigates the grating lobe levels through optimizing the locations of transducer elements in the bin-based random array. Compared with the baseline 3-D SASB, the proposed method increases the volume rate from 8.56 to 34.2 volumes/s without increasing the front-end computation requirement. Field-II-based cyst simulations show that the proposed method achieves imaging quality comparable with baseline 3-D SASB in both shallow and deep regions. [ABSTRACT FROM AUTHOR]

Volumetric flow rate estimation is an important ultrasound medical imaging modality that is used for diagnosing cardiovascular diseases. Flow rates are obtained by integrating velocity estimates over a cross-sectional plane. Speckle tracking is a promising approach that overcomes the angle dependency of traditional Doppler methods, but suffers from poor lateral resolution. Recent work improves lateral velocity estimation accuracy by reconstructing a synthetic lateral phase (SLP) signal. However, the estimation accuracy of such approaches is compromised by the presence of clutter. Eigen-based clutter filtering has been shown to be effective in removing the clutter signal; but it is computationally expensive, precluding its use at high volume rates. In this paper, we propose low-complexity schemes for both velocity estimation and clutter filtering. We use a two-tiered motion estimation scheme to combine the low complexity sum-of-absolute-difference and SLP methods to achieve subpixel lateral accuracy. We reduce the complexity of eigen-based clutter filtering by processing in subgroups and replacing singular value decomposition with less compute-intensive power iteration and subspace iteration methods. Finally, to improve flow rate estimation accuracy, we use kernel power weighting when integrating the velocity estimates. We evaluate our method for fast- and slow-moving clutter for beam-to-flow angles of 90° and 60° using Field II simulations, demonstrating high estimation accuracy across scenarios. For instance, for a beam-to-flow angle of 90° and fast-moving clutter, our estimation method provides a bias of −8.8% and standard deviation of 3.1% relative to the actual flow rate. [ABSTRACT FROM PUBLISHER]

Intermittent high intensity ultrasound pulses with circulating contrast agent microbubbles can induce scattered cavitation caused myocardial microlesions of potential value for tissue reduction therapy. Here, computer-aided histological evaluation of the effective treated volume was implemented to optimize ultrasound pulse parameters, exposure duration, and contrast agent dose. Rats were treated with 1.5 MHz focused ultrasound bursts and Evans blue staining indicates lethal cardiomyocytic injury. Each heart was sectioned to provide samples covering the entire exposed myocardial volume. Both brightfield and fluorescence images were taken for up to 40 tissue sections. Tissue identification and microlesion detection were first done based on 2-D images to form microlesion masks containing the outline of the heart and the stained cell regions. Image registration was then performed on the microlesion masks to reconstruct a volume-based model according to the morphology of the heart. The therapeutic beam path was estimated from the 3-D stacked microlesions, and finally the total microlesion volume, here termed macrolesion, was characterized along the therapeutic beam axis. Radially symmetric fractional macrolesions were characterized via stepping disks of variable radius determined by the local distribution of microlesions. Treated groups showed significant macrolesions of a median volume of 87.3 μL, 2.7 mm radius, 4.8 mm length, and 14.0% lesion density compared to zero radius, length, and lesion density for sham. The proposed radially symmetric lesion model is a robust evaluation for myocardial cavitation-enabled therapy. Future work will include validating the proposed method with varying acoustic exposures and optimizing involved parameters to provide macrolesion characterization [ABSTRACT FROM PUBLISHER]

The vaporization of a superheated droplet emulsion into gas bubbles using ultrasound--termed acoustic droplet vaporization (ADV)--has potential therapeutic applications in embolotherapy and drug delivery. The optimization of ADV for therapeutic applications can be enhanced by understanding the physical mechanisms underlying ADV, which are currently not clearly elucidated. Acoustic cavitation is one possible mechanism. This paper investigates the relationship between ADV and inertial cavitation (IC) thresholds (measured as peak rarefactional pressures) by studying parameters that are known to influence the IC threshold. These parameters include bulk fluid properties such as gas saturation, temperature, viscosity, and surface tension; droplet parameters such as degree of superheat, surfactant type, and size; and acoustic properties such as pulse repetition frequency and pulse width. In all cares the ADV threshold occurred at a lower rarefactional pressure than the IC threshold, indicating that the phase transition curs before IC events. The viscosity and temperature of the bulk fluid are shown to influence both thresholds directly and inversely, respectively. An inverse trend is observed between threshold and diameter for droplets in the 1 to 2.5 μm range. Based on a choice of experimental parameters, it is possible to achieve ADV with or without IC. [ABSTRACT FROM AUTHOR]

The use of superheated liquid perfluorocarbon droplets encased in albumin shells has been proposed as a minimally invasive alternative to current treatment of cancer by means of occlusion therapy. In response to an applied acoustic field, these droplets, which are small enough to pass through capillaries, vaporize into large gas bubbles that subsequently lodge in the vasculature. This technique, known as acoustic droplet vaporization (ADV) has been shown to successfully reduce blood flow in vivo, but for in situ conditions where attenuation is present, lower acoustic frequency and ADV threshold may be desirable. Thus, two methods to lower the ADV threshold at a lower 1.44 MHz were explored. The first part of this study investigated the role of pulse duration on ADV. The second part investigated the role of inertial cavitation (IC) external to a droplet by lowering the IC threshold in the host liquid with the presence of Definity® contrast agent (CA). The threshold was found to be 5.5-5.9 MPa for short microsecond pulses and decreased for millisecond pulses (3.8-4.6 MPa). When CAs were present and long millisecond pulses were used, the ADV threshold decreased to values as low as 0.41 MPa. [ABSTRACT FROM AUTHOR]

Acoustic droplet vaporization (ADV) has been introduced with the potential application of tumor treatment via occlusion and subsequent necrosis. New Zealand White rabbits were anesthetized, and their left kidney was externalized. All imaging array and single-element transducer were positioned in a tank with direct access to the kidney's vasculature and renal artery. Filtered droplet emulsions (diameter <6 μm) were injected intra-arterially (IA) into the left heart during insonification of the renal artery, and the extent of blood flow reduction by ADV was compared to the untreated right kidney. Flow cytometry (using colored microspheres) of kidney tissue samples and reference blood from the femoral artery allowed the quantitative estimation of regional blood flow. A maximum regional blood flow reduction in the treated region of >90% and an average organ perfusion reduction of >70% was achieved using ADV. After treatment of the left kidney, the control kidney on the contralateral side showed a maximum decrease in regional blood flow of 18% relative to the pre-ADV baseline. Image-based hyper-echogenicity from ADV of IA injections was monitored for approximately 90 minutes, and cortex perfusion was reduced by >60% of its original value for more than 1 hour. This could be enough time for the onset of cell death and possible tumor treatment via ischemic necrosis. Moreover, currently used radiofrequency tissue ablation-based tumor treatment could benefit from ADV due to the decreased heat loss via vascular cooling. [ABSTRACT FROM AUTHOR]

Presents a study that tested whether gas bubbles, made by the acoustic vaporization of albumin-stabilized dodecafluoropentane (DDFP) droplets are feasible as point beacons for both phase aberration correction and occlusion therapy. Information on perfluorocarbons; Discussion on embolotheraphy; Results of the in vitro verification of acoustic droplet verification; Possible counteragent for the use of DDFP emulsion for occlusion therapy.

Ultrasound test objects containing reference point targets could be useful for evaluating ultrasound systems and phase aberration correction methods. Polyacrylamide gels containing albumin-stabilized droplets (3.6 μm mean diameter) of dodecafluoropentane (DDFP) are being developed for this purpose. Perturbation by ultrasound causes spontaneous vaporization of the superheated droplets to form gas bubbles, a process termed acoustic droplet vaporization (ADV). The resulting bubbles (20 to 160 μmm diameter) are small compared with acoustic wavelengths in diagnostic ultrasound and are theoretically suitable for use as point targets (phase errors <20° for typical f-numbers). Bubbles distributed throughout the material are convenient for determining the point spread function in an imaging plane or volume. Cooling the gel causes condensation of the DDFP droplets, which may be useful for storage. Studying ADV in such viscoelastic media could provide insight into potential bioeffects from rapid bubble formation. [ABSTRACT FROM AUTHOR]

Acoustic droplet vaporization (ADV) is an ultrasound method for converting biocompatible microdroplets into microbubbles. The objective is to demonstrate that ADV bubbles can enhance high intensity focused ultrasound (HIFU) therapy by controlling and increasing energy absorption at the focus. Thermal phantoms were made with or without droplets. Compound lesions were formed in the phantoms by 5-second exposures with 5-second delays. Center to center spacing of individual lesions was 5.5 mm in either a linear pattern or a spiral pattern. Prior to the HIFU, 10 cycle tone bursts with 0.25% duty cycle were used to vaporize the droplets, forming an "acoustic trench" within 30 seconds. The transducer was then focused in the middle of the back bubble wall to form thermal lesions in the trench. All lesions were imaged optically and with 2T MRI. With the use of ADV and the acoustic trench, a uniform thermal ablation volume of 15 cm(3) was achieved in 4 minutes; without ADV only less than 15% of this volume was filled. The commonly seen tadpole shape characteristic of bubble-enhanced HIFU lesions was not evident with the acoustic trench. In conclusion, ADV shows promise for the spatial control and dramatic acceleration of thermal lesion production by HIFU.