Coded excitation for diverging wave cardiac imaging: a feasibility study

Optimized Virtual Sources Distributions for 3-D Ultrafast Diverging Wave Compounding Imaging: A Simulation Study

Ultrafast ultrasound imaging allows observing rapid phenomena; combined with 3-D imaging it has the potential to provide a more accurate analysis of organs which leads, in the end, to better diagnosis. Coherent compounding using diverging waves is commonly used to reconstruct high-quality images on large volumes while keeping the frame rate high enough to allow dynamic analysis. In practice, the virtual sources (VSs) that drive the diverging waves are often distributed in a deterministic way: following a regular grid, concentric rings, and spirals. Even though those deterministic distributions can offer various tradeoffs in terms of imaging performance, other distributions can be considered to improve imaging performance. It is herein suggested to look at alternative VSs distributions for optimizing the lateral resolution and the secondary lobes level (SLL) on several point spread functions (PSFs) by means of a multiobjective genetic algorithm. The optimization framework has led to seven pseudo-irregular distributions of VSs distributions that have not yet been found in the literature. An analysis of the imaging performance with a simulated phantom shows that these new distributions offer different tradeoffs between lateral resolution and contrast, respectively, measured on point-like reflectors and anechoic cysts. As an example, one of these optimized distributions improves the lateral resolution by 16% and gives equivalent contrast values on cysts and PSF isotropy properties, when compared to a concentric-rings-based distribution.

Spatially segmented SVD clutter filtering in cardiac blood flow imaging with diverging waves

Ultrafast ultrasound imaging enables the visualization of rapidly changing blood flow dynamics in the chambers of the heart. Singular value decomposition (SVD) filters outperform conventional high pass clutter rejection filters for ultrafast blood flow imaging of small and shallow fields of view (e.g., functional imaging of brain activity). However, implementing SVD filters can be challenging in cardiac imaging due to the complex spatially and temporally varying tissue characteristics. To address this challenge, we describe a method that involves excluding the proximal portion of the image (near the chest wall) and divides the reduced field of view into overlapped segments, within which tissue signals are expected to be spatially and temporally coherent. SVD filtering with automatic selection of cut-off singular vector orders to remove tissue and noise signals is implemented for each segment. Auto-thresholding is based on the coherence of spatial singular vectors, delineating tissue, blood, and noise subspaces within a spatial similarity matrix calculated for each segment. Filtered blood flow signals from the segments are reconstructed and then combined and Doppler processing is used to form a set of blood flow images. Preliminary experimental results suggest that the spatially segmented approach improves the separation of the tissue and blood subsets in the spatial similarity matrix so that automatic thresholding is significantly improved, and tissue clutter can then be rejected more effectively in cardiac ultrafast imaging, compared to using the full field of view. In the case studied, spatially segmented SVD improved the rate of correct automatic selection of thresholds from 78% to 98.7% for the investigated cases and improved the post-filter power of blood signals by an average of more than 10 dB during a cardiac cycle.

Signal-to-noise ratio of diverging waves in multiscattering media: Effects of signal duration and divergence angle

In this paper, SNR maximization in coded diverging waves is studied, and experimental verification of the results is presented. Complementary Golay sequences and binary phase shift keying modulation are used to code the transmitted signal. The SNR in speckle and pin targets is maximized with respect to chip signal length. The maximum SNR is obtained in diverging wave transmission when the chip signal is as short a duration as the array permits. We determined the optimum diverging wave profile to confine the transmitted ultrasound energy in the imaging sector. The optimized profile also contributes to the SNR maximization. The SNR performances of the optimized coded diverging wave and conventional single-focused phased array imaging are compared on a single frame basis. The SNR of the optimized coded diverging wave is higher than that of the conventional single-focused phased array imaging at all depths and regions.

Ultrasound Planar Array Imaging Metric Analysis

Planar array design makes the tradeoff between the 3-D ultrasound image quality and the system complexity based on the imaging metrics. The -6 dB mainlobe width (MW), mainlobe-to-sidelobe energy ratio (MSR), peak sidelobe level (PSL), and average sidelobe level (ASL) are the common imaging metrics for linear array design. MW is used for lateral resolution evaluation, while MSR, PSL, and ASL are adopted for contrast resolution evaluation. However, simulation results show that these metrics cannot fully evaluate the planar array performance. This article proposes several new imaging metrics for planar array: averaged mainlobe acoustic energy level and mainlobe energy density curve are the lateral resolution metrics, while mainlobe-to-sidelobe energy density ratio is the contrast resolution metric. The new metrics take into account the influence of the mainlobe area on the planar array performance evaluation. PSF analysis and simulated images show that the proposed metrics can evaluate planar array performance more accurately than the existing metrics. Moreover, uniform planar arrays with different scales and random sparse arrays are tested to show how to use the proposed metrics in planar array design.

In Vivo Evaluation of Plane Wave Imaging for Abdominal Ultrasonography

Although plane wave imaging (PWI) has been extensively employed for ultrafast ultrasound imaging, its potential for sectorial B-mode imaging with a convex array transducer has not yet been widely recognized. Recently, we reported an optimized PWI approach for sector scanning that exploits the dynamic transmit focusing capability. In this paper, we first report the clinical applicability of the optimized PWI for abdominal ultrasonography by in vivo image and video evaluations and compare it with conventional focusing (CF) and diverging wave imaging (DWI), which is another dynamic transmit focusing technique generally used for sectorial imaging. In vivo images and videos of the liver, kidney, and gallbladder were obtained from 30 healthy volunteers using PWI, DWI, and CF. Three radiologists assessed the phantom images, 156 in vivo images, and 66 in vivo videos. PWI showed significantly enhanced (p < 0.05) spatial resolution, contrast, and noise and artifact reduction, and a 4-fold higher acquisition rate compared to CF and provided similar performances compared to DWI. Because the computations required for PWI are considerably lower than that for DWI, PWI may represent a promising technique for sectorial imaging in abdominal ultrasonography that provides better image quality and eliminates the need for focal depth adjustment.

Contrast and Volume Rate Enhancement of 3-D Ultrasound Imaging Using Aperiodic Plane Wave Angles: A Simulation Study

Three-dimensional plane wave imaging (PWI) with a 2-D array has been studied for ultrafast volumetric imaging in medical ultrasound. Compared to 2-D PWI, 3-D PWI requires the transmission of an increased number of plane waves (PWs) to scan a volume of interest and achieve transmit dynamic focusing in both the lateral and elevational directions. To reduce the number of PW angles for a given 2-D angular range by mitigating the grating lobe level, we propose two aperiodic patterns of PW angles: concentric rings with a uniform radial interval and the well-known sunflower pattern. Both patterns are validated to provide uniform angle distributions without regular periodicity, and thereby reduce the grating lobe level compared to a periodic angle distribution with the same number of PW angles. Simulation studies show that the aperiodic patterns enhance the contrast of B-mode images by approximately 3-6 dB over all depths. This enhancement implies that the aperiodic angle sets can increase the volume rate by approximately 2-6 times compared to the periodic angle set at the same contrast and spatial resolution.

Wide Field-of-View Ultrafast Curved Array Imaging Using Diverging Waves

Ultrafast ultrasound imaging provides great opportunities for very high frame rate applications, such as shear wave elastography and microvascular imaging. However, ultrafast imaging with curved array transducers remains challenging in terms of element directivity and a limited field-of-view (FOV) for a fully synthetic area. In this paper, a wide FOV ultrafast curved array imaging method based on diverging wave transmissions is presented for high frame rate abdominal ultrasound applications. For this method, a theoretical model for a diverging wave solution based on a virtual point source originating from a circular line is proposed, and the FOV and element directivity are analyzed by this model. Furthermore, an integrated model for plane wave and diverging wave imaging along the location of the virtual point source is derived. The proposed method was evaluated with simulation, phantom, and in vivo studies. In the simulation and phantom studies, the image quality (i.e., spatial resolution, cystic resolution, and contrast-to-noise ratio), and effective FOV were assessed. For the in vivo study, a preliminary result from abdominal microvascular imaging, where diverging wave excitation was utilized to depict the vasculature, was also presented. In the renal cortex microvessels, the diverging wave imaging yielded a higher signal-to-clutter ratio value than the plane wave imaging, i.e., 6.35 vs. 4.26 dB, due to the wider synthetic field. These studies demonstrated that the proposed ultrafast curved array imaging technique based on diverging wave excitation allowed for an extended FOV with high spatiotemporal resolution.

Natural Shear Wave Imaging in the Human Heart: Normal Values, Feasibility, and Reproducibility

Left ventricular myocardial stiffness could offer superior quantification of cardiac systolic and diastolic function when compared to the current diagnostic tools. Shear wave elastography in combination with acoustic radiation force has been widely proposed to noninvasively assess tissue stiffness. Interestingly, shear waves can also result from intrinsic cardiac mechanical events (e.g., closure of valves) without the need for external excitation. However, it remains unknown whether these natural shear waves always occur, how reproducible they can be detected and what the normal range of shear wave propagation speed is. The present study, therefore, aimed at establishing the feasibility of detecting shear waves created after mitral valve closure (MVC) and aortic valve closure (AVC), the variability of the measurements, and at reporting the normal values of propagation velocity. Hereto, a group of 30 healthy volunteers was scanned with high-frame rate imaging (>1000 Hz) using an experimental ultrasound system transmitting a diverging wave sequence. Tissue Doppler velocity and acceleration were used to create septal color M-modes, on which the shear waves were tracked and their velocities measured. Overall, the methodology was capable of detecting the transient vibrations that spread throughout the intraventricular septum in response to the closure of the cardiac valves in 92% of the recordings. Reference velocities of 3.2±0.6 m/s at MVC and 3.5±0.6 m/s at AVC were obtained. Moreover, in order to show the diagnostic potential of this approach, two patients (one with cardiac amyloidosis and one undergoing a dobutamine stress echocardiography) were scanned with the same protocol and showed markedly higher propagation speeds: the former presented velocities of 6.6 and 5.6 m/s; the latter revealed normal propagation velocities at baseline, and largely increased during the dobutamine infusion (>15 m/s). Both cases showed values consistent with the expected changes in stiffness and cardiac loading conditions.

Ultrasonic sector imaging using plane wave synthetic focusing with a convex array transducer

Synthetic transmit focusing (STF) methods using unfocused waves or plane waves (PWs) have recently been investigated extensively. However, STF using PWs with a convex array (PWSTF-CA) has not been rigorously studied for high-resolution sector imaging. In this paper, the authors suggest an analytical model for accurate characterization of the spatial beam patterns of PWSTF-CA using a large range of either uniformly or non-uniformly distributed PW angles. On the basis of the model, a frame-based PWSTF-CA approach with non-uniform PW angles is suggested to achieve superior image quality at a higher frame rate than conventional transmit focusing (CTF). The analytical model can also be used for optimal selection of a set of PW angles to scan the entire sectorial field of view and its subsets employed for STF at each imaging point. The authors also investigate how to select transmit subarrays for each of the PWs to obtain the best spatial resolution. A theoretical analysis and simulations are conducted for the verification of the analytical model and the optimal utilization strategy of PWSTF-CA. The results indicate that the PWSTF-CA improves not only the frame rate but also the contrast, signal-to-noise ratio, and resolution compared with the CTF, as in the case of PWSTF with linear arrays.

Methods for Grating Lobe Suppression in Ultrasound Plane Wave Imaging

Plane wave imaging has been proven to provide transmit beams with a narrow and uniform beam width throughout the imaging depth. The transmit beam pattern, however, exhibits strong grating lobes that have to be suppressed by a tightly focused receive beam pattern. In this paper, we present the conditions of grating lobe occurrence by analyzing the synthetic transmit beam pattern. Based on the analysis, the threshold of the angle interval is presented to completely eliminate grating lobe problems when using uniformly distributed plane wave angles. However, this threshold requires a very small angle interval (or, equivalently, too many angles). We propose the use of non-uniform plane wave angles to disperse the grating lobes in the spatial domain. In this paper, we present an approach using two uniform angle sets with different intervals to generate a non-uniform angle set. The proposed methods were verified by continuous-wave transmit beam patterns and broad-band 2D point spread functions obtained by computer simulations.

Diverging wave compounding with spatio-temporal encoding using orthogonal Golay pairs for high frame rate imaging

Golay coded excitation for diverging wave compounding (DWC) has been demonstrated to increase the signal-to-noise ratio (SNR) and contrast for high frame rate cardiac imaging. However, the complementary codes need to be transmitted in two consecutive firings for decoding, which reduces the frame rate by 2 folds. This paper proposes an orthogonal Golay pairs coded (OGPs-coded) DWC sequence to overcome this problem, which implements spatio-temporal encoding for DWC. Two diverging waves (DWs) at different steering angles coded by an orthogonal Golay pair are transmitted simultaneously, thus compensating the frame rate reduction caused by transmissions of complementary codes. The two DWs can be separated based on the orthogonality of Golay pairs. To test the feasibility of the proposed sequence, we performed simulations of point targets and tissue phantoms in both static and moving states. Compared with non-coded DWC at the same frame rate, OGPs-coded DWC obtains comparable resolution, SNR gains of 7.5-10 dB and contrast gains of 3-5 dB. The OGPs-coded DWC sequence was also tested experimentally on a tissue-mimicking phantom. Compared with non-coded DWC, OGPs-coded DWC achieves improvements in the SNR (3-6 dB) and contrast (1-2 dB). Preliminary in vivo results show brighter myocardium and larger penetration depth with the proposed method. The proposed OGPs-coded DWC sequence has potential for high frame rate and high quality cardiac imaging.

Optimization of Transmit Parameters in Cardiac Strain Imaging With Full and Partial Aperture Coherent Compounding

Coherent compounding methods using the full or partial transmit aperture have been investigated as a possible means of increasing strain measurement accuracy in cardiac strain imaging; however, the optimal transmit parameters in either compounding approach have yet to be determined. The relationship between strain estimation accuracy and transmit parameters-specifically the subaperture, angular aperture, tilt angle, number of virtual sources, and frame rate-in partial aperture (subaperture compounding) and full aperture (steered compounding) fundamental mode cardiac imaging was thus investigated and compared. Field II simulation of a 3-D cylindrical annulus undergoing deformation and twist was developed to evaluate accuracy of 2-D strain estimation in cross-sectional views. The tradeoff between frame rate and number of virtual sources was then investigated via transthoracic imaging in the parasternal short-axis view of five healthy human subjects, using the strain filter to quantify estimation precision. Finally, the optimized subaperture compounding sequence (25-element subperture, 90° angular aperture, 10 virtual sources, 300-Hz frame rate) was compared to the optimized steered compounding sequence (60° angular aperture, 15° tilt, 10 virtual sources, 300-Hz frame rate) via transthoracic imaging of five healthy subjects. Both approaches were determined to estimate cumulative radial strain with statistically equivalent precision (subaperture compounding E(SNRe %) = 3.56, and steered compounding E(SNRe %) = 4.26).