Elevational Directions Research Articles

Distortion map and correction for ultrasound localization microscopy imaging with a row–column array

The row–column array is an emerging technology which can enable large fields of view without the need for a large number of electronic channels. In ultrasound localization microscopy, sparse microbubbles are localized and tracked to produce high-resolution images of the microvasculature. During the localization of the microbubbles, it is typically assumed that the center of the target observed can be used as a substitute for the true microbubble location. However, the true microbubble will be located at the onset of the received signal. This distortion between the true location and the observed location will result in a shift of the microbubble away from the true location, thus altering the accuracy of the images produced. In most forms of ultrasound imaging, this shift mostly manifests as a vertical shift so can often be safely ignored. However, we have observed that in row–column array imaging this distortion will be spatially variable and will result in distortions in the axial, elevational, and lateral directions, which can be on the order of a few wavelengths. Consequently, this distortion, if ignored, will result in inaccuracies in the resulting ultrasound localization microscopy images. Here, we present a technique to map and correct for these distortions.

A 1.5-D Array for Acoustic Radiation Force (ARF)-Induced Peak Displacement-Based Tissue Anisotropy Assessment With a Row-Column Excitation Method.

Many biological tissues, including muscle or kidney, are mechanically anisotropic, and the degree of anisotropy (DoA) in mechanical properties is diagnostically relevant. DoA can be assessed either using the ratio of shear wave velocities (SWVs) or acoustic radio forced impulse (ARFI)-induced peak displacements (PD) measured longitudinal over transverse orientations. Whether using SWV or PD as a basis, DoA expressed as the ratio of values requires 90° transducer rotation when a linear array is employed. This large rotation angle is prone to misalignment errors. One solution is the use of a fully sampled matrix array for electronic rotation of point spread function (PSF). However, the challenges of matrix array are its high fabrication cost and complicated fabrication procedures. The cheaper and simpler alternative of matrix array is the use of a row-column array. A 3×64 elements 1.5-D array with a row-column excitation mode is proposed to assess DoA in mechanical properties using the PD ratio. Different numbers of elements in elevational and lateral directions were selected to have orthogonal ARFI excitation beams without rotating the transducer. A custom-designed flex circuit was used to fabricate the array with a simpler electrode connection than a fully sampled matrix array. The performance of the array was evaluated in Field II simulation and experiment. The output pressure was 0.57-MPa output under a 40- [Formula: see text] excitation with a -6-dB point spread dimension of 14×4 mm2 in orthogonal directions. The PD was measured to be [Formula: see text] in an isotropic elastic phantom with Young's modulus of 5.4 kPa. These results suggest that the array is capable of assessing DoA using PD ratio without physical rotation of the transducer. The array has the potential to reduce the misalignment errors for DoA assessment.

Characterization and registration of 3D ultrasound for use in permanent breast seed implant brachytherapy treatment planning

PurposePermanent breast seed implant (PBSI) brachytherapy is a novel technique for early-stage breast cancer. Computed tomography (CT) images are used for treatment planning and freehand 2D ultrasound for implant guidance. The multimodality imaging approach leads to discrepancies in target identification. To address this, a prototype 3D ultrasound (3DUS) system was recently developed for PBSI. In this study, we characterize the 3DUS system performance, establish QA baselines, and develop and test a method to register 3DUS images to CT images for PBSI planning. Methods and Materials3DUS system performance was characterized by testing distance and volume measurement accuracy, and needle template alignment accuracy. 3DUS-CT registration was achieved through point-based registration using a 3D-printed model designed and constructed to provide visible landmarks on both images and tested on an in-house made gel breast phantom. ResultsThe 3DUS system mean distance measurement accuracy was within 1% in axial, lateral, and elevational directions. A volumetric error of 3% was observed. The mean needle template alignment error was 1.0° ± 0.3 ° and 1.3 ± 0.5 mm. The mean 3DUS-CT registration error was within 3 mm when imaging at the breast centre or across all breast quadrants. ConclusionsThis study provided baseline data to characterize the performance of a prototype 3DUS system for PBSI planning and developed and tested a method to obtain accurate 3DUS-CT image registration for PBSI planning. Future work will focus on system validation and characterization in a clinical context as well as the assessment of impact on treatment plans.

Improved Acoustic Radiation Force Impulse Imaging Using Split-Focused Ultrasound Transducer With Phase Inversion Technique

Echo decorrelation artifacts decrease the resolution of the acoustic radiation force impulse (ARFI) images. Reducing echo decorrelation requires that the lateral and the elevational beamwidths of the pushing beam are broad compared to the detection beam. In this study, we propose the split-focused ultrasound based on phase inversion technique to achieve this goal. The proposed technique can be implemented with a cross-segmented transducer composed of four rectangular elements driven by signals having different phases. Four independent focal lobes were generated in the lateral and elevational directions, extending the beamwidths in both directions. A finite element analysis (FEA) simulation was conducted to verify the performance of the proposed technique, and an experimental demonstration was performed using the fabricated prototype transducer and a tissue-mimicking gelatin phantom. Compared to the conventional model, the proposed method reduced the jitter by up to 4.7 times at the peak mean displacement, improving the accuracy of the measured displacement, and increased the peak contrast-to-noise ratio by 2.1 times. Thus, the proposed technique can contribute to improve the resolution of the ARFI imaging.

Design of a Volumetric Imaging Sequence Using a Vantage-256 Ultrasound Research Platform Multiplexed With a 1024-Element Fully Sampled Matrix Array.

Ultrasound imaging using a matrix array allows real-time multi-planar volumetric imaging. To enhance image quality, the matrix array should provide fast volumetric ultrasound imaging with spatially consistent focusing in the lateral and elevational directions. However, because of the significantly increased data size, dealing with massive and continuous data acquisition is a significant challenge. We have designed an imaging acquisition sequence that handles volumetric data efficiently using a single 256-channel Verasonics ultrasound research platform multiplexed with a 1024-element matrix array. The developed sequence has been applied for building an ultrasonic pupilometer. Our results demonstrate the capability of the developed approach for structural visualization of an ex vivo porcine eye and the temporal response of the modeled eye pupil with moving iris at the volume rate of 30 Hz. Our study provides a fundamental ground for researchers to establish their own volumetric ultrasound imaging platform and could stimulate the development of new volumetric ultrasound approaches and applications.

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.

Design and Fabrication of a Miniaturized Convex Array for Combined Ultrasound and Photoacoustic Imaging of the Prostate.

Although transrectal ultrasound (TRUS) imaging is widely used for screening and diagnosing prostate cancer, it is often not found on TRUS images, depending on its stage, size, and location. In addition, due to the weak echo signal and the low contrast of TRUS images, it is difficult to diagnose early-stage prostate cancers and distinguish malignant tumors from benign prostatic hyperplasia. For this reason, TRUS image-guided biopsy is mandatory to confirm the malignancy of the suspicious tumor, but the diagnostic accuracy of initial biopsy is only 20%-30%, so that the patients inevitably undergo repeated biopsies. TRUS-photoacoustic (TRUS-PA) imaging is one way to resolve those problems. However, the development of a TRUS-PA probe, in which an ultrasound array transducer and optical fibers are integrated, is demanding because the overall size of the probe should be as small as possible for the convenience of the patients, while providing the desired performances. Here, we report a recently developed TRUS-PA probe. The core element of the TRUS-PA is a miniaturized 128-element, 7-MHz convex array transducer of which size in the lateral and elevational directions is 11.4 and 5 mm, respectively. A new concept of a flexible printed circuit board was also developed to limit the size of the TRUS-PA probe to less than 15 mm. From the performance evaluation, it was found that the developed array with a field-of-view of 134° has a center frequency of 6.75 MHz, a -6-dB fractional bandwidth of 66%, and a crosstalk of less than -45 dB. In the tissue-mimicking phantom test and ex vivo experiments, the miniaturized convex array proved to be capable of providing combined US and PA images with acceptable imaging quality in spite of its small size.

Lateral ultrasound strain imaging using subband processing

Since most biological tissues are nearly incompressible, the axial compression leads to expansion in the lateral and elevational directions. Although axial strain is the main component of three dimensional strain field, estimation of the lateral strain may provide important additional information on the tissue mechanical properties. In this paper, we employed the idea and principle of image compounding and proposed a subband processing method to estimate lateral strain. To keep lateral radio freqency (RF) signal bandwidth and strain resolution, we split axial RF signal into several subband signals and then estimate lateral strains of these subband signals along lateral direction, finally average these strains to get a lateral compounded strain image. The simulation results demonstrate that the elastographic signal-to-noise ratio of the lateral compounded strain image is improved by 48% using this subband processing method, compared with the conventional method.

Exploring potential mechanisms responsible for observed changes of ultrasonic backscattered energy with temperature variations.

Previous studies have provided the observation that the ultrasonic backscattered energy from a tissue region will change due to a change of temperature. The mechanism responsible for the changes in backscattered energy (CBE) with temperature has been hypothesized to be from the changes in scattering properties of local aqueous and lipid scatterers. An alternative mechanism is hypothesized here to be capable of producing similar CBE curves, i.e., changes in speckle resulting from changes in summation of scattered wavelets. Both simulations and experiments were conducted with a 5.5 MHz, 128-element linear array and synthetic and physical phantoms containing randomly spaced scatterers. The speckle pattern resulting from summation of scattered wavelets was changed in simulations and experiments by directly increasing the background sound speed from 1520 to 1540 m/s, and changing the temperature from 37 °C to 48 °C, respectively. Shifts in the backscattered signal were compensated using 2D cross-correlation techniques. Excellent agreement between simulations and experiments was observed, with each pixel in the CBE images on average undergoing either a monotonic increase (up to 3.2 dB) or a monotonic decrease (down to -1.9 dB) with increasing sound speed or temperature. Similar CBE curves were also produced by shifting the image plane in the elevational and axial directions even after correcting for apparent motion. CBE curves were produced by changing the sound speed or temperature in tissue mimicking phantoms or by shifting the image plane in the elevational and axial directions and the production of these CBE curves did not require the presence of lipid and aqueous scatterers.

The effect of object speed and direction on the performance of 3D speckle tracking using a 3D swept-volume ultrasound probe

Three-dimensional (3D) soft tissue tracking using 3D ultrasound is of interest for monitoring organ motion during therapy. Previously we demonstrated feature tracking of respiration-induced liver motion in vivo using a 3D swept-volume ultrasound probe. The aim of this study was to investigate how object speed affects the accuracy of tracking ultrasonic speckle in the absence of any structural information, which mimics the situation in homogenous tissue for motion in the azimuthal and elevational directions. For object motion prograde and retrograde to the sweep direction of the transducer, the spatial sampling frequency increases or decreases with object speed, respectively. We examined the effect object motion direction of the transducer on tracking accuracy. We imaged a homogenous ultrasound speckle phantom whilst moving the probe with linear motion at a speed of 0–35 mm s−1. Tracking accuracy and precision were investigated as a function of speed, depth and direction of motion for fixed displacements of 2 and 4 mm. For the azimuthal direction, accuracy was better than 0.1 and 0.15 mm for displacements of 2 and 4 mm, respectively. For a 2 mm displacement in the elevational direction, accuracy was better than 0.5 mm for most speeds. For 4 mm elevational displacement with retrograde motion, accuracy and precision reduced with speed and tracking failure was observed at speeds of greater than 14 mm s−1. Tracking failure was attributed to speckle de-correlation as a result of decreasing spatial sampling frequency with increasing speed of retrograde motion. For prograde motion, tracking failure was not observed. For inter-volume displacements greater than 2 mm, only prograde motion should be tracked which will decrease temporal resolution by a factor of 2. Tracking errors of the order of 0.5 mm for prograde motion in the elevational direction indicates that using the swept probe technology speckle tracking accuracy is currently too poor to track homogenous tissue over a series of volume images as these errors will accumulate. Improvements could be made through increased spatial sampling in the elevational direction.

WE‐D‐220‐03: The Effect of Object Speed on the Performance of 3D Speckle Tracking Using a 3D Swept‐Volume Probe for the Purpose of Ultrasound‐Guided Radiotherapy

Purpose: This study investigated how object speed affects 3D speckle tracking accuracy and precision in the azimuthal and elevational directions when using a mechanically swept 3D ultrasound probe. Methods: A 3D probe was used to acquire B‐mode volumetric ultrasound images of a homogenous tissue‐mimicking ultrasound phantom. Object motion was simulated by moving the probe in the azimuthal and elevational directions using a translational stage with 3‐axis of motion whilst volumetric images were continuously acquired. To form volumetric images the transducer was mechanically swept back and forth and therefore object motion was prograde or retrograde to the transducerˈs sweep motion. Displacement between volumes was estimated using 3D correlation‐based speckle tracking of a region of interest (ROI) located at various positions within the phantom. We investigated tracking accuracy and precision for speed of 0 to 35 mm s‐1 for fixed displacements of 2mm and 4mm, for prograde and retrograde motion and as a function of depth. Results: For the azimuthal direction, both accuracy and precision were better than 0.1mm and 0.15mm for 2mm and 4mm displacements respectively and no significant correlation was found between accuracy and speed and between accuracy and depth. For the elevational direction and 2mm displacement, accuracy was 0.25mm and 0.3mm for prograde and retrograde motion respectively. Accuracy and precision were greatest at the elevational focus (60mm). For a 4mm elevational displacement with retrograde motion, accuracy and precision reduced with speed and tracking failure was observed at speeds > 14mms‐1, this was not observed for prograde motion. Conclusion: For mechanically swept 3D probes tracking in the elevational direction is poor for retrograde motion at speeds of greater than 14mms‐1. Thus for inter‐volume displacements greater than 2mm only prograde motion should be tracked which will decrease temporal resolution by a factor of 2.

A broadband Fabry–Pérot cavity antenna designed using an improved resonance prediction method

AbstractWe have proposed a wideband Fabry–Pérot cavity (FPC) base station antenna for wireless broadband Internet (WiBro) service in Korea. To widen the radiation bandwidth of our antenna, unit cells printed on a superstrate have been gradually tapered according to a proposed linear tapering equation. Also, all openings in the lateral directions have been enclosed using four metallic sidewalls to enhance the overall performance of the antenna by reducing unnecessary wave leakage. Furthermore, the radiation aperture has been designed in a rectangular shape to create sharp and sectoral beam shapes in the elevational and azimuthal directions, respectively. For a more accurate determination of the FPC resonance, an improved prediction formula has been used. We have demonstrated the basic principle of bandwidth widening using higher mode behavior, which appears at a higher frequency region by means of tapered cells on the superstrate. In addition, to show the effectiveness of simple tapering of unit cells, we have compared the performance of two antennas covered with the tapered and uniform (nontapered) superstrate. Tapering the unit cells has expanded the fractional radiation bandwidth about four times wider than that of the antenna using uniform unit cells. The measured behaviors of our antenna are in good agreement with the simulated data, which confirms the validity of our design and analysis approach. The measured behaviors of our antenna are in good agreement with the simulated data, which confirms the validity of our design and analysis approach. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1065–1069, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25898

Ultrasound Color Flow Imaging At High Frame Rates

A method is provided to improve the frame-rate in color-flow ultrasound imaging using simultaneous spatially-distinct transmit beams with one or more frequency bands per transmit beam. Pulses of different center frequencies are used simultaneously in different (lateral and/or elevational) directions, thereby reducing the scanning time and improving the frame-rates. Optionally, a multi-modal pulse is used, and flow is estimated separately for the different frequencies. The flow estimates for these pulses are appropriately combined to improve low-velocity sensitivity and to reduce aliasing. A flow sample count with two or more different pulse repetition intervals can be used to further improve low-flow sensitivity and minimize aliasing.

Sub-sample displacement estimation from digitized ultrasound RF signals using multi-dimensional polynomial fitting of the cross-correlation function

A widely used time-domain technique for motion or delay estimation between digitized ultrasound RF signals involves the maximization of a discrete pattern-matching function, usually the cross-correlation. To achieve sub-sample accuracy, the discrete pattern-matching function is interpolated using the values at the discrete maximizer and adjacent samples. In prior work, only 1-D fit, applied separately along the axial, lateral, and elevational axes, has been used to estimate the sub-sample motion in 1-D, 2-D, and 3-D. In this paper, we explore the use of 2-D and 3-D polynomial fitting for this purpose. We quantify the estimation error in noise-free simulations using Field II and experiments with a commercial ultrasound machine. In simulated 2-D translational motions, function fitting with quartic spline polynomials leads to maximum bias of 0.2% of the sample spacing in the axial direction and 0.4% of the sample spacing in the lateral direction, corresponding to 38 nm and 1.31 μm, respectively. The maximum standard deviations were approximately 1% of the sample spacing in both the axial and the lateral directions, corresponding to 193 nm axially and 4.43 μm laterally. In simulated 1% axial strain, the same function fitting leads to mean absolute displacement estimation errors of 255 nm in the axial direction and 4.77 μm in the lateral direction. In experiments with a linear array transducer, 2-D quartic spline fitting leads to maximum bias of 458 nm and 6.27 μm in the axial and the lateral directions, respectively. These results are more than one order of magnitude smaller than those obtained with separate 1-D fit when applied to the same data set. Simulations and experiments in 3-D yield similar results when comparing 3-D polynomial fitting with 1-D fitting along the axial, lateral, and elevational directions.

Principal component analysis of shear strain effects

Shear stresses are always present during quasi-static strain imaging, since tissue slippage occurs along the lateral and elevational directions during an axial deformation. Shear stress components along the axial deformation axes add to the axial deformation while perpendicular components introduce both lateral and elevational rigid motion and deformation artifacts into the estimated axial and lateral strain tensor images. A clear understanding of these artifacts introduced into the normal and shear strain tensor images with shear deformations is essential. In addition, signal processing techniques for improved depiction of the strain distribution is required. In this paper, we evaluate the impact of artifacts introduced due to lateral shear deformations on the normal strain tensors estimated by varying the lateral shear angle during an axial deformation. Shear strains are quantified using the lateral shear angle during the applied deformation. Simulation and experimental validation using uniformly elastic and single inclusion phantoms were performed. Variations in the elastographic signal-to-noise and contrast-to-noise ratios for axial deformations ranging from 0% to 5%, and for lateral deformations ranging from 0 to 5° were evaluated. Our results demonstrate that the first and second principal component strain images provide higher signal-to-noise ratios of 20 dB with simulations and 10 dB under experimental conditions and contrast-to-noise ratio levels that are at least 20 dB higher when compared to the axial and lateral strain tensor images, when only lateral shear deformations are applied. For small axial deformations, the lateral shear deformations significantly reduces strain image quality, however the first principal component provides about a 1–2 dB improvement over the axial strain tensor image. Lateral shear deformations also significantly increase the noise level in the axial and lateral strain tensor images with larger axial deformations. Improved elastographic signal and contrast-to-noise ratios in the first principal component strain image are always obtained for both simulation and experimental data when compared to the corresponding axial strain tensor images in the presence of both axial and lateral shear deformations.

Filtering and processing of panoramic images obtained using a camera and a wide-angle-imaging reflective surface

There is an increasing interest in wide-angle imaging of the environment using curved reflective surfaces. With this comes the need for appropriate filtering and processing of the acquired images. Here we present a technique for homogeneous, fast filtering of panoramic images captured using a camera and a wide-angle-imaging reflective surface. Imaging of the panoramic environment onto a two-dimensional (2-D) plane necessarily introduces spatial distortions such as stretching and bending that vary with the viewing direction. Therefore, if the panoramic image is to be filtered homogeneously in all viewing directions, it is necessary to match the filtering to the distortions. We show how this can be accomplished. The image acquired by the camera is first digitally unwarped and represented in Cartesian coordinates representing azimuth and elevation. The mappings of patches of uniform size and shape on the viewsphere are then established. Next, for each filter patch the local mappings of great circles along two principal axes (along the local longitudinal and elevational directions) on the image plane are determined. The mappings of these great circles are used to perform the 2-D convolution required by the filtering operation. Convolution along the directions of local, mutually orthogonal great circles permits the filtering to be carried out in a quasi-separable fashion, resulting in increased computational speed and efficiency. Examples of homogeneous filtering using this procedure are presented.

A computational model of depth perception based on headcentric disparity.

It is now well established that depth is coded by local horizontal disparity and global vertical disparity. We present a computational model which explains how depth is extracted from these two types of disparities. The model uses the two (one for each eye) headcentric directions of binocular targets, derived from retinal signals and oculomotor signals. Headcentric disparity is defined as the difference between headcentric directions of corresponding features in the left and right eye's images. Using Helmholtz's coordinate systems we decompose headcentric disparity into azimuthal and elevational disparity. Elevational disparities of real objects are zero if the signals which contribute to headcentric disparity do not contain any errors. Azimuthal headcentric disparity is a 1D quantity from which an exact equation relating distance and disparity can be derived. The equation is valid for all headcentric directions and for all binocular fixation positions. Such an equation does not exist if disparity is expressed in retinal coordinates. Possible types of errors in oculomotor signals (six) produce global elevational disparity fields which are characterised by different gradients in the azimuthal and elevational directions. Computations show that the elevational disparity fields uniquely characterise both the type and size of the errors in oculomotor signals. Our model uses a measure of the global elevational disparity field together with local azimuthal disparity to accurately derive headcentric distance throughout the visual field. The model explains existing data on whole-field disparity transformations as well as hitherto unexplained aspects of stereoscopic depth perception.