Harmonic leakage and image quality degradation in tissue harmonic imaging

Image quality degradation caused by harmonic leakage was studied for finite amplitude distortion-based harmonic imaging. Various sources of harmonic leakage, including transmit waveform, signal bandwidth, and system nonlinearity, were investigated using both simulations and hydrophone measurements. Effects of harmonic leakage in the presence of sound velocity inhomogeneities were also considered. Results indicated that sidelobe levels of the harmonic beam pattern were directly affected by harmonic leakage when the harmonic signal was obtained by filtering out the fundamental signal. Because sidelobe levels also increase with the bandwidth of the transmitted signal, a trade-off exists between axial resolution and contrast resolution. It is concluded that accurate control of the frequency content of the waveform prior to propagation is necessary to optimize imaging performance of tissue harmonic imaging. The filtering technique was also compared with the pulse inversion technique. It was shown that the pulse inversion technique effectively suppresses harmonic leakage at the cost of imaging frame rate and potential motion artifacts     

Pulse-inversion-based fundamental imaging for contrast detection

Pulse-inversion-based fundamental imaging was experimentally investigated for the enhancement of contrast detection. The pulse-inversion technique involves two firings with inverted waveforms. When the returning echoes from the two firings are summed, the residue signal is limited to even-order harmonics for tissue. However, when the returning echoes are from microbubbles, the fundamental signal is not completely cancelled because the reaction of the bubbles under compression is different from that under rarefaction. Thus, with the application of pulse-inversion technique, the fundamental signal can be used to enhance the contrast-to-tissue ratio. In this paper, B-mode, pulse-inversion-based fundamental images were constructed with various transmit waveforms. Motion artifacts also were studied. The results indicate that the contrast-to-tissue ratio was significantly enhanced compared to that obtained using either conventional, fundamental imaging or second-harmonic imaging. Longer transmit pulses resulted in a better signal-to-noise ratio, but did not noticeably affect the nonlinear response of the bubbles. In addition, the optimal ratio of the magnitude of the positive pulse to that of the negative pulse was unity, in terms of avoiding the uncancelled, third-order response in the fundamental frequency range. It also was found that the pulse-inversion fundamental technique is highly sensitive to tissue motion because the fundamental tissue signal is not cancelled when motion is present.     

Pulse inversion sequences for mechanically scanned transducers

Mechanically scanned transducers are currently used for tissue harmonic imaging (THI) and nonlinear microbubble imaging at high frequencies. The pulse inversion (PI) technique is widely used for suppressing the fundamental signal, but its effectiveness is reduced by relative tissue/ transducer motion. In this paper, we investigate multipulse inversion (MPI) sequences that achieve a significant improvement on the fundamental suppression for mechanically scanned single-element transducers. MPI was subsequently applied on simulated and measured RF-data and relative fundamental suppression was compared with the 2-pulse PI technique. Simulations showed, for example, an increased fundamental suppression of 6 and 10 dB for MPI-sequences that combined 3 and 7 pulses, respectively, for a rotating intravascular ultrasound transducer with an interpulse angle of 0.15deg. Initial application of MPI sequences on RF-data from in vivo acquisitions resulted in similar fundamental suppression levels. The investigated MPI technique will help to reduce relative tissue/transducer motion effects and might lead to improved sensitivity and spatial resolution in nonlinear tissue imaging and improved microbubble detection in contrast imaging for mechanically scanned transducers.     

Comparison of pulse subtraction Doppler and pulse inversion Doppler

In this paper, a new Doppler technique based on pulse subtraction imaging (PSD) is described and compared with pulse inversion Doppler (PID). Combining a nonlinear contrast agent imaging technique with a Doppler process provides a tool for detecting motion of both contrast agents and tissues. This has potential in targeted imaging in which attached microbubbles need to be separated from moving ones and surrounding tissues. The results from both simulation and experiment show that PSD is able to differentiate bubble motion from tissue motion. For Doppler processing conducted at the fundamental frequency, the contrast-to-tissue ratio (CTR) in PSD was 3.3 (±0.4) times higher on average than PID at a mechanical index (MI) of 0.1. At the harmonic frequency, PID was shown to have a 3.1 (±0.4) times higher CTR than PSD. Overall, taken in their optimum processing conditions, PID has a CTR up to 1.9 (±0.4) times higher than PSD. The CTRs for both techniques have also been shown to increase with increasing MI. However, for the same axial Doppler resolution. PSD also allows less energy to be transmitted into the medium, which makes it less disruptive. The relative performances of PSD and PID in terms of the bandwidth of the imaging system are also discussed.     

Pulse inversion Doppler: a new method for detecting nonlinear echoes from microbubble contrast agents

A novel technique for the selective detection of ultrasound contrast agents, called pulse inversion Doppler, has been developed. In this technique, a conventional Doppler or color Doppler pulse sequence is modified by inverting every second transmit pulse. Either conventional or harmonic Doppler processing is then performed on the received echoes. In the resulting Doppler spectra, Doppler shifts from linear and nonlinear scattering are separated into two distinct regions that can be analyzed separately or combined to estimate the ratio of nonlinear to linear scattering from a region of tissue. The maximum Doppler shift that can be detected is 1/2 the normal Nyquist limit. This has the advantage over conventional harmonic Doppler that it can function over the entire bandwidth of the echo signal, thus achieving superior spatial resolution in the Doppler image. In vitro measurements comparing flowing agent and cellulose particles suggest that pulse inversion Doppler can provide 3 to 10 dB more agent to tissue contrast than harmonic imaging with similar pulses. Similar measurements suggest that broadband pulse inversion Doppler can provide up to 16 dB more contrast than broadband conventional Doppler. Nonlinear propagation effects limit the maximum contrast obtainable with both harmonic and pulse inversion Doppler techniques.     

Strain compounding: a new approach for speckle reduction

A new compounding technique for reducing speckle brightness variations is proposed. This method exploits the decorrelation between signals under different strain states. The different strain states can be created using externally applied forces such as the ones used in sonoelastography. Such forces produce three-dimensional tissue motion. By correcting only the in-plane (i.e., axial and lateral) motion, the images under different strain states have similar characteristics except for speckle appearance caused by the uncorrected out-of-plane (i.e., elevational) motion. Additional speckle decorrelation is also introduced through tissue motion correction caused by the change of effective in-plane sample volume geometry. Therefore, these images can be combined for speckle reduction with less degradation in in-plane spatial resolution than conventional approaches. In this paper, three-dimensional tissue motion under various strain conditions were simulated. It was found that significant speckle decorrelation existed at strains achievable in some clinical situations. Experiments were also conducted to test efficacy of this approach. Pulse-echo data from a gelatin-based phantom were acquired using a 5-MHz, single crystal transducer, and both conventional and compound B-mode images were formed. Results indicated that speckle brightness variations were reduced, and detectability of low contrast objects was enhanced. Performance limitations and fundamental differences between the proposed technique and existing techniques are discussed     

Vibration analysis of thermoelastic double porous microbeam subjected to laser pulse

The present work deals with vibration phenomenon of a homogeneous, isotropic thermoelastic double porous microbeam induced by laser pulse heating, in context of Lord–Shulman theory of thermoelasticity with one relaxation time. Laplace transform technique has been used to obtain the expressions for lateral deflection, axial stress, axial displacement, volume fraction field and temperature distribution. The resulting quantities are obtained in the physical domain by applying a numerical inversion technique. Variations of axial displacement, axial stress, lateral deflection, volume fraction field and temperature distribution with axial distance are depicted graphically to show the effects of porosity and laser intensity parameter. Some particular cases are also deduced.     

Optical tomographic image reconstruction from ultrafast time-sliced transmission measurements

Optical imaging and localization of objects inside a highly scattering medium, such as a tumor in the breast, is a challenging problem with many practical applications. Conventional imaging methods generally provide only two-dimensional (2-D) images of limited spatial resolution with little diagnostic ability. Here we present an inversion algorithm that uses time-resolved transillumination measurements in the form of a sequence of picosecond-duration intensity patterns of transmitted ultrashort light pulses to reconstruct three-dimensional (3-D) images of an absorbing object located inside a slab of a highly scattering medium. The experimental arrangement used a 3-mm-diameter collimated beam of 800-nm, 150-fs, 1-kHz repetition rate light pulses from a Ti:sapphire laser and amplifier system to illuminate one side of the slab sample. An ultrafast gated intensified camera system that provides a minimum FWHM gate width of 80 ps recorded the 2-D intensity patterns of the light transmitted through the opposite side of the slab. The gate position was varied in steps of 100 ps over a 5-ns range to obtain a sequence of 2-D transmitted light intensity patterns of both less-scattered and multiple-scattered light for image reconstruction. The inversion algorithm is based on the diffusion approximation of the radiative transfer theory for photon transport in a turbid medium. It uses a Green s function perturbative approach under the Rytov approximation and combines a 2-D matrix inversion with a one-dimensional Fourier-transform inversion to achieve speedy 3-D image reconstruction. In addition to the lateral position, the method provides information about the axial position of the object as well, whereas the 2-D reconstruction methods yield only lateral position.     

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.     

Three-dimensional tissue motion and its effect on image noise inelastography

In elastography both high correlation coefficient between pre- and post-compression RF signals and high applied strain are required to achieve the best quality in elastograms. Because the elastogram is computed using a 1-D cross-correlation technique applied to a 1-D ultrasound signal, it is assumed that tissue motion occurs only within the axis of compression (axis of the acoustic wave propagation), or at least that the scatterers remain within the acoustic beam during tissue motion. In practice, soft tissues are incompressible and, therefore, the lateral and elevational (out-of-plane) tissue strains are 50% of the applied strain. Therefore, tissue scatterers may move across the beam due to the applied compression. In this paper we address the degradation of the elastographic quality due to the lateral and elevational motion of the scatterers in uniformly elastic media. A full 3-D model predicting the correlation coefficient as measured using 1-D cross-correlations is proposed. It is shown that the signal-to-noise ratio in elastograms (SNR_e) is nonstationary, and that it depends on the beamwidth and on the applied strain. In order to achieve a higher stationary SNR_e, it is proposed to confine the tissue in the lateral direction. Phantom experiments are used to corroborate the theoretical developments     

A fundamental limit on the performance of correlation based phasecorrection and flow estimation techniques

Cross correlation and similar operations are used in ultrasonic imaging to estimate blood or soft tissue motion in one or more dimensions and to measure echo arrival time differences for phase aberration correction. These estimates are subject to large errors known as false peaks and smaller magnitude errors known as jitter. While false peaks can sometimes be removed through nonlinear processing, jitter errors place a fundamental limit on the performance of delay estimation techniques. This paper applies the Cramer-Rao Lower Bound to derive analytical expressions which predict the magnitude of jitter for 1-D and 2-D problems using both radio frequency (RF) and envelope detected data. One-dimensional simulation results are presented which closely match theoretical predictions. These results indicate that for typical clinical conditions axial jitter for detected data is approximately five times greater than that for RF data. Lateral jitter is approximately ten times greater than axial jitter for RF data. Examples are presented which utilize these results to predict the performance of phase aberration correction and flow estimation systems     

Improving IVUS palpography by incorporation of motion compensation based on block matching and optical flow

Intravascular ultrasound (IVUS) strain imaging of the luminal layer in coronary arteries, coined as IVUS palpography, utilizes conventional radio frequency (RF) signals acquired at 2 different levels of a compressional load. The signals are cross-correlated to obtain the microscopic tissue displacements, which can be directly translated into local strain of the vessel wall. However, (apparent) tissue motion and nonuniform deformation of the vessel wall, due to catheter wiggling, reduce signal correlation and result in invalid strain estimates. Implications of probe motion were studied on the tissue-mimicking phantom. The measured circumferential tissue displacement and level of the speckle decorrelation amounted to 12° and 0.58, respectively, for the catheter displacement of 456 μm. To compensate for the motion artifacts in IVUS palpography, a novel method based on the feature-based scale-space optical flow (OF), and classical block matching (BM) algorithm, were employed. The computed OF vector and BM displacement fields quantify the amount of local tissue misalignment in consecutive frames. Subsequently, the extracted circumferential displacements are used to realign the signals before strain computation. Motion compensation reduces the RF signal decorrelation and increases the number of valid strain estimates. The advantage of applying the motion correction in IVUS palpography was demonstrated in a midscale validation study on 14 in vivo pullbacks. Both methods substantially increase the number of valid strain estimates in the partial and compounded palpograms. Mean relative improvement in the number of valid strain estimates with motion compensation in comparison to one without motion compensation amounts to 28% and 14%, respectively. Implementation of motion compensation methods boosts the diagnostic value of IVUS palpography.     

Numerical sampling rules for paraxial regime pulse diffraction calculations

Sampling rules for numerically calculating ultrashort pulse fields are discussed. Such pulses are not monochromatic but rather have a finite spectral distribution about some central (temporal) frequency. Accordingly, the diffraction pattern for many spectral components must be considered. From a numerical implementation viewpoint, one may ask how many of these spectral components are needed to accurately calculate the pulse field. Using an analytical expression for the Fresnel diffraction from a 1-D slit, we examine this question by varying the number of contributing spectral components. We show how undersampling the spectral profile produces erroneous numerical artifacts (aliasing) in the spatial-temporal domain. A guideline, based on graphical considerations, is proposed that determines appropriate sampling conditions. We show that there is a relationship between this sampling rule and a diffraction wave that emerges from the aperture edge; comparisons are drawn with boundary diffraction waves. Numerical results for 2-D square and circular apertures are presented and discussed, and a potentially time-saving calculation technique that relates pulse distributions in different z planes is described.     

Third harmonic transmit phasing for tissue harmonic generation

Generation of tissue harmonic signals during acoustic propagation is based on the combined effect of two different spectral interactions of the transmit signal. One produces harmonic whose frequency is the sum of transmit frequencies. The other results in harmonic at difference frequency of the transmit signals. Both the frequency-sum component and the frequency-difference component are sensitive to the phase of their constitutive spectral signals. In this study, a novel approach for modifying the amplitude of tissue harmonic signal is proposed based on phasing these two components to achieve either harmonic enhancement or suppression. Both experiments and simulations were performed to investigate the effects of 3f0 transmit phasing on tissue harmonic generation. Results indicate that the relative phasing between the frequency-sum component and the frequency-difference component markedly changes the amplitude of the second harmonic signal. For harmonic enhancement, approximate 6 dB increase of second harmonic amplitude can be achieved while the lateral harmonic beam pattern also is improved as compared to conventional situations in which only the frequency-sum component is considered. For harmonic suppression, the second harmonic signal also could be significantly reduced by about 11 dB when the frequency-difference component is out of phase with the frequency-sum component. Hence, the method of 3f0 transmit phasing has potentials for both improving signal-to-noise ratio in tissue harmonic imaging and enhancing image contrast in contrast-agent imaging by suppression of tissue harmonic background.     

Artifact reduction in photoplethysmography

Motion artifact reduction in photoplethysmography, and therefore by implication in pulse oximetry, is achieved with a novel nonlinear methodology. The physical origins of the photoplethysmographic signals are explored in relation to a nonlinear measure of the observed intensity fluctuations. It is demonstrated that the nonlinearity renormalizes the received pulsations with optical information in a manner that aids physical interpretation. A heuristic physical model for the motion artifact is introduced and experimentally justified, with an inversion for artifact reduction being simplified by the nonlinearity. A practical implementation technique is discussed with emphasis placed on the resultant rescaling of the static and the dynamic portions of the signals. It is noted that this implementation also has the desirable effect of reducing any residual ambient artifact. The scope and power of this methodology is investigated with the presentation of results from a practical electronic system.     

2-D high-frame-rate dynamic elastography using delay compensated and angularly compounded motion vectors: preliminary results

This paper describes a new ultrasound-based system for high-frame-rate measurement of periodic motion in 2-D for tissue elasticity imaging. Similarly to conventional 2-D flow vector imaging, the system acquires the RF signals from the region of interest at multiple steering angles. A custom sector subdivision technique is used to increase the temporal resolution while keeping the total acquisition time within the range suitable for real-time applications. Within each sector, 1-D motion is estimated along the beam direction. The intra- and inter-sector delays are compensated using our recently introduced delay compensation algorithm. In-plane 2-D motion vectors are then reconstructed from these delay-compensated 1-D motions. We show that Young's modulus images can be reconstructed from these 2-D motion vectors using local inversion algorithms. The performance of the system is validated quantitatively using a commercial flow phantom and a commercial elasticity phantom. At the frame rate of 1667 Hz, the estimated flow velocities with the system are in agreement with the velocity measured with a pulsed-wave Doppler imaging mode of a commercial ultrasound machine with manual angle correction. At the frame rate of 1250 Hz, phantom Young's moduli of 29, 6, and 54 kPa for the background, the soft inclusion, and the hard inclusion, are estimated to be 30, 11, and 53 kPa, respectively.     

Acoustic signatures of submicron contrast agents

Previous studies have revealed that hard-shelled submicron contrast agents exhibit large relative expansions and strong acoustical echoes that can be observed experimentally, and predicted by theoretical simulations. In this paper, we study harmonic imaging and pulse-pair imaging techniques designed to assist in the differentiation of these contrast agents from tissue. For harmonic imaging, we apply a high-sensitivity, narrowband strategy that differentiates the microbubble from tissue based on the generation of strong harmonic echoes. For pulse-pair imaging, we apply high spatial resolution, wideband strategies using phase inversion, which relies on the frequency differences observed in response to phase-inverted pulses, and signal subtraction, which takes advantage of the amplitude differences in response to identical pulses. The bubble-to-phantom signal amplitude ratio in the absence of motion approaches 20 dB using phase inversion and 30 dB using signal subtraction; both techniques are robust for up to 50 microm of simulated motion. With the experience gained in these studies, we hope to advance the development of multi-pulse or shaped-pulse techniques that are optimized for specific clinical applications.     

Two-Dimensional Image Transmission Based on the Ultrafast Optical Data Format Conversion between a Temporal Signal and a Two-Dimensional Spatial Signal

We have experimentally demonstrated a two-dimensional (2-D) image transmission based on the ultrafast optical data format conversion between a temporal signal and a spatial signal with an ultrashort optical pulse. In the proposed system we adopt a spectral holography technique to transmit a one-dimensional (1-D) spatial signal and use a spatial-domain time-frequency transform to realize a transform between 1-D and 2-D spatial signals. By use of these techniques, a low-optical-loss transmission system can be constructed. To demonstrate a 2-D image transmission with this technique, we achieved experimentally transmission of the alphabet letter T as a 3 x 3 pixel 2-D spatial image.