Second harmonic inversion for ultrasound contrast harmonic imaging

A Review of Clinical Applications for Super-resolution Ultrasound Localization Microscopy

Microvascular structure and hemodynamics are important indicators for the diagnosis and assessment of many diseases and pathologies. The structural and functional imaging of tissue microvasculature in vivo is a clinically significant objective for the development of many imaging modalities. Contrast-enhanced ultrasound (CEUS) is a popular clinical tool for characterizing tissue microvasculature, due to the moderate cost, wide accessibility, and absence of ionizing radiation of ultrasound. However, in practice, it remains challenging to demonstrate microvasculature using CEUS, due to the resolution limit of conventional ultrasound imaging. In addition, the quantification of tissue perfusion by CEUS remains hindered by high operator-dependency and poor reproducibility. Inspired by super-resolution optical microscopy, super-resolution ultrasound localization microscopy (ULM) was recently developed. ULM uses the same ultrasound contrast agent (i.e. microbubbles) in CEUS. However, different from CEUS, ULM uses the location of the microbubbles to construct images, instead of using the backscattering intensity of microbubbles. Hence, ULM overcomes the classic compromise between imaging resolution and penetration, allowing for the visualization of capillary-scale microvasculature deep within tissues. To date, many in vivo ULM results have been reported, including both animal (kidney, brain, spinal cord, xenografted tumor, and ear) and human studies (prostate, tibialis anterior muscle, and breast cancer tumors). Furthermore, a variety of useful biomarkers have been derived from using ULM for different preclinical and clinical applications. Due to the high spatial resolution and accurate blood flow speed estimation (approximately 1 mm/s to several cm/s), ULM presents as an enticing alternative to CEUS for characterizing tissue microvasculature in vivo. This review summarizes the principles and present applications of CEUS and ULM, and discusses areas where ULM can potentially provide a better alternative to CEUS in clinical practice and areas where ULM may not be a better alternative. The objective of the study is to provide clinicians with an up-to-date review of ULM technology, and a practical guide for implementing ULM in clinical research and practice.

Post-processing radio-frequency signal based on deep learning method for ultrasonic microbubble imaging

Background

Improving imaging quality is a fundamental problem in ultrasound contrast agent imaging (UCAI) research. Plane wave imaging (PWI) has been deemed as a potential method for UCAI due to its’ high frame rate and low mechanical index. High frame rate can improve the temporal resolution of UCAI. Meanwhile, low mechanical index is essential to UCAI since microbubbles can be easily broken under high mechanical index conditions. However, the clinical practice of ultrasound contrast agent plane wave imaging (UCPWI) is still limited by poor imaging quality for lack of transmit focus. The purpose of this study was to propose and validate a new post-processing method that combined with deep learning to improve the imaging quality of UCPWI. The proposed method consists of three stages: (1) first, a deep learning approach based on U-net was trained to differentiate the microbubble and tissue radio frequency (RF) signals; (2) then, to eliminate the remaining tissue RF signals, the bubble approximated wavelet transform (BAWT) combined with maximum eigenvalue threshold was employed. BAWT can enhance the UCA area brightness, and eigenvalue threshold can be set to eliminate the interference areas due to the large difference of maximum eigenvalue between UCA and tissue areas; (3) finally, the accurate microbubble imaging were obtained through eigenspace-based minimum variance (ESBMV).

Results

The proposed method was validated by both phantom and in vivo rabbit experiment results. Compared with UCPWI based on delay and sum (DAS), the imaging contrast-to-tissue ratio (CTR) and contrast-to-noise ratio (CNR) was improved by 21.3 dB and 10.4 dB in the phantom experiment, and the corresponding improvements were 22.3 dB and 42.8 dB in the rabbit experiment.

Conclusions

Our method illustrates superior imaging performance and high reproducibility, and thus is promising in improving the contrast image quality and the clinical value of UCPWI.

High-Order Hadamard-Encoded Transmission for Tissue Background Suppression in Ultrasound Contrast Imaging: Memory Effect and Decoding Schemes

Hadamard-encoded multipulses (HEM) transmit has recently been utilized for tissue background suppression in ultrasound contrast imaging to enhance the contrast-to-tissue ratio (CTR). Nonetheless, the second-harmonic component in HEM transmit results in residual tissue background after decoding and, thus, compromises the detection of contrast microbubbles. Theoretically, high-order HEM transmit can produce harmonic-free background but the memory effect, which considers the nonlinear contribution of previous bit waveform into the next one in the progress of harmonic generation, may limit the achievable tissue suppression. In this paper, three possible harmonic-free pairs using time-shifted subtraction (SH1, SH2, and SH3) in the fourth-order Hadamard decoding are analyzed and experimentally compared using hydrophone measurement and B-mode imaging. Moreover, the orthogonal decoding (OD) of HEM transmit is also proposed with pulse-inversion harmonic suppression (PIHS) to remedy memory effect on the tissue background. Results show that SH3, which utilizes the third and fourth rows for decoding, provides the lowest magnitude of tissue background among all possible decoding pairs and performs comparably to the reference PI and amplitude-modulation sequence in terms of CTR. For PIHS-OD, the pulse subtraction effectively removes the harmonic interferences from memory effect and, thus, further improves the CTR by 5.4 dB compared to SH3. For high-order HEM transmit, PIHS-OD can help to eliminate the residual tissue background due to memory effect and is comparable to Hadamard decoding in temporal resolution and possible motion artifacts.

Nonlinear X-wave ultrasound imaging of acoustic biomolecules

The basic physics of sound waves enables ultrasound to visualize biological tissues with high spatial and temporal resolution. Recently, this capability was enhanced with the development of acoustic biomolecules - proteins with physical properties enabling them to scatter sound. The expression of these unique air-filled proteins, known as gas vesicles (GVs), in cells allows ultrasound to image cellular functions such as gene expression in vivo, providing ultrasound with its analog of optical fluorescent proteins. Acoustical methods for the in vivo detection of GVs are now required to maximize the impact of this technology in biology and medicine. We previously engineered GVs exhibiting a nonlinear scattering behavior in response to acoustic pressures above 300 kPa, and showed that amplitude-modulated (AM) ultrasound pulse sequences that both excite the linear and nonlinear GV scattering regimes were highly effective at distinguishing GVs from linear scatterers like soft biological tissues. Unfortunately, the in vivo specificity of AM ultrasound imaging is systematically compromised by the nonlinearity added by the GVs to propagating waves, resulting in strong image artifacts from linear scatterers downstream of GV inclusions. To address this issue, we present an imaging paradigm, cross-amplitude modulation (xAM), which relies on cross-propagating plane-wave transmissions of finite aperture X-waves to achieve quasi artifact-free in vivo imaging of GVs. The xAM method derives from counter-propagating wave interaction theory which predicts that, in media exhibiting quadratic elastic nonlinearity like biological tissue, the nonlinear interaction of counter-propagating acoustic waves is inefficient. By transmitting cross-propagating plane-waves, we minimize cumulative nonlinear interaction effects due to collinear wave propagation, while generating a transient wave-amplitude modulation at the two plane-waves' intersection. We show in both simulations and experiments that residual xAM nonlinearity due to wave propagation decreases as the plane-wave cross-propagation angle increases. We demonstrate in tissue-mimicking phantoms that imaging artifacts distal to GV inclusions decrease as the plane-wave cross-propagation angle opens, nearing complete extinction at angles above 16.5 degrees. Finally, we demonstrate that xAM enables highly specific in vivo imaging of GVs located in the gastrointestinal tract, a target of prime interest for future cellular imaging. These results advance the physical facet of the emerging field of biomolecular ultrasound, and are also relevant to synthetic ultrasound contrast agents.

A Column-Row-Parallel Ultrasound Imaging Architecture for 3D Plane-wave Imaging and Tx 2nd-Order Harmonic Distortion (HD2) Reduction

We propose a Column-Row-Parallel imaging frontend architecture for integrated and low-power 3D medical ultrasound imaging. The Column-Row-Parallel architecture offers linear-scaling interconnection, acquisition and programming time with row-by-row or column-by-column operations, while supporting volumetric imaging functionality and fault-tolerance against possible transducer element defects with per-element controls. The combination of column-parallel selection logic, row-parallel selection logic, and per-element selection logic reaches a balance between flexible imaging aperture definition and manageable imaging data / control interface to a 2D array. A 16×16 CMUT-ASIC Column-Row-Parallel prototype is fabricated and assembled with a flip-chip bonding process. It facilitates the 3D plane-wave coherent compounding algorithm for volumetric imaging with a fast frame rate of 62.5 Hz and 46% improved lateral resolution with 10-angle compounding and a field of view volume of 2.3mm in both azimuth and elevation, 8.5mm in depth. At a hypothetically scaled up 64x64 array size, the frame rate can still be kept at 31.2 Hz for a volume of 40mm in both azimuth and elevation, 150mm in depth. An interleaved checker board pattern with in-phase (I) and quadrature (Q) excitations is also demonstrated for reducing CMUT second harmonic distortion (HD2) emission by up to 25 dB at the loss of 3 dB fundamental energy reduction. The method reduces nonlinear effects from both transducers and circuits and is a wide band technique that is applicable to arbitrary pulse shapes.

Hadamard-Encoded Multipulses for Contrast-Enhanced Ultrasound Imaging

The development of contrast-enhanced ultrasound (CEUS) imaging offers great opportunities for new ultrasound clinical applications such as myocardial perfusion imaging and abdominal lesion characterization. In CEUS imaging, the contrast agents (i.e., microbubbles) are utilized to improve the contrast between blood and tissue based on their high nonlinearity under low ultrasound pressure. In this paper, we propose a new CEUS pulse sequence by combining Hadamard-encoded multipulses (HEM) with fundamental frequency bandpass filter (i.e., filter centered on transmit frequency). HEM consecutively emits multipulses encoded by a second-order Hadamard matrix in each of the two transmission events (i.e., pulse-echo events), as opposed to conventional CEUS methods which emit individual pulses in two separate transmission events (i.e., pulse inversion (PI), amplitude modulation (AM), and PIAM). In HEM imaging, the microbubble responses can be improved by the longer transmit pulse, and the tissue harmonics can be suppressed by the fundamental frequency filter, leading to significantly improved contrast-to-tissue ratio (CTR) and signal-to-noise ratio (SNR). In addition, the fast polarity change between consecutive coded pulse emissions excites strong nonlinear microbubble echoes, further enhancing the CEUS image quality. The spatial resolution of HEM image is compromised as compared to other microbubble imaging methods due to the longer transmit pulses and the lower imaging frequency (i.e., fundamental frequency). However, the resolution loss was shown to be negligible and could be offset by the significantly enhanced CTR, SNR, and penetration depth. These properties of HEM can potentially facilitate robust CEUS imaging for many clinical applications, especially for deep abdominal organs and heart.

Nonlinear ultrasound imaging of nanoscale acoustic biomolecules

Ultrasound imaging is widely used to probe the mechanical structure of tissues and visualize blood flow. However, the ability of ultrasound to observe specific molecular and cellular signals is limited. Recently, a unique class of gas-filled protein nanostructures called gas vesicles (GVs) was introduced as nanoscale (∼250 nm) contrast agents for ultrasound, accompanied by the possibilities of genetic engineering, imaging of targets outside the vasculature and monitoring of cellular signals such as gene expression. These possibilities would be aided by methods to discriminate GV-generated ultrasound signals from anatomical background. Here, we show that the nonlinear response of engineered GVs to acoustic pressure enables selective imaging of these nanostructures using a tailored amplitude modulation strategy. Finite element modeling predicted a strongly nonlinear mechanical deformation and acoustic response to ultrasound in engineered GVs. This response was confirmed with ultrasound measurements in the range of 10 to 25 MHz. An amplitude modulation pulse sequence based on this nonlinear response allows engineered GVs to be distinguished from linear scatterers and other GV types with a contrast ratio greater than 11.5 dB. We demonstrate the effectiveness of this nonlinear imaging strategy in vitro, in cellulo, and in vivo.

3-D Ultrasound Localization Microscopy for Identifying Microvascular Morphology Features of Tumor Angiogenesis at a Resolution Beyond the Diffraction Limit of Conventional Ultrasound

Angiogenesis has been known as a hallmark of solid tumor cancers for decades, yet ultrasound has been limited in its ability to detect the microvascular changes associated with malignancy. Here, we demonstrate the potential of 'ultrasound localization microscopy' applied volumetrically in combination with quantitative analysis of microvascular morphology, as an approach to overcome this limitation. This pilot study demonstrates our ability to image complex microvascular patterns associated with tumor angiogenesis in-vivo at a resolution of tens of microns - substantially better than the diffraction limit of traditional clinical ultrasound, yet using an 8 MHz clinical ultrasound probe. Furthermore, it is observed that data from healthy and tumor-bearing tissue exhibit significant differences in microvascular pattern and density. Results suggests that with continued development of these novel technologies, ultrasound has the potential to detect biomarkers of cancer based on the microvascular 'fingerprint' of malignant angiogenesis rather than through imaging of blood flow dynamics or the tumor mass itself.

A multi-frequency sparse hemispherical ultrasound phased array for microbubble-mediated transcranial therapy and simultaneous cavitation mapping

Focused ultrasound (FUS) phased arrays show promise for non-invasive brain therapy. However, the majority of them are limited to a single transmit/receive frequency and therefore lack the versatility to expose and monitor the treatment volume. Multi-frequency arrays could offer variable transmit focal sizes under a fixed aperture, and detect different spectral content on receive for imaging purposes. Here, a three-frequency (306, 612, and 1224 kHz) sparse hemispherical ultrasound phased array (31.8 cm aperture; 128 transducer modules) was constructed and evaluated for microbubble-mediated transcranial therapy and simultaneous cavitation mapping. The array is able to perform effective electronic beam steering over a volume spanning (-40, 40) and (-30, 50) mm in the lateral and axial directions, respectively. The focal size at the geometric center is approximately 0.9 (2.1) mm, 1.7 (3.9) mm, and 3.1 (6.5) mm in lateral (axial) pressure full width at half maximum (FWHM) at 1224, 612, and 306 kHz, respectively. The array was also found capable of dual-frequency excitation and simultaneous multi-foci sonication, which enables the future exploration of more complex exposure strategies. Passive acoustic mapping of dilute microbubble clouds demonstrated that the point spread function of the receive array has a lateral (axial) intensity FWHM between 0.8-3.5 mm (1.7-11.7 mm) over a volume spanning (-25, 25) mm in both the lateral and axial directions, depending on the transmit/receive frequency combination and the imaging location. The device enabled both half and second harmonic imaging through the intact skull, which may be useful for improving the contrast-to-tissue ratio or imaging resolution, respectively. Preliminary in vivo experiments demonstrated the system's ability to induce blood-brain barrier opening and simultaneously spatially map microbubble cavitation activity in a rat model. This work presents a tool to investigate optimal strategies for non-thermal FUS brain therapy and concurrent microbubble cavitation monitoring through the availability of multiple frequencies.

Phase and Amplitude Modulation Methods for Nonlinear Ultrasound Imaging With CMUTs

Conventional amplitude and phase modulated pulse sequences for selective imaging of nonlinear tissue and ultrasound contrast agents are designed for piezoelectric transducers that behave linearly. Inherent nonlinearity of capacitive micromachined ultrasonic transducers (CMUTs), especially during large-signal operation, renders these methods inapplicable. In this paper, we present different pulse sequences for nonlinear imaging that are valid for small- and large-signal CMUT operations. For small-signal operation, two-pulse amplitude and phase modulation methods for microbubble and tissue harmonic imaging are presented, where CMUT nonlinearity is compensated via subharmonic excitation. In the large-signal regime, using a nonlinear model, we first show that there is a simple linear relationship between the phases of each harmonic distortion component generated and the input drive signal. Based on this observation, we demonstrate a pulse sequence using N+1 consecutive phase modulated transmit events to extract N harmonics of the nonlinear contrast agent echo content uncorrupted by CMUT nonlinearity. The proposed methods assume no apriori information about the transducer and, therefore, are applicable to any CMUT. The phase modulation method is also valid for piezoelectric transducers and systems with nonlinearities described by Taylor series where the same phase relationship between the input signal and the harmonic content is valid. The proof of principle experiments using a commercial contrast agent validates the phase modulated pulse sequences for CMUTs, operating in a highly nonlinear collapse-snapback mode and for piezoelectric transducers.

Cumulative phase delay imaging for contrast-enhanced ultrasound tomography

Standard dynamic-contrast enhanced ultrasound (DCE-US) imaging detects and estimates ultrasound-contrast-agent (UCA) concentration based on the amplitude of the nonlinear (harmonic) components generated during ultrasound (US) propagation through UCAs. However, harmonic components generation is not specific to UCAs, as it also occurs for US propagating through tissue. Moreover, nonlinear artifacts affect standard DCE-US imaging, causing contrast to tissue ratio reduction, and resulting in possible misclassification of tissue and misinterpretation of UCA concentration. Furthermore, no contrast-specific modality exists for DCE-US tomography; in particular speed-of-sound changes due to UCAs are well within those caused by different tissue types. Recently, a new marker for UCAs has been introduced. A cumulative phase delay (CPD) between the second harmonic and fundamental component is in fact observable for US propagating through UCAs, and is absent in tissue. In this paper, tomographic US images based on CPD are for the first time presented and compared to speed-of-sound US tomography. Results show the applicability of this marker for contrast specific US imaging, with cumulative phase delay imaging (CPDI) showing superior capabilities in detecting and localizing UCA, as compared to speed-of-sound US tomography. Cavities (filled with UCA) which were down to 1 mm in diameter were clearly detectable. Moreover, CPDI is free of the above mentioned nonlinear artifacts. These results open important possibilities to DCE-US tomography, with potential applications to breast imaging for cancer localization.

Generalization of Multipulse Transmission Techniques for Ultrasound Imaging

To increase the contrast-to-tissue ratio (CTR) in contrast imaging or the signal-to-noise ratio (SNR) in tissue harmonic imaging, many multipulse transmission techniques have been suggested. This article first recalls the various imaging techniques proposed in the literature and then presents a mathematical background to synthesize and generalize most of the multipulse ultrasound imaging techniques. The formulation presented can be used to predict the relative amplitude of the nonlinear components in each frequency band and to design new transmission sequences to either increase or decrease specified nonlinear components in each harmonic band. Simulation results on several multipulse techniques agree with the results from previous studies.

Correction of Non-Linear Propagation Artifact in Contrast-Enhanced Ultrasound Imaging of Carotid Arteries: Methods and in Vitro Evaluation

Non-linear propagation of ultrasound creates artifacts in contrast-enhanced ultrasound images that significantly affect both qualitative and quantitative assessments of tissue perfusion. This article describes the development and evaluation of a new algorithm to correct for this artifact. The correction is a post-processing method that estimates and removes non-linear artifact in the contrast-specific image using the simultaneously acquired B-mode image data. The method is evaluated on carotid artery flow phantoms with large and small vessels containing microbubbles of various concentrations at different acoustic pressures. The algorithm significantly reduces non-linear artifacts while maintaining the contrast signal from bubbles to increase the contrast-to-tissue ratio by up to 11 dB. Contrast signal from a small vessel 600 μm in diameter buried in tissue artifacts before correction was recovered after the correction.

Evaluation of bias voltage modulation sequence for nonlinear contrast agent imaging using a capacitive micromachined ultrasonic transducer array

Many clinical diagnoses have now been improved thanks to the development of new techniques dedicated to contrast agent nonlinear imaging. Over the past few years, Capacitive Micromachined Ultrasonic Transducers (cMUTs) have emerged as a promising alternative to traditional piezoelectric transducers. One notable advantage of cMUTs is their wide frequency bandwidth. However, their use in nonlinear imaging approaches such as those used to detect contrast agents have been challenging due their intrinsic nonlinear character. We propose a new contrast imaging sequence, called bias voltage modulation (BVM), specifically developed for cMUTs to suppress their inherent nonlinear behavior. Theoretical and experimental results show that a complete cancellation of the nonlinear signal from the source can be reached when the BVM sequence is implemented. In-vitro validation of the sequence is performed using a cMUT probe connected to an open scanner and a flow phantom setup containing SonoVue microbubbles. Compared to the standard amplitude modulation imaging mode, a 6 dB increase of contrast-to-tissue ratio was achieved when the BVM sequence is applied. These results reveal that the problem of cMUT nonlinearity can be addressed, thus expanding the potential of this new transducer technology for nonlinear contrast agent detection and imaging.

CREANUIS: a non-linear radiofrequency ultrasound image simulator

Nonlinear ultrasound methods are widely used in clinical applications for tissue or contrast harmonic imaging. Accurate non-linear imaging simulation tools are required in research studies for the development of new methods. However, in existing simulators, the possible inhomogeneity of the coefficient of non-linearity, which is generally observed in tissue and in particular when contrast agents are involved, has not yet been implemented. This article describes a new ultrasound simulator, called CREANUIS, devoted to the computation of B-mode images where both linear and non-linear propagation in media is considered, with a possible inhomogeneous coefficient of non-linearity. The resulting fundamental images, based on a spatially variant and non-linear point spread function, are in accordance with those obtained through the reference linear FieldII simulator, with computation time reduced by a factor of at least 1.8. Non-linear images of media exhibiting inhomogeneous coefficients of non-linearity are also provided. The simulation software can be freely downloaded from our website.

Ultrasound contrast imaging: influence of scatterer motion in multi-pulse techniques

In ultrasound contrast imaging, many techniques based on multiple transmissions have been proposed to increase the contrast-to-tissue ratio (CTR). They are generally based on the response of static scatterers inside the imaged region. However, scatterer motion, for example in blood vessels, has an inevitable influence on multi-pulse techniques, which can either enhance or degrade the technique involved. This paper investigates the response of static nonlinear media insonated by multi-pulses with various phase shifts, and the influence of scatterer motion on multi-pulse techniques. Simulations and experimental results from a single bubble and clouds of bubbles show that the phase shift of the echoes backscattered from bubbles is dependent on the transmissions' phase shift, and that the bubble motion influences the efficiency of multi-pulse techniques: fundamental and second-harmonic amplitudes of the processed signal change periodically, exhibiting maximum or minimum values, according to scatterer motion. Furthermore, experimental results based on the second-harmonic inversion (SHI) technique reveal that bubble motion can be taken into account to regulate the pulse repetition frequency (PRF). With the optimal PRF, the CTR of SHI images can be improved by about 12 dB compared with second-harmonic images.

Real-time monitoring of HIFU treatment using pulse inversion

Ultrasound (US) imaging is widely used for the real-time guidance of high-intensity focused ultrasound (HIFU) treatment at a relatively low cost. However, ultrasound image guided HIFU (USgHIFU) is limited in the real-time monitoring of HIFU treatment due to the large amplitude HIFU signals received by the US imaging transducer. The amplitude of the HIFU scattered signal is generally much higher than the amplitude of the pulse-echo signal received by the imaging transducer. This creates an interference pattern obscuring the image of the tissue. As such, it is difficult to monitor lesion location. This paper proposes a real-time monitoring method to be performed concurrently with the HIFU insonation, but without HIFU interference, which allows for the improvement of treatment accuracy and safety in USgHIFU. The proposed method utilizes the physical properties of pulse inversion which is capable of removing the fundamental and odd harmonic components of the HIFU interference. Therefore, it is possible to secure the desired spectral bandwidth used to construct US images for HIFU treatment monitoring. The performance of the proposed method was evaluated through experiments with both a bovine serum albumin phantom and a chicken breast. The results demonstrated that the proposed method is capable of providing interference-free US images, thus successfully allowing for US imaging during HIFU treatment.

Contrast-enhanced intravascular ultrasound pulse sequences for bandwidth-limited transducers

We demonstrate two methods for vasa vasorum imaging using contrast-enhanced intravascular ultrasound, which can be performed using commercial catheters. Plaque neovascularization was recognized as an independent marker of coronary artery plaque vulnerability. IVUS-based methods to image the microvessels available to date require high bandwidth (-6 dB relative frequency bandwidth >70%), which are not routinely available commercially. We explored the potential of ultraharmonic imaging and chirp reversal imaging for vasa vasorum imaging. In vitro recordings were performed on a tissue-mimicking phantom using a commercial ultrasound contrast agent and a transducer with a center frequency of 34 MHz and a -6 dB relative bandwidth of 56%. Acoustic peak pressures <500 kPa were used. A tissue-mimicking phantom with channels down to 200 μm in diameter was successfully imaged by the two contrast detection sequences while the smallest channel stayed invisible in conventional intravascular ultrasound images. Ultraharmonic imaging provided the best contrast agent detection.

Dual-frequency tissue harmonic suppression using phase-coded pulse sequence: proof of concept using a phantom

BACKGROUND

The presence of tissue harmonic generation during acoustic propagation is one major limitation in nonlinear detection of microbubble contrast agents. However, conventional solutions for tissue harmonic suppression are not applicable in dual-frequency (DF) harmonic imaging. In DF harmonic imaging, the second harmonic signal at second harmonic (2f(0)) frequency and the inter-modulation harmonic signal at fundamental (f(0)) frequency are simultaneously generated for imaging and both need to be suppressed to improve contrast-to-tissue ratio (CTR). In this study, a novel phase-coded pulse sequence is developed to accomplish DF tissue harmonic suppression.

METHODS

Phase-coded pulse sequence utilizes multiple firings with equidistant transmit phase for harmonic cancellation in the sum of respective echoes. For the f(0) transmit component, the transmit phase comes from the equidistant set of {-2π/3, 0, 2π/3} to suppress the second harmonic signal at 2f(0) frequency. Moreover, in order to provide the inter-modulation harmonic suppression at f(0) frequency, the 2f(0) transmit phase has to be particularly manipulated for the corresponding f(0) transmit phase.

RESULTS

The proposed three-pulse sequence can remove not only the second-order harmonic signal but also other higher-order counterparts at both f(0) and 2f(0) frequencies. Measurements were performed at f(0) equal to 2.25 MHz and using hydrophone in water and contrast agents in tissue phantom. Experimental results indicate that the sequence reduces the tissue harmonic magnitude by about 20 dB along the entire axial depths and the corresponding CTR improves at both frequencies.

CONCLUSION

In DF harmonic imaging, the proposed phase-coded sequence can effectively remove the tissue harmonic background at both f(0) and 2f(0) frequencies for improvement of contrast detection.