Ultrasound Tissue Harmonic Imaging Research Articles

An Adaptive Harmonic Separation Technique for Ultrasound Harmonic Imaging

ObjectiveAn adaptive harmonic separation (HS) technique is proposed to overcome the limitations in conventional filtering techniques for ultrasound (US) tissue harmonic imaging (THI). MethodsBased on expectation-maximization source separation, the proposed HS technique adaptively models the depth-varying fundamental and harmonic components in the frequency domain and separates the two by applying their calculated posterior probabilities. Phantom experiments with a Tx center frequency of 2 MHz are conducted to evaluate the proposed HS-based US THI schemes. ResultsThe phantom images show that the proposed single-pulse THI scheme utilizing the HS technique provides not only an average improvement of 19.2% in axial resolution compared to the conventional bandpass filtering scheme but also similar image quality to that of the conventional pulse-inversion (PI) scheme which requires two Tx/Rx sequences for each scan line. Furthermore, when combined with the PI technique, the HS technique provides a uniform axial resolution over the entire 170 mm imaging depth with an average improvement of 17.1% compared to the conventional PI scheme. ConclusionThese results show that the proposed adaptive HS technique is capable of improving both the frame rate and the image quality of US THI.

An efficient adaptive method based on empirical wavelet transform for ultrasound tissue harmonic imaging

In this study, an efficient adaptive method based on the empirical wavelet transform (EA_EWT) was developed to separate harmonics from ultrasonic radio frequency (RF) echo signals for B-mode imaging. The spectrum for each RF scan line was calculated, and a meaningful partition based on the scale-space representation was performed on the preprocessed spectrum, from which the harmonic boundaries were automatically determined. Thereafter, the harmonic components were obtained using the empirical scaling function and empirical wavelets. Finally, harmonic signals were obtained after separating all RF scan lines in a frame for imaging. In the experiments, the EA_EWT method was used to separate the ultrasonic RF signals simulated by CREANUIS and obtained from the human carotid artery. The results were compared with separation based on bandpass filtering (BPF), pulse inversion (PI), and complete ensemble empirical mode decomposition with adaptive noise (S_CEEMDAN) methods. The EA_EWT-based simulation results were superior to the PI-based simulation results with shifted scatterer models and comparable to the BPF- and S_CEEMDAN-based results. In the in vivo measurements, the EA_EWT-based harmonic images exhibited a mean contrast, contrast-to-noise ratio, and tissue-to-clutter ratio of 0.896, 3.973, and 8.306, respectively. The processing times of the BPF, PI, and EA_EWT methods were 0.06 s, 0.01 s, and 0.214 s, respectively, which were significantly shorter than that of the S_CEEMDAN method (113.26 s). The EA_EWT approach may be used in practice to generate more precise diagnostic data since it effectively executes a better separation of harmonic components for B-mode imaging.

An Efficient Adaptive Method Based on Empirical Wavelet Transform for Ultrasound Tissue Harmonic Imaging

An Efficient Adaptive Method Based on Empirical Wavelet Transform for Ultrasound Tissue Harmonic Imaging

Intravascular Ultrasound Transducer by Using Polarization Inversion Technique for Tissue Harmonic Imaging: Modeling and Experiments.

Intravascular ultrasound (IVUS) tissue harmonic imaging (THI) is a useful vessel imaging technique that can provide deep penetration depth as well as high spatial and contrast resolution. Typically, a high-frequency IVUS transducer for THI requires a broad bandwidth or dual-frequency bandwidth. However, it is very difficult to make an IVUS transducer with a frequency bandwidth covering from the fundamental frequency to the second harmonic or a dual-peak at the desired frequency. To solve this problem, in this study, we applied the polarization inversion technique (PIT) to the IVUS transducer for THI. The PIT makes it relatively easy to design IVUS transducers with suitable frequency characteristics for THI depending on the inversion ratio of the piezoelectric layer and specifications of the passive materials. In this study, two types of IVUS transducers based on the PIT were developed for THI. One is a front-side inversion layer (FSIL) transducer with a broad bandwidth, and the other is a back-side inversion layer (BSIL) transducer with a dual-frequency bandwidth. These transducers were designed using finite element analysis (FEA)-based simulation, and the prototype transducers were fabricated. Subsequently, the performance was evaluated by not only electrical impedance and pulse-echo response tests but also B-mode imaging tests with a 25μm tungsten wire and tissue-mimicking gelatin phantoms. The FEA simulation and experimental results show that the proposed scheme can successfully implement the tissue harmonic IVUS image, and thus it can be one of the promising techniques for developing IVUS transducers for THI.

Interleaved Array Transducer with Polarization Inversion Technique to Implement Ultrasound Tissue Harmonic Imaging.

In ultrasound tissue harmonic imaging (THI), it is preferred that the bandwidth of the array transducer covers at least the fundamental frequency f0 for transmission and the second harmonic frequency 2f0 for reception. However, it is challenging to develop an array transducer with a broad bandwidth due to the single resonance characteristics of piezoelectric materials. In this study, we present an improved interleaved array transducer suitable for THI and a dedicated transducer fabrication scheme. The proposed array transducer has a novel structure in which conventional elements exhibiting f0 resonant frequency and polarization-inverted elements exhibiting 2f0 resonant frequency are alternately located, and the thicknesses of all piezoelectric elements are identical. The performance of the proposed method was demonstrated by finite element analysis (FEA) simulations and experiments using a fabricated prototype array transducer. Using the proposed technique, f0 and 2f0 frequency ultrasounds can be efficiently transmitted and received, respectively, resulting in a 90% broad bandwidth feature of the transducer. Thus, the proposed technique can be one of the potential ways to implement high resolution THI.

Diagnosis of Biliary Tract Disease Based on Ultrasound Tissue Harmonic Imaging

With the deepening of ultrasound imaging research, studying nonlinear phenomena in medical ultrasound will help people to further improve the existing diagnostic level. Harmonic imaging technology generated in recent years is an effective new technology for nonlinear acoustics in ultrasonic diagnosis. Patients with biliary tract lesions generally have few symptoms, which are often detected by chance during physical examination. Ultrasonography is currently the preferred diagnostic method for biliary tract disease. The application of tissue harmonic imaging (THI) can effectively eliminate many near-field misdiagnosed images, enhance the tissue echo of deep tissues or organs, and significantly improve signal-to-noise ratio and resolution. At the same time, it can better display the subtle features of biliary diseases, thereby reducing the rate of missed diagnosis and improving the accuracy of diagnosis. Therefore, this study aimed to observe the changes of ultrasound images in the lesions and surrounding tissues of several biliary lesions by ultrasound tissue harmonic imaging technology, and realized the diagnosis in the early stage of the disease by means of tissue harmonic imaging technology, then provided objective quantitative indicators for the diagnosis of biliary diseases. Also, this study confirmed that tissue harmonic imaging technology can significantly improve the ultrasound image resolution of biliary tract disease, clearly showing the sub-acute gallbladder wall, gallbladder wall, gallbladder cavity in the acute and chronic inflammation and the presence of subtle changes in the tumor. This can provide a rich diagnostic value for the clinic and has practical significance for clinical treatment.

Adaptive Ultrasound Tissue Harmonic Imaging Based on an Improved Ensemble Empirical Mode Decomposition Algorithm

Complete and accurate separation of harmonic components from the ultrasonic radio frequency (RF) echo signals is essential to improve the quality of harmonic imaging. There are limitations in the existing two commonly used separation methods, that is, the subjectivity for the high-pass filtering (S_HPF) method and motion artifacts for the pulse inversion (S_PI) method. A novel separation method called S_CEEMDAN, based on the complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) algorithm, is proposed to adaptively separate the second harmonic components for ultrasound tissue harmonic imaging. First, the ensemble size of the CEEMDAN algorithm is calculated adaptively according to the standard deviation of the added white noise. A set of intrinsic mode functions (IMFs) is then obtained by the CEEMDAN algorithm from the ultrasonic RF echo signals. According to the IMF spectra, the IMFs that contain both fundamental and harmonic components are further decomposed. The separation process is performed until all the obtained IMFs have been divided into either fundamental or harmonic categories. Finally, the fundamental and harmonic RF echo signals are obtained from the accumulations of signals from these two categories, respectively. In simulation experiments based on CREANUIS, the S_CEEMDAN-based results are similar to the S_HPF-based results, but better than the S_PI-based results. For the dynamic carotid artery measurements, the contrasts, contrast-to-noise ratios (CNRs), and tissue-to-clutter ratios (TCRs) of the harmonic images based on the S_CEEMDAN are averagely increased by 31.43% and 50.82%, 18.96% and 10.83%, as well as 34.23% and 44.18%, respectively, compared with those based on the S_HPF and S_PI methods. In conclusion, the S_CEEMDAN method provides improved harmonic images owing to its good adaptivity and lower motion artifacts, and is thus a potential alternative to the current methods for ultrasonic harmonic imaging.

Development of Dual-Frequency Oblong-Shaped-Focused Transducers for Intravascular Ultrasound Tissue Harmonic Imaging.

Tissue harmonic imaging (THI), an essential mode of commercial ultrasound imaging scanners, can provide images with high spatial and contrast resolutions. For THI, the frequency spectrum of a transducer is generally divided for the transmission of fundamental signal and the reception of its second harmonic. Therefore, it is difficult to use the THI mode for intravascular ultrasound (IVUS) imaging because typical IVUS transducers have a narrow -6-dB fractional bandwidth of about 50%. Due to its small aperture (about 0.5 mm) and the strength of IVUS being too weak, it is difficult to construct a high-quality tissue harmonic image. In this paper, we report a recently developed dual-frequency oblong-shaped-focused IVUS transducer for high-quality intravascular THI; the transducer consists of three elements arranged side by side in the horizontal (i.e., elevation) direction. The two outer elements with a center frequency of 35 MHz are responsible for ultrasound transmission and the center element has a center frequency of 70 MHz for the reception of the second-harmonic signals. All three elements have a spherical shape with a radius of 3 mm to efficiently generate harmonics in the region of interest. This configuration of the developed IVUS transducer was determined to facilitate high-quality THI, which was based on the results of Field II simulation and finite-element analysis. The images of wires and a tissue-mimicking phantom indicated that the tissue harmonic images produced by the developed transducer have not only a high spatial resolution but also a deep imaging depth, compared to the 35- and 70-MHz fundamental images.

An Adaptive Synthetic Aperture Method Applied to Ultrasound Tissue Harmonic Imaging.

In recent years, the minimum variance (MV) beamformer has been highly regarded since it provides high resolution and contrast in B-mode ultrasound imaging compared with nonadaptive delay-and-sum (DAS) beamformer. However, the performance of MV beamformer is degraded in the presence of the noise due to inaccurate estimation of the covariance matrix resulting in low-quality images. The conventional tissue harmonic imaging (THI) offers multiple advantages over conventional pulse-echo ultrasound imaging, including enhanced contrast resolution and improved axial and lateral resolutions, but low signal-to-noise ratio (SNR) is a major problem facing this imaging method, which uses a fixed transmit focus and dynamic receive focusing (DRF). In this paper, a synthetic aperture method based on the virtual source, namely, bidirectional pixel-based focusing (BiPBF), has been combined with the MV beamformer and then applied to second-harmonic ultrasound imaging. The main objective is suppressing the noise level to enhance the performance of the MV beamformer in the harmonic imaging, especially in lower and deeper depths where the SNR is low. In addition, combining the BiPBF and MV weighting results in simultaneous improvement in imaging resolution and contrast, in comparison with the conventional methods: DRF (DAS), BiPBF (DAS), and DRF (MV). The performance of the proposed method is evaluated on simulated and experimental RF data. The THI is achieved using the pulse-inversion technique. The results of the simulated wire phantom demonstrate that the proposed beamformer can achieve the best lateral resolution, along different depths, compared with DRF (DAS), BiPBF (DAS), and DRF (MV) methods. The results of the simulated and experimental cyst phantoms show that the new beamformer improves the contrast ratio (CR) and contrast-to-noise ratio (CNR) of the resulting images. In results of simulated cyst phantom, in average, the new beamformer improves the CR and CNR of the cyst about (7.4 dB, 49%), (3.2 dB, 16%), and (5 dB, 26%) compared with DRF (DAS), BiPBF (DAS), and DRF (MV), respectively. In results of experimental cyst phantom, these relative improvements are about (4.2 dB, 22%), (1.7 dB, 7%), and (2.6 dB, 15%). In addition, BiPBF (MV) method offers improved edge definition of cysts in comparison with the other methods.

Ultrasound Tissue Harmonic Imaging based on Wavelet Correlation Analysis

In this paper, an ultrasound tissue harmonic imaging method based on wavelet transform has been proposed. Wavelet transform is utilized to analyze the correlation between mother wavelet and the received echoes. In order to obtain a better correlation, a new mother wavelet named ‘Tissue wavelet’ is constructed according to the KZK equation. The proposed method was validated by experiments performed on an ultrasound flow phantom. Results showed that the contrast and axial resolution have been improved effectively.

Optimization of contrast resolution by genetic algorithm in ultrasound tissue harmonic imaging

The development of ultrasound imaging techniques such as pulse inversion has improved tissue harmonic imaging. Nevertheless, no recommendation has been made to date for the design of the waveform transmitted through the medium being explored. Our aim was therefore to find automatically the optimal “imaging” wave which maximized the contrast resolution without a priori information. To overcome assumption regarding the waveform, a genetic algorithm investigated the medium thanks to the transmission of stochastic “explorer” waves. Moreover, these stochastic signals could be constrained by the type of generator available (bipolar or arbitrary). To implement it, we changed the current pulse inversion imaging system by including feedback. Thus the method optimized the contrast resolution by adaptively selecting the samples of the excitation. In simulation, we benchmarked the contrast effectiveness of the best found transmitted stochastic commands and the usual fixed-frequency command. The optimization method converged quickly after around 300 iterations in the same optimal area. These results were confirmed experimentally. In the experimental case, the contrast resolution measured on a radiofrequency line could be improved by 6% with a bipolar generator and it could still increase by 15% with an arbitrary waveform generator.

Is tissue harmonic ultrasound imaging (THI) of the prostatic urethra and rectum superior to brightness (B) mode imaging? An observer study

Quality ultrasound images are an essential part of prostate brachytherapy procedure. The authors have previously reported that tissue harmonic ultrasound images (THI) are superior to brightness (B) mode for the prostate. The objective of the current study was to compare both imaging modes for visualization of the prostatic urethra and rectum.B and THI mode transrectal ultrasound images were acquired for ten patients. The prostatic urethra and rectal wall were contoured by a radiation oncologist (RO) and five observers on randomly presented images. The contours on one patient were repeated four additional times by four observers. All the images were qualitatively scored using a five-level Likert scale.The values of the Pearson product-moment correlation coefficients showed that the observers were in close agreement with the RO. Two sample paired student t-test showed that the rectum volumes with THI were significantly smaller than B-mode, but no significant difference for urethra. Two-factor analysis of variances showed significant observer variability in defining the rectum and urethra in both imaging modes. Observer consistency of the rectum volumes, estimated by standard deviations as percentages of means was significantly improved for THI. The Likert scale based qualitative assessment supported quantitative observations.The significant improvement in image quality of the prostate (reported previously) and rectum with THI may offer better-quality treatment plans for prostate brachytherapy and potential improvement in local control.

Improved Shear Wave Motion Detection Using Pulse-Inversion Harmonic Imaging With a Phased Array Transducer

Ultrasound tissue harmonic imaging is widely used to improve ultrasound B-mode imaging quality thanks to its effectiveness in suppressing imaging artifacts associated with ultrasound reverberation, phase aberration, and clutter noise. In ultrasound shear wave elastography (SWE), because the shear wave motion signal is extracted from the ultrasound signal, these noise sources can significantly deteriorate the shear wave motion tracking process and consequently result in noisy and biased shear wave motion detection. This situation is exacerbated in in vivo SWE applications such as heart, liver, and kidney. This paper, therefore, investigated the possibility of implementing harmonic imaging, specifically pulse-inversion harmonic imaging, in shear wave tracking, with the hypothesis that harmonic imaging can improve shear wave motion detection based on the same principles that apply to general harmonic B-mode imaging. We first designed an experiment with a gelatin phantom covered by an excised piece of pork belly and show that harmonic imaging can significantly improve shear wave motion detection by producing less underestimated shear wave motion and more consistent shear wave speed measurements than fundamental imaging. Then, a transthoracic heart experiment on a freshly sacrificed pig showed that harmonic imaging could robustly track the shear wave motion and give consistent shear wave speed measurements of the left ventricular myocardium while fundamental imaging could not. Finally, an in vivo transthoracic study of seven healthy volunteers showed that the proposed harmonic imaging tracking sequence could provide consistent estimates of the left ventricular myocardium stiffness in end-diastole with a general success rate of 80% and a success rate of 93.3% when excluding the subject with Body Mass Index higher than 25. These promising results indicate that pulse-inversion harmonic imaging can significantly improve shear wave motion tracking and thus potentially facilitate more robust assessment of tissue elasticity by SWE.

Automatic Optimization of Chirp Setting Parameters in Medical Ultrasound Contrast Imaging

Medical ultrasound contrast imaging is a powerful modality undergoing successive developments in the last decade to date. Lately, pulse inversion has been used in both ultrasound tissue harmonic and contrast imaging. However, there was a tradeoff between resolution and penetration. Chirp excitations partially solved the tradeoff, but the chirp setting parameters were not optimized. The present work proposes for the first time combining chirp inversion with ultrasound contrast imaging, with the motivation to improve the contrast, by automatically optimizing the setting parameters of chirp excitation, it is thus an optimal command problem. Linear chirps, 5 ?m diameter microbubbles and gradient ascent algorithm were simulated to optimize the chirp setting parameters. Simulations exhibited a gain of 5 dB by automatic optimization of chirp inversion relative to pulse inversion. The automatic optimization process was quite fast. Combining chirp inversion with ultrasound contrast imaging led to a maximum backscattered power permitting high contrast outcomes and optimum parameters.

Chirp-encoded excitation for dual-frequency ultrasound tissue harmonic imaging

Dual-frequency (DF) transmit waveforms comprise signals at two different frequencies. With a DF transmit waveform operating at both fundamental frequency (f(0)) and second-harmonic frequency (2f(0)), tissue harmonic imaging can be simultaneously performed using not only the conventional 2f(0) second-harmonic signal but also using the f(0 )frequency-difference harmonic signal. Nonetheless, when chirp excitation is incorporated into the DF transmit waveform for harmonic SNR improvement, a particular waveform design is required to maintain the bandwidth of the f(0) harmonic signal. In this study, two different DF chirp waveforms are proposed to produce equal harmonic bandwidth at both the f(0) and 2f(0) frequencies to achieve speckle reduction by harmonic spectral compounding and to increase harmonic SNR for enhanced penetration and sensitivity. The UU13 waveform comprises an up-sweeping f(0) chirp and an up-sweeping 2f(0) chirp with triple bandwidth, whereas the UD11 waveform includes an up-sweeping f(0) chirp and a down-sweeping 2f(0) chirp with equal bandwidth. Experimental results indicate that the UU13 tends to suffer from a high range side lobe level resulting from 3f(0) interference. Consequently, the 2f(0) harmonic envelopes of the UD11 and the UU13 waveforms have compression qualities of 87% and 77%, respectively, when the signal bandwidth is 30%. When the bandwidth increases to 50%, the compression quality of the 2f(0) harmonic envelope degrades to 78% and 54%, respectively, for the UD11 and the UU13 waveforms. The compression quality value of the f0 harmonic envelope remains similar between the two DF transmit waveforms for all signal bandwidths. B-mode harmonic images also show that the UD11 is less contaminated by range side lobe artifacts than is the UU13. Compared with a short pulse with equal bandwidth, the UD11 waveform not only preserves the same spatial resolution after compression but also improves the image SNR by about 10 dB. Moreover, the image contrast-to-noise ratio (CNR), defined as the ratio of the mean to the standard deviation of image intensity in the speckle region, can be increased from 1.0 to about 1.2 when DF spectral compounding is performed. Therefore, it is concluded that the UD11 waveform is a potential solution for chirp-encoded DF harmonic imaging.

SU-E-U-06: Is Tissue Harmonic Ultrasound Imaging of the Prostate and Critical Organs Superior to B Mode Imaging? An Observer Study

Purpose: The objective of the current work is to assess the relative quality of tissue harmonic (H) transrectal ultrasoundimages of the prostate and neighbouring critical organs by comparing it with brightness (B) mode, using quantitative and qualitative statistical tools. Methods: Image sets containing prostate and critical organs were acquired using the B and H mode capable transrectal probe B‐K medical 8848, for ten patients undergoing radioactive seed implants at our centre. The prostate, urethra and rectum were contoured by a radiation oncologist (RO) and five similarly trained observers for all image sets. These observers also qualitatively scored randomly presented images using a five‐level Likert scale. The prostate, rectum and urethra volumes and qualitative scores of images were used for inter imaging mode comparison and observer group versus RO and among the observers (interobserver) studies. Results: Pearson correlation coefficients showed that all volumes outlined by observers were in close agreement with that of the RO. Two sample paired student t‐tests showed that the prostate (p=0.008) and rectum (p=0.009) volumes with H mode were significantly smaller than B mode. However there was no significant difference (p=0.29) in urethral volumes between the two modes. Interobserver consistency was estimated based on standard deviations as percentages of means. Two‐factor analysis of variances (ANOVA) showed significant interobserver variability (p<0.003) in defining the prostate and critical organs, in both imaging modes. Interobserver consistency for small prostates and rectum improved in H mode, but there is no significant improvement for the urethra. Qualitative assessment of image clarity agreed well with these quantitative observations. Conclusions: Except for the urethra, the volumes of prostates and critical organs defined with H mode are significantly smaller compared to B mode. H mode improves interobserver consistency in defining the prostate and rectum as a result of superior image clarity.

Coded excitation for ultrasound tissue harmonic imaging

Coded excitation can improve the signal-to-noise ratio (SNR) in ultrasound tissue harmonic imaging (THI). However, it could suffer from the increased sidelobe artifact caused by incomplete pulse compression due to the spectral overlap between the fundamental and harmonic components of ultrasound signal after nonlinear propagation in tissues. In this paper, three coded tissue harmonic imaging (CTHI) techniques based on bandpass filtering, power modulation and pulse inversion (i.e., CTHI-BF, CTHI-PM, and CTHI-PI) were evaluated by measuring the peak range sidelobe level (PRSL) with varying frequency bandwidths. From simulation and in vitro studies, the CTHI-PI outperforms the CTHI-BF and CTHI-PM methods in terms of the PRSL, e.g., −43.5 dB vs. −24.8 dB and −23.0 dB, respectively.

Poster — Wed Eve—01: The Characterization of Tissue Harmonic Ultrasound Imaging for Potential Use in Prostate Brachytherapy

A phantom has been designed and fabricated to explore the potential benefits of tissue harmonic ultrasound imaging (THI) in prostate brachytherapy. Transverse and sagittal images of the phantom were acquired using 6, 9 and 12 MHz ultrasound frequencies in brightness (B) mode and 10 and 12 MHz in THI mode. The imaging parameters such as dead zone, depth of penetration, geometrical accuracy, axial and lateral resolution, contrast resolution and signal to noise ratio (SNR) in B‐mode were compared with those in THI‐mode. It was found that the dead zone is 1mm at all frequencies of both B‐mode and THI‐mode. The depth of penetration decreases insignificantly in THI mode, while contrast resolution and SNR improves. The axial resolution in THI mode (12 MHz) is ∼45% higher than B‐mode (6 MHz). The lateral resolution in THI mode (12 MHz) is ∼50% higher in the near zone and ∼10% higher in the far field than corresponding values in B‐mode (6 MHz). In general harmonic images are clearer, less noisy and display fewer artifacts than B‐mode images. This phantom study demonstrates that THI‐mode has the potential to provide better images of the prostate, which may help in prostate volume identification and improved treatment planning. An improvement in target localization has a potential to improve the treatment outcomes and reduction in the treatment related complications.

Reproducibility of sonographic measurements of carotid intima-media thickness

Carotid intima-media thickness (IMT), measured with B-mode ultrasound, is a marker for atherosclerosis and widely used as an outcome in intervention trials. To evaluate the reproducibility of carotid intima-media thickness measurements using tissue harmonic imaging (THI) sonography between two observers with different levels of experience. IMT measurement was performed in 40 patients: 26 males and 14 females, with a mean age of 62.6 years. The measurements were taken on the posterior wall of the right common carotid artery using a standardized protocol. Observer measurement of the IMT was independent and blind. Statistical analysis was done with parametric tests and Pearson's correlation coefficient. The measurements of the IMT were taken satisfactorily in all cases. IMT measurement time was 5 min/study. No significant differences were noted between the means obtained by the two observers. The interobserver correlation of the measurements was very high (97.2%). The results of this study show that carotid IMT measurement using THI sonography and a standardized protocol is accurate and reproducible.

Intravascular ultrasound tissue harmonic imaging in vivo

Tissue harmonic imaging (THI) has been shown to increase image quality of medical ultrasound in the frequency range from 2 to 10 MHz and might, therefore, also be used to improve image quality in intravascular ultrasound (IVUS). In this study we constructed a prototype IVUS system that could operate in both fundamental frequency and second harmonic imaging modes. This system uses a conventional, continuously rotating, single-element IVUS catheter and was operated in fundamental 20 MHz, fundamental 40 MHz, and harmonic 40 MHz modes (transmit 20 MHz, receive 40 MHz). Hydrophone beam characterization measurements demonstrated the build-up of a second harmonic signal as a function of increasing pressure. Imaging experiments were conducted in both a tissue-mimicking phantom and in an atherosclerotic animal model in vivo. Acquisitions of fundamental 20 and 40 MHz and second harmonic acquisitions resulted in cross sections of the phantom and a rabbit aorta. The harmonic results of the imaging experiments showed the feasibility of intravascular THI with a conventional IVUS catheter both in a phantom and in vivo. The harmonic acquisitions also showed the potential of THI to reduce image artifacts compared to fundamental imaging.