Delay-and-sum (DAS) beamforming is widely used for generation of B-mode images from echo signals obtained with an array probe composed of transducer elements. However, the resolution and contrast ...achieved with DAS beamforming are determined by the physical specifications of the array, e.g., size and pitch of elements. To overcome this limitation, adaptive imaging methods have recently been explored extensively thanks to the dissemination of digital and programmable ultrasound systems. On the other hand, it is also important to evaluate the performance of such adaptive imaging methods quantitatively to validate whether the modification of the image characteristics resulting from the developed method is appropriate. Since many adaptive imaging methods have been developed and they often alter image characteristics, attempts have also been made to update the methods for quantitative assessment of image quality. This article provides a review of recent developments in adaptive imaging and image quality assessment.
The regional elastic property of the arterial wall is a useful marker for the diagnosis of atherosclerosis because an atherosclerotic lesion is formed locally. Pulse wave velocity is a noninvasive ...index for the evaluation of the elasticity of the arterial wall. However, a high temporal resolution is required for the measurement of the arterial wall motion because the pulse wave propagates along the artery at several meters per second. In the present study, the arterial wall motion was measured at a high temporal resolution at 1302 frames per second (fps) using plane-wave ultrafast ultrasound. Also, a method of estimating the temporal and spatial frequencies of the pulse wave was developed for the determination of the regional pulse wave velocity in a short segment of about 20 mm along the artery. The accuracy of the proposed method was evaluated by simulation experiments and its feasibility was examined in the in vivo measurement of the human carotid artery.
Purpose
Echocardiography is a widely used modality for diagnosis of the heart. It enables observation of the shape of the heart and estimation of global heart function based on B-mode and M-mode ...imaging. Subsequently, methods for estimating myocardial strain and strain rate have been developed to evaluate regional heart function. Furthermore, it has recently been shown that measurements of transmural transition of myocardial contraction/relaxation and propagation of vibration caused by closure of a heart valve would be useful for evaluation of myocardial function and viscoelasticity. However, such measurements require a frame rate much higher than that achieved by conventional ultrasonic diagnostic equipment. In the present study, a method based on parallel receive beamforming was developed to achieve high-frame-rate (over 300 Hz) echocardiography.
Methods
To increase the frame rate, the number of transmits was reduced to 15 with angular intervals of 6°, and 16 receiving beams were created for each transmission to obtain the same number and density of scan lines as realized by conventional sector scanning. In addition, several transmits were compounded to obtain each scan line to reduce the differences in transmit–receive sensitivities among scan lines. The number of transmits for compounding was determined by considering the width of the transmit beam. For transmission, plane waves and diverging waves were investigated. Diverging waves showed better performance than plane waves because the widths of plane waves did not increase with the range distance from the ultrasonic probe, whereas lateral intervals of scan lines increased with range distance.
Results
The spatial resolution of the proposed method was validated using fine nylon wires. Although the widths at half-maxima of the point spread functions obtained by diverging waves were slightly larger than those obtained by conventional beamforming and parallel beamforming with plane waves, point spread functions very similar to those obtained by conventional beamforming could be realized by parallel beamforming with diverging beams and compounding. However, there was an increase in the lateral sidelobe level in the case of parallel beamforming with plane and diverging waves. Furthermore, the heart of a 23-year-old healthy male was measured.
Conclusion
Although the contrast of the B-mode image obtained by the proposed method was degraded due to the increased sidelobe level, a frame rate of 316 Hz, much higher than that realized by conventional sector scanning of several tens of Hertz, was realized with a full lateral field of view of 90°.
Apodized adaptive beamformer Hasegawa, Hideyuki
Journal of medical ultrasonics (2001),
04/2017, Letnik:
44, Številka:
2
Journal Article
Recenzirano
Purpose
A number of studies aimed at improvement of ultrasound image quality, such as spatial resolution and contrast, have been conducted. Apodization is known as an important factor that determines ...image quality. However, in the case of amplitude and phase estimation (APES) beamforming, a kind of adaptive beamformer that has been employed in medical ultrasound recently, only rectangular apodization has been used in the previous studies. In this study, apodization was employed in adaptive beamforming, and its effects on image quality were examined in phantom experiments.
Methods
We recently proposed a modified APES beamformer that reduces the computational complexity significantly using sub-aperture beamforming. In this study, the total receiving aperture was divided into four sub-apertures, and the APES beamforming was applied to the output from the four sub-apertures. Before the delay-and-sum (DAS) beamforming in each sub-aperture, echoes received by individual transducer elements were apodized with rectangular, Gaussian, and two Hanning functions, where the apodization with two Hanning functions realized lateral modulation of the ultrasonic field. The lateral spatial resolution was evaluated by the full width at half maximum of an echo from a string phantom, and the image contrast was evaluated using a cyst phantom.
Results
The modified APES beamformer realized a significantly better spatial resolution of 0.38 mm than that of the conventional delay-and-sum beamformer (0.67 mm), even with rectangular apodization. Using Gaussian apodization, the spatial resolution was further improved to 0.34 mm, and contrast was also improved from 4.3 to 5.1 dB. Furthermore, an image obtained by the modified APES beamformer with apodization consisting of two Hanning functions was better “tagged” as compared with the conventional DAS beamformer with the same apodization.
Conclusion
Apodization was shown to be effective in adaptive beamforming, and an image obtained by the adaptive beamformer with lateral modulation seemed to have potential for improvement of the accuracy in measurement of tissue lateral motion.
Recently, high-frame-rate ultrasound has been extensively studied for measurement of tissue dynamics, such as pulsations of the carotid artery and heart. Motion estimators are very important for such ...measurements of tissue dynamics. In high-frame-rate ultrasound, the tissue displacement between frames becomes very small owing to the high temporal resolution. Under such conditions, the speckle tracking method requires high levels of interpolation to estimate such a small displacement. A phase-sensitive motion estimator is feasible because it does not suffer from the aliasing effect by such a small displacement and does not require interpolation to estimate a sub-sample displacement. In the present study, two phase-sensitive 2D motion estimators, namely, paired 1D motion estimators and 2D motion estimator with shifted cross spectra, were developed. Phase-sensitive motion estimators using frequency spectra of ultrasonic echoes have already been proposed in previous studies. However, such methods had not taken into account the ambiguity of the frequency of each component of the spectrum. We have proposed a method, which estimates the mean frequency of each component of the spectrum, and the proposed method was validated by a phantom experiment. The experimental results showed that the bias errors in the estimated motion velocities of the phantom were less than or equal to (11.5% in lateral, 2.0% in axial) by the proposed 1D paired motion estimators and (3.0%, 2.0%) by the proposed 2D motion estimators, both of which were significantly smaller than (14.0%, 3.0%) of the conventional phase-sensitive 2D motion estimator.
Complex-valued neural networks (CVNNs) have shown their excellent efficiency compared to their real counter-parts in speech enhancement, image and signal processing. Researchers throughout the years ...have made many efforts to improve the learning algorithms and activation functions of CVNNs. Since CVNNs have proven to have better performance in handling the naturally complex-valued data and signals, this area of study will grow and expect the arrival of some effective improvements in the future. Therefore, there exists an obvious reason to provide a comprehensive survey paper that systematically collects and categorizes the advancement of CVNNs. In this paper, we discuss and summarize the recent advances based on their learning algorithms, activation functions, which is the most challenging part of building a CVNN, and applications. Besides, we outline the structure and applications of complex-valued convolutional, residual and recurrent neural networks. Finally, we also present some challenges and future research directions to facilitate the exploration of the ability of CVNNs.
Purpose
Spatial resolution is one of the important factors that determines ultrasound image quality. In the present study, methods using the phase variance of ultrasonic echoes received by individual ...transducer elements have been examined for improvement of spatial resolution.
Method
An imaging method, i.e., phase coherence imaging, which uses the phase coherence factor (PCF) obtained from the phase variance of received ultrasonic echoes, was recently proposed. Spatial resolution is improved by weighting ultrasonic RF signals obtained by delay-and-sum (DAS) beam forming using PCF. In the present study, alternative PCFs, i.e., exponential PCF, harmonic PCF, and Gaussian PCF, have been proposed and examined for further improvement of spatial resolution.
Result
Spatial resolutions realized by the proposed PCFs were evaluated by an experiment using a phantom. The full widths at half maxima of the lateral profiles of an echo from a string phantom were 2.61 mm (DAS only), 1.46 mm (conventional PCF), and 0.48–0.62 mm (proposed PCFs).
Conclusion
The PCFs newly proposed in the present study showed better spatial resolutions than the conventional PCF. The proposed PCFs also realized better visualization of echoes from a diffuse scattering medium than the conventional PCF.
The multi-angle Doppler method was introduced for the estimation of velocity vectors by measuring axial velocities from multiple directions. We have recently reported that the autocorrelation-based ...velocity vector estimation could be ameliorated significantly by estimating the wavenumbers in two dimensions. Since two-dimensional wavenumber estimation requires a snapshot of an ultrasonic field, the method was first implemented in plane wave imaging. Although plane wave imaging is predominantly useful for examining blood flows at an extremely high temporal resolution, it was reported that the contrast in a B-mode image obtained with a few plane wave emissions was lower than that obtained with focused beams. In this study, the two-dimensional wavenumber analysis was first implemented in a framework with focused transmit beams. The simulations showed that the proposed method achieved an accuracy in velocity estimation comparable to that of the method with plane wave imaging. Furthermore, the performances of the methods implemented in focused beam and plane wave imaging were compared by measuring human common carotid arteries in vivo. Image contrasts were analyzed in normal and clutter-filtered B-mode images. The method with focused beam imaging achieved a better contrast in normal B-mode imaging, and similar velocity magnitudes and angles were obtained by both the methods with focused beam and plane wave imaging. In contrast, the method with plane wave imaging gave a better contrast in a clutter-filtered B-mode image and smaller variances in velocity magnitudes than those with focused beams.
Effect of subaperture beamforming on phase coherence imaging Hasegawa, Hideyuki; Kanai, Hiroshi
IEEE transactions on ultrasonics, ferroelectrics, and frequency control,
2014-November, 2014-Nov, 2014-11-00, 20141101, Letnik:
61, Številka:
11
Journal Article
Recenzirano
High-frame-rate echocardiography using unfocused transmit beams and parallel receive beamforming is a promising method for evaluation of cardiac function, such as imaging of rapid propagation of ...vibration of the heart wall resulting from electrical stimulation of the myocardium. In this technique, high temporal resolution is realized at the expense of spatial resolution and contrast. The phase coherence factor has been developed to improve spatial resolution and contrast in ultrasonography. It evaluates the variance in phases of echo signals received by individual transducer elements after delay compensation, as in the conventional delay-andsum beamforming process. However, the phase coherence factor suppresses speckle echoes because phases of speckle echoes fluctuate as a result of interference of echoes. In the present study, the receiving aperture was divided into several subapertures, and conventional delay-and-sum beamforming was performed with respect to each subaperture to suppress echoes from scatterers except for that at a focal point. After subaperture beamforming, the phase coherence factor was obtained from beamformed RF signals from respective subapertures. By means of this procedure, undesirable echoes, which can interfere with the echo from a focal point, can be suppressed by subaperture beamforming, and the suppression of the phase coherence factor resulting from phase fluctuation caused by such interference can be avoided. In the present study, the effect of subaperture beamforming in high-frame-rate echocardiography with the phase coherence factor was evaluated using a phantom. By applying subaperture beamforming, the average intensity of speckle echoes from a diffuse scattering medium was significantly higher (-39.9 dB) than that obtained without subaperture beamforming (-48.7 dB). As for spatial resolution, the width at half-maximum of the lateral echo amplitude profile obtained without the phase coherence factor was 1.06 mm. By using the phase coherence factor, spatial resolution was improved significantly, and subaperture beamforming achieved a better spatial resolution of 0.75 mm than that of 0.78 mm obtained without subaperture beamforming.
Purpose
Ultrasound beamforming is required to obtain clinical ultrasound images. In the beamforming procedure, the distance between the receiving focal point and each transducer element is determined ...based on the assumed speed of sound in the tissue. However, the actual speed of sound in tissue is unknown and varies depending on the tissue type. To improve the performance of an ultrasonic beamformer by evaluating its focusing quality, the coherence factor (CF) was introduced in medical ultrasound imaging. The CF may be used to estimate the speed of sound in tissue because it can identify focusing errors in beamforming. In the present study, the feasibility of CF for estimating the speed of sound was examined through phantom experiments.
Method
To evaluate the dependency of CF on the assumed speed of sound in ultrasound beamforming, beamformed ultrasonic radio frequency (RF) signals and CFs were obtained at different assumed speeds of sound. CF is highest when the assumed speed of sound matches the true speed of sound in the medium. Therefore, the speed of sound in the medium was determined as the assumed speed of sound, which gives the highest CF. The proposed method was validated in a conventional line-by-line sequence with a focused transmit beam and ultrafast plane wave imaging.
Results
A homogeneous phantom (diffuse scattering medium) with a known speed of sound of 1540 m/s was used for validating the proposed method. Beamformed ultrasonic RF signals and CFs were obtained at an assumed speed of sound from 1480 to 1600 m/s varied at a pitch of 5 m/s. In the line-by-line sequence, CF reached the maximum at an assumed speed of sound of 1525.0 m/s (0.97% difference from the true value) when CFs at all spatial points in the region of interest (ROI) were averaged. On the other hand, the speed of sound was determined to be 1528.5 m/s (0.75% difference) when CFs at spatial points with CF-weighted echo amplitudes were larger than 20% of the maximum value. In plane wave imaging, the speed of sound was estimated to be 1544.5 m/s (0.29% difference) using CFs with CF-weighted echo amplitudes larger than 20% of the maximum value.
Conclusion
The speed of sound of a homogeneous medium could be determined by the proposed method with errors of less than 1% using CFs obtained from ultrasonic echo signals selected based on the CF-weighted echo amplitudes, i.e., when echo signals with better signal-to-noise ratios (SNRs) were used.
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