Postfilter for Dual Channel Speech Enhancement Using Coherence and Statistical Model-Based Noise Estimation.

A multichannel speech enhancement system usually consists of spatial filters such as adaptive beamformers followed by postfilters, which suppress remaining noise. Accurate estimation of the power spectral density (PSD) of the residual noise is crucial for successful noise reduction in the postfilters. In this paper, we propose a postfilter utilizing proposed a posteriori speech presence probability (SPP) and noise PSD estimators, which are based on both the coherence and the statistical models. We model the coherence-based a posteriori SPP as a simple function of the magnitude of coherence between two microphone signals and combine it with a single-channel SPP based on statistical models. The coherence-based estimator for the PSD of the noise remaining in the beamformer output in the presence of speech is derived using the pseudo-coherence considering the effect of the beamformers, which is used to construct the coherence-based noise PSD estimator. Then, the final noise PSD estimator is obtained by combining the coherence-based and statistical model-based noise PSD estimators with the proposed SPP. The spectral gain function is also modified, incorporating the proposed SPP. Experimental results demonstrate that the proposed method led to more accurate noise PSD estimation and perceptual evaluation of speech quality scores in various diffuse noise environments, and did not degrade the speech quality under the presence of directional interference, although the proposed method utilizes the coherence information.

Standard versus no post-filter ionized calcium monitoring in regional citrate anticoagulation for continuous renal replacement therapy (NPC trial).

Background: Current guidelines recommend monitoring of post-filter ionized calcium (pfCa) when using regional citrate anticoagulation during continuous renal replacement therapy (RCA-CRRT) to determine citrate efficiency for the prevention of filter clotting. However, the reliability of pfCa raises the question of whether routine monitoring is required. Reducing the frequency of pfCa monitoring could potentially reduce costs and workload. Our objective was to test the efficacy and safety of no pfCa monitoring among critically ill patients receiving RCA-CRRT. Methods: This study was a non-inferiority randomized controlled trial conducted between January 2021 and October 2021 at King Chulalongkorn Memorial Hospital, Thailand. Critically ill patients who were treated with RCA-CRRT were randomized to receive either standard pfCa monitoring (aiming pfCa level of 0.25-0.35 mmol/L), or no pfCa monitoring, in which a constant rate of citrate infusion was maintained at pre-determined citrate concentrations of 4 mmol/L with blinding of pfCa levels to treating clinicians. The primary outcome was the filter lifespan. Non-inferiority would be demonstrated if the upper limit of the 95% confidence interval (CI) for the difference in filter lifespan between the groups was less than 20 h. Results: Fifty patients were randomized to the standard pfCa monitoring group (n = 25) or no pfCa monitoring group (n = 25). The mean filter lifespan was 54 ± 20 h in the standard pfCa monitoring group and 47 ± 23 h in the no pfCa monitoring group (absolute difference 7.1 h; 95% CI -5.3, 19.5, P = .25). When restricting the analysis to circuits reaching the maximum duration of circuit lifespan at 72 h and clotted filters, the filter lifespan was 61 ± 17 h in the standard pfCa group vs 60 ± 19 h in the no pfCa monitoring group (absolute difference 0.9 h; 95% CI -11.5, 13.4, P = .88). Compared with the no pfCa monitoring group, the standard pfCa monitoring group had a significantly higher mean citrate concentrations (4.43 ± 0.32 vs 4 mmol/L, P < .001) and a higher rate of severe hypocalcemia (44% vs 20%, P = .13). No statistical differences were found in filter clotting, citrate accumulation, citrate overload and mortality between the two groups. Conclusions: Among critically ill patients receiving RCA-CRRT, no pfCa monitoring by maintaining the citrate concentrations of 4 mmol/L is feasible. Larger randomized controlled trials should be conducted to ensure the efficacy, safety and cost-effectiveness of this strategy. Trial registration: ClinicalTrials.gov: NCT04792424 (registered 11 March 2021).

Generalized sidelobe canceler beamforming combined with Eigenspace-Wiener postfilter for medical ultrasound imaging.

BACKGROUND: The beamforming algorithm is key to the image quality of the medical ultrasound system. The generalized sidelobe canceler (GSC) beamforming can improve the image quality in lateral resolution, but the contrast is not improved correspondingly. OBJECTIVE: In our research, we try to optimize the generalized sidelobe canceler to obtain images that achieve an improvement in both lateral resolution and contrast. METHODS: We put forward a new beamforming algorithm which combines the generalized sidelobe canceler and Eigenspace-Wiener postfilter. According to eigenspace decomposition of the covariance matrix of the received data, the components of the Wiener postfilter can be calculated from the signal matrix and the noise matrix. Then, the adaptive weight vector of GSC is further constrained by the Eigenspace-Wiener postfilter, which make the output energy of the receiving array closer to the desired signal than the conventional GSC output. RESULTS: We compare the new beamforming algorithm with delay-and-sum (DS) beamforming, synthetic aperture (SA) beamforming, and GSC beamforming using the simulated and experimental data sets. The quantitative results show that our method reduces the FWHM by 85.5%, 80.5%, and 38.9% while improving the CR by 123.6%, 47.7%, 84.4% on basis of DS, SA, and GSC beamforming, respectively. CONCLUSIONS: The new beamforming algorithm can obviously improve the imaging quality of medical ultrasound imaging systems in both lateral resolution and contrast.

Ultrasound far-focused pixel-based imaging using Wiener postfilter scaled by adjustable zero-cross factor.

Synthetic aperture (SA) imaging can provide a uniform lateral resolution but an insufficient signal-to-noise ratio (SNR). SA method with bidirectional pixel-based focusing (SA-BiPBF) has the ability to obtain a higher quality image than conventional SA imaging. In this paper, an enhanced SA-BiPBF named full aperture received far-focused pixel-based (FrFPB) is firstly proposed to obtain a high resolution image. An adjustable zero-cross factor scaled Wiener postfilter (AZFsW) is then implemented in FrFPB for improving contrast ratio (CR). The adjustable zero-cross factor is calculated using the polarity of echo signals sequence with an adjustable coefficient σ to estimate the signal coherence, and it is combined with Wiener postfilter to obtain a good capability of noise reduction and background speckle pattern preservation. Simulation and experiments have been conducted to evaluate the imaging performance of the proposed methods. Results show that FrFPB can obviously improve the resolution in comparison with SA-BiPBF, and contrast-to-noise ratio (CNR) and speckle signal-to-noise ratio (sSNR) are retained. In addition, AZFsW can achieve a much higher CR than SA-BiPBF. When σ is 0.6, the CR improvement is 96.7% in simulation, 78.7% in phantom experiment, and 49.2% in in-vivo experiment. To evaluate the imaging performance of AZFsW, coherence factor, conventional Wiener postfilter, and scaled Wiener postfilter are implemented. The imaging results show that when σ is in the range of [0.6, 0.7], AZFsW exhibits a satisfying comprehensive imaging performance.

United Wiener postfilter for plane wave compounding ultrasound imaging.

Plane wave compounding (PWC) is a valid method for ultrafast ultrasound imaging. Its imaging quality depends on the beamforming method. The coherence factor (CF) and Wiener postfilter are effective signal processing schemes for aberration correction. However, the CF usually causes over-suppression and brings artifacts. Additionally, the conventional CF and Wiener postfilter cannot fully utilize the spatial coherence in the PWC, which limits the imaging performance and increases the computation. In this paper, we propose a united Wiener postfilter specially for the PWC. The signal and noise power are both estimated through the echo signal matrix, rather than array signal vectors. The method also accords with the theoretical relationship between the CF and Wiener. To evaluate the performance of the proposed method, we conduct simulations, phantom and in vivo experiments and make comparisons with the delay-and-sum (DAS), the CF, the generalized coherence factor (GCF), the conventional Wiener and the scaled Wiener beamformers. Results indicate that our method can offer the better resolution and contrast than the DAS and Wiener. It also solves the over-suppression drawback of the CF. Specifically, the contrast ratio and contrast-to-noise ratio achieve 26.7% and 25.2% improvements in simulations, 28.7% and 32.4% in phantom experiments, respectively. The proposed method also performs well in terms of the speckle signal-to-noise ratio and the generalized contrast-to-noise ratio. Consequently, we believe that the proposed method is effective in enhancing the imaging quality of the PWC.

Subspace-Based Blood Power Spectral Capon Combined with Wiener Postfilter to Provide a High-Quality Velocity Waveform with Low Mathematical Complexity.

In Doppler analysis, the power spectral density (PSD), which accounts for the axial velocity distribution of the blood scatterers, is estimated. The conventional spectral estimator is Welch's method, which suffers from frequency leakage at small observation window length. The performance of adaptive techniques such as blood power Capon (BPC) has been promising at the cost of higher computation complexity. Reducing the computational complexity while retaining the benefits of BPC would be necessary for real-time implementation. The purpose of the work described here was to investigate whether it is possible to decrease the computation load in BPC and still obtain acceptable results. The computation complexity in BPC is owing primarily to the matrix inversion required for computing the PSD estimate. We here propose the subspace blood power Capon technique, which employs a data covariance matrix with reduced number of rows in estimation of the weight vector. In maximum velocity estimation in the spectra, the signal noise slope intersection envelop estimator that makes use of the integrated power spectrum is employed. The evaluations are made based on both simulated and in vivo data. The results indicate that it is possible to reduce the order of complexity to almost 12.25% at the cost of 2.31% and 2.24% increases in the relative standard deviation and relative bias of the estimates. Moreover, the Wiener post-filter as a post-weighting factor, which will be multiplied by the final weight vector of the spectral estimator, estimates the power of the desired signal and the power of the interference plus noise to improve the contrast. The proposed estimator has exhibited a promising performance at beam-to-flow angles of 45°, 60° and 75°. Furthermore, the robust performance of the proposed estimator against variation in the flow rate is also documented.

Intensive monitoring of post filter ionized calcium concentrations during CVVHD with regional citrate anticoagulation: A retrospective study.

BACKGROUND: The aim of the present study was to assess the predictive value of post-filter ionized calcium (pfCa) levels for filter-clotting during continuous veno-venous hemodialysis (CVVHD) with regional citrate anticoagulation (RCA). METHODS: Retrospective analysis of a database derived from 6 intensive care units (ICU) at a university hospital. During the 3-year period 1070 patients were treated with RCA-CVVHD with a citrate starting dose of 4 mmol/L blood and a target-range for pfCa of 0.25-0.35 mmol/L. RESULTS: The pfCa concentrations at RCA-CVVHD initiation were within the target range in 69.7% of patients. Within 12 h the fraction of patients with pfCa above target-range decreased significantly from 13.1% to 7.8% (p < .001). There was no significant difference in filter survival between patients with a pfCa initially below, within, or above the target-range (83.7%, 89.5% and 90.4%; p = .228) and no significant correlation between the last pfCa and the incidence of filter clotting (rho 0.018, p = .572 and -0.054, p = .104; respectively). CONCLUSIONS: CVVHD with a citrate starting dose of 4 mmol/L blood resulted in a pfCa within target in the majority of patients. The observation that pfCa was not associated with the incidence of circuit clotting suggests that less frequent measurements of pfCA might be safe.

Adaptive scaling Wiener postfilter using generalized coherence factor for coherent plane-wave compounding.

This paper proposes an adaptive scaling Wiener postfilter (AScW) for coherent plane-wave compounding (CPWC) to improve the image quality. AScW introduces an adaptive scale factor dependent on the signal incoherence to maintain the performance balance between good noise suppression and good robustness. AScW utilizes several plane waves with a small angular difference to calculate generalized coherence factor (GCF) for the estimation of signal incoherence and noise power. And a standard depth parameter is used to further adjust the beamforming performance of AScW and improve speckle quality. The proposed method was tested on datasets acquired from simulation, experimental phantom and in-vivo study provided by the Plane-wave Imaging Challenge in Medical Ultrasound (PICMUS). Results show that AScW applied to CPWC can provide a good speckle quality and lateral resolution, while attaining a satisfying contrast. AScW can achieve maximal improvements of speckle signal-to-noise ratio (sSNR) by 36.4% (simulation) and 44.8% (experiment) compared with GCF, while 14.4% (simulation) and 12.5% (experiment) compared with the scaled Wiener postfilter (ScW). And the maximal lateral full width at half maximum (FWHM) improvements are 51.9% upon GCF and 53.7% upon ScW. Meanwhile, the contrast of AScW is comparable to that of GCF. From the results of in-vivo study, AScW also shows its clinical application potential for the visualization of anatomical structures and hyper echoic structures in ultrasound imaging.

The possibility of using effluent ionized calcium to assess regional citrate anticoagulation in continuous renal replacement therapy.

AIM: This study aimed to investigate whether effluent ionized calcium was an appropriate indicator to assess anticoagulant effect in continuous renal replacement therapy with regional citrate anticoagulation instead of post-filter ionized calcium. METHODS: In total, 48 paired samples of effluent fluid and post-filter blood were obtained from critically ill patients who required continuous renal replacement therapy. All samples were taken for ionized calcium measurements and were assessed by point-of-care analyzer. Correlations and agreements between two methods were performed by Pearson linear analysis and Bland-Altman analysis accordingly. RESULTS: The mean post-filter ionized calcium was 0.42 ± 0.12 mmol/L, and mean ionized calcium level of effluent fluid was 0.39 ± 0.11 mmol/L. The ionized calcium level of effluent fluid was significantly correlated with post-filter ionized calcium in all continuous renal replacement therapy patients. Bland-Altman analysis showed that the mean difference of ionized calcium between two sampling sites in all continuous renal replacement therapy patients was -0.02 mmol/L with 95% confidence interval ranging from -0.09 to 0.04 mmol/L. The significant correlations and agreements were also demonstrated in continuous veno-venous hemofiltration, continuous veno-venous hemodialysis, and continuous veno-venous hemodiafiltration modalities separately. CONCLUSION: The effluent ionized calcium could be a considerable substitute for post-filter ionized calcium to monitor the validity of regional citrate anticoagulation in continuous renal replacement therapy with less blood loss.

Subarray coherence based postfilter for eigenspace based minimum variance beamformer in ultrasound plane-wave imaging.

This paper introduces a new beamformer, which combines the eigenspace based minimum variance (ESBMV) beamformer with a subarray coherence based postfilter (SCBP), for improving the quality of ultrasound plane-wave imaging. The ESBMV beamformer has been validated in improving the imaging contrast, but the difficulty in dividing the signal subspace limits the usage of it in the low signal-to-noise ratio (SNR) scenarios. Coherence factor (CF) based methods could optimize the output of a distortionless beamformer to reduce sidelobes, but the influence by the subarray decorrelation technique on the postfilter design has not attracted enough concern before. Accordingly, an ESBMV-SCBP beamformer was proposed in this paper, which used the coherence of the subarray signal to compute an SCBP to optimize the ESBMV results. Simulated and experimental data were used to evaluate the performance of the proposed method. The results showed that the ESBMV-SCBP method achieved an improved imaging quality compared with the ESBMV beamformer. In the simulation study, the contrast ratio (CR) for an anechoic cyst was improved by 9.88 dB and the contrast-to-noise ratio (CNR) was improved by 0.97 over the ESBMV. In the experimental study, the CR improvements for two anechoic cysts were 7.32 dB and 9.45 dB, while the CNRs were improved by 1.27 and 0.66, respectively. The ESBMV-SCBP also showed advantages over the ESBMV-Wiener beamformer in preserving a less grainy speckle, which is closer to that of distortionless beamformers and benefits the imaging contrast. With a relatively small extra computational load, the proposed method has potential to enhance the quality of the ultrasound plane-wave imaging.

Amplitude and phase estimator combined with the Wiener postfilter for medical ultrasound imaging.

BACKGROUND: The adaptive amplitude and phase estimator (APES) has been introduced in medical ultrasound imaging to calculate the amplitude of the desired signal more robustly than other adaptive beamformers like minimum variance (MV). This beamformer minimizes the optimization problem of MV by replacing the estimated array covariance matrix by the interferences plus noise covariance matrix. On the other hand, the Wiener postfilter as a post-weighting factor, which will be multiplied to the final weight vector of the beamformer, estimates the power of the desired signal and the power of the interferences plus noise to improve the contrast. METHOD: The proposed method is a combination of the APES beamformer with the Wiener postfilter which uses the capabilities of the APES beamformer for accurate estimation of the amplitude of the desired signal and the Wiener postfilter in suppressing sidelobes. Specifically, we used the interferences plus the noise covariance matrix estimated in the APES beamformer to obtain an APES-based Wiener postfilter and obtained the APES + Wiener weight vector by multiplying the APES-based Wiener postfilter to the standard APES weight vector. RESULTS: To evaluate the proposed APES + Wiener beamformer, we tested the proposed method on simulated and experimental datasets. The results of a simulated wire phantom demonstrate that the proposed beamformer can resolve two point scatterers better than the standard APES beamformer, even if the points are placed near each other. Simulating a cyst phantom shows that the APES + Wiener beamformer improves the contrast of the resulting images by about 4.5 dB by estimating the interior of the cyst better than the standard APES. CONCLUSION: The evaluation of the proposed beamformer on an experimental dataset confirms the results of simulations, in which the proposed beamformer improves the resolution and contrast in comparison with the standard APES beamformer.