Echo Correction Research Articles

Correction of ionosphere phase contamination of high‐frequency hybrid sky‐surface wave radar using wavelet transform

AbstractIonospheric phase contamination (IPC) is a major contributor to the imprecise retrieval of ocean dynamics parameters by the high‐frequency hybrid sky‐surface wave radar (HFHSSW). Since the direct wave and sea echo of HFHSSW have comparable IPC, the information of ocean dynamics parameters can be preserved by using the IPC extracted from the direct waves to correct the sea echoes. In this article, a wavelet transform (WT) algorithm for HFHSSW sea echo correction is proposed. The direct waves are decomposed using a series of wavelet functions constructed with complex Morlet wavelets in order to extract the IPC and correct the sea echoes. The authors enumerated two examples and counted the correction results for 50 echoes to validate the performance of the WT algorithm. The results demonstrate that the WT algorithm enhances the signal‐to‐noise ratio (SNR) of the sea echoes and suppresses the broadening of the sea echoes. The 6‐dB bandwidth of the positive and negative first‐order Bragg peaks (B+, B−) decreases by 32.96% and 35.04% respectively, while the SNR increases by 1.16 and 1.21 dB respectively. In comparison to the generalised S‐transform (GST algorithm), the computation speed of the WT algorithm is approximately 3.2 times that of the GST algorithm, making it more suitable for practical operation.

Pengaruh Penerapan Quality Control Data Radar Terhadap Akurasi Estimasi Curah Hujan Di Wilayah Pontianak Dan Sekitarnya

Weather radar operational have a limitation that leads to a quality reduction of radar rainfall estimation, such as interference at Pontianak’s weather radar. The application of 3D Clutter Map and Data Processing (3DCDP), Brightband Echo Correction (BBC), and Z–Based Attenuation Correction (ZATC) which include in post–processing QC is the common procedure of radar limitation mitigation. This procedure applied to data of Pontianak weather radar over 1 January 2019 – 31 December 2019. The QC implemetation effect observed qualitatively and quantitatively by compare radar’s product before and after QC implementation. Data of radar rainfall estimation compared with rainfall observation data from AWS/ARG, and validated with RMSE and ME values. The comparison of the weather radar image shows that the interference and clutter around the weather radar is reduced. According to 19 validation value on the 10 cases of chosen rain, 18 value shows QC’s increased the accuracy of radar rainfall estimation, though insignificant and still underestimate.

Terahertz Multiple Echoes Correction and Non-Destructive Testing Based on Improved Wavelet Multi-Scale Analysis

During terahertz (THz) non-destructive testing (NDT), multiple echoes from the sample interface reflection signals are mixed with the detection signals, resulting in signal distortion and affecting the accuracy of the THz NDT results. Combined with the frequency property of multiple echoes, an improved wavelet multi-scale analysis is put forth in this paper to correct multiple echoes, allowing the maximum retention of detailed signal information in contrast with the existing echo correction methods. The results showed that the improved wavelet multi-scale analysis enhanced the continuity and smoothness of the image at least twice in testing adhesive layer thickness, prevented missing judgments and misjudgments in identifying characteristic defects, and ensured accurate detection results. Hence, it is of great significance for evaluating the THz NDT results.

Investigation of BOLD contrast in fMRI using multi‐shot EPI

https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1099-1492(199706/08)10:4/5%3C179::AID-NBM463%3E3.0.CO;2-X Menon, R.S., Thomas, C.G. and Gati, J.S. (1997), Investigation of BOLD contrast in fMRI using multi-shot EPI. NMR Biomed., 10: 179– 182. https://doi.org/10.1002/(SICI)1099-1492(199706/08)10:4/5<179::AID-NBM463>3.0.CO;2-X The following text in the Results and Discussion section: This procedure has the effect of linearizing the plots, effectively showing what the contrast sensitivity would be if the same number of images were acquired for each trial. Under these conditions, it becomes quite clear that the contrast sensitivity is still strongly dependent on TR. Thus in doing a multi-slice fMRI experiment with EPI, it is better to acquire all the segments for a single slice with the shortest possible TR before going on to the next slice. Note that this is exactly opposite to the strategy that one would use in anatomic imaging, where interleaving the segments and slices would yield superior SNR. Examination of the points for etl = 32 shows that the biggest drop in normalized contrast sensitivity occurs when the total acquisition time of the image exceeds 0.5 s. This is likely because the physiological sources of motion that are effectively frozen at the shorter TR values now start to modulate the intensity of the individual segments. Is replaced by the following: This procedure has the effect of showing what the contrast sensitivity would be if the same number of images were acquired for each trial. Under these conditions, it becomes quite clear that the contrast sensitivity is still strongly dependent on TR. Thus in doing a multi-slice fMRI experiment with EPI, it is better to interleave the segments and slices to yield superior SNR, as would be done in anatomic imaging. Examination of the points for etl = 32 and etl = 16 show that greater CSBOLD is realized at longer TR times which allow for higher flip angles resulting in higher SNR. Use of a short TR allows for collection of a larger number of images over the same period of total imaging time, hence yielding the best statistical detection thresholds, but the cost is paid in decreased SNR. Physiological sources of motion become more important to modulation of the image intensity over time. The following text in the Conclusion section: For a fixed number of slices, the best approach is to perform the acquisition of all the segments in a single slice before continuing on to the rest of the slices. Is replaced by the following: For a fixed number of slices, the best approach is to interleave the acquisition of the segments and slice across the entire imaging volume. The final sentence of the Conclusion should be supplemented with: It is important to note that no navigator echo was used in the data acquisition of this experiment, consequently these results do not necessarily apply if navigator echo correction was implemented.

Transverse relaxometry with transmit field-constrained stimulated echo compensation.

Purely exponential decay is rarely observed in conventional mono-exponential T2 mapping due to transmit field inhomogeneity and calibration errors, which collectively introduce stimulated and indirect echo pathways. Stimulated echo correction (SEC) requires an additional fit parameter for the transmit field, resulting in greater uncertainty in T2 relative to mono-exponential fitting. The aim of this study was to develop an accurate and precise method for T2 mapping using SEC. The proposed method, called two-step SEC (tSEC), leverages spatial correlations in the transmit field to reduce the number of fully independent fitting parameters from three to two. The method involves a two-pass fit: the first pass involves a fast but standard SEC fit. The initially estimated transmit field is smoothed and provided as a fixed input to the second pass. Simulations and in vivo experiments demonstrated up to 38% and 27% decreases in relative T2 variance with tSEC relative to SEC. Average T2 values were unchanged between tSEC and SEC fits. The proposed method uses the same input data as SEC and exponential fits, so it is applicable to existing data. The proposed method generates reliable and reproducible quantitative T2 maps and should be considered for future relaxometry studies.

Evaluating the accuracy of multicomponent T2 parameters for luminal water imaging of the prostate with acceleration using inner-volume 3D GRASE.

Prostate cancer can be detected using a multicomponent T2 mapping technique termed luminal water imaging. The purpose of this study is twofold: 1) To accelerate the luminal water imaging acquisition by using inner volume selection as part of a gradient and spin echo sequence, and 2) to evaluate the accuracy of luminal water fractions and multicomponent T2 relaxation times. The accuracy of parameter estimates was assessed using Monte Carlo simulations, in phantom experiments and in the prostate (in 5 healthy subjects). Two fitting methods, nonnegative least squares and biexponential fitting with stimulated echo correction, were compared. Results demonstrate that inner volume selection in a gradient and spin echo sequence is effective for accelerating prostate luminal water imaging by at least threefold. Evaluation of the accuracy shows that the estimated luminal water fractions are relatively accurate, but the short- and long-T2 relaxation times should be interpreted with caution in noisy scenarios (SNR < 100) and when the corresponding fractions are small ( < 0.5). The mean luminal water fractions obtained at SNR above 100 are 0.27 ± 0.07 for the peripheral zone for both fitting methods, 0.16 ± 0.04 for the transition zone with nonnegative least squares, and 0.16 ± 0.03 for the transition zone with biexponential fitting including stimulated echo correction. The shortened scan duration allows the luminal water imaging sequence to be easily integrated into a standard multiparametric prostate MRI protocol.

Genome assembly and annotation for red clover (Trifolium pratense; Fabaceae)

Red clover (Trifolium pratense) is an important forage plant from the legume family with great importance in agronomy and livestock nourishment. Nevertheless, assembling its medium-sized genome presents a challenge, given current hardware and software possibilities. Next-generation sequencing technologies enable us to generate large amounts of sequence data at low cost. In this study, the genome assembly and red clover genome features are presented. First, assembly software was assessed using data sets from a closely related species to find the best possible combination of assembler plus error correction program to assemble the red clover genome. The newly sequenced genome was characterized by repetitive content, number of protein-coding and nonprotein-coding genes, and gene families and functions. Genome features were also compared with those of other sequenced plant species. Abyss with Echo correction was used for de novo assembly of the red clover genome. The presented assembly comprises ∼314.6 Mbp. In contrast to leguminous species with comparable genome sizes, the genome of T. pratense contains a larger repetitive portion and more abundant retrotransposons and DNA transposons. Overall, 47 398 protein-coding genes were annotated from 64 761 predicted genes. Comparative analysis revealed several gene families that are characteristic for T. pratense. Resistance genes, leghemoglobins, and nodule-specific cystein-rich peptides were identified and compared with other sequenced species. The presented red clover genomic data constitute a resource for improvement through molecular breeding and for comparison to other sequenced plant species.

A pulse-length correction to improve energy-based seabed classification in coastal areas

Sea bottom classification using echosounders is an active field of research, where many different methods (to define echo features or perform their statistical classification) have been proposed and tested. Here we propose a new echo correction method suitable for use in coastal waters, where large relative depth variations occur. The idea is based on scaling the pulse length with depth as suggested by Pouliquen (Preston et al., 2004. Proceedings of the Seventh European Conference on Underwater Acoustics, ECUA 2004), but instead of in real time, in postprocessing. We investigate, in particular, the benefits of this correction for a classification based on the energy integrals of first and second return echoes, relying on the correction to find an optimal definition for those energy integrals. The method is tested in a coastal area survey with substrates varying over small scales (less than 200m) and with large relative changes in water depth (5–40m and slopes of up to 0.2). We show that the unsupervised classification bears a good agreement with divers groundtruthing (85% agreement with a Cohen׳s kappa, κ=0.74 for a 4-class map) and with previous knowledge of the study area.

Multicenter measurements of myelin water fraction and geometric mean T2: Intra‐ and intersite reproducibility

To assess the reproducibility of myelin water fraction (MWF) and geometric mean T2 (GMT2 ), which are in vivo markers of pathological changes underlying disability and progression in diseases such as multiple sclerosis. Five healthy volunteers were scanned twice within 24 hours at six different sites using the same manufacturer's 3T magnetic resonance (MR) system. T2 distributions were produced by fitting multiecho 3D T2 data using non-negative least squares, with stimulated echo correction. MWF, the fraction of signal with T2 between 15 and 40 msec to the entire signal, and GMT2 , the mean T2 on a logarithmic scale from T2 between 40 and 200 msec, were examined in white matter. Intrasite coefficients of variation (COVs) were low (mean 3.99% for MWF and 0.51% for GMT2 ), as were intersite COVs (mean 4.68% for MWF, 0.31% for GMT2 ). Scan-rescan intraclass correlation coefficients (ICCs) (0.76 for MWF and 0.93 for GMT2 ) and Bland-Altman plots indicated good agreement between single site scans. Intersite ICCs were relatively high (0.69 for MWF and 0.92 for GMT2 ), revealing good intersite reliability. MWF and GMT2 measures are reproducible between scans and across sites with an equivalent MR scanner and sequence protocol. Multicenter clinical trials using quantitative T2 relaxation are feasible.

Intramolecular zero‐quantum‐coherence 2D NMR spectroscopy of lipids in the human breast at 7 T

Intramolecular zero-quantum-coherences (ZQCs) are intrinsically insensitive toward magnetic field inhomogeneity. This fact is used to quantify and characterize lipid signals in the human breast at 7 T despite the presence of severe magnetic field inhomogeneity caused by water-lipid susceptibility boundaries. A novel 3D localized 2D ZQC method is presented. The combination of cardiac/respiratory triggering and post-acquisition navigator echo correction provides high-quality 2D NMR spectra in vivo. The lipid profile of the human breast could be quantified by 2D ZQC NMR in 100% of the subjects despite a wide range of magnetic field homogeneity. With conventional 1D (1)H MRS, the magnetic field homogeneity was only adequate in 60% of the subjects. The results from 2D ZQC NMR and 1D NMR are in good correspondence, both in vitro and in vivo. It has been demonstrated that high quality and quantitative 2D ZQC NMR spectra can be acquired from human breast tissue at 7 T. While the simplicity and sensitivity of 1D MRS are preferable when the magnetic field homogeneity is adequate, the 2D ZQC method provides a viable alternative in cases where this requirement cannot be met.

Sparse deconvolution based on regularisation for echo correction in antenna measurement

Antenna measurement may be affected by multipath effects when it is carried out in a semi-anechoic chamber or when a fault in the chamber arises. In this study, a sparse deconvolution method is applied to identify clearly the direct and reflected contributions in the time response of the measurement chamber. It is possible to recover the distortion-less radiation pattern by simply removing the reflected wave. This method requires half the bandwidth of other methods. Some significant results are presented to show the accuracy of the proposed approach.

Hybrid two-dimensional navigator correction: A new technique to suppress respiratory-induced physiological noise in multi-shot echo-planar functional MRI

A troublesome source of physiological noise in functional magnetic resonance imaging (fMRI) is due to the spatio-temporal modulation of the magnetic field in the brain caused by normal subject respiration. fMRI data acquired using echo-planar imaging are very sensitive to these respiratory-induced frequency offsets, which cause significant geometric distortions in images. Because these effects increase with main magnetic field, they can nullify the gains in statistical power expected by the use of higher magnetic fields. As a study of existing navigator correction techniques for echo-planar fMRI has shown that further improvements can be made in the suppression of respiratory-induced physiological noise, a new hybrid two-dimensional (2D) navigator is proposed. Using a priori knowledge of the slow spatial variations of these induced frequency offsets, 2D field maps are constructed for each shot using spatial frequencies between ±0.5 cm−1 in k-space. For multi-shot fMRI experiments, we estimate that the improvement of hybrid 2D navigator correction over the best performance of one-dimensional navigator echo correction translates into a 15% increase in the volume of activation, 6% and 10% increases in the maximum and average t-statistics, respectively, for regions with high t-statistics, and 71% and 56% increases in the maximum and average t-statistics, respectively, in regions with low t-statistics due to contamination by residual physiological noise.

Modeling and suppression of respiration‐related physiological noise in echo‐planar functional magnetic resonance imaging using global and one‐dimensional navigator echo correction

A major source of noise in functional magnetic resonance imaging (fMRI) arises from modulations in the local magnetic field in the head due to motion of the subject's chest through the respiratory cycle, and this physiologic noise can nullify the gains in statistical power expected by the use of higher magnetic fields for fMRI. In particular, fMRI data acquired using echo-planar imaging (EPI) are very sensitive to these spatially and temporally varying respiration-induced frequency offsets. In this study, accurate 3D magnetic field maps in the head were measured and used to determine the frequency offsets at the two extremes of the respiratory cycle. From these maps, spatially dependent frequency variations from about -1.0 Hz to +1.5 Hz were measured in the brain through the respiratory cycle. Simulations of a typical axial EPI fMRI experiment acquired in the presence of this measured field variation were performed, demonstrating regional image intensity variations between 1 and 5% in single pixel time series. The inadequacy of either global or 1D navigator echo corrections to measure and suppress respiratory-induced noise in fMRI time series is demonstrated. The nature of the spatial variations observed suggests that 2D approaches should be considered.

SPATIAL MAP BUILDING USING FAST TEXTURE ANALYSIS OF ROTATING SONAR SENSOR DATA FOR MOBILE ROBOTS

This paper presents a novel, fast algorithm for accurate detection of the shape of targets around a mobile robot using a single rotating sonar element. The rotating sonar yields an image built up by the reflections of an ultrasonic beam directed at different scan angles. The image is then interpreted with an image-understanding approach based on texture analysis. Several important tasks are performed in this way, such as noise removal, echo correction and restoration. All these processes are obtained by estimating and restoring the degree of texture continuity. Texture analysis, in fact, allows us to look at the image on a large scale thus giving the possibility to infer the overall behavior of the reflection process. The algorithm has been integrated in a mobile robot. However, the algorithm is not suitable for working during the mobile robot movement, rather it can be used during the period when the robot stays in a fixed position.

Two-step navigatorless correction algorithm for radial k-space MRI acquisitions.

A new way to correct magnetic resonance image artifacts resulting from view-dependent phase variations and view-dependent variations in rigid body object translation is presented by exploiting basic properties of the trajectory of radial k-space acquisitions. Simulations, phantom studies, and in vivo experiments are used to demonstrate the feasibility and the utility of this method. While somewhat analogous to navigator echo correction, in which special gradients are interleaved into the imaging sequence so echoes at the center of k-space can be acquired prior to or after collection of the image data, the current method does not require additional new gradient structures within the pulse sequence or increases in scan time. The new method uses the phase information from all collected radial k-space data points rather than only the navigator echo, which permits correction of multiple sources of view-dependent phase variation in the image data. The resultant effect is improved image quality in radial MRI acquisitions. Magn Reson Med 45:277-288, 2001.

Comparison of multiple sclerosis clinical subgroups using navigated spin echo diffusion-weighted imaging

The apparent diffusion coefficient (ADC) of tissue provides an indication of the size, shape, and orientation of the water spaces in tissue. Thus, pathologic differences between lesions in multiple sclerosis (MS) patients with different clinical courses may be reflected by changes in ADC measurements in lesions and white matter. Twelve healthy subjects and 35 MS patients with a relapsing-remitting ( n = 10), benign ( n = 8), secondary progressive ( n = 8) and primary progressive ( n = 9) clinical course were studied. T 2-weighted and post-gadolinium T 1-weighted images were obtained using a 1.5 T Signa Echospeed magnetic resonance imaging (MRI) system. Diffusion-weighted imaging was implemented using a pulsed gradient spin echo (PGSE) sequence with diffusion gradients applied in turn along three orthogonal directions in order to obtain the average apparent diffusion coefficient (ADC av). Navigator echo correction and cardiac gating were used to reduce motion artifact. ADC maps were derived using a two point calculation based on the Stejskal-Tanner formula. Diffusion anisotropy was estimated using the van Gelderen formula to calculate an anisotropy index. MS lesions had a higher ADC and reduced anisotropy compared with normal appearing white matter. Highest ADC values were found in gadolinium enhancing lesions and non-enhancing hypointense lesions on T 1-weighted imaging. MS white matter had a slightly higher ADC and lower anisotropy than white matter of healthy subjects. Lesion and white matter ADC values did not differ between patients with different clinical courses of MS. There was no correlation between lesion ADC and disability. Diffusion-weighted imaging with measurement of ADC using the PGSE method provides quantitative information on acute edematous MS lesions and chronic lesions associated with demyelination and axonal loss but does not distinguish between clinical subtypes of MS.

K-Space detection and correction of physiological artifacts in fMRI.

Signal phase variations caused by physiology are a major source of instability in fMRI images produced by multiple RF pulses. k-Space phase variation maps show cyclic phase variations at the frequency of respiration combined with a cardiac variation of lower amplitude. The amplitude of the variation increases with gradient echo time and proximity to the chest, suggesting that the dominant cause of the phase variation is a B0 shift (approximately 0.01 ppm) produced by movement of organs in the chest. A simple k-space phase correction method is proposed and demonstrated for FLASH fMRI. The retrospective method requires no pulse sequence modification, and is more effective than navigator echo correction. Physiological noise is dramatically reduced, especially at inferior slice locations.

4993414 Moving material projection imaging system using nuclear magnetic resonance

A troublesome source of physiological noise in functional magnetic resonance imaging (fMRI) is due to the spatio-temporal modulation of the magnetic field in the brain caused by normal subject respiration. fMRI data acquired using echo-planar imaging are very sensitive to these respiratory-induced frequency offsets, which cause significant geometric distortions in images. Because these effects increase with main magnetic field, they can nullify the gains in statistical power expected by the use of higher magnetic fields. As a study of existing navigator correction techniques for echo-planar fMRI has shown that further improvements can be made in the suppression of respiratory-induced physiological noise, a new hybrid two-dimensional (2D) navigator is proposed. Using a priori knowledge of the slow spatial variations of these induced frequency offsets, 2D field maps are constructed for each shot using spatial frequencies between ± 0.5 cm− 1 in k-space. For multi-shot fMRI experiments, we estimate that the improvement of hybrid 2D navigator correction over the best performance of one-dimensional navigator echo correction translates into a 15% increase in the volume of activation, 6% and 10% increases in the maximum and average t-statistics, respectively, for regions with high t-statistics, and 71% and 56% increases in the maximum and average t-statistics, respectively, in regions with low t-statistics due to contamination by residual physiological noise.