Showing papers on "Artifact (error) published in 2004"

Automatic removal of eye movement and blink artifacts from EEG data using blind component separation

Abstract: Signals from eye movements and blinks can be orders of magnitude larger than brain-generated electrical potentials and are one of the main sources of artifacts in electroencephalographic (EEG) data. Rejecting contaminated trials causes substantial data loss, and restricting eye movements/blinks limits the experimental designs possible and may impact the cognitive processes under investigation. This article presents a method based on blind source separation (BSS) for automatic removal of electroocular artifacts from EEG data. BBS is a signal-processing methodology that includes independent component analysis (ICA). In contrast to previously explored ICA-based methods for artifact removal, this method is automated. Moreover, the BSS algorithm described herein can isolate correlated electroocular components with a high degree of accuracy. Although the focus is on eliminating ocular artifacts in EEG data, the approach can be extended to other sources of EEG contamination such as cardiac signals, environmental noise, and electrode drift, and adapted for use with magnetoencephalographic (MEG) data, a magnetic correlate of EEG.

Evaluation of Image Artifact Produced by Optical Coherence Tomography of Retinal Pathology

Abstract: Purpose To determine the frequency and type of optical coherence tomography (OCT) fast macular thickness map (FMTM) scan artifacts, and whether these artifacts depend on patient diagnosis, demographics, and ocular therapy. Design Retrospective observational case series. Methods Records from patients who underwent an ophthalmologic evaluation by a member of the Duke University Eye Center vitreoretinal faculty and had an OCT scan produced by the FMTM protocol between July 7, 2003 and July 31, 2003 were reviewed. The relationships between OCT scan artifacts and ocular diagnosis, ocular treatment, and patient demographics were determined. Logistic regression was used to relate OCT scan artifacts simultaneously with ocular diagnosis and treatment. Results Scans from 171 eyes were analyzed. Retinal scan artifacts, though not observed in normal eyes, were identified frequently in eyes with macular pathology ( P = .049). Artifacts were observed in 43.2% of all scans, and of these, an erroneous retinal thickness measurement was obtained in 62.2%. Six types of OCT surface map artifacts were observed. Of these, inner and outer retinal misidentification, degraded image artifact, and "off center" artifact were significantly associated with central thickness calculation errors ( P P = .002, .022, and Conclusion Optical coherence tomography scan artifacts are seen surprisingly frequently, adversely affect retinal thickness measurements in a high proportion of cases, and are diagnosis-dependent. Recognition of these artifacts will improve retinal thickness measurement accuracy, and will prevent faulty treatment decisions that are based on inaccurate retinal thickness measurements.

Virtual image artifact detection

Abstract: Artifacts are detected in a cyclopean virtual image generated from stereo images. A disparity map is generated from the stereo images. Individual projected images are determined based on the disparity map and the corresponding stereo images. A difference map is computed between the individual projected images to indicate the artifacts. A source patch in the virtual image is defined relative to an artifact. A replacement target patch is generated using a split-patch search technique as a composite of a background exemplar patch and a foreground exemplar patch. Each exemplar patch may be identified from an image patch selected from at least one of the stereo images. The source patch of the virtual image is replaced by the replacement target patch to correct the detected artifact.

UNFOLD-SENSE: a parallel MRI method with self-calibration and artifact suppression.

Abstract: This work aims at improving the performance of parallel imaging by using it with our "unaliasing by Fourier-encoding the overlaps in the temporal dimension" (UNFOLD) temporal strategy. A self-calibration method called "self, hybrid referencing with UNFOLD and GRAPPA" (SHRUG) is presented. SHRUG combines the UNFOLD-based sensitivity mapping strategy introduced in the TSENSE method by Kellman et al. (5), with the strategy introduced in the GRAPPA method by Griswold et al. (10). SHRUG merges the two approaches to alleviate their respective limitations, and provides fast self-calibration at any given acceleration factor. UNFOLD-SENSE further includes an UNFOLD artifact suppression scheme to significantly suppress artifacts and amplified noise produced by parallel imaging. This suppression scheme, which was published previously (4), is related to another method that was presented independently as part of TSENSE. While the two are equivalent at accelerations 2.0, with up to double the artifact suppression at high accelerations. Furthermore, a slight modification of Cartesian SENSE is introduced, which allows departures from purely Cartesian sampling grids. This technique, termed variable-density SENSE (vdSENSE), allows the variable-density data required by SHRUG to be reconstructed with the simplicity and fast processing of Cartesian SENSE. UNFOLD-SENSE is given by the combination of SHRUG for sensitivity mapping, vdSENSE for reconstruction, and UNFOLD for artifact/amplified noise suppression. The method was implemented, with online reconstruction, on both an SSFP and a myocardium-perfusion sequence. The results from six patients scanned with UNFOLD-SENSE are presented.

Artifact in the bispectral index in a patient with severe ischemic brain injury.

Abstract: The electroencephalogram (EEG) has been used to predict neurological outcome in patients with anoxic-ischemic brain injury. The bispectral index (BIS) may be a useful alternative. A persistently low BIS associated with burst-suppression of the raw EEG in the setting of minimal hypnotic drug administ

Quantitative evaluation of metal artifact reduction techniques

Abstract: Purpose To develop a technique to quantify artifact, and to use it to compare the effectiveness of several approaches to metal artifact reduction, including view angle tilting and increasing the slice select and image bandwidths (BWs), in terms of metal artifact reduction, noise, and blur. Materials and Methods Nonmetallic replicas of two metal implants (stainless steel and titanium/chromium-cobalt femoral prostheses) were fabricated from wax, and MR images were obtained of each component immersed in water. The differences between the images of each metal prosthesis and its wax counterpart were measured. The contributions from noise and blur were isolated, resulting in a measure of the metal artifact. Several off-resonance artifact reduction techniques were assessed in terms of metal artifact reduction capability, as well as signal to noise ratio and blur. Results Increasing the image BW from ±16 kHz to ±64 kHz was found to reduce the artifact by an average of 60%, while employing view angle tilting (VAT) alone was found to reduce the artifact by an average of 63%. The metal artifact reduction sequence (MARS), which combines several susceptibility artifact reduction techniques, resulted in the least amount of image distortion, reducing the artifact by an average of 79%. Conclusion The results indicate that while VAT alone (with an image BW of ±16 kHz) resulted in the smallest amount of total energy and no reduction in the signal-to-noise ratio compared to a conventional spin-echo pulse sequence, MARS resulted in significantly less artifact and dramatically less blur. J. Magn. Reson. Imaging 2004;20:487–495. © 2004 Wiley-Liss, Inc.

Independent Component Analysis Applied in Biomedical Signal Processing

Abstract: The EEG is composed of electrical potentials arising from several sources. Each source (including separate neural clusters, blink artifact, or pulse artifact) projects a unique topography onto the scalp-'scalp maps'. These maps are mixed according to the principle of linear superposition. Independent component analysis (ICA) attempts to reverse the superposition by separating the EEG into mutually independent scalp maps, or components so that the method used is the study is independent component analysis (ICA). The Polysomnogram (PSG), a complex of more signals recorded over night for the patients with respiratory problems contains also redundant information that can be removed using the ICA.

Neural-ICA and wavelet transform for artifacts removal in surface EMG

Abstract: Recent works have shown that artifacts removal in biomedical signals, like electromyographic (EMG) or electroencephalographic (EEG) recordings, can be performed by using discrete wavelet transform (DWT) or independent component analysis (ICA). Often, the removal of some artifacts is very hard because they are superimposed on the recordings and they corrupt biomedical signals also in frequency domain. In these cases DWT and ICA methods cannot perform artifacts cancellation. We present a method based on the joint use of wavelet transform and independent component analysis. We show the obtained results and the comparisons among the proposed method, DWT and ICA techniques. In this preliminary study, a user interface is needed to identify the artifact.

A wavelet based de-noising technique for ocular artifact correction of the electroencephalogram

Abstract: In this study, the technique of wavelet de-noising of the electroencephalogram (EEG) signal to correct ocular artifact (OA) is proposed. The Donoho technique is used in work as base method: 1) a wavelet transform is applied to the EEG record; 2) the thresholding of the coefficients in the lower frequency bands is performed; 3) the de-noised signal is reconstructed. The estimation of reached de-noising is under construction. The proposed technique allows to free from any limitation in using in experiments EEG contaminated with eye movements.

Measurement and correction of respiration-induced B0 variations in breast 1H MRS at 4 Tesla.

Abstract: Respiratory motion is well known to cause artifacts in magnetic resonance spectroscopy (MRS). In MRS of the breast, the dominant artifact is not due to motion of the breast itself, but rather it is produced by B0 field distortions associated with respiratory motion of tissues in the chest and abdomen. This susceptibility artifact has been reported to occur in the brain, but it is more apparent in the breast due to the anatomic proximity of the lungs. In the breast, these B0 distortions cause shot-to-shot frequency shifts, which vary an average of 24 Hz during a typical 1 H MRS scan at 4 T. This variation can be corrected retrospectively by frequency shifting individual spectra prior to averaging. If not corrected, these shifts reduce spectral resolution and increase peak fitting errors. This work demonstrates the artifact, describes a method for correcting it, and evaluates its impact on quantitative spectroscopy. When the artifact is not corrected, quantification errors increase by an average of 28%, which dramatically impacts the ability to measure metabolite resonances at low signal-to-noise ratios. Magn Reson Med 52:1239 –1245, 2004. © 2004 Wiley-Liss, Inc.

Removal of phase artifacts from fMRI data using a Stockwell transform filter improves brain activity detection.

Abstract: A novel and automated technique is described for removing fMRI image artifacts resulting from motion outside of the imaging field of view. The technique is based on the Stockwell transform (ST), a mathematical operation that provides the frequency content at each time point within a time-varying signal. Using this technique, 1D Fourier transforms (FTs) are performed on raw image data to obtain phase profiles. The time series of phase magnitude for each and every point in the phase profile is then subjected to the ST to obtain a time-frequency spectrum. The temporal location of an artifact is identified based on the magnitude of a frequency component relative to the median magnitude of that frequency's occurrence over all time points. After each artifact frequency is removed by replacing its magnitude with the median magnitude, an inverse ST is applied to regain the MR signal. Brain activity detection within fMRI datasets is improved by significantly reducing image artifacts that overlap anatomical regions of interest. The major advantage of ST-filtering is that artifact frequencies may be removed within a narrow time-window, while preserving the frequency information at all other time points.

Using independent component analysis to remove artifact from electroencephalographic measured during stuttered speech.

Abstract: The electro-encephalographic (EEG) activity of people who stutter could provide invaluable information about the association of neural processing and stuttering. However, the EEG has never been adequately studied during speech in which stuttering naturally occurs. This is owing, in part, to the masking of the EEG signal by artifact from sources such as the speech musculature and from ocular activity. The aim of this paper was to demonstrate the ability of independent component analysis (ICA) to remove artifact from the EEG of stuttering children recorded while they are speaking and stuttering. The EEG of 16 male children who stuttered and 16 who did not stutter was recorded during a reading task. The recorded EEG that contained artifact was then subjected to ICA. The results demonstrated that the EEG assessed during stuttered speech had substantially more noise than the EEG of speech that did not contain stuttering (p<0.01). Furthermore, it was shown that ICA could effectively remove this artifact in all 16 children (p<0.01). The results from one child highlight the findings that ICA can be used to remove dominant artifact that has prevented the study of EEG activity during stuttered speech in children.

Apparatus and method for detecting blood flow signal free from motion artifact and stress test apparatus using the same

Abstract: An apparatus and method to detect a blood flow signal free from a motion artifact, and a stress test apparatus using the same, enhance data reliability of the blood flow signal by removing the motion artifact from the blood flow signal detected by photo-plethysmography. The apparatus to detect the blood flow signal includes a base pattern correlation coefficient calculating unit to determine peak points in the blood flow signal sensed from a body of an examinee using a blood flow sensing unit, and to calculate correlation coefficients of each peak point using a predetermined base pattern, and a motion artifact processing unit to determine the motion artifact using the calculated correlation coefficients and to remove the motion artifact from the blood flow signal. Thus, reliability of the blood flow signal is enhanced by effectively removing the motion artifact from the blood flow signal detected by the photo-plethysmography.

CPM 2: A Revised Core Product Model for Representing Design Information

Abstract: This report presents a revised version of the Core Product Model (CPM) initially reported in [1]. The initial CPM was intended to provide a base-level product model that is: not tied to any specific application or software; open; non-proprietary; simple; generic; expandable; independent of any product development process; and capable of capturing all product information shared throughout the product’s lifecycle. The revisions presented continue to support these intentions. The objectives of the report are: (1) to document the changes in the CPM relative to the initial version; (2) to describe in detail the revised CPM, represented as a UML class diagram; (3) to show, through Java and XML implementations, how the CPM can be used as the basis, or organizing principle, of a product information-modeling framework that can support the full range of product design information; and (4) to present a rational, model-based process for converting a CPM supporting the early conceptual phases of design into an implementation-level operational database support system. UML, XML and Java representations of the model are presented so as to provide interoperability with other models. A case study example is discussed and its XML representation is presented and analyzed to illustrate the principal elements of the revised CPM.

The SENSE ghost: Field-of-view restrictions for SENSE imaging

Abstract: Purpose: To describe a known (but undocumented) limitation in parallel imaging using simulation and experiment. This limitation consists of an artifact that appears when the imaging field of view (FOV) is less than the object size. This study demonstrates this artifact in the phase- and partition-encoding dimensions. Materials and Methods: One-dimensional simulations as well as in vivo experiments were performed with FOVs greater and less than the object being imaged. Full-FOV, reduced-FOV, and SENSE reconstructions were visually compared. Results: Image artifacts occurred when the final SENSE FOV was smaller than the object being imaged. This artifact, termed the SENSE ghost, was a residual fold-over/ aliasing artifact. Its location was in the central portion of the image rather than at the edges of the image. Conclusion: This image artifact results from an FOV being smaller than the imaged object. The SENSE reconstruction cannot unfold this additional fold-over, and will place it in a predictable image location based on the SENSE reduction factor. Knowledge of this artifact is necessary when prescribing SENSE acquisitions and interpreting the resulting

Magnetic resonance imaging with ultra short echo times

Abstract: Systems and techniques for imagining samples including components with small values of (T2). Optionally, the systems and techniques may provide suppression of unwanted signals, enhanced contrast, and artifact control in imaging samples with small values of (T2).

Using chest velocity to process physiological signals to remove chest compression artifacts

Abstract: A method of analyzing a physiological (e.g., an ECG) signal during application of chest compressions. The method includes acquiring a physiological signal during application of chest compressions; acquiring the output of a sensor from which information on the velocity of chest compressions can be determined; and using the information on the velocity to reduce at least one signal artifact in the physiological signal resulting from the chest compressions.

Document authentication combining digital signature verification and visual comparison

Abstract: A document authentication system and method combine digital and non-electronic (or visual) authentication methodologies in an integrated, unified manner. As well as providing indicia of digital authentication, the invention generates a physical artifact that can be validated by unaided human visual perception. The present invention thus provides an opportunity to improve the level of trust in authentication of documents, while preserving the advantages of both traditional and digital authentication mechanisms.

A Framework for Evaluating ICA Methods of Artifact Removal from Multichannel EEG

Abstract: We present a method for evaluating ICA separation of artifacts from EEG (electroencephalographic) data. Two algorithms, Infomax and FastICA, were applied to "synthetic data," created by superimposing simulated blinks on a blink-free EEG. To examine sensitivity to different data characteristics, mul- tiple datasets were constructed by varying properties of the simulated blinks. ICA was used to decompose the data, and each source was cross-correlated with a blink template. Different thresholds for correlation were used to assess stability of the algorithms. When a match between the blink-template and a component was obtained, the contribution of the source was subtracted from the EEG. Since the original data were known a priori to be blink-free, it was possi- ble to compute the correlation between these "baseline" data and the results of different decompositions. By averaging the filtered data, time-locked to the simulated blinks, we illustrate effects of different outcomes for EEG waveform and topographic analysis.

7.2: Measuring Color Breakup of Stationary Images in Field-Sequential-Color Displays

Abstract: Color breakup is an artifact perceivable on field-sequential color (FSC) displays, both in stationary and in moving images. In this work a unique device for measuring color breakup on stationary images is presented. Target FSC display can be a virtual display, a direct-view display or a projector. Turning the view of a high-speed measurement camera in milliseconds simulates saccadic eye movement. Captured images can be used e.g. in studies of color breakup theories.

Landsat thematic mapper reflective-band radiometric artifacts

Abstract: Radiometric "artifacts" are known to be present to varying degrees in the reflective-band imagery from both Landsat-4 and Lansat-5 Thematic Mappers (TMs). The most common artifacts are known as scan-correlated shift (SCS), memory effect (ME), and coherent noise (CN). The characterization and correction of these artifacts has been performed for both the Landsat-4 and Lansat-5 TMs. SCS is a sudden shift in bias that can be as large as 2 DN. However, this artifact can be accurately quantified and easily removed from imagery using a line-by-line bias subtraction. ME causes the detector response to undershoot after a sudden transition from a bright target to a dark target. For large transitions, this can cause a 2% radiometric error. This artifact can be removed through a spatial filtering operation. Lastly, CN is a periodic pattern that is most often seen in homogeneous portions of TM imagery. The amplitude of this noise artifact is quite small, less than 0.5 DN. While CN has been accurately characterized, a correction procedure is not recommended, due to the small amplitude of this artifact. Recommendations are given for proper processing of TM imagery to remove the effects of these artifacts.

Artifact analysis and reconstruction improvement in helical cardiac cone beam CT

Abstract: With the introduction of cone beam (CB) scanners, cardiac volumetric computed tomography (CT) imaging has the potential to become a noninvasive imaging tool in clinical routine for the diagnosis of various heart diseases. Heart rate adaptive reconstruction schemes enable the reconstruction of high-resolution volumetric data sets of the heart. Artifacts, caused by strong heart rate variations, high heart rates and obesity, decrease the image quality and the diagnostic value of the images. The image quality suffers from streak artifacts if suboptimal scan and reconstruction parameters are chosen, demanding improved gating techniques. In this paper, an artifact analysis is carried out which addresses the artifacts due to the gating when using a three-dimensional CB cardiac reconstruction technique. An automatic and patient specific cardiac weighting technique is presented in order to improve the image quality. Based on the properties of the reconstruction algorithm, several assessment techniques are introduced which enable the quantitative determination of the cycle-to-cycle transition smoothness and phase homogeneity of the image reconstruction. Projection data of four patients were acquired using a 16-slice CBCT system in low pitch helical mode with parallel electrocardiogram recording. For each patient, image results are presented and discussed in combination with the assessment criteria.

Cine Magnetic Resonance Microscopy of the Rat Heart Using Cardiorespiratory-Synchronous Projection Reconstruction

Abstract: Purpose: To tailor a cardiac magnetic resonance (MR) microscopy technique for the rat that combines improvements in pulse sequence design and physiologic control to acquire high-resolution images of cardiac structure and function. Materials and Methods: Projection reconstruction (PR) was compared to conventional Cartesian techniques in point-spread function simulations and experimental studies to evaluate its artifact sensitivity. Female Sprague-Dawley rats were imaged at 2.0 T using PR with direct encoding of the free induction decay. Specialized physiologic support and monitoring equipment ensured consistency of biological motion and permitted synchronization of imaging with the cardiac and respiratory cycles. Results: The reduced artifact sensitivity of PR offered improved delineation of cardiac and pulmonary structures. Ventilatory synchronization further increased the signal-to-noise ratio by reducing inter-view variability. High-quality shortaxis and long-axis cine images of the rat heart were acquired with 10-msec temporal resolution and microscopic spatial resolution down to 175 m 175 m 1 mm. Conclusion: Integrating careful biological control with an optimized pulse sequence significantly limits both the source and impact of image artifacts. This work represents a novel integration of techniques designed to support measurement of cardiac morphology and function in rodent models of cardiovascular disease.