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PubMed2007

Artifact reduction in moving-table acquisitions using parallel imaging and multiple averages.

Two-dimensional (2D) axial continuously-moving-table imaging has to deal with artifacts due to gradient nonlinearity and breathing motion, and has to provide the highest scan efficiency. Parallel imaging techniques (e.g., generalized autocalibrating partially parallel acquisition GRAPPA)) are used to reduce such artifacts and avoid ghosting artifacts. The latter occur in T(2)-weighted multi-spin-echo (SE) acquisitions that omit an additional excitation prior to imaging scans for presaturation purposes. Multiple images are reconstructed from subdivisions of a fully sampled k-space data set, each of which is acquired in a single SE train. These images are then averaged. GRAPPA coil weights are estimated without additional measurements. Compared to conventional image reconstruction, inconsistencies between different subsets of k-space induce less artifacts when each k-space part is reconstructed separately and the multiple images are averaged afterwards. These inconsistencies may lead to inaccurate GRAPPA coil weights using the proposed intrinsic GRAPPA calibration. It is shown that aliasing artifacts in single images are canceled out after averaging. Phantom and in vivo studies demonstrate the benefit of the proposed reconstruction scheme for free-breathing axial continuously-moving-table imaging using fast multi-SE sequences.

Artifacts↗

PubMed1997

Anatomic localization and quantitative analysis of gradient refocused echo-planar fMRI susceptibility artifacts.

Functional magnetic resonance imaging (fMRI) techniques, such as echo-planar imaging, can permit rapid, sensitive, whole-brain measurements of local blood flow-induced MR signal changes seen during cognitive paradigms. Changes in blood oxygenation due to mismatch of flow and oxygen metabolism cause dynamic variations in microscopic susceptibility effects, leading to the blood oxygenation level-dependent (BOLD) signal measured by fMRI techniques. A related static macroscopic susceptibility effect is known to cause artifacts that attenuate the MR signal, leading to "blind spots" in some regions of brain adjacent to bone and air sinuses. The anatomical location, spatial extent, and magnitude of signal loss artifact are quantitated for a common whole-brain fMRI technique. Resting gradient-echo EPI studies were obtained in four healthy volunteers. Signal loss was primarily localized to inferior frontal regions (medial orbital gyri and gyrus rectus) and to inferior lateral temporal lobe (including part of fusiform gyrus) bilaterally. Increased echo time (TE) uniformly produced larger artifacts. The orientation of acquired slices and choice of phase-encoding direction influenced the location, shape, and extent of the artifacts. Regions of the brain with severe artifact may have attenuated activation signal, with potential implications for the design and interpretation of fMRI studies targeting activations in these areas.

Adult↗

PubMed1998

Identification of EEG events in the MR scanner: the problem of pulse artifact and a method for its subtraction.

Triggering functional MRI (fMRI) image acquisition immediately after an EEG event can provide information on the location of the event generator. However, EEG artifact associated with pulsatile blood flow in a subject inside the scanner may obscure EEG events. This pulse artifact (PA) has been widely recognized as a significant problem, although its characteristics are unpredictable. We have investigated the amplitude, distribution on the scalp, and frequency of occurrence of this artifact. This showed large interindividual variations in amplitude, although PA is normally largest in the frontal region. In five of six subjects, PA was greater than 50 microV in at least one of the temporal, parasagittal, and central channels analyzed. Therefore, we developed and validated a method for removing PA. This subtracts an averaged PA waveform calculated for each electrode during the previous 10 s. Particular attention has been given to reliable ECG peak detection and ensuring that the average PA waveform is free of other EEG artifacts. Comparison of frequency spectra for EEG recorded outside and inside the scanner, with and without PA subtraction, showed a clear reduction in artifact after PA subtraction for all four frequency ranges analyzed. As further validation, lateralized epileptiform spikes were added to recordings from inside and outside the scanner: PA subtraction significantly increased the proportion of these spikes that were correctly identified and decreased the number of false spike detections. We conclude that in some subjects, EEG/fMRI studies will be feasible only using PA subtraction.

Adult↗

PubMed1992

Usefulness of an acoustic edge artifact in assessment of the Ilizarov corticotomy interval.

Thirty-three ultrasound examinations of the corticotomy interval of patients undergoing Ilizarov procedures were retrospectively evaluated for the presence or absence of an acoustic edge artifact. This artifact, consisting of a fine anechoic band, has been previously described in phantom models and is presumed to be due to phase cancellation effects. We demonstrated this artifact in 8 of 33 examinations. The artifact proved helpful in identifying the location of the corticotomy margin, even when this margin was obscured by the presence of developing periosteal new bone. Attention to technical factors is, however, important. We believe that this artifact may have a useful role in the routine monitoring of the Ilizarov patient.

Adolescent↗

PubMed2004

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

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.

Adolescent↗

PubMed1996

Intention, history, and artifact concepts.

What determines our intuitions as to which objects are members of specific artifact kinds? Prior research suggests that factors such as physical appearance, current use, and intended function are not at the core of concepts such as chair, clock and pawn. The theory presented here, based on Levinson's (1993) intentional-historical theory of our concept of art, is that we determine that something is a member of a given artifact kind by inferring that it was successfully created with the intention to belong to that kind. This theory can explain why some properties (such as shape) are more important than others (such as color) when we determine kind membership and can account for why certain objects are judged to be members of artifact kinds even though they are highly dissimilar from other members of the kinds. It can also provide a framework for explaining the conditions under which broken objects cease to be members of their kinds and new artifacts can come into existence. This account of our understanding of artifact concepts is argued to be consistent with more general "essentialist" theories of our understanding of concepts corresponding to proper names and natural kind terms.

Art↗

PubMed1992

Spectral characterization of the effect of viscosity on Fura-2 fluorescence: excitation wavelength optimization abolishes the viscosity artifact.

A systematic study of the spectral characteristics of the viscosity artifact in Fura-2 based [Ca2+] measurements reveals that, by selecting excitation wavelengths approximately 10 nm longer than those routinely employed and modestly reducing excitation bandpasses, the magnitude of the artifact can be reduced to experimentally undetectable levels without greatly impairing [Ca2+] measurements. The feasibility of this approach was confirmed on a ratio imaging microscope; the magnitude of the artifact observed in dextran-conjugated Fura-2 solutions prepared in water or in 50% sucrose was not statistically significant using an excitation wavelength pair of 361/389 nm, whereas at 350/380 nm [Ca2+] was underestimated by 34% in the higher viscosity solution. Thus, provided potential pitfalls are taken into account, a simple change in imaging protocol can avoid the viscosity artifact without recourse to correction factors. This approach may be employed either routinely, or else merely to test whether apparent [Ca2+]i differences observed at more conventional wavelengths arise from the viscosity artifact.

Artifacts↗

PubMed1994

High dose-rate induced temperature artifacts: thermometry considerations for simultaneous interstitial thermoradiotherapy.

PURPOSE: The goal of the present study was to investigate the effect of high dose-rate radiation on a flouroptic thermometry system commonly used during microwave hyperthermia. METHODS AND MATERIALS: Measurements were performed by placing the flouroptic thermometry sensors at distances of < or = 1.5, 5, 10, and 15 mm from a remote afterloading high dose-rate 192Ir source in a water bath (at two different temperatures) and in a tissue equivalent radiation bolus medium. A simulated volumetric clinical setup using a radiation bolus medium was performed with thermometry sensors placed at 1.5, 7.5, 8.4, and 10.6 mm from a scanning high dose-rate source. RESULTS: It was found that high dose-rate radiation caused thermometry artifacts greater than 1.5 degrees C within 2 min for flouroptic thermometers placed 1.5 mm from a 5 Ci activity high dose-rate source. Simple calculations showed that artifacts of this magnitude could not be due to any heating caused by the energy deposited by the high dose-rate source. The artifact decayed, but was still evident 24 h after the exposure. The effect strongly depended on distance with a 0.7 degrees C artifactual increase in temperature seen for the probe 5 mm from the high dose-rate source. Moreover, experiments performed under conditions that represented a clinical setup with a 7 Ci high dose-rate source showed that for exposure times of 10 s, at distances of 1.5 mm, significant artifacts (> 0.5 degrees C) are produced. CONCLUSIONS: These findings indicate that high dose-rate-induced temperature artifacts should be taken into account in the quality assurance procedures for the treatment of patients with simultaneous interstitial thermoradiotherapy.

Artifacts↗

PubMed2005

A hybrid approach to reducing computed tomography metal artifacts in intracavitary brachytherapy.

PURPOSE: To develop a hybrid approach to reduce computed tomography (CT) metal artifacts caused by the Fletcher-Suit applicator. METHODS AND MATERIALS: Using the acquired raw projection data, the hybrid algorithm determines the separate contributions from the metal and non-metal objects. Next, reconstructions of metal and non-metal images are separately obtained from their estimated projections. A final image is formed by appropriately combining the individual images. Phantom and patient data are used to evaluate the performance of this method. RESULTS: CT images reconstructed by use of the standard filtered-backprojection (FBP) algorithm contain significant artifacts even for small metal objects. However, images reconstructed using the hybrid algorithm contain virtually no artifacts. For metal objects with complex structures, the hybrid algorithm can also yield images containing fewer severe streak artifacts than those reconstructed using the FBP algorithm alone. CONCLUSIONS: These studies demonstrate that the hybrid approach can effectively reduce CT metal artifacts caused by the Fletcher-Suit applicator.

Algorithms↗

PubMed2006

A system for automatic artifact removal in ictal scalp EEG based on independent component analysis and Bayesian classification.

OBJECTIVE: To devise an automated system to remove artifacts from ictal scalp EEG, using independent component analysis (ICA). METHODS: A Bayesian classifier was used to determine the probability that 2s epochs of seizure segments decomposed by ICA represented EEG activity, as opposed to artifact. The classifier was trained using numerous statistical, spectral, and spatial features. The system's performance was then assessed using separate validation data. RESULTS: The classifier identified epochs representing EEG activity in the validation dataset with a sensitivity of 82.4% and a specificity of 83.3%. An ICA component was considered to represent EEG activity if the sum of the probabilities that its epochs represented EEG exceeded a threshold predetermined using the training data. Otherwise, the component represented artifact. Using this threshold on the validation set, the identification of EEG components was performed with a sensitivity of 87.6% and a specificity of 70.2%. Most misclassified components were a mixture of EEG and artifactual activity. CONCLUSIONS: The automated system successfully rejected a good proportion of artifactual components extracted by ICA, while preserving almost all EEG components. The misclassification rate was comparable to the variability observed in human classification. SIGNIFICANCE: Current ICA methods of artifact removal require a tedious visual classification of the components. The proposed system automates this process and removes simultaneously multiple types of artifacts.

Algorithms↗

PubMed2006

Minimization of cochlear implant stimulus artifact in cortical auditory evoked potentials.

OBJECTIVE: To compare two methods of minimizing cochlear implant artifact in cortical auditory evoked potential (CAEP) recordings. METHODS: Two experiments were conducted. In the first, we assessed the use of independent component analysis (ICA) as a pre-processing filter. In the second, we explored the use of an optimized differential reference (ODR) for minimizing artifacts. RESULTS: Both ICA and the ODR can minimize the artifact and allow measurement of CAEP responses. CONCLUSIONS: When using a large number of recording electrodes ICA can be used to minimize the implant artifact. When using a single electrode montage an optimized differential reference is adequate to minimize the artifact. SIGNIFICANCE: The use of an optimized differential reference could allow cortical evoked potentials to be used in routine clinical assessment of auditory pathway development in children and adults fit with cochlear implants.

Artifacts↗

PubMed2005

Reduction in flow artifacts by using interleaved data acquisition in segmented balanced steady-state free precession cardiac MRI.

Balanced steady-state free precession (SSFP) magnetic resonance (MR) imaging is feasible for cine cardiac images because of the high contrast between myocardium and blood pool and robustness to rapid blood flow. Nonetheless, the flow artifacts are often observed because of off-resonance effects and to in-flow effects of the blood flow. Although reshimming the gradients or readjusting the center frequency reduces the artifacts, the technique can be susceptible for respiratory and cardiac motion and operator-dependent. The purpose of this study is to use another MR imaging technique for the reduction in the flow artifacts in the heart: odd-even interleaved data acquisition in segmented balanced SSFP imaging. The flow artifacts in the ventricle, ghost outside the heart, and visualization of the myocardial border were visually compared between sequential and odd-even interleaved k-space data acquisitions in cine balanced SSFP cardiac MR imaging. The odd-even interleaved k-space data acquisition significantly reduced dark flow artifacts in the left ventricle, improved the visualization of the myocardial border, and was easily installed. This imaging technique should be applied to cine segmented balanced SSFP cardiac MR imaging.

Adult↗

PubMed2004

Differential electrocardiographic artifact from implanted thalamic stimulator.

Electrocardiographic (ECG) artifacts may interfere in ECG interpretation. Body movement, tremors, poor skin-electrode contact, recorder malfunction, electromagnetic interference and implantable electronic devices are the main reasons for ECG artifacts. Transcutaneous nerve and implanted spinal cord stimulators have been reported to result in ECG artifacts. With availability of newer implantable electronic devices, different ECG artifact patterns are being seen. Tremor control device is a newer implanted device used for suppression of tremors in patients with essential tremors or Parkinsonian tremors not adequately controlled by medications and where the tremor causes a significant functional incapacity. A differential pattern of ECG artifacts due to use of an implanted tremor control device is reported.

Aged↗

PubMed2004

Automatic correction of ocular artifacts in the EEG: a comparison of regression-based and component-based methods.

A variety of procedures have been proposed to correct ocular artifacts in the electroencephalogram (EEG), including methods based on regression, principal components analysis (PCA) and independent component analysis (ICA). The current study compared these three methods, and it evaluated a modified regression approach using Bayesian adaptive regression splines to filter the electrooculogram (EOG) before computing correction factors. We applied each artifact correction procedure to real and simulated EEG data of varying epoch lengths and then quantified the impact of correction on spectral parameters of the EEG. We found that the adaptive filter improved regression-based artifact correction. An automated PCA method effectively reduced ocular artifacts and resulted in minimal spectral distortion, whereas ICA correction appeared to distort power between 5 and 20 Hz. In general, reducing the epoch length improved the accuracy of estimating spectral power in the alpha (7.5-12.5 Hz) and beta (12.5-19.5 Hz) bands, but it worsened the accuracy for power in the theta (3.5-7.5 Hz) band and distorted time domain features. Results supported the use of regression-based and PCA-based ocular artifact correction and suggested a need for further studies examining possible spectral distortion from ICA-based correction procedures.

Adult↗

PubMed2005

Artifacts and collaborative work in healthcare: methodological, theoretical, and technological implications of the tangible.

Although modeled as knowledge work with emphasis on data flow and decision making, healthcare is delivered in the context of a highly structured physical environment, with much effort and emphasis placed on physical and spatial arrangement and re-arrangement of workers, patients, and materials. The tangible aspects of highly collaborative healthcare work have profound implications for research and development of information and communication technology (ICT) despite the tendency to model work as flow of abstract data items. This article reviews field studies in healthcare and other domains on the role of artifacts in collaborative work and draws implications in three areas: methodological, theoretical, and technological. In regard to methodologies, assessment of new ICT and development of user requirements should take into account how artifacts are used and exploited to facilitate collaborative work. In regard to theories, the framework of distributed cognition provides a starting point for modeling the contribution and exploitation of physical artifacts in supporting collaborative work. In regard to technology, design and deployment of new technology should support the functions provided by physical artifacts replaced or disrupted by new technology, and profitable ways for new technology to support collaborative work by embedding ICT into existing infrastructure of physical artifacts.

Artifacts↗

PubMed2006

The effect of artifacts on dependence measurement in fMRI.

The study of effective connectivity by means of neuroimaging depends on the measurement of similarity between activity patterns at different locations in the brain, without necessarily presupposing a particular model for this dependence. When these interactions are measured using functional magnetic resonance imaging (fMRI) techniques, however, imaging and physiological artifacts create patterns of dependence that may be unrelated to cortical activity. We demonstrate some of these effects through the measurement of short-range dependencies present in fMRI scans of the primary visual cortex (V1) in the anaesthetized macaque monkey. High-field (4.7 T) fMRI scans were conducted to measure responses based on the blood oxygen level-dependent contrast mechanism, during periods of no sensory stimulation and of visual stimulation with rotating polar-transformed checkerboard gratings. Dependence between the haemodynamic activity at different spatial locations (i.e., different voxels) was measured using correlation, mutual information and functional covariance. Particular attention was paid to understanding the sources of spurious dependence that may be observed during such investigations. Two main effects were detected: (a) short-range correlations introduced by the process of image reconstruction and (b) perturbations in the haemodynamic response caused by breathing. The image reconstruction artifacts were shown to create an artificially high short-range dependence in the readout direction of the scan, and the breathing artifacts caused enhanced short-range dependence in both the readout and phase-encode directions. Additional dependence in the phase-encode direction due to image-ghosting is also possible but will not be discussed in this report, as it can be alleviated by fine adjustment of preemphasis (elimination of eddy currents). A technique is described for removing breathing artifacts, and the effect of breathing on the apparent dependence between voxels is illustrated. The correlation of haemodynamic activity with the stimulus was found to be affected by breathing, although this effect can be neutralised by averaging the haemodynamic responses over many repetitions of the stimulus. Nonetheless, patterns of dependent activity between voxels may be lost in this averaging process, which makes the removal of breathing artifacts necessary if statistical dependence and the study of effective connectivity is the primary aim of an investigation.

Animals↗

PubMedDate not supplied

Contributions of subdiaphragmatic activity, attenuation, and diaphragmatic motion to inferior wall artifact in attenuation-corrected Tc-99m myocardial perfusion SPECT.

BACKGROUND: Subdiaphragmatic activity and diaphragmatic motion both contribute to inferior wall artifacts in technetium 99m myocardial perfusion single photon emission computed tomography (SPECT). METHODS AND RESULTS: We used an anthropomorphic phantom with ventricular wall activity, liver/spleen inserts containing variable Tc-99m activity, and variable vertical (diaphragmatic) motion amplitude. SPECT and transmission scans were obtained on a GE Optima NX camera. Data were processed by use of filtered backprojection or attenuation correction (AC). Resulting myocardial activity maps were analyzed with standardized inferior-anterior and anterior-lateral wall ratios. At a subdiaphragmatic-myocardial activity ratio of 0.5:1, inferior wall attenuation predominates, producing a cold artifact. AC corrects inferior wall activity to the level of the anterior wall irrespective of diaphragmatic motion. At a subdiaphragmatic-myocardial activity ratio of 1:1, inferior wall counts vary widely depending on the proximity of subdiaphragmatic activity to the ventricle. With increasing diaphragmatic amplitude, the overlap of subdiaphragmatic activity and inferior wall worsens, leading to a complex mixture of cold and hot artifacts, not corrected by AC. CONCLUSIONS: Concentration and proximity of subdiaphragmatic Tc-99m activity relative to myocardium comprise a major factor in the nature and severity of inferior wall artifacts. If the subdiaphragmatic Tc-99m concentration is equivalent to that in the myocardium, complex, potentially uninterpretable hot and cold inferior wall artifacts are produced.

Artifacts↗

PubMed1996

Stimulus artifact reduction in evoked potential measurements.

OBJECTIVE: To investigate the main coupling mechanisms involved in stimulus artifact contamination of evoked potential recordings and to suggest techniques that minimize this interference. DESIGN: A before-after trial of a single subject. SETTING: Measurements were obtained at a university biomedical engineering laboratory. PARTICIPANTS: Data were obtained from one volunteer subject. INTERVENTION: An electrical stimulus was used to depolarize the posterior tibial nerve at the ankle. Various recording electrode configurations were used to demonstrate stimulus artifact recordings. RESULTS: Three mechanisms are defined as contributing significantly to stimulus artifact contamination of evoked potential data. These are: the volume conducted component, the displacement current component, and the electromagnetic coupling component. When each component is maximally controlled, the problem of stimulus artifact is greatly reduced. CONCLUSION: Three major factors that contribute to stimulus artifact contamination of the evoked potential waveform can be identified and minimized by relatively simple clinical techniques.

Artifacts↗