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At least 361 records · Page 20Linked to original sources
PubMed1983

Intravenous infusion device artifact in the EEG-confusion in the diagnosis of electrocerebral silence.

Intravenous infusion devices are routinely used in the intensive care unit to accurately regulate the delivery of various intravenous fluids and vasoactive drugs. These devices have been well described in the literature as the cause of various electrocardiographic artifacts. There has been little documentation in the literature implicating these devices as the etiology of artifacts in the electroencephalogram. The association of intravenous infusion devices with electroencephalographic artifacts became relevant during the brain death evaluation of two patients. The electroencephalograms, which were done for documentation of electrocerebral silence clearly showed activity in both patients which disappeared when the intravenous infusion devices were stopped. Possible mechanisms responsible for producing these artifacts include piezoelectric current, poor electrode contact, inadequate skin preparation, current leakage, static charges and electromagnetic activity. In the evaluation of patients for electrocerebral silence, it is important to both recognize and eliminate this artifact so that it is not confused with true electrocerebral activity.

Brain Death↗
PubMed1990

Acoustic artifacts and reverberation shadows in gallbladder sonograms: their cause and clinical implications.

From among more than 7000 sonographic examinations of the gallbladder performed over a period of 2 years, different types of acoustic artifacts were observed in 42 patients. Artifacts originated in the gallbladder wall in 37 cases and within the lumen in 5 cases. Eighteen patients had symptoms related to the hepatobiliary area. Twenty-four patients were fully asymptomatic although cholelithiasis was demonstrated in 8. In 21 cases (50%) some type of gallbladder pathology was associated with the presence of acoustic artifacts. Thirteen patients were cholecystectomized and the associated gallbladder pathology consisted of cholelithiasis (N = 5), cholesterolosis (N = 2), adenomyomatosis (N = 2), emphysematous cholecystitis (N = 2), and acute cholecystitis (N = 2). In 3 of these patients a sonographic/pathologic correlation was established. Only 5 of the 18 symptomatic patients presented acoustic artifacts in an otherwise normal gallbladder. Recognition of those pathological conditions favorable to the appearance of artifacts and reverberation shadows is extremely useful for differentiating merely clinically irrelevant anomalies from those that require prompt management.

Acoustics↗
PubMed1991

Characterization of an actin flow artifact that occurs during the presteady state dissociation of actosubfragment-1.

Models for the activation of the myosin subfragment-1 (S-1) ATPase activity by actin describe transitions that occur between kinetic intermediate states during steady state hydrolysis of ATP. These states consist of myosin-nucleotide complexes in rapid equilibrium binding with actin, but steady state measurements of actin binding during hydrolysis lead only to a weighted average of the individual binding constants involved. In the current work, in order to determine the individual binding constants involved in the activation process, we have investigated the presteady state kinetics of the dissociation of actomyosin by ATP. We find that an actin flow artifact appears to dominate the time course of dissociation, and characterization of this artifact reveals that its magnitude rises linearly (approximately) with the concentration of bound S-1. Attempts to subtract the actin flow artifact from the actoS-1 dissociation signal were not entirely successful due at least partially to the transient nature of the bound S-1 concentration in the first few milliseconds. However, further studies reveal that if the order of addition of actin, ATP, and S-1 are varied, the observed light scattering transients are essentially superimposable. One possible explanation of these data is that the binding constants for myosin-ATP and myosin-ADP-Pi to actin are equal. However, it is also possible that the flow artifact is so large that further analysis is precluded. In addition, we show that the actin flow artifact has little effect on the fluorescence measurements of the phosphate burst reported previously. Therefore, the prior interpretation of the fluorescence data remains unchanged.

Actins↗
PubMed1991

A method for eliminating the stimulus artifact from digital recordings of the direct cortical response.

A computer algorithm which removes the electrical pulse stimulus artifact from digitized electrophysiological recordings is described. The technique has been applied to recordings of the direct cortical response (DCR). The process removes the stimulus artifact without removing the overlapping physiological signal. The results from recordings of the DCR in the rat before and after the artifact removal process are shown. The validity of the method is demonstrated from simultaneous recording of the same response with a minimal and maximal artifact. An examination of the residual artifact and degree of signal distortion after the algorithm has been applied is presented.

Algorithms↗
PubMed1993

Adaptive cancellation of the respiratory artifact in surface recording of small intestinal electrical activity.

In the human small intestine there is omnipresent electrical activity with a frequency of 0.15-0.2 Hz. The electrical activity of the small intestine can be measured by surface electrodes placed on the abdominal skin. The most annoying problem in the surface electrical recording is the respiratory artifact which is not discernible from the small intestinal signal. The frequency of the respiration is about 0.2-0.4 Hz, which is very close to that of small intestinal activity, making the use of the conventional bandpass filtering impractical. In this paper a selective frequency domain adaptive filter was proposed for the cancellation of the respiratory artifact. The basic principle of the selective frequency domain adaptive filter is that only selected filter weights are adapted based on the frequency characteristics of the respiratory artifact. Therefore, a substantial reduction of computation is achieved. A series of computer simulations was conducted for the optimization of the system parameters and for the investigation of the system performance. It was demonstrated in this paper that the selective frequency domain adaptive filter is as effective as, but more efficient than, the conventional frequency domain adaptive filter. The adaptive system for the cancellation of the respiratory artifact based on the selective frequency domain adaptive filter is very efficient in computation, has a fast convergence (about 100 adaptations), substantial reduction of the respiratory artifact and little effect (or distortion) on the small intestinal electrical signal.

Computer Simulation↗
PubMed1984

EMG artifact minimization during clinical EEG recordings by special analog filtering.

A multichannel analog filter for minimizing EMG artifact in routine EEG recordings is described. A 20-channel readily portable version has been implemented which can be easily interfaced with standard electroencephalographs at the auxiliary input/output connectors, in such a way that the filter can be inserted or removed for all channels simultaneously, or, selected channels can be filtered individually. That such an on-line filter be explored was suggested by the remarkable results for minimizing EMG artifact from EEG recordings at seizure onset by off-line computer-based digital filtering recently reported by Gotman, Ives and Gloor. A 4-pole Butterworth filter, i.e., having a maximally flat amplitude response in the passband and having a cut-off frequency, i.e., 30% attenuation point, of 12.5 Hz was selected, the same cut-off frequency as that of the digital filter used by Gotman et al. With this cut-off frequency, which represents a compromise, residual EMG artifact is relatively small. With higher cut-off frequencies, EMG artifact can appear as pseudo-beta activity. Minimization of pseudo-beta activity is obtained, however, at the expense of the elimination of true beta activity. On the other hand, activity of alpha frequencies or lower can be readily identified in the filter output, even with severe EMG artifact in the unfiltered EEG. Other problems in EMG filtering are discussed. Since the analog filter has a non-linear phase characteristic, in contrast to the linear phase characteristic of the finite-impulse-response digital filter used by Gotman et al., the possibility existed that distortion of the wave form of EEG spikes could result, as well as attenuation of their amplitudes. However, the wave forms of filtered spikes in the write-out of the analog filter were found to be essentially identical with those from a one-channel on-line finite-impulse-response computer-based filter having a linear phase characteristic. This finding indicates that phase shift in the analog filter is not a problem, at least for clinical EEG.

Electroencephalography↗
PubMed1985

A general-purpose automatic multichannel electronic switch for EEG artifact elimination.

A general-purpose automatic multichannel electronic switch is described for eliminating artifact from EEG recordings. Although primarily intended for computer processing of EEG data, the device may on occasion be useful in clinical EEG (e.g., during electro-cautery). The switch is activated from a separate artifact-monitoring channel of the electroencephalograph (e.g., accelerometrically monitored head movement, electro-oculographically monitored eye movements or blinks), or from an EEG channel itself, if the artifact in the latter is very marked. Optionally, a sine wave can replace the EEG during the artifact period, and a separate output signal (e.g., for interrupting computer processing) is available for indicating the occurrence of artifact.

Computers↗
PubMed1987

Elimination of EKG artifacts from EEG records: a new method of non-cephalic referential EEG recording.

A new method for eliminating EKG artifacts from EEGs was reported. Based on the simultaneously recorded EEGs and EKG, the procedure consisted of 4 steps: synchronized partition of the raw EEG record into segments with respect to the QRS complex of the EKG, averaging of the segments time-locked to EKG, repetition of the average artifact synchronized to EKG and subtraction of the estimate of the artifacts from the raw data. This method enabled the elimination of EKG artifacts from EEGs recorded by using the chin or the hand electrode as a reference. Features of the proposed method include: all signals are processed in digital instead of analogue form, the average EKG wave form is computed throughout the subtraction part of the recording instead of being computed only before the subtraction starts, in case of an on-line algorithm, the average EKG wave form is always updated to the point immediately preceding the real time of EKG subtraction, and by using the recursive form of exponential weighted averaging equation, the average EKG wave form can adapt appropriately to a change of the artifact wave form. By the high speed implementation of the procedure, the processed data of multi-channel EEGs can be obtained on-line with a delay of only 200 msec.

Adult↗
PubMedDate not supplied

Computed tomographic evaluation of surgical clip artifact: tissue phantom and experimental animal assessment.

Surgical clips in postoperative patients create streak artifact on computed tomographic (CT) studies which often significantly degrade image quality. We have evaluated three types of surgical clips: tantalum, stainless steel, and titanium in a phantom and animal model to assess their relative CT artifact. Tantalum clips showed the greatest artifact, stainless steel intermediate, and titanium the least artifact. Artifact could be reduced with the use of faster scan times. The type of clip used may enter into surgical decision making depending upon the need for high resolution scanning to detect small pathological processes in the area of surgical clips.

Animals↗
PubMedDate not supplied

Effect of field strength on susceptibility artifacts in magnetic resonance imaging.

In magnetic resonance imaging susceptibility artifacts occur at the interface of substances with large magnetic susceptibility differences, resulting in geometric distortions of the image at those boundaries. The susceptibility artifacts are often subtle on clinical images and if not carefully examined they may lead to misdiagnosis. Magnetic susceptibility artifacts are prevalent on the boundary of air-containing paranasal sinuses, as well as bone-soft tissue interfaces in the spinal canal. The appearance of these artifacts on images from three different magnetic field strength instruments, 0.3, 0.5, and 1.5 Tesla were studied. T1- and T2-weighted spin echo and gradient recalled echo pulse sequences were selected to image a water phantom containing substances of varying susceptibilities. The effects were also studied in MR images of the head in a normal human volunteer. At any given field strength the artifacts were more prominent in the gradient echo imaging than in the corresponding spin echo pulse sequence. As expected, the distortions were also greater at higher field strengths. The results in human subjects paralleled the findings in the phantom study.

Head↗
PubMed1997

Separate detection of moving luminance and contrast modulations: fact or artifact?

We have investigated first-order artifacts in second-order motion perception. Subjects were required to identify the orientation and direction of a drifting sinusoidal contrast modulation. When the carrier consisted of static two-dimensional noise, performance often reflected the use of first-order artifacts that arise from stochastic local biases in the noise, rather than the detection of the contrast modulation per se. This stimulus, which has been used widely for studying second-order motion, therefore appears to be inappropriate for that purpose. In contrast, global distortion products arising from luminance non-linearities do not appear to provide usable artifacts. Two manipulations were employed to eliminate local first-order artifacts: the use of dynamic noise and the use of high-pass filtered static noise. These two manipulations gave similar results, which were quite different from those obtained with broadband static noise. We argue that performance with both of these image types reflects the activity of a true second-order motion mechanism. A characteristic property of this mechanism is that it cannot specify direction at the threshold for detecting orientation. Direction thresholds are around 50% higher than orientation thresholds when first-order artifacts are eliminated.

Contrast Sensitivity↗
PubMed2003

Computerized processing of EEG-EOG-EMG artifacts for multi-centric studies in EEG oscillations and event-related potentials.

The aim of this study is to present a package including standard software for the electroencephalographic (EEG), electro-oculographic (EOG) and electromyographic (EMG) preliminary data analysis, which may be suitable to standardize the results of many EEG research centers studies (i.e. multi-centric studies) especially focused on event-related potentials. In particular, our software package includes (semi)automatic procedures for (i) EOG artifact detection and correction, (ii) EMG analysis, (iii) EEG artifact analysis, (iv) optimization of the ratio between artifact-free EEG channels and trials to be rejected. The performances of the software package on EOG-EEG-EMG data related to cognitive-motor tasks were evaluated with respect to the preliminary data analysis performed by two expert electroencephalographists (gold standard). Due to its extreme importance for multi-centric EEG studies, we compared the performances of two representative "regression" methods for the EOG correction in time and frequency domains. The aim was the selection of the most suitable method in the perspective of a multi-centric EEG study. The results showed an acceptable agreement of approximately 95% between the human and software behaviors, for the detection of vertical and horizontal EOG artifacts, the measurement of hand EMG responses for a cognitive-motor paradigm, the detection of involuntary mirror movements, and the detection of EEG artifacts. Furthermore, our results indicated a particular reliability of a 'regression' EOG correction method operating in time domain (i.e. ordinary least squares). These results suggest that such a software package could be used for multi-centric EEG studies.

Algorithms↗
PubMed1997

In vivo animal tests of an artifact-free contrast agent for gastrointestinal MRI.

A variety of shortcomings are associated with most currently used gastrointestinal contrast agents for magnetic resonance imaging (MRI). Artifacts resulting from peristalsis and other motions in the abdominal region are produced by many positive contrast agents (which increase signal intensity). Although this is not a problem for negative contrast agents (which decrease signal intensity), some negative contrast agents produce magnetic susceptibility artifacts that are especially pronounced at high field strength and with gradient echo pulse sequences. These susceptibility artifacts are produced by both paramagnetic and diamagnetic agents. It has been demonstrated in phantoms, however, that susceptibility matching can be used to produce contrast agents with desirable relaxation and contrast properties but without deleterious susceptibility artifacts. We now report results of animal tests of such an oral contrast agent, consisting of a suspension of superparamagnetic iron oxide particles and diamagnetic barium sulfate particles, compared to individual suspensions of the iron oxide and of the barium sulfate. Iron oxide was the least effective and the matched susceptibility mixture was the most effective for the intestine, which has traditionally been the most difficult region of the GI tract to visualize clearly. Matched susceptibility mixtures, which are inherently able to yield images free of susceptibility artifacts without compromising contrast, show promise of being improved oral negative contrast agents for use in gastrointestinal MRI.

Administration, Oral↗
PubMed1980

Two new artifacts in automated coagulation testing.

Two artifacts were noted during the routine use of the Sherwood Lancer Coagulyzer. The first was that prothrombin times for 44 patients were greater than 90 sec on the Sherwood instrument, but were significantly shorter using reference methods. All 44 patients had subnormal fibrinogen values. Continuous optical density tracings during clotting performed for 11 patients demonstrated low slopes and amplitudes. For the three patients tested, the addition of fibrinogen corrected prothrombin times to values similar to those obtained on the Bio-Data instrument. Twenty-four of the patients subsequently died, 20 within three weeks of the observation of the artifact. The second artifact was that activated partial thromboplastin times for 12 patients were 9 sec on the Sherwood instrument, but were either normal or prolonged when using reference methods. Continuous tracings performed for nine patients demonstrated early optical density increases before clotting, which were responsible for the spurious values. Eleven of the 12 patients were hypoalbuminemic, and nine of the ten tested had elevated IgM levels. For seven patients, the artifact disappeared with in-vitro correction of hypoalbuminemia. Far from being weaknesses of the Coagulyzer, these artifacts may lead to the discovery of unsuspected, clinically significant laboratory information.

Blood Coagulation Tests↗
PubMed2002

Artifact analysis of approximate helical cone-beam CT reconstruction algorithms.

In this paper, four approximate cone-beam CT reconstruction algorithms are compared: Advanced single slice rebinning (ASSR) as a representative of algorithms employing a two dimensional approximation, PI, PI-SLANT, and 3-PI which all use a proper three dimensional back-projection. A detailed analysis of the image artifacts produced by these techniques shows that aliasing in the z-direction is the predominant source of artifacts for a 16-row scanner with 1.25 mm nominal slice thickness. For a detector with isotropic resolution of 0.5 mm, we found that ASSR and PI produce different kinds of artifacts which are almost at the same level, while PI-SLANT produces none of these artifacts. It is shown that the use of redundant data in the 3-PI method suppresses aliasing artifacts efficiently for both scanners.

Algorithms↗
PubMedDate not supplied

A stimulated echo artifact from slice interference in magnetic resonance imaging.

When multiple slices are imaged with a short time between slice acquisitions, a disturbing line artifact along the direction of frequency encoding is often seen across the center of the images. The artifact consists of alternating bright and dark pixel intensities. In this paper, we show that the artifact is due to slice interference, and is caused by stimulated echoes that are produced in the regions of overlap between slices. A theoretical analysis of the formation of these stimulated echoes leads to ways of reducing the artifact, which are verified experimentally. The artifact can be suppressed most conveniently by extending the duration of the read gradient beyond the sampling window.

Humans↗
PubMed2000

Efficient correction for CT image artifacts caused by objects extending outside the scan field of view.

The purpose of this paper is to develop a method of eliminating CT image artifacts generated by objects extending outside the scan field of view, such as obese or inadequately positioned patients. CT projection data are measured only within the scan field of view and thus are abruptly discontinuous at the projection boundaries if the scanned object extends outside the scan field of view. This data discontinuity causes an artifact that consists of a bright peripheral band that obscures objects near the boundary of the scan field of view. An adaptive mathematical extrapolation scheme with low computational expense was applied to reduce the data discontinuity prior to convolution in a filtered backprojection reconstruction. Despite extended projection length, the convolution length was not increased and thus the reconstruction time was not affected. Raw projection data from ten patients whose bodies extended beyond the scan field of view were reconstructed using a conventional method and our extended reconstruction method. Limitations of the algorithm are investigated and extensions for further improvement are discussed. The images reconstructed by conventional filtered backprojection demonstrated peripheral bright-band artifacts near the boundary of the scan field of view. Images reconstructed with our technique were free of such artifacts and clearly showed the anatomy at the periphery of the scan field of view with correct attenuation values. We conclude that bright-band artifacts generated by obese patients whose bodies extend beyond the scan field of view were eliminated with our reconstruction method, which reduces boundary data discontinuity. The algorithm can be generalized to objects with inhomogeneous peripheral density and to true "Region of Interest Reconstruction" from truncated projections.

Algorithms↗
PubMed1983

Potential hazards and artifacts of ferromagnetic and nonferromagnetic surgical and dental materials and devices in nuclear magnetic resonance imaging.

The risks to patients with metal surgical implants who are undergoing nuclear magnetic resonance (NMR) imaging and the artifacts caused by such implants were studied. Twenty-one aneurysm and other hemostatic clips and a variety of other materials (e.g., dental amalgam, 14 karat gold) were used. Longitudinal forces and torques were found to be exerted upon 16 of the 21 clips. With five aneurysm clips, forces and torques sufficient to produce risk of hemorrhage from dislocation of the clip from the vessel or aneurysm, or cerebral injury by clip displacement without dislodgement were identified. The induced ferromagnetism was shown to be related to the composition of the alloys from which the clips were manufactured. Clips with 10-14% nickel are evidently without sufficient induced ferromagnetism to cause hazard. The extent of NMR imaging artifacts was greater for materials with measurable ferromagnetic properties, but metals without measurable ferromagnetism in our tests also resulted in significant artifacts. Dental amalgam and 14 karat gold produced no imaging artifacts, but stainless steels in dentures and orthodontic braces produced extensive artifacts in the facial region.

Aneurysm↗