Showing papers on "Artifact (error) published in 1999"

Novel real-time R-wave detection algorithm based on the vectorcardiogram for accurate gated magnetic resonance acquisitions.

Abstract: Electrocardiograph (ECG) triggered or gated magnetic resonance methods are used in many imaging applications. Therefore, a reliable trigger signal derived from to the R-wave of the ECG is essential, especially in cardiac imaging. However, currently available methods often fail mainly due to the artifacts in the ECG generated by the MR scanner itself, such as the magnetohydrodynamic effect and gradient switching noise. The purpose this study was to characterize the accuracy of selected R-wave detection algorithms in an MR environment, and to develop novel approaches to eliminate imprecise triggering. Vectorcardiograms (VCG) in 12 healthy volunteers exposed to 1.5 T magnetic field were digitized and used as a reference data set including manually corrected onsets of R-waves. To define the magnetohydrodynamic effect, the VCGs were characterized in time, frequency, and spatial domains. The selected real-time R-wave detection algorithms, and a new "target-distance" VCG-based algorithm were applied either to standard surface leads calculated from the recorded VCG or to the VCG directly. The flow related artifact was higher in amplitude than the R-wave in 28% of the investigated VCGs which yielded up to 9-16%false positive detected QRS complexes for traditional algorithms. The "target-distance" R-wave detection algorithm yielded a score of 100% for detection with 0.2% false positives and was superior to all the other selected methods. Thus, the VCG of subjects exposed to a strong magnetic field can be use to separate the magnetohydrodynamic artifact and the actual R-wave, and markedly improves the trigger accuracy in gated magnetic resonance scans. Magn Reson Med 42:361-370, 1999.

Clinical consequences of electrocardiographic artifact mimicking ventricular tachycardia

Abstract: Electrocardiographic artifact can simulate ventricular tachycardia. The literature regarding electrocardiographic artifact is limited to case reports,1–7 proposed classifications,8,9 and diagnostic criteria.10 There is little information regarding the clinical implications of the misdiagnosis of artifact as ventricular tachycardia. We describe 12 patients who underwent unnecessary diagnostic or therapeutic interventions as a result of such a misdiagnosis. Methods We included in this series patients who were seen in consultation by a cardiac electrophysiologist at our institution between 1995 and 1999 and who underwent a diagnostic procedure or received treatment unnecessarily, solely as a result of the misdiagnosis of artifact as . . .

Cone-beam computed tomography with a flat-panel imager: effects of image lag.

Abstract: A system for cone-beam computed tomography (CBCT) has been developed based upon the technology of active matrix flat-panel imagers (FPIs), and the system has demonstrated the potential for fully three-dimensional volumetric imaging with high spatial and contrast resolution. This paper investigates the effects of image lag (arising from charge trapping and release in the FPI pixels) upon CBCT reconstructions. Hypotheses were derived based upon a simple, geometrical/physical model, suggesting that image lag in the projection data results primarily in two artifacts: a spatial blurring artifact in the direction opposite to the direction of rotation (called a "comet") and a line artifact along the direction of the first few projections (called a "streak"). The hypotheses were tested by means of computer simulations and experimental measurements that yielded CBCT images of a simple cylindrical water phantom containing an attenuating rod of varying size and composition. The computer simulations generated projection images based upon analysis of the system geometry and a simple model of the FPI that allowed free adjustment of the image lag. Experimental measurements involved CBCT scans of the phantom under various conditions and modes of acquisition followed by examination of the resulting CBCT axial slices for lag artifacts. Measurements were performed as a function of exposure level, position and contrast of the rod, and for three modes of acquisition designed to isolate and/or minimize the two hypothesized artifacts. The results clearly illustrate the comet and streak artifacts, particularly in relation to high-contrast objects imaged at high exposure levels. The significance of such artifacts under clinical conditions is expected to be small, considering the magnitude of the effect relative to the morphology and composition of typical anatomy. The artifacts may become appreciable, however, in the presence of high-contrast objects, such as marker BBs, dental fillings, and metal prosthetics. A procedural method of reducing lag artifacts is demonstrated.

Visual evoked potential (VEP) measured by simultaneous 64-channel EEG and 3T fMRI.

Abstract: We present the first simultaneous measurements of evoked potentials (EPs) and fMRI hemodynamic responses to visual stimulation. Visual evoked potentials (VEPs) were recorded both inside and outside the static 3T magnetic field, and during fMRI examination. We designed, constructed, and tested a non-magnetic 64-channel EEG recording cap. By using a large number of EEG channels it is possible to design a spatial filter capable of removing the artifact noise present when recording EEG/EPs within a strong magnetic field. We show that the designed spatial filter is capable of recovering the ballistocardiogram-contaminated original EEG signal. Isopotential plots of the electrode array recordings at the peak of the VEP response (approximately 100ms) correspond well with simultaneous fMRI observed activated areas of primary and secondary visual cortices.

Multivariable artifact assessment

Abstract: An apparatus detects corruption of an event signal by two or more non-event artifacts by using multivariable artifact assessment. The apparatus comprises: 1) a sensor for providing an input signal comprising an event signal coexisting with two or more non-event signals, 2) a measurement system including a receiver for receiving the input signal from the sensor and separating the received input signal into its constituent parts, 3) detectors for detecting both the event signal and non-event signals from the received input signal, and 4) an inference processor for analyzing the correlation signals to produce an indication of signal corruption. In a second embodiment, the apparatus comprises: 1) a plurality of sensors for providing additional non-event input signals, 2) a corresponding measurement system having a plurality of receivers for receiving the additional input signals, 3) detectors for detecting the non-event signals from the received additional input signals, 4) correlators for comparing the detected event signal with each of the detected non-event signals to produce respective correlation signals, and 5) an inference processor for analyzing the correlation signals to produce an indication of signal corruption. The apparatus may be a patient monitoring system or a defibrillator. Methods of operating the apparatus are also described.

Adaptive cancellation of ring-down artifact in IVUS imaging

Abstract: In an intravascular ultrasonic (IVUS) imaging system ring-down artifact is reduced or eliminated by dynamically enhancing the ring-down over a plurality of scans, determining the ring-down range by keying on a ring-down-to-blood transition characterized by a rapid change from high amplitude to low amplitude echoes, and using a spectral pattern for a single or several A-scans within the ring-down range, using for example an FFT analysis, as the basis of selectively filtering current or subsequent images using the recently computed ring-down pattern.

K-space sampling strategies.

Abstract: The k-space algorithm offers a comprehensive way for classification and understanding of the imaging properties of all commonly used MR sequences. This presentation describes the basic concepts of k-space and its most relevant properties for MR imaging. The ramifications of k-space sampling is discussed for the most commonly used groups of MR sequences including gradient-echo techniques, echo-planar imaging, spin echo, and rapid acquisition relation enhanced imaging (e. g., turbo spin echo, fast spin echo). In addition, the basic problems and properties of sequences based on non-rectilinear k-space sampling, such as spiral imaging, are discussed. Their artifact behavior is significantly different from rectilinear scans, which project all imperfections along the phase-encoding directions, whereas the artifact produced by spirals are more complex and not always easily recognizable as such. An understanding of the k-space sampling offers important insight into the basic properties of a given sequence regarding signal-to-noise ratio, image distortion, resolution and contrast. It is demonstrated that the ultimate limitation in imaging speed is given by the loss of signal-to-noise ratio inherent to faster data sampling.

Effect of color Doppler system on the twinkling sign associated with urinary tract calculi

Abstract: Purpose The aim of this study was to assess the effect of the color Doppler system on the artifact known as the “twinkling sign.” Methods Sixty-five patients with at least 1 urinary tract calculus seen on sonography were examined with color Doppler sonography. Two color Doppler systems of 2 different generations were used. Examinations included gray-scale, color Doppler, power Doppler, and spectral evaluations of the calculus with various settings of Doppler gain, velocity range, color filter, focal depth, and acoustic power. Results A color artifact was present within and/or distal to 29 (39%) of 75 urinary tract calculi when the old generation system was used. Use of the new generation system produced this color artifact within and/or distal to 72 (96%) of the urinary tract calculi. The intensity of the artifact was affected by the acoustic output of the equipment. Conclusions The twinkling sign is an artifact that can not only lead to misdiagnosis of vascular flow within a structure but also help differentiate a very small stone from other small echogenic structures when later generation “all-digital” systems are used. The color sensitivity of the system and acoustic power used are important technical factors that affect the production of this artifact. © 1999 John Wiley & Sons, Inc. J Clin Ultrasound 27:433–439, 1999.

Quantitative optical densitometry with scanning-laser film digitizers.

Abstract: A new process for eliminating two types of artifacts inherent in commercially available transmission scanning-laser film digitizers is presented. The first kind of artifact results in nonreproducible interference-pattern fluctuations as large as 7%. The second kind results in spreading of transmitted light from low-to-high optical density (OD) in regions with rapidly varying ODs, producing errors as large as 50%. These OD artifacts cause the loss of precision for films with low-OD regions (first type) and the loss of accuracy for films with regions of high-OD near high-OD gradients (second type). Test radiochromic films, produced by uniform exposure to a 6 MV photon beam and a high dose rate 192 Ir brachytherapy source, along with test radiographic films were used to characterize the artifacts of a commercially available scanning-laser film digitizer. The interference-pattern artifact was eliminated by digitizing the films on a masked diffusing ground-glass scanning bed. The light-transmission artifact was eliminated through discrete-fast-Fourier-transform (DFFT) deconvolution of transmission profiles with measured digitizer line-spread functions. Obtaining precise OD distributions after the DFFT deconvolution required prior removal of the interference-pattern artifact and application of a low-pass Wiener noise filter. Light-transmission artifacts are particularly significant for applications requiring measurement of high-gradient OD distributions, such as brachytherapy or conformal photon-beam film dosimetry and quantitation of two-dimensional electrophoresis gels. Errors as large as 15%–35% occur in OD distributions representative of these applications. The data collection and correction process developed in this study successfully removes these artifacts.

The effect of artifact rejection by signal-space projection on source localization accuracy in MEG measurements

Abstract: The consequences of artifact suppression by means of signal-space projection on dipole localization accuracy for magnetoencephalography measurements are studied. Approximate analytical formulas, equivalent to the Cramer-Rao bound, are presented and verified by Monte Carlo simulations which relate the increase of localization error for individual coordinates to the similarity of the artifact field and respective (contravariant) quadrupole fields obtained by differentiating the dipole field with respect to its origin. The expressions simplify significantly for dipoles placed below the center of the measuring system giving rise to highly symmetric field patterns. Formulas are presented both for single- and for multiple-artifact rejection. As illustrative examples artifact fields are constructed which (a) lead to highly decreasing signal-to-noise ratio and goodness-of-fit (GOF), while the localization error is unaffected for all coordinates and (b) lead to an increase of localization error while the SNR and the GOF stays constant. Finally, the rich structure of localization error increase is demonstrated for a class of artifact fields originating from artifact current dipoles.

Artifact rejector for repetitive physiologic-event-signal data

Abstract: An artifact rejector for repetitive physiologic-event-signal data generated from electronically-controlled physiologic-event-measuring equipment includes a physiologic-event-signal averager in communication with such physiologic-event-measuring equipment. The artifact rejector is constructed to generate and store repetitive averaged physiologic-event-signal data based upon a substantially stable time relationship between corresponding physiologic-event-signal data and heart-beat-related-signal data. The repetitive averaged physiologic-event-signal data includes less noise than the repetitive physiologic-event-signal data. The artifact rejector generates and continuously updates an averaged-data template by storing such repetitive averaged physiologic-event-signal data for a preselected number of measured physiologic events. The artifact rejector also includes a physiologic-event-noise estimator and a physiologic-event-noise monitor in communication with such physiologic-event-measuring equipment, and capable of instructing the monitor to adjust the measurement cycle based upon estimated, monitored noise. The averager preferably includes a sharp roll-off, low pass filter and examples include a fourth-order Bessel filter, two cascaded, identical second-order Bessel filters, an elliptic filter, a Tchetschebyscheff filter, or finite impulse response filters. The heart-beat-related signal is preferably an ECG signal. A method of artifact rejection includes generating and storing repetitive averaged physiologic-event-signal data based upon a substantially stable time relationship between corresponding physiologic-event-signal data and heart-beat-related-signal data, with the repetitive averaged physiologic-event-signal data including less noise than the repetitive physiologic-event-signal data.

System and method for performing image-based diagnosis over a network

Abstract: A system for performing image-based diagnosis of a machine includes a database containing a plurality of historical images taken from a plurality of machines, a diagnostic unit configured to diagnose a new artifact image from the machine and to communicate historical and non-historical images or data associated with the system to a remote facility. The plurality of historical images include a plurality of ideal images generated from the plurality of machines using all possible machine settings and a plurality of artifact images generated from the plurality of machines, each of the artifact images having known faults associated therewith and a corresponding corrective action for repairing the faults. The diagnostic unit includes a diagnostic image processor and a diagnostic fault isolator. The diagnostic image processor includes means for finding an ideal image from the plurality of historical images that most closely matches the new artifact image, means for assigning an artifact category to the new artifact image based on the matched ideal image, and means for extracting an artifact feature from the new artifact image according to the assigned category. The diagnostic fault isolator includes means for generating a plurality of metrics for the extracted artifact feature and means for applying the plurality of metrics to identify an artifact image from the plurality of historical images that most closely matches the new artifact image and a corrective action for repairing the unknown fault.

Time-reversal aspect of the point interactions in one-dimensional quantum mechanics

Abstract: It is known that there is a four-parameter family of point interactions in one-dimensional quantum mechanics. We point out that, as far as physics is concerned, it is sufficient to use three of the four parameters. The fourth parameter is redundant. The apparent violation of time-reversal invariance in the presence of the fourth parameter is an artifact.

Accuracy of a miniature intracranial pressure monitor, its function during magnetic resonance scanning, and assessment of image artifact generation.

Abstract: OBJECTIVE: We examined the accuracy and repeatability of an intracranial pressure (ICP) monitor (Codman MicroSensor; Johnson & Johnson Professional, Inc., Raynham, MA) in a nonmagnetic environment and during magnetic resonance imaging (MRI). The resulting image artifact generation was calculated. ICP monitoring is essential in management of severe head injury, but few ICP monitoring devices are compatible with use in an MRI scanner. The use of MRI to assess head injury is increasing, and developing safe methods of continuously monitoring ICP may improve patient care. METHODS: A water manometer was used as the standard for comparison. We assessed pressure readings from the ICP monitor in a nonmagnetic environment using a standard and a long connector cable between the pressure transducer and display unit. This long cable permitted testing during MRI sequences because the display unit could be distanced from the magnet. Accuracy was determined during T2-weighted imaging, proton spectroscopy, and diffusion-weighted imaging, and artifact generation was assessed. RESULTS: We found a high degree of accuracy for repeated measurements over a clinical pressure range using both standard and long connector cables outside the MRI room. During MRI scanning, the ICP monitor was accurate during T2 and proton spectroscopy sequences. Accuracy during diffusion-weighted imaging, however, was clinically unacceptable. This ICP monitor creates a reduction in signal-to-noise ratio in the received signal during T2-weighted imaging and proton spectroscopic imaging, with the obtained images still radiologically interpretable. CONCLUSION: The Codman ICP monitor is sufficiently accurate and free of artifact generation to be used during most clinical MRI applications. This could enhance patient monitoring and safety.

The removal of ocular artifact from the EEG

Abstract: Ocular artifact is a major source of contamination of the EEG This artifact causes serious difficulties in EEG interpretation, and although methods to overcome these difficulties have been sought since the late 1960s, there is no consensus on how best to do this A widely used means of removing ocular artifact is termed 'EOG correction', where a portion of EOG is removed from the EEG There are a number of ways of performing this 'correction' This thesis is an attempt to resolve these differences A review of the literature suggested that the main discrepancies between EOG correction findings (different rates of EOG propagation for different eye-movement types and/or frequencies) could be removed if EOG magnitude was held constant A simulation study found that low EOG magnitude significantly inflated propagation estimates (Bs), and a second study found that the same inflation pattern pertained to subject data It was then found that if interference was removed, differences between Bs for different eye-movement types could also be removed Thus it was suggested that propagation does not vary between eye-movement types and/or frequencies A means of averaging eye-movements was thus suggested to overcome the effects of interference in EOG and EEG channels (the AAA method) When tested with simulation data, AAA was found to be relatively unaffected by interference A more easily implemented version of the AAA (NERP) was tested empirically, and found to produce equivalent Bs It was also found that at least 40 epochs should be included in such an averaging procedure Due to difficulties with correcting blink data with saccade Bs under certain circumstances, the role of the radial EOG channel (mean

Three-dimensional artifact induced by projection weighting and misalignment

Abstract: In recent years, the use of computer graphic techniques to produce three-dimensional (3-D) and reformatted images from a set of axial computed tomography (CT) images has gained significant interest. In most cases, the CT images are generated with the projection data set weighted prior to reconstruction, to combat motion artifacts, data inconsistency, or redundant data samples. Here, the author investigates the potential bias introduced to the reconstruction as a result of the interaction of the projection weights and the isocenter misalignment (ISM). The author demonstrates that when the weights applied to the conjugate rays are significantly different, bias will result which favors the sample with a higher weight. Although the error is not easily detected in axial CT images, it can be quite visible in 3-D or multiplanar reformatted (MPR) images. In this paper, the authors first present a theoretical framework to analyze and predict the bias. The theoretical prediction is validated by both computer simulations and phantom experiments. Several schemes to combat this artifact are subsequently presented; and their effectiveness is demonstrated.

Error assessment due to coronary stents in flow-encoded phase contrast MR angiography: a phantom study.

Abstract: Phase contrast magnetic resonance imaging (PC MRI) is a promising method for assessing coronary flow. MR angiography images in the presence of coronary stents display artifacts because of the metal present in the stent. Using a flow phantom, the goal of this in vitro study was to assess quantitatively the effects of flow dephasing caused by magnetic susceptibility in velocity measurements in a region where the artifact is not visualized in a magnitude image. The results showed that for high velocities, significant errors in measurements exist around the stent, outside the susceptibility artifact visible on a magnitude image. J. Magn. Reson. Imaging 1999;10:899–902. © 1999 Wiley-Liss, Inc.

Removal of blotches and line scratches from film and video sequences using a digital restoration chain.

Abstract: Line scratches and blotches are two of the most common and annoying artifacts of image sequences. A digital restoration chain for the removal of these artifacts was conceived and implemented. The basic elements of the chain are the digital encoder, the artifact generator, the artifact detectors, the interpolators and the quality measurer. The digital encoder has the mission to convert analogue sequences into digital ones and, if necessary, to make a format conversion. The artifact generator allows quantitative and qualitative tests of the restoration algorithms. If we have access to the original non-degraded sequence we can add, in a controlled way, artifacts with the generator, and then measure efficiently the quality of the restoration algorithms, since we have the original, the degraded and the restored sequences to compare. In a real restoration process, the degraded sequence is directly applied to the artifact detector. The artifact detector finds the degraded regions in the image and, somehow, marks them. The interpolator replaces the marked degraded pixels with others that are the result of an interpolation algorithm. Finally, the quality measurer rates the quality of the restored sequence.

Artifact detection in the PO2 and PCO2 time series monitoring data from preterm infants.

Abstract: Background. Artifacts in clinical intensive care monitoring lead to false alarms and complicate later data analysis. Artifacts must be identified and processed to obtain clear information. In this paper, we present a method for detecting artifacts in PCO2 and PO2 physiological monitoring data from preterm infants. Patients and data. Monitored PO2 and PCO2 data (1 value per minute) from 10 preterm infants requiring intensive care were used for these experiments. A domain expert was used to review and confirm the detected artifact. Methods.Three different classes of artifact detectors (i.e., limit-based detectors, deviation-based detectors, and correlation-based detectors) were designed and used. Each identified artifacts from a different perspective. Integrating the individual detectors, we developed a parametric artifact detector, called ArtiDetect. By an exhaustive search in the space of ArtiDetect instances, we successfully discovered an optimal instance, denoted as ArtiDetector. Results. The sensitivity and specificity of ArtiDetector for PO2 artifacts is 95.0% (SD = 4.5%) and 94.2% (SD = 4.5%), respectively. The sensitivity and specificity of ArtiDetector for PCO2 artifacts is 97.2% (SD = 3.6%) and 94.1% (SD = 4.2%), respectively. Moreover, 97.0% and 98.0% of the artifactual episodes in the PO2 and PCO2 channels respectively are confirmed by ArtiDetector. Conclusions. Based on the judgement of the expert, our detection method detects most PO2 and PCO2 artifacts and artifactual episodes in the 10 randomly selected preterm infants. The method makes little use of domain knowledge, and can be easily extended to detect artifacts in other monitoring channels.

Detecting film-screen artifacts in mammography using a model-based approach

Abstract: Microcalcifications can be one of the earliest signs of breast cancer. Unfortunately, their appearance in mammograms can be mimicked by dust and dirt entering the imaging process and this has been shown previously to lead to false positives. The authors use a model of the imaging process and, in particular, the blurring functions inherent within it to detect the film-screen artifacts caused by dust and dirt and, thus, reduce false-positives. A crucial facet of the work is the choice of the correct image representation upon which to perform the image processing. After extensive testing, the authors' algorithm has identified no microcalcifications as being artifacts and has an artifact detection rate of approaching 96%.

Sine wave artifact as a means of calibrating structured light systems

Abstract: Structured light systems made to provide dense data over full image fields present a unique challenge to the task of calibration Localized artifacts made for CMM or hard gages are often a poor fit for testing actual 3D performance This paper considers the use of a sine wave artifact to provide a mapping of a calibration matched to full-field capabilities The sine wave offers the advantages of a continuous function across the full field, with a well defined and easy to analyze shape Changes in scale in all dimensions, as well as localized variations can be mapped in clear detail using this method