Showing papers on "Artifact (error) published in 1998"

Removing electroencephalographic artifacts: comparison between ICA and PCA

Abstract: Pervasive electroencephalographic (EEG) artifacts associated with blinks, eye-movements, muscle noise, cardiac signals, and line noise poses a major challenge for EEG interpretation and analysis. Here, we propose a generally applicable method for removing a wide variety of artifacts from EEG records based on an extended version of the independent component analysis (ICA) algorithm for performing blind source separation on linear mixtures of independent source signals. Our results show that ICA can effectively separate and remove contamination from a wide variety of artifact sources in EEG records with results comparing favourably to those obtained using principal component analysis (PCA).

Dealing with artifacts : the eog contamination of the event-related brain potential

Abstract: Eye movements and blinks represent a major source of artifacts in the electroencephalogram (EEG) and event-related brain potentials (ERPs). The origin of this artifact is the large difference in potential that exists between the cornea and the retina. Eye movements and blinks produce shifts of the electric fields that propagate across the whole head and that can be several times larger than the activity generated by the brain. Ocular activity can be monitored by electrodes located near the eyes (electrooculogram, or EOG). The electric fields associated with eye movements and blinks are somewhat different. The simplest procedure for dealing with ocular artifacts is to eliminate trials on which EOG activity is detected (rejection). However, this technique may result in data loss and biased data samples, especially when one is comparing clinical populations or tasks involving large amounts of eye movements. Another approach involves estimation and correction of the ocular artifact on the EEG and ERP traces. Several techniques have been proposed. Some of them are reviewed in the present paper. Issues related to the accuracy of the various techniques, as well as other advantages and limitations, are also discussed. Finally, general guidelines for how to deal with ocular artifacts are proposed.

Image input device and method for providing scanning artifact detection

Abstract: A system and method which enables the location and orientation of an image to be determined without introducing scanning artifacts into the rendered image. The system utilizes a separate set of sensors on a scanning bar that are sensitive to a wavelength of light outside the range of wavelengths of the typical sensors found on the scanning bar, such as red, green, or blue. Moreover, the system includes a light source which is capable of emitting the wavelength of light that is outside this range of wavelength of light. Thus, whenever the additional set of sensors detect light, the system would determine that the light was due to a scanning artifact, such as a hole, rip, or dog ear in the original document being scanned. Furthermore, the system would include a process which measures the variations of the image signal in the fastscan and slowscan directions, as well as, the gray level of the video signal from these various measurements, the system would develop a cost value as to the certainty as whether the signal represents a scanning artifact. In such a system, the backing would be embedded with a pattern which would have a predetermined variation in a fastscan direction but have little or no variation in a slowscan direction.

Minimizing the ghosting artifact in scene-based nonuniformity correction

Abstract: Current infra-red focal point arrays are limited by their inability to calibrate out component variations. Nonuniformity correction (NUC) techniques have been developed and implemented in off-board digital hardware to perform the necessary calibration for most IR sensing applications. There are two possible types of NUC that can be considered for focal-plane integration: (1) Two-point correction using calibrated images on startup and (2) Scene- based techniques that continually recalibrate the sensor for parameter drifts. The problems with the two-point methods have been well-documented in the literature (parameter drift, expense, etc.) We address the two major problems of scene-based techniques: (1) a more difficult hardware implementation and (2) ghosting artifacts in the corrected images. We have previously addressed the implementation problems by developing and demonstrating special purpose analog hardware as well as an efficient digital algorithm that incorporates the constant statistics model. The ghosting artifact occurs in all scene-based techniques when an object that does not move enough tends to `burn in' and can remain visible for thousands of images after the object has left the field of view. We have improved our model to eliminate much of the ghosting artifact that plagues all scene-based NUC algorithms. By modifying the correction update during ghosting situations, we are able to significantly remove the ghosting artifact and improve the overall accuracy of the correction procedure. We demonstrate these results on real and synthetic image sequences.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A comparison of magnetic resonance and computed tomographic image quality after the implantation of tantalum and titanium spinal instrumentation.

Abstract: Study design Tantalum- and titanium-based lumbar interbody fusion devices were implanted into two fresh human cadavers, and magnetic resonance and computed tomographic imaging were performed to evaluate adjacent spinal structures and the amount of metallic artifact. Objective The objective of this study was to prospectively compare the preliminary results of magnetic resonance imaging and computed tomography scanning image quality after the implantation of both titanium and tantalum spinal implants. Summary of background data The availability of tantalum and titanium spinal implants brings theoretical magnetic resonance imaging compatibility along with several other desirable characteristics. The magnetic resonance imaging and computed tomographic imaging of tantalum spinal instrumentation has never been studied previously or compared with titanium instrumentation. Methods Titanium and tantalum spinal implants produced for anterior spinal fusion were each placed at two levels in the lumbar spine of two fresh cadaver specimens. Sequential spin echo T1-weighted and T2-weighted magnetic resonance imaging studies and computed tomographic scans were obtained. The resulting images were then graded to describe and compare the behavior of tantalum metal in magnetic resonance imaging and computed tomographic studies. Results Good T1 and T2 images were obtained that allowed visualization of the neural structures with minimal artifact. The optimal T1 images for tantalum metal were similar in quality to the optimal T1 parameters for titanium metal. T2 images for both tantalum and titanium metal were obtained with similar results for both metals. Gradient echo magnetic resonance imaging scans of both were poorly imaged with a large amount of artifact. Computed tomographic studies of tantalum implants produced a large amount of metal artifact when compared with computed tomographic studies of titanium implants. Conclusions High-quality magnetic resonance imaging studies can be obtained after the implantation of both titanium and tantalum spinal instrumentation. Both of the metals produce similar images on magnetic resonance imaging studies with comparable amounts of metallic artifact. High-quality computed tomographic scans of titanium implants can be obtained with minimal distortion secondary to artifact. However, computed tomographic scanning is not the imaging modality of choice for the tantalum spinal implants because of the large amounts of artifact.

The Effects of Digitization on Nonstationary Stochastic Signals with Applications to Pulsar Signal Baseband Recording

Abstract: The process of digitizing a stochastic signal introduces systematic distortions into the resulting digitized data. Further processing of these data may result in the appearance of unwanted artifacts, especially when the input signal was generated by a nonstationary stochastic process. In this paper the magnitude of these distortions are calculated analytically and the results are applied to a specific example found in pulsar signal processing. A pulsar signal is an excellent example of a nonstationary stochastic process. When analyzing pulsar data, the effects of interstellar medium (ISM) dispersion must be removed by digitally filtering the received signal. The distortions introduced through the digitization process cause unwanted artifacts to appear in the final "dedispersed" signal. These artifacts are demonstrated using actual 2-bit (4-level) digitized data of the pulsar PSR B083345 (Vela). Techniques are introduced that simultaneously minimize these artifacts and maximize the signal-to-noise ratio of the digitized data. The distortion analysis and artifact removal techniques described in this paper hold for an arbitrary number of input digitization thresholds (i.e., number of bits). Also presented are tables of the optimum digitizer thresholds for both uniform and nonuniform input threshold digitizers.

Image coding ringing artifact reduction using morphological post-filtering

Abstract: Ringing is an annoying artifact frequently encountered in low bit-rate transform and subband decomposition based compression of different media such as image, intra frame video and graphics. A mathematical morphology based post-processing algorithm is presented in this paper for image ringing artifact suppression. First, we use binary morphological operators to isolate the regions of an image where the ringing artifact is most prominent to the human visual system (HVS) while preserving genuine edges and other (high-frequency) fine details present in the image. Then, a gray-level morphological nonlinear smoothing filter is applied to the unmasked regions of the image under the filtering mask to eliminate ringing within this constraint region. To gauge the effectiveness of this approach, we propose an HVS compatible objective measure of the ringing artifact. Preliminary simulations indicate that the proposed method is capable of significantly reducing the ringing artifact on both subjective and objective basis.

Hybrid ordered phase encoding (HOPE): An improved approach for respiratory artifact reduction

Abstract: Respiration causes continuous change in cardiac position, which leads to image degradation. Phase-encode reordering methods are often used to reduce these artifacts. An improved method for suppressing motion artifacts by reordering the acquisition of k space has been developed that is less sensitive to change of breathing patterns and bulk movement. We describe the theory behind the new approach and compare its results with those of existing methods by use of a phantom with simulated and actual acquired breathing patterns. The comparison was also made in vivo; cardiac scans were performed in 15 subjects with image planes that are known to be particularly susceptible to respiratory artifact. A significant improvement in image quality was achieved compared with conventional nonreordered and existing reordering methods.

Recording of electrical brain activity in a magnetic resonance environment: Distorting effects of the static magnetic field

Abstract: The technical limitations of electroencephalography (EEG) and flashed visually-evoked potentials (VEP) recordings in the static magnetic field of the MR system were systematically studied. A main artifact occurring in the magnetic field was found to be correlated with the heart cycle and had amplitudes in the range of EEG and VEP signals. For VEP recordings, a substantial reduction of this effect was achieved by subtraction of the averaged artifact from the averaged composed signal (VEP and artifact) resulting in the VEP signal alone. However, for continuous EEG recordings, there is no such solution, since the observed effect is not sufficiently constant in amplitude, and the standard deviation of the amplitude of the effect is often larger than the EEG amplitude.

System and method for compensating for motion artifacts in a strong magnetic field

Abstract: A system for monitoring an ECG during a patient examination in a strong magnetic field, such as that created by an MRI machine, which eliminates motion artifacts affecting the accurate reading of the ECG. The system modulates the strong magnetic field and then generates an error correction signal substantially equal to the motion artifact signal in timed relationship with the modulated signal that is subtracted from the measured ECG signal to produce a more accurate ECG signal.

Reduction of blocking effect in DCT-coded images based on a visual perception criterion

Abstract: Blocking effect constitutes one of the main drawbacks of the actual DCT-based compression methods. We propose in this paper a new block reduction technique; it is based on a space-variant non-linear filtering operation of the blocking artifacts present in the image to be reconstructed. To account for the perceptual importance of the distortion, the amount of smoothing is adapted to the visibility of the blocking effect. A visibility parameter is computed for each artifact using the psychovisual properties of the human visual system (HVS). The postprocessing algorithm is in conformity with actual existing compression standards; it provides a way to greatly reduce the artifacts without degrading high-frequency information of the original image. First the proposed method is described and then experimental results are presented, showing the effectiveness of the correction.

Correction algorithm for bone-induced spectral artifacts in computed tomograph imaging

Abstract: A spectral correction algorithm for correcting dense object-induced spectral artifacts is described. In one embodiment, a calibration object, representative of typical head scanning conditions is scanned and the data reconstructed to provide an image. A water or water-equivalent cylinder of about the same diameter also is scanned and reconstructed, on the same display field of view (DFOV). These two images are designated respectively by BWEQ and WEQ. The ratio of images BWEQ and WEQ is then evaluated, and a region of interest extracted by multiplying the ratio by a function II(r), to obtain a calibration pattern CP. The calibration pattern is then averaged in azimuth to obtain a calibration vector. This calibration vector is fitted with low--order polynomial, and then divided by the fitting polynomial, to take out from the vector the low frequency components that, for instance, would be introduced on an "ideal" scanner. By subtracting 1.0 from the ratio, and multiplying by a CT number scale factor (ctscale) and an apodizing window Aw(r), a calibration error vector CEV is obtained that is representative of the circularly symmetric image error introduced by the non-corrected bone-induced artifact. The corresponding error calibration vector can be expanded into a circularly symmetric image error pattern I[CEV(r)], and subtracted from the calibration image, to provide a substantially artifact free image. The method can be extended to extract and correlate error vectors on an image segment basis such that the resulting error image pattern is not circularly symmetric.

Ghost artifact reduction

Abstract: A method of and apparatus for reducing ghost artifact in image data are disclosed, for use with a Magnetic Resonance Imaging apparatus in which one or more coherent ghost artifacts occur in the image. The apparatus comprises means for converting the sampled data into the image data, and means for analysing the sampled data to determine a correction to reduce the ghost artifact. The method of the invention comprises analysing the sampled data to determine a correction to reduce the ghost artifact.

Image processing system for reducing vertically disposed patterns on images produced by scanning

Abstract: A digital image processing system applies an edge-based adaptive thresholding technique with different contrast parameters to convert an image into first and second binary images. In the digital processing system of the invention, the first and second binary images are compared and a map of the difference between the first and second binary images is generated. A vertical profile of this map is projected and local peaks in the vertical profile which correspond to a vertical pattern or artifact in the map is detected. Pixels in the first binary image are reversed based on the vertical pattern or artifact in the map. This system is effective in reducing vertical artifacts produced by scanners.

Muscle artifact noise detector for ECG signals

Abstract: A filter system for detecting and removing small amplitude, high frequency signals such as muscle artifact signals is provided. One form of the system includes a baseline wander filter, high and low pass filters, an adaptive line noise canceler and various noise detectors to identify, signal and remove muscle artifact or loose electrode signal contamination from an ECG signal wherein the ECG signal is conditioned to remove various portions of the ECG signal, such as the QRS portion of the ECG signal, prior to processing in various noise detectors while minimizing the signal conditioning effect of the filters on the ECG signal and while further providing the operator with the ability to manually or automatically activate the filters and to indicate the status of the filters on a printout or display.

Sleep states and neonatal pulse oximetry.

Abstract: Aim: Pulse oximetry is a quick, easy, noninvasive method widely used for monitoring oxygen saturation (SaO 2 ) in the neonatal period. The greatest recognized problem with SaO 2 readings measured from the oximeter is artifact, arising from an inability to record accurately during movement. The aim of this study was to determine the proportion of pulse oximeter readings affected by movement artifact during sleep in term and preterm infants. Method: Polygraphic recordings were obtained from 11 term and 6 preterm infants at postconceptional ages (SD) of 39.8 (0.8) and 39.3 (1.5) weeks respectively. The polygraphic and computer recordings were divided into epochs of 30 seconds duration and identified as active sleep (AS), quiet sleep (QS), indeterminate sleep (IS), or wakefulness (AW), using electrophysiological parameters and behavioral observations. Movement artifact was identified by visual examination of polygraphic computer recordings using strict criteria. Signal containing artifact was removed from recordings and the percentage of artifact time present per recording calculated. Results: Signal artifact was present in recordings of all infants studied, comprising an average state time of 19% during quiet sleep, 49% of active sleep, 49% of indeterminate sleep and 91% of wakefulness. A significant difference in the proportion of artifact present in recordings of term and preterm was observed only during quiet sleep. Conclusion: Movement artifact during pulse oximetry recordings is dependent on behavioral state, and overall affects up to 50% of recorded traces. A reliable and more accurate noninvasive method of recording oxygen saturation is thus needed, for use in both neonatal nurseries and in sleep studies, to aid in accurate clinical decision-making.

Apparatus and method for degrading the quality of unauthorized copies of color images and video sequences

Abstract: Image-distorting artifacts are added to selected areas of a color image. A first artifact change is made to a luminance component of a selected area, and at least one other artifact change is made to chrominance components of the selected area. The artifact changes are perceptively balanced such that the artifacts are not perceptible when the image is displayed on a monitor. For example, changes in saturation are selected to balance changes in intensity of luminance. However, when the image is filtered such that one of the artifacts is modified, the remaining artifact becomes visible. Such filtering might be performed by an RGB-to-NTSC converter. Thus, if the RGB-to-NTSC converter is used between an RGB output of DVD-ROM electronics and a computer monitor (in an attempt to make an unauthorized copy), the video signal leaving the RGB-to-NTSC converter will contain perceptible artifacts and, therefore, images of reduced viewability. Consequently, attempts to make unauthorized copies of DVD discs onto NTSC-type videocassettes will be discouraged.