Showing papers on "Artifact (error) published in 1996"

Color Doppler twinkling artifact in hyperechoic regions.

Abstract: PURPOSE: To investigate a new color Doppler ultrasound (US) artifact that manifested as a rapidly changing mixture of red and blue behind a strongly reflecting structure. MATERIALS AND METHODS: In 140 consecutive patients with parenchymal calcifications seen at US, the presence of color signal was assessed in calcified areas relative to adjacent noncalcified tissue. The artifact, called the twinkling color artifact, was stimulated with various strongly reflecting structures immersed in still water. RESULTS: The artifact was found in 42 parenchymal calcifications. In vitro experiments showed that the twinkling artifact was present in granular structures, whereas no color signal was noted in smooth surfaces. The "twinkling sign" appeared to be generated by a strongly reflecting medium composed of individual reflectors. CONCLUSION: The presence of a color signal close to calcifications should be interpreted with caution, and a flow spectrum should always be recorded to eliminate the twinkling artifact.

Muscle artifacts in the sleep EEG: Automated detection and effect on all-night EEG power spectra

Abstract: Owing to the use of scalp electrodes in human sleep recordings, cortical EEG signals are inevitably intermingled with the electrical activity of the muscle tissue on the skull. Muscle artifacts are characterized by surges in high frequency activity and are readily identified because of their outlying high values relative to the local background activity. To detect bursts of myogenic activity a simple algorithm is introduced that compares high frequency activity (26.25-32.0 Hz) in each 4-s epoch with the activity level in a local 3-min window. A 4-s value was considered artifactual if it exceeded the local background activity by a certain factor. Sensitivity and specificity of the artifact detection algorithm were empirically adjusted by applying different factors as artifact thresholds. In an analysis of sleep EEG signals recorded from 25 healthy young adults 2.3% (SEM: 0.16) of all 4-s epochs during sleep were identified as artifacts when a threshold factor of four was applied. Contamination of the EEG by muscle activity was more frequent towards the end of non-REM sleep episodes when EEG slow wave activity declined. Within and across REM sleep episodes muscle artifacts were evenly distributed. When the EEG signal was cleared of muscle artifacts, the all-night EEG power spectrum showed significant reductions in power density for all frequencies from 0.25-32.0 Hz. Between 15 and 32 Hz, muscle artifacts made up a substantial part (20-70%) of all-night EEG power density. It is concluded that elimination of short-lasting muscle artifacts reduces the confound between cortical and myogenic activity and is important in interpreting quantitative EEG data. Quantitative approaches in defining and detecting transient events in the EEG signal may help to determine which EEG phenomena constitute clinically significant arousals.

Comparing stress ECG enhancement algorithms

Abstract: There are two predominant types of noise that contaminate the electrocardiogram (EGG) acquired during a stress test: the baseline wander noise (BW) and electrode motion artifact, and electromyogram-induced noise (EMG). BW noise is at a lower frequency, caused by respiration and motion of the subject or the leads. The frequency components of BW noise are usually below 0.5 Hz, and extend into the frequency range of the ST segment during a stress test. EMG noise, on the other hand, is predominantly at higher frequencies, caused by increased muscle activity and by mechanical forces acting on the electrodes. The frequency spectrum of the EMG noise overlaps that of the ECG signal and extends even higher in the frequency domain. In this article, the authors review some of the published ECG enhancing techniques to overcome the noise problems, and compare their performance on stress ECG signals under adverse noise scenarios. They also describe the filter bank-based ECG enhancing algorithm.

Artifacts in magnetic resonance imaging from metals

Abstract: Metallic biomedical implants, such as aneurysm clips, endoprostheses, and internal orthopedic devices give rise to artifacts in the magnetic resonanceimage(MRI) of patients. Such artifacts impair the information contained in the image in precisely the region of most interest, namely near the metallic device. Ferromagnetic materials are contraindicated because of the hazards associated with their movement during the MRI procedure. In less‐magnetic metals, it has been suggested that the extent of the artifact is related to the magnetic susceptibility of the metal, but no systematic data appear to be available. When the susceptibility is sufficiently small, an additional artifact due to electrical conductivity is observed. We present an initial systematic study of MRI artifacts produced by two low susceptibility metals, titanium (relative permeability μ r ≊1.0002) and copper (μ r ≊0.99998), including experimental, theoretical, and computer simulation results.

Neuropsychological deficits in chronic fatigue syndrome: artifact or reality?

Abstract: Chronic fatigue syndrome is an illness characterised by extreme fatigue of uncertain origin which has been present for at least six months. The fatigue should have a sudden onset and be severe enough to substantially reduce the patient's day to day activities.' A number of other symptoms are associated with chronic fatigue syndrome including complaints of impaired memory, difficulty making decisions, poor attention, and reduced concentration. ' Many patients complain that it is these cognitive symptoms which cause them the greatest frustration and disability. Over the past six years several researchers have conducted neuropsychological tests on patients with chronic fatigue syndrome in an attempt to objectively quantify the nature and extent of these cognitive difficulties. Despite the objective aims of these studies the pattern of results has been contradictory and often confusing. The aim of this review is to provide a comprehensive summary and discussion of the results from neuropsychological studies which have compared patients with chronic fatigue syndrome with controls or normative data, to provide both some clarity in the area and directions for future research. Studies published since the Centre for Disease Control issued the first diagnostic criteria for chronic fatigue syndrome2 were accessed through a search on the MEDLINE and PsycLIT databases. Only fully reported studies were reviewed, as we were unable to reliably assess the limited information presented in the abstracts. The results of the search are grouped into five neuropsychological categories; global intellectual functioning, receptive functioning (encompassing sensation and perception), mental activity (subgrouped into activity rate and attentional functions), memory, and cognition (which incorporates the higher mental functions of planning, organising, problem solving, and abstracting).3 We also examine the relations between cognitive impairment in chronic fatigue syndrome and both psychological and physical factors.

Multigated Doppler Ultrasound in the Detection of Emboli in a Flow Model and Embolic Signals in Patients

Abstract: Background and purpose The ability to detect asymptomatic circulating cerebral emboli may contribute to the management of patients with stroke, but its clinical usefulness will depend on effective systems for automatically detecting embolic signals (ES) and differentiating them from artifact. A new method involves the use of a multidepth probe that allows recording from both distal and proximal sample volumes along the same vessel. Theoretically, an embolus should appear sequentially, with a time delay, between the two channels, whereas an artifact should appear simultaneously in the two channels. Methods We evaluated this method in an in vitro flow model and in patients. In an in vitro model, with a flow pattern mimicking intracerebral flow, 181 air bubbles and 193 thrombus emboli were compared with the signals resulting from 368 episodes of artifact; a sample volume of 5 mm and a channel separation of 10 mm were used. ES from two groups of patients-those with carotid artery stenosis (141 ES) and those with mechanical prosthetic cardiac valves (125 ES)-were studied and compared with 222 episodes of artifact produced in the same patients. Results In the model the mean (SD) time delay was 17.32 (9.94) ms for air emboli and 17.78 (10.66) ms for thrombus emboli compared with -0.01 (0.39) ms for artifact (air and thrombus emboli versus artifact, P 2 ms was used for an embolus. The method allowed equally good detection of those air emboli that resulted in receiver overload and aliasing. In patients the mean (SD) time delay was 29.6 (28.2) ms for valve ES and 14.9 (15.42) for carotid ES compared with 0.00 (0.46) for artifact (carotid and valve ES versus artifact, P Conclusions The multigated technique offers a new method of detecting ES and differentiating them from artifact and is the first reliable method for differentiating intense ES resulting in receiver overload from artifact. Occasionally its sensitivity is reduced when ES do not appear in the distal channel, probably because they pass down a side branch; this may be reduced by reducing gate separation. Some less intense carotid ES can be difficult to detect if the amplitude increase is small compared with the amplitude of the background Doppler signal.

Post-processing device for eliminating blocking artifact and method therefor

Abstract: A post-processing device for eliminating a blocking artifact generated upon reconstructing an image compressed by block transform operation and a method thereof minimize a blocking artifact at block boundaries by selecting a predetermined discrete cosine transform (DCT), estimating transform coefficients with respect to the information lost upon quantization or inverse quantization to have the highest continuity with respect to adjacent blocks, performing an inverse transform operation on the estimated transform coefficients, and adding the thus-obtained adjustment value to an inverse-transform-operated reconstructed image signal.

Theoretical comparison of Fourier and wavelet encoding in magnetic resonance imaging

Abstract: A theoretical comparison of the wavelet and Fourier encoding methods is made with respect to resolution, sensitivity to artifact, and signal-to-noise ratios (SNR's). A general mathematical description is developed in which magnetic resonance (MR) image encoding is represented by a projection of the function representing the "MR signal density" onto an approximation subspace of the finite energy functions. Characteristics of the subspace are used to define a "generalized" point-spread function for space-variant systems. Using the formal model of MR image encoding it is shown that wavelet encoding approaches the resolution limit defined by Fourier encoding. Artifact is treated according to whether or not the source of the variation in the measured data is stationary. Nonstationary imperfections perturb the projection operation and result in encoding method-dependent effects which can be modeled by a distortion matrix suitable for treating shift variant systems. A framework is developed in which to derive expressions for SNR's applicable to a general class of MR encoding methods.

MRI artifact cancellation due to rigid motion in the imaging plane

Abstract: A post-processing technique has been developed to suppress the magnetic resonance imaging (MRI) artifact arising from object planar rigid motion. In two-dimensional Fourier transform (2-DFT) MRI, rotational and translational motions of the target during magnetic resonance magnetic resonance (MR) scan respectively impose nonuniform sampling and a phase error an the collected MRI signal. The artifact correction method introduced considers the following three conditions: (1) for planar rigid motion with known parameters, a reconstruction algorithm based on bilinear interpolation and the super-position method is employed to remove the MRI artifact, (2) for planar rigid motion with known rotation angle and unknown translational motion (including an unknown rotation center), first, a super-position bilinear interpolation algorithm is used to eliminate artifact due to rotation about the center of the imaging plane, following which a phase correction algorithm is applied to reduce the remaining phase error of the MRI signal, and (3) to estimate unknown parameters of a rigid motion, a minimum energy method is proposed which utilizes the fact that planar rigid motion increases the measured energy of an ideal MR image outside the boundary of the imaging object; by using this property all unknown parameters of a typical rigid motion are accurately estimated in the presence of noise. To confirm the feasibility of employing the proposed method in a clinical setting, the technique was used to reduce unknown rigid motion artifact arising from the head movements of two volunteers.

Method and system for the extraction of cardiac artifacts from EEG signals

Abstract: The disclosed filter removes cardiac artifacts from signals representative of a patient's cerebral activity. The filter preferably replaces portions of the signal including artifacts with temporally adjacent artifact free portions.

Controlled-artifact magnetic resonance instruments

Abstract: The present invention provides materials and methods for preparing medical instruments which cause reduced or enhanced artifact on diagnostic images. According to the invention, a reduced artifact instrument may be prepared using titanium alloy compositions or plastic compositions disclosed herein. An enhanced artifact instrument may be prepared by incorporating artifact causing materials into artifact free instruments. Such instruments are suitable for use during diagnostic imaging procedures, for example, magnetic resonance imaging. The invention also provides a new penetrating end for penetrating instruments used to perform medical and surgical diagnostic and therapeutic procedures.

CT artifact correction: an image-processing approach

Abstract: This paper presents an image processing approach for correcting streaking artifacts in ComputedTomography (CT) images and compares it to physics based and composite approaches. In CT, thereare two major sources for streaking artifacts: beam hardening and nonlinear partial volume averaging.The physics based approach uses a physical model for the beam hardening. Inaccuracy of the model andnonlinear partial volume averaging result in an incomplete correction of the artifacts by this approach.The proposed image processing approach identifies and corrects the artifacts using their frequency char-acteristics. It corrects the artifacts regardless of their source. The composite approach also utilizes thephysical model for the beam hardening. Although correcting some of the artifacts, it is computationallymore intense than the image processing approach. We illustrate the methods and compare their perfor-mance using a computer simulation and CT images of a phantom and a human brain. We show thatthe streaking artifacts resulting from nonlinear partial volume averaging which are not corrected by thephysics based method are corrected by the image processing approach.Key Words: Artifact Correction; Computed Tomography (CT); Image Enhancement; Image Seg-mentation; and Image Analysis.

Motion artifact suppression filter for use in computed tomography systems

Abstract: The disclosed motion artifact filter generates motion corrected signals from projection data signals to compensate for motion artifacts in CT images Each measurement of the projection data signals is representative of the density of all the mass occupying a volume between an X-ray source and a detector at a particular projection angle The motion artifact filter includes a high pass filter for generating high frequency signals representative of patient movement Each measurement of the high frequency signals is generated by combining groups of measurements of the projection data signals corresponding to volumes that are proximal to each other Each measurement of the motion corrected signals is generated by subtracting a portion of a measurement of a high frequency signal from its associated measurement of a projection data signal The portions increase as the projection angle approaches a starting angle and the portions decrease as the projection angle approaches an intermediate angle

Magnetic susceptibility artifacts in magnetic resonance imaging: calculation of the magnetic field disturbances

Abstract: In magnetic resonance imaging, inhomogeneities of the static magnetic field lead to perturbations in the resulting images, called artifacts. The authors' goal is to compute numerically the disturbances induced by a material having magnetic properties different from that of the surrounding tissues. The method is linked to an artifact reconstruction model to get simulated images. This model needs very accurate results in a fine three-dimensional grid around the implant. The use of standard methods would lead to solve very large systems. The authors' method is based on a surface integral representation of the magnetic field. To implement it in an efficient way, an analytical expression is derived when the boundary of the domain can be meshed in flat panels. For curved surfaces a numerical quadrature scheme is implemented.

Truncation artifacts in tomographic reconstructions from projections.

Abstract: The artifacts in tomographic reconstructions from truncated sets of projections are analyzed. The shift-variant impulse response of the tomographic system for parallel-beam geometry is derived. A number of propositions are made describing the observed artifacts. A graphical scheme for the prediction of the location and shape of the truncation artifacts is presented and applied to reconstructions from simulated projections. The artifact analysis is applied to images obtained with the commonly used convolution backprojection reconstruction algorithms, and it is extended to reconstructions from fan-beam projections. The analysis is performed for the continuous imaging domain so as to separate the clipping artifacts clearly from those attributed to the digital implementation of the reconstruction algorithms.

Investigations into possible causes of hot inferior wall artifacts in attenuation corrected cardiac perfusion images

Abstract: Through our investigations with simulated images, we have identified several causes of hot inferior wall artifacts in attenuation corrected SPECT cardiac perfusion images. With an insufficient number of iterations, the non-uniform resolution recovered in three dimensions and slow convergence rate of ML-EM reconstruction can cause this artifact when the heart is at a shallow angle (the axis of the heart is close to being horizontal) or when there exists significant background activity. Increasing the number of iterations does not help in correcting this artifact when the attenuation map used has poor resolution or its attenuation coefficients used are not accurate. We also notice that the hot inferior wall artifact is more pronounced when body contouring acquisition or when 180/spl deg/ angular sampling is used.