Showing papers on "Artifact (error) published in 1995"

Motion Artifacts in fMRI: Comparison of 2DFT with PR and Spiral Scan Methods

Abstract: Activation signals based on BOLD contrast changes consequent to neuronal stimulation typically produce cortical intensity differences of < 10% at 1.5T. Hemodynamically driven pulsation of the brain can cause highly pulsatile phase shifts, which in turn result in motion artifacts whose intensity is larger than the activation signals in 2DFT scan methods. This paper presents a theoretical and experimental comparison of the magnitude of such artifacts for 2DFT and two other methods using non-Cartesian k-space trajectories. It is shown that artifacts increase with TR for 2DFT methods, and that projection reconstruction (PR) and spiral methods have significantly reduced artifact intensities, because these trajectories collect low spatial frequencies with every view. The spiral technique is found to be superior in terms of efficiency and motion insensitivity.

Method and apparatus for removing artifact and noise from pulse oximetry

Abstract: Motion compensation is based on analysis of intensity signals received by detectors, without separately measuring a motion signal, without providing feedback to cancel the motion signal and without attempting to mathematically eliminate the motion signal. Instead, the present invention mathematically recognizes the presence of the motion signal and recognizes a few key characteristics of the motion signal and makes corresponding assumptions. First, it is recognized that the motion/noise in each wavelength signal is proportional. Second, it is assumed that the blood pulse signal is not affected by motion.

Nonsusceptibility artifacts due to metallic objects in MR imaging.

Abstract: The authors investigated eddy current artifacts due to metallic objects within the magnetic resonance imaging field. The problem was simplified by using a circular copper loop as a model for the more complex eddy current pathways present in a metallic implant. With this simple geometry, the authors show that radio-frequency (RF)-induced eddy currents in the metal produce a significant local artifact. However, no appreciable artifacts due to the switching magnetic field gradients were observed. A detailed quantitative analysis of the mechanism of RF-induced eddy current artifact due to the the copper loop was performed. The artifact was demonstrated experimentally to result from perturbations of the transmit and receive sensitivities of the RF coil. Theoretical calculations of these perturbations showed excellent agreement with experimental results. With an understanding of the artifact mechanism, methods for correcting the RF-induced eddy current artifact were applied.

A study of the liver-heart artifact in emission tomography

Abstract: UNLABELLED With the introduction of 99mTc-teboroxime, a previously undocumented artifact has shown up in cardiac SPECT imaging. In the images, the uptake values near the inferior wall are lower than expected. The artifact has been reported in the literature, but an adequate explanation has not yet been provided. The high uptake of 99mTc-teboroxime in the liver has been demonstrated to be the cause of this artifact. METHODS With simulations we show that an artifact can be reproduced by applying filtered backprojection (without corrections for attenuation) of attenuated and blurred projections. The conclusions from the simulations are validated with SPECT and PET phantom measurements. Maximum likelihood expectation maximization (ML-EM) reconstruction is applied to evaluate the effect of accurate attenuation correction. The influence of the high liver uptake on the convergence of ML-EM was also evaluated. RESULTS The artifact results mainly when the photon attenuation during reconstruction is ignored. This results in a distorted reconstruction of the liver. These distortions affect the neighboring inferior wall of the myocardium. While the use of opposite projections reduces the effect, accurate attenuation correction nearly eliminates it. A small additional deformation is caused by the position dependence of the spatial resolution of the gamma camera. It was also noted that the presence of the liver slows down the convergence of ML-EM in the heart region. CONCLUSION The liver-heart artifact is an attenuation effect and is eliminated by attenuation correction. The local convergence of ML-EM is affected by the total image content.

Systems and methods for adaptive filtering artifacts from composite signals

Abstract: Analog or digital systems or methods filter an artifact from a signal derived from a biological event comprising sample values arranged with respect to time. The systems and methods generate a variable expressing a template of the artifact and reduce the derived biological signal by the variable to derive a filtered derived biological signal. The systems and methods change the variable over time based upon the energy of the filtered derived biological signal to minimize the energy of the artifact remaining in the filtered derived biological signal over time. The systems and methods output the filtered derived biological signal. The systems and methods remove artifacts from signals derived from a biological event; for example, electrograms, electrocardiograms, electroencephalograms, electrogastrograms, electromyograms, and respiratory signals.

Imaging artifacts in diagnostic ultrasound—a review

Abstract: Imaging artifacts commonly occur during routine ultrasonographic evaluation of patients and contribute to image inaccuracies. These artifacts are classified into artifacts arising from factors controllable prior to imaging or from sound beam interactions within the patient which may result in clinically useful or confusing artifacts. Artifact formation, and how this influences the anatomical image location, echogenicity, size and shape, is described. The clinical interpretation of artifacts is explained in order to gain maximum diagnostic information from an ultrasonographic image.

Adaptive cancelation of motion artifact in photoplethysmyographic blood volume pulse measurements for exercise evaluation

Abstract: An adaptive transversal filter was designed to minimize the impact of motion artifact in the measurement of photoplethysmographic Blood Volume pulse (BVP) in an exercising subject. The rationale of the design is introduced and results from off-line testing with signals recorded from an experimental setup are presented.

EEG during MR imaging Differentiation of movement artifact from paroxysmal cortical activity

Abstract: It is possible to record scalp EEG during the acquisition of gradient-echo/echo-planar MR images or MR spectra without significant degradation of the MR data. [1] Functional MR imaging or spectroscopy with simultaneous EEG recording allows the study of hemodynamic and metabolic changes in vivo in human subjects during cognitive tasks in normal subjects and in patients with paroxysmal epileptiform discharges or ictal events. [2] EEG registration in the MR scanner, however, is very susceptible to a variety of artifacts, particularly patient head movement and ballistocardiogram; the latter is related to head and body movements produced during systole. Despite having the head securely restrained, very small movements of the head produce high-amplitude, paroxysmal bursts on EEG that are impossible to differentiate from paroxysmal cortical epileptiform activity. This artifact results from currents produced by small movements of the EEG electrodes and wires …

Clinical evaluation of a method for automatic detection and removal of artifacts in auditory evoked potential monitoring

Abstract: Objective. The objective of our study was to evaluate the method for detection and removal of artifacts in evoked potential monitoring described earlier by Cluitmans and colleagues in a clinical setting.Methods. The method for detection and removal of artifacts by Cluitmans and colleagues is based on the assumption that a sweep of the recorded electroencephalogram (EEG) signal contains artifacts if one or more variables derived from the signal deviates strongly from the normal range of values. Once these normal ranges are defined, all future EEG recordings that are recorded under comparable circumstances can be automatically evaluated for artifacts by tracking when one or more signal variables falls outside the normal range. To assess the performance of this method in a clinical setting, recordings from a learning set were visually evaluated for artifacts. From the empirical distribution functions of the signal variables, the thresholds for automatic detection of artifacts were determined. The auditory evoked potential (AEP) waveforms resulting after automatic screening were compared with the waveforms obtained after visual evaluation of the raw signal combined with manual exclusion of signal periods containing artifacts.Results. The quality of the resulting waveform was improved by our method of automatic detection and removal of artifacts in 97% of partly contaminated recordings. In only 2% of the recordings, automatic screening slightly degraded the resulting waveform.Conclusions. We conclude that the described method of automatic detection and removal of artifacts in AEP recordings effectively improves the quality of the resulting AEP waveform, without excessive rejection of artifact-free signal periods. The signal variables used in this method seem appropriate for distinguishing artifact-free signal periods from periods containing artifacts for the types of artifact that were studied.

Resolved: Multiple Personality Disorder Is an Individually and Socially Created Artifact

Abstract: Where's hysteria now that we need it? With DSM-IV, psychiatrists have developed a common language and a common approach to diagnosis. But in the process of operationalizing diagnoses, we may have lost some concepts about patient behavior. The term \"hysteria\" disappeared when DSM-III was published; without it, psychiatrists have been deprived of a scientific concept essential to the development of new ideas: the null hypothesis. This loss hits home with the epidemic of multiple personality disorder (MPD). The work of Talcott Parsons (1964), David Mechanic (1978), and Isidore Pilowsky (1969) taught psychiatrists to appreciate that phenomena such as hysterical paralyses, blindness, and pseudoseizures were actually behaviors with a goal: achieving the \"sick role.\" Inspired by Parsons, Mechanic and Pilowsky used the term \"abnormal illness behavior\" in lieu of hysteria. Their approach eliminated the stigma of malingering that had been implied in hysteria and indicated that patients could take on such behavior without fraudulent intent. They were describing an old reality of medical experience. Some people-experiencing emotional distress in the face ofa variety oflife circumstances and conflictscomplain to doctors about physical or psychological symptoms that they claim are signs of illness. Sometimes they display gross impairments of movement or consciousness; sometimes the features are subtle and changing. These complaints prompt doctors to launch investigations in laboratories, to conduct elaborate and sometimes dangerous studies of the brain or body, and

Experimental study: brachial motion artifact reduction in the ECG

Abstract: This study focuses on a dual-input adaptive noise reduction technique by investigation of brachial motion artifact in the ECG under a special experimental protocol. The ECG and motion artifact signals are acquired from a three-electrode system. Primary input is obtained from the standard EGG lead II. Because limbs function like fixed resistors in ECG measurement, we obtain EGG-free brachial motion noise between two electrodes located on the arm, near the right biceps muscle. The separation distance of the electrodes is 5 mm to acquire the motion noise signal, and this signal is the auxiliary input for adaptive filtering. The results show that the LMS algorithm has a very slow rate of convergence. Comparatively, an RLS algorithm converges almost immediately once motion artifact appears and performs satisfactorily in reducing even rapidly varying brachial artifact. It also significantly improves the low-frequency baseline drift. Although the RLS algorithm imposes a large computational burden, a 33-MHz PC486 can execute the algorithm, written in C language, in real time. To prevent the ill-conditioning matrix in the RLS algorithm when the noise is very small, we add white noise to the auxiliary input The experiment shows that this approach can significantly improve the condition of the matrix.

An improved method for MRI artifact correction due to translational motion in the imaging plane

Abstract: A computer postprocessing technique is developed to remove MRI artifact arising from unknown translational motion in the imaging plane. Based on previous artifact correction methods, the improved technique uses two successive steps to reduce read out and phase-encoding direction artifacts: First, the spectrum shift method is applied to remove read-out axis translational motion. Then, the phase retrieval method is employed to eliminate the remaining subpixel motion of the read-out axis and the entire motion of the phase-encoding axis. In the presence of noise, to protect edge detection (in the spectrum shift method), two high-density gray-level markers are added, one to each side of the imaging object. Experimental results with an actual MR scan confirmed the ability of the method to correct the artifact of an MR image caused by unknown translational motion in the imaging plane. >

Reconstruction artifacts in digital video compression

Abstract: This paper surveys the visual distortions introduced by a compression scheme into the reconstruction of a video sequence. Specifically, the paper will concentrate on systems utilizing motion compensation (MC), differential pulse code modulation (DPCM), and the discrete cosine transform (DCT). Such systems are exemplified by the CCITT H.261 and MPEG standards. In addition to the artifacts that have already been widely considered, such as `blocking' and `mosquito' effects, new classifications of artifacts will be presented. A concise characterization and demonstration of each artifact will be provided. This will include the specification of the distribution of the artifact within the reconstructed frames, and its correlation with the local spatial/temporal features within the sequence. Also noted will be the specific causes of the artifacts, with relation to the MC/DPCM/DCT components. Since the human visual system is, typically, the final judge of the quality of the reconstructed sequence, it is also important to note the level of severity of the artifacts that make the artifacts visually noticeable.

Signal-to-motion artifact ratio versus frequency for impedance pneumography

Abstract: The authors measured transthoracic impedance between 12.5 and 185 kHz in nine adults. They used a system with two impedance channels, both simultaneously detecting the real part of impedance at two different frequencies. They used only two electrodes in the midaxillary line, connecting both channels in parallel. The amplitude relation between the two channels was measured for different maneuvers and frequencies. Results show for normal breathing an increase of the signal of 20% and a decrease in motion artifacts from 12.5 to 185 kHz. It is concluded that, for the maneuvers studied, it is better to work at higher frequencies than the ones commonly used. Also, the authors suggest a method to further increase the signal-to-motion artifact ratio based on measurement at two frequencies. >

Brood-Size-Dependent Offspring Mortality in Common Goldeneyes Reconsidered: Fact or Artifact?

Abstract: Clutch-sizeand brood-size-dependent offspring mortality has important effects on the evolution of clutch size (reviews in Lessells 1991; Rohwer 1992; Stearns 1992) and on intraspecific brood parasitism (reviews in Andersson 1984; Eadie et al. 1988; Petrie and M0ller 1991; Sayler 1992). In altricial bird species, in which parents feed their young, there is convincing evidence that the young in larger families suffer from increased mortality (reviews in Linden and M0ller 1989; Dijkstra et al. 1990; Lessells 1991). By contrast, in precocial species in which parents usually do not feed their young, brood-size-dependent mortality may be questioned. Indeed, the brood size manipulation experiments have shown no effect of brood size on offspring survival (Rohwer 1985; Lessells 1986; but see Safriel 1975). The common goldeneye (Bucephala clangula) is a precocial species with selffeeding young. Earlier studies of this species have reached different conclusions regarding brood-size-dependent mortality. Based on the observations of unmanipulated and unmarked broods in a Swedish common goldeneye population, Andersson and Eriksson (1982) found that mortality increased with the brood size. In contrast, by analyzing the ringing recovery data from another Swedish common goldeneye population, Dow and Fredga (1984) did not find an association between brood size and mortality. We find it important o reconsider this controversy using better data, especially because brood-size-dependent mortality is a critical component in a model Andersson and Eriksson (1982; see also Andersson 1984) presented to explain host response to conspecific nest parasitism. Moreover, Andersson and Eriksson's (1982) result has frequently been considered when evaluating costs of brood parasitism in general (Andersson 1984; Eadie et al. 1988; Godfray et al. 1991; Petrie and M0ller 1991; Eadie and Fryxell 1992). Our note has two aims. The first is to study whether there is any brood-sizedependent mortality in three Finnish common goldeneye populations using both experimental and observational data. The second aim is to identify possible pitfalls of the earlier brood size/offspring mortality analyses. Unfortunately, as we will show, Andersson and Eriksson's (1982) result may be a methodological artifact.

Methodologic considerations for spiral k-space functional MRI

Abstract: Magnetic resonance imaging has recently been developed as a tool to image brain function based on regional changes in blood oxygenation. In this article, we examine the methodologic considerations for functional brain mapping using the spiral k-space acquisition technique. Gradient design and image reconstruction methods as well as considerations for the selection of imaging parameters are presented. Several sources of image artifacts, along with artifact compensation strategies, are examined. Functional maps of activation in primary motor cortex are usd to demonstrate the effect of image parameter selection and of artifact compensation procedures. These results demonstrate that the spiral k-space acquisition technique is a rapid, flexible, and robust method for imaging brain activity.

Non-iterative post-processing technique for transform coded image sequence

Abstract: In this paper, we present a new post-processing algorithm, based on the theory of projections onto convex sets (POCS), to enhance the quality of decoded images degraded by the blocking artifact. In our approach, by introducing the notion of the global slope, block slope and local slope, a novel linear projection operator is induced. The operator makes the block slopes of two adjacent blocks be equal to the global slope, to alleviate the blocking artifact, while maintaining the local slopes inside the blocks, to preserve most original high frequency components. Computer simulation results on still and moving images show that in the viewpoint of the objective and the subjective quality, the proposed algorithm makes decoded images converge without iteration to the post-processed images, in which the blocking artifact is alleviated effectively, while the conventional algorithms based on the POCS converge with a number of iterations.

Reducing artifact in bioelectric signal monitoring

Abstract: Systems and methods of conditioning skin to reduce artifact in bioelectric signal monitoring by passing electrical energy into the skin through the medical electrodes. An electrode having electrolyte including an agent to reduce skin discomfort.

Methods and apparatus for reducing image artifacts

Abstract: The present invention, in one form, is a method for improving image quality in CT helical scan systems by using a post reconstruction filtering algorithm. In accordance with one embodiment of the algorithm, image data is segmented into sharp structure data and background data. The background data is then filtered to remove reconstruction and data artifacts. The filtered background data is then combined with the sharp structure data. The combined data can then be used to generate an artifact reduced image.

MRI center point artifact elimination using realtime receiver phase control

Abstract: A method and apparatus for eliminating baseline error artifacts in NMR images produced using ultrafast pulse sequences including alternating the phase of a reference signal during the reception of NMR echo signals to invert alternate views in the acquisition. The inverted views are re-inverted prior to image reconstruction such that any dc levels introduced during acquisition are converted to a high spacial frequency which moves the resulting artifacts to the borders of the image reconstructed using a Fourier transformation.

Artifact removal from physiological signals

Abstract: A measured magnetocardiography signal has a relatively small heart signal mixed with a large noise signal. To produce a heart signal having a reduced noise content, the times of occurrence of a time-series of isoelectric intervals of the measured signal is first determined and a time-series isoelectric artifact curve is formed from the measurements made at those times of occurrence. A time-series non-isoelectric artifact curve for other times is determined from this information. The time-series isoelectric and nonisoelectric artifact curve is subtracted from the measured magnetocardiography time-series signal to yield a time-series heart amplitude signal having reduced noise and undistorted form.

Comparison of Artifact from Craniomaxillofacial Internal Fixation Devices: Magnetic Resonance Imaging

Abstract: This study compares artifact from craniomaxillofacial internal fixation devices in magnetic resonance images and examines heating and magnetic deflection effects on these devices. Stainless steel wires, microfixation plates of vitallium and titanium, and minifixation and mandibular reconstruction plates of stainless steel, vitallium, and titanium were evaluated. The plates were mounted on a dry skull and submerged in dilute copper sulfate solution. All images were obtained in duplicate by two independent, nonblinded teams of observers with a 1.5-T Signa magnetic resonance system. Each team ranked artifact size for each material by computerized measurement. The rank order of artifact size for each material within each fixation system group and between fixation groups was identical between the two teams. Bath and plate temperatures were recorded before and after imaging. Artifact production was related to hardware size and composition. Titanium hardware caused the least amount of "black-hole" artifact. Vitallium and stainless steel fixation devices of similar size produced significantly more artifact. No significant heating or magnetic deflection effects were seen with any of the fixation devices.

Statistical Artifacts in the Ratio of Discrete Quantities

Abstract: The ratio is a familiar statistic, but it is often misused. One frequently overlooked problem occurs when ratioing two discrete (digitized) variables. Fine structure appears in the histogram of the ratio that can be very subtle, or can sometimes even dominate the histogram. It disappears when the numerator and/or denominator become continuous. This statistical artifact is not a binning error, nor is it removed by taking more data. It is important to be aware of the artifact in order to avoid misinterpretation of ratio data. We provide examples of the statistical artifact (including one from baseball) and discuss ways to avoid or minimize the problems it can cause.

Suppression of sawtooth artifacts in the display of a progressively scanned television signal

Abstract: Jagged vertical or diagonal transition artifacts in interlaced to line doubled progressive scan converted television signals are detected and areas of the television picture having such artifacts are subjected to vertical averaging such that the resolution of the jagged transitions is reduced, thereby softening, 'greying' or 'fuzzing' the jagged transitions and causing them to appear smooth. Thus, highly contrasted lines appearing in a sawtooth pattern, which are extremely preceptible, are replaced with a flash of what might be characterized as 'fuzziness', which is not perceptible. The eye no longer perceives the jagged artifacts. Moreover, areas of reduced vertical and horizontal resolution, resulting from the vertical averaging, do not create any new artifacts and are not perceived by the eye. Artifacts are detected by comparing pixels in corresponding positions in the scan lines of the line doubled progressively scanned television signal. The detection scheme looks for sawtooth patterns by determining if there are differences among pixels in more than one set of three adjacent lines. When a sawtooth pattern is detected, the uncorrected signal in the sawtooth region of a scan line is faded to a vertically filtered or averaged signal just prior to the onset of the sawtooth artifact region and then faded back to the uncorrected signal just subsequent to the sawtooth artifact region. The sawtooth correction signal, which controls fading to and from a filtered video signal, is expanded in all directions horizontally and vertically somewhat beyond the region of the detected artifact in order to insure that all of the artifact is suppressed.