Showing papers on "Artifact (error) published in 1991"

Removal of the ocular artifact from the EEG: a comparison of time and frequency domain methods with simulated and real data

Abstract: Constructed 2 data sets from simulated data, in which the transfer from electrooculogram (EOG) to EEG was either frequency-independent (constant gain) or frequency-dependent. Three different correction methods were applied: simple regression analysis (RA) in the time domain, a multiple-lag RA in the time domain, and an RA in the frequency domain. For Data Set 1, the 3 methods constructed the original EEG equally well. With Data Set 2, reconstruction of the original EEG was achieved reasonably well with the frequency domain method and the time domain multiple-lag method, but not with simple time domain regression. These 3 correction procedures were also applied to real data, consisting of concomitantly recorded EEG and high-variance EOG series. There were no differences in outcome of the 3 methods.

ECG muscle artifact filter system

Abstract: A filter system for removing small amplitude, high frequency signals such as muscle artifact signals from an ECG signal is provided. The filter system includes a low pass filter with variable cutoff frequencies, an electronic delay and a system for detecting the R wave in the ECG signal and determining the variable cutoff frequency in response to the detection of the R wave. A digitized input ECG signal is simultaneously presented to the electronic delay and the system for detecting the R wave. During the portion of the ECG signal exclusive of the QRS complex, the filter is operated at a low cutoff frequency to filter the muscle artifact signals for a maximum smoothing effect. At a time slightly prior to the on set of the QRS complex, the cutoff frequency is rapidly incrementally increased to a higher cutoff frequency to pass the QRS complex with a minimum reduction of amplitude of the QRS signal. At the end of the QRS signal, the cutoff frequency of the filter is rapidly incrementally returned to the low cutoff frequency.

Three-dimensional time-of-flight MR angiography: applications in the abdomen and thorax.

Abstract: The application of three-dimensional (3D) time-of-flight (TOF) magnetic resonance (MR) angiographic techniques to the vasculature of the abdomen and thorax has not, to the authors' knowledge, been previously reported; this is possibly due to the large amount of physiologic motion in these regions along with the anticipated sensitivity of the 3D acquisition scheme to image degradation caused by motion artifact. The authors describe an asymmetric short-echo velocity-compensated 3D TOF technique that minimized the effects of physiologic motion on image quality and provided rapid high-resolution 3D MR arteriograms of the abdomen and thorax. Contiguous 3D volumes were often combined to provide sufficient anatomic coverage. Benefits include high spatial resolution and minimization of signal loss; limitations include sensitivity to motion artifact and progressive spin saturation.

Motion artifact correction in MRI using generalized projections

Abstract: An algorithm that suppresses translational motion artifacts in magnetic resonance imaging (MRI) by using post processing on a standard spin-warp image is presented. It is shown that translational motion causes an additional phase factor in the detected signal and that this phase error can be removed using an iterative algorithm of generalized projections. The method has been tested using computer simulations and it successfully removed most of the artifact. The algorithm converges even in the presence of severe noise. >

Data extrapolation for truncation artifact removal.

Abstract: Clinicians typically obtain high-resolution clinical MR images in an effort to avoid the truncation artifacts that often arise in Fourier transform reconstruction of limited data. A method for reducing these artifacts in MR images, at the reconstruction stage, would allow for reduced imaging times, through the collection of fewer phase encode steps and increased signal-to-noise ratios, through increased pixel size. The approach to reducing truncation artifacts in MR images is developed and a simple algorithm is presented which significantly reduces truncation artifacts in images with as few as 96 phase encode steps. The algorithm is compared with a more sophisticated method of reconstructing truncation-free images and is shown to be equivalently effective. Three clinical examples are shown illustrating the success of the method.

Post-processing technique for reducing metallic clip artifacts in CT images

Abstract: Radiation from an x-ray source (14) is collected by radiation detectors (16) to generate an original set of CT line integral data (18). The original CT data is subject to a first order correction and reconstructed (20) to generate a first order corrected image representation (22). A first artifact image representation (26) is generated (24) from the first order corrected image. The artifact image representation and the first order corrected image representation are subtractively combined (28) to generate a second order corrected image representation (30). A bone and high density clip image representation (142) is generated from the second order corrected image. The bone and clip image representation is forward projected (144) to generate a first high contrast line integral data set (146). High contrast model line integral data corresponding to rays which traverse the high density clip are topologically transposed (148) for corresponding rays of the original CT data. The topologically transposed CT data is reconstructed (160) to generate a third order corrected image representation (34). A second artifact image representation (38) is derived (36) from the third order image representation. The second artifact image representation and the third order corrected image representation are subtractively combined (40) to produce a fourth order corrected image representation (42) which is displayed on a video monitor (44) or the like.

Cranial CT artifacts and gantry angulation.

Abstract: Computed tomographic investigation of the brain is frequently accomplished by scanning parallel to the canthomeatal line. In these scans, evaluation of the brain stem and neighboring structures is often compromised by streak artifacts from bone-induced beam hardening. These artifacts may be reduced by introducing a change in gantry angulation. Results from a phantom study as well as from a clinical trial offer evidence that may be helpful in assessing different scanning planes. An inclination of 5 degrees below Reid baseline is suggested to minimize the interpetrous artifact. Variations in slice thickness and exposure parameters do not appear to affect significantly this typical posterior fossa artifact.

An improved algorithm for 2-D translational motion artifact correction

Abstract: The quality of magnetic resonance imaging systems has improved to the point that motion is a major limitation in many examinations. Translational motion in the imaging plane causes the phase of the data to be corrupted. An algorithm using computer post-processing is proposed to correct the phase of the data, and hence remove the artifact. This algorithm has superior convergence properties to an earlier algorithm, which is achieved by incorporating additional prior information specific to the situation. The algorithm is verified using a Shepp and Logan phantom with simulated motion in the imaging plane. It is shown that the algorithm can correct both periodic and random motion, and that the algorithm is not significantly degraded when noise is present. >

Segmentation of brain CT images using the concept of region growing.

Abstract: A method is described for extracting and isolating cerebrospinal fluid and tissue areas of brain images obtained with computed tomography. The classification of the pixels into components is based on region growing and nearest neighbor principles. To aid the performance of this method, the algorithm utilizes a priori information on the anatomic composition of the brain, and reduces the ‘cupping effect’ in the CT image that is attributed to beam hardening artifacts. In order to avoid subjectivity, the performance of the algorithm was tested by superimposing five computer-simulated circular lesions on different areas of the original CT scans, 8 mm thick. These images were taken at different levels in the brain, thereby accommodating different anatomy as well as the apical artifact of CT scanning. In this exploratory investigation, the false negative error of segmentation for lesions having diameter of 20 pixels was found in the order of 25% at an estimated partial volume (PV) effect of 50% that decrease further to about 5% for a PV of 80%. At that point the false positive error becomes the dominant error in the analysis.

Technique for reduction of truncation artifact in chemical shift images

Abstract: The authors describe a technique to reduce truncation artifacts in chemical shift images taken for in vivo magnetic resonance spectroscopy by using boundary information available from anatomic images. In this approach, high spatial frequency components are extrapolated from the observed data by making use of the boundary information. This technique has been tested with simulations and phantom experiments. Results obtained show that the technique works very well. The authors present technical details and results of computer simulations and phantom studies. >

Motion artifact simulating aortic dissection on CT.

Abstract: We recently imaged two patients clinically suspected of having aortic dissection whose contrast-enhanced CT examinations, obtained on a new scanner with a 1-sec scanning time, showed findings suggesting an ascending aortic dissection. The subsequent clinical course and evaluation implied that the CT findings were predominantly artifactual. We identified identical artifacts in 18% of 50 consecutive contrast-enhanced CT examinations performed for a variety of indications on the same scanner. The double-lumen artifact, simulating an intimal flap, occurs in the proximal ascending aorta and is limited to one or two contiguous transaxial images. The artifact was not detected on two other CT units. We believe the artifact arises from motion of the aortic wall and the surrounding pericardial recesses during image acquisition.

Artifact and method for verifying accuracy of a positioning apparatus

Abstract: A gage master artifact and method for verifying calibration of machine tools and other positioning apparatus such as a coordinate measuring machine wherein measurements are taken of bore sizes and lengths of the artifact in different orientations within the coordinate field of the apparatus by a probe of that apparatus and comparing such measurements with known measurements of the artifact to verify calibration accuracy of the apparatus.

A modified Gerchberg-Saxton algorithm for one-dimensional motion artifact correction in MRI

Abstract: A novel method for the suppression of one-dimensional translational motion artifacts in two-dimensional Fourier transform magnetic resonance images is presented. It is shown that the motion causes an additional phase factor in the detected signal and that this phase error can be removed using a modified Gerchberg-Saxton (1972) algorithm. The major differences between this algorithm and other phase retrieval algorithms are: (1) the phase information is not totally unavailable, but in a corrupted form; and (2) the algorithm does not try to recover the entire phase information from the magnitude of the detected signal, but rather to correct the distorted phase using the average phase error. The method has been successfully tested using computer simulations. >

Method and apparatus for deriving a respiration signal and/or artifact signal from a physiological signal

Abstract: A method for deriving a respiration signal and/or a cardiac artifact signal from a physiological signal, in particular an impedance pneumography signal, includes detection of a heartbeat moment, storing the amplitude of the physiological signal at the heartbeat moment, and calculating and storing a learning signal, the learning signal being a function of a previous learning signal and the difference between the instantaneous amplitude of the physiological signal and the stored amplitude thereof. The learning signal is either read out starting each time with the particular heartbeat moment, for deriving the cardiac artifact signal, or subtracted from the physiological signal at each heartbeat moment for deriving the respiration signal.

Magnetic resonance artifact in the postoperative cervical spine. A potential pitfall.

Abstract: An magnetic resonance imaging artifact that stimulates hypertrophic bone formation is described in patients who have had an anterior cervical discectomy. The magnetic resonance images of 26 patients with anterior cervical discectomy were retrospectively reviewed. Comparison was made to the available concurrent computed tomographic scans, computed tomographic myelograms, and operative reports. A bovine spine was drilled with a drill only at one level and with a metal suction tip in close proximity to the drill at another level, and magnetic resonance images were obtained. Artifact was present in 12 patients and absent in 14; this was confirmed in the 8 patients with comparison studies. Close correlation was seen with the prospective reading of the presence of artifact and operative drill use in the seven patients with available operative reports. The bovine spine model showed no artifact at the drill-only level and significant artifact at the level where the metal suction tip was positioned next to the drill. Small metal flecks were seen grossly at the second level, but not on plain roentgenograms. The metallic magnetic resonance artifact seen in postoperative cervical spines is probably from small bits of metal from the metal suction tip as it occasionally hits the drill. Bone abnormalities seen on magnetic resonance imaging at the level of a previous anterior cervical discectomy may need a cervical computed tomogram to confirm the findings.

Experiments for two MR imaging theories of motion phase sensitivity.

Abstract: Two theories of motion-sensitive phase shifts in magnetic resonance (MR) imaging result in different mathematical predictions of the observed effects of gradient modulation-induced motion artifacts The consequences are critical for gradient waveform designed to minimize motion artifact contaminations from time-dependent motion sensitivity To resolve this discrepancy with a test case (the monopolar waveform of a commonly used, discretely pulsed encoding phase gradient), computer integration of the fundamental Bloch equations for MR imaging with motion was performed Simulation images for constant and erratic motion showed almost complete agreement with the predictions of the transport integral solutions for motion phase sensitivity; the artifact was solely time-of-flight oblique flow misregistration Conventional method-of-moments gradient moment nulling compensations produced greater motion artifacts in experiments than did use of no waveform compensation at all Transport equation solutions implied sec

Self-Consistency and Transitivity in Self-Calibration Procedures

Abstract: Self-calibration refers to the use of an uncalibrated measuring instrument and an uncalibrated object called an artifact, such as a rigid marked plate, to simultaneously measure the artifact and calibrate the instrument. Typically, the artifact is measured in more than one position, and the required information is derived from comparisons of the various measurements. The problems of self-calibration are surprisingly subtle. This paper develops concepts and vocabulary for dealing with such problems in one and two dimensions and uses simple (non-optimal) measurement procedures to reveal the underlying principles. The approach in two dimensions is mathematically constructive: procedures are described for measuring an uncalibrated artifact in several stages, involving progressive transformations of the instrument''s uncalibrated coordinate system, until correct coordinates for the artifact are obtained and calibration of the instrument is achieved. Self-consistency and transitivity, as defined within, emerge as key concepts. It is shown that self-consistency and transitivity are necessary conditions for self-calibration. Consequently, in general, it is impossible to calibrate a two dimenstional measuring instrument by simply rotating and measureing a calibration plate about a fixed center.

Pacemaker generator pseudomalfunction: an artifact of Holter monitoring.

Abstract: Ambulatory ECG (Holter) monitoring is often useful for assessing patients with cardiac pacemakers. Pacer system malfunction, including overor undersensing, failure to capture, and the rare case of primary generator malfunction can be diagnosed with the assistance of Holter monitoring. '̂̂ However, one must he aware of sources of artifact when such Holters are interpreted.^ Early Holter systems, hecause of limited frequency response, often failed to record the very hrief pacemaker stimuli making analysis of pacing system function more difficult. To circumvent attenuation of Holter-recorded pacer pulses, many modern Holter systems employ a separate channel, snch as descrihed hy Kelen et al.,'' which detects pacemaker stimuli and places a mark on the tape coincident with the sensed stimulus. While such an EGG interpretation is often useful, it represents another possihie source of error in interpretation. We describe a case in which failure to recognize this source of error could have led to an inaccurate diagnosis of generator circuitry malfunction.

Methods Of Arrhythmia And Artifact Removal In Heart Rate Variability Analysis

Abstract: Analysis of heart rate variability with Holter monitoring during the acute phase of myocardial infarction is often difficult due to excessive artifacts and arrhythmias. Detection of noisy epochs of data or arrhythmias and their substitution by interpolation has enabled further analysis on epochs which otherwise would have been discarded. The present paper discusses various strategies for artifact and arrhythmia detection. Although none of the methods solved all varieties of artifacts, a method based on differences from both moving mean and from the last normal value performs well in the majority of cases.

The EINTHOVEN system: toward an improved cardiac arrhythmia monitor.

Abstract: Contemporary cardiac arrhythmia monitors, used commonly in intensive care settings, are highly sensitive to artifact, resulting in high false alarm rates, inability to detect P waves reliably, and crude rhythm interpretation. We report on two new approaches that address these problems: a noise preprocessor that characterizes the type and degree of artifact in an ECG, and a model-based rhythm interpretation algorithm.

An explanation of anomalous artifact intensity in fully 1H–1H decoupled two‐dimensional chemical shift correlation spectroscopy

Abstract: Two-dimensional heteronuclear chemical shift correlation experiments which attempt decoupling in f 1 have been shown to give artifacts in strongly coupled systems. In particular, COLOC and HETRES experiments on methylene groups show an artifact midway between the two proton lines. This paper provides an explanation of the artifact intensity behaviour with respect to the strength of coupling between the methylene protons. Experimental results on 1,2-dibromoethylbenzene are presented, and theoretical calculations were carried out on the COLOC experiment and a mathematical model was developed which predicted the artifact appearance and intensity. Simulations were also performed using the simulation program SIMPLTN to verify the experimental and theoretical results