Showing papers on "Artifact (error) published in 1987"

Spatial presaturation: a method for suppressing flow artifacts and improving depiction of vascular anatomy in MR imaging.

Abstract: In clinical magnetic resonance (MR) imaging, the diagnostic quality of examinations is often degraded by streaklike flow artifacts that obscure anatomic details and reduce contrast. In addition, vascular structures are often not depicted clearly because the desired flow voids are obliterated by spurious intraluminal signals. On the basis of analysis of the physical mechanism of flow artifact formation, the authors developed a new technique for suppressing these artifacts. This applies interleaved, spectrally shaped radio frequency pulses to selectively saturate spins located in regions outside the image volume. In phantom, volunteer, and clinical imaging studies, the technique has proved to be effective by yielding a striking reduction in flow artifacts and markedly improving the reliability with which arterial and venous structures are imaged. The method has few drawbacks: It is applicable to most MR pulse sequences and, in principle, can be implemented on most imagers. It is particularly helpful for high-resolution surface coil studies of the neck, mediastinal imaging, gated cardiac imaging, and for detecting thrombus and other intravascular lesions such as dissections.

Optimizing electrocardiograph electrode placement for cardiac-gated magnetic resonance imaging.

Abstract: Electrocardiographs recorded in a magnetic field for cardiac-gating in magnetic resonance imaging (MRI) are complicated by blood flow-induced potentials. This study examines which lead of the standard 12-lead ECG maximizes the QRS while minimizing flow-induced interference. Twelve-lead ECGs were performed on normal volunteers (n = 9) and patients (n = 13) in and out of the bore of a 1.5 Tesla imaging magnet. The amplitude of the major flow-induced potentials was measured, and the vectors of largest induced potential and the QRS axis were plotted for each subject. ECGs obtained outside and inside the magnet were digitized and subtracted (in magnet ECG--out of magnet ECG = artifact ECG) and the peaks of the resultant curves measured. Superimposed potentials were largest in the early T wave and late S-T segment in leads I, II, V1, and V2, and smallest in III and AVF. A low-amplitude 7-to 10-Hz signal occurred in most leads. In the frontal plane, QRS axes and flow potential vectors were closely clustered. In the transverse plane, QRS axes generally followed leads V5 or V6, whereas the flow potential vectors followed leads V1, V2, or V3. The normal and patient groups did not differ. Although leads III and AVP showed the smallest superimposed potentials, V5, V6, or a left posterior chest lead may maximize QRS and reduce artifact most consistently. A 7- to 10-Hz frequency filter may help eliminate artifacts in some subjects.

Apparatus and method for filtering electrocardiograph signals

Abstract: Apparatus and method for filtering the electrocardiograph (ECG) signal of a patient of unwanted signals, such as contamination signals produced by the use of nuclear magnetic resonance imaging system or muscle artifact signals. In a preferred embodiment, variable filter (2) is used to filter an input ECG signal (1). The filtering characteristics applied to the ECG signal by variable filter (2) vary in response to the logical state of a synchronization signal indicating a patient's biophysical state which is input to variable filter (2) via line (3). In alternative embodiments, a QRS detector (22), a delay circuit (20), and a pulse stretcher (21) may be used to provide a synchronization signal. In another alternative embodiment, a microprocessor (24) with an input ECG signal and with input data is used to control the filtering of the ECG signal in synchronization with a patient's ECG signal.

Gibbs artifact removal in magnetic resonance imaging.

Abstract: Gibbs artifact in magnetic resonance imaging results when band-limited interpolation is used. This is typically done when there are more reconstructed pixels in the phase encoding direction of the image than corresponding phase encoding measurements. Such sampling is effectively an ideal (in a noise sense) low-pass filter which provides a maximal improvement in contrast resolution at the expense of a decrease in spatial resolution. In this paper we demonstrate that an alternate low-pass filter can be used to improve contrast resolution with a loss in spatial resolution and yet not result in Gibbs artifact. We show that the noise performance of this filter can be made to approach that of an ideal filter by properly specifying the number of samples averaged for each phase encoding index.

The management and dynamics of anomalies in scientific work

Abstract: This article examines the dynamics of anomalies as part of scientific work. Several types of anomaly are identified: mistakes, artifacts, fraud, and discovery; and typical trajectories for artifact...

Measurement of artifact provenience with an electronic theodolite

Abstract: An electronic theodolite and electronic distance meter can be used to map absolute provenance (cartesian coordinates) of artifacts as they are excavated. The system and its application at la quina (france) are described. The coordinates are transmitted into a hand held computer, which can also be programmed for other survey functions. Diagram, photographs, references.

Topographic mapping of EEG artifacts.

Abstract: The topographic distribution of the artifacts of gritting the teeth, eye movements and blinking were studied in 32 channel recordings. In records from resting normal controls with all artifacts deleted, little EEG activity is present below 1.2 Hz or above 22 Hz. Gritting the teeth does not produce any apparent artifact in the alpha band or lower frequencies. However, it increases fast activity at 13 Hz and above across the scalp, especially in the temporal region bilaterally. The activity of eye movements was strongly present in low frequency maps in the frontal area and disappeared above 9 Hz. Blinking influences all four frequency bands greatly. Knowledge about the frequency range within the power spectrum and the areas on the map in which artifact appear is important in interpreting clinical EEG maps.

An evaluation of different techniques for removing eye-blink artifact from visual evoked response recordings.

Abstract: This investigation compared four methods used to control eye-blink artifact in visual evoked response (VER) recordings using the augmenting-reducing paradigm: rejecting trials containing blinks, presenting the stimuli through closed eyelids, and two different techniques for subtracting a proportion of the averaged electro-oculogram (FOG) from the averaged VER. The two subtraction procedures differed in the way in which they estimated the amount of EOG to be subtracted and in the case with which they could be applied. Twenty normal, female subjects were exposed to a series of four different intensity light flashes twice with eyes open and once with eyes closed. Blinking was monitored by recording the EOG and EEG was recorded from three midline electrodes. The results indicated that the two subtraction techniques were very similar. They effectively removed ocular artifact and produced corrected VERs that showed high retest stability. Presenting the flashes through closed lids yielded idiosyncratic data, and rejecting trials contaminated by blinks was found to have limited utility with the VER paradigm used in the present study.

Image processing method and apparatus

Abstract: A filtering process is carried out on an original image, which is constituted of a lattice-like array of picture elements, that are located at predetermined intervals along the rows and columns of the lattice-like array, by using a mask having a predetermined size and with respect to each of the picture elements of the original image. The mean value of the values of picture elements falling within the mask is calculated with the filtering process, and a smoothed image represented by the thus calculated mean values is thereby obtained. The filtering process is carried out repeatedly on the smoothed image. Processing for emphasizing high frequency components of the original image is then carried out in accordance with the smoothed image, which has thereby been obtained. A processed image, in which an artifact is not perceptible, is thereby obtained.

A stimulated echo artifact from slice interference in magnetic resonance imaging

Abstract: When multiple slices are imaged with a short time between slice acquisitions, a disturbing line artifact along the direction of frequency encoding is often seen across the center of the images. The artifact consists of alternating bright and dark pixel intensities. In this paper, we show that the artifact is due to slice interference, and is caused by stimulated echoes that are produced in the regions of overlap between slices. A theoretical analysis of the formation of these stimulated echoes leads to ways of reducing the artifact, which are verified experimentally. The artifact can be suppressed most conveniently by extending the duration of the read gradient beyond the sampling window.

Slice-shape artifact changes with precession angle in rapid MR imaging.

Abstract: One possible method of measuring TI in rapidly acquired magnetic resonance images using variants of the FLASH technique is by the manipulation of the magnetization precession angle. It is suggested here that this method is as prone to slice-shape artifacts as many other approaches. © 1987 Academic Press, Inc.

Analysis of inhalation rCBF data.

Abstract: Relative to other approaches that have been recommended, fitting all head data, solving for a time shift, and including an air passage artifact term in the model significantly improved the estimate of gray matter blood flow by the inhalation technique. A robust algorithm, which incorporates these features, has been developed. Formulas which facilitate implementation of this algorithm are reported. An artifact from large scalp arteries was not significant and does not need to be included in the model.

Rotational artifact in phase imaging of cardiac scans: potential pitfalls in diagnosis.

Abstract: In the past few years, we have occasionally observed linear bands in the phase images of gated cardiac blood-pool scans along the interventricular septum region among patients with normal septal motions. Our retrospective study investigated the cause of out-of-phase bands of 12 patients. We documented through review of cines, computer analysis of gated cardiac blood-pool scintigraphy data, and correlation with echocardiograms that this phenomenon was artifact introduced by rotational movements of the heart. It is important for nuclear physicians to recognize this rotational artifact on the phase analysis image in order to avoid the erroneous misdiagnosis of wall motion abnormalities of the septum.

Postoperative magnetic resonance imaging artifacts: Report of three cases

Abstract: Artifacts are occasionally encountered on magnetic resonance imaging after operation. These may be due to minute metallic particles from neurosurgical instruments. Particles not detectable on plain x-ray films or computerized tomography scans may cause local change of magnetic resonance activity, resulting in a deceptive magnetic resonance appearance. Three illustrative case reports are presented.

Controlled eddy currents: applications to MR imaging.

Abstract: We describe how the artifact caused by eddy currents generated in free standing copper coils may be controlled and used to advantage in magnetic resonance (MR) imaging. The eddy currents distort the excitation field of the MR imager in the vicinity of the coil; the coils may then be used to reduce the signal intensity of tissues adjacent to them. Two examples of how this signal reduction may be used to advantage are given. In one example a coil was used to eliminate an aliasing artifact by removing the signal from an unwanted object. In another example a coil was placed on the anterior abdominal wall of a subject, thereby reducing the high-intensity signal from subcutaneous fat and the resulting ghosting from respiratory motion.

Nuclear magnetic resonance imaging method and apparatus for realizing same

Abstract: Before beginning a prior art imaging sequence, at first parts producing artifact are excited by applying a high frequency magnetic field thereto in a frequency band, which includes the parts emitting artifact and doesn't include a desired slice plane. Next, the phase of the excited spin is extremely rotated by applying at least one of gradient magnetic fields G x , G y , G z . When the prior art imaging sequence is effected thereafter, the produced artifact is imaged as a high spatial frequency component of an object body to be examined. Since a usual object body to be examined contains many low spatial frequency components and the power of high spatial frequency components is extremely small, it becomes possible to reduce the artifact in such an extent that it cannot be practically recognized.

Lidblink artifacts in recordings of horizontal saccadic eye movements with the infrared reflection method: Analysis and presentation of a new monitoring technique

Abstract: In order to test the accuracy of lidblink artifact monitoring with EOG during recordings of horizontal saccadic eye movements by means of Infrared Reflection Oculography (IROG), the influences of EOG-recorded blinks of different size on amplitude, peak velocity, and duration of horizontal saccades were studied in ten healthy volunteers. During the study, a simpler blink monitoring technique was developed using IROG itself. in six subjects the conventional ‘retinoelectric’ (EOG) and the new ‘photoelectric’ (IROG) blink artifact monitoring methods were simultaneously employed. A comparison of the two methods showed that photoelectric blink monitoring not only is much simpler to install, but also yields less false positive, as well as less false negative results than retinoelectric monitoring. in addition, photoelectric monitoring gives a continuous control of the placement of the photo cells with respect to the horizontal plane, which is important to avoid asymmetric blink artifacts in the IROG recordings d...

Unusual MR metallic artifact due to steel threads.

Abstract: Metallic objects are a well known source of artifact in magnetic resonance imaging. We report a case of artifacts created by metallic fibers woven into surgical scrub pants worn by a patient during the examination.