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At least 775 records · Page 43Linked to original sources
PubMed1998

[A trial to reduce cardiac motion artifact on HR-CT images of the lung with the use of subsecond scan and special cine reconstruction algorithm].

We describe our trial to reduce cardiac motion artifacts on HR-CT images caused by cardiac pulsation by combining use of subsecond CT (scan time 0.8 s) and a special cine reconstruction algorithm (cine reconstruction algorithm with 180-degree helical interpolation). Eleven to 51 HR-CT images were reconstructed with the special cine reconstruction algorithm at the pitch of 0.1 (0.08 s) from the data obtained by two to six contagious rotation scans at the same level. Images with the fewest cardiac motion artifacts were selected for evaluation. These images were compared with those reconstructed with a conventional cine reconstruction algorithm and step-by-step scan. In spite of its increased radiation exposure, technical complexity and slight degradation of spatial resolution, our method was useful in reducing cardiac motion artifacts on HR-CT images in regions adjacent to the heart.

Algorithms↗
PubMed1997

Magnetic resonance imaging and the reduction of motion artifacts: review of the principles.

Magnetic resonance (MR) imaging is a non-invasive diagnostic tool which is widely used nowadays. In this paper, the basic principles of MR imaging are explained and it is shown how images can be reconstructed in case of standard 2D Fourier Transform (2DFT) imaging. Several aspects of MR signal encoding are described. Unfortunately, motion of the patient during a magnetic resonance experiment often causes severe artifacts in the images. For example, in 2DFT imaging blurring and ghosting are seen and the appearance of motion artifacts remains one of the major drawbacks in MR imaging. Several methods to reduce motion artifacts in MR imaging have been proposed in the past. An overview of the principles on which these methods are based is given in this paper. Both post-processing methods and techniques that rely on gating or the use of alternative acquisition schemes such as projection reconstruction are discussed.

Artifacts↗
PubMed1998

[Artifacts in magnetic resonance angiography].

Magnetic resonance angiography (MRA) has become a major tool in the management of vascular diseases. However, many artifacts that may lead to misinterpretations are encountered with this imaging modality. The purpose of this paper was to enumerate, explain and illustrate each kind of artifact encountered with "time of flight" or "phase contrast" imaging methods. For each artifact, we tried to provide one or several techniques to minimize its consequences.

Artifacts↗
PubMed1998

Simulated frame-loss artifacts in myocardial perfusion SPECT imaging: the difference between single- and dual-headed systems.

OBJECTIVE: Right-angle dual-headed tomography has increased cardiac SPECT utility by cutting acquisition time in half which enhances gating capabilities. When gating, however, a deceleration in heart rate, due to a return to baseline rate after stress or lessened anxiety at the end of a study, may significantly affect the last step(s) of a gated study with possible frame or information loss. The purpose of this study was to illustrate the artifacts produced in myocardial perfusion studies when a frame or frames are lost in single- and dual-detector SPECT imaging methodologies. METHODS: A near-normal 99mTc-sestamibi study was obtained using a dual-headed camera system fitted with high-resolution, long-bore collimators. The normal study was processed including all frames (1-32). To demonstrate the effect of losing frames on a dual-headed system, the study was processed three different ways to simulate frame loss. RESULTS: Loss of frames at the end of a SPECT acquisition results in significant inferoseptal wall defects, left ventricular lumen narrowing, as well as thinning of the anterior and lateral walls. The overall appearance of the heart is a more oval-shaped heart with decreased perfusion. The effect of losing the last frame in a dual-headed camera system as opposed to losing the last frame in a single-headed camera system is more substantial. CONCLUSION: A scan resulting in the loss of a frame in either a single- or a dual-headed camera system creates artifacts in the myocardial wall and should always be repeated. It is significant to note that artifacts present in a dual-headed system are much more prevalent than in a single-headed system due to the nature of dual-headed acquisition parameters.

Artifacts↗
PubMed1975

Common artifacts in ultrasound scanning.

Numerous artifacts can occur during contact B scanning of the abdomen. These artifacts may give anatomically inaccurate scans and thus lead the interpreter into making an inaccurate diagnosis. This paper describes and illustrates artifacts resulting from improper scanning techniques, improperly set gain levels, motion related organ distortion, and acoustic reverberations.

Abdomen↗
PubMed1986

Boundary artifact in inversion-recovery images.

Modulus reconstruction of inversion-recovery images causes a pixel value to be assigned the absolute magnitude of the signal amplitude. Consequently, contrast between tissues is reduced if they have amplitudes of opposite sign. When contrast is reduced between adjacent tissues, dark line artifacts are created at the tissue boundaries. The artifacts may enable tissues to be spatially distinguished even if there is no contrast between them. However, the artifacts may lead to misinterpretation of the image since they may be difficult to distinguish from real anatomical structures.

Data Display↗
PubMed1987

Pseudo-gating: elimination of periodic motion artifacts in magnetic resonance imaging without gating.

This note explains and illustrates a technique of reducing artifacts produced by periodic motion without using physiologic gating. The method is simple and can be applied on any standard MRI unit. For periodic motion, effective gating can be attained by setting the product of the number of acquisitions (at each phase-encoding step), N, times the repeat time, TR, equal to the period of the motion, T. This pseudo-gating can be used with any TR but, if changes in the period occur, is most robust with short TR values. The method is also applicable to multislice and volume imaging. For fast field echo methods, the above rule can be used if the period of the motion is smaller than the total scan time. Otherwise, the scan need only be repeated N times to avoid artifacts and improve signal-to-noise. The method has been implemented to remove both respiratory and/or cardiac motion artifacts.

Heart↗
PubMed1988

Acquisition order and motional artifact reduction in spin warp images.

Multiple averaging can be a powerful tool against motional artifacts if significant motion occurs between the redundant acquisitions taken at a given gradient strength. However, if the time delay between these redundant measurements is too short, data or images depicting the patient is exactly the same position will be combined. Pooling such identical data has no effect on motional artifacts. This problem can be solved by increasing TR, increasing the number of redundant acquisitions, or changing the order in which acquisitions are taken. Usually all acquisitions at a particular value of the warp gradient are taken before proceeding to the next gradient value. This order minimizes motion between redundant acquisitions and so maximizes artifacts. The effect of other acquisition orders on both periodic and nonrepetitive motion is discussed. Human images for breathing and phantom results for single-occurrence motions are presented.

Humans↗
PubMed1990

Flow artifact reduction in MRI: a review of the roles of gradient moment nulling and spatial presaturation.

In the past, flow artifacts and inconsistent depiction of vascular anatomy have represented significant problems in clinical MRI. These difficulties are now generally well addressed by the techniques of gradient moment nulling and spatial presaturation. Gradient moment nulling (GMN) is an effective method for eliminating flow artifacts in gradient echo images, while presaturation is more applicable to the same task in spin echo acquisitions. The GMN technique also has useful applications in spin echo imaging such as combating the effects of tissue and CSF motion in long TE sequences. In contrast to presaturation, however, GMN is not suitable for suppressing artifacts due to pulsatile blood flow in spin echo images.

Blood Circulation↗
PubMed1991

Reduction of flow artifacts in NMR diffusion imaging using view-angle tilted line-integral projection reconstruction.

Most of the diffusion imaging techniques employ strong diffusion gradient pulses of long duration in order to achieve appreciable signal attenuation through the diffusion effect. However, these strong and long gradient pulses make the resultant images extremely sensitive to the motion or flow of the object. Fourier imaging, with which most of the current NMR imaging is performed, is especially sensitive to the fluctuating flow and the images are usually obscured by severe flow artifacts smeared in the phase-encoding direction. In this paper, we have proposed a diffusion imaging technique which reduces the flow artifacts by use of the line-integral projection reconstruction (LPR) imaging method. Furthermore, the inhomogeneity artifacts expected to occur in LPR imaging have been corrected by application of the view-angle tilting technique. The pulse sequence of the view-angle tilted LPR diffusion imaging is designed in such a way that it works for both isotropic and anisotropic diffusion. Experimental results are presented along with the experimental procedures.

Brain↗
PubMed1997

Reducing flow artifacts in echo-planar imaging.

Echo-planar imaging (EPI) is very susceptible to flow artifacts. Two ways to improve its flow properties are presented. First, "partial flyback" is proposed to reduce artifacts arising from flow in the readout direction. Near the center of k-space, only the even echoes of the EPI echo-train are used. Partial flyback is shown to improve the readout-flow properties at the expense of a slight worsening of the phase-encode flow and off-resonance properties. We recommend that the flyback region acquire 95% of the energy in k-space. Second, "inside-out" EPI is used to reduce artifacts arising from flow in the phase-encode direction. Data collection begins at the center of k-space, with separate interleaves to acquire the top and bottom, halves of k-space. Partial flyback is combined with partial-Fourier EPI and inside-out EPI. Partial-flyback inside-out EPI has worse off-resonance properties than partial-flyback partial-Fourier EPI but demonstrates better flow properties and does not require partial k-space reconstruction.

Heart↗
PubMed1988

Stimulus artifact compensation using biphasic stimulation.

The feasibility of using biphasic stimulation as a method of reducing stimulus artifact was explored in several different circumstances. Sensory and muscle evoked potentials were compared using monophasic and biphasic constant-current stimuli. The monophasic stimulus was a negative rectangular pulse with a duration of 0.1-0.2 msec. The biphasic stimulus was a negative rectangular pulse of 0.1-0.2 msec duration followed, after a variable delay, by a positive rectangular pulse of the same duration and variable amplitude. When the conducting distances were long, stimulus artifact did not disturb either monophasically or biphasically evoked potentials. When the conducting distances were short, stimulus artifact occurred with monophasic stimulation but could be diminished readily using biphasic stimulation. The method can be accomplished easily using standard instruments with dual stimulators.

Electric Stimulation↗
PubMed1982

Cobalt mapping of the nervous system: how to avoid artifacts.

Infusion of cobalt ions into cut axons is an established method for tracing neuron projections in the central nervous system. Artifacts, where unintended neurons are stained, however, have been reported, leading to difficulties in interpretation. Experiments in the locust Schistocerca gregaria Forskål show that such artifacts can be induced through damage to axons caused by cutting peripheral nerves and by using high cobalt chloride concentrations (0.4M and above). Mixtures of cobalt and nickel chlorides and nickel chloride alone were introduced into different branches of the same nerve and developed with rubeanic acid to give precipitates of different colors in the two sets of axons. Preparations were examined with the light microscope, where mixing of ions would appear as intermediate colors, and by x-ray probe microanalysis. No evidence for leakage of metal ions from the filled axons or for ion uptake by other axons could be detected, provided that low concentrations of cobalt and nickel chlorides were used and nerve cutting was reduced to a minimum by making preparations in vivo. If extreme conditions are avoided when making the preparation, the risk of producing artifacts is minimized, thus enabling the cobalt method to be used with greater confidence for describing neuronal projections.

Animals↗
PubMed1998

Rapid-Pulsing Artifact-Free Double-Quantum-Filtered Homonuclear Spectroscopy.

Rapid pulsing artifacts are observed in the conventional phase-cycled carbon-13 2D INADEQUATE experiment. By using the product operator formalism, it is shown that they result from the effects of imperfect 90 degrees and 180 degrees excitation pulses on the most abundant molecules containing only one isolated carbon-13 nucleus. The labeled longitudinal magnetization remaining at the end of one scan is recycled by the subsequent acquisition, giving rise to multiple-quantum (p = 0, +/-1, +/-2, ellipsis) artifacts in the F1 dimension. By considering pairs of scans instead of single scans, a new phase cycle is proposed. It is based on a scheme for compensating for imperfections in the excitation cluster by a proper combination of the pulse phases in two consecutive scans. Because the artifacts are 90 degrees out of phase compared to the desired signal, a concomitant rearrangement of the receiver phase achieves suppression of all unwanted signals. Experiments are presented on menthol dissolved in CDCl3 as a test compound. Improvements in spectrum quality as well as increased sensitivity are discussed. Copyright 1998 Academic Press.

Journal Article↗
PubMed1987

Artifacts in camera based single photon emission tomography due to time activity variation.

Image quality in single photon emission computed tomography (SPECT) using a rotating gamma camera is dependent on the time course of the tracer in the field of view. If acquisition times are slow compared to the tracer turnover, artifacts may occur in the reconstructed images. The properties of such artifacts were studied by computer simulation. Experimental projection data of point sources, cylindrical phantoms, and an anatomically realistic brain phantom were altered by sequentially weighting the projections with a function that varied exponentially or linearly with time. The observed distortion in the reconstructed images could be related to the ratio between the object activity variation and the camera rotation time. If the tracer concentration changed less than a factor of two during one camera rotation then little image distortion was visible although quantitatively the resolution was degraded. If the object's activity variation with time is fast enough to produce noticeable distortion, the artifacts can be reduced by performing multiple, rapid, camera rotations, instead of one rotation, for the same total acquisition time. The proposed procedure is computer storage space intensive and takes longer to produce the transaxial images, but improves image quality.

Image Enhancement↗
PubMed1993

Ocular artifacts in EEG and event-related potentials. I: Scalp topography.

The ocular artifacts that contaminate the EEG derive from the potential difference between the cornea and the fundus of the eye. This corneofundal or corneoretinal potential can be considered as an equivalent dipole with its positive pole directed toward the cornea. The cornea shows a steady DC potential of approximately +13 mV relative to the forehead. Blink potentials are caused by the eyelids sliding down over the positively charged cornea. The artifacts from eye-movements result from changes in orientation of the corneo-fundal potential. The scalp-distribution of the ocular artifacts can be described in terms of propagation factors--the fraction of the EOG signal at periocular electrodes that is recorded at a particular scalp location. These factors vary with the location of the scalp electrode. Propagation factors for blinks and upward eye-movements are significantly different.

Adolescent↗
PubMed1986

Common artifacts of pulmonary artery and pulmonary artery wedge pressures: recognition and interpretation.

Bedside measurement of pulmonary artery pressure and pulmonary artery wedge pressure has an important role in the management of critically ill patients. Unfortunately, waveform abnormalities and artifacts commonly distort numeric values and lead to incorrect therapeutic decisions. The clinical impact of these artifacts is magnified by the digital pressure displays used in most intensive care units. We present here an atlas and an analysis of the artifacts that commonly occur. Use of analog rather than digital pulmonary artery wedge pressure data, when combined with an understanding of the physiological characteristics of patients, can prevent critical errors in patient management.

Cardiac Catheterization↗
PubMed1985

Artifacts, variants, and factors degrading image quality in pediatric magnetic resonance imaging.

The experience from 123 pediatric MRI examinations disclosed several different types of artifacts and age-related variants that could have caused pitfalls in MRI interpretation. Artifacts were caused by metallic objects, patient touching the RF coil, and patient motion. The potential pitfalls are caused by the MRI variant appearances of anatomical structures specific to the growing body. Recommendations concerning preparation and immobilization of children prior to and during MRI examinations to minimize artifacts are given.

Adolescent↗