Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “artifacts”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 685 records · Page 38Linked to original sources
PubMed2000

Attenuation smear: a 'paradoxical' increase in counts due to attenuation artifact.

BACKGROUND: Attenuation is a well recognized cause of reconstruction artifacts in SPECT imaging. Occasionally, we have noted an increase in activity extending from the apical septal portion of the ventricle in women with significant breast attenuation. Although the idea that attenuation can produce an increase in activity on the reconstructed images seems paradoxical at first, it is consistent with the process of filtered back projection. METHODS: We filled a cardiac phantom with 1 mCi of Technetium-99m, placed it in a water filled anthropomorphic torso phantom and imaged it over a 180 degree orbit. Next, a breast phantom designed to simulate a significant degree of breast attenuation was placed on the torso phantom and imaging was repeated. The images were reconstructed first using conventional filtered back projection then with maximum likelihood. RESULTS: When the phantoms with and without breast attenuation were reconstructed using filtered back projection and compared, the phantom with breast attenuation had a large 'smear' of activity extending anteriorly from the apical septal wall which was very similar to the abnormalities previously noted in clinical images; the phantom without breast attenuation had no such defect. This artifact was significantly less prominent when the images were reconstructed using the maximum likelihood technique. CONCLUSIONS: Attenuation artifact can also produce a seemingly paradoxical increase in counts on the reconstructed image but this phenomenon is consistent with the workings of filtered back projection.

Artifacts↗
PubMed2003

Gadolinium susceptibility artifact causing false positive stenosis isolated to the proximal common carotid artery in 3D dynamic contrast medium enhanced MR angiography of the thorax--a brief review of causes and prevention.

Due to the close proximity of arteries and veins in the superior mediastinum and upper extremities, T2* shortening effects of the gadolinium within the central veins may cause artifactual vascular stenosis (susceptibility artifact) on arterial-phase MR angiographic images of the major branches of the aortic arch. We report a case of artifactual stenosis isolated to the origin of the left common carotid artery on arterial-phase MR angiography, secondary to susceptibility artifact from non-diluted gadolinium in the adjacent brachiocephalic vein. The cause of the artifact, its identification and prevention is reviewed.

Adult↗
PubMed2006

Effects of artifact rejection and bayesian weighting on the auditory brainstem response during quiet and active behavioral conditions.

PURPOSE: To evaluate the effects of 2 noise reduction techniques on the auditory brainstem response (ABR). METHOD: ABRs of 20 normal hearing adults were recorded during quiet and active behavioral conditions using 2 stimulus intensity levels. Wave V amplitudes and residual noise root-mean-square values were measured following the offline application of artifact rejection and Bayesian weighting. Repeated measures analysis of variance and Bonferroni adjusted pairwise t tests were utilized to evaluate significant main effects and interactions between the 2 noise reduction techniques. RESULTS: ABRs recorded during the quiet behavioral condition resulted in minimal differences in wave V amplitude and noise reduction improvement, suggesting that the 2 techniques were equally effective under ideal recording situations. During the active behavioral condition, however, the techniques differed significantly in the ability to preserve the evoked potential and reduce noise. Consequently, strict artifact rejection levels resulted in an inherent underestimation of wave V amplitudes when compared with the Bayesian approach. CONCLUSION: Artifact rejection had a detrimental effect on waveform morphology of the ABR. This could lead to difficulty in ABR interpretation when patients are active and ultimately result in diagnostic errors.

Adult↗
PubMed2000

Artifacts and the visualization of fetal distal extremities using three-dimensional ultrasound.

OBJECTIVES: To demonstrate that acoustic shadowing in 3D US may give rise to artifacts simulating limb defects and provide a solution to eliminate its occurrence. METHODS: Twenty second trimester fetuses (gestational age 15-24 weeks) were scanned with three-dimensional ultrasound (3D US) using a sagittal acquisition plane. Fetal tibia/fibula and radius/ulna pairs were assessed for completeness of imaging. A further 20 fetuses (gestational age 20-26 weeks) were scanned in both axial and sagittal planes and the results compared to verify clear visualization of both bones. RESULTS: Shadowing from adjacent structures produced an apparent limb defect in 55% of the first 20 fetuses imaged only sagittally (18% of limb pairs). Acquiring data from more than one orientation avoided this artifact. CONCLUSIONS: The 3D US is subject to the same artifacts as two-dimensional (2D US) in terms of acoustic shadowing, although their presentation may be different. Awareness of this fact is essential for correct interpretation of 3D US studies. Three-dimensional scanning protocols should be modified to ensure that fetal structures are adequately visualized by acquiring volume data in more than one acquisition orientation.

Artifacts↗
PubMed2000

[The double aorta--rarity or artifact of ultrasound].

The double aorta is very rare. Only two cases are described in the literature. More frequent is a duplication artifact which is misinterpreted as being the image of a double aorta. This is caused by a refraction artifact which results from the difference in the velocity of sound between muscle tissue and fat tissue. Furthermore, a fat depot between parts of the rectus abdominis muscle has a prism-like effect. In most cases, this artifact can be avoided by lateral shifting of the transducer.

Adipose Tissue↗
PubMed2001

[Artifacts in MR Imaging due to ventilation tubes].

UNLABELLED: Artifacts in MR Imaging due to ventilation tubes. PURPOSE: MR examinations may be necessary in patients under artificial ventilation. Possible imaging artifacts originating from ventilation tubing made from silicone are demonstrated. MATERIALS AND METHODS: In silicone carbohydrate residues are bound to silicon atoms. In silicone gels they give strong signals in MR imaging. MR signals from more solid silicone rubber are only possible if the carbohydrate residues in the molecular chain retain a sufficient mobility. To investigate whether silicone tubes give signals in MR images, different tubes for ventilation made from silicone and polyester were examined at 1.5 T (Gyroscan ACS NT, Powertrak 6000 gradients) with MR imaging and spectroscopy (MRS: PRESS, TE = 20 ms; relaxation time measurements: MIXED sequence; imaging: FFE [gradient echo] and TSE sequences). RESULTS: Using short TE (</= 4 ms), tubes and sockets made from silicone could be imaged with FFE and SE sequences. An SE sequence with a short TE = 4 ms provided a sufficient signal intensity to depict the tube clearly besides brain tissue of a volunteer. When foldover occurred, the signal of the tube was projected onto the brain tissue. MR spectra confirmed that the image signal originated from the carbohydrate residues of the silicone molecule chains, not from residual water in the tube wall material. T(2) determinations indicate a multiexponential relaxation including a large component with a short TE (</= 15 ms). Tubes made from a polyester elastomere did not show up in MR images. CONCLUSION: In MR images acquired with very short echo times silicone in ventilation tubes may produce significant artifacts.

Adult↗
PubMed1999

[MR-compatible and conventional marker wires in breast diagnosis--experimental studies on their dislocatability and artifact size in MRT].

PURPOSE: Localization of non-palpable suspicious breast lesions with tumor localization wires is commonly used before surgical excision. Dislocation of the wire is possible. The possibility of dislocation for different types of tumor localization wires was examined experimentally. METHODS: 22 different tumor localization wires were inserted into the subcutaneous fat-tissue of a pig. The wire tip was recorded under fluoroscopy. Traction was applied to the wire using a spring-balance. The artifact size of MR-compatible wires was examined in a 1.0 Tesla MR imager. RESULTS: The localization wires with an X-configuration and long hook could not be dislocated by a force up to the maximum of 12 Newton. The retractable localization wires were easily dislocated at traction forces of 1 Newton. Other wires with only one hook could withstand a traction from 2 up to 7 Newton. The wires with a Z-configuration and wires with short hooks showed a highly variable rate of resistance. The artifact size of localization systems was between 10.1 and 11.0 mm for gradient-echo sequences and between 6.7 and 7.1 mm for spin-echo sequences. CONCLUSION: Tumor localization wires with an X-configuration and long hooks were best anchored in the tissue. The advantage of repositioning retractable wires is confined by the high risk of accidental dislocation. No reliable fixation in the tissue could be achieved using wires with a Z-configuration and wires with short hooks. MR-compatible wires were comparable to non-MR-compatible wires concerning the rate of resistance. The size of the artifacts of MR-compatible systems showed no relevant differences.

Adipose Tissue↗
PubMed1996

[Effect of various electrode configurations and voluntary, reproducible artifacts in computer-assisted analysis of nystagmus].

BACKGROUND: There is no detailed information in the current literature about the best position for the electrodes with regard to electronystagmography. The computer nystagmograph registers potential changes resulting from electrodes on the forearm as nystagmus (pseudonystagmus). This phenomenon led us to investigate more closely the effect of various electrode arrangements and artifacts on the result of computer nystagmography. METHODS: The following four electrode arrangements were investigated on 20 human subjects: distance between the horizontal electrode and the lateral canthus (1 cm versus 2 cm), diagonal arrangement of the lateral electrodes, and changes to the lower vertical electrode. Recorded parameters included calibration potential, rotary nystagmus, post-rotary nystagmus, and optokinetic nystagmus. After determining the basic activity with closed eyes, the influence of five human artifacts on the computer nystagmography was investigated, namely blinking, contractions of the masticatory muscles, swallowing, facial expression, and squinting. RESULTS: The arrangement in which the horizontal electrodes were placed 1 cm away from the lateral canthus showed the greatest calibration potential and the smallest degree of human influence with regard to frequency and amplitude of the pseudonystagmus since a larger potential does not have to be amplified as much as a smaller one to achieve the same needle deflection. For this reason, the artifacts experience a smaller degree of amplification. CONCLUSIONS: We think the following arrangement of the electrodes guarantees the best signal reproduction: a distance between horizontal electrodes and the lateral canthus of 1 cm. The medial electrode is centered between the eyes on the bridge of the nose. The upper vertical electrode is placed above the left eyebrow in the line with the middle of the pupil. The lower vertical electrode is also in line with the middle of the pupil, 1 cm below the lower eyelid. The result is a clear recording of a physiological nystagmus, e.g. on rotation. In the absence of nystagmus, the analysis program is too unreliable and produces a multiplicity of pseudonystagmi. To avoid this we have to improve the recognition of nystagmus in the analysis program.

Adult↗
PubMed2005

Artifacts in dynamical simulations of coarse-grained model lipid bilayers.

With special focus on dissipative particle dynamics simulations of anisotropic and complex soft matter, such as lipid bilayers in water, we have investigated the occurrence of artifacts in the results obtained from dynamical simulations of coarse-grained particle-based models. The particles are modeled by beads that interact via soft repulsive conservative forces (as defined in dissipative particle dynamics simulations), harmonic bond potentials, as well as bending potentials imparting stiffness to the lipid tails. Two different update schemes are investigated: dissipative particle dynamics with a velocity-Verlet-like integration scheme [G. Besold, I. Vattulainen, M. Karttunen, and J. M. Polson, Phys. Rev. E 63, R7611 (2000)] and Lowe-Andersen thermostatting [C. P. Lowe, Europhys. Lett. 47, 145 (1999)] with the standard velocity-Verlet integration algorithm. By varying the integration time step, we examine various physical quantities, in particular pressure profiles and kinetic bead temperatures, for their sensitivity to artifacts caused by the specific combination of integration technique and the thermostat. We then propose a simple fingerprint method that allows monitoring the presence of simulation artifacts.

Artifacts↗
PubMed1998

Artifacts in magnetic resonance images following anterior cervical discectomy and fusion: report of two cases.

Magnetic susceptibility artifacts in two patients who underwent anterior cervical discectomy with fusion for cervical intervertebral disc prolapse are described. These artifacts located at the previously operated level suggested severe ventral compression of the dural tube. Computed tomography (CT) confirmed the artifactual nature of the MR findings and delineated the possible cause for the recurrence of symptoms in these patients. Elements and factors that can possibly lead to MR susceptibility artifacts in post operative imaging are elucidated. The danger of using MR imaging alone in directing the management of these patients is highlighted.

Adult↗
PubMed2006

Influence of artifacts and pass/refer criteria on otoacoustic emission hearing screening.

A screening device for otoacoustic emission (OAE) testing was evaluated via measurements of click evoked OAEs (CEOAEs) and/or distortion product OAEs (DPOAEs) obtained in normally hearing young adults and newborns. Moreover, measurements were performed in a passive cavity and in ears with severe sensorineural hearing loss, in which the occurrence of a pass result was assumed to indicate artifacts. Different parameter settings were investigated. In comparison to normally hearing young adults, overall pass rates in newborns were similar using CEOAEs (93 to 100%), but lower using DPOAEs (65 to 95% vs. 83 to 100%). Pass rates in ears with severe hearing loss were 10% at 2 kHz, 13% at 3 kHz, and lower at other frequencies. Pass rates in the test cavity were generally low. The influence of test frequency on pass rates and calculations using computer simulation indicated the presence of artifacts. The discrimination of such artifacts from biological signals is difficult or impossible, and they may represent a serious problem for efficient OAE screening.

Adolescent↗
PubMed2001

Prosthetic heart valves and annuloplasty rings: assessment of magnetic field interactions, heating, and artifacts at 1.5 Tesla.

The purpose of this study was to determine the magnetic resonance (MR) safety aspects and artifacts for three different heart valve prostheses and two different annuloplasty rings that have not been evaluated previously in association with the 1.5-T MR environment. Ex vivo testing was performed using previously described techniques for the evaluation of magnetic field interactions (deflection angle and torque), heating (gel-filled phantom and fluoroptic thermometry; 15 min of MR imaging at a whole body-averaged specific absorption rate of 1.2 W/kg), and artifacts (using T1-weighted, spin echo, and gradient echo pulse sequences). One heart valve prosthesis and one annuloplasty ring showed no magnetic field interactions. Two heart valve prostheses and one annuloplasty ring displayed relatively minor magnetic field interactions (i.e., deflection angle < or = 6 degrees, torque, +1). Heating was < or = 0.7 degrees C for the five different implants. Artifacts varied depending on the amount and type of metal used to make the implants. For the three heart valve prostheses and two annuloplasty rings, the lack of substantial magnetic field interactions and relatively minor hearing indicated that MR procedures may be conducted safetly in patients with these implants using MR systems operating with static magneticfields of 1.5 T or less. Notably, these findings essentially apply to 54 different heart valve prostheses and 37 different annuloplasty rings (i.e., based on the various models and sizes available for these implants).

Artifacts↗
PubMed1992

Truncation artifact suppression in cone-beam radionuclide transmission CT using maximum likelihood techniques: evaluation with human subjects.

Transverse image truncation can be a serious problem for human imaging using cone-beam transmission CT (CB-CT) implemented on a conventional rotating gamma camera. If this problem can be solved, CB-CT will be useful for attenuation compensation of SPECT images. This paper presents a reconstruction method to reduce or eliminate the artifacts resulting from the truncation. The method uses a previously published transmission maximum likelihood EM algorithm, adapted to the cone-beam geometry. The reconstruction method is evaluated qualitatively using three human subjects of various dimensions and various degrees of truncation. For the two smaller subjects, with moderate truncation, the maximum likelihood method is very successful, nearly eliminating the artifacts seen with conventional filtered backprojection of truncated geometries. The use of an expanded reconstructed space, which contains the entire transverse slice of the subject, is necessary for optimal truncation removal. For the largest subject investigated, the truncation was substantial, and the artifacts were only partially removed by the maximum likelihood reconstruction. Nonetheless, the images were qualitatively superior to those obtained with filtered backprojection. An added elliptical support prior moderately increased the rate of convergence, and helped to force a reasonable body contour.

Adult↗
PubMed1995

Avoidance of upward creep artifact during TI-201 myocardial perfusion tomography using adenosine infusion. A case report.

Upward motion of the heart during tomographic data acquisition of stress TI-201 myocardial perfusion images may lead to artifacts in the reconstructed tomograms. The authors report a patient in whom this occurred, giving the impression of reversible perfusion defects. Although correction of the raw data abolished the artifact, a separate study using adenosine vasodilation produced normal images. The authors conclude that adenosine vasodilation should be considered in preference to dynamic exercise in patients at risk of this artifact, such as those with a high exercise tolerance.

Adenosine↗
PubMed1995

Incorporation of right ventricular counts as a bullseye artifact.

Although extensive information regarding right ventricular uptake of thallium and a variety of bullseye artifacts exist, previous reports have not recognized a bullseye artifact due to right ventricular uptake of thallium. This case report again emphasizes the need for careful inspection of tomographic images while interpreting the bullseye. It is the first reported case of a bullseye artifact due to incorporation of right ventricular counts into the bullseye representation of left ventricular septal wall counts.

Aged↗
PubMed2000

Measurement of hepatic perfusion with dynamic computed tomography: assessment of normal values and comparison of two methods to compensate for motion artifacts.

RATIONALE AND OBJECTIVES: To assess normal values of hepatic perfusion by dynamic, single-section computed tomography, to compare two methods of data processing (a smoothing with a fitting procedure), and to evaluate the influence of motion artifacts. METHODS: Twenty-five volunteers with no history or suspicion of liver disease were examined (age range, 32.8-81.1 years). All examinations were subjectively ranked into groups 1 through 3 according to the degree of motion artifacts (negligible, moderate, severe). All data were processed with a smoothing procedure and a pharmacokinetic fitting procedure (TopFit). The arterial, portal venous, and total hepatic perfusion; the hepatic perfusion index (HPI); and the arterial/portal venous ratio (A/P ratio) were calculated with both procedures. RESULTS: Mean hepatic perfusion, as assessed with the fitting procedure and the smoothing procedure, respectively, was as follows: arterial, 0.20 and 0.22 mL x min(-1) x mL(-1); portal venous, 1.02 and 1.24 mL x min(-1) x mL(-1); total perfusion, 1.22 and 1.47 mL x min(-1) x mL(-1); HPI, 16.4% and 15.4%; and A/P ratio, 0.20 and 0.19. The differences were significant for the portal venous and total hepatic perfusion. The portal venous and total hepatic perfusion values showed significant differences between group 1 and groups 2 and 3 for both procedures. HPI and the A/P ratio showed no significant differences at all. CONCLUSIONS: Motion artifacts and the type of data processing influence the assessment of the arterial, portal venous, and total hepatic perfusion but do not influence measurement of the HPI and the A/P ratio.

Adult↗
PubMed1996

Cardiac artifacts in magnetoencephalogram.

We studied cardiac contamination of magnetoencephalographic signals in eight healthy volunteers. The signals were recorded in a magnetically shielded room while the subject was sitting under a whole-scalp neuromagnetometer and were averaged time-locked to the R wave of the electrocardiogram. The maximum amplitude of the cardiac artifact varied between the subjects and was on average 130 fT/cm. The number of significantly contaminated channels was higher over the left than the right hemisphere. The electric and magnetic signals varied over time in the same way, implying that the artifacts are generated by cardiac currents, without any significant contribution from blood-flow-related pulsations or body movements. These artifacts may have a considerable effect on unaveraged data, i.e., recordings of spontaneous brain activity, and thus should be taken into account in the analysis.

Adult↗