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At least 883 records · Page 49Linked to original sources
PubMed1972

Artifacts in ultracentrifugal estimation of aqueous fatty acid concentration.

Ultracentrifugation for determination of isotropic concentrations of fatty acids is widely used. However, several artifacts, which would affect the isotropic concentration, could occur if care is not taken. These include sedimentation of micelles, incomplete flotation of unsolubilized oil, and uptake of labeled fatty acid by the walls of centrifuge tubes. The first artifact can be overcome by sampling large volumes from the ultracentrifuged sample, and the second by ultracentrifugation for long periods; a force-duration of 7.2 X 10(7) g-min is suitable. The third artifact cannot be eliminated but may be made constant if the duration of exposure of lipid mixtures to centrifuge tubes is kept constant.

Carbon Isotopes↗
PubMed1970

Production of an artifact during methanolysis of lipids by boron trifluoride-methanol.

The production of an artifact by the methanolysis of total lipids of Ascaridia galli with boron trifluoride-methanol was dependent on the reaction time and the age of the reagent used. It was shown that this artifact was produced from oleic acid. Methanolysis of total lipids with sodium methoxide was complete in 10 min, and no artifact was produced.

Animals↗
PubMed1984

[Detection of EMG artifacts for use in automatic routine analysis of clinical EEG's].

Automatic computerized EEG-evaluation in several stages requires information about whether or not EMG-artifacts are present. Furthermore, algorithms for recognition of EMG-artifacts may be helpful while reproducing low pass filtered EEG-signals. Depending on the kind of application three algorithms for the recognition of EMG-artifacts are proposed, taking into account the different kind of results required as well as the different nature of input data. The algorithms have been constructed under the guideline of numerical simplicity to match the requirements of routine EEG-evaluation. They have shown to be a sufficient tool for any EMG-recognition task arising from computerized analysis of clinical EEG-signals.

Computers↗
PubMed1983

Fluid-level motion artifact in computed tomography.

Ingestion of oral contrast material as a routine part of abdominal computerized tomographic scanning creates numerous gas and fluid interfaces within the gastrointestinal tract. Following deep inspiration or expiration, fluid motion induced by shifting intra-abdominal contents persists for several seconds. This causes radial streak artifacts to arise from air-fluid interfaces, even though respiration is suspended while the scan is made. Such artifacts can be reduced if the beginning of a scan is delayed to allow fluid motion to stop. An alternative is to re-scan an area of interest in the lateral decubitus position so as to shift air-fluid levels and their associated artifacts away from any region in question.

Contrast Media↗
PubMed1977

Biosynthesis of mitochondrial DNA. Is 8 S DNA an artifact?

Sucrose density gradient fractionation of isolated rat liver mitochondrial DNA ordinarily yields two peaks, one at 39 S, the other at 27 S. However, when these mitochondria are first incubated with a labeled DNA precursor, a labeled peak at about 8 S is also observed. Is this low molecular weight 8 S DNA merely an artifact of contamination or breakdown, or is it a functioning part of the mitochondrial genome? That it is not a nuclear contaminant is shown by: (a) the absence of nuclei or nuclear fragments in active mitochondrial preparations; (b) the insensitivity of 8 S DNA synthesis to treatment of mitochondria with DNase and RNase; (c) the ability of inner membrane preparations to synthesize this DNA; (d) the ability of atractyloside to inhibit incorporation of [3H]dATP into 8 S and 39 S or 27 S DNA equally; (e) the labeling of 8 S DNA (as well as 39 S and 27 S DNA) but not of nuclear DNA after the administration in vivo of [3H]thymidine. The evidence that 8 S DNA is not an artifact resulting from DNA breakdown during mitochondrial incubation or DNA isolation is as follows: (a) 8 S DNA can be isolated from unincubated mitochondrial; (b) 8 S DNA becomes labeled when labeled DNA precursors are administered in vivo; (c) 8 S DNA biosynthesis continues in the complete absence of labeled 39 S or 27 S DNA (whose synthesis is repressed by ethidium bromide), making it unlikely that 8 S DNA is formed from the breakdown of 39 S or 27 S DNA; (d) substitution of milder methods of DNA extraction does not decrease 8 S DNA labeling; moreover, the usual conditions of extraction, when applied to purified 39 S and 27 S DNA, do not generate 8 S DNA, nor does an additional mitochondrial washing cycle; (e) the specific radioactivity of 8 S DNA is higher than that of 39 S or 27 S DNA, making it improbable that the latter forms are precursors of 8 S DNA. Since 8 S DNA is double-stranded, it is not identical to the 7 S fragment of D loop DNA. The hypothesis that the artifactual nicking of those DNA molecules which contain opposing D loops leads to the release of double-stranded fragments was tested. The DNA which was released was predominantly (and probably completely) single-stranded. We conclude that 8 S DNA is probably not an artifact and studies are in progress on its function.

Animals↗
PubMedDate not supplied

An artifact in acoustic reflex measurement: some further observations.

This study was designed to clarify characteristics of an artifact that has been reported to be a potential source of misleading information when determing acoustic reflex thresholds elicited by low-frequency tones. The study was conducted in two parts. Experiment 1 was concerned with an investigation of acoustic interactions in a sealed cavity between the probe tone from a commercial electroacoustic impedance bridge and interfering tones of various frequencies. Experiment 2 consisted of measurements of sound pressure levels in the sealed external ear canal of the probe ear when the contralateral ear was stimulated by air-conducted pure tones. The results indicated that acoustic reflex thresholds in 250-Hz tones probably cannot be determined for most persons unless special measures are taken to prevent the artifact. The artifact should not occur when the eliciting stimulus is 500 Hz, however.

Acoustic Stimulation↗
PubMed1999

MR imaging of focal lung lesions: elimination of flow and motion artifact by breath-hold ECG-gated and black-blood techniques on T2-weighted turbo SE and STIR sequences.

Respiratory and cardiac motion correction may result in better turbo spin-echo (SE) imaging of the lung. To compare breath-hold cardiac-gated black-blood T2-weighted turbo SE and turbo short-inversion-time inversion-recovery (STIR) magnetic resonance (MR) imaging pulse sequences with conventional breath-hold turbo SE and half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences for lesion conspicuity of focal lung lesions, 42 patients with focal lung lesions were prospectively studied with MR imaging at 1.5 T. Helical computed tomography was used as a reference. In comparison with the conventional breath-hold turbo SE sequence, all black-blood sequences had fewer image artifacts arising from the heart and blood flow. The overall image quality for the black-blood turbo SE and turbo STIR sequences was superior to that for the breath-hold turbo SE and HASTE sequence (P < 0.01). Not only focal lung lesions but also surrounding inflammatory changes were clearly visualized with these two sequences. With the HASTE sequence, although several slices could be obtained in one breath-hold, both the tumor and vessels appeared blurred. We conclude that T2-weighted turbo SE and turbo STIR imaging of the lung with effective suppression of flow and motion artifacts provide high-quality images in patients with focal lung lesions.

Artifacts↗
PubMed1999

Improved field of view-reducing gradient insert: artifacts and application to cardiac imaging.

An improved homogeneity-spoiling local gradient insert has been constructed and built into the patient bed to reduce its impact on bore size and setup time. It allows the field of view to be reduced without introducing aliasing artifacts. Image quality and artifacts were evaluated for pulse sequences relevant to cardiac imaging. The utility of the insert is demonstrated by a contrast-enhanced perfusion study, in which the reduced field of view allowed a 25% increase in resolution and one more slice to be imaged per heartbeat. J. Magn. Reson. Imaging 1999;10:209-215.

Adult↗
PubMed1999

Chemical shift artifact-free microscopy: spectroscopic microimaging of the human skin.

A spectroscopic imaging technique with high spatial resolution was used for the study of human skin in vivo. The measurements were performed using a whole-body magnetic resonance system (1.5 T) with standard gradients and a standard 8-cm diameter circular surface coil. A decisive gain in signal-to-noise ratio was achieved by reducing the receiver bandwidth of the imaging system to values less than +/-5 kHz. The chemical shift misregistration was eliminated by post-detection data processing. The method was tested on different kinds of skin, on the foot sole and head. Water, fat, and chemical shift artifact-free images were obtained with resolution 0.107 x 0.143 mm in plane and slice thickness 1 mm. A major advantage of the spectroscopic imaging procedure is that the pulse sequence can be optimized for the maximum signal-to-noise ratio. There is no need for special modification of the sequence to circumvent the chemical shift artifacts (water, fat suppression, etc.).

Adipose Tissue↗
PubMed2001

Fluorescence lifetime imaging: multi-point calibration, minimum resolvable differences, and artifact suppression.

BACKGROUND: Frequency-domain fluorescence lifetime imaging microscopy (FLIM) is finding increasing use in the analysis of biological systems. However, the calibration, determination of resolvable lifetime differences, and evaluation of artifacts have not been extensively treated. We describe a multi-point method for calibrating a frequency-domain FLIM system, characterize the minimum detectable heterogeneity and intra- and inter-image lifetime differences, discuss the statistical treatment of FLIM data, and suggest methods for minimizing artifacts. METHODS: A set of solutions exhibiting single-component lifetimes suffice for accurately calibrating a reference material with a single-component lifetime, even in the absence of accurate data on the lifetimes of the individual solutions or the reference material. We used a set of rhodamine 6G solutions quenched with varying concentrations of iodide, leading to lifetimes of 0.5--4.0 ns, to calibrate a 1 microM reference solution of rhodamine 6G in water. RESULTS: We measured a value of 4.11 ns with an estimated absolute error of +/-0.05 ns for the rhodamine 6G reference solution. With 57.7 MHz modulation, the minimum detectable inter-image lifetime difference was 0.1--0.15 ns and the minimum detectable intra-image lifetime difference was 4--5 ps, allowing solutions differing in lifetime by 40 and 70 ps to be easily distinguished. The minimum detectable lifetime heterogeneity was 50--80 ps. Evaluation of replicate measurements of single solutions demonstrated that inter-image instrument errors exceeded those predicted from intra-image statistics by more than an order of magnitude. We also measured lifetimes and heterogeneity in 4 GFP variants (WTGFP, EGFP, S65T, and EYFP) with the technique. CONCLUSION: The multi-point calibration method is applicable to any system consisting of single-component lifetimes. Applying the method in our FLIM microscope allowed us to demonstrate a previously unreported degree of lifetime resolution in a FLIM microscope. Cytometry 43:248-260;2001.

Animals↗
PubMed2000

Prosthetic heart valves: evaluation of magnetic field interactions, heating, and artifacts at 1.5 T.

The purpose of this study was to use ex vivo testing techniques to determine the magnetic resonance imaging (MRI) safety aspects for 32 different heart valve prostheses that had 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 minutes of MRI at a specific absorption rate (SAR) of 1.1 W/kg], and artifacts (using gradient echo and T1-weighted spin-echo pulse sequences). Thirteen heart valve prostheses displayed interactions with the magnetic field. However, these magnetic field interactions were considered relatively minor. Heating was < or =0.8 degrees C for these implants. Artifacts varied from mild to severe depending on the amount and type of metal used to make the particular heart valve prosthesis. For these 32 different heart valve prostheses, the relative lack of substantial magnetic field interactions and negligible heating indicate that MR procedures may be conducted safely in individuals with these implants using MR systems with static magnetic fields of 1.5 T or less.

Artifacts↗
PubMed2000

MR imaging in the presence of vascular stents: A systematic assessment of artifacts for various stent orientations, sequence types, and field strengths.

A systematic evaluation of the potential quality of magnetic resonance images recorded in the presence of metallic stents was performed on a low-field open imager operating at 0.2 T and on a high-field closed unit operating at 1.0 T. Eight different stent types were examined by two-dimensional gradient-echo sequences with echo times of 4 and 10 msec and by a fast spin-echo technique. In addition, a three-dimensional gradient-echo sequence was applied with an echo time of 2.4 msec. A set of sequence and slice parameters was used on both scanners. Thus, artifacts due to susceptibility effects depending on the magnetic field strength could be distinguished from radiofrequency shielding effects in the lumen of the stents (independent of the field strength). Nine different orthogonal orientations of the stent axis and the image (in terms of slice, read, and phase-encoding direction) were tested, and the artifacts (extension of signal void and visibility of the lumen) were compared. The optimal strategy for visualization of vascular and perivascular regions outside the stents was fast spin-echo imaging with the stent axis and read direction parallel to the static field. Susceptibility-induced signal void in gradient-echo images was minimal using the three-dimensional approach. Increased transmitter amplitudes above usual values provided clearly improved insight in the lumen using gradient-echo sequences.

Alloys↗
PubMed2000

Statistical artifacts in diffusion tensor MRI (DT-MRI) caused by background noise.

This work helps elucidate how background noise introduces statistical artifacts in the distribution of the sorted eigenvalues and eigenvectors in diffusion tensor MRI (DT-MRI) data. Although it was known that sorting eigenvalues (principal diffusivities) by magnitude introduces a bias in their sample mean within a homogeneous region of interest (ROI), here it is shown that magnitude sorting also introduces a significant bias in the variance of the sample mean eigenvalues. New methods are presented to calculate the mean and variance of the eigenvectors of the diffusion tensor, based on a dyadic tensor representation of eigenvalue-eigenvector pairs. Based on their use it is shown that sorting eigenvalues by magnitude also introduces a bias in the mean and the variance of the sample eigenvectors (principal directions). This required the development of new methods to calculate the mean and variance of the eigenvectors of the diffusion tensor, based on a dyadic tensor representation of eigenvalue-eigenvector pairs. Moreover, a new approach is proposed to order these pairs within an ROI. To do this, a correspondence between each principal axis of the diffusion ellipsoid, an eigenvalue-eigenvector pair, and a dyadic tensor constructed from it is exploited. A measure of overlap between principal axes of diffusion ellipsoids in different voxels is defined that employs projections between these dyadic tensors. The optimal eigenvalue assignment within an ROI maximizes this overlap. Bias in the estimate of the mean and of the variance of the eigenvalues and of their corresponding eigenvectors is reduced in DT-MRI experiments and in Monte Carlo simulations of such experiments. Improvement is most significant in isotropic regions, but some is also observed in anisotropic regions. This statistical framework should enhance our ability to characterize microstructure and architecture of healthy tissue, and help to assess its changes in development, disease, and degeneration. Mitigating these artifacts should also improve the characterization of diffusion anisotropy and the elucidation of fiber-tract trajectories in the brain and in other fibrous tissues. Magn Reson Med 44:41-50, 2000. Published 2000 Wiley-Liss, Inc.

Artifacts↗
PubMed2001

Correcting partial volume artifacts of the arterial input function in quantitative cerebral perfusion MRI.

To quantify cerebral perfusion with dynamic susceptibility contrast MRI (DSC-MRI), one needs to measure the arterial input function (AIF). Conventionally, one derives the contrast concentration from the DSC sequence by monitoring changes in either the amplitude or the phase signal on the assumption that the signal arises completely from blood. In practice, partial volume artifacts are inevitable because a compromise has to be reached between the temporal and spatial resolution of the DSC acquisition. As the concentration of the contrast agent increases, the vector of the complex blood signal follows a spiral-like trajectory. In the case of a partial-volume voxel, the spiral is located around the static contribution of the surrounding tissue. If the static contribution of the background tissue is disregarded, estimations of the contrast concentration will be incorrect. By optimizing the correspondence between phase information and amplitude information one can estimate the origin of the spiral, and thereupon correct for partial volume artifacts. This correction is shown to be accurate at low spatial resolutions for phantom data and to improve the AIF determination in a clinical example. Magn Reson Med 45:477-485, 2001.

Artifacts↗
PubMed2002

Dye exclusion artifact in flow cytometers.

BACKGROUND: Cells exclude their own volume of dye solution in the sample flow which carries them through the flow chamber of the flow cytometer, thereby affecting the otherwise constant signal arising from the fluorescence of this solution. Under certain conditions, this phenomenon may significantly influence the fluorescence signal of the cells. MATERIALS AND METHODS: Using the slit scan technique, we studied this phenomenon as observed for monodisperse polystyrene particles in fluorescein solution. RESULTS: The measurements show that dye solution accumulates just in front of the particle and just behind it, with a relative void in between. This phenomenon is most likely caused by the rapid constriction of the flow as it enters the orifice of the nozzle or flow chamber, giving rise to a pulse of fluorescence which adds to that of the particle or cell itself. The magnitude of this artifact depends on the design and dimensions of the nozzle/flow chamber as well as on the rate of sample flow. CONCLUSIONS: The dye exclusion artifact may affect measurements of cells when they are in a dye solution having a fluorescence per unit volume which is significant compared to that of the cells, especially at low sample flow rates.

Animals↗
PubMed2003

Mirror-image artifact mimicking epidural hematoma: usefulness of power Doppler sonography.

During sonographic examination, a mirror-like interface can scatter the ultrasound beam, producing mirror-image artifacts. We describe a case of bilateral parietal cephalhematoma in the parietal region of a 2-day-old neonate that mimicked bilateral epidural hematoma on routine sonographic examination. Forceps had been used during delivery, and the neonate had been experiencing episodes of apnea since birth. Gray-scale coronal sonograms showed anechoic structures resembling epidural hematomas in the left and right parietal regions. However, color and power Doppler sonography showed normal vascular findings, thus permitting identification of the mirror-image artifact.

Artifacts↗
PubMed2005

Color Doppler twinkling artifact in a calcified liver mass.

Twinkling artifact in color Doppler imaging is related both to the presence of a strongly reflective structure and to the scanner's settings. This artifact has been described in association with calculi and various foreign bodies, but to our knowledge it has not been reported in a hepatic mass. If a calcified hepatic mass is examined with color Doppler Mapping without using spectral Doppler analysis, misinterpreted as real blood flow and lead to the erroneous of hypervascular mass.

Adult↗
PubMedDate not supplied

Ineffectiveness of averaging for reducing motion artifacts in half-Fourier MR imaging.

Two data sets for half-Fourier imaging (HFI) can be collected in the same time as one data set for conventional full Fourier imaging (FFI). The hypothesis is that averaging twice as much data in HFI does not make ghost artifacts caused by motion have less signal intensity than in FFI. This hypothesis was tested with images of a human subject by measuring the standard deviation within regions of interest containing ghosts. The control experiment involved measuring the standard deviation on images from the same data reconstructed with FFI. The images were formed after averaging of one to eight data sets from a collection of nine data sets acquired sequentially. Background ghosts or those in other regions of low intensity were less intense on images from HFI after twice as much averaging as in FFI, but this was not the case for ghosts superimposed on anatomic structures. This observation is explained by showing that an image obtained by means of FFI can be expressed in terms of two images obtained by means of HFI applied to the top and bottom halves of the data. The use of HFI to allow twice as much averaging without prolonging data acquisition time is not advantageous for reducing ghost artifacts caused by motion.

Artifacts↗