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At least 667 records · Page 37Linked to original sources
PubMed2005

Optimizing T2-weighted magnetic resonance sequences for surface coil microimaging of the eye with regard to lid, eyeball and head moving artifacts.

PURPOSE: To acquire high-resolution magnetic resonance (MR) images, we developed a new blinking artifact reduced pulse (BARP) sequence with a surface coil specialized for microscopic imaging (47 mm in diameter). MATERIALS AND METHODS: To reduce eye movement, we ascertained that the subjects' eyes were kept open and fixated to the target in the 1.5-T MR gantry. To reduce motion artifacts from blinking, we inserted rest periods for blinking (1.5 s within every 5 s) during MR scanning (T2-weighted fast spin echo; repetition time, 5 s; echo time, 100 ms; echo train, 11; matrix, 256 x 128; field of view, 5 cm; 1-mm thickness x 30 slices). Three scans (100 s x 3) were performed for each normal subject, and they were added together after automatic adjustment for location to reduce quality loss caused by head motion. RESULTS: T2-weighted MR images were acquired with a high resolution and a high signal-to-noise ratio. Motion artifacts were reduced with BARP, as compared with those with random blinking. Intraocular structures such as the iris and ciliary muscles were clearly visualized. Because the whole eye can be covered with a 1-mm thickness by this method, three-dimensional maps can easily be generated from the obtained images. CONCLUSION: The application of BARP with a surface coil of the human eye might become a useful and widely adopted procedure for MR microimaging.

Algorithms↗
PubMed2005

Detecting and adjusting for artifacts in fMRI time series data.

We present a new method to detect and adjust for noise and artifacts in functional MRI time series data. We note that the assumption of stationary variance, which is central to the theoretical treatment of fMRI time series data, is often violated in practice. Sporadic events such as eye, mouth, or arm movements can increase noise in a spatially global pattern throughout an image, leading to a non-stationary noise process. We derive a restricted maximum likelihood (ReML) algorithm that estimates the variance of the noise for each image in the time series. These variance parameters are then used to obtain a weighted least squares estimate of the regression parameters of a linear model. We apply this approach to a typical fMRI experiment with a block design and show that the noise estimates strongly vary across different images and that our method detects and appropriately weights images that are affected by artifacts. Furthermore, we show that the noise process has a global spatial distribution and that the variance increase is multiplicative rather than additive. The new algorithm results in significantly increased sensitivity in the ability to detect regions of activation. The new method may be particularly useful for studies that involve special populations (e.g., children or elderly) where sporadic, artifact-generating events are more likely.

Algorithms↗
PubMed2006

Metal artifact reduction in musculoskeletal magnetic resonance imaging.

Many strategies can be employed to reduce the size and scale of metal susceptibility artifacts in the vicinity of orthopedic hardware; factors include selection of metal hardware material, patient positioning, and MRI sequence adjustments and techniques. The adjustments to sequence parameters include high spatial resolution fast spin echo sequences with minimal interecho spacing and increased receiver bandwidth. Frequency and phase encoding gradients can be orientated so misregistration artifacts arising from the metal are directed away from areas of anticipated clinical diagnostic interest. Complications arising in the vicinity of metallic hardware, including loosening, can be assessed after implication of these metal artifact reduction techniques.

Artifacts↗
PubMed1996

Radiopharmaceutical-related pitfalls and artifacts.

The primary goal of this review article is to increase the reader's knowledge and understanding of problems associated with the radiopharmaceuticals commonly used in daily practice. To achieve this objective, problems related to the commonly used radiopharmaceuticals are divided into pitfalls and artifacts related to radiopharmaceutical preparation (technetium-99m [99mTc]-labeled and non-99mTc-labeled radiopharmaceutical) and those related to radiopharmaceutical administration. For the radiopharmaceutical formulation-associated pitfalls and artifacts, problems are discussed in terms of factor categories, such as factors associated with radionuclides, factors associated with components, factors associated with preparation procedures, and miscellaneous factors. As for the pitfalls and artifacts caused by radiopharmaceutical administration, these problems are categorized into errors associated with administration technique and nontechnical errors. Clinical manifestations (ie, appearance upon imaging) from the numerous literature-based examples are presented. The effect of the causative factors and the reason each factor can result in radiopharmaceutical preparation and administration problems are discussed. In addition, the possible preventive actions are presented for each group. However, the cause of some pharmaceutical related problems may not be easily recognized, and thus it is difficult to develop preventive and/or corrective plans for these cases.

Artifacts↗
PubMed1998

Why are different features central for natural kinds and artifacts?: the role of causal status in determining feature centrality.

Ahn and Lassaline [Ahn, W., Lassaline, M.E., 1995. Causal structure in categorization. Proceedings of the Seventeenth Annual Conference of the Cognitive Science Society, Pittsburgh, PA, pp. 521-526] recently proposed a causal status hypothesis which states that features that play a causal role in a relational structure are more central than their effects. This hypothesis can account for previous research demonstrating that compositional features are generally important for natural kinds but functional features are generally important for artifacts. The causal status hypothesis explains this category-feature interaction effect in terms of differences in the causal status of compositional and functional features between natural kinds and artifacts. Experiments 1 and 2 examined real-life categories used in previous studies, and found positive correlations between the causal status of the features and their centrality across natural and artifactual kinds. Experiments 3 and 4 manipulated the causal status of compositional and functional features in artificial categories, and showed that it was causal status rather than the interaction between the type of feature and the type of category per se that accounted for feature centrality. The implications of these results on the distinctions between natural kinds and artifacts are discussed.

Adult↗
PubMed2002

The effects of automatic artifact rejection on evoked potential recordings.

An analysis of the effects of automatic artifact rejection on the evoked potential recordings is made using both a theoretical analysis and computer simulations. It is demonstrated that when the noise at each measurement point is independently distributed with maximum probability near zero amplitude, the bias and changes in variance induced by the automatic artifact rejection are small unless more than 80% of traces are rejected. When high-amplitude noise transients occur, there can be significant bias in the evoked potential when the artifact rejection threshold is set near the amplitude of the noise transients. Even more substantial distortions in the evoked potential occur when the noise is not independently distributed at each measurement point. It is important to recognize the possibility of these effects during clinical evoked potential recording.

Artifacts↗
PubMedDate not supplied

Mucolytic-antifoam solution for reduction of artifacts during endoscopic ultrasonography: a randomized controlled trial.

Gastric mucus and air bubbles are frequent sources of artifacts during endoscopic ultrasonography (EUS). To reduce these artifacts, we prepared a mucolytic-antifoam solution consisting of simethicone, acetylcysteine, and sodium bicarbonate and performed a prospective randomized and controlled trial comparing this solution (21 patients) to a placebo (22 patients) during EUS. Eight patients in the drug group received a perfect score for both mucolytic and antifoam effect compared with none in the placebo group (p less than 0.001). For overall rating, the drug group received a total of 10 excellents and 9 goods compared with 9 goods, 10 satisfactories, and 1 unacceptable in the placebo group (p less than 0.001). There were no complications associated with the solution. In conclusion, a mucolytic-antifoam solution improved endoscopic visualization and reduced artifacts during EUS study. Its routine use during EUS procedures is therefore recommended.

Acetylcysteine↗
PubMed1999

Artifact processing during exercise testing.

In signal processing of exercise electrocardiograms (ECGs), artifacts are a recurring problem. It is still difficult to discriminate the ECG curves from artifacts, especially in exercise ECGs and particularly in the high exercise phase. We focused on the artifact problem and worked on two new topics: the Finite Impulse Response Residual Filtering (FRF) algorithm and the Intelligent Lead Switch algorithm. The FRF algorithm reduces the baseline wander and muscle noise in the ECG stream, with much less distortion of the QRS complexes. It subtracts a continuously updated median beat from the current ECG, filters the residual signal with a high-pass and a low-pass filter, and adds the median beat to the filtered residual signal. The Intelligent Lead Switch algorithm takes advantage of the redundancy of a multilead system (eg, standard leads), which is nowadays used during exercise testing. It selects the best leads for QRS detection and thus improves the heart rate calculation, ST segment evaluation, and arrhythmia classification.

Algorithms↗
PubMed2000

Reduction of ECG and gradient related artifacts in simultaneously recorded human EEG/MRI data.

Nowadays, electroencephalography signals can be acquired from a patient lying in a magnetic resonance imaging system. It is even possible to acquire EEG signals during an MR imaging sequence. However, such EEG signals are severely distorted by artifacts originating from various effects (e.g., MR gradients, ECG). In this paper, a simple method is presented to reduce such artifacts. Thereby, special attention is focused on artifacts related to the patient's electrocardiogram. The method is shown to be effective, adaptive, and automatic.

Artifacts↗
PubMed2003

Fast T2-weighted MR imaging: impact of variation in pulse sequence parameters on image quality and artifacts.

The purpose of this study was to quantitatively evaluate in a phantom model the practical impact of alteration of key imaging parameters on image quality and artifacts for the most commonly used fast T(2)-weighted MR sequences. These include fast spin-echo (FSE), single shot fast spin-echo (SSFSE), and spin-echo echo-planar imaging (EPI) pulse sequences. We developed a composite phantom with different T1 and T2 values, which was evaluated while stationary as well as during periodic motion. Experiments involved controlled variations in key parameters including effective TE, TR, echo spacing (ESP), receive bandwidth (BW), echo train length (ETL), and shot number (SN). Quantitative analysis consisted of signal-to-noise ratio (SNR), image nonuniformity, full-width-at-half-maximum (i.e., blurring or geometric distortion) and ghosting ratio. Among the fast T(2)-weighted sequences, EPI was most sensitive to alterations in imaging parameters. Among imaging parameters that we tested, effective TE, ETL, and shot number most prominently affected image quality and artifacts. Short T(2) objects were more sensitive to alterations in imaging parameters in terms of image quality and artifacts. Optimal clinical application of these fast T(2)-weighted imaging pulse sequences requires careful attention to selection of imaging parameters.

Artifacts↗
PubMed1996

MR imaging and cervical fixation devices: evaluation of ferromagnetism, heating, and artifacts at 1.5 Tesla.

The purpose of this study was to assess ferromagnetism, heating, and artifacts for cervical fixation devices exposed to a 1.5 T MR system. Cervical fixation devices (three halos, one tong and two halo vests) were evaluated for compatibility with MR procedures. Ferromagnetism was determined using a previously described technique. Heating was evaluated by measuring temperatures at various positions on the cervical fixation devices while applied to a volunteer subject before and during the use of various pulse sequences, including an magnetization transfer contrast (MTC) sequence. Artifacts associated with routine clinical MR imaging of the cervical spine were qualitatively evaluated with the cervical fixation devices applied to a volunteer subject. None of the devices displayed attraction to the magnetic field. The temperature changes were +/-1.5 degrees C in each instance. The MTC pulse sequence produced a sensation of "heating" the skull pins that may have been caused by vibration of the cervical fixation device. The MR images of the cervical spine were obtained without apparent artifacts using each routine, clinical pulse sequence. The lack of ferromagnetism, negligible heating, and capability of obtaining diagnostically acceptable studies of the cervical spine indicate that MR imaging performed at 1.5 T or less may be conducted safely in patients with each of the cervical fixation devices tested using conventional pulse sequences.

Artifacts↗
PubMed1999

Restoration of MR-induced artifacts in simultaneously recorded MR/EEG data.

During a Magnetic Resonance sequence, simultaneously acquired ElectroEncephaloGraphy (EEG) data are compromised by severe pollution due to artifacts originating from the switching of the magnetic field gradients. In this work, it is shown how these artifacts can be strongly reduced or even removed through application of an adaptive artifact restoration scheme. The method has proved to be fully automatic and to retain high frequency EEG information, which is indispensable for many EEG applications.

Animals↗
PubMedDate not supplied

Metallic artifacts of coronary and iliac arteries stents in MR angiography and contrast-enhanced CT.

Metallic artifacts of intravascular stents were assessed with MR angiography and contrast-enhanced spiral CT. Stainless steel showed less metal artifact than tantalum stent in CT. Metallic artifact in coronary and iliac arteries treated with tantalum stent was not remarkable in MR angiography. Contrast-enhanced CT might be preferable to assess patency of arteries treated with stainless steel stent. while MR angiography was useful in depicting intraluminal signal in tantalum stent.

Aged↗
PubMed2001

The cell in absence of aggregation artifacts.

Eduard Kellenberger understood that the conventional resin-embedding, he helped to develop (Ryter, A., Kellenberger, E., 1958. L'inclusion au polyester pour l'ultramicrotomie. J. Ultrastruct. Res. , 2, 200-214), was prone to aggregation artifacts (Kellenberger, E., 1987. The response to biological macromolecules and supramolecular structures to the physics of specimen cryo-preparation. In: Steinbrecht, R.A., Zierold, K. (Eds.), Cryo-techniques in Biological Electron Microscopy, Springer, Berlin, pp. 35-63). He was instrumental in developing various methods to overcome this limitation, for instance, by using low temperature-embedding and partially hydrophilic resins (Carlemalm, E., Garavito, R.M., Villiger, W., 1982. Resin development for electron microscopy and an analysis of embedding at low temperature. J. Microstruct., 126, 123-143; Villiger,W., 1993. Low temperature-embedding with Lowicryl resins. In: Robards, A.W., Wilson, A.J. (Eds.), Procedures in electron microscopy, Wiley, Chichester, UK, pp. 16:7.3-16:7.6). In principle, cryo-electron microscopy of vitreous sections is free of any aggregation artifact since the material remains fully hydrated and is free of chemical fixation or staining. The method is technically difficult still, but recent progress has made it amenable to routine practical applications. We compare here electron microscopical aspects of Zea mays meristem cells prepared by: (1) conventional resin-embedding and sectioning; (2) low temperature-embedding and sectioning of freeze substituted samples; and (3) cryo-sections of vitrified samples. The appearance of the extra-cellular space, the cytoplasm and the nucleoplasm are very different in conditions (1) and (3). They appear as compact, irregular and well delineated structures in conventional resin sections, whereas they are more diffuse and homogeneous in the vitreous sections. In the resin sections, the material seems to form a complex matrix, whereas it looks more like a thick soup in the vitreous sample. Low temperature-embedding (condition 2) shows an intermediate appearance. We suggest that regardless of the difference due to staining and different sectioning conditions, the other image differences are the consequence of aggregation artifacts in the resin-embedded specimens.

Artifacts↗
PubMedDate not supplied

The "hurricane sign": evidence of patient motion artifact on cardiac single-photon emission computed tomographic imaging.

Single-photon emission computed tomography (SPECT) is frequently used with myocardial perfusion imaging in the assessment of patients with known or suspected coronary artery disease. The susceptibility of SPECT imaging to artifact, especially that caused by patient motion, has long been recognized. We report a characteristic artifact of patient motion, which we have termed the "hurricane sign" because of its similarity to the National Weather Service Symbol for a hurricane. This artifact is caused by varying contributions of different portions of the heart during image acquisition and their misalignment produced by patient motion.

Artifacts↗
PubMed1999

Filtering of electromyogram artifacts from the electrocardiogram.

Electromyogram (EMG) artifacts often contaminate the electrocardiogram (ECG). They are more difficult to suppress or eliminate, compared for example to the power line interference, due to their random character and to the considerable overlapping of the frequency spectra of ECG and EMG signals obtained from the same pair of electrodes. The usually applied low-pass filtering (cutoff frequency of minimum 35 Hz) results in limited suppression of the EMG artifact and considerable reduction of sharp Q, R and S ECG wave amplitudes. A solution to this problem is proposed by applying approximation filtering with dynamically varied number of samples and weighting coefficients, depending on the ECG signal slope. The slope measure used is the absolute value of the product of the tilts of two adjacent 10 ms segments sliding along the signal. The results obtained show a slight widening of some sharper QRS complexes, but a virtual preservation of their amplitudes and a considerable reduction of the EMG artifact.

Artifacts↗
PubMed2004

Investigation of conventional membrane and tangential flow ultrafiltration artifacts and their application to the characterization of freshwater colloids.

Artifacts associated with the fractionation of colloids in a freshwater sample were investigated for conventional membrane filtration (0.45 microm cutoff), and two tangential flow ultrafiltration cartridges (0.1 microm cutoff and 3000 MW cutoff). Membrane clogging during conventional filtration removed some colloids smaller than 0.1 microm in diameter, much smaller than the nominal size of the filter. For certain constituents (e.g., Fe), filter clogging had a significant effect on filtrate concentrations, while artifacts associated with tangential flow ultrafiltration using a 0.1 microm cutoff were minimal. Artifacts occurred during tangential flow ultrafiltration with a 3000 MW cutoff, but did not deviate from predicted changes in concentration based on a standard permeation model. Comparison of filtrate concentrations for membrane filtration (at 1.0 and 0.45 microm) and tangential flow ultrafiltration (at 0.1 microm) for a large number of samples from Connecticut rivers shows that significant and consistent differences exist between their separation characteristics. Results for organic carbon, Fe, Mn, Al, Cu, and Pb demonstrate the magnitude of the effects of the fractionation technique on filtrate element concentration, show variability by element and flow condition, and highlight the importance of larger colloids to freshwater metal speciation. One implication of the research is that tangential flow ultrafiltration with large size cutoff membranes (e.g., approximately 0.1 microm) may be superior to conventional filtration with filters in the same size discrimination range, and potentially more appropriate for the fractionation of natural water samples.

Artifacts↗
PubMed2001

Comparison of methylation procedures for conjugated linoleic acid and artifact formation by commercial (trimethylsilyl) diazomethane.

Four different methods for methylating conjugated linoleic acid (CLA) were compared. The HCl/MeOH and BF(3)/MeOH methods were tested under different time and temperature combinations. Increasing temperature and/or incubation time for either method decreased the cis-9,trans-11 and trans-10,cis-12 isomers, but trans-9,trans-11/trans-10,trans-12 isomers and artifacts (allylic methoxide) were increased. In addition, the triacylglyceride form of CLA was tested using the above methods and NaOMe at various temperatures for 20 min. The NaOMe did not generate methoxy artifacts. However, there were impurities in GC after methylation with NaOMe as well as with BF(3)/MeOH. The (trimethylsilyl)diazomethane method, which is a mild and easy alternative, was tested. Free forms of fatty acids were easily, but not completely, methylated by this method. Also, the method generated artifacts (trimethylsilyl CLA esters) and impurities (trimethylsilyl) that would interfere with short-chain fatty-acid analysis by GC.

Artifacts↗