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At least 937 records · Page 52Linked to original sources
PubMed2000

Burst imaging: rotation artifacts and how to correct them.

The effect of coherent rotational motion on images acquired with the ultrafast single-shot spin-echo Burst sequence has been analyzed. Previous experience has demonstrated that sample rotation during Burst experiments has the potential to cause severe image artifacts. In this paper we show that no distortions are visible when the readout gradient is parallel to the rotation axis, but that there is a very distinctive behavior for the case of the rotation axis orthogonal to the imaging plane. The mathematical expression that describes the resulting signal is presented and is used as a basis for a method of correcting the k-space data. The conditions under which undistorted images may be recovered are discussed. It is shown that there is an asymmetry, dependent on the rotation direction, in both the manifestation of the artifact and the range of angular velocities over which one can correct the images. Data from an agar gel phantom rotating at a known rate are used to show how the theory is successful at reconstructing images, with no free parameters. The range of angular velocities over which correction is possible depends on the timing parameters of the pulse sequence, but for these data was -0.016 < omega less, similar 0.1 revolutions/s. Volunteer experiments have confirmed that the theory is applicable to patient motion and can correct motional distortion even when the exact rate is not known a priori. By optimizing the reconstruction to restore a known sample geometry/aspect ratio, an estimate of the rotation angular frequency is obtained with a precision of +/-10%.

Artifacts↗
PubMed1998

Problems and artifacts of microwave accelerated procedures in neurohistotechnology and resolutions.

Microwaving artifacts in histoprocessing and staining arise from the acceleration of diffusional and reactive processes. Because such accelerations provide the advantages of microwaving, and because microwave ovens cannot distinguish desirable from undesirable accelerations, artifacts are inevitable. Such microwaving problems can be categorized as follows: histoprocessing and staining reagents may be lost or altered; staining targets may move away from their in vivo sites, may be totally lost from the specimen, or may be altered; physical characteristics such as permeability of specimens or embedding resins may be changed; and staining processes themselves are sometimes different at elevated temperatures. The most general tips for detecting and/or avoiding such problems are to monitor and control the temperature of the reagents and the specimen, to standardize the procedures, and to observe the specimen and reagents carefully during microwaving when a new procedure is being introduced to the laboratory.

Animals↗
PubMed2005

Plaque and stent artifact reduction in subtraction CT angiography using nonrigid registration and a volume penalty.

Computed tomography angiography (CTA) is an established tool for vessel imaging. Yet, high-intense structures in the contrast image can seriously hamper luminal visualisation. This can be solved by subtraction CTA, where a native image is subtracted from the contrast image. However, patient and organ motion limit the application of this technique. Within this paper, a fully automated intensity-based nonrigid 3D registration algorithm for subtraction CT angiography is presented, using a penalty term to avoid volume change during registration. Visual and automated validation on four clinical datasets clearly show that the algorithm strongly reduces motion artifacts in subtraction CTA. With our method, 39% to 99% of the artifacts disappear, also those caused by minimal displacement of stents or calcified plaques. This results in a better visualisation of the vessel lumen, also of the smaller vessels, allowing a faster and more accurate inspection of the whole vascular structure, especially in case of stenosis.

Algorithms↗
PubMed1991

Eliminating diathermy-induced artifacts during intraoperative monitoring of somatosensory-evoked potentials: a hardware solution.

Somatosensory-evoked potentials (SEPs) recorded intraoperatively are often contaminated by artifacts arising from unipolar diathermy. This increases variability in serial SEP recordings and decreases the reliability of SEPs for intraoperative monitoring. Rejection of single sweeps on the basis of a voltage criterion can only partially prevent the inclusion of sweeps containing artifacts. We describe a low-cost hardware system that will halt SEP acquisition automatically before diathermy becomes active. Activation of the diathermy switch will halt SEP acquisition by sending a digital inhibit signal to the evoked potential machine via a fiberoptic cable. A timer delays the activation of the diathermy until the last sweep has been completed. To allow the electroencephalographic amplifiers to recover from any overload condition resulting from the high-voltage/high-frequency input during diathermy, acquisition is resumed only after a user-selectable time interval. The system was evaluated in 50 patients. It effectively interrupted SEP acquisition each time diathermy was used. By eliminating diathermy noise from the averaged waveforms, the system will allow evoked potential monitoring even during periods of intensive use of diathermy.

Artifacts↗
PubMed1996

Motion and image artifacts of various intracranial aneurysm clips in a magnetic field.

Deflection angles of 36 types of Sugita intracranial aneurysm clips (Elgiloy), 2 types of Yasargil clips (Phynox), a Kirschner chip (SUS316) and a silver clip were examined by suspending each in a 1.5-tesla MRI (TOSHIBA MRT-200/FXII). Image artifacts of 5 types of intracranial aneurysm clips were also studied quantitatively with respect to their ratio of artifact/actual clip size using a spin-echo sequence with T1-weighted (Time of repetition (TR) = 500 ms, Time of echo (TE) = 20 ms) and T2-weighted (TR = 2000 ms, TE = 80 ms) images. All the Sugita clips from mini type to long type showed a deflection angle of less than 1.0 degree, the Yasargil clips less than 2.0 degrees, the Kirschner chip less than 6.0 degrees, and the silver chip did not move at all. Both brands of clips are therefore suitable for intracranial applications. The artifact size was directly proportional to the clip size and weight, though the ratio of artifact/actual width tended to decrease proportionately as the actual size and weight increased. The ratio of the width seemed slightly bigger than that of the length, but the difference did not reach statistical significance. There was no significant difference in the ratio of image artifacts/clip size between T1- and T2-weighted images.

Artifacts↗
PubMed2000

Artifact reduction in electrogastrogram based on empirical mode decomposition method.

Severe contamination of the gastric signal in electrogastrogram (EGG) analysis by respiratory, motion, cardiac artifacts, and possible myoelectrical activity from other organs, poses a major challenge to EGG interpretation and analysis. A generally applicable method for removing a variety of artifacts from EGG recordings is proposed based on the empirical mode decomposition (EMD) method. This decomposition technique is adaptive, and appears to be uniquely suitable for nonlinear, non-stationary data analysis. The results show that this method, combined with instantaneous frequency analysis, effectively separate, identify and remove contamination from a wide variety of artifactual sources in EGG recordings.

Artifacts↗
PubMed2005

Removal of eye blinking artifact from the electro-encephalogram, incorporating a new constrained blind source separation algorithm.

A robust constrained blind source separation (CBSS) algorithm has been developed as an effective means to remove ocular artifacts (OAs) from electro-encephalograms (EEGs). Currently, clinicians reject a data segment if the patient blinked or spoke during the observation interval. The rejected data segment could contain important information masked by the artifact. In the CBSS technique, a reference signal was exploited as a constraint. The constrained problem was then converted to an unconstrained problem by means of non-linear penalty functions weighted by the penalty terms. This led to the modification of the overall cost function, which was then minimised with the natural gradient algorithm. The effectiveness of the algorithm was also examined for the removal of other interfering signals such as electrocardiograms. The CBSS algorithm was tested with ten sets of data containing OAs. The proposed algorithm yielded, on average, a 19% performance improvement over Parra's BSS algorithm for removing OAs.

Algorithms↗
PubMed1999

Removal of cardiac beat artifact in oesophageal pressure measurement by frequency analysis.

Oesophageal pressure (Pes) measurements are important in medical research and useful in clinical diagnosis. Measurements, however, are contaminated heavily by cardiac artifacts. The spectrum and waveform of the Pes signal is obtained from the oesophageal balloon. Adaptive finite impulse response (AFIR) filter and modified adaptive noise cancellation (MANC) methods are adopted to filter out cardiac beat interference. These results are compared. In the frequency domain, frequency variations and spectral overlap between the Pes components and cardiac beat signal components impact on the performance of the filter. From our experimental results on power strength, the fourth or higher harmonics did not have any significant effect on the filter performance. However, the second harmonics of these signals had a significant effect on the filtering result. Thus, in the design of AFIR filters, attention is needed to minimise these effects. In frequency analysis, these harmonics or overlapping frequencies do not affect MANC. MANC was the better method for eliminating cardiac beat artifact in Pes measurement. The dynamic compliance (Cdyn) was also used to evaluate the performance of MANC and AFIR. The standard deviation of Cdyn was less than 0.15 using MANC, compared with standard deviations as high as 0.57 for AFIR. We conclude that MANC performs better than AFIR.

Animals↗
PubMed2006

Goldvalve detachable balloon: "in vitro" assessment of safety and imaging artifacts in a 3-T MR system.

The Goldvalve balloon is the only currently available detachable balloon. We undertook a study to determine safety and imaging artifacts in a MR environment. We found no evidence for heating or deflection of the balloon in a comprehensive series of in vitro experiments at 3 T. MR imaging at field strengths up to 3 T of patients with implanted Goldvalve balloons is safe. Imaging artifacts are minimal.

Artifacts↗
PubMed2001

MRI of spinal hardware: comparison of conventional T1-weighted sequence with a new metal artifact reduction sequence.

OBJECTIVE: This study was designed to compare diagnostic quality of MR images of patients with spinal hardware acquired using a conventional T1-weighted spin-echo sequence and a new metal artifact reduction sequence (MARS). CONCLUSION: The new MARS sequence effectively reduces the degree of tissue-obscuring artifact produced by spinal fixation hardware and subjectively improves image quality compared with the conventional T1-weighted spin-echo sequence.

Adult↗
PubMed2005

Use of twinkling artifact in sonographic detection of intestinal pneumatosis.

Intestinal pneumatosis is a relatively rare condition that has been associated with a wide variety of abnormalities. Traditionally, this entity has been diagnosed by radiographic films, contrast series, or more accurately by computed tomography. However, reports describing findings of intestinal pneumatosis on sonographic examination, which is an important initial diagnostic tool for abdominal diseases, are limited. We suggest that twinkling artifact may be of value in the sonographic diagnosis of this condition. We report two cases of intestinal pneumatosis that were diagnosed initially by sonography with the aid of twinkling artifact.

Aged↗
PubMed2002

A retrospectively ECG-gated multislice spiral CT scan and reconstruction technique with suppression of heart pulsation artifacts for cardio-thoracic imaging with extended volume coverage.

A method for cardio-thoracic multislice spiral CT imaging with ECG gating for suppression of heart pulsation artifacts is introduced. The proposed technique offers extended volume coverage compared with standard ECG-gated spiral scan and reconstruction approaches for cardiac applications: Thin-slice data of the entire thorax can be acquired within one breath-hold period using a four-slice CT system. The extended volume coverage is enabled by a modified approach for ECG-gated image reconstruction. For a CT system with 0.5-s gantry rotation time, images are reconstructed with 250-ms image temporal resolution. Instead of selecting scan data acquired in exactly the same phase of the cardiac cycle for each image as in standard ECG-gated reconstruction techniques, the patient's ECG signal is used to omit scan data acquired during the systolic phase of highest cardiac motion. With this approach cardiac pulsation artifacts in CT studies of the aorta, of paracardiac lung segments, and of coronary bypass grafts can be effectively reduced.

Aortic Diseases↗
PubMed2003

Flow ghost artifact in slice-encoding direction mimicking vestibular schwannoma in contrast-enhanced 3D spoiled gradient-echo sequence.

The value of MR imaging by post-contrast T1-weighted 3D spoiled gradient-echo (3D SPGR) is well established for the detection of small vestibular schwannomas in the cerebellopontine angle region. We describe a case in which a flow ghost artifact in the slice-encoding direction mimicked a vestibular schwannoma and heavily T2-weighted MR cisternography and multiplanar reconstruction images helped us to reach the correct diagnosis. In addition, we conducted a volunteer study to demonstrate that changing the k-space trajectory can reduce this artifact in post-contrast 3D SPGR images.

Adult↗
PubMed2006

Impact of the z-flying focal spot on resolution and artifact behavior for a 64-slice spiral CT scanner.

The effect of the z-flying focal spot (zFFS) technology was evaluated by simulations and measurements with respect to resolution and artifact behavior for a 64-slice spiral cone-beam computed tomography (CT) scanner. The zFFS alternates between two z-positions of the X-ray focal spot, acquiring two slices per detector row, which results in double sampling in the z-direction. We implemented a modified reconstruction that is able to obtain images as they would be without zFFS. A delta phantom equipped with a thin gold disc was used to measure slice sensitivity profiles (SSP), and a high-contrast bar phantom was used to quantify the resolution in the x/z-plane with and without zFFS. The zFFS decreases the full width at half maximum (FWHM) of the SSPs by a factor of about 1.4. The double z-sampling allows the separation of 0.4 mm bars in the z-direction compared with 0.6 mm in the case without zFFS. The zFFS effectively reduces windmill artifacts in the reconstructed images while maintaining the transverse resolution, even at the largest available pitch value of 1.5.

Artifacts↗
PubMed1996

Clinical application of automatic integrative interpretation of awake background EEG: quantitative interpretation, report making, and detection of artifacts and reduced vigilance level.

Methods for the automatic detection of artifacts and vigilance level of the EEG record were developed for a preprocessing procedure for the automatic integrative interpretation of awake background EEG. All equations for the detection were derived such that they would conform to the procedure that an EEGer adopts for visual EEG inspection. The automatic EEG interpretation system was improved by adding the proposed preprocessing procedure and gave satisfactory interpretation results for the EEG data even contaminated with artifacts or in the drowsy state. The automatic EEG interpretation method proposed here is suggested for clinical use as an assistant tool for EEGers and physicians.

Arousal↗
PubMed1992

Filters for the reduction of baseline wander and muscle artifact in the ECG.

Contamination of the electrocardiographic (ECG) signal by extraneous electrical potentials is a significant problem in exercise electrocardiography. Baseline wander and muscle artifact are particularly troublesome sources of interference. This paper describes two digital filters that were constructed and found effective in reducing these two types of signal contamination. The baseline wander filter presented here is a linear phase high-pass filter having a cutoff frequency lower than the heart rate. The low computational overhead makes this filter practical for multiple channel implementation on a low-cost digital signal processor. Because the muscle artifact signal, generated by skeletal muscle activity, occupies the same spectrum space as the heart-generated signal, spectral filtering is not very effective in eliminating it. Described here is a time-varying filter that employs a combination of linear and nonlinear filtering techniques.

Action Potentials↗
PubMed1995

Errors and artifacts in coupled spectrophotometric assays of enzyme activity.

This paper considers problems associated with the accurate determination of enzyme activity using coupled spectrophotometric assays. Criteria for establishing optimum assay conditions and ensuring that the coupled assay accurately reflects enzyme activity are presented. The susceptibility of such assays to interference is illustrated by five instances in which contamination of specific assay components has produced misleading estimates of phosphofructokinase and pyrophosphate:fructose 6-phosphate phosphotransferase activity. Such artifacts have resulted in publication of spurious biochemical and physiological conclusions. These examples suggest that problems associated with contaminants are likely to be widespread in coupled spectrophotometric assays, and are likely to confound interpretation of the measurements. Strategies for identifying artifacts resulting from contaminants in coupled assays are proposed.

Artifacts↗
PubMed1994

Pseudoinvasion of vascular spaces: report of an artifact caused by cervical lidocaine injection prior to loop diathermy.

Cervical loop diathermy is a relatively new procedure performed by gynecologists to diagnose and treat squamous intraepithelial lesions. We report a case of pseudoinvasion of vascular spaces noted in an excisional biopsy of a high-grade squamous intraepithelial lesion of the cervix. The neoplastic epithelium was forced into the cervical stroma by injection of local anesthetic through the lesion prior to loop diathermy. The identification of this pseudovascular space invasion as artifact had important prognostic and therapeutic value. With the increasing use of loop diathermy and local anesthesia this type of artifact may be seen more commonly.

Adult↗