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At least 559 records · Page 31Linked to original sources
PubMedDate not supplied

A two-parameter method for the estimation of ultrasound-induced temperature artifacts.

A two-parameter method for the estimation of ultrasound-induced temperature artifacts was evaluated and compared with other commonly applied methods using analytical solutions to the bioheat equation. The two parameters are the exponent of the assumed temperature decay curve after power is turned off and the baseline temperature. These parameters are found by optimizing the fit of the temperature data from 30 to 60s after power is turned off. The artifact is modelled as a point source at the centre of a Gaussian temperature distribution. The blood flow, baseline temperature, and variance of the Gaussian temperature distribution were varied to simulate different clinical situations. Noise was added to the model to investigate the effects of thermometry resolution and sampling intervals. It was found that for artifacts of < 2 degrees C the two-parameter method had errors of less than 0.25 degrees C, whereas other methods generally had greater errors depending on the conduction rate and blood flow rate. The effects of the temperature sampling interval and resolution on the ability of the methods to estimate the artifact were also investigated, and it was found that the two-parameter method was much more sensitive to these parameters than other commonly applied methods.

Blood Flow Velocity↗
PubMed2005

On the Ewald artifacts in computer simulations. The test-case of the octaalanine peptide with charged termini.

The treatment of electrostatic interactions in molecular simulations is of fundamental importance. Ewald and related methods are being increasingly used to the detriment of cutoff schemes, which are known to produce several artifacts. A potential drawback of the Ewald method is the spatial periodicity that is imposed to the system, which could produce artifacts when applied in the simulation of liquids. In this work we analyze the octaalanine peptide with charged termini in explicit solvent, for which severe effects due to the use of Ewald sums were predicted using continuum electrostatics. Molecular Dynamics simulations for a total of 158 nanoseconds were performed in cells of different sizes. From the comparison of the results of different system sizes, no significant periodicity-induced artifacts were observed. It is argued that in current biomolecular simulations, the incomplete sampling is likely to affect the results to a larger extent than the artifacts induced by the use of Ewald sums.

Alanine↗
PubMed1984

Bone marrow biopsy technic. Artifact induced by aspiration.

Bone marrow aspiration and trephine biopsy specimens are of additive value when assessing various marrow disorders. Many physicians obtain both an aspirate and a trephine core when taking a biopsy of bone marrow, but opinions differ about which biopsy specimen should be obtained first. Some authors mention the possibility of artifactual hypocellularity and hemorrhage in the trephine biopsy specimen when trephine biopsy follows aspiration at the same site. This report describes the occurrence and significance of bone marrow aspiration artifact. The artifact usually occupies a relatively small area of the biopsy specimen and satisfactory evaluation of the marrow is still possible. However, the artifact occasionally hinders marrow evaluation, particularly when the trephine biopsy specimen is relatively short. Because the artifact is not observed in specimens obtained away from the aspiration site, it is best to perform the trephine biopsy before aspiration, or away from the aspiration site, especially when technical problems prevent obtaining a large amount of trephine material.

Biopsy, Needle↗
PubMed1978

Polychromatic streak artifacts in computed tomography images.

A series of artifacts in computed tomography (CT) images, consisting of streaks coming off edges and flares surrounding the object being scanned, is shown to be due to polychromaticity of the X-ray beam. These artifacts are seldom seen on conventional CT brain scans. They are helpful in understanding artifacts such as those seen around the petrous bone of the skull. Following this, a comparison is made between "single energy" and "dual energy" correction schemes for poly-artifacts. The theoretical assumptions underlying either method are analyzed, and the importance of these concepts in dual energy reconstructions is stressed. A new test phantom is suggested to evaluate different poly-correction schemes. All the simulated projection data in this study are noise free and are reconstructed using an industrial convolution-back projection technique. In addition, reconstructions are shown of the phantom using real data taken with and without a water bag path length compensator.

Mathematics↗
PubMedDate not supplied

Artifacts in computed tomography of the posterior fossa: a comparative phantom study.

A phantom consisting of a human skull fixed in a cylinder containing water was used to study the occurrence of artifacts and their influence on image quality in CT of the posterior fossa. Only minor differences in interpetrous bone artifact and in deviation in CT numbers in other parts of the posterior fossa were found between eight tested scanner models. Interpetrous bone artifact was not significantly affected by slice thickness but it could be reduced to some extent by proper selection of the scan angle and almost eliminated by application of a dual-energy technique. Other bone-related artifacts, however, were found to be dependent on slice thickness.

Evaluation Studies as Topic↗
PubMedDate not supplied

Artifacts in MR imaging after surgical intervention.

Artifacts are encountered repeatedly in magnetic resonance (MR) imaging after neurological orthopedic surgery, although there is no evidence of metallic foreign bodies on radiography and CT. Experiments demonstrated that surgical filling materials, such as bone cement and bone transplants are not the cause of the artifacts. On the other hand, we have been able to show that short-term contact between a diamond drill and untempered operating instruments can cause abrasions and set free minute metallic fragments. Those particles are missed by X-ray procedures including CT but cause MR artifacts by local disturbance of the homogeneity of the magnetic field. Therefore, MR represents a highly sensitive method to trace extremely small amounts of magnetic substances. Postsurgical MR, however, may be of limited value because of the disturbing artifacts.

Animals↗
PubMedDate not supplied

Evaluation of CT techniques for reducing artifacts in the presence of metallic orthopedic implants.

Metallic intramedullary orthopedic implants generate artifacts that can markedly degrade transaxial CT images. The artifacts, typically seen as starburst streaking, result primarily from reconstructions involving missing projection data. Two approaches are clinically available to reduce the artifacts around orthopedic implants. These are (a) the imaging of implants with lower attenuation coefficients or smaller path lengths (less attenuating objects) and (b) the planar reformatting of image data. The sizing accuracy of these two approaches was quantified using phantoms and the efficacy using cadaveric femoral specimens. Results demonstrated that metal artifacts may be reduced and accurate bony dimensional data obtained.

Evaluation Studies as Topic↗
PubMedDate not supplied

Cranial CT artifacts and gantry angulation.

Computed tomographic investigation of the brain is frequently accomplished by scanning parallel to the canthomeatal line. In these scans, evaluation of the brain stem and neighboring structures is often compromised by streak artifacts from bone-induced beam hardening. These artifacts may be reduced by introducing a change in gantry angulation. Results from a phantom study as well as from a clinical trial offer evidence that may be helpful in assessing different scanning planes. An inclination of 5 degrees below Reid baseline is suggested to minimize the interpetrous artifact. Variations in slice thickness and exposure parameters do not appear to affect significantly this typical posterior fossa artifact.

Algorithms↗
PubMed2001

Is it real or is it an artifact.

Sonographic artifacts are inherent to the modality and occur to some extent in every image. This is despite optimal technique and scanning parameters. Artifacts occur in both gray-scale and color Doppler imaging, and though many are readily recognizable as such, occasionally, an artifact may obscure the diagnosis or even create false pathology. This article illustrates the common and some of the more unusual sonographic artifacts that may be encountered, reviews the factors leading to their formation, and discusses strategies to recognize and minimize these misleading echoes.

Journal Article↗
PubMed2000

CPR artifact removal from human ECG using optimal multichannel filtering.

The purpose of this study was to assess whether the artifacts presented by precordial compressions during cardiopulmonary resuscitation could be removed from the human electrocardiogram (ECG) using a filtering approach. This would allow analysis and defibrillator charging during ongoing precordial compressions yielding a very important clinical improvement to the treatment of cardiac arrest patients. In this investigation we started with noise-free human ECGs with ventricular fibrillation (VF) and ventricular tachycardia (VT) records. To simulate a realistic resuscitation situation, we added a weighted artifact signal to the human ECG, where the weight factor was chosen to provide the desired signal-to-noise ratio (SNR) level. As artifact signals we used ECGs recorded from animals in asystole during precordial compressions at rates 60, 90, and 120 compressions/min. The compression depth and the thorax impedance was also recorded. In a real-life situation such reference signals are available and, using an adaptive multichannel Wiener filter, we construct an estimate of the artifact signal, which subsequently can be subtracted from the noisy human ECG signal. The success of the proposed method is demonstrated through graphic examples, SNR, and rhythm classification evaluations.

Biomedical Engineering↗
PubMed2005

Young children's rapid learning about artifacts.

Tool use is central to interdisciplinary debates about the evolution and distinctiveness of human intelligence, yet little is actually known about how human conceptions of artifacts develop. Results across these two studies show that even 2-year-olds approach artifacts in ways distinct from captive tool-using monkeys. Contrary to adult intuition, children do not treat all objects with appropriate properties as equally good means to an end. Instead, they use social information to rapidly form enduring artifact categories. After only one exposure to an artifact's functional use, children will construe the tool as 'for' that particular purpose and, furthermore, avoid using it for another feasible purpose. This teleo-functional tendency to categorize tools by intentional use represents a precursor to the design stance - the adult-like tendency to understand objects in terms of intended function - and provides an early foundation for apparently distinctive human abilities in efficient long-term tool use and design.

Adult↗
PubMedDate not supplied

Nonlinear partial volume artifacts in x-ray computed tomography.

Dark streaks connecting the petrous bones are often observed in cranial transverse section CT reconstructions. These artifacts are usually only slightly diminished by two-pass beam hardening corrections. However, it is found that by narrowing the slice thickness the artifacts are substantially reduced. In this paper, it is shown that axial partial volume effects can account for the presence of the artifacts. These axial partial volume effects occur when, at any point in the slice, the object has axial variations in attenuation. In such cases the logarithm of the integrated intensity measured by the detector is not a linear function of the integrated attenuation (even for monochromatic beams). This nonlinearity causes inconsistencies in the data set which in turn can cause streaks in the image. We have studied the partial volume effect using computer simulation. Algorithms are presented whose purpose is to correct for these effects by estimating the axial variation using neighboring slices. These correction algorithms are successful in computer simulation cases but failed with clinical data. It is concluded that no practical correction method is viable unless overlap scanning is employed. However, thin-slice scanning for sections where these artifacts are common is perhaps a more preferable solution.

Brain↗
PubMedDate not supplied

An algorithm for the reduction of metal clip artifacts in CT reconstructions.

Implanted surgical metal clips often produce objectionable artifacts in CT reconstructions. The artifacts appear as streaks which emanate radially from the site of the clip. It is shown in this paper that these artifacts stem primarily from motion of the clip during the scan. An algorithm is described which reduces the intensity of these artifacts. The procedure attempts to remove the metal object entirely from the scan data by replacing the measured projection values of rays that passed through a neighborhood of the clip with calculated values consistent with an object whose density is an average of the surround. Examples are given for head and body scans as well as for computer simulations which show substantial reduction of the streak intensity.

Humans↗
PubMedDate not supplied

An analytical approach to quantify uniformity artifacts for circular and noncircular detector motion in single photon emission computed tomography imaging.

Uniformity artifacts in rotating gamma camera tomography will result if there are errors in the correction factors which are routinely calculated from a static uniformity flood image. The accuracy of the correction factors is a function of the statistics in the collected flood image. Since the factors are applied to each projection view, an error in a correction factor will propagate as a projection error at the same pixel location for each view. For circular detector motion, the error in each projection is reconstructed as a ring whose maximum amplitude varies approximately inversely proportional to the square root of the distance of the projection error from the center of rotation. For noncircular detector motion the artifacts are not rings but are more complicated geometric curves. Simulations show that statistical fluctuations in the reconstructed image will mask the uniformity artifacts provided the correction flood satisfies minimum count requirements. An analytical expression is derived for the percent root-mean-square (% rms) error in the reconstruction and is compared with the percent relative amplitude error (% RAE) of the reconstructed artifacts in order to obtain expressions for uniformity flood counting statistics. For an elliptical source distribution with total counts equal to CT, the uniformity statistics required to reconstruct elliptical disks is inversely proportional to the square root of the area: U greater than or equal to KCT/area 1/2. The constant K depends on the filter function and type of detector motion.

Biophysical Phenomena↗
PubMed1995

A comparison of beam-hardening artifacts in x-ray computerized tomography with gadolinium and iodine contrast agents.

Lanthanide-based compounds such as Gd-DTPA are currently used as contrast agents in MRI. Recent experiments using CT and transmission radiography show that Gd agents can increase image contrast by up to a factor of 2 relative to more commonly used iodinated agents on an equi-molar basis. It has also been suggested that beam hardening artifacts may be reduced with Gd. This hypothesis was experimentally tested on three different CT scanners using a circular water equivalent phantom with contrast filled tube inserts. It was found that the artifacts were a factor of 1.3-1.8 more pronounced with the iodinated contrast compared with Gd-DTPA. A theoretical model which uses an experimentally derived photon energy spectrum is proposed which relates the strength of beam hardening artifacts to the variance (over the energy spectrum) of the attenuation coefficient of the contrast agent. This allows easy assessment of the relative magnitudes of the artifact for different contrast agents.

Biophysical Phenomena↗
PubMed1985

Quantitative determination of microbial activity and community nutritional status in estuarine sediments: evidence for a disturbance artifact.

In estuarine sediments with a high degree of vertical heterogeneity in reduced substrate and terminal electron acceptor concentrations, the method of exposure of the microbiota to labeled substrates can introduce a "disturbance artifact" into measures of metabolic activity. The detection of this artifact is based on quantitative measurement of the relative rates of incorporation of [14C]acetate into phospholipid fatty acids (PLFA) and endogenous storage lipid, poly-beta-hydroxyalkanoate (PHA). Previous studies have shown that PLFA synthesis measures cellular growth and that PHA synthesis measures carbon accumulation (unbalanced growth). The "disturbance artifact" of exposure to [14C]acetate was demonstrated by comparing injection of a core with the usual or pore-water replacement or slurry techniques. Only injection of labeled substrate allowed detection of preassay disturbance of the sediment with a garden rake. The raking increased PLFA synthesis with little effect to differences in concentration or distribution of [14C]acetate in the 10-min incubation. Bioturbation induced by sand dollar feeding in estuarine sediment could be detected in an increased PLFA/PHA ratio which was due to decreased PHA synthesis if the addition of labeled substrate was by the injection technique. Addition of labeled precursors to sediment by slurry or pore-water replacement induces greater disturbance artifacts than injection techniques.

Acetates↗
PubMed1987

Spatial presaturation: a method for suppressing flow artifacts and improving depiction of vascular anatomy in MR imaging.

In clinical magnetic resonance (MR) imaging, the diagnostic quality of examinations is often degraded by streaklike flow artifacts that obscure anatomic details and reduce contrast. In addition, vascular structures are often not depicted clearly because the desired flow voids are obliterated by spurious intraluminal signals. On the basis of analysis of the physical mechanism of flow artifact formation, the authors developed a new technique for suppressing these artifacts. This applies interleaved, spectrally shaped radio frequency pulses to selectively saturate spins located in regions outside the image volume. In phantom, volunteer, and clinical imaging studies, the technique has proved to be effective by yielding a striking reduction in flow artifacts and markedly improving the reliability with which arterial and venous structures are imaged. The method has few drawbacks: It is applicable to most MR pulse sequences and, in principle, can be implemented on most imagers. It is particularly helpful for high-resolution surface coil studies of the neck, mediastinal imaging, gated cardiac imaging, and for detecting thrombus and other intravascular lesions such as dissections.

Blood Vessels↗
PubMed1988

MR imaging artifacts, ferromagnetism, and magnetic torque of intravascular filters, stents, and coils.

Experiments were conducted in which various intravascular filters, stents, and coils were imaged using magnetic resonance (MR) spin-echo technique at 0.35 T. These devices were also evaluated for ferromagnetism (at 0.35, 1.5, and 4.7 T), magnetic torque (at 0.35 and 1.5 T), and magnetically induced migration within a plastic tube (at 0.35 and 1.5 T for the Greenfield filter [GF]). The stainless-steel GF was evaluated in vitro for its propensity to perforate canine inferior venae cavae (IVC). Magnetic force and torque at 1.5 T did not dislodge the GF or result in perforation of canine IVC by the GF. Beta-3 titanium alloy (used in a new percutaneous version of the GF) is apparently one of the best-suited metals for use with MR imaging because of its lack of ferromagnetism (up to 4.7 T) and absence of MR imaging artifacts (at 0.35 T). Devices composed of Elgiloy (Mobin-Uddin filter), nitinol, and MP32-N (Amplatz filter) alloys all created mild artifacts. Devices fashioned from 304 and 316L (GF and Palmaz stent) stainless-steel alloys created severe "black-hole" artifacts, with the 304 alloy devices also showing marked image distortion. Generally, the greater the ferromagnetism of a device, the greater its magnetic susceptibility artifact.

Filtration↗