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At least 469 records · Page 26Linked to original sources
PubMed2005

Artifact reduction in magnetogastrography using fast independent component analysis.

The analysis of magnetogastrographic (MGG) signals has been limited to epochs of data with limited interference from extraneous signal components that are often present and may even dominate MGG data. Such artifacts can be of both biological (cardiac, intestinal and muscular activities, motion artifacts, etc) and non-biological (environmental noise) origin. Conventional methods-such as Butterworth and Tchebyshev filters-can be of great use, but there are many disadvantages associated with them as well as with other typical filtering methods because a large amount of useful biological information can be lost, and there are many trade-offs between various filtering methods. Moreover, conventional filtering cannot always fully address the physicality of the signal-processing problem in terms of extracting specific signals due to particular biological sources of interest such as the stomach, heart and bowel. In this paper, we demonstrate the use of fast independent component analysis (FICA) for the removal of both biological and non-biological artifacts from multi-channel MGG recordings acquired using a superconducting quantum intereference device (SQUID) magnetometer. Specifically, we show that the signal of gastric electrical control activity (ECA) can be isolated from SQUID data as an independent component even in the presence of severe motion, cardiac and respiratory artifacts. The accuracy of the method is analyzed by comparing FICA-extracted versus electrode-measured respiratory signals. It is concluded that, with this method, reliable results may be obtained for a wide array of magnetic recording scenarios.

Action Potentials↗
PubMed1998

Biopsy bag artifact.

We describe a distinctive tissue artifact that results from the use of biopsy bags for processing small impressionable pieces of tissue. In its fully developed form, the artifact produces a tic-tac-toe pattern, while in less pronounced cases it may produce elongated oval spaces in the tissue or a serrated contour at the periphery of the tissue. The artifact was observed in 60% of endometrial specimens, 55% of endocervical curettings, and sporadically in other small specimens. In the endometrial and endocervical specimens, the artifact was focal and did not interfere with the diagnosis. In occasional small lung and prostate specimens, there was focally significant distortion that potentially could compromise the diagnosis.

Artifacts↗
PubMed1999

The use of modified primers to eliminate cycle sequencing artifacts.

Cycle sequencing is the workhorse of DNA sequencing projects, allowing the production of large amounts of product from relatively little template. This cycling regime, which is aimed at linear growth of the desired products, can also produce artifacts by exponential amplification of minor side-products. These artifacts can interfere with sequence determination. In an attempt to allow linear but prevent exponential growth of products, and thus eliminate artifacts, we have investigated the use of primers containing modified residues that cannot be replicated by DNA polymerase. Specifically, we have used primers containing 2'- O -methyl RNA residues or abasic residues. Oligomers consisting of six DNA residues and 20 2'- O -methyl RNA residues, with the DNA residues located at the 3'-end, primed as efficiently as DNA primers but would not support exponential amplification. Oligonucleotides containing fewer DNA residues were not used as efficiently as primers. DNA primers containing a single abasic site located six residues from the 3'-end also showed efficient priming ability without yielding exponential amplification products. Together these results demonstrate that certain types of modified primers can be used to eliminate artifacts in DNA sequencing. The technique should be particularly useful in protocols involving large numbers of cycles, such as direct sequencing of BAC and genomic DNA.

Artifacts↗
PubMedDate not supplied

Transcutaneous electrical nerve stimulator-induced electrocardiogram artifact. A brief report.

Transcutaneous electrical nerve stimulation (TENS) is an effective modality for the treatment of pain. TENS use is well tolerated and has relatively few complications. Potentially serious complications of TENS use include disruption of cardiac pacemaker function and electrical artifact during electrocardiographic monitoring. Electrical interference with cardiac monitoring using parasternal electrodes and electrocardiographic (EKG) artifact with an epidural stimulator has been reported. We report the case of EKG artifact produced by surface TENS electrodes applied to the thoracic and lumbar regions. The electrical signal produced by the TENS unit created an electrocardiographic artifact resembling a pacemaker spike on routine EKG. This was interpreted as a malfunctioning cardiac pacemaker. The easily misinterpreted electrical spikes that can result from TENS use as seen in this case highlights the need for physician awareness of this potential complication.

Aged↗
PubMed1994

The "count loss" artifact in Tc-99m HMPAO leukocyte SPECT of the abdomen.

During SPECT studies of the abdomen with Tc-99m labeled autologous leukocytes, the authors had observed false defects involving the vertebral bodies of the lower thoracic spine in two patients. To investigate this "count loss" artifact, a phantom simulating liver, spleen, and spine was constructed. Four clinical situations were simulated by positioning cylinders in the different locations: normal spine-spleen model, normal liver-spine-spleen model, spine-enlarged spleen model, and enlarged liver-spine-enlarged spleen model. SPECT studies were performed at different concentration ratios between the spleen and the spine on the disparate models with varying distance from the spleen to the spine. The results showed that the count loss artifact is not only related to the concentration ratio between the structures but also is related to the size, position, and distance between the structures. The correction steps for this artifact include: 1) identifying average count value of the spine and spleen from ECT sinogram count data, and 2) replacing the original high-count value in the spleen by the count value close to the average value at the region of the spine in the 120 raw projection data. Using these new modified ECT data, the reconstructed orthogonal images are almost free of the count loss artifact.

Abdomen↗
PubMed2000

SPECT attenuation artifacts in normal and overweight persons: insights from a retrospective comparison of Rb-82 positron emission tomography and TI-201 SPECT myocardial perfusion imaging.

PURPOSE: Myocardial perfusion imaging can be performed using SPECT or positron emission tomography (PET). SPECT has lower specificity than PET, largely as a result of attenuation artifacts; however, it is more widely available. The authors describe a study of the effect of sex and body weight on the incidence of SPECT attenuation artifacts using a retrospective comparison of TI-201 SPECT and Rb PET. METHODS: One hundred sixty-one persons (101 men, 60 women; 81 normal weight, 80 overweight) underwent TI-201 SPECT and Rb PET. The incidence of observed perfusion defects was studied in territories of the three major coronary arteries. SPECT and PET results were also compared with those of angiography in a subset of 75 patients. RESULTS: One hundred fourteen defects were reported on Rb PET compared with 176 defects with TI-201 SPECT. Excess TI-201 SPECT defects occurred in male and female, normal-weight and overweight persons. The average specificity was 64% for TI-201 SPECT and 84% for Rb PET, reflecting this difference. CONCLUSIONS: Attenuation artifacts in TI-201 SPECT occur frequently and are not confined to easily identifiable subgroups of patients. Therefore, measures to improve specificity of SPECT (e.g., prone or gated imaging) or alternative imaging techniques such as PET have potential advantages for everyone, not simply for obese patients and women with large breasts. In addition, awareness of the prevalence of SPECT attenuation artifacts, in both sexes and all weight categories, may contribute to improved accuracy of interpretation.

Artifacts↗
PubMed2000

An improved method of reducing stimulus artifact in the electrically evoked whole-nerve potential.

OBJECTIVE: Recording a compound action potential in response to electrical stimulation requires attention to minimize contamination due to electrical stimulus artifact. In patients implanted with the Nucleus 24 device, the electrically evoked whole-nerve potential (EAP) is recorded using a neural response telemetry (NRT) system. This system employs a forward-masking technique that greatly reduces stimulus artifact. However, theoretical considerations and experimental animal data suggest that the technique may distort the acquired EAP waveform under some situations. We proposed and evaluated a modification to the forward-masking technique that addresses this concern, particularly during collection of refractory recovery data. DESIGN: We first examined neural responses of the electrically stimulated auditory nerve using cat preparations. Through single-fiber recordings from cats, we demonstrated underlying physiological limitations likely encountered with the "standard" forward masking technique. We then recorded feline EAP waveforms using both the standard technique and our proposed, modified, technique. Finally, we collected EAP data from human cochlear implant patients using both artifact reduction methods. These comparisons allowed us to evaluate the effectiveness of our modification. RESULTS: The cat EAP data demonstrated that the standard forward-masking technique currently in use in the Nucleus NRT system can distort the EAP waveform when the nerve is partially refractory. In the cat, this distortion resulted in forward-masking recovery curves with artifactually prolonged recovery times and inaccurate latency trends. Similar effects were observed in the comparison of human recovery curves obtained using both the standard and modified techniques. In some cases, the modified technique produced EAP waveforms with more clearly defined peaks than were obtainable with the standard method. CONCLUSIONS: Consideration should be given to implementing our modified forward-masking artifact reduction scheme, because it introduces less distortion of the EAP waveform and accordingly provides for more accurate assessment of the refractory properties of the electrically stimulated nerve.

Action Potentials↗
PubMed1994

Low-density supradiaphragmatic band artifact on advanced multiple-beam equalization radiography of the chest.

RATIONALE AND OBJECTIVES: A low-density dark band artifact was found above the diaphragm on advanced multiple-beam equalization radiography (AMBER) of the chest and was evaluated. METHODS: Fifty consecutive chest radiographs were evaluated for the presence of the artifact. AMBER radiographs of a phantom were scanned with a microdensitometer. RESULTS AND CONCLUSION: The presence of the artifact is confirmed on the patient chest radiographs (present in 84% on posteroanterior view; 94% on lateral view) and on the phantom images. The artifact was probably caused by a delayed system response to scanning across a sharp boundary.

Artifacts↗
PubMed2002

A novel EEG artifact in the intensive care unit.

The authors describe a novel EEG artifact in two patients whose records were otherwise suppressed. The saw-toothed waveform, containing 5.5- and 11-Hz rhythms, was induced as a movement artifact by a hemoperfusion machine connected to the patient. The artifact is highly characteristic, but could be easily misinterpreted as a cerebral rhythm if one is not aware of the entity. The artifact should be suspected when invariant, saw-toothed rhythms appear beyond the temporal and occipital lobes, and also contaminate the electrocardiographic channel. It can be proved by its disappearance on stopping and reemergence with starting the rotary pump action of the device.

Amitriptyline↗
PubMedDate not supplied

MR artifacts after anterior cervical diskectomy and fusion: a cadaver study.

OBJECTIVE: To identify the source of metallic artifacts observed on cervical spine MR studies following anterior cervical diskectomy and fusion (ACD&F), preoperative and postoperative MR studies were done on cadavers using standard spin echo T1, T2, and GRASS imaging. MATERIALS AND METHODS: An ACD&F was performed on each cadaver at various levels using different combinations of drill burrs, curettes, and suction tips. RESULTS: Results showed that contact between the drill burr and suction tip produced the worst artifacts; however, artifacts still occurred in cases in which the suction tip was not introduced. CONCLUSION: This observation suggests that metal flakes from the curettes also produced artifacts. Image distortion was greater on GRASS imaging than on the spin echo imaging.

Artifacts↗
PubMedDate not supplied

CT of metal implants: reduction of artifacts using an extended CT scale technique.

PURPOSE: The purpose of this work was to use an extended CT scale technique (ECTS) to reduce artifacts due to metal implants and to optimize CT imaging parameters for metal implants using an experimental model. METHOD: Osteotomies were performed in 20 porcine femur specimens. One hundred cobalt-base screws and 24 steel plates were used for osteosynthesis in these specimens. Artificial lesions were produced in 50 screws, such as osteolysis near the screws (mimicking lysis due to infection, tumor, or loosening), displacement of the screws, as well as fractures of the screws. All specimens were examined using eight different CT protocols: four conventional (CCT) and four spiral (SCT) CT protocols with different milliampere-second values (130 and 480 mAs for CCT, 130 and 300 mAs for SCT), kilovolt potentials (120 and 140 kVp), and slice thicknesses (2 and 5 mm). The images were analyzed by three observers using a standard window (maximum window width 4,000 HU) and ECTS (maximum window width 40,000 HU). Receiver operating characteristic analysis was performed, and image quality was assessed according to a five level scale. RESULTS: Metal artifacts were significantly reduced using ECTS (p < 0.05). The highest diagnostic performance was obtained using ECTS with the thinnest slice thickness. Metal artifacts were more pronounced using SCT. In this experimental model, exposure dose and kilovolt potential had no significant impact on diagnostic performance (p > 0.05). CONCLUSION: ECTS improved imaging of metal implants. In this study, no significant effects of exposure dose and kilovolt potential were noted. Metal artifacts were more prominent using SCT than using CCT.

Animals↗
PubMed2004

Contrast dynamics during CT pulmonary angiogram: analysis of an inspiration associated artifact.

OBJECTIVE: Several artifacts have been observed during contrast-enhanced CT of the pulmonary arteries. We describe a physiological artifact caused by a transient interruption of the contrast column in the pulmonary arteries associated with inspiration immediately prior to imaging. This results from a variable inflow of unopacified blood from the inferior vena cava (IVC). MATERIALS AND METHODS: From 327 consecutive pulmonary CT-angiograms, all performed on a single detector scanner at 3 mm collimation (1.5 mm incremental reconstruction), 50 positive studies, 46 indeterminate studies, and 33 negative studies (129 exams) were retrospectively reviewed by a blinded observer to determine the frequency of the described contrast interruption, its severity (mild, moderate, or severe), and its possible contribution to misinterpretation of studies. The numerical change in Hounsfield units was assigned within the right ventricular chamber for each examination to correlate with the subjective evaluation of severity. Statistical significance was determined with P = 0.05%. RESULTS: The artifact was present in 48 (37.2%) of the 129 evaluated studies. It was greater in frequency (50.0%) with the negative studies. The presence was 25% with positive studies and 36.7% with indeterminate exams. The interruption was more often mild (<100 HU change) in severity (45.8%). Three (6.6%) definite false positives were detected where the misinterpretation was directly attributed to the artifact. Three (6.6%) other examinations called positive were also directly related to the interrupted contrast column. However, since no further pulmonary vascular evaluation was performed, these examinations can only be considered indeterminate. Two of the latter 3 studies demonstrated a severe (>150 HU change) and the other study demonstrated a moderate (100-150 HU) interruption of contrast opacification. CONCLUSIONS: During inspiration, there is a variable increase in unopacified venous blood from the IVC, briefly diluting the contrast column entering from the SVC. This interruption is common, though usually mild in severity. However, a short severe interruption of vascular opacification can lead to misinterpretation as a pulmonary embolus or contribute to an indeterminate examination.

Adult↗
PubMed2001

Clinical measurement, artifact, and data analysis in dioptric power space.

It appears now to be recognized that traditional clinical representations of astigmatic power, including sphere, cylinder, and axis, in particular, do not lend themselves directly to satisfactory quantitative analysis. For purposes of analysis, the clinical representations need first to be transformed into representations in dioptric power space. It turns out, however, that characteristics of the clinical measurements carried over into dioptric power space can be a source of spurious conclusions reached in such studies. The source of the problem lies in the nature of sphere, cylinder, and axis and in the discreteness (multiples of 0.25 D, usually, in sphere and cylinder and 1 or 5 degrees in axis) of the clinical measurements. As a consequence, scatter plots of clinical measurements in dioptric power space may show patterns and structures, including clusters, arcs, and moiré effects. All the structures are artifacts of the discreteness in sphere, cylinder, and axis; they have no other physical basis. Furthermore, the clinical measurements can show departures from normality that are also purely artifact. By learning to recognize artifact in the scatter plots, the researcher can overcome some of the problems. But because of the assumption of normality underlying many statistical procedures, the problem of distortions in the distributions remains. The distortions may weaken the confidence with which inferences can be made or even lead to erroneous conclusions. The purpose of this paper is to draw attention to the problems of artifact in analyses of clinical data and to suggest ways of overcoming it or avoiding it at least in part.

Artifacts↗
PubMed1998

Metallic artifacts in magnetic resonance imaging of patients with spinal fusion. A comparison of implant materials and imaging sequences.

STUDY DESIGN: Devices for spinal fusion were compared with respect to their influence on magnetic resonance images. In addition, different magnetic resonance pulse sequences were evaluated to elicit their susceptibility to imaging artifacts. OBJECTIVES: To determine the implants with the least imaging artifacts as a recommendation for the spine surgeon and to assess the best imaging strategy for the radiologist. SUMMARY OF BACKGROUND DATA: For patients who have had surgical spinal fusion with instrumentation, magnetic resonance imaging is the most favorable diagnostic method. Unfortunately, metallic implants lead to severe degradation of image quality. These artifacts depend on the material of the implant and on the choice of the pulse sequence. METHODS: The fusion devices were mounted on a simple plastic phantom in various combinations and were imaged on 1.5-T magnetic resonance units. Frequently used types of plates and screws made of titanium or steel in various alloys were examined on the phantom with routinely used pulse sequences. The results of these examinations were compared with those in patient studies involving the same implants as well as the same pulse sequences. RESULTS: The least imaging artifacts were caused by titanium implants, especially when using shorter screws, wider screw placement, and thinner titanium plates. Nevertheless, there were distinct image distortions that could lead to erroneous image interpretation. The best images were acquired by spin echo (T1), turbo spin echo (T1, T2), and half Fourier single shot turbo spin echo (HASTE) sequences. Sequences containing any gradient echo components (gradient echo or turbo gradient and spin echo sequence or frequency-selective fat saturation techniques) resulted in the highest amount of image degradation. CONCLUSION: By choosing appropriate spinal fusion devices as well as pulse sequences, postoperative magnetic resonance imaging examinations can give acceptable results, in spite of the presence of metallic implants.

Adolescent↗
PubMed2004

New metallic MR stents for artifact-free coronary MR angiography: feasibility study in a swine model.

RATIONALE AND OBJECTIVES: The objective of this study was to investigate the potential for artifact-free coronary magnetic resonance angiography (cMRA) in the presence of dedicated metallic MR stents in vitro and in a swine model. METHODS: All investigations were performed at 1.5 T, applying a standard cMRA gradient echo sequence with a T2 preparation pulse. Two prototypes of each hand-woven, mechanically woven, and lasered Aachen Resonance Coronary MR Stents made out of an MR-compatible metallic alloy and dilated to 2.5 mm and 4 mm were examined in a water bath. RESULTS: Artifact behavior was judged independently by 2 radiologists as showing "no artifacts" for all tested stent types. Signal-to-noise ratios inside and outside of the stents were measured yielding a Pearson correlation coefficient of 0.98 (y = 1.22 + 0.92x). Nineteen stents (8 hand woven, 3 mechanically woven, 8 lasered) were deployed in coronary arteries of 19 domestic pigs and were examined by cMRA. Artifact behavior of the stents was analyzed by measuring the signal-to-noise ratio at the stent positions and compared with signal-to-noise ratio measurements outside of the stents, yielding a Pearson correlation coefficient of 0.90 (y = -0.75 + 1.06x). CONCLUSIONS: All 3 prototypes of coronary MR stents allowed complete visualization of the stent lumen and consequently determination of stent patency by cMRA.

Animals↗
PubMed2005

In vivo evaluation of the carotid wallstent on three-dimensional contrast material-enhanced MR angiography: influence of artifacts on the visibility of stent lumina.

PURPOSE: Contrast material-enhanced magnetic resonance (MR) angiography is increasingly used in postinterventional imaging after implantation of endovascular stents. The main limitations are stent-related artifacts compromising the visibility of the stent lumen. The aim of this in vivo study is the evaluation of contrast-enhanced MR angiography imaging characteristics of the carotid Wallstent. MATERIALS AND METHODS: The carotid arteries of 29 patients were examined with contrast-enhanced MR angiography 3-6 days and/or 7-23 months after implantation of a carotid Wallstent into the internal carotid artery. Images were evaluated with regard to the diameter and signal intensity (SI) of the visible stent lumen. Digital subtraction angiography (DSA) was used as the standard of reference. RESULTS: Stent-related artifacts on contrast-enhanced MR angiography caused an artificial lumen narrowing and a reduction of the SI within the stent. Artifacts were pronounced on imaging 3-6 days after stent implantation, but 68% of stents imaged 7-23 months after stent implantation presented with a significantly decreased artificial signal reduction and an improved visibility of the stent lumen. CONCLUSIONS: The results of this study indicate that a reliable evaluation of the stent lumen is limited as a result of an artificial decrease of the SI inside the stent. However, in follow-up examinations 7-23 months after stent implantation, visibility of the stent lumen was improved and diagnostic reliability of contrast-enhanced MR angiography was markedly increased. A probable explanation for this phenomenon might be the formation of a neointimal layer covering the stent struts and thereby reducing stent-related artifacts.

Adult↗
PubMed1994

Computing genetic similarity coefficients from RAPD data: the effects of PCR artifacts.

Random amplified polymorphic DNA (RAPD) markers have been used for many types of genetic analyses, including genome mapping, genotype fingerprinting, phylogeny reconstruction, and measuring genetic similarities. They suffer from one potential limitation, however, because the PCR that is used to produce informative amplification products often produces artifactual products as well. Optimization of PCR protocols to eliminate artifactual bands completely is often too costly or too time-consuming to be practical. Other methods for handling RAPD artifacts, such as deleting inconsistent or faint bands or using only those bands that are reproducible, introduce false negatives into the data. Simply ignoring artifacts and using all bands introduces false positives. When RAPD data are used to compute genetic similarity coefficients, such artifacts can cause significant bias in the estimation. The three coefficients most widely used with RAPD data, the simple matching coefficient, Jaccard's coefficient and Nei and Li's coefficient, differ in the amount of bias produced by a given level of artifactual bands. The simple matching coefficient and Nei and Li's coefficient always exhibit less percent bias than Jaccard's coefficient. For closely related organisms, Nei and Li's coefficient displays less percent bias than the simple matching coefficient. If new DNA samples possessing RAPD markers not present in the previously analyzed samples are added to a study, values of the simple matching coefficient will need to be computed for all samples, not just the new ones. Jaccard's and Nei and Li's coefficients, however, will not need to be recomputed. Furthermore, only Nei and Li's coefficient has a direct biological meaning (it is an estimate of the expected proportion of amplified fragments shared by two samples because they were inherited from a common ancestor). On the basis of these results, Nei and Li's coefficient is recommended for routine computation of genetic similarities using RAPD data, particularly if PCR artifacts are present.

Artifacts↗
PubMed2000

X-ray CT metal artifact reduction using wavelets: an application for imaging total hip prostheses.

Traditional computed tomography (CT) reconstructions of total joint prostheses are limited by metal artifacts from corrupted projection data. Published metal artifact reduction methods are based on the assumption that severe attenuation of X-rays by prostheses renders corresponding portions of projection data unavailable, hence the "missing" data are either avoided (in iterative reconstruction) or interpolated (in filtered backprojection with data completion; typically, with filling data "gaps" via linear functions). In this paper, we propose a wavelet-based multiresolution analysis method for metal artifact reduction, in which information is extracted from corrupted projection data. The wavelet method improves image quality by a successive interpolation in the wavelet domain. Theoretical analysis and experimental results demonstrate that the metal artifacts due to both photon starving and beam hardening can be effectively suppressed using our method. As compared to the filtered backprojection after linear interpolation, the wavelet-based reconstruction is significantly more accurate for depiction of anatomical structures, especially in the immediate neighborhood of the prostheses. This superior imaging precision is highly advantageous in geometric modeling for fitting hip prostheses.

Artifacts↗