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PubMed1997

Artifacts in recording immediate mandibular translation: a laboratory investigation.

STATEMENT OF PROBLEM: Even though immediate mandibular translation has been extensively studied and analyzed, and its clinical significance emphasized, there is controversy as to whether it actually exists or whether it is just an artifact of the pantograph. PURPOSE: The aim of this study was to determine whether what appears to be pantographic evidence of immediate mandibular translation can actually be an artifact and, if so, to find a method to avoid it. MATERIAL AND METHODS: The first part of this article explains geometrically with computer vector graphics how pure rotation can produce a pantographic tracing on the horizontal plate that is identical to what would be seen for immediate mandibular translation. The second part of the article presents a technique that uses a modified pantograph that eliminates the rotational artifact and thus permits proper interpretation of true immediate mandibular translation. RESULTS: This study shows that pure rotation about the sagittal axis mimics immediate mandibular translation on a pantographic tracing when the plates are inferior to the transverse horizontal axis, produces scribings in an opposite direction for plates in a superior position, and produces no scribing when the plates are level with it. By modifying the pantograph so the tip of the scribing pin of the horizontal plate is level with the transverse horizontal axis, true immediate mandibular translation can most easily be differentiated from rotational artifact. CONCLUSIONS AND CLINICAL SIGNIFICANCE: Rotation can cause artifacts that mimic immediate mandibular translation. A technique to avoid this problem is presented. This technique provides the foundation for a valid evaluation of patients to determine whether true immediate mandibular translation exists and whether it is clinically important.

Artifacts↗
PubMed1999

A hybrid technique for spectroscopic imaging with reduced truncation artifact.

Traditionally, Fourier spectroscopic imaging is associated with a small k-space coverage which leads to truncation artifacts such as "bleeding" and ringing in the resultant image. Because substantial truncation artifacts mainly arise from regions having intense signals, such as the subcutaneous lipid in the head, effective reduction of truncation artifacts can be achieved by obtaining an extended k-space coverage for these regions. In this paper, a hybrid technique which employs phase-encoded spectroscopic imaging (SI) to cover the central portion of the k-space and echo-planar spectroscopic imaging (EPSI) to measure the peripheral portion of the k-space is developed. EPSI, despite its inherently low SNR characteristics, provides a sufficient SNR for outer high-spatial frequency components of the aforementioned high signal regions and supplies an extended k-space coverage of these regions for the reduction of truncation artifacts. The data processing includes steps designed to remove inconsistency between the two types of data and a previously described technique for selectively retaining only outer k-space information for the high signal regions during the reconstruction. Experimental studies, in both phantoms and normal volunteers, demonstrate that the hybrid technique provides significant reduction in truncation artifacts.

Artifacts↗
PubMed2000

Multiple signal integration by decision tree induction to detect artifacts in the neonatal intensive care unit.

The high incidence of false alarms in the intensive care unit (ICU) necessitates the development of improved alarming techniques. This study aimed to detect artifact patterns across multiple physiologic data signals from a neonatal ICU using decision tree induction. Approximately 200 h of bedside data were analyzed. Artifacts in the data streams were visually located and annotated retrospectively by an experienced clinician. Derived values were calculated for successively overlapping time intervals of raw values, and then used as feature attributes for the induction of models trying to classify 'artifact' versus 'not artifact' cases. The results are very promising, indicating that integration of multiple signals by applying a classification system to sets of values derived from physiologic data streams may be a viable approach to detecting artifacts in neonatal ICU data.

Artifacts↗
PubMed2000

Fast iterative algorithm for metal artifact reduction in X-ray CT.

RATIONALE AND OBJECTIVES: The reduction of metal artifacts in x-ray computed tomography (CT) has important clinical applications. An iterative method adapted from the expectation maximization (EM) formula for emission CT was shown to be effective for metal artifact reduction, but its computational speed is slow. The goal of this project was to accelerate that iterative method for metal artifact reduction. MATERIALS AND METHODS: Using the row-action/ordered-subset (EM) formula for emission CT as a basis, the authors developed a fast iterative algorithm for metal artifact reduction. In each iteration of this algorithm, both reprojection from an intermediate image and backprojection from discrepancy data are performed. RESULTS: The feasibility of the fast iterative algorithm was demonstrated in numerical and phantom experiments. In comparison with the nonaccelerated iterative algorithm, the speed of iterative metal artifact reduction is improved by an order of magnitude given image quality in terms of visual inspection, I-divergence in the projection domain, and the euclidean distance in the image domain. CONCLUSION: The fast iterative algorithm corrects intermediate reconstruction according to subsets of projections and produces satisfactory image quality at a much faster speed than the previously published iterative algorithm. This algorithm has important potential in clinical applications, such as orthopedic, oncologic, and dental imaging.

Algorithms↗
PubMed2003

Artifacts in T(1rho)-weighted imaging: correction with a self-compensating spin-locking pulse.

Significant artifacts arise in T(1rho)-weighted imaging when nutation angles suffer small deviations from their expected values. These artifacts vary with spin-locking time and amplitude, severely limiting attempts to perform quantitative imaging or measurement of T(1rho) relaxation times. A theoretical model explaining the origin of these artifacts is presented in the context of a T(1rho)-prepared fast spin-echo imaging sequence. Experimentally obtained artifacts are compared to those predicted by theory and related to B(1) inhomogeneity. Finally, a "self-compensating" spin-locking preparatory pulse cluster is presented, in which the second half of the spin-locking pulse is phase-shifted by 180 degrees. Use of this pulse sequence maintains relatively uniform signal intensity despite large variations in flip angle, greatly reducing artifacts in T(1rho)-weighted imaging.

Artifacts↗
PubMed2000

Removal of eye activity artifacts from visual event-related potentials in normal and clinical subjects.

OBJECTIVES: Electrical potentials produced by blinks and eye movements present serious problems for electroencephalographic (EEG) and event-related potential (ERP) data interpretation and analysis, particularly for analysis of data from some clinical populations. Often, all epochs contaminated by large eye artifacts are rejected as unusable, though this may prove unacceptable when blinks and eye movements occur frequently. METHODS: Frontal channels are often used as reference signals to regress out eye artifacts, but inevitably portions of relevant EEG signals also appearing in EOG channels are thereby eliminated or mixed into other scalp channels. A generally applicable adaptive method for removing artifacts from EEG records based on blind source separation by independent component analysis (ICA) (Neural Computation 7 (1995) 1129; Neural Computation 10(8) (1998) 2103; Neural Computation 11(2) (1999) 606) overcomes these limitations. RESULTS: Results on EEG data collected from 28 normal controls and 22 clinical subjects performing a visual selective attention task show that ICA can be used to effectively detect, separate and remove ocular artifacts from even strongly contaminated EEG recordings. The results compare favorably to those obtained using rejection or regression methods. CONCLUSIONS: The ICA method can preserve ERP contributions from all of the recorded trials and all the recorded data channels, even when none of the single trials are artifact-free.

Adult↗
PubMed2001

Gas/solid partitioning of semivolatile organic compounds (SOCs) to air filters. 3. An analysis of gas adsorption artifacts in measurements of atmospheric SOCs and organic carbon (OC). WHen using teflon membrane filters and quartz fiber filters.

Adsorption of gaseous semivolatile organic compounds (SOCs) onto the filter(s) of a filter/sorbent sampler is a potential source of measurement error when determining specific SOCs as well as organic carbon (OC) levels in the atmosphere. This work examines partitioning to both Teflon membrane filters (TMFs) and quartz fiber filters (QFFs) for purposes of predicting the magnitude of the compound-dependent gas adsorption artifact as a function of various sampling parameters. The examination is based on values of Kp,face (m3 cm(-2)), the gas/filter partition coefficient expressed as [ng sorbed per cm2 of filter face]/[ng per m3 in the gas phase]. Values of Kp,face were calculated based on literature values of the gas/solid partition coefficient Kp,s [ng sorbed per m2 of filter]/[ng per m3 in gas phase] for the adsorption of various polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzodioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) to TMFs, and for the adsorption of PAHs to QFFs. At relative humidity (RH) values below approximately 50%, the Kp,face values for PAHs are lower on TMFs than on ambient-backup QFFs. The gas adsorption artifact will therefore be lower for PAHs with TMFs than with QFFs under these conditions. In the past, corrections for the gas/filter adsorption artifact have been made by using a backup filter, and subtracting the mass amount of each compound found on the backup filter from the total (particle phase + sorbed on filter) amount found on the front filter. This procedure assumes that the ng cm(-2) amounts of each SOC sorbed on the front and backup filters are equal. That assumption will only be valid after both filters have reached equilibrium with each of the gaseous SOCs in the incoming sample air. The front filter will reach equilibrium first. The minimum air sample volume Vmin,f+b required to reach gas/filter sorption equilibrium with a pair of filters is 2Kp,face Afilter where Afilter (cm2) is the per-filter face area. Kp,face values, and therefore Vmin,f+b values, depend on the compound, relative humidity (RH), temperature, and filter type. Compound-dependent Vmin,f+b values are presented for PAHs and PCDD/Fs on both TMFs and QFFs. Compound-dependent equations which give the magnitude of the filter adsorption artifact are presented for a range of different sampling arrangements and circumstances. The equations are not intended for use in actually correcting field data because of uncertainties in actual field values of relevant parameters such as the compound-dependent Kp,face and gas/particle Kp values, and because of the fact that the equations assume ideal step-function chromatographic movement of gas-phase compounds through the adsorbing filter. Rather, the main utility of the equations is as guidance tools in designing field sampling efforts that utilize filter/sorbent samplers and in evaluating prior work. The results indicate that some backup-filter-based corrections described in the literature were carried out using sample volumes that were too small to allow proper correction for the gas adsorption artifactfor some specific SOCs of interest. Similar conclusions are reached regarding artifacts associated with the measurement of gaseous and particulate OC.

Adsorption↗
PubMed2001

MRI artifact masquerading as orbital disease.

OBJECTIVES: To describe the diagnostic challenge associated with magnetic resonance imaging (MRI) artifact and to describe methods of minimizing this artifact. METHODS: Gadolinium enhancement was demonstrated to be artifact by using an inversion recovery pulse sequence technique. RESULTS: A patient with complaints of painless loss of vision also had radiologic abnormalities that led to subsequent referral to a tertiary care facility. Increased signals on the fat-suppressed contrast enhanced MRI was demonstrated to be artifact by using an inversion recovery technique which produced a normal MRI of the orbit. CONCLUSIONS: Clinical confusion leading to unnecessary and costly evaluation may be avoided if one considers the possibility of artifact when reviewing magnetic resonance studies. When magnetic susceptibility is suspected, the use of an inversion recovery sequence can better delineate the true nature of the abnormality.

Adult↗
PubMed1999

Metallic residues as a source of artifacts in magnetic resonance imaging of the temporomandibular joint.

OBJECTIVES: To establish the source of metallic artifacts on postoperative MRI of the temporomandibular joint after arthroplasty. METHODS: An experiment was carried out to evaluate MR artifacts which were thought to have been caused by minute metallic particles deposited at surgery. Four types of cutting instruments were used to cut an extracted tooth: a diamond bur attached to a dental air turbine handpiece, a steel bur and bone bur attached to a dental micromotor handpiece and a bone file. The tooth fragments were collected in plastic centrifuge tubes in water. These samples were evaluated with six MR sequences. RESULTS: MR artifacts were clearly demonstrated in samples using the bone bur and the bone file but not the diamond and steel burs. CONCLUSIONS: The artifacts seen on postoperative MRI were derived from minute metal particles from the bone bur or file used for condylar arthroplasty. Metal artifacts should be considered when interpreting postoperative MR images.

Adult↗
PubMed2005

Artifacts in 3D rotational angiography: an experimental study.

PURPOSE: To investigate artifacts in three-dimensional rotational angiography (3D-RA) in an experimental model and to evaluate which parameters influence their distribution. MATERIAL AND METHODS: 3D-RA was carried out in a circular vessel phantom filled with contrast medium. Two different rotational angulations were used: 160 degrees causing 64 images and 180 degrees causing 90 or 120 images. The images were transferred to one workstation for reconstruction of axial slices and then to another workstation for 3D reconstructions. The 3D reconstructions were compared with standardized threshold settings. RESULTS: The artifacts occurred where the vessel had a longer path parallel to the rotation plane and became increasingly pronounced when the threshold level was raised. The artifacts decreased in size when rotation angle and number of projections were increased. CONCLUSION: The quality of the 3D reconstructions from RA was degraded by beam-hardening and sampling artifacts. The sampling artifacts were diminished by increasing both the rotation angle and the number of projections. The distortions in the 3D reconstructions caused by beam-hardening remain to be resolved. The threshold values also had a considerable influence on the 3D reconstructions.

Angiography↗
PubMed2005

Three-dimensional rotational angiography of transplanted renal arteries: influence of an extended angle of rotation on beam-hardening artifacts.

PURPOSE: To investigate whether three-dimensional rotational angiography (3D-RA) of the transplant renal artery performed with an extended angle of rotation can reduce beam-hardening artifacts in 3D reconstructed images without image quality being lost or side effects to the transplanted kidney being increased. MATERIAL AND METHODS: 3D-RA with a C-arm rotation of 180 degrees was performed consecutively in 12 renal transplanted patients with suspicion of renal artery stenosis. A 1.7-mm balloon occlusion catheter was placed using the crossover technique and this was compared to a protocol with 160 degrees rotation and a traditional 1.4-mm catheter in 10 patients. The occurrence of beam-hardening artifacts was registered and the effects of the reduced contrast load on image quality and of arterial occlusion on renal function were assessed. RESULTS: The extended angle of rotation, from 160 degrees to 180 degrees, reduced the beam-hardening artifacts. Artifacts were observed in 4/11 patients (36%) in the study group and in all 10 (100%) of the controls. There was no statistical difference regarding image quality between the two protocols. Renal function was equally affected in both protocols. CONCLUSION: 3D-RA with an extended C-arm rotation reduced the beam-hardening artifacts. Image quality was not reduced despite the reduced contrast medium load. The different protocols had no effect on patient outcome.

Adult↗
PubMed1993

Detection of a GC/MS artifact peak as methamphetamine.

Recently, certain laboratories reported the presence of methamphetamine in several negative urine specimens. We have demonstrated an artifact peak, by SIM GC/MS, in negative urine specimens, that frequently matches the retention time and identity ion ratio criteria for methamphetamine. This peak resulted from the presence of high levels of pseudoephedrine (PS) or ephedrine (EP) and was produced after derivatization with 4-carbethoxyhexafluorobutyryl chloride (CB), heptafluorobutyric anhydride (HFB), and N-trifluoroacetyl-l-prolyl chloride (TFAP). Use of N-methyl trideuterated PS and two deuterated EP compounds in spiked samples provided additional proof that PS and EP can produce the artifact peak. The above results were achieved using a GC injection port temperature of 300 degrees C and low split and isothermal conditions. Thermal transformation in the injector is involved because lowering and raising the injector temperature results in the disappearance and subsequent reappearance of the artifact peak. Proof that the artifact peak is actually methamphetamine was provided by full-scan GC/MS data of the artifact peak produced after use of all three derivatizing reagents. GC/MS analysis has demonstrated that urine (65 specimens) with concentrations of PS or EP ranging from 100,000 to 1,100,000 ng/mL will test negative for methamphetamine, while the same specimens will trigger positive results by radioimmunoassay and the Emit monoclonal assay. Full-scan GC/MS of the CB derivative for PS and EP reveals what appears to be both mono- and diderivatized products. SIM analysis can differentiate between PS and EP in urine specimens because the relative amounts of mono- and diderivatized products are very different for PS than for EP.(ABSTRACT TRUNCATED AT 250 WORDS)

Artifacts↗
PubMedDate not supplied

Conductance artifacts in a novel in vitro model of ventriculothoracic electrical coupling.

The utility of open chest conductance (COND) ventriculography is limited by artifacts altering the relationship between COND and left ventricular (LV) volume. Pressure-COND loops often lean to the left during LV volume reduction by caval occlusion. Time varying alterations in the pericardial-LV contact area affect electrical coupling in the open chest during the cardiac cycle, producing COND artifacts. In this study, an open-mediastinum model was constructed. Components represented the LV, blood, pericardium, and thoracic contents. Varying ventriculothoracic coupling was simulated by changing the volume of pericardial saline (0, 30, 60 ml). Raw dual field COND was repeatedly (n = 20) compared with volumes of normal saline from 60 to 120 ml at 5 ml intervals. Groups were compared by linear regression and repeated measures ANOVA. Artifacts significantly (p < 0.01) altered parallel COND, indicated by the y-intercept, with the exception of 0 versus 30 ml. The slope constant also changed significantly, with the exception of 30 versus 60 ml. These results suggest that variable pericardial-LV contact can cause time varying artifacts in COND in the open chest. Therefore, posterior insulation may reduce artifacts in COND ventriculography and should be tested for this effect.

Analysis of Variance↗
PubMed2000

Reduction of motion artifacts in magnetic resonance imaging of the neck and cervical spine by long-term averaging.

RATIONALE AND OBJECTIVES: We performed a prospective comparison of T1-weighted turbo spin-echo (TSE) imaging with standard averaging and with the long-term averaging method (LOTA), comparing the effects on signal-to-artifact noise ratio (S/aN) and motion artifacts. METHODS: In 30 consecutive patients undergoing imaging of the neck or cervical spine, a transverse T1-weighted TSE sequence was applied with and without LOTA by using identical sequence parameters. Quantitative image analysis was done by calculating S/Ns in the phase-encoding direction (S/aN). Visual image analysis was performed by four independent, masked readers using a standardized score sheet for anatomic and pathological findings. RESULTS: The LOTA sequence yielded significantly superior S/aN values compared with the standard averaging sequence. In the subjective evaluation, the LOTA sequence showed significantly fewer motion artifacts and better visualization of normal anatomy of the neck, cervical spine, and spinal cord, as well as of the pathological findings. CONCLUSIONS: LOTA is a valuable method for increasing S/aN in magnetic resonance imaging of the neck and cervical spine. It reduces motion artifacts and increases the conspicuity of pathology without increasing acquisition time. No additional hardware is needed, and this technique can be combined with other artifact-reducing methods.

Adult↗
PubMedDate not supplied

Total hip prosthesis metal-artifact suppression using iterative deblurring reconstruction.

PURPOSE: CT of total joint prostheses is limited by metal artifact produced mainly by missing projection data. Iterative deblurring reconstruction is less sensitive to missing projection data than filtered backprojection. A software CT simulator was used to compare total hip prosthesis images reconstructed using standard filtered backprojection, filtered backprojection after linear interpolation of missing data, and iterative deblurring. METHOD: Unilateral and bilateral total hip replacements with metal-backed or all-polyethylene acetabular prostheses were simulated using bone, metal, and polyethylene annuli and circles of metal and water (soft tissue). Material attenuation properties were taken from the literature. The simulation assumed that no X-rays penetrated the metal. Simulated projection data were reconstructed using filtered backprojection, filtered backprojection after linear interpolation of missing data, and iterative deblurring. Visual observations and objective region-of-interest analyses were made. RESULTS: Even with no X-rays penetrating the metal, iterative deblurring produced almost no visible artifact within the bone or soft tissues. Bone edge detection and sizing were more easily and accurately done from the iterative deblurring images. All reconstruction techniques underestimated bone and water CT attenuation values. Metal artifact was worse for the bilateral or metal-backed prostheses. CONCLUSION: Iterative deblurring generated nearly metal-artifact-free images in this simulation. Filtered backprojection, even after linear interpolation, produced typical clinical metal-artifact images.

Artifacts↗
PubMedDate not supplied

Hand exercise during contrast medium delivery at thoracic helical CT: a simple method to minimize perivenous artifact.

PURPOSE: The purpose of this work was to assess the effects of hand exercise on perivenous artifact caused by undiluted venous contrast material at thoracic helical CT. METHOD: Eighty patients were prospectively and randomly assigned to thoracic helical CT with (n = 42) or without (n = 38) intermittent squeezing of a hand-sized ball during delivery of a contrast material. Two radiologists graded perivenous artifact and arterial enhancement in a blind fashion. CT attenuation values were obtained by region-of-interest measurements from arteries and veins. RESULTS: Both qualitative and quantitative analyses showed statistically significant differences in the assessment of perivenous artifact (p < 0.01). Perivenous artifact from the subclavian vein was significantly reduced in patients subjected to thoracic helical CT by using a hand exercise method. CONCLUSION: Hand exercise during contrast material delivery at thoracic helical CT minimizes perivenous artifact and improves image quality.

Adult↗
PubMed2001

A new method for pulse oximetry possessing inherent insensitivity to artifact.

A new method for pulse oximetry is presented that possesses an inherent insensitivity to corruption by motion artifact, a primary limitation in the practical accuracy and clinical applicability of current technology. Artifact corruption of the underlying photoplethysmographic signals is reduced in real time, using an electronic processing methodology that is based upon inversion of a physical artifact model. This fundamental approach has the potential to provide uninterrupted output and superior accuracy under conditions of sustained subject motion, therefore, widening the clinical scope of this useful measurement. A new calibration technique for oxygen saturation is developed for use with these processed signals, which is shown to be a generalization of the classical interpretation. The detailed theoretical and practical issues of implementation are then explored, highlighting important engineering simplifications implicit in this new approach. A quantitative investigation of the degree of insensitivity to artifact is also undertaken, with the aid of a custom electronic system and commercial pulse oximeter probes, which is compared and contrasted with the performance of a conventional implementation. It is demonstrated that this new methodology results in a reduced sensitivity to common classes of motion artifact, while retaining the generality to be combined with conventional signal processing techniques.

Artifacts↗
PubMed2006

Removal of ballistocardiogram artifacts from simultaneously recorded EEG and fMRI data using independent component analysis.

Simultaneous recording of electroencephalogram (EEG) and functional magnetic resonance imaging (fMRI) has been studied to identify areas related to EEG events. EEG data recorded in the magnetic resonance (MR) scanner with MR imaging is suffered from two specific artifacts, imaging artifact, and ballistocardiogram (BCG). In this paper, we focus on BCG. In preceding studies, average subtraction was often used for this purpose. However, average subtraction requires an assumption that BCG waveforms are precisely periodic, which seems unrealistic because BCG is a biomedical artifact. We propose the application of independent component analysis (ICA) with a postprocessing of high-pass filtering for the removal of BCG. With this approach, it is not necessary to assume that the BCG waveform is periodic. Empirically, we show that our proposed method removes BCG artifacts as well as does the average subtraction method. Power spectral density analysis of the two approaches shows that, with ICA, distortion of recovered EEG data is also as small as that associated with the average subtraction approach. We also propose a hypothesis for how head movement causes BCGs and show why ICA can remove BCG artifacts arising from this source.

Algorithms↗