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At least 163 records · Page 9Linked to original sources
PubMed2000

Influence of image acquisition parameters on CT artifacts and polyp depiction in spiral CT colonography: in vitro evaluation.

PURPOSE: To quantify the effects of spiral computed tomographic (CT) acquisition parameters on the magnitude of three-dimensional (3D) rippling artifacts and polyp depiction. MATERIALS AND METHODS: An in vitro colon phantom was constructed with air-filled acrylic cylinders that contained synthetic polyps of 3-13 mm. The phantom was submerged in fluid and positioned at four angles of inclination relative to the z axis. Image data were acquired at collimation and pitch combinations of 3 mm and 1.67 and 5 mm and 1.6, respectively. Rippling artifacts were quantified by measuring the longitudinal variation of in-plane phantom edge width, and the influence of these artifacts on the depiction of pedunculated and sessile polyps was assessed qualitatively. RESULTS: The in-plane magnitude of the rippling artifact was a function of the angle of inclination relative to the longitudinal axis and the table increment. The through-plane periodicity of the artifact was equal to one-half the table increment. CONCLUSION: The table increment and angle of inclination of the surface of the object relative to the z axis determine the periodicity and magnitude of the rippling artifact at 3D spiral CT colonography. Although the depiction of small pedunculated polyps was not compromised, some sessile polyps were degraded by the artifact.

Artifacts↗
PubMed1993

Computerized detection of cerebral emboli and discrimination from artifact using Doppler ultrasound.

BACKGROUND AND PURPOSE: Transcranial Doppler ultrasound can detect circulating cerebral emboli. Monitoring of patients with potential embolic sources may allow identification of high-risk patients who can then be selected for prophylactic treatment. However, practical patient monitoring will require automated programs that can detect emboli and differentiate them from artifact. METHODS: A new off-line algorithm for the detection of emboli, which detects the characteristic relative power increase occurring with an embolus, was evaluated in both an animal model and in patients. (1) In a sheep model, solid embolic materials (thrombus, platelet aggregates, and atheroma) were introduced into the proximal carotid artery while the distal carotid artery or a major branch was insonated. The signals resulting from 77 emboli (mean size, 1.77 mm) were studied and compared with the Doppler signals resulting from artifact. (2) In patients, 100 embolic signals occurring in three patients were analyzed and compared with signals associated with artifact in the same patients. RESULTS: (1) In the sheep model, emboli resulted in a short-duration, high-intensity signal, but intensity increase alone did not distinguish between emboli and artifact. In contrast, the algorithm discriminated embolus from artifact with a sensitivity of 98.7% and a specificity of 98.0%. (2) In patient studies, embolic signals were differentiated from artifact with a sensitivity of 97.2% and a specificity of 97.0% by the algorithm. CONCLUSIONS: Using such an algorithm, detection of cerebral emboli and discrimination from artifact are possible with a high sensitivity and specificity. Incorporation of such an algorithm into an on-line system should make prolonged patient monitoring practical.

Algorithms↗
PubMed2006

Effects of high-resolution CT of the lung using partial versus full reconstruction on motion artifacts and image noise.

OBJECTIVE: The purpose of our study was to evaluate the effects of 0.3-second high-resolution CT (HRCT) of the lung using partial reconstruction on cardiac motion artifacts and image noise. SUBJECTS AND METHODS: Thirty-seven pairs of 0.3-second (partial reconstruction) and 0.75-second (full reconstruction) HRCT images were obtained for the lower lung zone during full-inspiration breath-holding. Imaging parameters other than temporal resolution were identical for each patient. Two radiologists visually graded motion artifacts of the cardiac border, bronchi, pulmonary vessels, and fissure in the left lung on a 4-point scale (with 4 indicating no artifacts). The maximum width of motion along the left cardiac border and the area percentage of motion artifacts in the left lung were calculated. Image noise in the air and lung was also determined. Cardiac motion artifacts and image noises were compared between the two sets of CT images. RESULTS: Visual grades for the cardiac border (4 +/- 0), bronchi (3.8 +/- 0.7), pulmonary vessels (3.6 +/- 0.8), and fissure (3.9 +/- 0.5) were higher for 0.3-second images than for 0.75-second images (1.7 +/- 0.7, 2.0 +/- 1.0, 1.6 +/- 0.7, and 2.4 +/- 0.9, respectively) (p < 0.001). The maximum width of motion along the left cardiac border (0.1 +/- 0.5 mm) and the area percentage of motion artifacts in the left lung (6.7% +/- 18.4%) were smaller for 0.3-second images than for 0.75-second images (4.5 +/- 1.7 mm and 36.2% +/- 20.9%, respectively) (p < 0.001). Image noises in the air (38.0 +/- 9.2) and the lung (86.0 +/- 23.1) were greater for 0.3-second images than for 0.75-second images (35.6 +/- 9.6 and 76.0 +/- 20.3, respectively) (p < 0.01). CONCLUSION: Compared with 0.75-second HRCT using full reconstruction, 0.3-second HRCT using partial reconstruction substantially reduces cardiac motion artifacts in the lung at the expense of increasing image noise.

Adolescent↗
PubMed2006

[Artifact of indicators equipped to stereotactic body frame scanned by 16-row MDCT can be resolved].

We found an artifact of the Stereotactic Body Frame's (SBF) indicators when we used diagnostic 16-row multi-detector computed tomography (MDCT) as a simulator. Stereotactic radiotherapy (SRT) requires the accuracy of each millimeter. However, a gap of 3 mm at the maximum along the Z axis was caused by this artifact. We assessed the characteristics and cause of this artifact and searched for an imaging parameter to reduce the artifact in SRT. It was considered that the artifact was formed mainly by a stair-step artifact and cone-beam artifact. We can acquired accuracy, 1 mm minimizing the beam width and pitch (10 mm/0.5625). However, exposure time was extended because minimum pitch was selected. The influence on dose calculation was negligible. We decided to use a default parameter for treatment planning, and this parameter for determining the isocenter. We found the MDCT parameters to get acceptable positional accuracy for SRT with SBF.

Artifacts↗
PubMed2002

[Report on the 86th Scientific Assembly and Annual Meeting of the Radiological Society of North America: motion artifact caused by heart pulsation observed on an electron beam and a multidetector-row CT].

UNLABELLED: Recently developed technologies in CT imaging have allowed higher temporal resolution and less motion artifacts caused by heart pulsation. However, complete deletion of motion artifacts has not altogether been accomplished. In interpreting images, differentiation of motion artifacts and true lesion is essential. In this exhibition, the ascending aorta, outer contour of the heart, valve, lung adjacent to the heart, and other structures in the mediastinum are demonstrated focusing on the characteristics of motion artifact. Images obtained with an electron beam CT (Imatron 150XL) and a MDCT (LightSpeed QX/i, GEMS) are comparatively demonstrated. In electron beam CT, the temporal resolution ranged from 50 msec to 2,000 msec. In multidetector-row CT, half reconstruction algorithm (500 msec) and newly developed algorithm (130 msec) can be applied to reduce temporal resolution, although temporal resolution (800msec) of a full scan is fixed. Degree, shape, and site at which motion artifacts arise are analyzed with respect to the temporal resolution on two different CT systems. LEARNING OBJECTIVES: Motion artifacts are correlated with the temporal resolution of the CT scanner. Electron beam and multidetector row CT provides different kinds of solutions to reduce motion artifacts.

Algorithms↗
PubMed1999

Artifact detection in cardiovascular time series monitoring data from preterm infants.

Artifacts in clinical intensive care monitoring lead to false alarms and complicate data analysis. They must be identified and processed to obtain true information. In this paper, we present a method for detecting artifacts in heart-rate (HR) and mean blood-pressure (BP) data from a physiological monitoring system used in preterm infants. The method uses three different types of artifact detectors: limit-based detectors, deviation-based detectors, and correlation-based detectors. Each identifies artifacts in the monitoring data from a different perspective. By integrating the individual detectors, we develop a parametric artifact detector, called CVDetector. The CVDetector is parametric because its performance depends on the specific values for the parameters in its component detectors. In a huge space of CVDetector instances, we have successfully discovered an optimal CVDetector instance, denoted by CVDetector. The sensitivity and specificity of CVDetector for HR artifacts is 94.8% (SD = 7.6%) and 90.6% (SD = 6.9%), respectively. The sensitivity and specificity of CVDetector for BP artifacts is 94.2% (SD = 5.3%) and 80.0% (SD = 12.4%), respectively.

Artifacts↗
PubMed2000

Streak artifact reduction in filtered backprojection using a level line-based interpolation method.

UNLABELLED: Because of the limited number of projections, the mathematic reconstruction formula of the filtered backprojection (FBP) algorithm may create an artifact that streaks reconstructed images. This artifact can be imperfectly removed by replacing the ramp filter of the FBP with an ad hoc low-pass filter, the cost being the loss of contrast and definition. In this study, a solution was proposed to increase, by computational means, the number of projections to reduce the artifact at a lower cost. The cost was a postacquisition process, which was reasonably time consuming. METHODS: The process was called interpolation of projections by contouring (IPC). First, level lines were plotted on the sinogram to delimit isocount regions; then, the regions containing the interpolated points were found, and to each point was assigned the intensity of its isocount region. Using this process, the data could be resampled, allowing an increase in the number of projections or the number of pixels by projections. A phantom study of bone scintigraphy was performed to compare the slices obtained with and without the IPC process with the true image. A clinical case was also presented. RESULTS: The phantom study showed that with the IPC process, the reconstructed slice was closer to the model, inside and outside the body, when the sinogram was resampled to multiply by 2 or 3 the number of projections, with the same number of pixels per projection. In the clinical study, the streak artifact was reduced, especially outside the body, although only a ramp filter was used. CONCLUSION: The IPC process succeeded in reducing the streak artifact. This process did not require any modification in acquisition and was not operator dependent. The increase in the number of projections is likely a necessary but not a sufficient condition to reduce the streak artifact: if not corrected, the attenuation could be a limiting factor in the removal of this artifact when the number of projections increases.

Algorithms↗
PubMed2001

Reduction of CSF and blood flow artifacts on FLAIR images of the brain with k-space reordered by inversion time at each slice position (KRISP).

BACKGROUND AND PURPOSE: Our purpose was to test a new variant of the fluid-attenuated inversion-recovery (FLAIR) sequence that was designed to reduce CSF and blood flow artifacts by use of a non-slice-selective inversion pulse and k-space reordered by inversion time at each slice position (KRISP). METHODS: With the KRISP FLAIR sequence, the slice order was cycled so that each inversion time (TI) was associated with a region of k-space rather than a particular slice, and the effective inversion time (TI(eff)) was chosen to null the signal from CSF. Scans were obtained with both conventional and KRISP FLAIR sequences. Studies were performed in 20 adult patients with a variety of brain diseases. Images were evaluated for artifacts from patient motion, CSF, and blood flow, and scored on a four-point scale. The conspicuity of the cortex, meninges, ventricular system, brain stem, and cerebellum was evaluated, as was lesion number and conspicuity. RESULTS: The KRISP FLAIR sequence showed more patient motion artifacts but had a pronounced advantage over the conventional sequence in control of CSF artifacts around the foramen of Munro, in the third ventricle, aqueduct, and fourth ventricle, as well as in the basal cisterns and around the brain stem and cerebellum. Blood flow artifacts from the internal carotid, basilar, and vertebral arteries were also much better controlled. Spurious high signal in the sylvian branches of the middle cerebral artery was eliminated. The meninges, cortex, ventricular system, brain stem, and cerebellum were better seen due to improved artifact suppression and an edge enhancement effect. CONCLUSION: The KRISP FLAIR sequence can suppress CSF and blood flow artifacts and improve the conspicuity of the meninges, cortex, brain stem, and cerebellum. Its major disadvantage is its duration, which may be reducible with a fast spin-echo version.

Adolescent↗
PubMed1992

Motion associated susceptibility artifacts.

A bowel labeling agent is important for improving abdominal MR. Besides providing contrast between the bowel and other organs, the contrast agent itself is a potential source of artifacts. The artifacts created by superparamagnetic particles (SPP) subjected to motion have been studied in vitro at 0.5 T, and compared to artifacts created by a paramagnetic compound. Apart from the expected static effects of the SPP, movement induced additional artifacts were seen as signal displacements in the phase-encoding direction. The artifacts were obvious at an iron concentration of 1 mg Fe/ml, barely visible at 0.2 mg Fe/ml, and completely absent at 0.1 mg Fe/ml. Artifacts were also evident with the SPP outside the imaging slice. This further emphasizes the importance of choosing the lowest effective dose when using SPP contrast agents. For the paramagnetic agent, motion propagated artifacts consisted of high and low signal regions in a mosaic pattern.

Artifacts↗
PubMed1995

Radiographic artifacts.

Radiographic artifacts commonly occur, particularly with hand processing. The artifacts may originate between the X-ray tube and the cassette as extraneous material on the patient or contamination of positioning aids, or result from debris within the cassette, or damage to, or staining of the screens. These artifacts are white to grey, may have a constant or different position on follow-up radiographs, and their size and shape are reflective of the inciting cause. A number of artifacts may occur in the darkroom during handling, developing, fixing and drying of the film. White to shiny artifacts are caused by the contamination of films with fixer, inability of developer to reach parts of the film or loss of emulsion from the developed film. Black artifacts result from improper handling or storage of films, resulting in exposure to light, or from pressure marks or static electricity discharges. Dropped levels of hand-processing chemicals may result in a variety of tide-marks on films. Most radiographic artifacts can be prevented by proper storage and handling of films and by optimal darkroom technique.

Animals↗
PubMed1998

Removing non-random artifacts from patch clamp traces.

Analysis and interpretation of current records from cellular membranes or lipid bilayers is complicated by perturbations. These perturbations can originate from random noise or deterministic artifacts as capacitive currents caused by step depolarization's, drift, 50 Hz pick up or microphonics. Accurate removal of deterministic perturbations is a prerequisite for further analysis of ion channel kinetics. In this report we present two methods developed for parameter estimation of such artifacts in order to remove these perturbations from single channel patch clamp traces. For both methods artifact removal does not require sweeps lacking channel activity. The first method was designed to extract artifacts from sweeps showing moderate channel activity. Within a certain number of sweeps the artifacts should remain rather constant. The second method allows for artifact removal from one individual sweep with channel activity. With the second method, correction for drift, pick up and microphonics is possible without long periods of minor channel activity. To evaluate the correctness of artifact removal a specific test was set up. The methods were carefully examined using simulated records for a wide range of parameters found in single channel experiments. Further, the developed algorithms were applied to original records obtained from cardiac and recombinant L-type Ca2+ channels.

Algorithms↗
PubMed2006

Detecting the node-density artifact in phylogeny reconstruction.

The node-density effect is an artifact of phylogeny reconstruction that can cause branch lengths to be underestimated in areas of the tree with fewer taxa. Webster, Payne, and Pagel (2003, Science 301:478) introduced a statistical procedure (the "delta" test) to detect this artifact, and here we report the results of computer simulations that examine the test's performance. In a sample of 50,000 random data sets, we find that the delta test detects the artifact in 94.4% of cases in which it is present. When the artifact is not present (n = 10,000 simulated data sets) the test showed a type I error rate of approximately 1.69%, incorrectly reporting the artifact in 169 data sets. Three measures of tree shape or "balance" failed to predict the size of the node-density effect. This may reflect the relative homogeneity of our randomly generated topologies, but emphasizes that nearly any topology can suffer from the artifact, the effect not being confined only to highly unevenly sampled or otherwise imbalanced trees. The ability to screen phylogenies for the node-density artifact is important for phylogenetic inference and for researchers using phylogenetic trees to infer evolutionary processes, including their use in molecular clock dating.

Classification↗
PubMed2006

What do children want to know about animals and artifacts? Domain-specific requests for information.

Children's questions may reveal a great deal about the characteristics of objects they consider to be conceptually important. Thirty-two preschool children were given opportunities to ask questions about unfamiliar artifacts and animals. The children asked ambiguous questions such as "What is it?" about artifacts and animals alike. However, they were more likely to ask about the functions of artifacts, but about category membership, food choices, and typical locations of animals. They never asked questions about either artifacts or animals that would be considered inappropriate by adults. The results indicate that children hold different expectations about the types of information important for categorizing living and artifact kinds. Young children conceive of artifacts in terms of functions, but conceive of animals in terms of biologically appropriate characteristics. Such results speak to debates about the role of function in children's biological reasoning and to accounts of children's artifact concepts.

Animals↗
PubMed1985

Flow artifacts in double-contrast esophagography.

Artifacts related to barium flow during double-contrast esophagography may obscure mucosal surface details. Double-contrast esophagograms with flow artifacts of 35 patients were evaluated to determine the effect on radiographic interpretation and to assess the method of examination. Initial radiographs obtained during swallowing of barium were compared with those obtained after a slight delay while patients repeatedly dry swallowed. When severe surface flow artifacts were present, the extent of mucosal disease was underestimated in all cases. Mild surface flow artifacts interfered with the demonstration of the reticular pattern of Barrett esophagus, and luminal flow artifacts caused misinterpretation. The demonstration of strictures was unaffected by flow artifacts. This study suggests that the dry swallowing maneuver and some delay improve depiction of esophageal surface details on double-contrast radiographs and obviate interpretive error from barium flow artifacts.

Barium Sulfate↗
PubMed1988

FRODO pulse sequences: a new means of eliminating motion, flow, and wraparound artifacts.

Magnetic resonance images of the spine, chest, abdomen, and pelvis are commonly degraded by ghost artifacts. The authors have developed a new technique named FRODO (Flow and Respiratory artifact Obliteration with Directed Orthogonal pulses) to suppress these artifacts. Signal from tissues responsible for the artifacts is eliminated by use of radio frequency pulses specifically optimized for high selectivity to saturate proton magnetization over one or more independently defined slabs (large rectangular volumes) of tissue. Ghost artifacts from pulsatile flow in the heart and blood vessels, as well as from respiratory motion and swallowing, are suppressed. Additional applications of this technique include elimination of intraluminal signal in blood vessels and suppression of wraparound artifact along the phase-encoding axis. Preliminary clinical experience suggests that the FRODO technique, in conjunction with other flow compensation methods, may provide a definitive solution to the problem of motion in spine imaging. FRODO pulse sequences may also prove useful for imaging of blood vessels, heart, abdomen, and other areas where motion, flow, or wraparound artifacts limit image quality.

Abdomen↗
PubMedDate not supplied

Catheter induced temperature artifacts in ultrasound hyperthermia.

Temperature artifacts were evaluated at 72 different sensor locations in 10 different tumour sites heated by use of planar ultrasound transducers operated at 1 and 3 MHz. Thermometry was carried out by single- and multisensor thermocouple probes inserted into 19- and 16-gauge polyurethane catheters, respectively. Nearly all catheters were oriented approximately perpendicular to the ultrasound beam. The artifacts were determined by backward extrapolation of the thermal decay 30-60s after the power was turned off. The effective blood flow and specific absorption rate (SAR) at the sensor locations were determined from the rate of decay and the steady-state temperature. The sample mean steady-state temperature, effective blood flow, and SAR were 41.4 degrees C, 17.5 ml/100 g/min, and 46.3 W/kg, respectively. The most frequent artifact was in the range 0-0.2 degrees C and the mean artifact was 0.6 degrees C. Less than 15% of the artifacts were above 1 degree C. The magnitude of the artifact correlates with the SAR of ultrasonic power, the effective blood flow rate, and the steady-state temperature. These results indicate that the artifact produced at 1 MHz by a multisensor, Teflon-sheathed thermocouple inserted into a 16-gauge polyurethane catheter is 1.7 +/- 0.4 degrees at an SAR of 100 W/kg.

Body Temperature↗
PubMedDate not supplied

Artifacts from pulsatile flow in MR imaging.

Previous investigators have examined the effect of blood flow on the apparent blood vessel signal intensity. These studies reported flow brightening and darkening effects within vessels. In this paper we have investigated another type of flow artifact, which originates from the pulsatile nature of blood flow. These flow artifacts have characteristic bright and dark "ghosting" patterns which appear close to small vessels, usually arteries, which are bright in slow flow. Similar to the amplitude-of-motion artifacts caused by patient motion (e.g., breathing and cardiac motion) the ghosting artifacts due to pulsatile flow are best characterized as frequency modulated spectral sidebands. The pulsatile artifacts can have both dark and bright structures and usually appear close to the "moving" vessel that generates the artifact. In this paper we present a study of the chief features of these pulsatile flow artifacts, and we develop a theoretical description of their origins in terms of "accidental" velocity-encodings that occur strongly in most magnetic resonance imaging sequences.

Blood Flow Velocity↗
PubMed1977

Electrocardiograph monitor artifacts in a neonatal intensive care unit.

A wide variety of artifacts may be found when monitoring the ECG in a neonatal intensive care unit. Many of the artifacts resemble arrhythmias, and unless they are recognized as artifacts they may lead to serious errors of diagnosis and therapy. Many of the artifacts are caused by patient movement such as seizures, tremulousness, or hiccups. Others may be introduced by the monitor itself or be caused by electrical equipment in the vicinity. A group of ECG tracings is presented to illustrate the various artifacts encountered. Features that distinguish the artifacts from the arrhythmias they mimic are described, as are suggestions for elimination of the artifacts.

Arrhythmias, Cardiac↗