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PubMedDate not supplied

Artifacts due to residual magnetization in three-dimensional magnetic resonance imaging.

An artifact is identified in magnetic resonance images produced by the three-dimensional FLASH technique, which features a short repetition time TR. The artifact is caused by differential spoiling of transverse magnetization by the phase-encoding gradients. The image intensity in different slices becomes altered, especially for short TR and large flip angle, which are conditions for achieving strong T1-weighted contrast. The effectiveness of spoiler gradient and rephasing gradients in suppressing the artifact is evaluated experimentally in images of a uniform phantom. Spoiler gradients that are incremented in amplitude cause even more slices to deviate in intensity, and are therefore less effective than in two-dimensional techniques. Rephasing gradients make the slices uniformly intense, but also enhance the intensity of tissues that have longer T2. The further addition of constant spoiler gradients has reduced this intensity increase by one-half and allowed for an intensity difference between white matter and gray matter comparable to without a rephasing gradient.

Diagnostic Errors↗
PubMedDate not supplied

Phase-encode order and its effect on contrast and artifact in single-shot RARE sequences.

Substantial manipulation of tissue contrast can be achieved by varying the order in which phase-encode values are applied to individual echoes within a 128-echo single-shot rapid acquisition relaxation enhanced (RARE) sequence. Appropriate ordering can then permit imaging of short T2 species like muscle and white matter with single-shot RARE. For sequential phase encoding with an arbitrary initial phase-encode value, the timing of the zero phase (ZP) encoded echo is found to be analogous to the echo time (TE) of standard spin-echo sequences. This is demonstrated qualitatively with human brain images and is verified quantitatively with NiCl2 phantoms by correlating the time constant for signal decay with ZP echo time, with transverse relaxation times T2, as obtained with a 128-echo Carr-Purcell-Meiboom-Gill (CPMG) imaging sequence. Banding artifacts accompanying the discontinuous traverse through K space are experimentally demonstrated in a rectangular phantom and expressions are developed for determining the dependence of this artifact on the phase-encode gradient increments and durations, the ZP echo number, echo spacing, and T2. Simulations based on the expressions are shown to be useful for characterizing the observed "banding" artifacts perpendicular to the phase-encode direction and for predicting the extent of tissue-tissue overlap to be expected with the use of this ultrafast rf echo planar imaging method.

Brain↗
PubMed1999

Techniques to alleviate the effects of view aliasing artifacts in computed tomography.

Due to practical limitations in data acquisition, 3-D computed tomography systems must attempt to provide rapid reconstructions of acceptable quality from a limited number of views. The use of convolution backprojection (CBP) for image reconstruction from an inadequate number of projections, results in view aliasing artifacts. In this paper we investigate different post-processing methods of alleviating the effects of view aliasing artifacts. Two distinct methods and their variants are considered. The first, termed the intermediate view reprojection (IVR) method, involves estimating a set of intermediate views by reprojection, followed by a reconstruction using the augmented set of views. The second, termed the error-correction (EC) method, incorporates a correction on the initial reconstruction based on the projection-domain error. Suitable modifications and variants of the above methods are indicated. Of the methods discussed, the IVR method is simple, tends to reduce the effects of artifacts with less susceptibility to secondary effects, and is applicable to region-of-interest reconstructions.

Algorithms↗
PubMed2005

Transducer hysteresis contributes to "stimulus artifact" in the measurement of click-evoked otoacoustic emissions (L).

Click-evoked otoacoustic emissions from the human ear are typically several orders of magnitude smaller than the stimuli that elicit them--a measurement technique that attempts to cancel the stimulus signal from the recorded waveform is therefore typically employed. In practice, an imperfect cancellation of the stimulus is achieved, leaving a "stimulus artifact" that obscures the early part of the emission. Input-output nonlinearities of the transducers used in recording emissions are acknowledged as one source of the stimulus artifact. Here an additional source of this artifact, related to hysteresis in the magnetic "receivers" (loudspeakers) used in such recordings, is identified and discussed.

Acoustic Stimulation↗
PubMed1982

The importance of ultrasonic side-lobe artifacts.

Transducer side lobes consist of multiple low-intensity sound beams located outside of the main ultrasound beam. Although the echoes generated by side lobes originate outside of the main beam, they appear as specular or diffuse artifactual echoes within the beam. They are visible with both static and real-time equipment. Specular side-lobe artifacts occur near strong, curved, highly reflecting surfaces such as the diaphragm, or near large cystic masses such as the urinary bladder or gallbladder. Diffuse artifacts originate from bowel gas adjacent to cystic structures. These echoes can be confused with particulate material or sludge in organs such as the urinary bladder and gallbladder, or in abnormal cystic masses. We illustrate the genesis of these artifactual echoes and provide clinical examples of commonly visualized side-lobe artifacts.

Humans↗
PubMed1986

Artifacts seen during CT pelvimetry: implications for digital systems with scanning beams.

During computed tomography (CT) pelvimetry of two pregnant women, bony abnormalities of the fetuses were noted on the scout images that were not confirmed at delivery. To explore the cause of these artifacts, specimen long bones were manipulated in various ways during CT scout imaging. Artifacts like those seen during in vivo imaging were found to be caused by motion of the object. The CT scout view is an example of an image produced by a digital system that uses a scanning beam. This type of digital system is being used for several types of body imaging including screening for scoliosis and chest radiography. Attention to motion artifacts should decrease diagnostic errors and aid further development of these systems.

Adult↗
PubMed1986

Boundary effects from opposed magnetization artifact in IR images.

A cancellation of signal intensity at the interface separating selected tissue-equivalent materials is observed in inversion recovery proton MR images. The absence of signal intensity at the interface is always one pixel wide and appears only when the tissue-equivalent materials forming the interface differ substantially in their longitudinal relaxation times (T1). Images were obtained of various two-layer combinations of tissue-equivalent materials consisting of vegetable oil, animal fat, saline, aqueous Mn+2, or 2% agar doped with Mn+2. This type of boundary is compared with chemical shift artifacts, which at 0.15 T and 0.35 T produce a similar effect. A clinical example of the opposed magnetization artifact is also shown. Since tissues with substantially different T1s are found in vivo, it is expected that this effect could lead to an instrument-dependent artifact that could easily be misinterpreted.

Electron Spin Resonance Spectroscopy↗
PubMed1987

Motion artifact reduction with fast spin-echo imaging.

The influence of signal averaging (n), repetition time (TR), and echo delay (TE) on systematic noise (cardiac, vascular, respiratory, and peristaltic ghost artifacts) and statistical noise (thermal effects) was determined in eight healthy volunteers and 57 patients. Systematic noise was the dominant factor degrading abdominal magnetic resonance (MR) images. Signal averaging was the primary determinant of both statistical and systematic image noise, fitting a power function (n)b with b = 0.44 and -0.42, respectively, close to the expected b = -0.5 power function. All types of ghosting showed the same sensitivity to signal averaging. Normalized systematic noise increased slightly with TR (b = 0.16) and increased markedly with TE (b = 0.40). These data indicate that the short TR, short TE technique is a powerful method for reducing motion artifacts on breathhold images and can be combined with signal averaging to further suppress artifacts, improve signal-to-noise ratio, and maximize anatomic resolution.

Abdomen↗
PubMed1989

Eddy-current-induced artifacts caused by an "MR-compatible" halo device.

Traction or halo devices thought to be compatible with magnetic resonance (MR) imaging and made of nonferromagnetic metals may actually cause MR image artifacts. The authors have observed such artifacts caused by eddy currents generated in the conducting frame of the device by the radio-frequency excitation field. The severity of the artifact depends on the orientation of the device in the MR unit and can be effectively eliminated by disrupting the electrical continuity of the supporting frame of the device.

Diagnostic Errors↗
PubMed1990

Color and spectral Doppler mirror-image artifact of the subclavian artery.

The mirror-image artifact, which has been previously described in gray-scale sonographic imaging, is also readily visualized with both spectral and color Doppler flow imaging. In 10 consecutive healthy subjects, a mirror image of the subclavian artery was readily apparent at gray-scale, spectral Doppler, and color Doppler sonography. An experimental in vitro model was constructed to demonstrate that the lung apex, which is located immediately posterior to the subclavian artery, acts as the highly reflective acoustic interface to form this artifact. Knowledge of the presence of the mirror-image artifact should be helpful to the radiologist and sonographer in avoiding misinterpretation of this important pitfall in both spectral and color Doppler flow imaging of the subclavian region, where there is a plethora of branching vessels.

Humans↗
PubMed1990

Rapid acquisition spin-echo (RASE) MR imaging: a new technique for reduction of artifacts and acquisition time.

The rapid acquisition spin-echo (RASE) technique combines a short repetition time, a short echo time, and a single excitation pulse sequence with half-Fourier data sampling. This allows for acquisition of 11 strongly T1-weighted sections during a single 23-second breath-holding period. Measurements obtained from volunteers and with phantoms reveal that RASE images have a lower signal-to-noise ratio and contrast-to-noise ratio than do conventional multiacquisition spin-echo (SE) images due to reduced data acquisition. However, liver-spleen contrast and spatial resolution are not affected. Moreover, contrast-to-artifact (C/A) measurements are 77% greater with RASE. When normalized for imaging time, all parameters are significantly higher with RASE, with a C/A per unit time that was 338% higher. Randomized, blinded review of RASE and SE sequences from 20 patients was conducted to evaluate qualitative performance. Excellent to good performances for phase-encoding artifact reduction, edge sharpness, and overall image quality were recorded for 89%, 88%, and 86% of RASE examinations, respectively, versus 41%, 59%, and 47% of conventional SE examinations, respectively. All results were statistically significant with P less than .001. RASE is an easily implemented imaging technique that utilizes widely available existing technology. Its major benefits relate to significant reduction in imaging time, elimination of respiratory artifacts, and the potential for performing dynamic contrast material-enhanced screening examinations.

Abdomen↗
PubMed1992

Gantry angulation in brain CT: dosage implications, effect on posterior fossa artifacts, and current international practice.

The aim of this study was first to determine the effect of use of different scan planes at computed tomography (CT) of the brain on lens radiation dose and on severity of posterior fossa artifact. Twenty patients requiring nonenhanced and contrast material-enhanced brain CT scanning were selected. Each was scanned with two different beam angulations, one passing through the eyes and one avoiding them, and the resulting radiation doses were measured. Angling of the beam to avoid the orbit reduced the dose by 87%, while the severity of posterior fossa artifacts was not significantly different. Second, the implications of these results was assessed by means of an international questionnaire survey of current scanning technique. Only 32% of the respondents routinely avoided the eye during brain CT. No single scan plane was accepted by the respondents as being superior in terms of minimizing posterior fossa artifact. The authors conclude that the potential exists for a reduction in lens dose after reappraisal of radiologic practice in brain CT.

Brain↗
PubMed1994

Breast artifact in planar cardiac imaging: a comparison of 201T1 to sestamibi-99mTc.

Breast artifact occurrence during 201T1 myocardial image interpretation is a significant problem. We serially evaluated 3 female patients utilizing exercise treadmill perfusion studies in order to assess the potential role of 99mTc-Sestamibi (MIBI) imaging in the setting of documented 201T1 breast artifact. Both visual and quantitative analyses revealed no superiority of 99mTc-MIBI over 201T1 imaging in reference to avoidance of breast artifact. The variation in artifactual cardiac imaging defects in the 3 patients most probably reflected variations in breast positioning. Finally, the ability to assess ventricular wall motion via gated 99mTc-MIBI imaging may help to confirm an artifactual versus an actual myocardial perfusion abnormality.

Aged↗
PubMed2003

Younger achievement age predicts shorter life for governors: testing the precocity-longevity hypothesis with artifact controls.

McCann's precocity-longevity hypothesis suggests that the prerequisites, concomitants, and consequences of early peaks in career achievement may foster the conditions for premature death. In the present test of the precocity-longevity hypothesis, it was predicted that state governors elected at younger ages live shorter lives. Two competing explanatory frameworks, the life expectancy artifact and the selection bias artifact, also were tested. In a sample of 1,672 male governors, the precocity-longevity prediction was supported, and it was demonstrated with correlation, regression, and subsample construction strategies that the life expectancy and selection bias artifacts were not sufficient to account or the significant positive correlation between election age and death age. The positive correlation also was maintained when year of birth, years of service, span of service, and state of election were statistically controlled.

Achievement↗
PubMed1990

Artifacts in routinely immersion fixed nervous tissue.

In order to demonstrate the types of artifacts commonly seen in inadequately fixed central nervous system (CNS) tissues, rat brains were fixed by either perfusion or immersion in 10% neutral buffered formalin. Perfused brains were either removed immediately or left in situ for 1 hr prior to removal. Some of the brains destined for immersion fixation were first allowed to sit for either one-half or 1 hr in either room air or in an isotonic saline solution. Other brains were subjected to various handling procedures immediately after removal from the cranial vault in order to simulate the types of trauma which commonly occur during routine removal/dissection. Commonly observed artifacts included the presence of basophilic (dark) neurons, retraction spaces around neurons, vessels, and glial cells, displacement of the neuropil, and neuropil vacuolar change. The results of the fixation and handling procedures utilized in the collection of these brains substantiate the well documented fact that the least degree of artifact will be seen in brains fixed by perfusion and left in situ for a reasonable period of time (several hours) prior to removal.

Animals↗
PubMed1986

A cryoprotection method that facilitates cutting frozen sections of whole monkey brains for histological and histochemical processing without freezing artifact.

Cutting frozen sections of large (greater than 60 cc) blocks of monkey brain using the conventional procedures of infiltration with 30% sucrose as a cryoprotectant before freezing with pulverized dry ice often produces unacceptable levels of freezing artifact (FA) caused by displacement of tissue by ice crystals. Experiments investigating FA utilized perfusion-fixed brains from 46 monkeys and spanned combinations of cryoprotectants (glycerol, sucrose), freezing methods (dry ice or -75 degrees C isopentane), and fixatives (10% formalin, Karnovsky's or Timm's). The effects were evaluated by rating of FA severity in frozen sections of whole monkey brains. Minor FA appears as enlarged capillaries, more serious FA as large vacuoles, and both first appear midway between the periphery and center of the block. Stronger fixatives increased the severity of freezing artifact. The best method for eliminating FA was graded infiltration with up to 20% glycerol and 2% DMSO (in buffer or fixative), followed by rapid freezing in -75 degrees C isopentane. Although using a glycerol-DMSO infiltration before conventional freezing with pulverized dry ice or using conventional sucrose infiltration before freezing in isopentane gave better results than sucrose infiltration and dry-ice freezing, only the combination of glycerol-DMSO infiltration and freezing in isopentane produced consistently excellent results and virtually eliminated freezing artifact. To determine the effect of freezing with dry ice or isopentane on the rate of cooling in large blocks of CNS tissue, thermocouples were embedded in an 80-cc block of albumin-gelatin and frozen with the two methods. The rate of cooling (-3.5 degrees C/min) was twice as fast using isopentane.

Animals↗
PubMed2005

Reduction of motion artifact in pulse oximetry by smoothed pseudo Wigner-Ville distribution.

BACKGROUND: The pulse oximeter, a medical device capable of measuring blood oxygen saturation (SpO2), has been shown to be a valuable device for monitoring patients in critical conditions. In order to incorporate the technique into a wearable device which can be used in ambulatory settings, the influence of motion artifacts on the estimated SpO2 must be reduced. This study investigates the use of the smoothed psuedo Wigner-Ville distribution (SPWVD) for the reduction of motion artifacts affecting pulse oximetry. METHODS: The SPWVD approach is compared with two techniques currently used in this field, i.e. the weighted moving average (WMA) and the fast Fourier transform (FFT) approaches. SpO2 and pulse rate were estimated from a photoplethysmographic (PPG) signal recorded when subject is in a resting position as well as in the act of performing four types of motions: horizontal and vertical movements of the hand, and bending and pressing motions of the finger. For each condition, 24 sets of PPG signals collected from 6 subjects, each of 30 seconds, were studied with reference to the PPG signal recorded simultaneously from the subject's other hand, which was stationary at all times. RESULTS AND DISCUSSION: The SPWVD approach shows significant improvement (p < 0.05), as compared to traditional approaches, when subjects bend their finger or press their finger against the sensor. In addition, the SPWVD approach also reduces the mean absolute pulse rate error significantly (p < 0.05) from 16.4 bpm and 11.2 bpm for the WMA and FFT approaches, respectively, to 5.62 bpm. CONCLUSION: The results suggested that the SPWVD approach could potentially be used to reduce motion artifact on wearable pulse oximeters.

Journal Article↗
PubMed1990

Hearing loss from the acoustic artifact of the coil used in extracranial magnetic stimulation.

The stimulating coil used in extracranial magnetic field stimulation (EMFS) emits a high intensity impulse sound artifact that causes permanent threshold shifts in the unprotected ears of experimental animals. At magnetic stimulation levels of 50 to 100%, the magnetic coil acoustic artifact (MCAA) ranged from 145 to 157 dB peak sound pressure level at the eardrum. The magnetic field alone did not appear to cause hearing impairment since no threshold shifts were observed in ears that were plugged with ear protectors during exposure to the MCAA. These findings suggest that the acoustic artifact produced by EMFS in the clinic may pose some risk for hearing loss in patients and clinicians when held in close proximity to the unprotected ear. We recommend the use of ear protectors for the patient and clinician during EMFS as a precautionary measure to prevent hearing loss.

Animals↗