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PubMedDate not supplied

Stroboscopic artifact in digital video-EEG.

Combined digital video-EEG (DV-EEG) systems eliminate many familiar technical artifacts of older analog recorders; however, new and unanticipated technical issues are becoming evident. In this report, a case is described that identifies one of these technical limitations that could represent a pitfall to accurate data interpretation. An EEG was recorded on an 18-year-old man with history of photically sensitive generalized tonic-clonic seizures, revealing photoparoxysmal responses that appeared to outlast photic stimulation. However, in an attempted video-EEG correlation, the digital video recording showed variable appearance or absence of photic bursts that did not correlate with EEG photic tick marks, initially suggesting desynchronization between video and EEG signals. However, the absence of flashes seen on the video record resulted from stroboscopic artifact caused by mismatch between strobe frequency, video sampling rate, and video display characteristics. Stroboscopic aliasing is a DV-EEG specific artifact that can complicate accurate interpretation of photoparoxysmal responses.

Adolescent↗
PubMedDate not supplied

Motion artifact as a pitfall in diagnosis of meniscal tear on gradient reoriented MRI of the knee.

OBJECTIVE: Reorientation of the phase and frequency encoding gradients is frequently performed for knee MRI to improve visualization of the cruciate ligaments. This technique allows pulsation artifacts arising from the popliteal vessels to be redirected away from the intercondylar notch. The objective of this study was to evaluate the potential for this method to result in false-positive depictions of meniscal tears based on subtle patient motion. MATERIALS AND METHODS: Magnetic resonance imaging of the knee was performed in five patients using gradient reorientation. Repeat examinations were then performed following deliberate instruction to the patients regarding the importance of refraining from moving the extremity. RESULTS: The MRI performed with gradient reorientation demonstrated apparent meniscal tears using standard criteria. However, the repeat examinations following deliberate patient instructions to refrain from moving demonstrated the affected menisci to be normal in all patients. CONCLUSION: Knee motion during image acquisition may result in ghosting artifacts of relatively high signal intensity structures such as marrow within the femoral condyles and hyaline cartilage along the articular surfaces. These artifacts may traverse the menisci on gradient-reoriented images and result in confusion with meniscal tear. Whereas gradient reorientation is a valuable tool for improving depiction of the cruciate ligaments, one must be aware of the potential for this technique to result in simulation of meniscal tears.

Adult↗
PubMedDate not supplied

Rib artifacts in electron beam tomography: incidence and severity without and with the cone beam reconstruction algorithm.

Electron beam tomography (EBT) may be compromised by rib artifacts. Two hundred forty-seven abdominal studies were performed without (Group A, n = 222) or with (Group B, n = 25) the cone beam algorithm. One hundred eighty-six (83.8%) and nine (36%) studies of Groups A and B, respectively, displayed some level of artifact. In Groups A and B, major, minor, and no artifacts were found in 115 (51.5%) and 0 (0%), 71 (32.3%) and 9 (36%), and 36 (16.2%) and 16 (64%) patients, respectively (p < 0.01). The cone beam algorithm improves EBT studies of the abdomen.

Algorithms↗
PubMed1999

Accuracy of a miniature intracranial pressure monitor, its function during magnetic resonance scanning, and assessment of image artifact generation.

OBJECTIVE: We examined the accuracy and repeatability of an intracranial pressure (ICP) monitor (Codman MicroSensor; Johnson & Johnson Professional, Inc., Raynham, MA) in a nonmagnetic environment and during magnetic resonance imaging (MRI). The resulting image artifact generation was calculated. ICP monitoring is essential in management of severe head injury, but few ICP monitoring devices are compatible with use in an MRI scanner. The use of MRI to assess head injury is increasing, and developing safe methods of continuously monitoring ICP may improve patient care. METHODS: A water manometer was used as the standard for comparison. We assessed pressure readings from the ICP monitor in a nonmagnetic environment using a standard and a long connector cable between the pressure transducer and display unit. This long cable permitted testing during MRI sequences because the display unit could be distanced from the magnet. Accuracy was determined during T2-weighted imaging, proton spectroscopy, and diffusion-weighted imaging, and artifact generation was assessed. RESULTS: We found a high degree of accuracy for repeated measurements over a clinical pressure range using both standard and long connector cables outside the MRI room. During MRI scanning, the ICP monitor was accurate during T2 and proton spectroscopy sequences. Accuracy during diffusion-weighted imaging, however, was clinically unacceptable. This ICP monitor creates a reduction in signal-to-noise ratio in the received signal during T2-weighted imaging and proton spectroscopic imaging, with the obtained images still radiologically interpretable. CONCLUSION: The Codman ICP monitor is sufficiently accurate and free of artifact generation to be used during most clinical MRI applications. This could enhance patient monitoring and safety.

Artifacts↗
PubMed1997

The influence of chemical shift artifact on magnetic resonance imaging of the ligamentum flavum at 0.5 tesla.

STUDY DESIGN: Retrospective clinical magnetic resonance imaging study and prospective magnetic resonance imaging volunteer study of the appearance of the ligamentum flavum. OBJECTIVE: To demonstrate the effect of chemical shift artifact on the apparent thickness of the ligamentum flavum on axial magnetic resonance images. SUMMARY OF BACKGROUND DATA: The ligamentum flavum is a symmetric structure clearly seen on magnetic resonance images. Apparent unilateral thickening may be interpreted as indicating a pathologic process, and the influence of chemical shift artifact on the apparent thickness of the ligamentum flavum has not been emphasized. METHODS: Ligamenta flava thicknesses were measured from axial T1-weighted gradient echo magnetic resonance scans of 12 consecutive patients and various axial sequences in seven volunteers. RESULTS: The ligamentum flavum appeared consistently thicker on the lower side of the readout gradient field. This chemical shift effect could be manipulated by swapping phase and frequency or patient orientation in the magnet. CONCLUSIONS: Caution should be applied in attributing apparent asymmetry of the ligamenta flava to disease; the influence of chemical shift artifact should be considered.

Adolescent↗
PubMed2004

ECG artifacts during intraoperative high-field MRI scanning.

High-field magnetic resonance imaging (MRI) (1.5 T) has recently been introduced into the neurosurgical operating room for intraoperative resection control and functional neuronavigational guidance. However, long-lasting neurosurgical procedures in an operating room equipped with a high-field MRI scanner raise new challenges to the anesthesiologist. In particular, monitoring of vital signs during anesthesia requires equipment compatible with working in close vicinity to the strong magnetic field. However, even MRI-compatible electrocardiographic (ECG) monitoring interferes with electromagnetic fields, so several ECG artifacts can be observed in static and pulsed magnetic fields. As shown in this study, pulsed high-frequency fields induce characteristic field frequency-based artifacts in the ECG that can imitate malignant arrhythmia or provoke ST-segment abnormalities. The knowledge of possible and characteristic ECG artifacts during high-field MRI is therefore essential to prevent misinterpretation. Moreover, interference-free parameters such as pulse oximetry or invasive blood pressure curves are highly relevant during intraoperative MRI scans.

Adult↗
PubMed2003

A new algorithm for metal artifact reduction in computed tomography: in vitro and in vivo evaluation after total hip replacement.

RATIONALE AND OBJECTIVES: To evaluate a newly developed algorithm for metal artifact reduction (MAR) in Computed Tomography (CT). METHODS: A projection interpolation algorithm for MAR with threshold-based metal segmentation was developed. First, the algorithm was tested with a simulated hip phantom. On demand, the presence of metallic inserts was simulated, representing total hip endoprostheses. Second, CT data of 20 patient with total hip endoprosthesis were reconstructed with and without application of the MAR algorithm. Image quality was independently assessed by 2 experienced radiologists using a qualitative score. The results of the in vitro study were evaluated with the Student's t test. Results of the in vivo study were analyzed using a repeated-measure analysis of variance. RESULTS: Applying the MAR algorithm the phantom study showed no significant difference between images with and without simulated metal contributions. The patient study revealed improved image quality using the MAR algorithm. Results were statistically significant for fat (P=0.0097), vessels (P=0.0091), and bone (P=0.0005). Improvement of the image quality for muscle was not statistically significant (P=0.0287). CONCLUSIONS: A new algorithm for metal artifact reduction was successfully introduced into clinical routine. The algorithm led to a robust reduction of metal artifacts. The MAR algorithm may serve for an improvement in image quality in patients with metallic implants.

Aged↗
PubMed2004

Artifact reduction using parallel imaging methods.

Multiple receiver coils produce images with different but complementary views of a patient. This can be used to shorten scans times but there often remain image artifacts caused by patient motion or physiological processes such as flowing blood. This paper reviews how the extra information from the multiple coils can be used to reduce image artifacts. In one method, affected portions of data can be identified and discarded but enough information is still available to reconstruct an improved image. In other methods, the motion itself is determined and the corrupted data is then corrected, leading to an image with reduced artifacts. Results are presented from images corrupted by motion or by flowing blood.

Aortic Diseases↗
PubMed2000

Pulse oximetry: an improved in vitro model that reduces blood flow-related artifacts.

Artifacts may occur in many in vitro models of pulse oximetry due to the optical effects of synchronously oriented and/or deformed erythrocytes. Although these artifacts are most likely negligible in living superficial tissues, they are demonstrated to have considerable influence on the calibration curve obtainable from the in vitro simulation of pulse oximetry in such models, especially at low oxygen saturations. Therefore, we have developed a modified in vitro model which reduces the effect of these artifacts. This is achieved by excluding data obtained during pressure transients and by raising the blood flow velocity. As a result, the model more closely approximates in vivo pulse oximetry, particularly under clinically important conditions of low blood oxygen saturation levels.

Artifacts↗
PubMed2000

Nonlinear adaptive filtering of stimulus artifact.

Noninvasive measurements of somatosensory evoked potentials have both clinical and research applications. The electrical artifact which results from the stimulus is an interference which can distort the evoked signal, and introduce errors in response onset timing estimation. Given that this interference is synchronous with the evoked signal, it cannot be reduced by the conventional technique of ensemble averaging. The technique of adaptive noise cancelling has potential in this regard however, and has been used effectively in other similar problems. An adaptive noise cancelling filter which uses a neural network as the adaptive element is investigated in this application. The filter is implemented and performance determined in the cancelling of artifact for in vivo measurements on the median nerve. A technique of segmented neural network training is proposed in which the network is trained on that segment of the record time window which does not contain the evoked signal. The neural network is found to generalize well from this training to include the segment of the window containing the evoked signal. Both quantitative and qualitative measures show that significant stimulus artifact reduction is achieved.

Artifacts↗
PubMed1999

Detecting film-screen artifacts in mammography using a model-based approach.

Microcalcifications can be one of the earliest signs of breast cancer. Unfortunately, their appearance in mammograms can be mimicked by dust and dirt entering the imaging process and this has been shown previously to lead to false positives. We use a model of the imaging process and, in particular, the blurring functions inherent within it to detect the film-screen artifacts caused by dust and dirt and, thus, reduce false-positives. A crucial facet of the work is the choice of the correct image representation upon which to perform the image processing. After extensive testing, our algorithm has identified no microcalcifications as being artifacts and has an artifact detection rate of approaching 96%.

Algorithms↗
PubMed2001

Simultaneous correction of ghost and geometric distortion artifacts in EPI using a multiecho reference scan.

A computationally efficient technique is described for the simultaneous removal of ghosting and geometrical distortion artifacts in echo-planar imaging (EPI) utilizing a multiecho, gradient-echo reference scan. Nyquist ghosts occur in EPI reconstructions because odd and even lines of k-space are acquired with opposite polarity, and experimental imperfections such as gradient eddy currents, imperfect pulse sequence timing, B0 field inhomogeneity, susceptibility, and chemical shift result in the even and odd lines of k-space being offset by different amounts relative to the true center of the acquisition window. Geometrical distortion occurs due to the limited bandwidth of the EPI images in the phase-encode direction. This distortion can be problematic when attempting to overlay an activation map from a functional magnetic resonance imaging experiment generated from EPI data on a high-resolution anatomical image. The method described here corrects for geometrical distortion related to B0 inhomogeneity, gradient eddy currents, radio-frequency pulse frequency offset, and chemical shift effect. The algorithm for removing ghost artifacts utilizes phase information in two dimensions and is, thus, more robust than conventional one-dimensional methods. An additional reference scan is required which takes approximately 2 min for a matrix size of 64 X 64 and a repetition time of 2 s. Results from a water phantom and a human brain at 3 T demonstrate the effectiveness of the method for removing ghosts and geometric distortion artifacts.

Algorithms↗
PubMed2003

A simple system for detection of EEG artifacts in polysomnographic recordings.

We present an efficient parametric system for automatic detection of electroencephalogram (EEG) artifacts in polysomnographic recordings. For each of the selected types of artifacts, a relevant parameter was calculated for a given epoch. If any of these parameters exceeded a threshold, the epoch was marked as an artifact. Performance of the system, evaluated on 18 overnight polysomnographic recordings, revealed concordance with decisions of human experts close to the interexpert agreement and the repeatability of expert's decisions, assessed via a double-blind test. Complete software (Matlab source code) for the presented system is freely available from the Internet at http://brain.fuw.edu.pl/artifacts.

Algorithms↗
PubMed2004

Reduction of stimulus artifact in somatosensory evoked potentials: segmented versus subthreshold training.

A new approach to stimulus artifact cancellation is introduced, which attempts to model the process of stimulus artifact generation. This is done by training an estimator with multiple exemplars of the stimulus artifact at levels below the threshold of evoked response stimulation. Two estimators are formulated: one using a dynamic neural network and another using a linear estimator. The performance of these new approaches is compared to the segmented training approach, which has been previously demonstrated to be one of the most capable methods available. Performance assessment is carried out using a novel metric introduced in this paper, which focuses upon the relevant portion of the recorded waveform. The new cancellation schemes show distinct performance advantages over the segmented training approach.

Adult↗
PubMed2005

Fast image restoration without boundary artifacts.

Fast Fourier transform (FFT)-based restorations are fast, but at the expense of assuming that the blurring and deblurring are based on circular convolution. Unfortunately, when the opposite sides of the image do not match up well in intensity, this assumption can create significant artifacts across the image. If the pixels outside the measured image window are modeled as unknown values in the restored image, boundary artifacts are avoided. However, this approach destroys the structure that makes the use of the FFT directly applicable, since the unknown image is no longer the same size as the measured image. Thus, the restoration methods available for this problem no longer have the computational efficiency of the FFT. We propose a new restoration method for the unknown boundary approach that can be implemented in a fast and flexible manner. We decompose the restoration into a sum of two independent restorations. One restoration yields an image that comes directly from a modified FFT-based approach. The other restoration involves a set of unknowns whose number equals that of the unknown boundary values. By summing the two, the artifacts are canceled. Because the second restoration has a significantly reduced set of unknowns, it can be calculated very efficiently even though no circular convolution structure exists.

Algorithms↗
PubMed2005

Chirp imaging vibro-acoustography for removing the ultrasound standing wave artifact.

Vibro-acoustography (VA) is an imaging technique that uses the dynamic (oscillatory) radiation force of two continuous-wave (CW) ultrasound to image objects at low frequency (within the kHz range). In this technique, the dynamic radiation force is created by means of a confocused transducer emitting two ultrasound beams at slightly-shifted frequencies f1 and f2 = f1 + deltaf. It has been demonstrated previously that high-resolution images of various types of inclusions and tissues can be obtained using this technique. However, if the targeted object reflects ultrasound directly back to the transducer, standing waves are produced that result in an artifact in the VA image. The goal of this study is to remove the standing wave artifact and improve VA images by means of a new process called chirp imaging. The procedure consists of sweeping the frequencies of the primary ultrasound beams in a selected bandwidth while keeping deltaf constant during the sweep. The chirp image is produced by averaging the amplitude of the acoustic emission produced during the sweep. Vibro-acoustography chirp imaging experiments are performed on a stainless-steel sphere attached to a latex sheet in a tank of degassed water. The resulting chirp images demonstrate remarkable reduction of the standing wave artifact compared to the "fixed frequency" VA images.

Acoustics↗
PubMed2004

"Convulsive" nonepileptic seizures have a characteristic pattern of rhythmic artifact distinguishing them from convulsive epileptic seizures.

PURPOSE: Approximately 30% of patients admitted for video-EEG monitoring have psychogenic nonepileptic seizures (PNES). Differentiation of "convulsive" PNES from convulsive seizures can be difficult. The EEG often displays rhythmic movement artifact that may resemble seizure activity and confound the interpretation. We sought to determine whether time-frequency mapping of the rhythmic EEG artifact during "convulsive" PNES reveals a pattern that differs from that of epileptic seizures. METHODS: EEGs from 15 consecutive patients with "convulsive" PNESs were studied with time-frequency mapping by using NEUROSCAN and compared with 15 patients with convulsive epileptic seizures. Fast Fourier transforms (FFTs) were performed to determine the dominant frequency for 1- to 2-s windows every 2 s through the seizures. RESULTS: The dominant frequency remained stable within a narrow range for the duration of the PNES, whereas in the epileptic seizures, it evolved through a wide range. The coefficient of variation of the frequency during the seizures was considerably less for patients without epilepsy (median, 15.0%; range, 7.2-23.7% vs. median, 58.0%; range, 34.8-92.1%; p < 0.001). The median frequency did not differ significantly between groups (4.2 vs. 4.6 Hz; p = 0.290). CONCLUSIONS: "Convulsive" PNES display a characteristic pattern on time-frequency mapping of the EEG artifact, with a stable, nonevolving frequency that is different from the evolving pattern seen during an epileptic seizure.

Adult↗
PubMed2005

Safe scanning, but frequent artifacts mimicking bradycardia and tachycardia during magnetic resonance imaging (MRI) in patients with an implantable loop recorder (ILR).

BACKGROUND: Patients with implantable devices are generally not permitted to undergo magnetic resonance imaging (MRI) because of potentially deleterious interactions. Little has been reported regarding the safety and effects of MRI scanning of patients with implantable loop recorders (ILRs). We evaluated the safety of scanning patients with ILRs and the output of the ILR after undergoing MRI. METHODS: Ten patients underwent 11 MRI scanning events. All patients had Reveal Plus (Medtronic, Minneapolis, MN) ILRs. Seven cranial, two lumbar-spine, one shoulder, and one knee MRI were performed. All of the MRIs were performed with the understanding that the patient had an ILR. In each patient, the ILR was cleared moments before the scan and the integrity of the signal and time date stamp were verified. The devices were reinterrogated immediately after MRI in 10 patients and two days post MR scanning in one patient. Each patient was questioned post MRI regarding any symptoms experienced during the scan. RESULTS: Both tachy and bradyarrhythmias appeared as artifacts as a result of ILR exposure to MRI. Post MRI, none of the ILRs showed diminished signal integrity, altered programmed parameters, diminished battery status, inability to communicate or be reprogrammed. No sensations of tugging or warmth at the implant site were noted. CONCLUSION: MRI was performed in ILR patients without harm to the patient or permanent damage to the ILR. MRI scanning of the Reveal appears safe. Artifact mimicking an arrhythmia was common, however, and must be excluded in any ILR patient undergoing MRI to avoid mistakenly attributing a syncopal episode, or palpitations to the artifacts produced from MRI exposure.

Artifacts↗