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We describe an unusual electrocardiographic artifact in a patient with a transcutaneous electrical nerve stimulation unit. The artifact, consisting of low amplitude high frequency deflections, can be misinterpreted as a runaway pacemaker.
Analysis of heart rate variability (HRV) with Holter monitoring is often difficult due to excessive artifacts and arrhythmias. While short sudden surges are treated successfully by most methods, slow heart rate (HR) variations, nocturnal trapezoidally-shaped HR increases and special types of arrhythmias which are similar to normal HRV fluctuations may distort further time domain and spectral analysis. This paper examines the advantages and disadvantages of different methods for preprocessing of HR data. We have developed the following approach to the analysis of HRV. (1) A combination method based on the absolute difference between HR values and both the last normal HR value and an updated mean is used for removal of artifacts and arrhythmias. This method can detect both sudden surges in HR values as well as longer periods of noise combined with slow normal variations. An additional stage of wild point removal is then optionally applied. (2) Certain special problems such as large T-waves, bigeminal rhythm, slow HR variations and nocturnal trapezoidally-shaped HR increases are also identified. Although none of the algorithms can be applied successfully to all cases, the final computer analysis for preprocessing described in the present study has proved to be superior to the simplified methods which are usually used and provides more suitable data for HRV analysis.
The ability of an intravascular ultrasound catheter to give cross-sectional images of vessel walls and surrounding tissues, and the behavior of ultrasound in heterogeneous media, are at the origin of degradation of image quality. Qualitative and quantitative analyses of in vivo studies are then operator-dependent and are limited by artifacts. We investigated these limitations by an in vitro study on plexiglass phantoms and segments of fresh arteries. We used a 20 MHz transducer mounted on the tip of a 4.8 F catheter and an interventional ultrasound system. The ultrasound beam is reflected onto the rotating transducer at 600 rotations per minute (RPM), creating 360 degrees real-time images (10 images/second). We then observed, analyzed and interpreted the most specific reasons for image artifacts: geometric distortions, multiple echoes, the point spread function (PSF) of the imaging system, near-field effects, "petal-shaped" effect, and ultrasound speckle. Various practical implications have resulted from this study. Only a thorough knowledge of how to avoid some of the most obvious pitfalls will enable the user to obtain maximum benefits from intravascular ultrasound imaging, and to appreciate its limitations.
Isolated DNA fibers complexed with protamine (the chromosomal protein that packages DNA in mammalian sperm) have been produced by partially decondensing the highly compacted mouse sperm chromatin particle on a glass coverslip. These DNA fibers were then scanned with the atomic force microscope (AFM). While the smallest of the fibers appear in AFM images as ribbon-like structures 250-350 A wide and 10-25 A high, experiments indicate that these images are the result of a convolution of the imaging-tip's shape with the object's actual shape. In such convolutions the height of the object is affected only by the compressibility of the object, while the width is affected in addition by the sharpness of the tip. Images of polyamidoamine particles also appear to show this artifact. We have also deduced the tip's radius of curvature from images of sharp steps and attempt to demonstrate the artifacts associated with a relatively large imaging tip.
The role of susceptibility effects in the production of artifacts in chemical shift images generated by the selective excitation technique is discussed. The effects are demonstrated in images of phantom samples of agarose gels containing small air bubbles. The artifacts can lead to erroneous interpretations, in which the resonances to be resolved exhibit a relatively small chemical shift separation, such as that between water and soluble carbohydrates (sugars).
Moving objects (e.g., heart, lung, chest wall, etc.) typically cause artifacts to appear in two-dimensional Fourier transform ("spin warp") images. The rapid line scan (RLS) technique is a simple inexpensive technique that can rapidly produce artifact-free images of moving objects, without requiring enormous magnetic field gradients or periodic motions. The basic concepts and potential industrial applications of the RLS technique are described.
Cardiac gating and phase re-ordered acquisition are often employed to reduce pulsatile flow artifacts in MR angiography. When these techniques are used in conjunction with phase contrast angiography in the extremities they can produce arterial edge artifacts.
OBJECTIVE: Recording low amplitude electroencephalography (EEG) signals in the face of large gradient artifacts generated by changing functional magnetic resonance imaging (fMRI) magnetic fields continues to be a challenge. We present a new method of removing gradient artifacts with time-varying waveforms, and evaluate it in continuous (non-interleaved) simultaneous EEG-fMRI experiments. METHODS: The current method consists of an analog filter, an EEG-fMRI timing error correction algorithm, and a temporal principal component analysis based gradient noise removal algorithm. We conducted a phantom experiment and a visual oddball experiment to evaluate the method. RESULTS: The results from the phantom experiment showed that the current method reduced the number of averaged samples required to obtain high correlation between injected and recovered signals, compared to a conventional average waveform subtraction method with adaptive noise cancelling. For the oddball experiment, the results obtained from the two methods were very similar, except that the current method resulted in a higher P300 amplitude when the number of averaged trials was small. CONCLUSIONS: The current method enabled us to obtain high quality EEGs in continuous simultaneous EEG-fMRI experiments. SIGNIFICANCE: Continuous simultaneous EEG-fMRI acquisition enables efficient use of data acquisition time and better monitoring of rare EEG events.
Echocardiography is an essential diagnostic tool for the investigation of the cardiovascular system. However, the nature of the ultrasound beam may lead to artifacts such as doubling of cardiac structures because of refraction. Here we present two illustrative cases showing doubling of the aortic ring and double regurgitation through the pulmonary valve. Doubling by refraction is different from other artifacts generating double images, such as mirroring of the ultrasound beam (eg, by prosthetic valves). Anatomic structures between the transducer and the heart such as the pleura, pericardium, or rib cartilage may induce refraction of the ultrasound beam resulting in doubling of cardiac structures. The resulting doubling of anatomic structures must not be misdiagnosed.
OBJECTIVES: We sought to determine whether the flow-dependent changes in Doppler-derived valve effective orifice area (EOA) are real or due to artifact. BACKGROUND: It has frequently been reported that the EOA may vary with transvalvular flow in patients with aortic stenosis. However, the explanation of the flow dependence of EOA remains controversial and some studies have suggested that the EOA estimated by Doppler-echocardiography (EOA(Dop)) may underestimate the actual EOA at low flow rates. METHODS: One bioprosthetic valve and three rigid orifices were tested in a mock flow circulation model over a wide range of flow rates. The EOA(Dop) was compared with reference values obtained using particle image velocimetry (EOA(PIV)). RESULTS: There was excellent agreement between EOA(Dop) and EOA(PIV) (r2 = 0.94). For rigid orifices of 0.5 and 1.0 cm2, no significant change in the EOA was observed with increasing flow rate. However, substantial increases of both EOA(Dop) and EOA(PIV) were observed when stroke volume increased from 20 to 70 ml both in the 1.5 cm2 rigid orifice (+52% for EOA(Dop) and +54% for EOA(PIV)) and the bioprosthetic valve (+62% for EOA(Dop) and +63% for EOA(PIV)); such changes are explained either by the presence of unsteady effects at low flow rates and/or by an increase in valve leaflet opening. CONCLUSIONS: The flow-dependent changes in EOA(Dop) are not artifacts but represent real changes in EOA attributable either to unsteady effects at low flow rates and/or to changes in valve leaflet opening. Such changes in EOA(Dop) can be relied on for clinical judgment making.
A detailed theoretical and experimental analysis of the artifacts induced by homonuclear band-selective decoupling during CT frequency labeling is presented. The effects are discussed in the context of an amino-acid-type editing filter implemented in (1)H-(13)C CT-HSQC experiments of methyl groups in proteins. It is shown that both Bloch-Siegert shifts and modulation sidebands are efficiently suppressed by using additional off-resonance decoupling as proposed by Zhang and Gorenstein [J. Magn. Reson. 132 (1998) 81], and appropriate adjustment of a set of pulse sequence parameters. The theoretical predictions are confirmed by experiments performed on (13)C-labeled protein samples, yielding artifact-free amino-acid-type edited methyl spectra.
Recent successes in treating neurological disorders with electrical stimulation of the brain have spurred interest in studying the neuronal mechanisms by which such therapies work. However, microelectrode recordings can be confounded by stimulation artifact. Also, large microelectrode arrays now allow recording amounts of data that would otherwise overwhelm current analytic methods that depend heavily on human intervention and interpretation. A set of algorithms is described for automatically removing stimulus artifacts that minimize signal loss with minimum human involvement. Other algorithms automatically differentiate between the extracellular action potentials of individual neurons.
The purpose of this investigation was to quantitatively evaluate the practical impact of alteration of key imaging parameters on image quality and artifacts in fast multi-planar gradient echo (GRE) pulse sequences. These include multi-planar GRASS (MPGR) and fast multi-planar spoiled GRASS (FMPSPGR). We developed a composite phantom with different T(1) and T(2) values comprising the range of common biological tissues, which was also subjected to periodic motion in order to evaluate motion effects. Magnetic resonance imaging was performed on a GE Signa 1.5-T system. Experimental variations in key parameters included excitation flip angle (FL), echo time (TE), repetition time (TR), and receive bandwidth (BW). Quantitative analysis consisted of signal-to-noise-ratio (SNR) and contrast (CN), image nonuniformity (NU), full-width-at-half-maximum (FWHM) (i.e., blurring or geometric distortion), and ghosting ratio (GR). We found that flip angle, TE, and TR play particularly critical roles in determining image signal, homogeneity, and ghosting artifact with these sequences. Optimum clinical application of these pulse sequences requires careful attention to these imaging parameters and to their complex interactions.
We have investigated the use of two different image coregistration algorithms for identifying local regions of erroneously high fractional anisotropy (FA) as derived from diffusion tensor imaging (DTI) data sets in newborns. The first algorithm uses conventional affine registration of each of the diffusion-weighted images to the unweighted (b = 0) image for each slice, while the second algorithm uses second-order polynomial warping. Similarity between images was determined using the mutual information (MI) criterion, which is the preferred 'cost' criterion for coregistration of images with significantly different image intensity distributions. We have found that subtle differences exist in the FA values resulting from affine and second-order polynomial coregistration and demonstrate that nonlinear distortions introduce artifacts of spatial extent similar to real white matter structures in the newborn subcortex. We show that polynomial coregistration systematically reduces the presence of erroneous regions of high FA and that such artifacts can be identified by visual inspection of FA maps resulting from affine and polynomial coregistrations. Furthermore, we show that nonlinear distortions may be particularly pronounced when acquiring image slices of axial orientation at the height of the nasal cavity. Finally, we show that third-order polynomial MI coregistration (using the images resulting from second-order coregistration as input) has no observable effect on the resulting FA maps.
An actively detunable planar quadrature surface coil for human body imaging at 4 T has been constructed and compared with a conventional linear surface coil. The coil could be used as a transmit/receive or a receive-only device in combination with a volume transmit coil. Transmission, reception profiles and the corresponding images acquired with each coil, as well as with both individual modes of the quadrature coil, are presented. Data collected using a tissue equivalent loaded phantom recorded with the linear surface coil demonstrated significant intensity distortions due to RF penetration artifact. The quadrature surface coil, on the other hand, provided compensation of the artifact, separately in its transmission and reception profiles as well as in the resultant images. Substantial sensitivity gain was also observed for the quadrature coil compared to the linear device. Significant advantages of using the quadrature surface coil over the linear device at 4 T have, therefore, been demonstrated.
Fast imaging using the STimulated Echo Acquisition Mode (STEAM) sequence can produce cine images of the heart with black-blood contrast. Nevertheless, correction of deformation-related artifacts is required in order to maintain myocardial signal throughout the cardiac cycle. Recent work by our group has eliminated this artifact by combining two STEAM sequences acquired with two different demodulation gradients. Unfortunately, these two STEAM sequences were acquired on two separate breath-holds; thus, scan time doubled. In this work, we present a technique to reduce the total scan time by one half, without sacrificing image quality. The technique is based on interleaving two demodulations within one acquisition in order to obtain quality cine images of the heart in a single breath-hold. The technique was tested on animal models and human subjects, and the impact of interleaved acquisition on image quality was studied using quantitative and qualitative measures.
One of the standard applications of Independent Component Analysis (ICA) to EEG is removal of artifacts due to movements of the eye bulbs. Short blinks as well as slower saccadic movements are removed by subtracting respective independent components (ICs). EEG recorded from blind subjects poses special problems, since it shows a higher quantity of eye movements, which are also more irregular and very different across subjects. It is demonstrated that ICA can still be of use by comparing results from four blind subjects with results from one subject without eye bulbs who therefore does not show eye movement artifacts at all.
In functional cerebral studies, it has been established that co-registered electroencephalography (EEG) measurements and functional magnetic resonance imaging (fMRI) were complementary. However, EEG data recorded inside an MRI scanner are heavily distorted, mainly by the most prominent artifact, the cardiac pulse artifact (PA). We describe an original algorithm which yields a high-quality PA filter and demonstrates how this tool can be used to improve the quality of P300 ERP measurements during event-related fMRI (e-fMRI) experiments. EEG data were acquired in interleaved mode during e-fMRI while six healthy volunteers performed a visual odd-ball task, involving Distractors, Target and Novel stimuli, to elicit P300 components. The PA was corrected with the original algorithm. The temporal variations in the PA were evidenced using a principal component analysis (PCA), on each EEG channel. The procedure yielded several PA templates, which were regressed from the EEG data. The PA removal procedure was optimised, and then implemented to improve the measured P300 components. Regressing the most adequate PA template resulted in a high-quality reduction in spectral power at frequencies associated with the cardiac PA. More reliable P300 component measurements were obtained, evidencing higher amplitudes for Novels (9.76-11.20 microV) than for to Targets (6.3-9.09 microV) in centro-parietal and prefrontal areas. The improvement of the processing of EEG data acquired simultaneously with fMRI data provides a new tool and casts perspectives to study the functional organisation of the brain.