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At least 919 records · Page 51Linked to original sources
PubMed1996

Elimination of artifacts in short echo time H MR spectroscopy of the frontal lobe.

Localized 1H spectra from the frontal part of the human brain are often distorted by "ghost" artifacts, especially with short echo time PRESS sequences. These artifacts are caused by insufficient dephasing of unwanted coherences involving magnetization in the sinuses and the mouth. Because the various unwanted coherences of a PRESS sequence are dephased differently, the order of the slice gradients has a pronounced effect on the spectral quality. For frontal voxels, the last slice gradient has to be applied in the axial direction. With this slice order, the major metabolites can be quantified with an interindividual variability of about 10%.

Artifacts↗
PubMed1996

Biopsy needle susceptibility artifacts.

Understanding the appearance of thin metallic structures in magnetic resonance imaging is important for evaluating the potential role of MRI in guiding and monitoring percutaneous interventions. As most MR compatible instruments are made from materials with a susceptibility different from water, their visibility is enhanced beyond what is expected on the grounds of displaced water alone. Unfortunately, this artifactually enhanced visibility is not constant, but instead depends on a variety of factors. This article presents computer simulations of the image distortion resulting from magnetic susceptibility differences between a needle and the surrounding tissue. The simulations show not only an artifact size that is dependent on needle composition, orientation, and pulse sequence, but also a corresponding shift of the artifact center away from the actual center of the needle. These effects place limits on the accuracy of MRI-guided needle tip placement.

Artifacts↗
PubMed1997

Ghost artifact reduction for echo planar imaging using image phase correction.

An algorithm is described for reducing ghost artifacts in echo planar imaging (EPI) using phase corrections derived from images reconstructed using only even or odd k-space lines. The N/2 ghost, that arises principally from time-reversal of alternate k-space lines, was significantly reduced by this algorithm without the need for a calibration scan. In images obtained in eight subjects undergoing EPI for auditory functional MRI (fMRI) experiments, N/2 ghost intensity was reduced from 10.3% +/- 2.1% (range: 7.9-14.1%) to 4.5% +/- 0.2% (range: 4.1-4.9%) of parent image intensity, corresponding to a percent reduction in ghost intensity of 54% +/- 9% (range: 43-65%), and the algorithm restored this intensity to the parent image. It provided a significant improvement in image appearance, and increased the correlation coefficients related to neural activation in functional MRI studies. The algorithm provided reduction of artifacts from all polynomial orders of spatial phase errors in both spatial directions. The algorithm did not eliminate N/2 ghost intensity contributed by field inhomogeneities, susceptibility, or chemical shift.

Algorithms↗
PubMed1997

STAR-HASTE: perfusion imaging without magnetic susceptibility artifact.

A novel magnetic resonance imaging technique (STAR-HASTE) based on pulsed arterial spin labeling using a single shot acquisition method is described for perfusion imaging. The method is similar to EPISTAR in using STAR (Signal Targeting with Alternating Radiofrequency) technique for pulsed radiofrequency labeling of inflowing blood, but uses a half-Fourier single shot turbo spin-echo (HASTE) sequence for data acquisition instead of echo-planar imaging (EPI). Our preliminary studies show that STAR-HASTE permits perfusion imaging to be performed without many of the artifacts encountered with other imaging methods based on EPI acquisition. The novel method not only provides similar perfusion information to that obtained by EPISTAR, as demonstrated in the functional brain imaging study, but also eliminates magnetic susceptibility artifacts and image distortion commonly observed in EPI images. Furthermore, this technique can be readily implemented in MR systems without EPI capability.

Artifacts↗
PubMed1998

Removal of local field gradient artifacts in T2*-weighted images at high fields by gradient-echo slice excitation profile imaging.

Development of high magnetic field MRI techniques is hampered by the significant artifacts produced by B0 field inhomogeneities in the excited slices. A technique, gradient-echo slice excitation profile imaging (GESEPI), is presented for recovering the signal lost caused by intravoxel phase dispersion in T2*-weighted images. This technique superimposes an incremental gradient offset on the slice refocusing gradient to sample k-space over the full range of spatial frequencies of the excitation profile. A third Fourier transform of the initial two-dimensional image set generates an image set in which the artifacts produced by the low-order B0 inhomogeneity field gradients in the sample are separated and removed from the high-order microscopic field gradients responsible for T2* contrast. Application to high field brain imaging, at 3.0 T for human and at 9.4 T for immature rat imaging demonstrates the significant improvement in quality of the T2*-weighted contrast images.

Algorithms↗
PubMed2004

Spiral readout gradients for the reduction of motion artifacts in chemical shift imaging.

A motion artifact reduction method for proton chemical shift imaging (CSI) is presented. The method uses spiral-based readout gradients for data acquisition. A characteristic of spiral-based readout gradients is that data are repeatedly sampled at the kxy origin. These data points are used to estimate and correct for motion-induced phase variations. Both phantom and in vivo spectra reconstructed using the new motion artifact reduction algorithm showed significant signal-to-noise ratio (SNR) improvements as compared to uncorrected data.

Algorithms↗
PubMed2004

Displacement-encoded cardiac MRI using cosine and sine modulation to eliminate (CANSEL) artifact-generating echoes.

Displacement-encoded imaging with stimulated echoes (DENSE) and harmonic phase imaging (HARP) employ 1-1 spatial modulation of magnetization to cosine modulate the longitudinal magnetization as a function of position at end diastole. Later in the cardiac cycle they sample the cosine-modulated signal and compute myocardial strain from the signal phase. The sampled signal generally includes three distinct echoes: 1) a displacement-encoded stimulated echo, 2) the complex conjugate of the displacement-encoded echo, and 3) an echo arising from T1 relaxation. If the T1-relaxation and complex conjugate echoes are suppressed, then a phase image representing just the displacement-encoded echo can be reconstructed. In the present study, the use of cosine and sine modulation to eliminate (CANSEL) the T1-relaxation and complex conjugate echoes was investigated. With the use of CANSEL, it was demonstrated that DENSE accurately measures through-plane as well as in-plane components of tissue motion. Also, DENSE with CANSEL artifact suppression can provide increased signal-to-noise ratio (SNR) secondary to reduced intravoxel dephasing by using relatively low displacement-encoding frequencies. For applications that employ DENSE imaging with multiple acquisitions, the CANSEL technique can suppress artifact-generating echoes without placing constraints on the displacement-encoding frequency and direction.

Animals↗
PubMed2005

Artifact and noise suppression in GRAPPA imaging using improved k-space coil calibration and variable density sampling.

A parallel imaging technique, GRAPPA (GeneRalized Auto-calibrating Partially Parallel Acquisitions), has been used to improve temporal or spatial resolution. Coil calibration in GRAPPA is performed in central k-space by fitting a target signal using its adjacent signals. Missing signals in outer k-space are reconstructed. However, coil calibration operates with signals that exhibit large amplitude variation while reconstruction is performed using signals with small amplitude variation. Different signal variations in coil calibration and reconstruction may result in residual image artifact and noise. The purpose of this work was to improve GRAPPA coil calibration and variable density (VD) sampling for suppressing residual artifact and noise. The proposed coil calibration was performed in local k-space along both the phase and frequency encoding directions. Outer k-space was acquired with two different reduction factors. Phantom data were reconstructed by both the conventional GRAPPA and the improved technique for comparison at an acceleration of two. Under the same acceleration, optimal sampling and calibration parameters were determined. An in vivo image was reconstructed in the same way using the predetermined optimal parameters. The performance of GRAPPA was improved by the localized coil calibration and VD sampling scheme.

Algorithms↗
PubMed2005

Metal artifacts caused by gradient switching.

In metal parts, e.g., implants or instruments, eddy currents can be induced from gradient switching if positioned off-center inside the MR scanner. For the first time, a systematic analysis of related artifacts was performed. Current strength increases in conjunction with increasing size of the part, increasing electrical conductivity, distance from isocenter, and increasing gradient strengths. A xy-plane oriented copper ring (d(o) = 20 mm, d(i) = 15 mm, 2 mm thick) was examined at isocenter and at x = 15 cm, y = z = 0. Comparisons of xy-, xz-, and yz-slices, recorded for both possibilities to select encoding directions, revealed effects from ramp-down of the slice-selection and ramp-up of the read-out gradient. Near the metal part, temporary inhomogeneities were superimposed to the static field and spin-dephasing signal loss resulted, despite using spin-echo technique. Artifacts depended on excitation and read-out bandwidth. For an equivalent titanium ring, conductivity related effects could not be ascertained but distinct susceptibility effects occurred. MR compatibility of implants/instruments therefore requires both low susceptibility and low conductivity.

Artifacts↗
PubMed2005

Suppression of lipid artifacts in amide proton transfer imaging.

Amide proton transfer (APT) imaging is a type of chemical exchange saturation transfer imaging in which the amide protons of cellular proteins and peptides are saturated and detected via the water resonance. To study this effect, conventional magnetization transfer and direct saturation effects in the frequency-dependent water saturation spectrum (z-spectrum) need to be removed by asymmetry analysis with respect to the water frequency offset. When using echo planar imaging, it was found that unequal pericranial fat saturation at equidistant higher and lower frequencies with respect to water leads to a lipid artifact in APT asymmetry images. It is demonstrated that a chemical-shift-selective refocusing pulse in combination with crusher gradients can suppress this artifact and provide high-quality images.

Amides↗
PubMed2005

Ghost artifact removal using a parallel imaging approach.

Parallel imaging techniques, which use several receive coils simultaneously, have been shown to enable a significant scan time reduction by subsampling k-space. Nevertheless, the data acquired with multiple coils in parallel exhibit some redundancy if the number of receive coils exceeds the subsampling factor. This redundancy leads to an overdetermination of the reconstruction problem, which is generally used to optimize the signal-to-noise ratio (SNR). However, it can yield further information about the quality of the reconstructed image, and can thus be used to identify and correct image artifacts. While some known approaches try to solve this problem in k-space, this study addresses it in the spatial domain and uses a modified SENSE reconstruction to reduce or completely remove ghost-type artifacts arising from processes such as motion or flow during data acquisition. Phantom and in vivo studies show significant improvements in image quality after correction, and serve as a basis for the discussion of the performance and limitations of this new approach.

Algorithms↗
PubMed2005

On the dark rim artifact in dynamic contrast-enhanced MRI myocardial perfusion studies.

A dark band or rim along parts of the subendocardial border of the left ventricle (LV) and the myocardium has been noticed in some dynamic contrast-enhanced MR perfusion studies. The artifact is thought to be due to susceptibility effects from the gadolinium bolus, motion, or resolution, or a combination of these. Here motionless ex vivo hearts in which the cavity was filled with gadolinium are used to show that dark rim artifacts can be consistent with resolution effects alone.

Algorithms↗
PubMed2006

BOLD contrast sensitivity enhancement and artifact reduction with multiecho EPI: parallel-acquired inhomogeneity-desensitized fMRI.

Functional MRI (fMRI) generally employs gradient-echo echo-planar imaging (GE-EPI) to measure blood oxygen level-dependent (BOLD) signal changes that result from changes in tissue relaxation time T(*) (2) between activation and rest. Since T(*) (2) strongly varies across the brain and BOLD contrast is maximal only where the echo time (TE) equals the local T(*) (2), imaging at a single TE is a compromise in terms of overall sensitivity. Furthermore, the long echo train makes EPI very sensitive to main field inhomogeneities, causing strong image distortion. A method is presented that uses accelerated parallel imaging to reduce image artifacts and acquire images at multiple TEs following a single excitation, with no need to increase TR. Sensitivity gains from the broadened T(*) (2) coverage are optimized by pixelwise weighted echo summation based on local T(*) (2) or contrast-to-noise ratio (CNR) measurements. The method was evaluated using an approach that allows differential BOLD CNR to be calculated without stimulation, as well as with a Stroop experiment. Results obtained at 3 T showed that BOLD sensitivity improved by 11% or more in all brain regions, with larger gains in areas typically affected by strong susceptibility artifacts. The use of parallel imaging markedly reduces image distortion, and hence the method should find widespread application in functional brain imaging.

Adult↗
PubMed2004

Issues of spectral quality in clinical 1H-magnetic resonance spectroscopy and a gallery of artifacts.

In spite of the facts that magnetic resonance spectroscopy (MRS) is applied as clinical tool in non-specialized institutions and that semi-automatic acquisition and processing tools can be used to produce quantitative information from MRS exams without expert information, issues of spectral quality and quality assessment are neglected in the literature of MR spectroscopy. Even worse, there is no consensus among experts on concepts or detailed criteria of quality assessment for MR spectra. Furthermore, artifacts are not at all conspicuous in MRS and can easily be taken for true, interpretable features. This article aims to increase interest in issues of spectral quality and quality assessment, to start a larger debate on generally accepted criteria that spectra must fulfil to be clinically and scientifically acceptable, and to provide a sample gallery of artifacts, which can be used to raise awareness for potential pitfalls in MRS.

Algorithms↗
PubMed2001

Digital subtraction angiogram registration method with local distortion vectors to decrease motion artifact.

We have been investigating registration methods for improving digital subtraction angiography (DSA) images to extract blood vessels by reducing artifacts due to body motion, such as rotation, contraction, and dilation. In this paper, we propose a new and simple DSA registration algorithm with local distortion vectors to reduce artifacts. According to the results, the proposed method works well for vascular system around the nasal cavity and the orbit of the head and neck DSA images, which cannot be observed clearly by conventional methods. Additionally, we have applied the proposed method to abdominal and leg DSA images.

Algorithms↗
PubMed1994

Overcoming solvent saturation-transfer artifacts in protein NMR at neutral pH. Application of pulsed field gradients in measurements of 1H-15N Overhauser effects.

Artifacts due to solvent saturation-transfer effects result in incorrect measurements of 1H-15N heteronuclear NOE (HNOE). These artifacts are commonly observed in aqueous protein solutions at neutral pH. We describe the application of pulsed field gradients (PFGs) together with long recycle delays in overcoming errors in HNOE measurements which arise from H2O solvent preirradiation and solvent saturation transfer. Even in the absence of explicit solvent irradiation, the HNOE pulse sequence itself results in a nonequilibrium spin-state distribution of solvent nuclei which can then be transferred by chemical exchange into amide-proton sites. This effect can be avoided by using PFGs for suppression of solvent H2O together with a recycle delay sufficiently long for the magnetization of water to relax back to equilibrium values during the preparation period. These effects were studied in 15N-enriched human type alpha transforming growth factor at pH 7.1. Comparisons of PFG-HNOE experiments with and without selective H2O irradiation and with different recycle times provide estimates of the effects of solvent irradiation on HNOE measurements, which are different for different amide nitrogen-15 nuclei. The amplitudes of these artificial HNOE enhancements are roughly correlated with solvent accessibilities of amide sites in the three-dimensional structure of hTGF alpha.

Artifacts↗
PubMed1998

Image-based reduction of artifacts in multishot echo-planar imaging.

The method to reduce the ghost artifact in echo-planar imaging (EPI) using a phase correction derived from the image data (M. H. Buonocore and L. Gao, Magn. Reson. Med. 38, 89 (1997)) is generalized to multishot (interleaved) EPI, where the artifact takes the form of multiple ghosts. The method is shown to be much more sensitive to noise when applied to standard interleaved data than is the case with single-shot EPI, because the calculation must be based on high-order ghosts of low intensity. A modified interleaving scheme is proposed for multishot EPI in which the initial trajectory direction alternates in consecutive shots and the number of shots is odd. With this scheme, only a single ghost shifted by one-half of the field of view appears just as in the single-shot EPI, and the image-based phase correction can be applied with the usual sensitivity to noise.

Artifacts↗
PubMed1999

Artifacts introduced by zero order phase correction in proton NMR spectroscopy and a method of elimination by phase filtering.

In in-vivo applications of proton NMR spectroscopic imaging, an oscillatory "ringing" artifact has been observed in some of the spectra. The source of this artifact was found to be the presence of a harmonic "beating" effect in the amplitude of the water reference free induction decay (FID) which was used for zero order phase correction for B0 inhomogeneity and eddy current compensation. The source of the beats was found to be the presence of distinct populations of spins resonating at slightly different frequencies. When the common method of zero order phase correction was implemented using such an FID, the resulting phase-corrected, water-suppressed spectra displayed ringing. Examination of the unwrapped phase correction angle revealed unexpected jumps in phase at points in time corresponding to nodes in the amplitude of the FID. Low-pass filtering of the phase correction angle of the reference FID was found to smooth out these unanticipated phase jumps. When used as a reference for phase correction, the filtered phase information gave a phase-corrected, water-suppressed spectrum free from ringing.

Artifacts↗