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At least 397 records · Page 22Linked to original sources
PubMed2000

Implantable spinal fusion stimulator: assessment of MR safety and artifacts.

The objective of this investigation was to perform magnetic resonance (MR) imaging safety and artifact testing of an implantable spinal fusion stimulator. Magnetic field interactions, artifacts, and operational aspects of an implantable spinal fusion stimulator were evaluated in association with a 1.5 T MR system. Magnetic field-related translational attraction was measured using the deflection angle test. A special test apparatus was used to determine torque at 4.7 T. Artifacts were characterized using fast multiplanar spoiled gradient-echo, T1-weighted spin-echo, and T1-weighted fast spin-echo sequences. Operational aspects of the implantable spinal fusion stimulator before and after exposure to MR imaging at 1.5 T were assessed. In addition, nine patients (six lumbar spine and three cervical spine) with implantable spinal fusion stimulators underwent MR imaging. The findings indicated that magnetic field interactions were relatively minor, artifacts were well characterized and should not create diagnostic problems, and there were no changes in the operation of the spinal fusion stimulator. The nine patients underwent MR procedures without substantial adverse events or complaints. Based on the results of this investigation and in consideration of the findings from previous studies of MR imaging safety for the implantable spinal fusion stimulator, MR imaging may be performed safely in patients using MR systems operating at 1.5 T or less following specific recommendations and precautions.

Artifacts↗
PubMed2001

Cardiac susceptibility artifacts arising from the heart-lung interface.

Cardiac MRI studies often show susceptibility artifacts along the inferoapical myocardial margin in both human and in vivo animal experiments at field strengths of 1.5T and greater. This study was designed to determine the cause of these artifacts in porcine myocardium at 3T. Gradient echo images were obtained under various anatomic and physiologic conditions to systematically study potential sources of local susceptibility gradients. Lung resection in the open-chested, euthanized swine was the only intervention that eliminated the artifact. The data suggest that in the porcine model, the heart-lung interface is the primary cause of these artifacts. Magn Reson Med 45:341-345, 2001.

Animals↗
PubMedDate not supplied

Surgical sutures: MR artifacts and sequence dependence.

Artifact reduction is fundamental to the daily clinical application of magnetic resonance (MR) imaging. Imaging of the postoperative patient may be difficult because of surgically introduced materials that result in artifacts. The authors tested some commonly used types of surgical suture for MR susceptibility artifact with various imaging sequences. Ten different suture types were studied. Suture was immersed in vegetable oil in separate plastic test tubes. The sutures were also studied embedded in meat. All samples were studied with T1-weighted and T2-weighted spin-echo, STIR (short-inversion-time inversion-recovery), and two-dimensional and three-dimensional gradient-echo sequences. Silk suture produced the most artifact.

Animals↗
PubMedDate not supplied

Pulsation artifact in short TR MR imaging and angiography: exacerbation with signal averaging.

Averaging the signals from more than one excitation per phase-encoding view increases the signal-to-noise ratio and, in conventional spin-echo magnetic resonance imaging, reduces most motion artifacts. To determine the effects of signal averaging on two-dimensional gradient-echo images, acquisitions with different TRs and with no averaging versus multiple-signal averaging were compared in a pulsatile flow phantom and the human abdominal aorta. Intraview (each view repeated before changing the phase-encoding value) and interview (obtaining all views sequentially and then repeating the entire set) averaging methods were used. Pulsation artifacts were present on all images of the flow phantom and the aorta. Intraview signal averaging, the method most commonly used, exacerbated rather than ameliorated pulsation artifacts with short TR sequences. Pulsation artifacts on two-dimensional images obtained with a short TR can be minimized by completing the acquisition as rapidly as possible, avoiding signal averaging. If signal averaging is used for short TR images, it should be interview averaging.

Algorithms↗
PubMedDate not supplied

MR imaging and vascular access ports: ex vivo evaluation of ferromagnetism, heating, and artifacts at 1.5 T.

The purpose of our study was to assess ferromagnetism, heating, and artifacts associated with vascular access ports exposed to a 1.5-T MR system. Twenty-eight different vascular access ports were evaluated in this investigation. Ferromagnetism was determined by using two previously described techniques. Temperature changes were measured immediately before and after performing a pulse sequence on the vascular access ports for 60 min at a specific absorption rate of 3.1 W/kg. Artifacts were assessed in association with the use of a fast GRASS pulse sequence. None of the vascular access ports displayed ferromagnetism. Heating was 0.2 degrees C or less. The presence of artifacts varied, depending on the component materials. The lack of ferromagnetism and negligible heating indicates that MR imaging performed at 1.5 T or less may be conducted safely in patients with any of the vascular access ports tested. None of the associated artifacts produced by the vascular access ports is considered to pose a substantial problem for MR imaging.

Artifacts↗
PubMedDate not supplied

Combined analysis of spatial and velocity displacement artifacts in phase contrast measurements of complex flows.

MR phase contrast (PC) velocity imaging is a promising tool for quantifying blood flow velocity in vivo. PC velocity imaging is, however, susceptible to artifacts that result from the displacement of spins during the finite duration pulse sequences. Such displacement artifacts can lead to errors in velocity measurements, especially in the presence of oblique and accelerating flows, which are common throughout the cardiovascular system. By tracking particles (representing spins) through a computed velocity field, and assuming that spatial and velocity encodings occur at discrete times during the pulse sequence, we simulate the separate and combined effects of oblique and acceleration artifacts on PC velocity images. We demonstrate, both by simulation and MR measurement, the errors associated with such artifacts in PC velocity measurements in a representative flow geometry. Using example particle trajectories, we provide a fluid dynamic basis for characteristic phase-velocity image distortions that can arise when imaging complex, physiologically relevant flows.

Animals↗
PubMedDate not supplied

Centric ordering is superior to gradient moment nulling for motion artifact reduction in EPI.

Echo-planar imaging (EPI) is sensitive to motion despite its rapid data acquisition rate. Compared with traditional imaging techniques, it is more sensitive to motion or flow in the phase-encode direction, which can cause image artifacts such as ghosting, misregistration, and loss of spatial resolution. Consequently, EPI of dynamic structures (eg, the cardiovascular system) could benefit from methods that eliminate these artifacts. In this paper, two methods of artifact reduction for motion in the phase-encode direction are evaluated. First, the k-space trajectory is evaluated by comparing centric with top-down ordered sequences. Next, velocity gradient moment nulling (GMN) of the phase-encode direction is evaluated for each trajectory. Computer simulations and experiments in flow phantoms and rabbits in vivo show that uncompensated centric ordering produces the highest image quality. This is probably due to a shorter readout duration, which reduces T2* relaxation losses and off-resonance effects, and to the linear geometry of phantoms and vessels, which can obscure centric blurring artifacts.

Animals↗
PubMedDate not supplied

The effect of mechanical deformation on magnetic properties and MRI artifacts of type 304 and type 316L stainless steel.

The purpose of this study was to evaluate the influence of composition and deformation of biomedical stainless steels on mechanical properties, magnetic properties, and MRI artifacts. Type 304 and Type 316L samples were prepared using standard wire-drawing techniques. Mechanical properties were determined using standard test methods. The amount of ferromagnetic phase present was estimated using a Severn Gage and x-ray diffraction. Magnetic field attraction and artifacts were determined using previously described techniques. The strength of both steels increased significantly with increasing deformation. None of the type 316L wires transformed to the magnetic phase. The amount of magnetic phase in the type 304 wires increased with increasing deformation. There was no magnetic field attraction, and artifacts were minimal for all of type 316L wires and the undeformed type 304 wire. Deflection and artifacts were significant for the deformed type 304 stainless steel. These results provide guidance regarding the use of type 304 and type 316L stainless steels for bioimplants. In this regard, type 316L stainless steel seems to be a more acceptable material with respect to MR compatibility.

Artifacts↗
PubMed2006

An unusual cause of susceptibility artifact in magnetic resonance imaging.

Magnetic susceptibility artifact has been documented with various implants and devices, which require appropriate identification with screening of patients prior to subjecting them to magnetic resonance examination. We performed cardiovascular magnetic resonance (CMR) examination of the aorta in a 24-year-old woman in the setting of repaired aortic coarctation, and found magnetic susceptibility artifact arising from the stomach in the absence of known susceptibility artifact-producing material in this region. Further history revealed that she had ingested a prenatal vitamin prior to imaging, prompting experimental analyses that led us to conclude that iron-containing vitamins may be a source of magnetic susceptibility artifact.

Adult↗
PubMed2002

Band artifacts due to bulk motion.

Band artifacts due to bulk motion were investigated in images acquired with fast gradient echo sequences. A simple analytical calculation shows that the width of the artifacts has a square-root dependence on the velocity of the imaged object, the time taken to acquire each line of k-space and the field of view in the phase-encoding direction. The theory furthermore predicts that the artifact width can be reduced using parallel imaging by a factor equal to the square root of the acceleration parameter. The analysis and results are presented for motion in the phase- and frequency-encoding directions and comparisons are made between sequential and centric ordering. The theory is validated in phantom experiments, in which bulk motion is simulated in a controlled and reproducible manner by rocking the scan table back and forth along the bore axis. Preliminary cardiac studies in healthy human volunteers show that dark bands may be observed in the endocardium in images acquired with nonsegmented fast gradient echo sequences. The fact that the position of the bands changes with the phase-encoding direction suggests that they may be artifacts due to motion of the heart walls during the image acquisition period.

Artifacts↗
PubMed2004

Flow artifacts in steady-state free precession cine imaging.

Steady-state free precession (SSFP) cardiac cine images are frequently corrupted by dark flow artifacts, which can usually be eliminated by reshimming and retuning the scanner. A theoretical explanation for these artifacts is provided in terms of spins moving through an off-resonant point in the magnetic field, and the theory is validated using phantom experiments. The artifacts can be reproduced in vivo by detuning the center frequency by an amount in the range of half the inverse repetition time (TR). Since this offset is similar in magnitude to the frequency difference between the water and lipid peaks, a likely cause of the artifacts in vivo is that the center frequency is tuned incorrectly to the lipid peak rather than the water peak.

Artifacts↗
PubMed2004

Artifact suppression in imaging of myocardial infarction using B1-weighted phased-array combined phase-sensitive inversion recovery.

Regions of the body with long T1, such as cerebrospinal fluid (CSF), may create ghost artifacts on gadolinium-hyperenhanced images of myocardial infarction when inversion recovery (IR) sequences are used with a segmented acquisition. Oscillations in the transient approach to steady state for regions with long T1 may cause ghosts, with the number of ghosts being equal to the number of segments. B1-weighted phased-array combining provides an inherent degree of ghost artifact suppression because the ghost artifact is weighted less than the desired signal intensity by the coil sensitivity profiles. Example images are shown that illustrate the suppression of CSF ghost artifacts by the use of B1-weighted phased-array combining of multiple receiver coils.

Artifacts↗
PubMed1992

Partial RF echo planar imaging with the FAISE method. I. Experimental and theoretical assessment of artifact.

The fast acquisition interleaved spin-echo (FAISE) method is a partial RF echo-planar technique which utilizes a specific phase-encode reordering algorithm to manipulate image contrast (Melki et al., J. Magn. Reson. Imaging 1:319, 1991). The technique can generate "spin-echo" like images up to 16 times faster than conventional spin-echo methods. However, the presence of T2 decay throughout the variable k-space trajectories used to manipulate T2 contrast ensures the presence of image artifacts, especially along the phase-encode direction. In this work, we experimentally and theoretically examine the type and extent of artifacts associated with the FAISE technique. We demonstrate the existence of well-defined minima of phase-encode ghost noise for selected k-space trajectories, examine the extent of blurring and edge enhancement artifacts, demonstrate the influence of matrix size and number of echoes per train on phase-encode artifact, and show how proper choice of FAISE sequence parameters can lead to proton density brain images which are practically indistinguishable from conventional spin-echo proton density images. A comparison of contrast between FAISE and standard spin-echo methods is presented in a companion article referred to as II.

Algorithms↗
PubMed1993

Pulsatile flow artifacts in 3D magnetic resonance imaging.

Some of the important features of how pulsatile flow generates artifacts in three-dimensional magnetic resonance imaging are analyzed and demonstrated. Time variations in the magnetic resonance signal during the heart cycle lead to more complex patterns of artifacts in 3D imaging than in 2D imaging. The appearance and location of these artifacts within the image volume are shown to be describable as displacements along a line in a plane parallel to that defined by the phase and volume encode directions. The angle of the line in the plane depends solely upon the imaging parameters while the ghost displacement along the line is proportional to the signal modulation frequency. Aliasing of these ghosts leads to a variety of artifact patterns which are sensitive to the pulsation period and repetition time of the pulse sequence. Numerical simulations of these effects were found to be in good agreement with experimental images of an elastic model of a human carotid artery under simulated physiological conditions and with images of two human subjects.

Aged↗
PubMed1995

Motion artifacts in fMRI: comparison of 2DFT with PR and spiral scan methods.

Activation signals based on BOLD contrast changes consequent to neuronal stimulation typically produce cortical intensity differences of < 10% at 1.5T. Hemodynamically driven pulsation of the brain can cause highly pulsatile phase shifts, which in turn result in motion artifacts whose intensity is larger than the activation signals in 2DFT scan methods. This paper presents a theoretical and experimental comparison of the magnitude of such artifacts for 2DFT and two other methods using non-Cartesian k-space trajectories. It is shown that artifacts increase with TR for 2DFT methods, and that projection reconstruction (PR) and spiral methods have significantly reduced artifact intensities, because these trajectories collect low spatial frequencies with every view. The spiral technique is found to be superior in terms of efficiency and motion insensitivity.

Artifacts↗
PubMed2004

Removal of EPI Nyquist ghost artifacts with two-dimensional phase correction.

Odd-even echo inconsistencies result in Nyquist ghost artifacts in the reconstructed EPI images. The ghost artifacts reduce the image signal-to-noise ratio and make it difficult to correctly interpret the EPI data. In this article a new 2D phase mapping protocol and a postprocessing algorithm are presented for an effective Nyquist ghost artifacts removal. After an appropriate k-space data regrouping, a 2D map accurately encoding low- and high-order phase errors is derived from two phase-encoded reference scans, which were originally proposed by Hu and Le (Magn Reson Med 36:166-171;1996) for their 1D nonlinear correction method. The measured phase map can be used in the postprocessing algorithm developed to remove ghost artifacts in subsequent EPI experiments. Experimental results from phantom, animal, and human studies suggest that the new technique is more effective than previously reported methods and has a better tolerance to signal intensity differences between reference and actual EPI scans. The proposed method may potentially be applied to repeated EPI measurements without subject movements, such as functional MRI and diffusion coefficient mapping.

Animals↗
PubMed2004

Coherence-induced artifacts in large-flip-angle steady-state spin-echo imaging.

High-resolution imaging of trabecular bone aimed at analyzing the bone's microarchitecture is preferably performed with spin-echo-type pulse sequences. Unlike gradient echoes, spin-echoes are immune to artifactual broadening of trabeculae caused by local static field gradients near the bone-bone marrow interface and signal loss from chemical shift dephasing at k-space center. However, the previously practiced 3D fast large-angle spin-echo (FLASE) pulse sequence was found to be prone to a low-frequency modulation artifact in both the readout and slice direction. The artifact is caused by deviations in the effective flip angle of the nonselective 180 degrees pulse, which converts a fraction of the phase-encoded transverse magnetization to longitudinal magnetization. The latter recurs as transverse magnetization in the subsequent pulse sequence cycle forming a spurious stimulated echo. The objective of this work was to perform a k-space analysis of this steady-state artifact and propose two modifications of the original 3D FLASE that effectively remove it. The results of the simulations were in exact agreement with the experiments and the proposed remedy was found to eliminate the artifact.

Artifacts↗
PubMed2004

Coil-based artifact reduction.

Multiple MRI receiver coils provide extra information and can enable the reconstruction of multiple images using data from different combinations of coils. Comparison of these images shows that artifacts due to motion or flowing blood appear with different intensities due to the differing coil sensitivities. Typically, the artifact appears amplified in regions of low coil sensitivity. An optimization routine was developed to correct for the artifact by comparing reconstructions from various coil combinations and favoring a self-consistent solution. It is demonstrated that images artifacted by blood flowing in the aorta, or translational motion of the head, can be improved.

Algorithms↗