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At least 937 records · Page 52Linked to original sources
PubMed1990

[High-field magnetic resonance imaging of the liver compared to x-ray computed tomography. Preliminary study apropos of 39 cases].

Thirty nine patients with one or more focal hepatic lesions were examined by contrast enhanced computed tomography (CE-CT) and magnetic resonance imaging (MRI). A variety of pulse sequences--spin echo (SE), gradient echo (GE) and inversion recovery (IR)--have been reported in the literature on MRI concerning the detection and characterization of liver tumors. Multiple studies have compared MRI at different field strengths to CT. As controversy still exists concerning the optimal pulse sequence on MRI, CE-CT has been compared to T2 weighted SE sequence in this study. CT, as well as MRI, identified abnormalities in liver parenchyma in all patients. As far as detection of hepatic lesions is concerned, MRI and CE-CT were equal in 35 cases and MRI was superior in the other four cases. However, CT remains the examination of choice for detection of focal lesions, due to the short examination time, the low cost and the superiority in detection of extrahepatic pathology.

Adolescent↗
PubMed2004

Real-time cardiac MRI at 3 tesla.

Real-time cardiac and coronary MRI at 1.5T is relatively "signal starved" and the 3T platform is attractive for its immediate factor of two increase in magnetization. Cardiac imaging at 3T, however, is both subtly and significantly different from imaging at 1.5T because of increased susceptibility artifacts, differences in tissue relaxation, and RF homogeneity issues. New RF excitation and pulse sequence designs are presented which deal with the fat-suppression requirements and off-resonance issues at 3T. Real-time cardiac imaging at 3T is demonstrated with high blood SNR, blood-myocardium CNR, resolution, and image quality, using new spectral-spatial RF pulses and fast spiral gradient echo pulse sequences. The proposed sequence achieves 1.5 mm in-plane resolution over a 20 cm FOV, with a 5.52 mm measured slice thickness and 32 dB of lipid suppression. Complete images are acquired every 120 ms and are reconstructed and displayed at 24 frames/sec using a sliding window. Results from healthy volunteers show improved image quality, a 53% improvement in blood SNR efficiency, and a 232% improvement in blood-myocardium CNR efficiency compared to 1.5T.

Adipose Tissue↗
PubMed1999

Flow effects in localized quadratic, partial Fourier MRA.

A pulse sequence for inflow-enhanced magnetic resonance angiography, including localized quadratic encoding, partial-Fourier slice selection, and spiral in-plane encoding, is analyzed. The through-plane encoding method is discussed in a space-spatial frequency context to illustrate some of its properties. This pulse sequence has the advantages of being faster and more robust to turbulent flow than conventional inflow-enhanced methods. Simulations show the effect of different parameters on the modulation-transfer function of the resulting images. A flow phantom is used to verify some of the simulation results.

Computer Simulation↗
PubMedDate not supplied

MRI acoustic noise: sound pressure and frequency analysis.

The large gradient coils used in MRI generate, simultaneously with the pulsed radiofrequency (RF) wave, acoustic noise of high intensity that has raised concern regarding hearing safety. The sound pressure levels (SPLs) and power spectra of MRI acoustic noise were measured at the position of the human head in the isocenter of five MRI systems and with 10 different pulse sequences used in clinical MR scanning. Each protocol, including magnetization-prepared rapid gradient echo (MP-RAGE; 113 dB SPL linear), fast gradient echo turbo (114 dB SPL linear), and spin echo T1/2 mm (117 dB SPL linear), was found to have the high SPLs, rapid pulse rates, amplitude-modulated pulse envelopes, and multipeaked spectra. Since thickness and SPL were inversely related, the T1-weighted images generated more intense acoustic noise than the proton-dense T2-weighted measures. The unfiltered linear peak values provided more accurate measurements of the SPL and spectral content of the MRI acoustic noise than the commonly used dB A-weighted scale, which filters out the predominant low frequency components. Fourier analysis revealed predominantly low frequency energy peaks ranging from .05 to approximately 1 kHz, with a steep high frequency cutoff for each pulse sequence. Ear protectors of known attenuation ratings are recommended for all patients during MRI testing.

Calibration↗
PubMed2002

Spectral profiles of cultured neuronal and glial cells derived from HRMAS (1)H NMR spectroscopy.

In the investigations of brain function and pathology in vivo by magnetic resonance spectroscopy (MRS), a decrease in the relative concentration of N-acetyl aspartate (NAA) has been correlated with neuronal cell damage or loss, while a relative increase in the resonance intensity of creatine has been correlated with gliosis. However, neither metabolite is confined strictly to one cell-type. In this study, pattern recognition of spectra derived from high-resolution magic angle spinning (HRMAS) (1)H NMR spectroscopy was used to distinguish three neural cell types; cortical astrocytes, cerebellar neurones and O-2A progenitors. The intact cells contained significant amounts of lipid resonances (-CH(2)CH(3) and -CH(2)CH(2)CH(2)-) in all three cell-types, even when a T(2)-edited Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence was used, selectively attenuating resonances from macromolecules. Creatine was also detected in all three cell types. Principle component analysis (PCA) readily differentiated the NMR spectra, based on the individual metabolic profile derived from the cohort of cell type examined using conventional solvent-suppressed and CPMG pulse sequences. Creatine was not found to contribute to this separation. Moreover, the large lipid content of neuronal cells contributed most to the separation from the other cell types. This suggests that during MRS in vivo, where lipid resonances are commonly 'edited out' by T(2) delays, significant information may be sacrificed concerning relative contribution from individual cell types.

Amino Acids↗
PubMed1984

Contrast manipulation in NMR imaging.

The past few years have shown rapid growth of NMR imaging in both image quality and diagnostic usefulness. It has become apparent, as the images have been published, that both inter- and intra-group imaging of the same underlying pathology produces images which can have vastly differing appearance. This effect is mainly due to imaging techniques which use different pulse sequence types and timings thus varying the relative contribution of the protpn density, T1, and T2 properties of the tissues. In this paper we investigate the contrast manipulation effects and methods for SNR optimization for the saturation recovery, inversion recovery, spin echo, and inversion recovery spin echo pulse sequences when applied to three clinically relevant imaging tasks.

Cerebral Infarction↗
PubMed2001

Localization of post-traumatic trochlear nerve palsy associated with hemorrhage at the subarachnoid space by magnetic resonance imaging.

PURPOSE: To report evaluation of traumatic trochlear nerve palsy using head magnetic resonance imaging. DESIGN: Observational case reports. METHODS: We examined two cases involving trochlear nerve palsy after closed head injury. RESULTS: Using a fluid attenuated inversion recovery pulse sequence, MRI showed a high-intensity lesion consistent with subarachnoid hemorrhage at the trochlear nerve area in the ambient cisterns. CONCLUSION: An impact force directed toward the tentorium can be a mechanism of injury in some post-traumatic trochlear nerve palsies. Fluid attenuated inversion recovery pulse sequence is a sensitive method for detection of abnormalities in cases associated with head injury.

Adolescent↗
PubMed1989

Hepatic tumors: comparison of CT during arterial portography, delayed CT, and MR imaging for preoperative evaluation.

Forty-three patients with known primary or secondary neoplastic involvement of the liver underwent evaluation to determine the number, size, and location of focal lesions before possible tumor resection. Imaging studies included computed tomography (CT) during arterial portography (CTAP), delayed CT, and magnetic resonance (MR) imaging at various pulse sequences. Results of radiologic studies were compared with surgical and pathologic findings. In the combined group of surgical and nonsurgical patients, CTAP was significantly more sensitive (85%) than all other techniques except 1.5-T T2-weighted spin-echo imaging (64%). Combining the information from all MR pulse sequences yielded a cumulative sensitivity of 68%. Combining the information from two modalities yielded sensitivity of 96% for CTAP plus MR imaging, 85% for CTAP plus delayed CT, and 77% for delayed CT plus MR imaging. The authors conclude that when it is vital to know the precise number, size, and location of focal hepatic lesions before tumor resection, CTAP has the highest sensitivity, but MR imaging is an important adjuvant.

Adult↗
PubMed1998

Minimizing artifacts caused by metallic implants at MR imaging: experimental and clinical studies.

OBJECTIVE: The purpose of this study was to investigate the effect of metallic implant positioning on MR imaging artifacts, to determine the optimal imaging conditions for minimizing artifacts, and to show the usefulness of artifact-minimizing methods in imaging of the knee. MATERIALS AND METHODS: Using MR images of experimental phantoms (titanium alloy and stainless steel screws), we compared the magnitude of metal-induced artifacts for various pulse sequences, different imaging parameters for the fast spin-echo sequence, and different imaging parameters for several incremental angles between the long axis of the screw and the direction of the main magnetic field. In clinical MR imaging of knees with metallic implants (n = 19), we assessed geometric distortion of anatomic structures to compare the influence of different pulse sequences (n = 19), frequency-encoding directions (n = 7), and knee positions (n = 15). RESULTS: Titanium alloy screws consistently produced smaller artifacts than did stainless steel screws. In experimental MR studies, artifacts were reduced with fast spin-echo sequences, with a screw orientation as closely parallel to the main magnetic field as possible, and, particularly, with smaller voxels that correlated positively with artifact size (R2 = .88, p < .01). In clinical MR studies, fast spin-echo MR imaging obscured articular structures less than did spin-echo imaging (8/19 patients). In particular, the anterior-posterior frequency-encoding direction (3/7 patients) and the flexion position of the knee (5/15 patients) were effective in reducing artifacts. CONCLUSION: MR artifacts can be minimized by optimally positioning in the magnet subjects with metallic implants and by choosing fast spin-echo sequences with an anterior-posterior frequency-encoding direction and the smallest voxel size.

Adult↗
PubMed2005

Sequence design for magnetic resonance spectroscopic imaging of prostate cancer at 3 T.

Magnetic resonance spectroscopic imaging (MRSI) has proven to be a powerful tool for the metabolic characterization of prostate cancer in patients before and following therapy. The metabolites that are of particular interest are citrate and choline because an increased choline-to-citrate ratio can be used as a marker for cancer. High-field systems offer the advantage of improved spectral resolution as well as increased magnetization. Initial attempts at extending MRSI methods to 3 T have been confounded by the J-modulation of the citrate resonances. A new pulse sequence is presented that controls the J-modulation of citrate at 3 T such that citrate is upright, with high amplitude, at a practical echo time. The design of short (14 ms) spectral-spatial refocusing pulses and trains of nonselective refocusing pulses are described. Phantom studies and simulations showed that upright citrate with negligible sidebands is observed at an echo time of 85 ms. Studies in a human subject verified that this behavior is reproduced in vivo and demonstrated that the water and lipid suppression of the new pulse sequence are sufficient for application in prostate cancer patients.

Choline↗
PubMed1988

Focal hepatic lesions: detection by dynamic and delayed computed tomography versus short TE/TR spin echo and fast field echo magnetic resonance imaging.

Eighteen patients with focal hepatic lesions were evaluated with two computed tomographic (CT) techniques including dynamic sequential bolus contrast CT and delayed contrast CT, and 3 magnetic resonance (MR) techniques including a spin echo pulse sequence with TE/TR of 21/310 msec and 2 fast field echo sequences using a TE/TR of 15/300 msec and 80 degrees flip angle (T1-weighted) and TE/TR of 15/500 msec and 10-20 degrees flip angle (T2-weighted). We concluded that CT, using delayed contrast and dynamic sequential bolus contrast techniques, was consistently superior to the 3 MR pulse sequences used on our imagers in terms of number of lesions detected, lesion-to-liver contrast, and quality of scan.

Adult↗
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↗
PubMed2001

S(3)E-E.COSY methods for the measurement of (19)F associated scalar and dipolar coupling constants.

A (1)H-(19)F spin state selective excitation (S(3)E) pulse sequence element has been applied in combination with (1)H homonuclear mixing to create E.COSY-type experiments designed to measure scalar J(HF2') and J(HH2') and residual dipolar D(HF2') and D(HH2') couplings in 2'-deoxy-2'-fluoro-sugars. The (1)H-(19)F S(3)E pulse sequence element, which resembles a simple INEPT sequence, achieves spin-state-selective correlation between geminal (1)H-(19)F spin pairs by linear combination of in-phase (19)F magnetization and anti-phase magnetization evolved from (1)H. Since the S(3)E sequence converts both (19)F and (1)H steady-state polarization into observable coherences, an approximately twofold signal increase is observed for fully relaxed (1)H-(19)F spin pairs with respect to a standard (1)H coupled (19)F 1D experiment. The improved sensitivity and resolution afforded by the use of (1)H-(19)F S(3)E E.COSY-type experiments for measuring couplings is demonstrated on the nucleoside 9-(2',3'-dideoxy-2'-fluoro-beta-D-threo-pentofuranosyl)adenine (beta-FddA) and on a selectively 2'-fluorine labeled 21mer RNA oligonucleotide.

Deuterium↗
PubMedDate not supplied

Effect of field strength on susceptibility artifacts in magnetic resonance imaging.

In magnetic resonance imaging susceptibility artifacts occur at the interface of substances with large magnetic susceptibility differences, resulting in geometric distortions of the image at those boundaries. The susceptibility artifacts are often subtle on clinical images and if not carefully examined they may lead to misdiagnosis. Magnetic susceptibility artifacts are prevalent on the boundary of air-containing paranasal sinuses, as well as bone-soft tissue interfaces in the spinal canal. The appearance of these artifacts on images from three different magnetic field strength instruments, 0.3, 0.5, and 1.5 Tesla were studied. T1- and T2-weighted spin echo and gradient recalled echo pulse sequences were selected to image a water phantom containing substances of varying susceptibilities. The effects were also studied in MR images of the head in a normal human volunteer. At any given field strength the artifacts were more prominent in the gradient echo imaging than in the corresponding spin echo pulse sequence. As expected, the distortions were also greater at higher field strengths. The results in human subjects paralleled the findings in the phantom study.

Head↗
PubMedDate not supplied

MR imaging of coronary artery flow in isolated and in vivo hearts.

Methods for imaging flow in coronary arteries with magnetic resonance (MR) imaging techniques are demonstrated in isolated heart preparations and live animal models. Coronary artery flow was first imaged with a flow-compensated gradient-echo pulse sequence in isovolumic and working perfused rat hearts and then in vivo. A bolus tracking technique was used to measure flow velocity in the coronary arteries. Ultrafast gradient-echo imaging techniques were then applied, with high resolution obtained by combining the information from several cardiac cycles. A stimulated-echo pulse sequence was demonstrated as a method for performing coronary angiography by flow tagging in isovolumic perfused hearts. This report describes the results of coronary flow MR imaging in isolated rat hearts and live mice and rats. The general approach has proved useful in evaluating new methods for coronary MR angiography and should permit well-controlled studies of pathologic conditions. This ability to image coronary flow in isolated hearts and in small animals should permit integrated MR studies of coronary flow, myocardial perfusion, myocardial metabolism, and cellular ionic status.

Animals↗
PubMed1997

NMR Relaxation Mechanisms for Backbone Carbonyl Carbons in a 13 C, 15 N-Labeled Protein

The predominant relaxation mechanisms for backbone carbonyl carbon (13 C') relaxation in a 13 C, 15 N-doubly enriched sample of the thermostable Sso7d protein have been investigated. Pulse sequences for measurements of longitudinal and transverse 13 C' relaxation rates were implemented, and these rates were measured at magnetic fields of 11.7 and 14.1 T. The field dependence in measured rates is small and consistent with a predominant contribution from chemical-shift anisotropy (CSA) to 13 C' relaxation. A pulse sequence for measurement of {1 H}-13 C' cross-relaxation rates (steady-state NOEs) was also developed. This experiment reveals a significant NOE between protons and all 13 C', indicating that dipolar interactions between these nuclei contribute to 13 C' relaxation. Experiments designed to suppress cross correlation between CSA relaxation and dipole-dipole (DD) relaxation due to neighboring 13 Calpha indicate that this effect is negligible. A more quantitative treatment is also presented, in which backbone dynamics parameters are fitted to average 13 C' relaxation rates using Lipari-Szabo expressions for the spectral density. This fit, which reproduces well expected backbone dynamics parameters for a folded protein, is used to estimate the relative contributions of various mechanisms to 13 C' relaxation. It is found that both longitudinal and transverse relaxation rates are dominated by CSA relaxation and contain significant contributions due to DD relaxation induced by nearby protons. Contributions from DD relaxation due to covalently bound 13 Calpha and 15 N are comparably small. The predominant effects of CSA and 1 H-13 C' DD interactions, for which physical and geometrical parameters are uncertain, complicate the use of 13 C' relaxation as a sequence-specific probe for protein backbone dynamics.

Journal Article↗
PubMed1999

Three-dimensional 13C shift/1H-15N coupling/15N shift solid-state NMR correlation spectroscopy.

Triple-resonance experiments capable of correlating directly bonded and proximate carbon and nitrogen backbone sites of uniformly 13C- and 15N-labeled peptides in stationary oriented samples are described. The pulse sequences integrate cross-polarization from 1H to 13C and from 13C to 15N with flip-flop (phase and frequency switched) Lee-Goldburg irradiation for both 13C homonuclear decoupling and 1H-15N spin exchange at the magic angle. Because heteronuclear decoupling is applied throughout, the three-dimensional pulse sequence yields 13C shift/1H-15N coupling/15N shift correlation spectra with single-line resonances in all three frequency dimensions. Not only do the three-dimensional spectra correlate 13C and 15N resonances, they are well resolved due to the three independent frequency dimensions, and they can provide up to four orientationally dependent frequencies as input for structure determination. These experiments have the potential to make sequential backbone resonance assignments in uniformly 13C- and 15N-labeled proteins.

Carbon Isotopes↗
PubMed2005

A review of technical advances in interventional magnetic resonance imaging.

Initial research in the development of interventional magnetic resonance (MR) imaging in the late 1980s and early to mid-1990s focused on pulse sequences, devices, and clinical applications. This focus was largely a result of the limited number of areas in which the academic research community leading the development could provide innovation on the MR systems of the time. However, during the past decade, computational power, higher bandwidth graphical displays, faster computer networks, improved pulse sequence architectures, and improved technical specifications have accelerated the pace of development on modern MR systems. Today, it is the combination of multiple system factors that are enabling the future of interventional MR. These developments, their impact on the field, and newly emerging applications are described.

Animals↗