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At least 793 records · Page 44Linked to original sources
PubMed2004

Comparison of intracranial 3D-ToF-MRA with and without parallel acquisition techniques at 1.5T and 3.0T: preliminary results.

PURPOSE: To evaluate the performance of four 3D-ToF magnetic resonance angiography (MRA) sequences with and without integrated parallel acquisition techniques (iPAT) at 1.5T and 3.0T in imaging intracranial vessels. MATERIAL AND METHODS: Seven volunteers and 5 patients (4 aneurysms, 1 AVM) underwent 3D-ToF-MRA at 1.5T (Magnetom Sonata) and 3.0T (Magnetom Trio) with and without parallel acquisition techniques (iPAT) using similarly designed 8-channel phased-array head coils. Imaging time of the pulse sequences was set to 7.15 and 7.35 min, respectively. Images were analyzed quantitatively by calculating signal-to-noise (SNR) and contrast-to-noise (CNR) ratios of proximal M2 segments and qualitatively by using a 5-point scale. RESULTS: SNR and CNR were significantly higher for both 3D-ToF sequences at 3.0T compared with both pulse sequences at 1.5T. The highest SNR and CNR were obtained at 3.0T without iPAT. However, because of a higher spatial resolution (matrix 512 x 640) visualization of small vessel details was best at 3.0T with iPAT. CONCLUSION: Intracranial 3D-ToF-MRA at 3.0T offers superior image quality compared with 1.5T, particular in the delineation of smaller vessels. In contrast to 1.5T, implementation of iPAT at 3.0T is of additional benefit since the high SNR available at 3.0T allows for higher spatial resolution without prolongation of measurement time.

Artifacts↗
PubMed1996

Needle localization in MR-guided biopsy and aspiration: effects of field strength, sequence design, and magnetic field orientation.

OBJECTIVE: The purpose of this investigation was to evaluate the accuracy of MR Imaging for needle depiction at 0.2 and 1.5 T with multiple pulse sequences and needle orientations. The goal was to provide a framework for biopsy approach and imaging technique parameter selection that will ensure the safety and accuracy of MR-guided procedures. MATERIALS AND METHODS: Eight titanium and stainless steel alloy MR-compatible biopsy devices were immersed in fluid phantoms and placed into 1.5- and 0.2-T MR systems used for clinical imaging. Spin-echo, turbo spin-echo, and gradient-echo images were obtained with the needle shafts of the biopsy devices placed parallel to, perpendicular to, and at angles of 30 degrees and 60 degrees relative to the static magnetic field of the scanner. All images were obtained with the frequency-encoding direction parallel to and perpendicular to the needle shaft. Needle width and tip position were measured from images on a freestanding workstation, and the apparent tip position was compared with that obtained by direct measurement. The difference between these values was calculated for each needle type, imaging sequence, frequency-encoding direction, and needle orientation. RESULTS: Artifactual widening was much more apparent at 1.5 T than at 0.2 T, as was error in determining needle tip position. Artifacts at both field strengths were most pronounced with gradient-echo sequences, less so with turbo spin-echo sequences, and least of all with spin-echo sequences. For spin-echo and turbo spin-echo sequences, when the frequency-encoding axis was perpendicular to the needle shaft, the apparent width of the needle was larger, but error in needle tip position was smaller. Artifacts were much less apparent, but error in tip position increased, as the orientation of the needle shaft became more parallel to the direction of the magnetic field. CONCLUSION: Specific measurements differed with field strength, but needle tip localization within 1 mm was obtained at both 0.2 and 1.5 T with the appropriate frequency-encoding direction, pulse sequence, and imaging parameters. Orientation of the needle parallel to the magnetic field significantly reduced the apparent width of the needle at both field strengths but also decreased the accuracy of needle tip position localization.

Biopsy, Needle↗
PubMed2000

Separation of the ortho and para NMR signals in solid deuterium via DQ filtering

Double quantum (DQ) filtering is shown to lead to an effective separation of the NMR signals from the para (I = 1) and ortho (I = 2) molecules in solid deuterium. The separation is achieved by the pulse sequence 90(φ)( degrees )-t(pr)-90(φ)( degrees )-t(ev)-90(x)( degrees )-t, where the phase-cycled first two pulses create the DQ coherence. Two components are observed after the third pulse; the para signal shows the maximum at a short time t while the ortho signal reaches the maximum at a longer t. The observed signal can be expressed as (1/2) summation operator(I) [F(I)(t(pr) - t) - F(I)(t(pr) + t)], where F(I)(t) is a proper fitting function for the free induction signal of the para and ortho molecules (with I = 1 or 2, respectively). Numerical fits to experimental data at 4.2 and 2 K show that this method can be used to determine the ratio F(1)(0)/F(2)(0) and thus, because the initial value F(I)(0) is proportional to the respective magnetization before the pulse sequence, the ortho and para concentrations in solid deuterium. Copyright 2000 Academic Press.

Journal Article↗
PubMed1993

MRI simulation using the k-space formalism.

An MRI simulation method, together with a corresponding computer program, using the k-space formalism has been developed. It uses a FFT algorithm to generate the ideal NMR signal from a user defined object. The k-space trajectory given by a pulse sequence is calculated. And it is used to select elements from the ideal NMR signal. This selection of elements mimic the sampling of the signal in an actual MRI experiment. During the sampling procedure changes in signal amplitude due to relaxation and excitation are introduced as well as signal phase changes due to movement or flow. Artifacts due to stimulated echoes and transversal magnetization that propagate through several repetition periods are also handled. The usefulness of the method is demonstrated by calculations using standard spin-echo sequence as well as modifications introduced in order to generate angiographical images and flow phase images. Further more a fast pulse sequence, echo planar imaging (EPI), is also simulated. The method is faster than previously presented ones. It is capable of generating images (128 x 128 matrix), including more than eight different T1 and T2 combinations, in less than 3 min on a standard 386/387 type IBM compatible PC.

Algorithms↗
PubMed1995

New technical developments in magnetic resonance imaging of epilepsy.

Within the last several years a number of technical developments have been made in magnetic resonance imaging (MRI) that can potentially impact clinical and research MR imaging application in epilepsy. These include developments in instrumentation and in pulse sequences. Advances in instrumentation include higher capacity gradient systems and multiple receiver coils as directed to brain imaging. Advances in pulse sequence include use of fast or turbo-spin-echo techniques, variants of echo-planar imaging, and sequences such as fluid-attenuation inversion recovery (FLAIR) targeted to specific applications of brain imaging. The purpose of this paper is to review several of these developments.

Brain↗
PubMed1995

Dipolar local field measurements from indirect observation of 1H nuclei via cross-polarization 13C nuclear magnetic resonance spectroscopy.

An extension of the exploitation of a simple pulse sequence designed for indirect observation of 1H nuclei through 13C resonances is presented. It is shown that by using this pulse sequence under conditions of rapid magic-angle rotation and coherent energy transfer between directly bonded protons and carbons, an insight into the local dipolar interactions is available in typical organic solids.

Calcium↗
PubMed1999

13C-1H dipolar recoupling under very fast magic-angle spinning using virtual pulses.

A new solid-state NMR pulse sequence for recoupling 13C-1H dipolar interactions under magic-angle spinning is proposed, which works under a spinning speed of a few to several tens kilohertz. The sequence is composed of two different frequency switched Lee-Goldburg sequences, and the modulation of the spin part of the 13C-1H dipolar interaction is introduced by a virtual pulse sequence consisting of unitary operators connecting the rotating frame and the tilted rotating frame. When the cycle time of the spinning is equal to or twice the cycle time of the sequence, the 13C-1H dipolar interactions can be recoupled. The sequence is insensitive to experimental imperfections such as rf inhomogeneity or frequency offset, and the resulting lineshape can be represented by a simple analytical equation based on the zeroth-order average Hamiltonian. Experimental results for [2-(13)C] L-valine x HCl are reported.

Carbon Isotopes↗
PubMedDate not supplied

Effects of image orientation on the comparability of pediatric brain volumes using three-dimensional MR data.

PURPOSE: The purpose of this study was to examine the comparability of morphometric measurements made on pediatric data sets collected at five scanner locations, each using variations on a 3D spoiled gradient-recalled echo (SPGR) pulse sequence. METHOD: Archived MR data from 60 typically developing children were collected and separated into seven groups based on the pulse sequence used. A highly automated image-processing procedure was used to segment the brain data into white tissue, gray tissue, and CSF compartments and into various neuroanatomic regions of interest. RESULTS: Volumetric comparisons between groups revealed differences in areas of the temporal and occipital lobes. These differences were observed when comparing data sets with different image orientations and appeared to be due to partial volume averaging (PVA) and susceptibility-induced geometric distortions. CONCLUSION: Our results indicate that slice selection and image resolution should be controlled in volumetric studies using aggregated data from multiple centers to minimize the effects of PVA and susceptibility-induced geometric distortions.

Adolescent↗
PubMed1987

Soft-tissue masses: histologic basis for decreased signal (short T2) on T2-weighted MR images.

Most soft-tissue masses and tumors of various etiologies and histologies have high signal intensity on T2-weighted pulse sequences (long T2). Of 47 soft-tissue masses, seven had a low signal (short T2) on T2-weighted pulse sequences. All seven masses were tumors, and histologic review showed that their composition differed from that of the other 40 lesions with a long T2 in that the seven masses were relatively acellular and had more collagen. The tumors with a short T2 included one malignant and six benign soft-tissue tumors. Malignant fibrous histiocytoma and aggressive fibromatosis showed paradoxical signal intensities in that they showed both long and short T2. All of the tumors with low signal intensity on T2-weighted images had significant fibrous elements and marked hypocellularity. This study suggests that the less commonly encountered short T2 may be seen in both benign and malignant soft-tissue lesions. A part of the explanation for the low signal on T2-weighted sequences appears to be the relative acellularity and abundant collagen of these tumors in comparison with those that have the same histologic diagnoses but show a high signal. The histologic composition of the tumor rather than the histologic diagnosis appears to influence the MR signal on T2-weighted sequences.

Collagen↗
PubMed1992

Magnetic resonance imaging of the spine. Technology and technique.

Perhaps in no other area of radiology is the appearance of the images as dependent on the technique used to obtain them as is the case in magnetic resonance imaging (MRI). In some instances, such as decreasing signal-to-noise ratios at the periphery of surface coils or obvious difficulties in localizing the level of an abnormality detected in spinal images, the technical dependence is obvious. There are many instances, however, in which the influence of technology may be more subtle, although no less important. It is far less of a problem to have difficulty localizing a level when this difficulty is recognized than it is when ambiguity concerning the level of an abnormality goes undetected. Obsessive attention to accurate localization of levels is necessary to avoid potentially dangerous mistakes. Terms such as "T1-weighted" rarely provide an accurate description of the contrast characteristics of an MRI image. Careful use of terminology, or at a minimum clear thinking concerning the determinants of image contrast will avoid confusion in image interpretation. The "weighting" of an image usually cannot be defined in terms of a single parameter, and attempts to do so often obfuscate more than they clarify. As technology evolves, a variety of different "T1-weighted" pulse sequences may be introduced. Each of these may display different contrast characteristics. It will be important to identify the specific pulse sequence, rather than "weighting" to understand the signal intensities observed on images.

Artifacts↗
PubMed1994

Effects of slow flow on slice profile and NMR signal in fast imaging sequences.

A computer program has been developed to evaluate the selective-slice profiles obtained in the steady state for fast gradient-echo imaging. Both spoiled and refocused gradient-echo pulse sequences have been considered. By numerically solving the Bloch equations modified for the effects of flow, for a three-dimensional volume of spins, for realistic RF excitations and linear gradient combinations, the program permits the combined effects of flow and imaging variables on the magnetization slice profile to be assessed quantitatively. We have found that the gradient pattern in gradient-echo pulse sequences is a significant factor for determining the steady-state slice profiles and the strength of the NMR signal from the flowing spins.

Algorithms↗
PubMed2005

Exploiting the carboxylate chemical shift to resolve degenerate resonances in spectra of 13C-labelled glycosaminoglycans.

Glycosaminoglycans (GAG) are important vertebrate extracellular matrix polysaccharides that comprise repeated units of an acidic and an N-acetylated sugar. The constituent acidic sugars are central to their biological functions, but have been largely inaccessible to NMR because the (1)H resonances overlap with those from other residues. Here, pulse sequences that address this failure are developed using (13)C-enriched oligosaccharides of the glycosaminoglycan, hyaluronan, as model systems. Two pulse sequences are presented that exploit the unique chemical shifts and scalar couplings present at the carboxylate moiety to filter out coherences from the N-acetylated sugars and produce simple spectra containing only resonances from the acidic sugars. The first sequence uses one-bond couplings to correlate the carboxylate carbon with the adjacent carbon and its directly attached proton, while the second sequence exploits a long-range coupling to correlate the carboxylate carbon with the anomeric proton and carbon of the same residue. In addition, inclusion of an isotropic mixing block into these sequences allows resonances from the otherwise degenerate ring protons to be resolved. Spectra from the hyaluronan tetra- and hexasaccharides show that all glucuronic acid (GlcA) residues can be resolved from one another, allowing nuclei to be assigned in a sequence-specific manner. However, in some spectra, resonances are observed at positions not predicted by spin-operator analysis, and simulations reveal that these additional magnetisation transfers result from strong-coupling. These experiments represent a foundation from which new structural and biochemical information can be obtained in a sequence-specific manner for the acidic sugar residues in hyaluronan and other glycosaminoglycans.

Carbohydrate Conformation↗
PubMed2000

Dipolar interactions in molecules aligned by strong AC electric fields

We observed magnetization transfer and spectroscopic splittings due to dipolar couplings in the solution NMR spectra of neat nitrobenzene aligned using AC electric fields. Weak dipolar splittings have been previously observed for nitrobenzene in a DC electric field (T. M. Plantenga, et al., Chem. Phys. 66, 1-9, 1982); the use of homogeneous pulsed AC fields has allowed us to establish stable experimental conditions, which were more tolerable to sample impurities and required no sample purification, and to carry out multidimensional experiments. A pulse sequence is discussed in which the electric field is present only for the indirect dimension: this sequence records the dipolar splittings for each proton in the indirect dimension; the direct dimension presents the isotropic chemical shift. Another pulse sequence is discussed that uses the applied electric field only in the mixing period to produce cross peaks between dipolar coupled pairs and correlate their isotropic chemical shifts. The order parameter describing molecular alignment was in good agreement with that previously determined from deuterium quadrupolar measurements of deuterated nitrobenzene in a similar range of electric fields: S(mol) approximately 0.025% for a field strength of 7.0 MV/m (rms). The dipolar splittings for ortho-meta, meta-para, and ortho-para protons were in qualitative agreement with the known geometry. Copyright 2000 Academic Press.

Journal Article↗
PubMed1991

NMR angiography with enhanced quasi-half-echo scanning.

Flow dephasing effects in NMR images can be significantly reduced by the use of gradient quasi-half-echo signals. They can also be reduced by moment-nulling techniques. In this paper, an efficient imaging pulse sequence, the flow-insensitive enhanced quasi-half-echo method is developed in which these two techniques are combined. This pulse sequence is used to reduce dephasing effects in images acquired to enhance blood vessels in gradient echo subtraction angiography. Both phase corrected and uncorrected quasi-half-echo reconstruction techniques are used to determine the effect on image resolution and vessel enhancement.

Blood Vessels↗
PubMed2005

Gradient hysteresis in MRI and NMR experiments.

In this paper, we describe a gradient hysteresis effect that can modulate the current in gradient coils during MRI and NMR experiments. A simple pulse sequence is presented for the purpose of evaluating the resulting changes in the accumulated phase. Additionally, the nature of the gradient pulse shape changes is described. These experiments will be of interest to MRI and NMR scientists who are developing pulse sequences requiring precision gradient performance or who are currently seeking the source of unexplained NMR artifacts.

Animals↗
PubMed1999

Nuclear magnetic resonance spectroscopy of peracetylated oligosaccharides having 13C-labeled carbonyl groups in lieu of permethylation analysis for establishing linkage substitutions of sugars.

Peracetylation of free hydroxyl groups in model saccharides with [13C-carbonyl]acetic anhydride resulted in additional splittings of sugar ring proton signals in NMR spectra, due to 3-bond J couplings between each acetyl carbonyl carbon and a sugar ring proton at that position. Quantification of 144 of these 3-bond coupling constants in different saccharide structures showed a range between 2.5 and 4.7 Hz, whereas all possible 4-bond couplings between sugar ring protons and acetyl carbonyl carbons were within linewidth (< 0.5 Hz). Therefore, further splitting of sugar ring proton signals in the range of 2.5-4.7 Hz upon acetylation with a [13C-carbonyl]acetyl group identifies that position as (formerly) having a free hydroxyl group. This performs the same basic function as permethylation analysis, but does not require hydrolysis of glycosidic linkages. Additionally, proton-detected 2D heteronuclear multiple bond correlation (HMBC) experiments or proton-detected heteronuclear correlation spectroscopy (hetCOSY) enabled ring proton-carbonyl-13C 3-bond J connectivities to be correlated with high sensitivity. Modified NMR pulse sequences are reported that include frequency selective decoupling schemes to enable coupling constants to be determined from 2D data. The tailored pulse sequences resulted in higher spectral resolution and sensitivity for [13C-carbonyl]-ring proton correlations.

Acetylation↗
PubMed2001

1H-1H correlations across N-H...N hydrogen bonds in nucleic acids.

In 2HJ(NN)-COSY experiments, which correlate protons with donor/acceptor nitrogens across Nd...HNa bonds, the receptor nitrogen needs to be assigned in order to unambiguously identify the hydrogen bond. For many situations this is a non-trivial task which is further complicated by poor dispersion of (Na,Nd) resonances. To address these problems, we present pulse sequences to obtain direct, internucleotide correlations between protons in uniformly 13C/15N labeled nucleic acids containing Nd...HNa hydrogen bonds. Specifically, the pulse sequence H2(N1N3)H3 correlates H2(A,omega1):H3(U,omega2) protons across Watson-CrickA-U and mismatched G.A base pairs, the sequences H5(N3N1)H1/H6(N3N1)H1 correlate H5(C,omega1)/H6(C,omega1):H1(G,omega2) protons across Watson-Crick G-C base pairs, and the H2(N2N7)H8 sequence correlates NH2(G,A,C;omega1):H8(G,A;omega2) protons across G.G, A.A, sheared G.A and other mismatch pairs. These 1H-1H connectivities circumvent the need for independent assignment of the donor/acceptor nitrogen and related degeneracy issues associated with poorly dispersed nitrogen resonances. The methodology is demonstrated on uniformly 13C/15N labeled samples of (a) an RNA regulatory element involving the HIV-1 TAR RNA fragment, (b) a multi-stranded DNA architecture involving a G.(C-A) triad-containing G-quadruplex and (c) a peptide-RNA complex involving an evolved peptide bound to the HIV-1 Rev response element (RRE) RNA fragment.

Base Pair Mismatch↗
PubMedDate not supplied

Evaluation of super paramagnetic iron oxide-enhanced diffusion-weighted PROPELLER T2-fast spin echo magnetic resonance imaging: Preliminary experience.

OBJECTIVE: To evaluate the usefulness of super paramagnetic iron oxide-enhanced, diffusion-weighted, periodically rotated overlapping parallel lines with enhanced reconstruction (SPIO DWI PROPELLER) T2-fast spin echo (FSE) magnetic resonance imaging (MRI) for the detection of hepatic metastases. METHODS: Fourteen patients were examined with SPIO-enhanced T2-FSE (SPIO FSE) imaging and SPIO DWI PROPELLER T2-FSE imaging. The b-value of the diffusion-sensitizing gradient was 10 s/mm so as to suppress the signal of the hepatic vessels. Hepatic resections were performed on all patients, and the number of lesions on MRI was compared between the 2 pulse sequences with references from pathologic reports. RESULTS: Nearly all metastases 1 cm or larger, totalling 38 metastases, were detected with both pulse sequences. Among the 30 metastases less than 1 cm, more lesions were detected on SPIO DWI PROPELLER T2-FSE imaging than on SPIO FSE imaging (16 for SPIO FSE imaging and 24 for DWI PROPELLER T2-FSE imaging; P < 0.05, McNemar test). CONCLUSION: Super paramagnetic iron oxide-enhanced DWI PROPELLER T2-FSE is useful for detecting small hepatic metastases.

Adult↗