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At least 811 records · Page 45Linked to original sources
PubMed1992

Multiplanar gradient recalled images of the knee: comparison of different flip angles and echo times.

To determine the most appropriate combination of echo time (TE) and flip angle in gradient echo images of the knee, the authors used different TEs (10, 20 and 30 msec) and flip angles (10, 30, 50, 70, 90, 120 and 150 degrees) to perform a systematic study of MRI on 10 volunteer knees. Contrast-to-noise ratios of cartilage-fat, fluid-cartilage, fluid-fat and fluid-ligament were calculated and compared. Images with a 30-degree flip angle combined with 20 msec of TE were found to have the best contrast-to-noise ratios in both objective data analysis and subjective observation. Hyaline cartilage of the knee was hyperintense and was well delineated on this pulse sequence, appearing distinct in contrast to the intra-articular fluid. It is concluded that this T2-weighted gradient echo pulse sequence is an alternative to conventional spin echo T2-weighted imaging of the knee.

Echo-Planar Imaging↗
PubMed1988

Brain MR: pathologic correlation with gross and histopathology. 1. Lacunar infarction and Virchow-Robin spaces.

MR imaging was performed on 36 formalin-fixed brain specimens. For three of these specimens, in vivo MR studies had also been performed before death. Changes that take place in the MR appearance of the brain after fixation are discussed. Gross and microscopic pathology revealed 14 lacunar infarctions in seven cases and enlarged Virchow-Robin spaces (état criblé) in four. Both types of lesion were seen in specimens from predominantly elderly, hypertensive patients. Eight lacunae were in the deep gray-matter nuclei (four in the putamen with variable involvement of the internal capsule and caudate nuclei, two in the thalami, and two in the dentate nuclei), five were in the supratentorial white matter, and one was in the brainstem. Enlarged Virchow-Robin spaces were identified in the basal ganglia. All lesions were detected on MR. CT failed to disclose the brainstem and dentate lacunae and the enlarged Virchow-Robin spaces. On MR, all lacunae were slitlike or ovoid, except one that was round. They were less than 1 cm in greatest diameter in all but two cases. The lacunae were hyperintense relative to brain parenchyma on both long TR sequences (short and long TEs) in all cases except that of a chronic infarct that underwent cystic change and was isointense relative to CSF on all pulse sequences. In contrast, dilated Virchow-Robin spaces were isointense relative to CSF in vivo or to fluid in the subarachnoid space in the postmortem state on all pulse sequences in all four cases. They were round or linear, and in general were smaller than the lacunae, although some overlap in size did occur. They were seen at the level immediately above the bifurcation of the internal carotid into the middle and anterior cerebral arteries and were seen on successive axial sections in the putamen (most prominent along its lateral margin) and in the internal capsule. MR studies of gross and microscopic pathology of lacunae and dilated Virchow-Robin spaces are useful in correlating MR and pathologic findings. However, changes resulting from the fixation process must be considered when postmortem and in vivo MR findings are correlated.

Adult↗
PubMed1992

MR imaging of anisotropic and restricted diffusion by simultaneous use of spin and stimulated echoes.

A new magnetic resonance imaging technique for evaluating anisotropic and restricted diffusion effects in a single experiment is described. The method is based on a pulse sequence that simultaneously excites a spin echo (SE) and a stimulated echo (STE) and contains two pairs of diffusion-sensitive gradient pulses. The diffusion attenuation of the SE and STE can be controlled independently by separate adjustment of the parameters associated with each pair of the diffusion-sensitive gradient pulses. This method can be more time efficient than acquiring separate SE and STE images for the same study. Furthermore, the pulse sequence minimizes the effect of motion and instrumental and physiological variations that occur between separate image acquisitions which can give rise to artifactual results when difference imaging is employed. In addition to applications to anisotropic and restricted diffusion, this technique can be used for: (1) simultaneous acquisition of T2-weighted and diffusion-weighted images during cerebral ischemia studies; (2) calculation of diffusion coefficients from a pair of diffusion-weighted images; (3) separating fast and slow diffusion components in a single imaging experiment; and (4) two-dimensional velocity-encoded imaging.

Animals↗
PubMed1997

Discrimination between the different compartments in sciatic nerve by 2H double-quantum-filtered NMR.

The 2H double-quantum-filtered (DQF) NMR spectrum of isolated rat sciatic nerve, equilibrated with deuterated saline, is composed of three quadrupolar-split water signals. On the basis of the time course of their shift by Co-EDTA2- and CoCl2, the signals with quadrupolar splittings of about 120, 470, and 9 Hz were assigned to water in the epineurium, endoneurium, and intra-axonal compartments, respectively. The signal of the bulk water, which experiences isotropic motion, was eliminated by the DQF pulse sequence. As the maximum intensities of the water signals in the three anisotropic compartments occur at different creation times, in the DQF pulse sequence, it is possible to resolve the signals and measure their properties, such as relaxation times, independently, without perturbing the system with shift reagents.

Animals↗
PubMed1996

Vascular access ports and catheters: ex vivo testing of ferromagnetism, heating, and artifacts associated with MR imaging.

The purpose of this study was to evaluate ferromagnetic qualities, heating, and artifacts associated with MR imaging of implantable vascular access ports (IVAPs, N = 9) and catheters (N = 8). Ferromagnetism was determined using previously described techniques. Heating was assessed for the IVAPs by measuring temperature immediately before and after performing a 3D GRASS, MTC pulse sequence for 60 min at an SAR of 2.8 W/kg. Artifacts were evaluated in association with the use of a fast GRASS pulse sequence and graded according to the severity of image distortion. None of the IVAPs or catheters were attracted by the magnetic field of the MR system. The largest temperature change measured was -0.3 degree C. Artifacts varied, depending on the component materials used for the construction of the IVAPs and catheters. The lack of ferromagnetic qualities and negligible heating indicates that MR imaging performed at 1.5 T or less may be conducted safely in patients with each of the IVAPs and catheters tested. None of the artifacts produced by the presence of the IVAPs or catheters is considered to impair the diagnostic aspects of MR imaging, especially if the device is not positioned directly in the imaging area of interest.

Artifacts↗
PubMed2004

Sensitivity-enhanced IPAP experiments for measuring one-bond 13C'-13Calpha and 13Calpha-1Halpha residual dipolar couplings in proteins.

Sensitivity-enhanced 2D IPAP experiments using the accordion principle for measuring one-bond 13C'-13Calpha and 1Halpha-13Calpha dipolar couplings in proteins are presented. The resolution of the resulting spectra is identical to that of the decoupled HSQC spectra and the sensitivity of the corresponding 1D acquisitions are only slightly lower than those obtained with 3D HNCO and 3D HN(COCA)HA pulse sequences due to an additional delay 2Delta. For cases of limited resolution in the 2D 15N-1HN HSQC spectrum the current pulse sequences can easily be modified into 3D versions by introducing a poorly digitized third dimension, if so desired. The experiments described here are a valuable addition to the suites available for determination of residual dipolar couplings in biological systems.

Algorithms↗
PubMed2006

Alanine check points in HNN and HN(C)N spectra.

Rapid resonance assignment is a key requirement in structural genomics research by NMR. In this context we present here two new pulse sequences, namely, HNN-A and HN(C)N-A that have been developed by simple modification of the previously described pulse sequences, HNN and HN(C)N [S.C. Panchal, N.S. Bhavesh, R.V. Hosur, Improved 3D triple resonance experiments, HNN and HN(C)N, for H(N) and 15N sequential correlations in (13C, 15N) labeled proteins: application to unfolded proteins, J. Biomol. NMR, 20 (2001) 135-147]. These increase the number of start/check points in HNN and/or HN(C)N spectra and hence help in pacing up resonance assignment in proteins.

Alanine↗
PubMed1999

Proton magnetic resonance spectroscopy in the frontal and temporal lobes of neuroleptic naive patients with schizophrenia.

Studies with proton magnetic resonance spectroscopy (MRS) have reported abnormalities in N-acetyl-aspartate (NAA), amino acids (AA) and choline (Cho) to creatine (Cr) ratios associated with schizophrenia. We report data on the three ratios in a sample of 18 neuroleptic naive patients with first-episode schizophrenia (eight studied in the dorsolateral prefrontal and 10 in the midtemporal lobe) and 24 healthy controls (14 studied in prefrontal and 10 in midtemporal lobes). Frontal lobe proton spectra were acquired with the stimulated-echo acquisition mode (STEAM) pulse sequence (echo time 21 ms, repetition time 2 s). Temporal lobe proton spectra were acquired with the point-resolved spectroscopy (PRESS) pulse sequence (echo time 16-21 ms, repetition time 2 s). Upon comparison with normal controls, NAA/Cr ratios were reduced in patients both for the frontal and the temporal lobe. By contrast, Cho/Cr ratios were slightly elevated in frontal and reduced in temporal lobes; whereas, AA/Cr ratios were normal in frontal and markedly increased in the temporal lobe. The reduced NAA/Cr ratios suggest lower neuronal viability in patients and is consistent with findings of reduced brain volume in both frontal and temporal regions.

Adult↗
PubMed2001

An optimized fast protocol for magnetic resonance imaging of the temporomandibular joint.

OBJECTIVES: To provide a fast protocol for MR imaging of the TMJ with high contrast resolution of both soft tissue and joint fluid. METHODS: A fast turbo-spin echo (TSE) pulse sequence was developed. The new technique was compared with T1W conventional spin echo (CSE) and T2W TSE sequences in imaging 57 TMJs in 50 patients. Disc location and contour and bone delineation were assessed on three images of each TMJ by one observer. Presence and location of intra-articular fluid were evaluated on both standard T2W and new TSE images. Image quality was scored on a 3 point-scale by one observer. RESULTS: The net image acquisition time for one plane was 2 min with the new TSE sequence compared with approximately 5-10 min for T1W and 4 min for T2W. The new sequence provided equivalent diagnostic information to a combination of T1W CSE for disc position and contour and bone contour and T2W TSE sequence for joint fluid. CONCLUSIONS: The examination time for MRI of the TMJ can be considerably reduced with the new fast TSE pulse sequence without compromising image quality.

Adolescent↗
PubMed1994

Assessment of MR image deformation for stereotactic neurosurgery using a tagging sequence.

A tagging sequence is used to assess MR image deformations before a stereotactic neurosurgical procedure, in a test model and in two patients. This pulse sequence super-imposes narrow parallel orthogonal tag lines on an image, which can be used as an internal reference frame. Image deformation is directly related to surface area variations in the squares produced by the tagging-sequence pulses. Small spatial deformations of the tags can be detected on the images used for measuring stereotactic-target spatial coordinates. A threshold of 2 SD guarantees that the distortion is smaller than one pixel.

Biopsy↗
PubMed1990

The effects of the extracellular manganese concentration and variation of the interpulse delay time in the CPMG sequence on water exchange time across erythrocyte membranes.

There has been broad disagreement in the literature regarding the dependence of water exchange times (Te) across erythrocyte membranes studied by the 1H-NMR Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence on extracellular Mn2+ concentration. While some workers saw no change in Te with Mn2+, others reported a 35-50% decrease in Te with this extracellular paramagnetic relaxation agent. We present 1H-NMR evidence that a 30-50% change in Te can be produced by interdependence of the interpulse delay time of the CPMG pulse sequence and the external Mn2+ concentration. Such a large dependency is interpreted in terms of the diffusional effect as a major source. However, it is shown experimentally that if a large number of refocusing pi pulses are used, the observed transverse relaxation times are unaffected by Mn2+. Under these conditions excellent agreement of Te obtained in our study (13.0 +/- 0.64 ms (N = 36) at 21 degrees C) and that of 12.8 +/- 3.6 ms at 20-23 degrees C reported by the radiotracer method was found. Our findings suggest new and important implications for evaluating the previous reports of the 1H-NMR CPMG method concerning the [Mn2+] effect in the decrease of Te, and provide conditions where studies of water transport across erythrocyte membranes using this magnetic resonance method can be used with confidence.

Biological Transport↗
PubMed2000

Slice-selective proton double quantum filtered MRI of joint connective tissues.

1H double quantum filtered (DQF) imaging has been shown to highlight tendons. In this work the DQF magnetic resonance imaging pulse sequence is extended to include slice selection. The short transverse relaxation time of the 1H nuclear magnetic resonance in connective tissues, presents a stringent demand on the application of gradients and soft radiofrequency pulse lengths needed for slice selection. In the present work a slice selection pulse sequence is implemented by postponing the application of the slice refocusing gradient to the period after the last pulse just before the acquisition. Slice-selective DQF images of rat lower leg and knee are given to demonstrate the efficacy of the technique.

Animals↗
PubMed2005

Fault-tolerant quantum dynamical decoupling.

Dynamical decoupling pulse sequences have been used to extend coherence times in quantum systems ever since the discovery of the spin-echo effect. Here we introduce a method of recursively concatenated dynamical decoupling pulses, designed to overcome both decoherence and operational errors. This is important for coherent control of quantum systems such as quantum computers. For bounded-strength, non-Markovian environments, such as for the spin-bath that arises in electron- and nuclear-spin based solid-state quantum computer proposals, we show that it is strictly advantageous to use concatenated pulses, as opposed to standard periodic dynamical decoupling pulse sequences. Namely, the concatenated scheme is both fault tolerant and superpolynomially more efficient, at equal cost. We derive a condition on the pulse noise level below which concatenation is guaranteed to reduce decoherence.

Journal Article↗
PubMed2002

Real-time autoshimming for echo planar timecourse imaging.

Head motion within an applied magnetic field alters the effective shim within the brain, causing geometric distortions in echo planar imaging (EPI). Even if subtle, change in shim can lead to artifactual signal changes in timecourse EPI acquisitions, which are typically performed for functional MRI (fMRI) or diffusion tensor imaging. Magnetic field maps acquired before and after head motions of clinically realistic magnitude indicate that motion-induced changes in magnetic field may cause translations exceeding 3 mm in the phase-encoding direction of the EPI images. The field maps also demonstrate a trend toward linear variations in shim changes as a function of position within the head, suggesting that a real-time, first-order correction may compensate for motion-induced changes in magnetic field. This article presents a navigator pulse sequence and processing method, termed a "shim NAV," for real-time detection of linear shim changes, and a shim-compensated EPI pulse sequence for dynamic correction of linear shim changes. In vivo and phantom experiments demonstrate the detection accuracy of shim NAVs in the presence of applied gradient shims. Phantom experiments demonstrate reduction of geometric distortion and image artifact using shim-compensated EPI in the presence of applied gradient shims. In vivo experiments with intentional interimage subject motion demonstrate improved alignment of timecourse EPI images when using the shim NAV-detected values to update the shim-compensated EPI acquisition in real time.

Artifacts↗
PubMed1997

[Principle and clinical application of MRI].

Magnetic resonance imaging(MRI) is widely used for whole body diagnosis. Spin echo and gradient echo are principle pulse sequences for MRI. Fast scan imaging is now available and can obtained excellent imaging quality with reduced scan time. Using a fast scan pulse sequence, dynamic study, functional and perfusion imaging can be performed in routine clinical practices. Biopsy and thermal therapy of brain and other parts of body are now being performed under MR monitoring and control.

Humans↗
PubMed2005

Simplifying DOSY spectra with selective TOCSY edited preparation.

Diffusion-ordered NMR spectroscopy, while quite powerful, is limited by its inability to resolve signals that are severely overlapped in the proton spectrum. We present here a DOSY experiment that uses selective TOCSY as an editing/preparation period. With this method, well-resolved signals of the analytes are selectively excited and the magnetization subsequently transferred by isotropic mixing to resonances buried in the matrix background, which are then resolved by the ensuing DOSY sequence. Key to the success of our proposed method is the incorporation of a highly effective zero-quantum filter into the selective TOCSY preparation period, which prevents zero-quantum coherence from being carried into the DOSY part of the pulse sequence. Further improvement in spectral resolution can be obtained by expanding the proposed experiment into a 3D sequence and utilizing the homonuclear decoupling feature of the BASHD-TOCSY technique. Both pulse sequences were found to greatly simplify the DOSY spectrum of a 'dirty' sucrose/raffinose mixture, as the complex matrix background is no longer present to obscure or overlap with the signals of interests. Furthermore, complete resolution of the relevant signals was achieved with the 3D sequence.

Carbohydrates↗
PubMed2006

Three-dimensional delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) at 1.5T and 3.0T.

PURPOSE: To implement and validate a three-dimensional (3D) T1 measurement technique that is suitable for delayed gadolinium (Gd)-enhanced MRI of cartilage (dGEMRIC) and can be easily implemented with clinically available pulse sequences at 1.5T and 3.0T. MATERIALS AND METHODS: A 3D inversion-recovery prepared spoiled gradient-echo (IR-SPGR) imaging pulse sequence with variable TR was used to implement a 3D T1 measurement protocol. The 3D T1 measurements were validated against a gold-standard single-slice 2D IR T1 measurement protocol in both phantoms and in vivo, in both asymptomatic volunteers and volunteers with osteoarthritis (OA). RESULTS: T1 measurements in phantoms showed a statistically significant correlation between the 2D and 3D measurements at 1.5T (R2=0.993, P<0.001) and 3.0T (R2=0.996, P<0.001). In vivo application demonstrated the feasibility of using this 3D IR-SPGR sequence to evaluate the molecular status of articular cartilage throughout the knee joint with 0.63x0.63x3.0 mm spatial resolution within a 20-minute acquisition, even with the measurement parameters set for the higher T1(Gd) of cartilage at 3T (range=400-900 msec mean T1 within a region of interest (ROI) in cartilage, compared to 200-600 msec mean T1 at 1.5T). CONCLUSION: This 3D T1 measurement protocol may prove useful for the evaluation and follow-up of cartilage dGEMRIC indices in clinical studies of OA.

Cartilage, Articular↗
PubMed1992

Dynamics of methyl groups in proteins as studied by proton-detected 13C NMR spectroscopy. Application to the leucine residues of staphylococcal nuclease.

This paper describes the application of recently developed nuclear magnetic resonance (NMR) pulse sequences to obtain information about the internal dynamics of isotopically enriched hydrophobic side chains in proteins. The two-dimensional spectra provided by the pulse sequences enable one to make accurate measurements of nuclear Overhauser effects (NOE) and longitudinal (T1) and transverse (T2) relaxation times of enriched methyl carbons in proteins. Herein, these techniques are used to investigate the internal dynamics of the 11 leucine side chains of staphylococcal nuclease (SNase), a small enzyme having Mr = 16.8K, in the absence and presence of ligands thymidine 3',5'-bisphosphate (pdTp) and Ca2+. We report the synthesis of [5,5'-13C2]leucine, the preparation of SNase containing the labeled leucine, the sequential assignment of the leucine methyl carbons and protons in the liganded and unliganded proteins, and the measurement of the 13C T1, T2, and NOE values for the SNase leucine methyl carbons. Analysis of the relaxation parameters using the formalism of Lipari and Szabo shows that the internal motions of the leucine methyl carbons are characterized by effective correlation times tau f (5-80 ps) and tau s (less than 2 ns). The fast motion is identified with the rapid rotation of the methyl group about the C gamma-C delta bond axis, while the slow motion is associated with reorientation of the C gamma-C delta bond axis itself. The mean squared order parameters associated with the latter motion, Ss2, lie in the range 0.34-0.92. The values of Ss2 correlate reasonably well with the temperature factors of the leucine methyl carbons obtained from the crystal structures, but some are smaller than anticipated on the basis of the fact that nearly all leucine methyl carbons are buried and have temperature factors no larger than that of the leucine backbone atoms. Five leucine residues in liganded SNase and eight in unliganded SNase have values of Ss2 less than 0.71. These order parameters correspond to large amplitude motions (angular excursions of 27-67 degrees) of the C gamma-C delta bond axis. These results indicate that, in solution, the internal motions of the leucine side chains of SNase are significantly larger than suggested by the X-ray structures or by qualitative analysis of NOESY spectra. Comparison of Ss2 values obtained from liganded and unliganded SNase reveals a strong correlation between delta Ss2 and distance between the leucine methyl carbon and the ligands.(ABSTRACT TRUNCATED AT 400 WORDS)

Leucine↗