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At least 289 records · Page 16Linked to original sources
PubMed1990

Multiple slice FLASH imaging: an improved pulse sequence for contrast enhanced MR brain studies.

A multisclice gradient echo sequence (FLASH) was compared with a conventional spin-echo (SE) technique with regard to its value for contrast enhanced brain studies. In 50 patients with contrast enhancing intracranial lesions, MR studies (0.5 Tesla MR tomograph) were performed with SE images (SE 400/30; four images/3.4 min) and FLASH scans (FLASH 315/14, 90 degrees; 15 images/1.4 min) before and after Gd-DTPA. Based on visual and quantitative assessment diagnostic results of postcontrast SE- and FLASH images were equivalent with respect to contrast enhancement, lesion/brain-contrast, lesion/edema-contrast, and lesion delineation. Although image quality generally was excellent on postcontrast FLASH images, susceptibility artifacts were more severe on FLASH scans than on SE images. However, with the exception of postoperative patients with artifacts due to metal remains, diagnostic information was not decreased by artifacts on postcontrast FLASH images. In conclusion, because of the clearly higher efficiency of the multisclice FLASH technique, this pulse sequence offers the opportunity to speed up contrast enhanced brain imaging.

Adult↗
PubMed2006

MRI of articular cartilage in OA: novel pulse sequences and compositional/functional markers.

Osteoarthritis (OA) is a leading cause of disability worldwide. Magnetic resonance imaging (MRI), with its unique ability to image and characterize soft tissue non-invasively, has proven valuable in assessing cartilage in OA. The development of new, fast imaging methods with high contrast show promise to improve the magnetic resonance (MR) evaluation of this disease. In addition to morphologic MRI methods, MRI contrast mechanisms under development may reveal detailed information about the physiology of cartilage. It is anticipated that these and other MRI techniques will play an increasingly important role in assessing the success or failure of therapies for OA. On December 5 and 6, 2002, OMERACT (Outcome Measures in Rheumatology Clinical Trials) and OARSI (Osteoarthritis Research Society International) held a workshop in Bethesda, MD aiming at providing a state-of-the-art review of imaging outcome measures for OA of the knee to help guide scientists and pharmaceutical companies in the use of MRI in multi-site studies of OA. Applications of MRI were initially reviewed by a multidisciplinary, international panel of expert scientists and physicians from academia, the pharmaceutical industry and regulatory agencies. The findings of the panel were then presented to a wider group of participants for open discussion. The following report summarizes the results of these discussions with respect to novel MRI pulse sequences for evaluating articular cartilage of the knee in OA and notes any additional advances that have been made since.

Cartilage, Articular↗
PubMed1994

MR detection of white matter disease of the brain in patients with HIV infection: fast spin-echo vs conventional spin-echo pulse sequences.

OBJECTIVE: Although fast spin-echo images and slower spin-echo images have similar contrast characteristics, the two techniques have not yet been shown to be equivalent in all aspects of brain imaging. To determine if the two sequences are equivalent, we compared detection of white matter lesions, image quality, and artifact degradation on fast spin-echo and spin-echo proton density-weighted and T2-weighted MR images of the brain in prospectively selected patients who were seropositive for HIV. SUBJECTS AND METHODS: Fast spin-echo and spin-echo MR images of the brain were obtained in 153 consecutive subjects. The images were reviewed independently by three experienced neuroradiologists. The size, number, and location of white matter lesions were compared for the two techniques. Image quality, motion artifact, CSF flow artifact, and gray-white matter differentiation were graded on a five-point scale. RESULTS: No statistical difference was found in gray-white matter differentiation. Overall image quality, CSF flow artifacts, and motion artifacts were slightly worse on the fast spin-echo images (p < .05). Although some variability existed in the detection of lesions less than 5 mm in diameter, the differences was small, and all larger lesions were detected by both techniques. Agreement between fast spin-echo and conventional spin-echo techniques was nearly exact with respect to characterizing findings in brain as either normal or abnormal. CONCLUSIONS: Fast spin-echo and spin-echo MR of the brain produce images of similar quality and show white matter lesions equally well. These results support the replacement of slower, conventional spin-echo pulse sequences with faster fast spin-echo sequences.

AIDS Dementia Complex↗
PubMed2004

Controlling the effects of pulse transients and RF inhomogeneity in phase-modulated multiple-pulse sequences for homonuclear decoupling in solid-state proton NMR.

The effects of pulse imperfections and RF inhomogeneity on NMR spectra obtained with phase-modulated multiple-pulse NMR sequences are analyzed. The emphasis is on the combined effects of frequency offset, RF inhomogeneity, and pulse phase transients. To enable a theoretical description of the transients associated with phase changes under continuous RF irradiation, the nature of the transients is investigated in depth. As monitored in our 300 MHz spectrometer, they are found to be caused by linear elements of the RF circuitry. The validity of their representation as delta-function pulses and the significance of their decomposition into antisymmetric and symmetric components are discussed. A practical method for quantitative control of the antisymmetric phase transients is proposed. The linearity property allows the development of a theoretical description of the spin dynamics caused by the transients. This leads to a vector-Hamiltonian model for phase-modulated Lee-Goldburg experiments. It quantitatively predicts both the frequency shift and the line broadening caused by antisymmetric phase transients and their coupling with RF inhomogeneity. The model is shown to be equally applicable to frequency-switched Lee-Goldburg experiments. A noteworthy discovery is that for a given magnitude of the antisymmetric phase transients a frequency offset exists at which the inhomogeneity broadening is essentially canceled. This explains the common observation that for best resolution one side of resonance is preferred over the other. It also suggests a strategy for enhancing resolution without having to resort to severe sample volume restriction. Numerical calculations verified the theoretical predictions and allowed extension of the model to BLEW-12 and DUMBO-1. Experimental verification is presented. The deviations from theoretical predictions are discussed.

Journal Article↗
PubMedDate not supplied

Pulse sequence strategies for vascular contrast in time-of-flight carotid MR angiography.

A systematic evaluation in healthy volunteers of the relative efficacy of various techniques for background suppression to improve two-dimensional (2D) and three-dimensional (3D) time-of-flight magnetic resonance angiography of the cervical carotid arteries was performed. Conventional 2D and 3D FISP (fast imaging with steady-state precession) sequences with flow compensation were compared with modifications of these sequences, including a tracking saturation pulse (2D), prolonged absolute TEs for fat suppression based on T2* decay (2D and 3D), frequency-selective saturation of fat (2D and 3D), in-plane spatial saturation (2D), and magnetization transfer contrast (2D and 3D). The tracking saturation pulse and slight overlap of the excitation sections provided uniform background suppression without impairing depiction of the morphology of the cervical carotid arteries. Frequency-selective fat saturation was the most effective background suppression scheme among the 2D and 3D techniques but was occasionally compromised by local field inhomogeneities. Magnetization transfer contrast provided little suppression of stationary tissues in the neck because of the intrinsic limitations of the coil. In-plane spatial saturation yielded the highest background suppression but reduced apparent arterial diameters and could not be implemented in a 3D version. The T2* decay method not only reduced the apparent size of the vessels but also their signal intensity.

Adult↗
PubMed2000

HNCAN pulse sequences for sequential backbone resonance assignment across proline residues in perdeuterated proteins.

A TROSY-based triple-resonance pulse scheme is described which correlates backbone 1H and 15N chemical shifts of an amino acid residue with the 15N chemical shifts of both the sequentially preceding and following residues. The sequence employs 1J(NC alpha) and 2J(NC alpha) couplings in two sequential magnetization transfer steps in an 'out-and-back' manner. As a result, N,N connectivities are obtained irrespective of whether the neighbouring amide nitrogens are protonated or not, which makes the experiment suitable for the assignment of proline resonances. Two different three-dimensional variants of the pulse sequence are presented which differ in sensitivity and resolution to be achieved in one of the nitrogen dimensions. The new method is demonstrated with two uniformly 2H/13C/15N-labelled proteins in the 30-kDa range.

Amino Acid Sequence↗
PubMed2003

Calculation of diffusion effect for arbitrary pulse sequences.

A method is presented for calculating the nuclear spin magnetization created by an arbitrary number of short radio frequency pulses and of piecewise constant gradient applied in a selected direction. The isotropic diffusion, the transverse and longitudinal relaxations as well as the global transport are taken into account. A thorough analysis of the magnetization density evolution results in an algorithm for the analytical calculation of final NMR signal. Computationally, it requires only accumulating numerical coefficients in the found analytical structure. For arbitrary sequences this is done with a computer program. This approach, which can be classified as symbolical computations, results in a high performance and in a practically unlimited accuracy. Results for sample pulse sequences are presented.

Algorithms↗
PubMed1992

The design of pulse sequences employing spatial presaturation for the suppression of flow artifacts.

The use of spatial presaturation to suppress the signal, and therefore also the artifacts, from flowing blood has become an important tool in the arsenal of techniques to suppress pulsatile flow artifacts in magnetic resonance images. However, a detailed theoretical analysis of the behavior of these flow artifact suppression pulses and of the important aspects of implementing suppression pulses in combination with particular imaging sequences has yet to be presented. In this paper we present a general theoretical framework to describe the flow artifact suppression technique. This analysis addresses the following four major issues: (1) the spin washout characteristics of the imaging sequence, (2) the interference between the flow signal suppression pulses and the imaging sequence, (3) the flow velocity range for a single application of the suppression pulse, and (4) the total flow velocity range for a suppression pulse repeated with a constant time interval between applications of the pulse. The predictions of our theoretical model are confirmed by experimental measurements made with stationary and flow phantoms. The results of this investigation provide guidelines for the design of flow artifact suppression pulse sequences and, in addition, should aid in the future development and refinement of the spatial presaturation technique as applied to flow signal suppression.

Artifacts↗
PubMed1985

Sensitive-volume localization for in vivo NMR using heteronuclear spin-echo pulse sequences.

Heteronuclear spin-echo techniques, which require the application of an inversion pulse on the second heteronucleus, may be applied with inhomogeneous rf coils such as surface coils. There are important applications in vivo including the detection of 13C-labeled and 15N-labeled metabolites in the 1H NMR spectrum. Using a depth pulse scheme for the 1H spin-echo sequence, and particular single or composite 13C pulses, two sensitive volumes are generated by the 1H and the 13C rf coils and signal is only obtained from the region of overlap between the two sensitive volumes. This method of signal localization for in vivo applications can be extended to 1H homonuclear editing and selective polarization-transfer techniques. The off-resonance characteristics of several different composite pulses are explored.

Animals↗
PubMed2005

Engineering very-high-n polarized Rydberg states using tailored half-cycle-pulse sequences.

We show that strongly polarized very-high-n (n approximately 600) potassium Rydberg atoms can be produced by manipulating lower-n (n approximately 350) polarized atoms using a tailored sequence of ultrashort half-cycle pulses (HCPs). The protocol for this involves first a weak HCP that generates transient phase-space localization whereupon a second large HCP of opposite polarity excites the electron to a broad distribution of highly elongated states. This distribution is then refocused by a short periodic train of HCPs using the properties of (un)stable manifolds near fixed points in phase space.

Journal Article↗
PubMedDate not supplied

A multiple-pulse sequence for improved selective excitation in magnetic resonance imaging.

A new framework for selective excitation that offers simpler design and better performance than conventional excitation methods is introduced. The guidelines for choosing the appropriate radiofrequency (rf) pulse envelope in a conventional selective excitation sequence often rely on Fourier analysis, leading to less than desirable results. Although providing useful insight, Fourier analysis of the rf pulse envelope determines the resultant slice shape accurately only for small flip-angle excitations, and not for larger flip-angle excitations owing to the generally nonlinear behavior of the spin system. In the new excitation framework, additional excitation pulses (typically one) are applied in sequence with the conventional pulse to improve the performance (in phase characteristics and slice definition) over that achieved by the conventional pulse alone. Given a desired spatial spin distribution and an associated rf pulse (e.g., Fourier transform pairs), the Bloch equation is solved backwards to yield the starting distribution required for the conventional pulse to give exactly the desired output. If this residual distribution is a small flip angle away from the actual starting distribution, then Fourier analysis of the residual distribution leads to the necessary "setup" pulse. A gradient of opposite polarity during the setup obviates a refocusing interval after the setup pulse. Computer simulations have verified the efficacy of the multiple-pulse excitation sequence for both 90 degrees and 180 degrees excitations.

Biophysical Phenomena↗
PubMed1979

Discrimination of uniform spectrum pulse sequences.

Random polarity-modulated sequences were produced with a uniform short-term spectrum over defined sampling intervals by a method described by Pierce, Lipes, and Cheetham [J. Acoust. Soc. Am. 61, 1609-1621 (1977)]. These are identified as PLC sequences. By contrast, unconstrained random polarity-modulated pulse trains with a constant interpulse interval may depart from a short-term uniform spectrum. It is shown that listeners can clearly discriminate between PLC sequences and unconstrained random sequences, and can discriminate among different PLC sequences. This discrimination is more nearly related to the statistical redundancy of the PLC sequences. This discrimination is more nearly related to the statistical redundancy of the PLC sequences than to their run-length distribution. Such discrimination is relatively resistant to moderate degrees of temporal jitter and is obtained with other forms of information coding. Discrimination of PLC sequences is presumably based upon phase information.

Acoustic Stimulation↗
PubMed1991

An analysis of the intrinsic resonance offset dependence of magnetization generated by stimulated echo pulse sequences for noncoupled spins.

It is demonstrated that the basic radiofrequency pulse train used to generate stimulated echoes (90x-tau TE-90x-tau TM-90x-tau TE-Acq.) is in general characterized by strong amplitude and phase modulations of the transverse magnetization as a function of the resonance offset. Two dephasing techniques which eliminate the modulations are investigated both theoretically and experimentally, and a simple formula is derived for calculating the relative modulation across a spectrum as a function of gradient strength and duration, echo delay, and spectral linewidth.

Magnetic Resonance Spectroscopy↗
PubMed1991

Optimized pulse sequences for magnetic resonance measurement of aortic cross sectional areas.

This study was done to improve the ability of magnetic resonance (MR) imaging to provide clear cross-sectional images of the ascending and descending aorta in diastole. The study was motivated by interest in measuring the regional compliance of the ascending aorta, which requires determination of the change in cross sectional area of the vessel between systole and diastole. In diastolic images, residual signal from slow flowing blood and flow artifact consistently obscured the inner boundary of the aortic wall and precluded tracing and measurement of the cross sectional area. We concluded that cross sectional area measurement of the ascending aorta was impossible on our system using standard spin echo sequences. To improve wall delineation in diastolic images, SAT pulses were optimized with respect to pulse timing, slice thickness, and gap. Optimized SAT pulses greatly improved the delineation of the vessel wall by removing unwanted signal from flowing spins. Measurement precision was vastly improved by running two scans with and without flow compensation, and correlating visually and numerically the area measurements from each. We established that each image should be measured by two independent observers and traced three times by each. Using these procedures, diastolic cross-sectional areas of the mid-ascending aorta could be measured with a precision of 2.5%, and the change of cross-sectional area between systole and diastole could be measured with a precision of 10.8%. These measurements were precise enough to detect CAD patients with low aortic compliance from the age-matched controls previously reported in one study. The test based on cross sectional area measurement, with a false positive detection rate of 5%, had a false negative rate of 58%. Compliance measurements by MR at 1.5 T could become clinically useful if normal and abnormal populations are sufficiently separated.

Aorta↗
PubMed1984

Spin lattice relaxation time measurements in two-dimensional nuclear magnetic resonance imaging: corrections for plane selection and pulse sequence.

Accurate determination of relaxation times has become increasingly important in efforts to determine the diagnostic specificity of nuclear magnetic resonance (NMR) imaging. Techniques used in NMR imaging, not routinely employed in conventional NMR spectroscopy, can significantly affect the resulting relaxation time determinations. For the saturation recovery (SR) approach of T1 measurement used in our laboratory, these include selective excitation to define the image plane and magnetization refocusing for NMR signal acquisition. Computer modeling of the Bloch equations shows that errors well over 50% can be made in image derived T1 measurements if the conventional SR relation between signal intensity and the 90-90 degrees interpulse delay, tr, is used. However, corrected expressions can be derived for the actual pulse and gradient sequence used by our imaging system, and phantom data acquired in imaging experiments have verified the validity of these equations. This allows for the correction of T1 data to compensate for systematic bias introduced during imaging procedures and suggests a means whereby errors introduced by radio frequency inhomogeneities across the imaging volume can be reduced.

Computers↗
PubMed2004

Modified steady-state free precession pulse sequences for the detection of pure nuclear quadrupole resonance.

Modifications of the steady-state free precession multi-pulse technique for the effective detection of the nuclear quadrupole resonance (NQR) signals are proposed. These modifications are based on the use of composite pulses and enable the suppression of the coherent noise signals such as the magneto-acoustic and piezo-electric signals or the ringing signal from the NQR probe. Experimental results of applying the proposed technique to nitrogen-14 NQR in the sample of C6H12N4 are also presented and convincingly demonstrate its effectiveness.

Journal Article↗
PubMed1987

Sequential NMR assignments of labile protons in DNA using two-dimensional nuclear-Overhauser-enhancement spectroscopy with three jump-and-return pulse sequences.

Two-dimensional nuclear Overhauser enhancement (NOESY) spectra of labile protons were recorded in H2O solutions of a protein and of a DNA duplex, using a modification of the standard NOESY experiment with all three 90 degree pulses replaced by jump-and-return sequences. For the protein as well as the DNA fragment the strategically important spectral regions could be recorded with good sensitivity and free of artifacts. Using this procedure, sequence-specific assignments were obtained for the imino protons, C2H of adenine, and C4NH2 of cytosine in a 23-base-pair DNA duplex which includes the 17-base-pair OR3 repressor binding site of bacteriophage lambda. Based on comparison with previously published results on the isolated OR3 binding site, these data were used for a study of chain termination effects on the chemical shifts of imino proton resonances of DNA duplexes.

Base Composition↗