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At least 1,261 records · Page 70Linked to original sources
PubMed1996

Two-dimensional magic-angle spinning isotropic reconstruction sequences for quadrupolar nuclei.

Two-dimensional magic-angle spinning (triple quantum, single quantum) correlation pulse sequences and phase cycles based on the technique of Frydman and Harwood for the reconstruction of the isotropic spectrum of half-integer spin quadrupolar nuclei broadened to second-order are described. These sequences provide pure absorption mode two-dimensional lineshapes and increased sensitivity. Experimental examples on spin I = 3/2 (87Rb in RbNO3) and I = 5/2 (27Al in NaSi3AlO8) are presented. The isotropic chemical shift and quadrupolar coupling parameters could be obtained from a simple analysis of the triple quantum filtered single quantum magic-angle spinning cross-sections.

Absorption↗
PubMed1995

Echo-time-encoded burst imaging (EBI): a novel technique for spectroscopic imaging.

A new technique for rapid spectroscopic imaging is presented. The proposed experiment enables a complete mapping of the two-dimensional reciprocal space kx, k sigma, and thus the acquisition of a 1D spectroscopic image in a single scan. The properties of the pulse sequence, based on the use of a burst of low flip angle pulses, are analyzed in the framework of linear response theory, and it is shown that chemical shift information may be introduced into the spatially encoded echoes. First experimental results are presented demonstrating that 32 x 32 proton spectroscopic images may be acquired within less than 1 min with a conventional imaging system.

Humans↗
PubMed2000

SIMPSON: a general simulation program for solid-state NMR spectroscopy.

A computer program for fast and accurate numerical simulation of solid-state NMR experiments is described. The program is designed to emulate a NMR spectrometer by letting the user specify high-level NMR concepts such as spin systems, nuclear spin interactions, RF irradiation, free precession, phase cycling, coherence-order filtering, and implicit/explicit acquisition. These elements are implemented using the Tcl scripting language to ensure a minimum of programming overhead and direct interpretation without the need for compilation, while maintaining the flexibility of a full-featured programming language. Basically, there are no intrinsic limitations to the number of spins, types of interactions, sample conditions (static or spinning, powders, uniaxially oriented molecules, single crystals, or solutions), and the complexity or number of spectral dimensions for the pulse sequence. The applicability ranges from simple 1D experiments to advanced multiple-pulse and multiple-dimensional experiments, series of simulations, parameter scans, complex data manipulation/visualization, and iterative fitting of simulated to experimental spectra. A major effort has been devoted to optimizing the computation speed using state-of-the-art algorithms for the time-consuming parts of the calculations implemented in the core of the program using the C programming language. Modification and maintenance of the program are facilitated by releasing the program as open source software (General Public License) currently at http://nmr.imsb.au.dk. The general features of the program are demonstrated by numerical simulations of various aspects for REDOR, rotational resonance, DRAMA, DRAWS, HORROR, C7, TEDOR, POST-C7, CW decoupling, TPPM, F-SLG, SLF, SEMA-CP, PISEMA, RFDR, QCPMG-MAS, and MQ-MAS experiments.

Algorithms↗
PubMed2004

Separation and characterization of different signals from intermolecular three-spin orders in solution NMR.

In this paper, signals originating from a pure specific coherence of intermolecular three-spin orders were separated and characterized experimentally in highly polarized two-component spin systems. A modified CRAZED sequence with selective radio-frequency excitation was designed to separate the small signals from the strong conventional single-spin single-quantum signals. General theoretical expressions of the pulse sequence with arbitrary flip angle pulses were derived using dipolar field treatment. The expressions were used to predict the relaxation and diffusion properties and optimal experimental parameters such as flip angles. For the first time, relaxation and diffusion properties of pure intermolecular single-quantum, double-quantum, and triple-quantum coherences of three-spin orders were characterized and analyzed in one-dimensional experiments. All experimental observations are in excellent agreement with the theoretical predictions. The theoretical results show that the quantum-mechanical treatment leads to exactly the same predictions as the dipolar field treatment. The quantitative study of intermolecular multiple-quantum coherences of three-spin orders presented herein provides a better understanding of their mechanisms.

Journal Article↗
PubMed2006

Rapid MRI method for mapping the longitudinal relaxation time.

A novel method for mapping the longitudinal relaxation time in a clinically acceptable time is developed based on a recent proposal [J.-J. Hsu, I.J. Lowe, Spin-lattice relaxation and a fast T1-map acquisition method in MRI with transient-state magnetization, J. Magn. Reson. 169 (2004) 270-278] and the speed of the spiral pulse sequence. The method acquires multiple curve-fitting samples with one RF pulse train. It does not require RF pulses of specific flip angles (e.g., 90 degrees or 180 degrees ), nor are the long recovery waiting time and the measurement of the magnetization at thermal equilibrium needed. Given the value of the flip angle, the curve fitting is semi-logarithmic and not computationally intensive. On a heterogeneous phantom, the average percentage difference between measurements of the present method and those of an inversion-recovery method is below 2.7%. In mapping the human brain, the present method, for example, can obtain four curve-fitting samples for five 128 x 128 slices in less than 3.2s and the results are in agreement with other studies in the literature.

Benchmarking↗
PubMed2006

Intelligent compensation of friction, ripple, and hysteresis via a regulated chatter.

In this paper, a hybrid control scheme utilizing a PID feedback control with an additional regulated chatter signal is developed to compensate motion impeding influences such as the effects due to friction, force ripples, and hysteresis in linear piezoelectric motor. The regulated chatter signal is a pulse sequence superimposed on the PID control signal. It has a fixed amplitude, and a pulse width regulated via iterative learning control. As such, the scheme is expected to be useful for applications involving iterative motion sequences. An analysis of the closed-loop performance is presented in the paper. Simulation and experimental results are also furnished to demonstrate that the proposed control scheme can reduce tracking errors significantly.

Algorithms↗
PubMed2001

[Coded excitation medical ultrasound imaging]

Pulse compression techniques were developed mainly for radar systems. And the technology allows a radar to utilize a long pulse to achieve radiated energy, but simultaneously to obtain the range resolution of a short pulse. It accomplishes this by employing frequency or phase modulation to widen the signal bandwidth. Coded excitation is an ultrasound technique which has only been made commercially available in the last 3 years. It improves SNR without loss of resolution by transmitting a long binary phase-encoded pulse sequence and then compressing its echo into a short, high-amplitude pulse on receive. Different types of codes, e.g. Golay, may be used to suppress range side lobes. And a new technique called B-Flow for imaging blood flow also uses advanced digital signal encoding/decoding techniques to provide direct visualization of blood echoes. Coded excitation together with tissue equalization forms the basis for B-Flow, which extends the wideband resolution and high frame rate capabilities of B-mode to flow and tissue imaging. A key aspect of B-Flow is tissue equalization, which uses decoding to preferentially reduce non-moving tissue signals to the level of the flow signal such that both may be displayed simultaneously without machine prioritization and overlay (as in color flow).

Journal Article↗
PubMed2005

Selective measurement of heteronuclear 1H-13C dipolar couplings in motionally heterogeneous semicrystalline polymer systems.

A pulse sequence for the selective recoupling of heteronuclear dipolar interactions in mobile amorphous phase of powdered semicrystalline polymers is described. 1H-13C dipolar interactions are selectively measured by PISEMA-type sequence. Selection of 13C magnetization originating from amorphous phase is achieved by a train of saturation pulses followed by a short delay and a direct excitation pulse on 13C spins. The development of undesired net 13C magnetization during the recoupling sequence is prevented by the efficient "reverse" 13C --> 1H cross-polarization. The efficacy of the 2D method to measure 1H-13C dipolar couplings selectively for mobile components is demonstrated on powdered crystalline L-alanine, semicrystalline polyethylene, and nanocomposite polyamide-6/montmorillonite.

Alanine↗
PubMed1999

"Silent" MRI with soft gradient pulses.

A method to reduce the acoustic noise generated by gradient systems in magnetic resonance imaging (MRI) is proposed based on the linear response theory. Since the acoustic frequency response function of typical gradient coils is low in the range below 200 Hz, the noise level can be significantly reduced by using gradient pulse sequences whose spectra are limited to this frequency range. Such "soft," i.e., band-limited, pulse shapes can be designed using sinusoidal ramps individually adjusted to available delays. "Silent" versions of three basic MRI sequences [gradient-echo (GE), spin-echo (SE), and rapid acquisition with relaxation enhancement (RARE)] were programmed on 2 and 3 T whole-body scanners. High-quality images could be acquired at noise levels as low as 40 dBA (GE and SE) and 60 dBA (RARE).

Brain↗
PubMed2000

Diagnosis of acute ischaemic stroke with fluid-attenuated inversion recovery and diffusion-weighted sequences.

We evaluated the feasibility and use of diffusion-weighted and fluid-attenuated inversion-recovery pulse sequences performed as an emergency for patients with acute ischaemic stroke. A 5-min MRI session was designed as an emergency diagnostic procedure for patients admitted with suspected acute ischaemic stroke. We reviewed routine clinical implementation of the procedure, and its sensitivity and specificity for acute ischaemic stroke over the first 8 months. We imaged 91 patients (80 min to 48 h following the onset of stroke). Clinical deficit had resolved in less than 3 h in 15 patients, and the remaining 76 were classified as stroke (59) or stroke-like (17) after hospital discharge. Sensitivity of MRI for acute ischaemic stroke was 98%, specificity 100%. MRI provided an immediate and accurate picture of the number, site, size and age of ischaemic lesions in stroke and simplified diagnosis in stroke-like episodes. The feasibility and high diagnostic accuracy of emergency MRI in acute stroke strongly support its routine use in a stroke centre.

Adult↗
PubMed1998

[Magnetic resonance tomography in plasmacytoma: ranking of various sequences in diffuse and focal infiltration patterns].

PURPOSE: The aim of this study was to compare various pulse-sequences employing the spine array coil while considering the different infiltration patterns in multiple myeloma, in order to develop a fast and cost effective screening method. MATERIALS AND METHODS: In a prospective study, 50 patients with histologically proven multiple myeloma were examined with the following standardised sequences: T1-weighted SE, T2-weighted FSE, opposed phase GRE, FSE STIR, T1-weighted SE post gadopentate dimeglumine without and with fat saturation. The images were evaluated in a ROC analysis by three radiologists in consensus with a 5-grade scale separatively for diffuse and focal involvement. The gold standard was bone marrow histology in cases of diffuse infiltration and the combination of all sequences with the staging system according to Durie and Salmon. RESULTS: For focal bone marrow involvement the area under curve was greatest for FSE STIR sequences, in cases of diffuse infiltration the area under curve was superior for unenhanced T1-w SE-Sequenz images. The sensitivity can be increased by contrast material application and signal intensity measurements. A combined focal and diffuse infiltration and a "salt and pepper" pattern can be diagnosed with the combination of T1-w SE-Sequenz and FSE STIR sequences. CONCLUSION: With T1-weighted SE-Sequenz sequences pre- and post-contrast and FSE STIR all therapeutical relevant infiltration patterns of plasmocytoma can be identified and bone marrow screening of the spine can be carried out with an acquisition time of 5.58 minutes.

Adult↗
PubMed1991

Spoiling of transverse magnetization in steady-state sequences.

A detailed analysis is presented of a method to eliminate transverse magnetization prior to each rf excitation in pulse sequences with TR less than T2. It is shown that artifact-free images with high T1 contrast can be obtained only if a phase shift that is incremented during each TR interval is applied to the transverse magnetization. Computer simulations are used to show that when this phase increment is 117 degrees, the steady-state transverse magnetization prior to each rf pulse is nulled over a wide range of T1, T2, and rf tip angles, resulting in optimal T1 contrast. Such nulling of steady-state transverse magnetization cannot be obtained by using large gradient pulses, or gradients of random or linearly incremented amplitude. Images of phantoms and human subjects confirm the theoretical predictions.

Artifacts↗
PubMedDate not supplied

High signal regions in normal white matter shown by heavily T2-weighted CSF nulled IR sequences.

Inversion recovery (IR) sequences with an inversion time (TI) designed to markedly reduce or null the signal from CSF (TI of approximately 2,100 ms at 1.0 T) and a very long echo time (TE) of 240 ms were used to image the brain of two normal adult volunteers, one 34-year-old man with an intrinsic tumor, and one 3-month-old infant with an infarct. Using these very heavily T2-weighted pulse sequences, adult gray and white matter showed similar signal intensity in many areas of the brain, but normal white matter in regions of the centrum semiovale, posterior internal capsule, parietopontile tract, occipitothalamic radiation, and brain stem showed a much higher signal intensity than surrounding gray or white matter. The infant displayed a low signal intensity in myelinated regions in the internal capsule and occipitothalamic radiation and a high signal in unmyelinated white matter. In many of the images there were strong similarities to the distribution of high signal within white matter seen with pulsed gradient spin echo sequences (TE 130 ms) designed to demonstrate effects due to anisotropic diffusion. Arguments are advanced to support the view that the high signal intensity in white matter tracts is due to one or more long T2 components that may be associated with unmyelinated or sparsely myelinated fibres within white matter. The resemblance to diffusion weighted images may reflect the fact that both employ long TEs and both produce a low signal from CSF. If myelin possessed a different susceptibility from axoplasm so that magnetic field gradients were generated around nerve fibres when their orientation was not parallel to B0, diffusion of water might then produce the observed dependence on fibre direction. The high signal regions in white matter are a potential source of confusion in image interpretation, and measurements of T2 in white matter need to be made with these regional variations in mind. The concept of normal appearing white matter also needs to be applied with a knowledge of these differences. The IR sequences used in this study provide a very high T2 dependence with a low signal from CSF and may be useful for detecting disease in the CNS of adults and children.

Adult↗
PubMedDate not supplied

Gadolinium-enhanced breath-hold three-dimensional time-of-flight MR angiography of the abdominal and pelvic vessels: the value of ultrafast MP-RAGE sequences.

MR angiography (MRA) was performed in 50 consecutive subjects (mean age, 59 years), who had been referred for abdominal MRA, on a 1.5-T superconductive unit that used a body phased-array coil. Three breath-hold three-dimensional sequences were evaluated both in phantom and clinical studies: (a) standard fast three-dimensional gradient-echo sequence (TR = 15, TE = 6; imaging time, 32 seconds), (b) ultrafast three-dimensional gradient-echo sequence (TR = 8.2, TE = 3; imaging time, 18 seconds), and (c) ultrafast magnetization-prepared (MP) rapid acquisition gradient echo (RAGE) (TR = 5.8, TE = 2.9, inversion time [TI] = 20; imaging time, 15 seconds). The initial 30 patients were randomized into three groups by three separate sequences. For the remaining 20 patients, ultrafast-gradient-echo and ultrafast MP-RAGE sequences were performed. Conventional angiography was performed on 36 patients. Signal measurements of the phantom and clinical images of the aorta, visceral branches of the aorta, iliac arteries, inferior vena cavae, and portal veins were performed. The overall image quality and background fatty tissue contrast of the vessels were rated subjectively. Comparison of images between MRA and conventional angiography also was performed. The contrast between the vessels and background fatty tissue was significantly higher in the ultrafast MP-RAGE sequence in both quantitative and qualitative analysis, and image-quality ultrafast MP-RAGE was superior to the other two sequences (P < .01). The aorta and iliac arteries could be visualized in all pulse sequences, and abnormalities of these vessels were diagnosed correctly. The renal artery was visualized more clearly with the two ultrafast sequences.

Angiography↗
PubMed2001

Real-time cardiac cine imaging with SPIDER: steady-state projection imaging with dynamic echo-train readout.

Steady-state projection imaging with dynamic echo-train readout (SPIDER) is a multiecho radial k-space trajectory TrueFISP sequence developed for real-time cine imaging of the heart. This new pulse sequence combines the superior SNR and blood-to-myocardium contrast of TrueFISP with the increased scan time efficiency of EPI and undersampled projection reconstruction. SPIDER sequence RF repetition time (TR) was minimized by limiting the echo-train to a length of three while acquiring the first and third echoes asymmetrically. A temporal resolution of 45 ms was achieved with TR/TE1/TE2/TE3 of 3.24/0.6/1.6/2.6 ms and a factor of 2 view sharing scheme. Phantom experiments showed little difference between the weighting of the signals acquired at each of the echo times but did show considerable off-resonance modulation between them. In vivo experiments demonstrated the feasibility of using the SPIDER sequence for real-time imaging in the cardiac short axis orientation.

Echo-Planar Imaging↗
PubMed1986

A new method for flow velocity measurement: frequency encoded NMR.

We demonstrate a new rapid NMR method to measure the average velocity and the velocity distribution of a flowing fluid. We use a truncated Carr-Purcel-Meiboom-Gill pulse sequence, pi/2-T-pi-2T-pi-T-echo, in the presence of a static magnetic field gradient. The pi/2 pulse is selective and tags a narrow slice of nuclei. The second echo from the tagged nuclei is Fourier transformed after the nuclei have evolved for a time 4T to give the velocity distribution modified by the initial slice selection profile. In this way, average velocities up to 54 cm/s were measured for water flowing in a plastic tube with either a plug or a developed velocity distribution. Each measurement required about 20 ms but this could be reduced with larger field gradients. Thus, this method is fast enough to apply to such problems as measuring pulsatile flow in arteries.

Animals↗
PubMed1996

Power efficient on-resonance saturation pulses for magnetization transfer in magnetic resonance imaging.

A family of new on-resonance saturation pulses for magnetization transfer in MRI is proposed. These pulses can be represented as a product of a shaped function and a cosine function. The shaped function can have many different forms, one of which is a Gaussian function. The experimental results on a 1.0 T whole body scanner show that the new on-resonance pulses are more efficient for magnetization transfer than either on-resonance binomial sequence pulses or off-resonance Gaussian pulses at the same power level.

Humans↗
PubMed2003

Magnetic resonance measurements of renal blood flow as a marker of disease severity in autosomal-dominant polycystic kidney disease.

BACKGROUND: Autosomal-dominant polycystic kidney disease (ADPKD) is an inherited disorder characterized by renal cyst growth, early development of hypertension, and late occurrence of renal insufficiency. Despite evidence for the importance of nephroangiosclerosis in the progression of renal insufficiency in ADPKD, evaluation of renal blood flow (RBF) as a surrogate marker of disease severity has received little attention. METHODS: Flow phantoms and repeat RBF measurements assessed accuracy and reproducibility. One hundred twenty-seven ADPKD subjects with creatinine clearances >70 mL/min underwent measurements of RBF, total, and cyst renal volumes, and % cyst volumes by magnetic resonance (MR) and of glomerular filtration rate (GFR). Renal vascular resistance (RVR) was calculated. MR blood flow sequences utilized a two-dimensional cine phase-contrast breath-hold pulse sequence perpendicular to the renal arteries. Flow rates were calculated utilizing FLOW software. Volumetric analysis was performed using stereology and region-based thresholding. RESULTS: Excellent accuracy and intraobserver and interobserver reproducibility were demonstrated. Anatomic (total kidney volume, total cyst volume, and % cyst volume), hemodynamic (RBF and RVR), and functional (GFR) parameters were strongly correlated. Left polycystic kidneys were larger and had more severe disease. Regression analysis showed that age, diagnosis of hypertension, anatomic parameters and hemodynamic parameters were significant predictors of GFR. Multiple linear regression analysis identified age and hemodynamic parameters only as separate predictors of GFR. Anatomic, hemodynamic, and functional parameters discriminated between normotensive and hypertensive subjects despite antihypertensive treatments. CONCLUSION: Renal hemodynamic parameters measured by MR correlate with anatomic and functional indices of disease severity, are the strongest predictors of renal function, and deserve further consideration as an outcome measure in clinical trials to guide therapy in ADPKD.

Adult↗