Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Pulse sequences”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,477 records · Page 82Linked to original sources
PubMed1986

Quadrature-multiplexed rf excitation for information doubling in NMR imaging.

A quadrature multiplexed pulse sequence to double the data information in a given acquisition time is proposed for NMR tomography. This technique takes advantage of the Hermitian property of the Fourier transform operation in NMR imaging. The proposed technique uses two rf pulses with 90 degrees phase difference simultaneously applied for the excitation of the spins in two different slices. The method results in truly simultaneous data acquisition coming from two different anatomical slices which can be separated into the real and imaginary parts of the image domain. This technique, therefore, also allows us either to reduce the experiment time by half or double the number of slices in the same examination time, depending on the applied imaging scheme. As a preliminary study, simultaneous dual-slice imaging of a human head using a conventional single-slice imaging scheme was performed.

Fourier Analysis↗
PubMed2002

A spin-state-selective experiment for measuring heteronuclear one-bond and homonuclear two-bond couplings from an HSQC-type spectrum.

Recently, a set of selective 1D experiments with spin-state-selective excitation for CH spin systems was introduced by Parella and Belloc (J. Magn. Reson., 148, 78-87 (2001)). We have expanded and generalized this concept further, and demonstrated that a very simple experiment utilizing spin-state-selective filtering can be used for simultaneous measurement of heteronuclear 1JNH (or 1JCH) and geminal 2JHH couplings from two-dimensional 15N-1H (or 13C-1H) correlation spectrum. The experiment has very high sensitivity owing to the preservation of equivalent coherence transfer pathways analogous to the sensitivity and gradient enhanced HSQC experiment. However, overall length of the pulse sequence is 1/(2J) shorter than the gradient selected SE-HSQC experiment. Furthermore, the spin-state-selection can be utilized between NH and NH2 (or CH and CH2) moieties by changing the phase of only one pulse. The pulse scheme will be useful for the measurement of scalar and residual dipolar couplings in wide variety of samples, due to its high sensitivity and artifact suppression efficiency. The method is tested on NH2 and CH2 moieties in 15N- and 15N/13C-labeled ubiquitin samples.

Carbon Isotopes↗
PubMed2006

MRI of the pancreas: tumours and tumour-simulating processes.

The most important issues in pancreatic imaging are the detection and staging of pancreatic cancer, differentiation between cancer and focal pancreatitis, the characterization of cystic lesions and the search for neuroendocrine tumours. Magnetic resonance (MR) units (1.5 T) with strong gradients and a phased-array torso coil should be used, making breath-hold imaging possible in order to avoid motion artifacts. Standard imaging sequences are T1-weighted (T1w) gradient recalled-echo (GRE) with and without fat saturation. For T2-weighted (T2w) imaging, axial single-shot turbo spin-echo (TSE) and coronal/oblique MR cholangio-pancreatography (MRCP) pulse sequences are preferable. As contrast agents either gadolinium agents or mangafodipir trisodium are used. Dynamic gadolinium-enhanced T1w fatsat 3D GRE images are helpful to delineate vessel infiltration by adenocarcinoma and to assess the aetiology of cystic masses. Mangafodipir-enhanced MRI has been found to be superior to helical computed tomography (CT) in the detection of small cancers and in the delineation of liver metastases. In cases of an equivocal pancreatic mass the presence of the "duct penetrating sign" at MRCP (i.e., the duct traversing the mass) is suggestive of an inflammatory pseudotumour. Hypoattenuation due to focal fatty infiltration may mimic a tumour at CT, but in-phase and opposedphased T1w imaging readily depicts the fat. Multi-detector CT has gained increasing popularity for pancreatic imaging because of the 3D visualization of the peripancreatic vessels. However, MR imaging is excellent in the delineation of small pancreatic tumours. Due to its superior soft tissue contrast, MR imaging is also the method of choice in the differential diagnosis between tumours and tumour-simulating conditions in patients with equivocal CT and to assess cystic lesions.

Humans↗
PubMed1998

Analysis of polymer materials by surface NMR via the MOUSE.

Applications are discussed of a novel NMR device, the NMR MOUSE (mobile universal surface explorer), for characterization of polymers. Different properties of elastomers can be related to an effective transverse relaxation parameter T2eff. Effects of multi-echo sequences influence the decay curve and can be described in terms of B0 inhomogeneity and spin-lock effects. Furthermore, the signal-to-noise ratio (SIN) can be improved by use of steady-state free precession (SSFP) pulse sequences modified for use in inhomogeneous magnetic fields.

Magnetic Resonance Spectroscopy↗
PubMed2002

Categories of coherence pathways for the CPMG sequence.

The CPMG sequence has been extremely useful for efficient measurements of NMR signal, spin-spin relaxation, and diffusion, particularly in inhomogeneous magnetic fields, such as when samples are outside the magnet and RF coil. Due to the inaccuracy of the pulses and the off-resonance effects, the CPMG echoes have contributions from the Hahn echo as well as signals that are similar to stimulated echoes. The systematic understanding of the CPMG pulse sequence requires decomposing the magnetization dynamics into different coherence pathways. In this paper, we describe a method to classify the CPMG coherence pathways and illustrate the nature of these types of pathways. This classification shows that direct echo and stimulated echoes are the major contribution to the CPMG signal. It also provides a clear understanding of the effect of restricted diffusion in porous media.

Journal Article↗
PubMed2006

Kinetics of RNA refolding in dynamic equilibrium by 1H-detected 15N exchange NMR spectroscopy.

By implementing new NMR methods that were designed to map very slow exchange processes we have investigated and characterized the refolding kinetics of a thermodynamically stable 34mer RNA sequence in dynamic equilibrium. The RNA sequence was designed to undergo a topologically favored conformational exchange between different hairpin folds, serving as a model to estimate the minimal time required for more complex RNA folding processes. Chemically prepared RNA sequences with sequence-selective (15)N labels provided the required signal separation and allowed a straightforward signal assignment of the imino protons by HNN correlation experiments. The 2D version of the new (1)H-detected (15)N exchange spectroscopy (EXSY) pulse sequence provided cross-peaks for resonances belonging to different folds that interchange on the time scale of longitudinal relaxation of (15)N nuclei bound to imino protons. The 34mer RNA sequence exhibits two folds which exchange on the observable time scale (tau(obs) approximately T(1){(15)Nu} < 5 s) and a third fold which is static on this time scale. A 1D version of the (15)N exchange experiment allowed the measurement of the exchange rates between the two exchanging folds as a function of temperature and the determination of the corresponding activation energies E(a) and frequency factors A. We found that the refolding rates are strongly affected by an entropically favorable preorientation of the replacing strand. The activation energies are comparable to values obtained for the slow refolding of RNA sequences of similar thermodynamic stability but less favorable topology.

Base Sequence↗
PubMed1983

Experimental analysis of alternative models of charge movement in frog skeletal muscle.

A series of pulse procedures was used to distinguish experimentally between a 'capacitative' (Schneider & Chandler, 1973) and a 'resistive' (Matthias, Levis & Eisenberg, 1980) model of 'charge movements' in skeletal muscle. A general condition describing the conservation of charge in a non-linear capacitor that was used as the basis for the experiments is derived in the Appendix. It was shown that earlier criteria concerning equality of 'on' and 'off' charge in response to large steps are insufficient to exclude resistive models. However, the capacitative, but not the resistive model successfully explained results bearing on charge conservation assessed through pulse procedures involving: (i) small, 10 mV voltage steps from a series of prepulse voltages, (ii) voltage steps to a fixed potential from a series of hyperpolarized voltages, (iii) pulse sequences incorporating a 'staircase' of voltage steps. It is concluded that the earlier use of 'on' and 'off' equality in response to large voltage steps is insufficient to exclude a resistive basis for the non-linear transient. However pulse procedures explicitly designed to distinguish the two models give results consistent with a capacitative model for the non-linear charge and at variance with a resistive one.

Animals↗
PubMed1990

Magnetic resonance imaging of bone marrow: diagnostic value in diffuse hematologic disorders.

Magnetic resonance imaging (MRI) has value in characterizing normal and abnormal bone marrow because of its ability to distinguish fat from other tissues. Due to this advantage, hematologic disorders resulting in alterations of the normal cellular and fatty marrow distribution can be appreciated. In this article, the role of MRI in diffuse hematologic disorders is emphasized. At birth, almost all marrow is cellular, but by age 25, cellular marrow is restricted to the axial skeleton and proximal femoral and humeral metaphysis. The remainder is fatty, consisting of 80% fat, 15% water, and 5% protein. With increased need for hematopoiesis, reconversion from fatty to cellular marrow occurs in many diffuse disease states. Diffuse diseases that affect bone marrow production are divided into four categories representing conditions that affect the pluripotent hematopoietic stem cell. These include stem cell failure resulting in aplastic anemia, uncontrolled stem cell proliferation as exemplified by polycythemia vera, stem cell dysplasia such as sickle cell anemia, and malignant transformations or replacement. The MRI appearance of these disorders is discussed in this article. The use of spin-echo (SE) sequences is the most common approach to bone marrow imaging. With T1-weighted SE images, fatty marrow will appear bright and cellular marrow, with lower fat content, will exhibit a lower density signal. With T2-weighted SE pulse sequences, contrast between fatty marrow and cellular marrow decreases. Contrast between fatty and cellular marrow is enhanced with chemical shift imaging, including Dixon out-of-phase imaging, as emphasized in this article. MRI presents a more global view of the bone marrow than biopsy material and should provide a better understanding of diffuse hematologic disease progression and resolution.

Bone Marrow↗
PubMed2001

Optimization and validation of a rapid high-resolution T1-w 3D FLASH water excitation MRI sequence for the quantitative assessment of articular cartilage volume and thickness.

In view of follow up, survey and development of therapeutic strategies for osteoarthritis where cartilage deterioration plays an important role, a non invasive, reliable and quantitative assessment of the articular cartilage is desirable. The currently available high resolution T(1)-weighted (T1-w) 3D FLASH pulse sequences with frequency selective fat suppression are very time consuming. We have 1) optimized a high resolution T1-w 3D FLASH water excitation (WE) sequence for short acquisition time and cartilage visualization, and 2) validated this sequence for cartilage volume and thickness quantification. The spectral fat presaturation was replaced by selective water excitation. The flip angle of the WE sequence was optimized for the contrast to noise (C/N(cart)) ratio of cartilage. Sagittal datasets (voxel size: 0.31 x 0.31 x 2 mm(3)) of the knees of nine healthy volunteers were acquired both, with the 3D FLASH WE (17.2/6.6/30 degrees ) sequence (WE) and a previously validated 3D FLASH fat saturated (42/11/30 degrees ) sequence (FS). For validation of the WE sequence, cartilage volume, mean and maximal cartilage thickness of the two sequences were compared. Reproducibility was assessed by calculating the coefficient of variation (COV %) of 4 consecutive WE data sets in the volunteers. The acquisition time was reduced from 16'30" (FS) down to 7'14" for the WE sequence. Image contrast and visualization of the cartilage was very similar, but delineation of the basal layer of the cartilage was slightly improved with the WE sequence. A flip angle of 30 degrees provided the best C/N(cart) ratios (WE). Reproducibility (COV) was between 1.9 and 5.9%. Cartilage volume and thickness agreed within 4% between FS and WE sequence. The WE sequence allows for rapid, valid and reproducible quantification of articular cartilage volume and thickness, prerequisites for follow-up examinations. The reduced acquisition time (50% of FS) enables routine clinical application and thus may contribute to a broader assessment of osteoarthritis.

Adult↗
PubMedDate not supplied

Integrated 3D display of brain anatomy and intracranial vasculature in MR imaging.

In many clinical situations neuroradiologists and neurosurgeons are keenly interested in images displaying both brain structures and intracranial vessels in an integrative manner. In this paper an approach to three-dimensional visualization of brain and vascular structures from magnetic resonance (MR) volume data is reported. It has been designed with the aim of providing a robust and reliable image processing tool for routine clinical applications. The method has been made possible by recent developments in MR image acquisition, especially MR angiography and rapid gradient-echo sequences (Turbo-fast low angle shot). On the basis of the ray-tracing principle, integration of brain and vessel anatomy into a single 3D image is achieved. Image data are acquired with flow-compensated gradient-echo pulse sequences. Even slow flow in venous structures may be demonstrated using a two-dimensional sequential-slice scanning method. Finally, if incorporated into an interactive image processing system, this technique may be used as a planning tool allowing a surgical "rehearsal" prior to actual operative exposure and resection of a lesion.

Brain↗
PubMed2000

Improvement of spectral editing in solids: A sequence for obtaining (13)CH + (13)CH(2)-only (13)C spectra

An improved spectral editing method for solids is described which allows one to obtain a set of subspectra in roughly two-thirds the amount of time as our original CPPI editing method for the same signal to noise. This improvement is afforded by a new pulse sequence that is used to acquire a (13)CH + (13)CH(2) spectrum which has very little (13)CH(3) or nonprotonated carbon contamination. By using this new sequence the (13)CH-only subspectrum is obtained much more efficiently. Criteria for optimizing the signal to noise in the edited subspectra are discussed. Copyright 2000 Academic Press.

Journal Article↗
PubMed1991

High-speed STEAM MRI of the human heart.

High-speed STEAM MR images of the normal human heart were obtained from single cardiac cycles using a 2.0-T whole-body system equipped with conventional 10 mT m-1 gradients. The single-shot 90 degrees-TE/2-90 degrees-TM-(alpha-TE/2-Acq)n pulse sequence acquires n differently phase-encoded stimulated echoes. Measuring times of 127-254 ms were achieved using a "repetition time" of 3.96 ms in conjunction with data matrices of 32-64 x 128 pixels covering a field-of-view of 250-350 mm. The sequence provides easy access to anatomical short-axis and long-axis views of the heart by single and double oblique rotation of the image orientation. STEAM images resemble the features of spin-echo images with respect to chemical shifts, susceptibilities, and flow. Thus, no additional techniques are required for the suppression of blood signals. EKG-triggered acquisitions demonstrate that slice-selective STEAM sequences using short TM intervals allow an unambiguous delineation of those parts of the myocardium that remain stationary within the selected plane throughout the entire imaging process. Neither spins leaving nor entering the slice defined by the initial 90 degrees RF pulses give rise to a stimulated echo and therefore do not contribute to the resulting image.

Heart↗
PubMed2005

Solvent-localized NMR spectroscopy using the distant dipolar field: a method for NMR separations with a single gradient.

Solvent-localized NMR (SOLO) is a new method which allows the separation of NMR spectra of substances dissolved in different solvents. It uses the selective HOMOGENIZED pulse sequence to produce a two-dimensional NMR spectrum resulting from intermolecular zero-quantum coherences in one distinct solvent. The detected signal is locally refocused by the action of the distant dipolar field, which is created by a frequency selective pulse only in regions containing the selected solvent. The prerequisites are that the different solvents have sufficiently different chemical shifts to be excited separately and that compartments with different solvents are spatially separated by more than the typical diffusion distance. Here, the method is demonstrated for the solvents water and DMSO on a length scale of 0.5 mm. Because signal in the spectra is refocused locally, SOLO is insensitive to variations in the magnetic field which may result from inhomogeneities or structures in the sample. This makes applications in strongly structured samples possible. SOLO is the first method that achieves localization of NMR signal with a single gradient pulse. Therefore, it can be used in conventional NMR spectrometers with one-axis gradient systems and lends itself to a wide range of applications including in vivo NMR.

Journal Article↗
PubMed2006

A 'just-in-time' HN(CA)CO experiment for the backbone assignment of large proteins with high sensitivity.

Among the suite of commonly used backbone experiments, HNCACO presents an unresolved sensitivity limitation due to fast 13CO transverse relaxation and passive 13Calpha-13Cbeta coupling. Here, we present a high-sensitivity 'just-in-time' (JIT) HN(CA)CO pulse sequence that uniformly refocuses 13Calpha-13Cbeta coupling while collecting 13CO shifts in real time. Sensitivity comparisons of the 3-D JIT HN(CA)CO, a CT-HMQC-based control, and a HSQC-based control with selective 13Calpha inversion pulses were performed using a 2H/13C/15N labeled sample of the 29 kDa HCA II protein at 15 degrees C. The JIT experiment shows a 42% signal enhancement over the CT-HMQC-based experiment. Compared to the HSQC-based experiment, the JIT experiment is 16% less sensitive for residues experiencing proper 13Calpha refocusing and13Calpha-13Cbeta decoupling. However, for the remaining residues, the JIT spectrum shows a 106% average sensitivity gain over the HSQC-based experiment. The high-sensitivity JIT HNCACO experiment should be particularly beneficial for studies of large proteins to provide 13CO resonance information regardless of residue type.

Carbon Isotopes↗
PubMed2005

Changes of pulmonary vein orifice size and location throughout the cardiac cycle: dynamic analysis using magnetic resonance cine imaging.

INTRODUCTION: Anatomically guided left atrial ablation is used increasingly for treatment of atrial fibrillation (AF). Three-dimensional mapping systems used for pulmonary veins (PV) encircling ablation procedures anticipate a stable size and position of the PV orifice. The aim of the current study was therefore to analyze changes of PV orifice size and location throughout the cardiac cycle using cine magnetic resonance imaging (MRI). METHODS AND RESULTS: Twenty-five healthy volunteers were studied using a 1.5 Tesla MRI system. MR angiograms were acquired with a breath-hold three-dimensional fast-spoiled gradient-echo imaging (3D FSPGR) sequence in the coronal plane before and after gadolinium injection. Maximum intensity projections and multiplanar reformations were performed to reconstruct images of the PV. Bright blood cine imaging in the axial view was acquired by a steady-state free precession pulse sequence. Twenty bright blood images were obtained per cardiac cycle. The axial (anterior-posterior) PV orifice diameter was measured in all 20 images. For analysis of PV movement the location of the orifice posterior edge was plotted on scale paper. PV orifice size depends on the stage of the cardiac cycle with the largest diameter in late atrial diastole and a mean decrease of 32.5% during atrial systole. Location changes of the PV orifice are in the range of up to 7.2 mm and larger in the coronal (lateral-medial) than in the sagittal (anterior-posterior) direction. CONCLUSION: PV orifice size and location is not as stable as anticipated by three-dimensional mapping systems used for PV encircling left atrial ablation procedures. RF application close to the presumed orifice location should therefore be avoided to minimize the risk of PV stenosis.

Adult↗
PubMed2002

Subvoxel image registration of multislice (2D) magnetic resonance images in patients with high-grade gliomas of the brain.

AIMS: To implement a multislice two-dimensional (2D) T2-weighted sequence suitable for subvoxel image registration and to assess its usefulness in detecting change in high-grade intracranial gliomas. MATERIALS AND METHODS: Twenty patients with high-grade gliomas were studied on two or more occasions. T2-weighted multislice pulse sequences with a Gaussian slice profile, 50% overlapping slices and nearly isotropic voxels were acquired. The images were registered and subtraction images were produced. The images were compared with three-dimensional (3D) T1-weighted registered images and conventional unregistered T2-weighted images. All images were scored for changes in the lesions and ventricular system. RESULTS: The 2D and 3D registered subtraction images were the most sensitive for detecting changes in both the lesions and other regions in the brain. The mean rank scores were significantly higher for the lesions (chi2=86.742; df=5, n=38, P<0.0001) and for the ventricles (chi2=63.837; df=5, n=35, P<0.0001) compared with the unregistered and registered anatomical images. The subtraction images were also most sensitive for detecting signal intensity changes irrespective of the direction of change. CONCLUSION: Rigid body subvoxel registration can be successfully performed with both multislice 2D and 3D imaging. In principle, virtually all forms of clinical MR images of the brain can be accurately registered and subtracted.

Adult↗
PubMed1987

Sequential or single pulse defibrillation? Investigations towards energy reduction in experimental animals.

With introduction of the automatic implantable cardioverter-defibrillator for treatment of medically refractory ventricular arrhythmias, many investigations are focussing on possibilities of reducing the energy necessary for defibrillating the heart to obtain a more adequate size and a longer durability of the generator. Several studies favour the sequential pulse delivery, using three electrodes, either endocardial, epicardial or subcutaneous plates, to improve defibrillation performance of low energy shocks. However, the validity of this conclusion remains equivocal since two different electrode configurations were used for single and sequential defibrillation. In the present study the influence of sequential pulse delivery, pulse sequence and resultant current pathways on defibrillation energy requirements were examined in comparison with single-shocks between the same epicardial electrode configuration as well as four orthogonally positioned patches. The energy requirement for 100% efficiency could be reduced by sequential pulsing with a time interval of 1 msec thus yielding significant superiority compared to single pulse defibrillation using three electrodes. The same reliable defibrillation and reduced threshold has been obtained by using four electrodes and one single shock. The addition of a third electrode alone had no influence on the energy requirements. Therefore, we have to discuss a different electrophysiological mechanism for sequential defibrillation than for single shocks with an extended current distribution over the heart using four electrodes. Possibly the action potential of the single cell of the fibrillating heart is of interest in reflecting the efficiency of sequential pulse defibrillation. By influencing the action potential pharmacologically in some animals, the optimal time interval for double shocks could be extended up to 7 msec.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2001

Accuracy for detection of simulated lesions: comparison of fluid-attenuated inversion-recovery, proton density--weighted, and T2-weighted synthetic brain MR imaging.

OBJECTIVE: The objective of our study was to determine the effects of MR sequence (fluid-attenuated inversion-recovery [FLAIR], proton density--weighted, and T2-weighted) and of lesion location on sensitivity and specificity of lesion detection. MATERIALS AND METHODS: We generated FLAIR, proton density-weighted, and T2-weighted brain images with 3-mm lesions using published parameters for acute multiple sclerosis plaques. Each image contained from zero to five lesions that were distributed among cortical-subcortical, periventricular, and deep white matter regions; on either side; and anterior or posterior in position. We presented images of 540 lesions, distributed among 2592 image regions, to six neuroradiologists. We constructed a contingency table for image regions with lesions and another for image regions without lesions (normal). Each table included the following: the reviewer's number (1--6); the MR sequence; the side, position, and region of the lesion; and the reviewer's response (lesion present or absent [normal]). We performed chi-square and log-linear analyses. RESULTS: The FLAIR sequence yielded the highest true-positive rates (p < 0.001) and the highest true-negative rates (p < 0.001). Regions also differed in reviewers' true-positive rates (p < 0.001) and true-negative rates (p = 0.002). The true-positive rate model generated by log-linear analysis contained an additional sequence-location interaction. The true-negative rate model generated by log-linear analysis confirmed these associations, but no higher order interactions were added. CONCLUSION: We developed software with which we can generate brain images of a wide range of pulse sequences and that allows us to specify the location, size, shape, and intrinsic characteristics of simulated lesions. We found that the use of FLAIR sequences increases detection accuracy for cortical-subcortical and periventricular lesions over that associated with proton density- and T2-weighted sequences.

Adult↗