Radiofrequency Pulse Sequences Research Articles

Fiberoptic Intracranial Pressure Monitoring during Magnetic Resonance Imaging

Fiberoptic Intracranial Pressure Monitoring during Magnetic Resonance Imaging

5541512 Method for the prevention of registration artifacts due to motion in magnetic resonance images

A magnetic resonance system employs a sequence of radio frequency pulses and magnetic field gradients to generate a flow-compensated image of a selected portion of a sample. Flow-compensation is performed with an oscillating readout gradient waveform which is comprised of two components. The first component is a constant amplitude gradient waveform whose amplitude is determined by the desired field-of-view and the bandwidth of the imaging system. The second component is an oscillating waveform whose amplitude, frequency and phase are chosen to obtain the desired degree of flow-compensation. The frequency of the oscillating waveform is typically chosen to match the sampling frequency of the imaging system. In effect, each acquired data point is preceded by the application of a bi-polar magnetic field gradient pulse which causes a phase shift in the acquired signal which is proportional to nuclear spin velocity. The amplitude is typically chosen to cause an incremental phase shift which when repeatedly added to the acquired MR response signal at the sampling rate causes a frequency modulation. This frequency modulation, in turn, induces a spatial displacement of signal intensity in the readout dimension which corresponds to the displacement of spin magnetization during the interval between the phase-encoding and frequency-encoding.

5523688 Method for operating a magnetic resonance imaging apparatus to obtain a high T2 contrast

Nuclear spins are excited in an examination subject by a sequence of radio-frequency pulses that are emitted in under the influence of slice selection gradients, with n different slices of the examination subject being excited. Due to read-out gradients, n gradient echoes respectively allocated to a slice are generated such that each gradient echo allocated to a radio-frequency pulse comes to lie between two successive radio-frequency pulses. Further slices can thus be excited within the echo time of a gradient echo of a specific slice. A T 2 * contrast image can thus be obtained with a significantly shorter examination time than was heretofore achievable.

Broadband Multiple-Quantum NMR Spectroscopy

Abstract A simple broadband radiofrequency pulse sequence for the excitation of multiple-quantum coherences in the presence of fast magic-angle spinning is introduced. This sequence involves back-to-back (BABA) 90° radiofrequency pulse cycles timed to span two rotor periods. It proved to be robust and insensitive to off-resonance effects, to isotropic chemical shifts, and to chemical-shift anisotropies. This is demonstrated on crystalline phosphates, composed of Q (1) , Q (2) , and Q (3) groups, which are characterized by a large chemical-shift anisotropy spanning more than 200 ppm. With these experiments, 31 P high-resolution double-quantum NMR spectroscopy is introduced as a tool for the direct investigation of dipolar connectivities between like or different Q ( n ) units in phosphates. 31 P dipolar connectivities in two samples, Mg 2 P 2 O 7 and MgP 4 O 11 were established. Therefore, double-quantum spectroscopy of this kind has potential for the investigation of disordered solids, for instance, phosphorus glasses.

Rotational resonance echoes in the nuclear magnetic resonance of spinning solids

A procedure is described which leads to the reversal of the rotor driven evolution of nuclear spins at rotational resonance, inducing the formation of a new class of spin echoes. The echoes are produced by radio-frequency pulse sequences which suspend the evolution of the nuclear spin states for a finite fraction of a sample rotation period. Experimental demonstrations are given. The echoes may be exploited to extract scalar J-couplings and to investigate the relaxation behavior of coupled spins in a rotating powdered solid.

Efficient dipolar recoupling in the NMR of rotating solids. A sevenfold symmetric radiofrequency pulse sequence

A new radiofrequency pulse sequence is introduced for the efficient reintroduction of magnetic dipolar couplings in the magic-angle-spinning NMR of solids. The sequence involves seven phase-shifted radiofrequency pulse cycles, timed to span two rotational periods of the sample. Double-quantum coherences are excited with high efficiency in a rotating powder sample of zinc acetate- 13C 2.

5363042 Methods for measurement of longitudinal spin relaxation times in moving liquids

A magnetic resonance imaging system employs a sequence of radio frequency pulses and magnetic field gradients to detect and measure the spin-relaxation time of moving blood within a subject. Spin-lattice relaxation times are determined by first inverting longitudinal spin magnetization and then detecting the recovery of this magnetization with a series of detection radio frequency pulses. The inversion pulse is applied to the entire subject, but the detection pulses are applied only to a selection portion of the subject. Blood motion causes the blood in the detection region to be replaced for each detection pulse, thereby increasing the accuracy of the measurement and permitting the use of multiple detection pulses after a single inversion pulse. In-vivo application of this invention can be used to assess renal function in individual kidneys.

Creation and detection of coherence and polarization in pulsed EPR

This paper makes an attempt to coordinate the various pulsed EPR experiments on a common ground. To that end we subdivide the pulse schemes into individual modules, each consisting of a sequence of microwave and/or radiofrequency pulses, or of another type of and external perturbation. The progress in time of both polarization and coherence transfer experiments, may then be considered as a sequence of such modules. This way of looking at a pulsed EPR experiment is illustrated by a number of examples of applications.

Double-quantum-filtered 23Na NMR study of intracellular sodium in the perfused liver

We acquired double-quantum-filtered 23Na NMR spectra from perfused liver, using a range of tau values from 0.2 to 24 ms, where tau is the separation between the first and second pi/2 pulses in the radio-frequency pulse sequence. For each tau value we compared the amplitude of the double-quantum-filtered 23Na NMR signal acquired from intracellular sodium ions when the liver was perfused with buffer containing the "shift reagent" Dy(PPP)2 to the amplitude of the total double-quantum-filtered 23Na NMR signal acquired when the liver was perfused with buffer containing no Dy(PPP)2. For tau < or = 4 ms, the average ratio of the two amplitudes was 0.98 +/- 0.03 (mean +/- SEM). For tau > or = 8 ms, the average ratio was significantly less than 1. These results demonstrate that double-quantum-filtered 23Na NMR signals acquired from perfused liver using short tau values arise almost exclusively from intracellular sodium ions, but double-quantum-filtered 23Na NMR signals acquired from perfused liver using long tau values contain contributions from both intracellular and extracellular sodium ions. This conclusion suggests that multiple-quantum-filtered 23Na NMR spectroscopy will be useful in studying intracellular sodium levels in the perfused liver, and possibly in the intact liver in vivo.

5214381 Method for selectively exciting nuclear spins in a radio-frequency field with predetermined strength

In an NMR spectrometer, nuclear spins are subjected to a radio-frequency (RF) pulse sequence consisting of repeating cycles, each having a first and a second pulse set. The first pulse set generates a first large-amplitude rotation of the spins in which the angle through which the spins rotate varies with RF field strength. The second pulse set generates a second rotation which has an axis orthogonal to the first rotation axis and the angle through which the spins rotate is independent of the RF field strength. When the nuclear spins are placed in a non-homogeneous RF field and the above pulse sequence is applied, those spins which are located in an area where the RF field strength causes the first pulse set to rotate the spins through an angle which is a multiple of 2 π, the second, uniform rotation accumulates with each repetition of the first and second pulse sets in order to selectively excite those spins.

5217016 Method for generating optimal pulses for magnetic resonance imaging using spatial modulation of magnetization

A technique for generating "optimal" radio frequency pulses for use in creating SPAMM imaging stripes. The MR imaging operator is given an opportunity to select the desired stripe parameters for the SPAMM imaging stripe so that the resulting SPAMM imaging stripe will have the desired narrowness, sharpness, flatness and the like. These parameters are then used by an optimal pulse generating system of the invention to generate the input radio frequency pulse sequence which will produce the desired stripes. This technique allows a large family of pulse sequences with fairly similar performance to be generated by simply adjusting the relative weightings of criteria such as the narrowness, sharpness, flatness and the like for the SPAMM stripes.

Does memory loss occur after MR imaging?

In four separate studies, the existence of specific memory loss after magnetic resonance (MR) imaging at 1.5 T was assessed by means of recognition and recall tests for faces, common objects, lexical items, and by digit span, in a pre-post paradigm. Although specific memory loss was demonstrated, it was shown that the loss was not due to the main magnetic field of the imager or to radio-frequency pulse sequences, but rather to probable psychological (not physical) factors. No gross or subtle memory changes could be attributed to MR imaging, because control groups showed similar patterns of memory loss.

31P and 1H NMR pulse sequences to measure lineshapes, T1Z and T2E relaxation times in biological membranes

The dynamics of model membrane phospholipids has been monitored by phosphorus-31 solid state nuclear magnetic resonance (31 P-NMR). Radio frequency pulse sequences were designed to monitor spectral lineshapes, longitudinal (T1Z ) and transverse (T2E ) relaxation times dominated by 31 P chemical shielding anisotropy. Lineshapes and T2E are obtained by means of Hahn-echo sequences whereas T1Z is measured either by inversion or saturation methods coupled with the spin-echo sequence. The dipolar proton-phosphorus interaction was either suppressed by proton spin-lock or estimated through measurement of the maximum nuclear Overhauser enhancement (NOE).

Spatially localized NMR with the VOISINER sequence

An analysis of the practical implementation of the VOISINER method for achieving spatially localized NMR is presented. The VOISINER (volume of interest by selective inversion, excitation, and refocusing) field gradient and radiofrequency pulse sequence relies upon the use of frequency-selective pulses in the presence of pulsed field gradients to enable easy control of the position and the size of the selected volume. Furthermore, it can be combined with conventional spin-echo imaging to facilitate the selection of the region of interest and optimization of the spatial selectivity of the localization process. Its performance is examined under various experimental conditions in order to evaluate the factors that influence the efficiency of its spatial selectivity and detection sensitivity. Potential extensions of the VOISINER technique for extracting high-resolution NMR parameters have also been explored.

Flow, radiofrequency pulse sequences, and gradient magnetic fields

The basic process of MRI consists of two essential, relatively independent components: (1) excitation in the form of a radiofrequency pulse sequence, and (2) signal sampling and localization, that is, forming the MR image through the use of field gradients. The presence of motion (blood flow) during either excitation or sampling results in two types of corresponding effects: (1) time-of-flight effects, and (2) spin phase phenomena. These effects can be manipulated through the use of special coils, pulse sequences, gradients, and postprocessing techniques to provide angiographic images in which simple motion provides the basis for contrast.

A new technique for solid NMR imaging and application to phosphorus imaging in solid bone

A new imaging technique for solids with lines broadened by inhomogeneous dipolar interactions is described. A model of loosely coupled spin-1/2 pairs is used to explain how the technique works. Using this technique, the radiofrequency pulse sequence induces a solid echo which effectively prolongs the lifetime of a transverse signal because of the independence of the echo amplitude from the dipolar interactions. In addition the refocused gradients turned on throughout the echo formation are capable of encoding spatial information. The imaging technique is illustrated with one-dimensional images of calcium phosphate mineral distribution in solid bone samples.

Measurement of nuclear magnetic dipole—dipole couplings in magic angle spinning NMR

We describe a method for measuring nuclear magnetic dipole—dipole couplings in NMR spectra of solids undergoing rapid magic angle spinning (MAS). We show in theory, simulations, and experiments that the couplings, which are averaged out by MAS alone, can be recovered by applying simple resonant radiofrequency pulse sequences in synchrony with the sample rotation. Experimental 13C dipolar powder pattern spectra of polycrystalline ( 13CH 3) 2C(OH)SO 3Na obtained in a two-dimensional experiment based on this method are presented. The method provides a means of determining internuclear distances in polycrystalline and noncrystalline solids while retaining the high resolution and sensitivity afforded by MAS.

Volume-selective and spectroscopically resolved NMR investigation of diffusion and relaxation in fertilised hen eggs

A radiofrequency and field-gradient pulse sequence is presented permitting the non-invasive, volume-selective and spectroscopically resolved determination of incoherent transport parameters with the aid of an NMR field-gradient method. With proton NMR, diffusion or (quasi-incoherent) perfusion coefficients as low as 10-12 m2 s-1 are accessible with gradients G100 mT m-1 for T20.3 s. If coherent motions like flow are superimposed, the gradient pulses can be compensated for phase shifts arising from uniform or accelerated motions. The efficiency of the suppression of the influence of coherent motions was demonstrated in test experiments with phantom sample arrangements. As a biophysical application, the local diffusion coefficients D and relaxation times T1 and T2 of hen eggs were studied during the first days of incubation. In the yolk, strongly non-exponential decay curves were observed for the CH2 line. The translational displacements of water were found to be restricted. The relaxation and diffusion parameters turned out to be constant during the incubation.

Determination of chemical-shift-anisotropy Lineshapes in a two-dimensional magic-angle-spinning NMR experiment

We describe a technique for measuring chemical-shift-anisotropy (CSA) lineshapes in NMR spectra of polycrystalline or amorphous solids. By combining magic-angle spinning (MAS) with a radiofrequency pulse sequence synchronized with the sample rotation in one time period of a two-dimensional experiment, we obtain two-dimensional spectra in which the CSA lineshapes appear along one axis and the normal MAS spectrum appears along the other axis. The CSA lines are thereby resolved as long as the inequivalent nuclei have resolved isotropic chemical shifts in the MAS spectrum. Our technique differs from previous, related techniques in that we employ pulse sequences designed so that the CSA lineshapes in the two-dimensional spectrum are precisely the same as those obtained from one-dimensional spectra of nonspinning samples in the absence of spectral overlap; the analysis of the spectra is thus simplified substantially. We describe the theory and experimental implementation of the technique in detail, and present resolved 13C CSA lineshapes for methyl-α- d-glucopyranoside. We analyze the effects of pulse imperfections on the observed lineshapes and show how such effects can be minimized.

Magnetic Resonance Imaging of the Orbit

Magnetic resonance has twin capabilities. It can provide anatomical (magnetic resonance imaging, MRI) and physiochemical (magnetic resonance spectroscopy) information. Nuclei with an odd mass number, particularly hydrogen in free water, have electromagnetic properties. When placed in a strong static magnetic field and excited by radiofrequency waves of a specific wavelength, these nuclei emit a signal. The MR signal can then be digitized, stored in a computer, and subsequently converted into an image. Factors that affect these images are tissue parameters (T1 and T2 time constants, proton density, and flow) and radiofrequency pulse sequences. Surface coils are useful for improving images of the orbit. The magnet can have a major and potentially dangerous influence on the surrounding environment, and access to the MRI area must be carefully controlled. Shielding of the MRI room prevents external factors from adversely affecting the MRI unit and the images produced.