Selective RF Pulses Research Articles

Design of adiabatic pulses for fat-suppression using analytic solutions of the Bloch equation.

Discrimination between signals produced by fat and by water is an important issue in MRI. One efficient approach is to perform fat-suppression by selective inversion. This technique exploits the transition region of a selective RF pulse to invert the longitudinal lipid magnetization while leaving the magnetization of the water protons untouched. The damaging effects of RF field inhomogeneity may be overcome by using pulses based on the adiabatic fast passage principle (AFP). In particular, the well-known sech/tanh adiabatic pulse is a robust and efficient pulse that is obtained as an analytic solution of the Bloch equation. In this paper, a wider class of analytic solutions of the Bloch equation is presented of which the sech/tanh driving function is merely a particular case. The new pulse exhibits an asymmetric distribution of magnetization with one transition sharper than the other. The sharper transition can be used to perform the required selective discrimination between signals. The resulting pulse features excellent adiabatic behavior. Moreover, the transition width of the new pulse can be reduced by a factor of about 2/3 with respect to an equal-duration sech/tanh pulse. The performance of the new pulse is compared with a similar sech/tanh pulse with the aid of a practical design example.

5572126 Reduced power selective excitation RF pulses

5572126 Reduced power selective excitation RF pulses

Frequency offset corrected inversion (FOCI) pulses for use in localized spectroscopy

Gradient localized spectroscopy techniques suffer from a well documented spatial localization error caused by the difference in chemical shifts between resonances. This results in the acquisition of spectra from partially overlapping spatial regions of the sample, with each resonance representing a different region. The image-selected in vivo spectroscopy technique uses hyperbolic secant inversion pulses, where the main limitation in reducing this error is in the RF power available for application of the selective RF pulse. This spatial localization error may be dramatically reduced by increasing, and temporally shaping, the gradient pulse during slice-selective spin inversion. The performance of these RF pulses have been experimentally verified.

Gradient-enhanced TOCSY experiments with improved sensitivity and solvent suppression

Gradient-enhanced versions of the homonuclear TOCSY experiment are described, with solvent suppression and sensitivity superior to that of a conventional TOCSY experiment. The pulse sequences are constructed by appending a WATERGATE module to a z-filtered TOCSY experiment. Pulsed-field gradients and appropriately phased selective rf pulses are used to maintain precise control of the water magnetization vector. Problems associated with radiation damping and spin-locking of the water magnetization are thus alleviated. The water magnetization is returned to equilibrium prior to each acquisition, which improves water suppression and minimizes signal losses due to saturation transfer.

Localized 1H NMR spectroscopy of rat spinal cord in vivo.

A movable, actively decoupled surface coil has been employed to obtain a localized 1H NMR spectrum from the lumbosacral spinal cord of a live Lewis rat. A volume selective 'VOSY' normally spelled out as 'volume selective spectroscopy' spectroscopy pulse sequence that incorporates 'phase ramped' selective RF pulses, has been used to minimize random phase jitter in the NMR signal as a result of the large frequency shifts required to locate the voxel in the center of the cord while using intense gradient pulses. Spectra from 13-microliters voxels in healthy rats and in rats inoculated with guinea pig spinal cord and complete Freund's adjuvant, resulting in experimental autoimmune encephalomyelitis, are shown.

5499629 Slice profile stabilization for segmented K-space magnetic resonance imaging

Selective RF pulses are applied for segmented k-space imaging sequences with the tip-angle profiles of the pulses varying for stabilizing the entire signal profile and reducing ghosting and blurring artifacts in slice images.

5349294 Two and three-dimensionally selective RF pulses for magnetic resonance imaging

A sequence controller (30) controls gradient pulse amplifiers (20) and a digital transmitter (24) to apply a conventional magnetic resonance imaging or spectroscopy sequence. One or more of the resonance excitation pulses includes a series of very small tip angle RF pulses (52, 70) applied in rapid succession substantially within the time interval of a normal RF excitation pulse (e.g. 10 msec.). A series of gradient pulses (58x, 58y, 72y, 72z) with linearly diminishing amplitudes and a repetition cycle that is an integer multiple of the duration of the very small tip angle RF pulses are applied such that an excitation trajectory in k-space follows a piecewise linear square spiral (FIG. 3 ) when gradients are applied along two axes or an octahedral spiral (FIG. 6 ) when a series of gradient pulses are applied along three axes. The subregion of resonance excitation is selectively shifted along one of the axes by applying a series of frequency offset pulses (66, 76) along one or more of the axes. In this manner, the position of the subregion of resonance excitation is shifted without changing the phase component of the RF pulses.

5410249 Method and apparatus for magnetic resonance imaging

In this study, a new strategy for slow flow imaging is proposed. The basic idea is to generate flow contrast on a microscopic level below the spatial resolution of an imaging experiment. Since a microscopic spin tagging scheme is used, this concept is called MiST (Microscopic Spin Tagging). MiST is not a single specific measurement sequence, but rather a new flow sensitive preparation concept which is highly flexible and can be carried out in many ways. The common principle in all possible realizations of MiST is a periodic tagging of magnetization in thin planes (100–200 μm) within the imaging voxels by means of spatially selective RF-pulses. Therefore, flow sensitivity occurs via inflow of fresh spins on a microscopic scale. With this approach, short evolution times are sufficient to introduce inflow contrast and a spatial dependence of inflow times is avoided. The flow sensitive preparation and image orientation are also not connected as they are in conventional time-of-flight techniques. Another powerful feature of MiST is that it can be designed as a non-subtraction method, which results in no signal from stationary spins. Here we present a first realization of the MiST concept and its validation in quantitative flow measurements to demonstrate the feasibility of the proposed preparation concept.

5347221 Truncated nuclear magnetic imaging probe

This paper considers a variety of problems in the design of selective RF-pulses. We apply a formula of Zakharov and Manakov to directly relate the energy of an RF-envelope to the magnetization profile and certain auxiliary parameters used in the inverse scattering transform (IST) approach to RF-pulse synthesis. This allows a determination of the minimum possible energy for a given magnetization profile. We give an algorithm to construct both the minimum energy RF-envelope as well as any other envelope that produces a given magnetization profile. This includes an algorithm for solving the Gel’fand–Levitan–Marchenko equations with bound states. The SLR method is analyzed in terms of traditional scattering data, and shown to be a special (singular) case of the IST approach. RF-envelopes are computed for a variety of examples.

A strategy for sampling on a sphere applied to 3D selective RF pulse design.

Conventional constant angular velocity sampling of the surface of a sphere results in a higher sampling density near the two poles relative to the equatorial region. More samples, and hence longer sampling time, are required to achieve a given sampling density in the equatorial region when compared with uniform sampling. This paper presents a simple expression for a continuous sample path through a nearly uniform distribution of points on the surface of a sphere. Sampling of concentric spherical shells in k-space with the new strategy is used to design 3D selective inversion and spin-echo pulses. These new 3D selective pulses have been implemented and verified experimentally.

The Effects of Frequency-Selective RF Pulses on J-Coupled Spin- Systems

The effects of solvent-suppression, sinc-Gauss, self-refocussed, and 2D selective RF pulses on J-coupled spin systems have been investigated by means of computer simulations. Depending on the pulse time, coupling constants, and chemical shifts, nonnegligible amounts of unwanted coherence may be generated, resulting in signal loss and deteriorated slice profiles. A time-inverted self-refocused frequency-selective pulse is useful for frequency-selective multiple-quantum generation and spectral editing. Volume-selective pulses with bandwidths in the order of the chemical-shift range may lead for the PRESS experiment to severe signal cancellation. The SADLOVE localization sequence does not have this effect. It is shown that except for multiple-pulse localization sequences, with RF bandwidths on the order of the chemical-shift range, the effects of J coupling can be neglected for in vivo use.

An integrated program for amplitude-modulated RF pulse generation and re-mapping with shaped gradients

Efficient generation of amplitude modulated, frequency selective RF pulses has been demonstrated by the Shinnar-Le Roux (SLR) algorithm. In the present article, we provide an overview of a relatively comprehensive computer program that includes a version of the SLR algorithm and also incorporates an algorithm for re-mapping a selective RF pulse onto a new dwell time with modulated gradients. The re-mapping may be used to reduce SAR, or to shorten the RF pulse time by increasing the gradient and RF strength in regions where the original RF pulse amplitude was low. The program includes additional useful features including a Bloch equations algorithm, and pulse scaling, to enable examination of pulse profiles under a variety of conditions such as RF inhomogeneity and even nuclear relaxation. The program, MATPULSE, was developed with the MATLAB for Windows programming language and makes extensive use of the MATLAB graphical user interface (GUI) features to generate a userfriendly interface. A number of examples are provided to illustrate the capabilities of the MATPULSE program.

Spatially selective RF pulses and the effects of digitization on their performance.

Spectrometers make use of D/A converters to generate RF and gradient shapes. This paper examines by exact simulations the time and amplitude digitization effects, inherent to the use of D/A converters, on the performance of amplitude modulated (AM) frequency selective RF pulses. By making use of Fourier theory and the small tip angle approximation, an approximate model of these effects on the magnetization slice profiles is derived and verified for several pulse types by computer simulations. This approximate model will be used to derive requirements for D/A converters with respect to spatial localization. The dynamics of the spin system allows pulse width modulation (PWM) as an alternative to AM for pulse envelope encoding. The effects of PWM on the slice profile are examined and compared with conventional AM pulses. It is shown by simulation and measurement that adiabatic PWM pulses can be found. In contrast to AM modulated adiabatic pulses, adiabatic PWM pulses have side bands with the same slice quality as the main slice and might therefore be useful as multislice selective pulses.

An application of snapshot FLASH MRI in localized NMR spectroscopy

Improvements in crossed images are presented by application of snapshot FLASH MRImaging which reduces the imaging time in localizedin vivo NMR spectroscopy. The method enables correlation of the volume of interest with its surroundings and determines the set of offset frequencies of the selective RF pulses to locate the volume of interest.

Properties of the PHase-Alternating Phase-Shift (PHAPS) multiple spin-echo protocol in MRI: A study of the effects of imperfect RF pulses

The effects of imperfect radiofrequency (FR) pulses on the echo amplitudes from the Carr-Purcell (CP), Carr-Purcell-Meiboom-Gill (CPMG), and the PHase-Alternating Phase-Shift (PHAPS; combination of CP and CPMG) multiple spin-echo schemes were studied. Properties of the PHAPS scheme for transverse relaxation time measurements was emphasized. Numerical simulations on non-relaxing spin systems were performed to assess the properties of selective (damped sinc shaped) and nonselective refocusing pulses in terms of effective spatial selectivity and generation of secondary echo signal. Analytical solutions of the Bloch equations were applied to study the generation and propagation of stimulated echo signal caused by nonideal 180° phase reversals, and the results were used to analyse the numerical simulations in terms of primary and stimulated echo components. Finally, the simulated echo train patterns from the different MSE schemes were compared with MR imaging measurements. It was found that the underestimation of T 2 values by the PHAPS protocol with selective refocusing pulses is mainly an effect of an “artificial” echo amplitude decay in the CP scheme, while the CPMG scheme produces a typical even-odd echo pattern (different from corresponding echo patterns in conventional high resolution NMR). Both effects are related to the flip angle error and phase dispersion along the slice selection direction from selective RF pulses, and are not significantly influenced by stimulated echo interference for nonrelaxing spin systems. However, the presence of stimulated echoes at the time of the primary echoes implies a dependence on T 1 of the PHAPS echo amplitudes. In the CPMG protocol, different gradient schemes have been implemented to defocus stimulated echoes. However, the results indicate that there exists stimulated components that will not be affected by such gradients, and that the optimization of the RF refocusing pulses then remain the main objective.

Practical applications of chemical shift imaging.

Methods of spectral localization are briefly reviewed and divided into two classes: those using phase encoding and those using frequency selective RF pulses in a constant gradient. A potentially troubling artifact in the latter case is the spatial misregistration of different compounds which causes serious errors in 31P spectra from smaller regions. Chemical shift imaging (CSI) is presented as a typical example of phase encoding techniques. An analytical expression for the relationship of the signal observed to the true signal (the point spread function) is derived. Examples of CSI in one, two, and three dimensions are used to illustrate the principles of this type of localization.

Application of self-refocusing band selective RF pulses for spectroscopic localization.

A new self-refocusing slice selection 90 degrees pulse is presented and its incorporation in the SPACE localization sequence described. Experimental comparisons are made with the self-refocusing pulse reported by Geen (H. Geen, S. Wimperis and R. Freeman, J. Magn. Reson. 85, 620 (1990)). The main source of localization error in the SPACE sequence is traced to the hard pi/2 pulse and the development of a shaped-pulse version of the sequence is described. This required the calculation of a slice-selective pulse capable of rotating coherent transverse magnetization to the z-axis. The RF power requirements for these experiments are also discussed.

Correction of chemical‐shift artifacts in multislice F‐19 imaging with perfluorooctyl bromide

Chemical-shift artifact correction methods for multislice F-19 imaging with perfluorooctyl bromide are described and results obtained with physical phantoms presented. Utilization of the two long-T2 spectral peaks of PFOB in multislice imaging enhances the imaging efficiency considerably. Simultaneous imaging of the adjacent slices is achieved by appropriately adjusting the slice selection gradient amplitude, the bandwidth, and the slice position offset frequency of the selective RF pulses in relation to the frequency separation between the two peaks.

4972148 Magnetic resonance tomography method and magnetic resonance tomography apparatus for performing the method

The invention relates to a magnetic resonance tomography method where two slice-selective rf pulses influence the nuclear magnetization in a slice. Further slice selective rf pulses which excite sub-slices extending perpendicularly to said slice generate stimulated echo signals in the strip-shaped zone of intersection between the relevant sub-slice and the slice. The FID signals associated with the further rf pulses are suppressed by means of a subsequently activated magnetic gradient field. In order to ensure that this field does not dephase the desired stimulated echo signals at the same time magnetic gradient field having the same time integral is activated between the first and the second rf pulse.

Correction for chemical‐shift artifacts in 19F imaging of PFOB: Simultaneous multislice imaging

One of the difficulties encountered in 19F NMR imaging of fluorinated blood substitutes is that these compounds often exhibit complex multipeak spectra. These peaks result in chemical-shift artifacts along the readout direction and blurred images. In addition, each peak excites a different slice (mis-selection) when a slice selection gradient is applied during the selective rf pulse. A simultaneous multislice imaging method has been developed to solve the inherent problem of mis-selection. The essence of this method is to use the two strongest peaks of the spectrum to excite controlled different multiple slices simultaneously, with or without a slice gap. The images corresponding to the two spectral lines are then separated from in- and out-of-phase images (Dixon method). This method corrects the problem of mis-selection and either improves the SNR or increases the number of slices over spectrally selective methods which image only one peak.