Generalized MAGSTE with bipolar diffusion-weighting gradient pulses

The generalized magic asymmetric gradient stimulated echo (generalized MAGSTE) sequence compensates background gradient cross-terms and can be adjusted to asymmetric timing boundary conditions which for instance are present in echo-planar MR imaging. However, its efficiency is not optimal because one of the two diffusion-weighting gradients applied in each interval usually must have a reduced amplitude to ensure the desired cross-term compensation. In this work, a modification of generalized MAGSTE is investigated where this gradient pulse is replaced by two gradient pulses with full amplitude but opposite polarities. It is shown that with these bipolar gradients (i) the sequence retains the cross-term compensation capability for an appropriate choice of the gradient pulse durations and (ii) the diffusion-weighting efficiency is improved, i.e. higher k and b values can be achieved without prolonging the echo time. These results are confirmed in MR imaging experiments on phantoms and in vivo in the human brain at 3 T using spin-echo and echo-planar MR imaging. In the examples shown, the b value could be increased between about 30% and 200% when using the bipolar gradient pulses. Thus, bipolar gradients may help to improve the applicability of the generalized MAGSTE sequence.     

Analysis of the magic asymmetric gradient stimulated echo sequence with shaped gradients

Recently a 13-interval magic asymmetrical gradient stimulated echo (MAGSTE) sequence has been proposed for accurate displacement measurements in the presence of spatially varying background gradients. In this paper, the commonly used trapezoidal and sine shaped gradients are studied for the MAGSTE sequence, and the magic asymmetrical gradient ratio and b-factor are provided. The derivation enables the MAGSTE sequence to be implemented on systems with non-negligible gradient rise times.     

Improved diffusion measurement in heterogeneous systems using the magic asymmetric gradient stimulated echo (MAGSTE) technique

A magic asymmetric gradient stimulated echo (MAGSTE) sequence was recently proposed to improve molecular diffusion measurements in the presence of spatially varying background gradients. Its effectiveness has been demonstrated previously with simulated background gradients and in phantoms that contain bulk susceptibility differences. In this study, we investigated the MAGSTE technique in microscopically heterogeneous systems, and compared it with the conventional bipolar pulsed gradient stimulated echo (bPGSTE) sequence. We demonstrated that the MASGTE measurements, compared to the bPGSTE method, varied significantly less when the diffusion encoding/decoding interval (delta) was changed. In addition, the MAGSTE technique provided good characterization of the surface area-to-volume ratio for heterogeneous systems investigated in this study. In sum, this study showed that the MAGSTE technique provided diffusion measurements superior to those of the bPGSTE sequence, especially in the presence of severe heterogeneous background gradients.     

MAG-PGSTE: a new STE-based PGSE NMR sequence for the determination of diffusion in magnetically inhomogeneous samples

A new stimulated echo based pulsed gradient spin-echo sequence, MAG-PGSTE, has been developed for the determination of self-diffusion in magnetically inhomogeneous samples. The sequence was tested on two glass bead samples (i.e., 212-300 and <106 microm glass bead packs). The MAG-PGSTE sequence was compared to the MAGSTE (or MPFG) (P.Z. Sun, J.G. Seland, D. Cory, Background gradient suppression in pulsed gradient stimulated echo measurements, J. Magn. Reson. 161 (2003) 168-173; P.Z. Sun, S.A. Smith, J. Zhou, Analysis of the magic asymmetric gradient stimulated echo sequence with shaped gradients, J. Magn. Reson. 171 (2004) 324-329; P.Z. Sun, Improved diffusion measurement in heterogeneous systems using the magic asymmetric gradient stimulated echo (MAGSTE) technique, J. Magn. Reson. 187 (2007) 177-183; P. Galvosas, F. Stallmach, J. K?rger, Background gradient suppression in stimulated echo NMR diffusion studies using magic pulsed field gradient ratios, J. Magn. Reson. 166 (2004) 164-173, P. Galvosas, PFG NMR-Diffusionsuntersuchungen mit ultra-hohen gepulsten magnetischen Feldgradienten an mikropor?sen Materialien, Ph.D. Thesis, Universit?t Leipzig, 2003, P.Z. Sun, Nuclear Magnetic Resonance Microscopy and Diffusion, Ph.D. Thesis, Massachusetts Institute of Technology, 2003] sequence and Cotts 13-interval [R.M. Cotts, M.J.R. Hoch, T. Sun, J.T. Marker, Pulsed field gradient stimulated echo methods for improved NMR diffusion measurements in heterogeneous systems, J. Magn. Reson. 83 (1989) 252-266] sequence using both glass bead samples. The MAG-PGSTE and MAGSTE (or MPFG) sequences outperformed the Cotts 13-interval sequence in the measurement of diffusion coefficients; more interestingly, for the sample with higher background gradients (i.e., the <106 microm glass bead sample), the MAG-PGSTE sequence provided higher signal-to-noise ratios and thus better diffusion measurements than the MAGSTE and Cotts 13-interval sequences. In addition, the MAG-PGSTE sequence provided good characterization of the surface-to-volume ratio for the glass bead samples.     

Improved diffusion-weighting efficiency of pulsed gradient stimulated echo MR measurements with background gradient cross-term suppression

Accurate diffusion measurements with pulsed gradient NMR are hampered by cross-terms of the diffusion-weighting and background gradients. For experiments based on a stimulated echo pulse sequence, that is preferred for samples with a T2 short compared to the diffusion time, a diffusion-weighting scheme has been presented that avoids these cross-terms in each of the en- and decoding periods separately. However, this approach suffers from a reduced diffusion-weighting efficiency because the two gradients applied in each of the periods have effectively opposite polarities leading to a partial cancellation. An extension of this scheme is presented that involves an additional gradient pulse in each period and delivers an improved diffusion-weighting efficiency without sacrificing the cross-term compensation. Analytical expressions for the gradient pulse lengths and amplitudes are given for arbitrary timing parameters. MR measurements with artificial (switched) background gradients were performed to test the cross-term compensation capability of the proposed extension. The results show that considerably higher q and b values can be achieved with the extension without changing the timing parameters. The MR measurements yielded identical diffusion coefficients without, with the same, and with different background gradients in the en- and decoding periods demonstrating the cross-term compensation of the presented approach.     

Symmetric echo acquisition for absolute-value display in solid-state NMR imaging

A method of solid-state NMR imaging that permits echo Fourier transformation (FT) has been devised using a magic echo train. The echo FT imaging can be implemented simply by modifying the gradient pulse sequence in the previous magic echo imaging (TREV-16TS) so that the one-dimensional k-space trajectory follows the sampling points which are symmetric about the k origin. The implemented ability of echo FT improves the performance of the magic echo imaging: the sensitivity gained by radical2, the phase correction is made unnecessary, and the digital resolution is doubled. One- and two-dimensional imaging experiments have been conducted on some solid samples, confirming the improved performance and revealing a TREV-16TS adjustment parameter that is critical for the successful echo FT imaging.     

Constant time steady gradient NMR diffusometry using the secondary stimulated echo

A pulse sequence producing a second stimulated echo is suggested for the compensation of relaxation and residual dipolar interaction effects in steady gradient spin echo diffusometry. Steady field gradients of considerable strength exist in the fringe field of NMR magnets, for instance. While the absolute echo time of the second stimulated echo is kept constant throughout the experiment, the interval between the first two radiofrequency pulses is augmented leading to a modulation of the amplitude of that second stimulated echo by self-diffusion only. The unique feature of this technique is that it is of a single-scan/single-echo-signal nature. That is, no reference signals neither of the same pulse sequence nor of separate experiments are needed. The new method was tested with poly(ethylene oxide) melts and proved to provide reliable data for (time dependent) self-diffusion coefficients down to the physical limit (D approximately 10(-15)m(2)/s) when flip-flop spin diffusion starts to become effective.     

Crossing fibers, diffractions and nonhomogeneous magnetic field: correction of artifacts by bipolar gradient pulses

In recent years, diffusion tensor imaging (DTI) and its variants have been used to describe fiber orientations and q-space diffusion MR was proposed as a means to obtain structural information on a micron scale. Therefore, there is an increasing need for complex phantoms with predictable microcharacteristics to challenge different indices extracted from the different diffusion MR techniques used. The present study examines the effect of diffusion pulse sequence on the signal decay and diffraction patterns observed in q-space diffusion MR performed on micron-scale phantoms of different geometries and homogeneities. We evaluated the effect of the pulse gradient stimulated-echo, the longitudinal eddy current delay (LED) and the bipolar LED (BPLED) pulse sequences. Interestingly, in the less homogeneous samples, the expected diffraction patterns were observed only when diffusion was measured with the BPLED sequence. We demonstrated the correction ability of bipolar diffusion gradients and showed that more accurate physical parameters are obtained when such a diffusion gradient scheme is used. These results suggest that bipolar gradient pulses may result in more accurate data if incorporated into conventional diffusion-weighted imaging and DTI.     

Diffusion studies in porous media with the "inside-out" technique

Diffusion measurements of water in a glass beads porous media were carried out by the "One-Sided Access" sensor. An approach to determine the long time mean square displacement of hydrogen molecules by measuring the ratio of stimulated echo to primary echo in a three pulse sequence in the presence of permanent static field gradient is analysed. The difference between the apparent diffusion coefficient in a pure water sample and in water/glass beads mixtures becomes visible.     

Mapping the intracellular fraction of water by varying the gradient pulse length in q-space diffusion MRI

Finite gradient pulse lengths are traditionally considered a nuisance in q-space diffusion NMR and MRI, since the simple Fourier relation between the acquired signal and the displacement probability is invalidated. Increasing the value of the pulse length leads to an apparently smaller value of the estimated compartment size. We propose that q-space data at different gradient pulse lengths, but with the same effective diffusion time, can be used to identify and quantify components with free or restricted diffusion from multiexponential echo decay curves obtained on cellular systems. The method is demonstrated with experiments on excised human brain white matter and a series of model systems with well-defined free, restricted, and combined free and restricted diffusion behavior. Time-resolved diffusion MRI experiments are used to map the spatial distribution of the intracellular fraction in a yeast cell suspension during sedimentation, and observe the disappearance of this fraction after a heat treatment.     

Fast imaging with the MMME sequence

The multiple-modulation-multiple-echo sequence, previously used for rapid measurement of diffusion, is extended to a method for single shot imaging. Removing the gradient switching requirement during the application of RF pulses by a constant frequency encoding gradient can shorten experiment time for ultrafast imaging. However, having the gradient on during the pulses gives rise to echo shape variations from off-resonance effects, which make the image reconstruction difficult. In this paper, we propose a simple method to deconvolve the echo shape variation from the true one-dimensional image. This method is extended to two-dimensional imaging by adding phase encoding gradients between echoes during the acquisition period to phase encode each echo separately. Slice selection is achieved by a frequency selective pulse at the beginning of the sequence. Imaging speed is mainly limited by the phase encoding gradients' switching times and echo overlap when echo spacing is very short. This technique can produce a single-shot image of sub-millimeter resolution in 5 ms.     

Manifestation of magic angle phenomenon: comparative study on effects of varying echo time and tendon orientation among various MR sequences

The purpose of this study was to systematically investigate the effect of varying the echo time (TE) values and angle of the tendon to the main magnetic field (B(o)) upon the signal intensity observed with the magic angle phenomenon in tendons among most commonly used MR pulse sequences, including conventional spin echo (CSE), fast spin echo (FSE) and gradient echo (GRE) sequences. The intact bovine Achilles tendon was imaged using a clinical 1.5-T MR scanner. Magic angle phenomenon occurs in CSE, FSE and GRE sequences with different grade, appearing most severe in CSE, middle in FSE, and weakest in GRE sequence. In addition, the tendon signal changes produced by the magic angle phenomenon could be greatly reduced by increasing the TE to above a certain critical value in all three sequences. These critical TE values were different among CSE (40 msec), FSE (70 msec), and GRE (30 msec) sequences.     

Background gradient suppression in stimulated echo NMR diffusion studies using magic pulsed field gradient ratios

By evaluating the spin echo attenuation for a generalized 13-interval PFG NMR sequence consisting of pulsed field gradients with four different effective intensities (F(p/r) and G(p/r)), magic pulsed field gradient (MPFG) ratios for the prepare (G(p)/F(p)) and the read (G(r)/F(r)) interval are derived, which suppress the cross term between background field gradients and the pulsed field gradients even in the cases where the background field gradients may change during the z-store interval of the pulse sequence. These MPFG ratios depend only on the timing of the pulsed gradients in the pulse sequence and allow a convenient experimental approach to background gradient suppression in NMR diffusion studies with heterogeneous systems, where the local properties of the (internal) background gradients are often unknown. If the pulsed field gradients are centered in the tau-intervals between the pi and pi/2 rf pulses, these two MPFG ratios coincide to eta=G(p/r)/F(p/r)=1-8/[1+(1/3)(delta/tau)(2)]. Since the width of the pulsed field gradients (delta) is bounded by 0< or =delta< or =tau, eta can only be in the range of 5< or =-eta< or =7. The predicted suppression of the unwanted cross terms is demonstrated experimentally using time-dependent external gradients which are controlled in the NMR experiment as well as spatially dependent internal background gradients generated by the magnetic properties of the sample itself. The theoretical and experimental results confirm and extend the approach of Sun et al. (J. Magn. Reson. 161 (2003) 168), who recently introduced a 13-interval type PFG NMR sequence with two asymmetric pulsed magnetic field gradients suitable to suppress unwanted cross terms with spatially dependent background field gradients.     

Magic Echo Solid-State NMR Imaging without a Rapidly Switchable Field Gradient

To relax the high-speed requirement imposed on the gradient system used in solid-state proton imaging, we propose two simple modifications of the magic echo imaging sequence, TREV-16TS. In the first modification, the applied gradient is inverted in the middle of the RF irradiation; the second modification utilizes a sinusoidal gradient synchronized with the RF sequence. It is estimated by experiments that as long as the RF amplitude is at least about 10 times stronger than the resonance offset induced by the gradient, the spatial resolution is not degraded significantly by the line narrowing deterioration due to the gradient applied during the on-resonance RF irradiation. The modifications allow commercially available standard gradients to be used for the magic echo imaging of solids.     

Quantitative diffusion coefficient maps using fast spin-echo MRI

In this work, we have evaluated the performance of a diffusion-sensitive fast spin-echo (FSE) pulse sequence. The proposed pulse sequence utilises velocity-compensating diffusion-encoding gradients and includes the collection of navigator echoes. Spoiler gradients were inserted in the slice-selecting direction to minimise effects from stimulated echoes. Calculations of the b values showed that cross-terms between imaging gradients and diffusion gradients only led to a marginal increase of b values. Pixel-wise calculation of apparent diffusion coefficient (ADC) maps was performed numerically, considering cross-terms between diffusion-encoding and imaging gradients. The sequences investigated used echo train lengths of 16, 8 and 4 echoes and were encoded in either the slice-, frequency- or phase-encoding direction. In order to allow for higher b values a pulse-sequence version using non-motion compensating diffusion-encoding gradients was written. Phantom measurements were performed and the diffusion coefficients of water and acetone were reasonable. Seven healthy volunteers (age 28–50 years) were examined and apparent diffusion coefficient values agreed well with expected values. Diffusion-weighted images, apparent diffusion coefficient maps and images corresponding to the trace of the diffusion tensor of good quality were retrieved in vivo.     

Pulse sequence optimization for T2-weighted MR imaging of the brain

The authors implemented bipolar velocity compensated pulse techniques for T2-weighted MR imaging of the brain. Signal-to-noise (S/N) and image quality was compared for pulse sequences with standard and optimized RF pulses, low and regular bandwidth versions and cardiac triggering. Images from bipolar velocity compensated sequences allowed better visualization of vessels and basilar cisterns and improved image quality relative to standard sequences without velocity compensation. The implementation of optimized RF pulses with bipolar sequences resulted in further improvement in image quality. Single echo sequences consistently had improved image quality and signal-to-noise relative to the second echo of a double echo sequence. Low bandwidth bipolar sequences with extended sampling period had 30% higher S/N, but at the cost of slight loss in edge definition. The highest image quality was obtained with the bipolar, optimized RF, single echo sequence. Using this technique contiguous high quality image slices could be obtained with velocity compensation. The addition of cardiac triggering to bipolar sequences resulted in slight improvement in image quality, but this difference was marginal and probably rarely necessary for MR imaging of the brain.     

New technical developments in magnetic resonance imaging of epilepsy

Within the last several years a number of technical developments have been made in magnetic resonance imaging (MRI) that can potentially impact clinical and research MR imaging applications in epilepsy. These include developments in instrumentation and in pulse sequences. Advances in instrumentation include higher capacity gradient systems and multiple receiver coils as directed to brain imaging. Advances in pulse sequence include use of fast or turbo-spin-echo techniques, variants of echo-planar imaging, and sequences such as fluid-attenuation inversion recovery (FLAIR) targeted to specific applications of brain imaging. The purpose of this paper is to review several of these developments.     

MQ-PGSTE: A new multi-quantum STE-based PGSE NMR sequence

A new multi-quantum stimulated echo based pulsed gradient spin-echo sequence, MQ-PGSTE, has been developed for measuring translational diffusion. The new sequence provides a higher signal-to-noise ratio than the (Hahn spin-echo based) MAXY-D sequence at long diffusion times, and thus potentially affords better diffusion measurements on macromolecule samples. Based on multi-quantum coherence encoding, the MQ-PGSTE sequence needs considerably lower gradient strengths for diffusion characterization compared to standard single quantum pulsed gradient spin-echo sequences. By using the new sequence, the diffusion coefficient of l-[3-¹³C]-alanine was found to be 8.1 ± 0.1 × 10⁻¹⁰ m² s⁻¹, which is in line with the value obtained by the use of the standard stimulated echo based pulsed gradient spin-echo sequence.     

Simulating the effects of time-varying magnetic fields with a realistic simulated scanner

Transient magnetic fields induce changes in magnetic resonance (MR) images ranging from small, visually undetectable effects (caused, for instance, by neuronal currents) to more significant ones, such as those created by the gradient fields and eddy currents. Accurately simulating these effects may assist in correcting or optimising MR imaging for many applications (e.g., diffusion imaging, current density imaging, use of magnetic contrast agents, neuronal current imaging, etc.). Here we have extended an existing MR simulator (POSSUM) with a model for changing magnetic fields at a very high-resolution time-scale. This simulator captures a realistic range of scanner and physiological artifacts by modeling the scanner environment, pulse sequence details and subject properties (e.g., brain geometry and air-tissue boundaries).     

How accurately can the parameters from a model of anisotropic 3He gas diffusion in lung acinar airways be estimated? Bayesian view

In the framework of a recently proposed method for in vivo lung morphometry, acinar lung airways are considered as a set of randomly oriented cylinders covered by alveolar sleeves. Diffusion of (3)He in each airway is anisotropic and can be described by distinct longitudinal and transverse diffusion coefficients. This macroscopically isotropic but microscopically anisotropic model allows estimation of these diffusion coefficients from multi b-value MR experiments despite the airways being too small to be resolved by direct imaging. Herein a Bayesian approach is used for analyzing the uncertainties in the model parameter estimates. The approach allows evaluation of relative errors of the parameter estimates as functions of the "true" values of the parameters, the signal-to-noise ratio, the maximum b-value and the total number of b-values used in the experiment. For a given set of the "true" diffusion parameters, the uncertainty in the estimated diffusion coefficients has a minimum as a function of maximum b-value and total number of data points. Choosing the MR pulse sequence parameters corresponding to this minimum optimizes the diffusion MR experiment and gives the best possible estimates of the diffusion coefficients. The mathematical approach presented can be generalized for models containing arbitrary numbers of estimated parameters.