The Experts below are selected from a list of 303 Experts worldwide ranked by ideXlab platform

Qin Qin - One of the best experts on this subject based on the ideXlab platform.

  • ensuring both velocity and spatial responses robust to b0 b1 field inhomogeneities for velocity selective arterial spin labeling through dynamic Phase Cycling
    Magnetic Resonance in Medicine, 2021
    Co-Authors: Dapeng Liu, Dan Zhu, Taehoon Shin, Qin Qin
    Abstract:

    PURPOSE To evaluate both velocity and spatial responses of velocity-selective arterial spin labeling (VS-ASL), using velocity-insensitive and velocity-compensated waveforms for control modules, as well as a novel dynamic Phase-Cycling approach, at different B0 / B1+ field inhomogeneities. METHODS In the presence of imperfect refocusing, the mechanism of Phase-Cycling the refocusing pulses through four dynamics was first theoretically analyzed with the conventional velocity-selective saturation (VSS) pulse train. Numerical simulations were then deployed to compare the performance of the Fourier-transform based velocity-selective inversion (FT-VSI) with these three different schemes in terms of both velocity and spatial responses under various B0 / B1+ conditions. Phantom and human brain scans were performed to evaluate the three methods at B1+ scales of 0.8, 1.0, and 1.2. RESULTS The simulations of FT-VSI showed that, under nonuniform B0 / B1+ conditions, the scheme with velocity-insensitive control was susceptible to DC bias of the static spins as systematic error, while the scheme with velocity-compensated control had deteriorated velocity-selective labeling profiles and, thus, reduced labeling efficiency. Through numerical simulation, phantom scans, and brain perfusion measurements, the dynamic Phase-Cycling method demonstrated considerable improvements over these issues. CONCLUSION The proposed dynamic Phase-Cycling approach was demonstrated for the velocity-selective label and control modules with both velocity and spatial responses robust to a wide range of B0 and B1+ field inhomogeneities.

Sean C L Deoni - One of the best experts on this subject based on the ideXlab platform.

  • correction of main and transmit magnetic field b0 and b1 inhomogeneity effects in multicomponent driven equilibrium single pulse observation of t1 and t2
    Magnetic Resonance in Medicine, 2011
    Co-Authors: Sean C L Deoni
    Abstract:

    Multicomponent analysis of longitudinal and transverse (T1 and T2) relaxation data provides information reflecting discrete subvoxel anatomical microstructure. Interrogation of multiecho T2 data in human brain parenchyma, e.g., has consistently revealed the presence of slow and fast relaxing species with T2 80 msec, respectively (1–4). Through investigations of the relative magnitudes of these components in grey and white matter, their variation in disease, and comparison with histological analysis (1–7), these species are broadly attributed to the water trapped within the myelin bilayer and to the less restricted intracellular and extracellular water, respectively. Conventionally, investigation of these microanatomical water pools is performed using the T2 spin-echo-based approach of Whittal et al. (4). This technique, uses a Poon–Henkelman multiple spin-echo imaging sequence (8) to acquire data over a range of echo times, provides discrete or continuous distribution estimates of each water compartment’s T2 relaxation time and relative volume fraction. Quantitative estimates of the fast relaxing component’s volume fraction corresponds strongly with histologically derived myelin content estimates (5–7), thus providing a noninvasive means of assessing this biologically important parameter. Multicomponent-driven equilibrium single-pulse observation of T1 and T2 (mcDESPOT) (9) is an extension of the DESPOT1 and DESPOT2 T1 and T2 mapping techniques (10,11) and is a clinically feasible alternative to SE-based multicomponent relaxation analysis approaches. The method combines spoiled and fully balanced steady-state imaging data (SPGR and bSSFP, respectively), acquired over a range of flip angles (α) and with constant repetition time (TR), to derive the free-water T1 and T2 (T1,F, T2,F); myelin-associated water T1 and T2 (T1,M, T2,M); relative myelin and free-water volume fractions (fM and fF); and the myelin and free-water proton residence times (τM and τF). Although these rapid and signal-to-noise efficient (SNR per unit scan time) three-dimensional (3D) imaging techniques enable whole-brain acquisitions with rapid scan times, SPGR and bSSFP are sensitive to main and transmit magnetic field (B0 and B1, respectively) inhomogeneity. These effects alter the measured SPGR and bSSFP signals from their theoretically predicted values (10, 12) and lead to substantive errors in the derived multicomponent parameters. Flip angle (B1) errors occur from tissue dielectric effects. In addition, if a volume selective pulse is applied alongside a slab-selection gradient, the gaussian or Sinc envelopes of typical radiofrequency (RF) excitation pulses will yield a nonuniform flip across the volume in the slab-select direction. The severity of this profile will depend on the specific attributes of the RF pulse [including duration and time-bandwidth (BW) product]. Main magnetic field (B0) inhomogeneity, and off-resonance effects (Phase precession of the transverse magnetization), observed in regions with steep susceptibility-induced gradients, such as near the sinuses, brainstem, or inner ear, and presented as areas of signal void in the bSSFP data (13). The affect of flip angle and off-resonance errors on the derived myelin fraction estimates is shown in Fig. 1 and clearly limits the research and diagnostic potential of the technique. FIG. 1 Example illustrations of the affects of B0 and B1 inhomogeneity on mcDESPOT derived myelin volume fraction maps. In this work, the aim was to mitigate the affect of B0 and B1 inhomogeneity through the incorporation of the DESPOT1-HIFI B1 mapping/flip angle calibration technique (14); and the inclusion of off-resonance modeling in the bSSFP signal coupled with the acquisition of bSSFP data with two or more RF Phase-Cycling patterns (13). The theoretical performance of these calibration techniques was first demonstrated through numerical simulations, which revealed substantial improvement in parameter estimate accuracy (less than 5% absolute error) even with substantial B0 and B1 inhomogeneity. The utility of the method was then established in vivo, demonstrating substantial artifact reduction in the derived multiparameter maps.

  • transverse relaxation time t2 mapping in the brain with off resonance correction using Phase cycled steady state free precession imaging
    Journal of Magnetic Resonance Imaging, 2009
    Co-Authors: Sean C L Deoni
    Abstract:

    Purpose To investigate a new approach for more completely accounting for off-resonance affects in the DESPOT2 (driven equilibrium single pulse observation of T2) mapping technique. Materials and Methods The DESPOT2 method derives T2 information from fully balanced steady-state free precession (bSSFP) images acquired over multiple flip angles. Off-resonance affects, which present as bands of altered signal intensity throughout the bSSFP images, results in erroneous T2 values in the corresponding calculated maps. Radiofrequency (RF) Phase-Cycling, in which the Phase of the RF pulse is incremented along the pulse train, offers a potential method for eliminating these artifacts. In this work we present a general method, referred to as DESPOT2, with full modeling (DESPOT2-FM), for deriving T2, as well as off-resonance frequency, from dual flip angle bSSFP data acquired with two RF Phase increments. Results The method is demonstrated in vivo through the acquisition of whole-brain, 1 mm3 isotropic T2 maps at 3T and shown to provide near artifact-free maps, even in areas with steep susceptibility-induced gradients. Conclusion DESPOT2-FM offers an efficient method for acquiring high spatial resolution, whole-brain T2 maps at 3T with high precision and free of artifact. J. Magn. Reson. Imaging 2009;30:411–417. © 2009 Wiley-Liss, Inc.

Dapeng Liu - One of the best experts on this subject based on the ideXlab platform.

  • ensuring both velocity and spatial responses robust to b0 b1 field inhomogeneities for velocity selective arterial spin labeling through dynamic Phase Cycling
    Magnetic Resonance in Medicine, 2021
    Co-Authors: Dapeng Liu, Dan Zhu, Taehoon Shin, Qin Qin
    Abstract:

    PURPOSE To evaluate both velocity and spatial responses of velocity-selective arterial spin labeling (VS-ASL), using velocity-insensitive and velocity-compensated waveforms for control modules, as well as a novel dynamic Phase-Cycling approach, at different B0 / B1+ field inhomogeneities. METHODS In the presence of imperfect refocusing, the mechanism of Phase-Cycling the refocusing pulses through four dynamics was first theoretically analyzed with the conventional velocity-selective saturation (VSS) pulse train. Numerical simulations were then deployed to compare the performance of the Fourier-transform based velocity-selective inversion (FT-VSI) with these three different schemes in terms of both velocity and spatial responses under various B0 / B1+ conditions. Phantom and human brain scans were performed to evaluate the three methods at B1+ scales of 0.8, 1.0, and 1.2. RESULTS The simulations of FT-VSI showed that, under nonuniform B0 / B1+ conditions, the scheme with velocity-insensitive control was susceptible to DC bias of the static spins as systematic error, while the scheme with velocity-compensated control had deteriorated velocity-selective labeling profiles and, thus, reduced labeling efficiency. Through numerical simulation, phantom scans, and brain perfusion measurements, the dynamic Phase-Cycling method demonstrated considerable improvements over these issues. CONCLUSION The proposed dynamic Phase-Cycling approach was demonstrated for the velocity-selective label and control modules with both velocity and spatial responses robust to a wide range of B0 and B1+ field inhomogeneities.

Matilde Inglese - One of the best experts on this subject based on the ideXlab platform.

  • global and regional brain concentration of intra and extra cellular sodium in ms a 7 tesla mri study p6 132
    Neurology, 2014
    Co-Authors: Lazar Fleysher, Niels Oesingmann, Roxana Teodorescu, Maria Petracca, Laura E Jonkman, Joseph Herbert, Matilde Inglese
    Abstract:

    OBJECTIVE: 1) to measure intracellular and extracellular sodium concentration (ISC and ESC) in MS patients using sodium MRI at 7T; 2) to investigate the associations between total sodium concentration (TSC), ISC and ESC and measures of lesion and brain volume; 3) to assess the clinical significance of ISC and ESC. BACKGROUND: Measurement of TSC obtained using single quantum sodium MRI is useful in the assessment of MS patients. However, it does not allow discrimination between ESC and ISC that might reflect delayed axonal damage DESIGN/METHODS: Twenty-four MS patients (14F; mean age: 43.5±12.2 yrs; median EDSS: 2.0) and 24 healthy CTRLs (14F; mean age: 41.6±11.8 yrs) underwent sodium MRI at 7T and proton MRI at 3T. The 7T MRI protocol included a modified GRE sequence with a new 12-step Phase-Cycling TQF scheme. The 3T MRI protocol included DE-TSE and 3D T1W-MPRAGE.TSC (mM), ISC (mmol/L) and ESC maps were calculated and tissue concentration were measured with a histogram analysis over the entire gray and white matter (GM, WM) and with a regional voxel-based approach using SPM8. Between groups comparison was performed with an ANCOVA test controlling for age, gender and intra-cranial volume RESULTS: Compared to CTRLs MS patients showed higher global GM, WM TSC and WM ISC, GM and WM ESC ( p<0.05). GM ISC was higher in patients than controls in the following regions: right (R) and left (L) thalamus, R superior temporal gyrus, forceps major and minor, L and R anterior thalamic radiation (ATR), R inferior longitudinal fasciculs (ILF) and R uncinate fasciculus (UF) while ESC resulted higher in all the above mentioned tracts. TSC in R UF correlated with brain volume (p<0.05); ISC in the L thalamus correlated with the T2LV (p<0.05) and ESC in the ATR and ILF was associated with EDSS (p<0.05). CONCLUSIONS: Brain ISC and ESC abnormalities are widespread in MS patients; while both metrics are moderately associated with lesion volume, only ESC correlates with EDSS Disclosure: Dr. Fleysher has nothing to disclose. Dr. Teodorescu has nothing to disclose. Dr. Petracca has nothing to disclose. Dr. Jonkman has nothing to disclose. Dr. Oesingmann has nothing to disclose. Dr. Herbert has received personal compensation for activities with Biogen Idec, Teva Neuroscience, Serono Inc., and Bayer Pharmaceuticals Inc. Dr. Herbert has received research support from Teva Neuroscience, Novartis, Biogen Idec, BioMS, and INC Research. Dr. Inglese has received research support from Novartis Pharmaceuticals.

  • b inhomogeneity insensitive triple quantum filtered sodium imaging using a 12 step Phase Cycling scheme
    NMR in Biomedicine, 2010
    Co-Authors: Lazar Fleysher, Niels Oesingmann, Matilde Inglese
    Abstract:

    Triple-quantum-filtered (TQF) sodium MRI can be used to separate sodium NMR signals from different physiological compartments. Although three-pulse triple-quantum filtering has been demonstrated to be better suited for in vivo imaging, the absence of the refocusing pulse in the filter increases its sensitivity to magnetic field inhomogeneities. Therefore, several TQF cycles have been developed previously to correct image distortions caused by B0 inhomogeneities. In this paper, we present a new 12-step Phase-Cycling TQF scheme based on three radiofrequency pulses which allows the compensation of B0 variations both with and without ancillary B0 map information. The method offers 40% higher signal-to-noise-ratio efficiency compared with the previously developed B0-correcting Phase-Cycling schemes. Copyright © 2010 John Wiley & Sons, Ltd.

A.j. Shaka - One of the best experts on this subject based on the ideXlab platform.

  • pure absorption electron spin echo envelope modulation spectra by using the filter diagonalization method for harmonic inversion
    Journal of Magnetic Resonance, 1999
    Co-Authors: Gunnar Jeschke, Vladimir A Mandelshtam, A.j. Shaka
    Abstract:

    Abstract Harmonic inversion of electron spin echo envelope (ESEEM) time-domain signals by filter diagonalization is investigated as an alternative to Fourier transformation. It is demonstrated that this method features enhanced resolution compared to Fourier-transform magnitude spectra, since it can eliminate dispersive contributions to the line shape, even if no linear Phase correction is possible. Furthermore, instrumental artifacts can be easily removed from the spectra if they are narrow either in time or frequency domain. This applies to echo crossings that are only incompletely eliminated by Phase Cycling and to spurious spectrometer frequencies, respectively. The method is computationally efficient and numerically stable and does not require extensive parameter adjustments or advance knowledge of the number of spectral lines. Experiments on γ-irradiated methyl-α- d -glucopyranoside show that more information can be obtained from typical ESEEM time-domain signals by filter-diagonalization than by Fourier transformation.

  • excitation sculpting in high resolution nuclear magnetic resonance spectroscopy application to selective noe experiments
    Journal of the American Chemical Society, 1995
    Co-Authors: Katherine Stott, James Keeler, Jonathan Stonehouse, Tsanglin Hwang, A.j. Shaka
    Abstract:

    Selective pulses are key elements in high-resolution NMR experiments, so great effort has been put into designing pulse shapes with desirable properties.'-' In this communication we describe a selective excitation technique which, judged by the usual criteria, outdistances existing methods. Our method gives constant Phase and amplitude excitation over an easily adjustable bandwidth, can achieve given selectivity in a shorter time than existing methods, has no out-of-band sidelobes, and in exciting a multiplet, refocuses the evolution of scalar coupling. The method is tolerant of radio-frequency (fl field inhomogeneity, and altering the net flip angle is easy. The use of pulsed field gradients (PFGS)~,~ results in these crucial properties being achieved in a single scan, without difference spectroscopy or Phase Cycling: magnetization from outside the desired bandwidth is destroyed, thus simplifying the subsequent manipulation of the excited magnetization. While PFGs have been used to tailor spectral response using single spin echoes, for example with the WATERGATE sequence,I0 and while selective 180" pulses have been used for selective excitation in conjunction with difference spectroscopy," the approach described here is more general. The heart of the method is the double PFG spin echo (DPFGSE) sequence (-GI -S-GI-G~-S-G~-), in which S is any sequence of 13'pulses of any kind and the Gi are PFGs. S induces the unitary transformation Us = R-,(/f?) R-y(t9) R,(a) R,(@ R,@) where, e.g., R,(y) represents a rotation through an angle y about an axis E , and a, @, and 8 are arbitrary angles. It can be shownI2 that applying a DPFGSE to isolated spins transforms a magnetization vector m = (m,,m,,m,) into a vector M with components

  • excitation sculpting in high resolution nuclear magnetic resonance spectroscopy application to selective noe experiments
    Journal of the American Chemical Society, 1995
    Co-Authors: Katherine Stott, James Keeler, Jonathan Stonehouse, Tsanglin Hwang, A.j. Shaka
    Abstract:

    Selective pulses are key elements in high-resolution NMR experiments, so great effort has been put into designing pulse shapes with desirable properties.'-' In this communication we describe a selective excitation technique which, judged by the usual criteria, outdistances existing methods. Our method gives constant Phase and amplitude excitation over an easily adjustable bandwidth, can achieve given selectivity in a shorter time than existing methods, has no out-of-band sidelobes, and in exciting a multiplet, refocuses the evolution of scalar coupling. The method is tolerant of radio-frequency (fl field inhomogeneity, and altering the net flip angle is easy. The use of pulsed field gradients (PFGS)~,~ results in these crucial properties being achieved in a single scan, without difference spectroscopy or Phase Cycling: magnetization from outside the desired bandwidth is destroyed, thus simplifying the subsequent manipulation of the excited magnetization. While PFGs have been used to tailor spectral response using single spin echoes, for example with the WATERGATE sequence,I0 and while selective 180" pulses have been used for selective excitation in conjunction with difference spectroscopy," the approach described here is more general. The heart of the method is the double PFG spin echo (DPFGSE) sequence (-GI -S-GI-G~-S-G~-), in which S is any sequence of 13'pulses of any kind and the Gi are PFGs. S induces the unitary transformation Us = R-,(/f?) R-y(t9) R,(a) R,(@ R,@) where, e.g., R,(y) represents a rotation through an angle y about an axis E , and a, @, and 8 are arbitrary angles. It can be shownI2 that applying a DPFGSE to isolated spins transforms a magnetization vector m = (m,,m,,m,) into a vector M with components