The Experts below are selected from a list of 31251 Experts worldwide ranked by ideXlab platform
Ruth Dixon - One of the best experts on this subject based on the ideXlab platform.
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an assessment of spin echo rotating frame imaging for spatially Localized determination of short t2 relaxation times in vivo
Magnetic Resonance in Medicine, 1993Co-Authors: Ruth Dixon, Peter StylesAbstract:The rotating frame method of Localized Spectroscopy can be augmented by the incluslon of a refocusing pulse to enable the measurement of T 2 relaxation times. This technique is particularly appropriate for determining short relaxation parameters due to the absence of time consuming switched B 0 field gradiente. We have evaluated the accuracy of this protocol by meaeuring Localized T 2 s in the range of 1 to 20 ms. Preliminary data obtained from muscle and liver of normal and iron overloaded human subjects are also presented
Ivan Tkac - One of the best experts on this subject based on the ideXlab platform.
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short echo single shot full intensity 1h mrs for neurochemical profiling at 4t validation in the cerebellum and brainstem
Magnetic Resonance in Medicine, 2011Co-Authors: Ivan TkacAbstract:Short echo time (TE) 1H MR Spectroscopy techniques are critical for extending the neurochemical information beyond NAA, creatine and choline as they facilitate the detection of brain metabolites with J-coupled spin systems, such as glutamate and glutamine. Short TE minimizes signal loss due to J-evolution and T2 relaxation, which is especially detrimental at high fields in the human brain where T2 values are relatively short (1,2). Neurochemical profiles have so far been mostly quantified using localization with the ultra-short TE stimulated-echo acquisition mode (STEAM) sequence (3–5), because T2 relaxation and J-evolution are negligible at ultra-short TE making metabolite quantification straightforward. However, the STEAM sequence utilizes only half of the available Mz magnetization, which limits the achievable spatial resolution of MRS that permits reliable metabolite quantification. Using the STEAM sequence to acquire spectra from small volumes in deep brain regions is even more difficult because the intrinsic sensitivity of volume RF coils is substantially lower than surface coils. While reasonably short TEs can be achieved with point resolved Spectroscopy (PRESS) sequences that also utilize the full available Mz magnetization (6,7), the limited bandwidth of 180° refocusing pulses in these sequences may result in substantial chemical shift displacement errors at high fields. Recently, a new localization pulse sequence termed SPECIAL (spin echo full intensity acquired localization) was introduced (8), which enables full signal intensity acquisition at ultra-short TEs. The feasibility of obtaining neurochemical profiles with the SPECIAL sequence was successfully demonstrated in the rat (9) and human (10) brain. However, localization with this hybrid ISIS/spin echo sequence relies on an add-subtract scheme. Single-shot methods simplify frequency and phase correction of individual FIDs (11) and are therefore desirable for Localized Spectroscopy, especially in clinical populations where motion artifacts are frequently encountered (12). The localization by adiabatic selective refocusing (LASER) sequence (13) is a single-shot technique and also enables localization with full signal intensity, but requires relatively longer TEs because of 3 pairs of adiabatic 180° pulses. The TE of the LASER sequence can be shortened by replacing one of the 180° pairs by a slice selective excitation pulse in the so-called semi-LASER sequence (14), which enables TEs as short as 30 ms with a surface coil and 50 ms with a volume coil at 7T (15). The LASER sequence has the advantage that apparent T2 relaxation times of metabolites are longer than those measured with conventional Hahn spin echo sequences (1), resulting in less signal attenuation at longer TEs. In addition, J-evolution is partially suppressed in LASER due to the series of 180° pulses, also favoring signal retention. Further shortening of the TE of semi-LASER is desirable, especially for volume RF coils as the limited B1(max) of these coils require longer RF pulses. In addition, neurochemical profiles obtained at the longer TEs of semi-LASER relative to STEAM need to be validated in multiple brain regions such that the sequence can be utilized for neurochemical profiling in clinical populations. Specifically, the acceptability of approximations, such as neglecting a correction for T2 relaxation, needs to be investigated for absolute metabolite quantification. The aims of this study were 1) to design and optimize a single-shot, semi-adiabatic localization method with full signal intensity, short TE and minimal chemical shift displacement error and 2) to validate neurochemical profiling using this new sequence in multiple, clinically relevant brain regions in humans. To achieve these goals, we modified the semi-LASER sequence to minimize TE and then tested its performance at 4T with a surface and a volume RF coil. To validate neurochemical profiles obtained with the newly developed semi-LASER sequence, we compared neurochemical profiles quantified from semi-LASER and STEAM spectra acquired from the cerebellum and brainstem, brain regions affected in various movement disorders (16).
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short echo single shot full intensity proton magnetic resonance Spectroscopy for neurochemical profiling at 4 t validation in the cerebellum and brainstem
Magnetic Resonance in Medicine, 2011Co-Authors: Ivan TkacAbstract:Short echo time (TE) 1H MR Spectroscopy techniques are critical for extending the neurochemical information beyond NAA, creatine and choline as they facilitate the detection of brain metabolites with J-coupled spin systems, such as glutamate and glutamine. Short TE minimizes signal loss due to J-evolution and T2 relaxation, which is especially detrimental at high fields in the human brain where T2 values are relatively short (1,2). Neurochemical profiles have so far been mostly quantified using localization with the ultra-short TE stimulated-echo acquisition mode (STEAM) sequence (3–5), because T2 relaxation and J-evolution are negligible at ultra-short TE making metabolite quantification straightforward. However, the STEAM sequence utilizes only half of the available Mz magnetization, which limits the achievable spatial resolution of MRS that permits reliable metabolite quantification. Using the STEAM sequence to acquire spectra from small volumes in deep brain regions is even more difficult because the intrinsic sensitivity of volume RF coils is substantially lower than surface coils. While reasonably short TEs can be achieved with point resolved Spectroscopy (PRESS) sequences that also utilize the full available Mz magnetization (6,7), the limited bandwidth of 180° refocusing pulses in these sequences may result in substantial chemical shift displacement errors at high fields. Recently, a new localization pulse sequence termed SPECIAL (spin echo full intensity acquired localization) was introduced (8), which enables full signal intensity acquisition at ultra-short TEs. The feasibility of obtaining neurochemical profiles with the SPECIAL sequence was successfully demonstrated in the rat (9) and human (10) brain. However, localization with this hybrid ISIS/spin echo sequence relies on an add-subtract scheme. Single-shot methods simplify frequency and phase correction of individual FIDs (11) and are therefore desirable for Localized Spectroscopy, especially in clinical populations where motion artifacts are frequently encountered (12). The localization by adiabatic selective refocusing (LASER) sequence (13) is a single-shot technique and also enables localization with full signal intensity, but requires relatively longer TEs because of 3 pairs of adiabatic 180° pulses. The TE of the LASER sequence can be shortened by replacing one of the 180° pairs by a slice selective excitation pulse in the so-called semi-LASER sequence (14), which enables TEs as short as 30 ms with a surface coil and 50 ms with a volume coil at 7T (15). The LASER sequence has the advantage that apparent T2 relaxation times of metabolites are longer than those measured with conventional Hahn spin echo sequences (1), resulting in less signal attenuation at longer TEs. In addition, J-evolution is partially suppressed in LASER due to the series of 180° pulses, also favoring signal retention. Further shortening of the TE of semi-LASER is desirable, especially for volume RF coils as the limited B1(max) of these coils require longer RF pulses. In addition, neurochemical profiles obtained at the longer TEs of semi-LASER relative to STEAM need to be validated in multiple brain regions such that the sequence can be utilized for neurochemical profiling in clinical populations. Specifically, the acceptability of approximations, such as neglecting a correction for T2 relaxation, needs to be investigated for absolute metabolite quantification. The aims of this study were 1) to design and optimize a single-shot, semi-adiabatic localization method with full signal intensity, short TE and minimal chemical shift displacement error and 2) to validate neurochemical profiling using this new sequence in multiple, clinically relevant brain regions in humans. To achieve these goals, we modified the semi-LASER sequence to minimize TE and then tested its performance at 4T with a surface and a volume RF coil. To validate neurochemical profiles obtained with the newly developed semi-LASER sequence, we compared neurochemical profiles quantified from semi-LASER and STEAM spectra acquired from the cerebellum and brainstem, brain regions affected in various movement disorders (16).
Michael Garwood - One of the best experts on this subject based on the ideXlab platform.
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Single-shot, B1-insensitive slice selection with a gradient-modulated adiabatic pulse, BISS-8.
Magnetic resonance in medicine, 1996Co-Authors: Robin A. De Graaf, Michael Garwood, Klaas NicolayAbstract:An adiabatic pulse has been developed to accomplish uniform slice-selective excitation with a spatially inhomogeneous B1. This new pulse can generate a uniform, arbitrary flip angle that is determined by four adjustable phase shifts in the pulse. Self-refocused slice selection is achieved by modulating a Bo gradient in concert with the pulse frequency (or phase) modulation. B1-compensated, self-refocused slice selection is demonstrated in computer simulations and phantom experiments using a surface transmitter/receiver coil. This adiabatic pulse can provide optimal performance in multislice MRI and Localized Spectroscopy when transmitting with an inhomogeneous B1.
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A New Localization Method Using an Adiabatic Pulse, BIR-4
Journal of magnetic resonance. Series B, 1995Co-Authors: R.a. Degraaf, Michael Garwood, Hellmut Merkle, Y. Luo, Melissa TerpstraAbstract:Abstract A new method is described for accomplishing Localized Spectroscopy with an adiabatic pulse, BIR-4. The method has advantages similar to previously described combinations of outer-volume suppression (OVS) and ISIS, with the additional advantages that localization is achieved with only three radiofrequency pulses and the localization remains accurate even in the presence of intense signals with short relaxation times. This new localization pulse sequence is referred to as integrated OVS-ISIS. Computer simulations, experimental images of the Localized volumes, and in vivo 1 H Spectroscopy measurements demonstrate the high degree of localization achievable with integrated OVS-ISIS.
Tad J Wieczorek - One of the best experts on this subject based on the ideXlab platform.
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relaxivity corrected response modulated excitation rme a t2 corrected technique achieving specified magnetization patterns from an rf pulse and a time varying magnetic field
Magnetic Resonance in Medicine, 1994Co-Authors: M Justin E D Pearlman, Tad J WieczorekAbstract:We have reworked the theory of RF excitation to enable correction for relaxivity while designing response-modulated excitation (RME) to achieve specified magnetization targets. This results in a significant improvement in the ability to achieve a specified target magnetization, especially if excitation time is long or T2 is short. The methods presented may also be used to improve the quality of spatial-spectral pulses as well as Localized Spectroscopy, real-time imaging, real-time Localized velocity, and noninvasive pressure measurement.
Peter Styles - One of the best experts on this subject based on the ideXlab platform.
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an assessment of spin echo rotating frame imaging for spatially Localized determination of short t2 relaxation times in vivo
Magnetic Resonance in Medicine, 1993Co-Authors: Ruth Dixon, Peter StylesAbstract:The rotating frame method of Localized Spectroscopy can be augmented by the incluslon of a refocusing pulse to enable the measurement of T 2 relaxation times. This technique is particularly appropriate for determining short relaxation parameters due to the absence of time consuming switched B 0 field gradiente. We have evaluated the accuracy of this protocol by meaeuring Localized T 2 s in the range of 1 to 20 ms. Preliminary data obtained from muscle and liver of normal and iron overloaded human subjects are also presented
