The Experts below are selected from a list of 294 Experts worldwide ranked by ideXlab platform
Albert Macovski - One of the best experts on this subject based on the ideXlab platform.
-
Short TE phosphorus spectroscopy using a spin‐Echo Pulse
Magnetic resonance in medicine, 1994Co-Authors: O M D Kelvin Lim, John M. Pauly, Peter Webb, Ralph E. Hurd, Albert MacovskiAbstract:In vivo phosphorus spectroscopy requires very short acquisition delays in order to capture the signal from components with short transverse relaxation times (T2). The Echo time typical of standard slice selective spin-Echo Pulses are too long for this application, so hard Pulse, free induction decay (FID) acquisitions have frequently been used instead. With FID, however, there is an interval between the time of coherence and data acquisition (acquisition delay), with resulting baseline distortions. In this paper we describe the design of a new short TE, slice-selective, composite spin-Echo Pulse with Echo times as short as 2.5 ms. With a long TR, fully relaxed, multislice spectra can be collected. This technique will be useful for assessing in vivo, changes in brain phospholipid activity associated with psychiatric and neurological diseases.
-
a reduced power selective adiabatic spin Echo Pulse sequence
Magnetic Resonance in Medicine, 1991Co-Authors: Steven M. Conolly, Dwight G. Nishimura, Gary H Glover, Albert MacovskiAbstract:We introduce a selective adiabatic Pulse sequence suitable for generating selective spin-Echoes for both MR imaging and spectroscopy. The technique is simple; one uses the Echo generated by any pair of identical selective adiabatic inversion Pulses. The nonlinear phase across the slice is compensated perfectly by the second pi Pulse. This compensation is immune to RF inhomogeneity and nonlinearity. For imaging applications, we concentrate on a reduced-power version of the Pulse sequence in which time is traded off variably for RF amplitude in the presence of a time-varying gradient. This technique, known as variable-rate excitation, mildly degrades the off-resonant slice profile when applied to amplitude-modulated Pulses. We present theoretical explanations and experimental results that show that the variable-rate adiabatic Pulses are immune to off-resonant degradation of the magnitude normally encountered in MR imaging.
Srinivas Sridhar - One of the best experts on this subject based on the ideXlab platform.
-
quantitative contrast enhanced mri with superparamagnetic nanoparticles using ultrashort time to Echo Pulse sequences
Magnetic Resonance in Medicine, 2015Co-Authors: Codi Gharagouzloo, Patrick N. Mcmahon, Srinivas SridharAbstract:Purpose: Conventional MRI using contrast agents is semiquantitative because it is inherently sensitive to extravoxular susceptibility artifacts, field inhomogeneity, partial voluming, perivascular effects, and motion/flow artifacts. Herein we demonstrate a quantitative contrast-enhanced MRI technique using ultrashort time-to-Echo Pulse sequences for measuring clinically relevant concentrations of ferumoxytol, a superparamagnetic iron oxide nanoparticle contrast agent with high sensitivity and precision in vitro and in vivo. Methods: The method achieves robust, reproducible results by using rapid signal acquisition at ultrashort time-to-Echo (UTE) to produce positive contrast images with pure T1 weighting and little T2* decay. The spoiled gradient Echo equation is used to transform UTE intensities directly into concentration using experimentally determined relaxivity constants and image acquisition parameters. Results: A multiparametric optimization of acquisition parameters revealed an optimal zone capable of producing highfidelity measurements. Clinically relevant intravascular concentrations of ferumoxytol were measured longitudinally in mice with high sensitivity and precision (� 7.1% error). MRI measurements were independently validated by elemental iron analysis of sequential blood draws. Automated segmentation of ferumoxytol concentration yielded high quality threedimensional images for visualization of perfusion. Conclusions: This ability to longitudinally quantify blood pool CA concentration is unique to quantitative UTE contrastenhanced (QUTE-CE) MRI and makes QUTE-CE MRI competitive with nuclear imaging. Magn Reson Med 000:000–000, 2014. V C 2014 Wiley Periodicals, Inc.
-
Quantitative contrast‐enhanced MRI with superparamagnetic nanoparticles using ultrashort time‐to‐Echo Pulse sequences
Magnetic resonance in medicine, 2014Co-Authors: Codi Gharagouzloo, Patrick N. Mcmahon, Srinivas SridharAbstract:Purpose: Conventional MRI using contrast agents is semiquantitative because it is inherently sensitive to extravoxular susceptibility artifacts, field inhomogeneity, partial voluming, perivascular effects, and motion/flow artifacts. Herein we demonstrate a quantitative contrast-enhanced MRI technique using ultrashort time-to-Echo Pulse sequences for measuring clinically relevant concentrations of ferumoxytol, a superparamagnetic iron oxide nanoparticle contrast agent with high sensitivity and precision in vitro and in vivo. Methods: The method achieves robust, reproducible results by using rapid signal acquisition at ultrashort time-to-Echo (UTE) to produce positive contrast images with pure T1 weighting and little T2* decay. The spoiled gradient Echo equation is used to transform UTE intensities directly into concentration using experimentally determined relaxivity constants and image acquisition parameters. Results: A multiparametric optimization of acquisition parameters revealed an optimal zone capable of producing highfidelity measurements. Clinically relevant intravascular concentrations of ferumoxytol were measured longitudinally in mice with high sensitivity and precision (� 7.1% error). MRI measurements were independently validated by elemental iron analysis of sequential blood draws. Automated segmentation of ferumoxytol concentration yielded high quality threedimensional images for visualization of perfusion. Conclusions: This ability to longitudinally quantify blood pool CA concentration is unique to quantitative UTE contrastenhanced (QUTE-CE) MRI and makes QUTE-CE MRI competitive with nuclear imaging. Magn Reson Med 000:000–000, 2014. V C 2014 Wiley Periodicals, Inc.
Maria I. Altbach - One of the best experts on this subject based on the ideXlab platform.
-
An efficient 3D stack-of-stars turbo spin Echo Pulse sequence for simultaneous T2-weighted imaging and T2 mapping.
Magnetic resonance in medicine, 2019Co-Authors: Mahesh Bharath Keerthivasan, Manojkumar Saranathan, Kevin Johnson, Craig Weinkauf, Diego R. Martin, Ali Bilgin, Maria I. AltbachAbstract:PURPOSE To design a Pulse sequence for efficient 3D T2-weighted imaging and T2 mapping. METHODS A stack-of-stars turbo spin Echo Pulse sequence with variable refocusing flip angles and a flexible pseudorandom view ordering is proposed for simultaneous T2-weighted imaging and T2 mapping. An analytical framework is introduced for the selection of refocusing flip angles to maximize relative tissue contrast while minimizing T2 estimation errors and maintaining low specific absorption rate. Images at different Echo times are generated using a subspace constrained iterative reconstruction algorithm. T2 maps are obtained by modeling the signal evolution using the extended phase graph model. The technique is evaluated using phantoms and demonstrated in vivo for brain, knee, and carotid imaging. RESULTS Numerical simulations demonstrate an improved point spread function with the proposed pseudorandom view ordering compared to golden angle view ordering. Phantom experiments show that T2 values estimated from the stack-of-stars turbo spin Echo Pulse sequence with variable refocusing flip angles have good concordance with spin Echo reference values. In vivo results show the proposed Pulse sequence can generate qualitatively comparable T2-weighted images as conventional Cartesian 3D SPACE in addition to simultaneously generating 3D T2 maps. CONCLUSION The proposed stack-of-stars turbo spin Echo Pulse sequence with pseudorandom view ordering and variable refocusing flip angles allows high resolution isotropic T2 mapping in clinically acceptable scan times. The optimization framework for the selection of refocusing flip angles improves T2 estimation accuracy while generating T2-weighted contrast comparable to conventional Cartesian imaging.
Roger C. Grimm - One of the best experts on this subject based on the ideXlab platform.
-
T2-weighted spin-Echo Pulse sequence with variable repetition and Echo times for reduction of MR image acquisition time.
Radiology, 1991Co-Authors: R K Butts, Stephen J. Riederer, Farhad Farzaneh, John N. Rydberg, Roger C. GrimmAbstract:Use of intraacquisition modification of Pulse-sequence parameters to reduce acquisition time for conventional T2-weighted spin-Echo images was evaluated. With this technique (variable-rate spin-Echo Pulse sequence), the repetition time and Echo time (TR msec/TE msec) were reduced during imaging as a function of the phase-encoding view. To maintain T2-based contrast, TR and TE for the low-spatial-frequency views were left at their prescribed values (eg, 2,000/80). TR and TE for the high-spatial-frequency views were progressively reduced during imaging (eg, to 1,000/20). Acquisition time was reduced by as much as 25%. In one Pulse sequence, the duration of multisection imaging nominally performed at TR 2,000 and with 256 phase-encoding views was reduced from 9 minutes 30 seconds to 6 minutes 30 seconds. In all sequences, edges and small structures were enhanced, and T2 contrast was somewhat decreased in high spatial frequencies. Filtering of the raw data before reconstruction can suppress these effects and ...
Codi Gharagouzloo - One of the best experts on this subject based on the ideXlab platform.
-
quantitative contrast enhanced mri with superparamagnetic nanoparticles using ultrashort time to Echo Pulse sequences
Magnetic Resonance in Medicine, 2015Co-Authors: Codi Gharagouzloo, Patrick N. Mcmahon, Srinivas SridharAbstract:Purpose: Conventional MRI using contrast agents is semiquantitative because it is inherently sensitive to extravoxular susceptibility artifacts, field inhomogeneity, partial voluming, perivascular effects, and motion/flow artifacts. Herein we demonstrate a quantitative contrast-enhanced MRI technique using ultrashort time-to-Echo Pulse sequences for measuring clinically relevant concentrations of ferumoxytol, a superparamagnetic iron oxide nanoparticle contrast agent with high sensitivity and precision in vitro and in vivo. Methods: The method achieves robust, reproducible results by using rapid signal acquisition at ultrashort time-to-Echo (UTE) to produce positive contrast images with pure T1 weighting and little T2* decay. The spoiled gradient Echo equation is used to transform UTE intensities directly into concentration using experimentally determined relaxivity constants and image acquisition parameters. Results: A multiparametric optimization of acquisition parameters revealed an optimal zone capable of producing highfidelity measurements. Clinically relevant intravascular concentrations of ferumoxytol were measured longitudinally in mice with high sensitivity and precision (� 7.1% error). MRI measurements were independently validated by elemental iron analysis of sequential blood draws. Automated segmentation of ferumoxytol concentration yielded high quality threedimensional images for visualization of perfusion. Conclusions: This ability to longitudinally quantify blood pool CA concentration is unique to quantitative UTE contrastenhanced (QUTE-CE) MRI and makes QUTE-CE MRI competitive with nuclear imaging. Magn Reson Med 000:000–000, 2014. V C 2014 Wiley Periodicals, Inc.
-
Quantitative contrast‐enhanced MRI with superparamagnetic nanoparticles using ultrashort time‐to‐Echo Pulse sequences
Magnetic resonance in medicine, 2014Co-Authors: Codi Gharagouzloo, Patrick N. Mcmahon, Srinivas SridharAbstract:Purpose: Conventional MRI using contrast agents is semiquantitative because it is inherently sensitive to extravoxular susceptibility artifacts, field inhomogeneity, partial voluming, perivascular effects, and motion/flow artifacts. Herein we demonstrate a quantitative contrast-enhanced MRI technique using ultrashort time-to-Echo Pulse sequences for measuring clinically relevant concentrations of ferumoxytol, a superparamagnetic iron oxide nanoparticle contrast agent with high sensitivity and precision in vitro and in vivo. Methods: The method achieves robust, reproducible results by using rapid signal acquisition at ultrashort time-to-Echo (UTE) to produce positive contrast images with pure T1 weighting and little T2* decay. The spoiled gradient Echo equation is used to transform UTE intensities directly into concentration using experimentally determined relaxivity constants and image acquisition parameters. Results: A multiparametric optimization of acquisition parameters revealed an optimal zone capable of producing highfidelity measurements. Clinically relevant intravascular concentrations of ferumoxytol were measured longitudinally in mice with high sensitivity and precision (� 7.1% error). MRI measurements were independently validated by elemental iron analysis of sequential blood draws. Automated segmentation of ferumoxytol concentration yielded high quality threedimensional images for visualization of perfusion. Conclusions: This ability to longitudinally quantify blood pool CA concentration is unique to quantitative UTE contrastenhanced (QUTE-CE) MRI and makes QUTE-CE MRI competitive with nuclear imaging. Magn Reson Med 000:000–000, 2014. V C 2014 Wiley Periodicals, Inc.
