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Yuval Zur - One of the best experts on this subject based on the ideXlab platform.
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is the sech tanh Adiabatic Pulse really Adiabatic
Journal of Magnetic Resonance, 1998Co-Authors: Daniel Rosenfeld, Yuval ZurAbstract:Abstract Adiabatic Pulses are most conveniently studied in the frequency frame which is a frame of reference rotating at the instantaneous frequency of the Pulse. In this frame the Adiabatic condition ‖γBeff‖ ⪢ |θ≳| sets an upper limit on the sweep rate θ≳ of the Beffvector. This, in turn, places a lower bound on the Pulse duration. Adiabatic behavior is studied at the threshold duration and two Pulses are examined: (i) a Pulse with a constant sweep rate ( cap Pulse) and (ii) a conventional sech/tanh Adiabatic Pulse. It is shown that the sech/tanh Pulse performs robust magnetization inversion although it seems to violate the Adiabatic condition. This puzzling phenomenon is solved by switching into a second-order rotating frame of reference (SORF) where it is shown that the Adiabatic condition is fulfilled. This frame coincides with the frequency frame at the beginning of the Pulse. Assuming an RF field along thex-axis of the frequency frame, the SORF then rotates about the commony-axis during the Pulse with thez-axis of the new frame aligned with the Beffvector. It is shown that Adiabatic motion may be performed in the SORF, in which the sweep rate is increased indefinitely; the Adiabatic condition is violated by this motion in the frequency frame but is fulfilled in the SORF. The lower bound on the sweep rate in the frequency frame is thereby lifted.
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Is the sech/tanh Adiabatic Pulse Really Adiabatic?
Journal of Magnetic Resonance, 1998Co-Authors: Daniel Rosenfeld, Yuval ZurAbstract:Abstract Adiabatic Pulses are most conveniently studied in the frequency frame which is a frame of reference rotating at the instantaneous frequency of the Pulse. In this frame the Adiabatic condition ‖γBeff‖ ⪢ |θ≳| sets an upper limit on the sweep rate θ≳ of the Beffvector. This, in turn, places a lower bound on the Pulse duration. Adiabatic behavior is studied at the threshold duration and two Pulses are examined: (i) a Pulse with a constant sweep rate ( cap Pulse) and (ii) a conventional sech/tanh Adiabatic Pulse. It is shown that the sech/tanh Pulse performs robust magnetization inversion although it seems to violate the Adiabatic condition. This puzzling phenomenon is solved by switching into a second-order rotating frame of reference (SORF) where it is shown that the Adiabatic condition is fulfilled. This frame coincides with the frequency frame at the beginning of the Pulse. Assuming an RF field along thex-axis of the frequency frame, the SORF then rotates about the commony-axis during the Pulse with thez-axis of the new frame aligned with the Beffvector. It is shown that Adiabatic motion may be performed in the SORF, in which the sweep rate is increased indefinitely; the Adiabatic condition is violated by this motion in the frequency frame but is fulfilled in the SORF. The lower bound on the sweep rate in the frequency frame is thereby lifted.
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Design of Selective Adiabatic Inversion Pulses Using the Adiabatic Condition
Journal of Magnetic Resonance, 1997Co-Authors: Daniel Rosenfeld, Shimon L. Panfil, Yuval ZurAbstract:Abstract Adiabatic RF Pulses play an important role in spin inversion due to their robust behavior in the presence of inhomogeneous RF fields. These Pulses are characterized by the trajectory swept by the tip of the B eff vector and the rate of motion along it. In this paper, we describe a method by which optimized modulation functions can be constructed to render insensitivity to B 1 inhomogeneity over a predetermined B 1 range and over a wide band of frequencies. This is accomplished by requiring that the optimized Pulse fulfill the Adiabatic condition over this range of B 1 inhomogeneity and over the desired frequency band for the complete duration of the Pulse. A trajectory similar to the well-known sech/tanh Adiabatic Pulse, i.e., a half-ellipse, is used. The optimization process improves the slice profile by optimizing the rate of motion along this trajectory. The optimized Pulse can be tailored to the specific design requirements; in particular, the transition sharpness may be traded off against the inverted bandwidth. Two design examples, including experimental results, demonstrate the superiority of the optimized Pulses over the conventional sech/tanh Pulse: in the first example, a large frequency band is to be inverted using a weak RF amplitude in a short time. In the second example, a Pulse with a very sharp transition is required.
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Design of Adiabatic Pulses for fat‐suppression using analytic solutions of the bloch equation
Magnetic resonance in medicine, 1997Co-Authors: Daniel Rosenfeld, Shimon L. Panfil, Yuval ZurAbstract: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.
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A new Adiabatic inversion Pulse
Magnetic resonance in medicine, 1996Co-Authors: Daniel Rosenfeld, Yuval ZurAbstract:Adiabatic Pulses play an important role in magnetization inversion in the presence of RF field inhomogeneity. In this work the authors present an efficient Adiabatic inversion Pulse that is able to selectively invert magnetization over a large frequency bandwidth in a short time. The Pulse is constructed in two steps: (i) the optimal trajectory is determined and (ii) the optimal rate of motion along that trajectory is determined. The resulting Pulse enables separately controlling and trading off the Pulse duration against the transition width. The superiority of this Pulse over the well known sech/tanh Adiabatic Pulse is demonstrated in a scenario where a large bandwidth should be inverted at a short time using limited B1 amplitude.
Daniel Rosenfeld - One of the best experts on this subject based on the ideXlab platform.
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is the sech tanh Adiabatic Pulse really Adiabatic
Journal of Magnetic Resonance, 1998Co-Authors: Daniel Rosenfeld, Yuval ZurAbstract:Abstract Adiabatic Pulses are most conveniently studied in the frequency frame which is a frame of reference rotating at the instantaneous frequency of the Pulse. In this frame the Adiabatic condition ‖γBeff‖ ⪢ |θ≳| sets an upper limit on the sweep rate θ≳ of the Beffvector. This, in turn, places a lower bound on the Pulse duration. Adiabatic behavior is studied at the threshold duration and two Pulses are examined: (i) a Pulse with a constant sweep rate ( cap Pulse) and (ii) a conventional sech/tanh Adiabatic Pulse. It is shown that the sech/tanh Pulse performs robust magnetization inversion although it seems to violate the Adiabatic condition. This puzzling phenomenon is solved by switching into a second-order rotating frame of reference (SORF) where it is shown that the Adiabatic condition is fulfilled. This frame coincides with the frequency frame at the beginning of the Pulse. Assuming an RF field along thex-axis of the frequency frame, the SORF then rotates about the commony-axis during the Pulse with thez-axis of the new frame aligned with the Beffvector. It is shown that Adiabatic motion may be performed in the SORF, in which the sweep rate is increased indefinitely; the Adiabatic condition is violated by this motion in the frequency frame but is fulfilled in the SORF. The lower bound on the sweep rate in the frequency frame is thereby lifted.
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Is the sech/tanh Adiabatic Pulse Really Adiabatic?
Journal of Magnetic Resonance, 1998Co-Authors: Daniel Rosenfeld, Yuval ZurAbstract:Abstract Adiabatic Pulses are most conveniently studied in the frequency frame which is a frame of reference rotating at the instantaneous frequency of the Pulse. In this frame the Adiabatic condition ‖γBeff‖ ⪢ |θ≳| sets an upper limit on the sweep rate θ≳ of the Beffvector. This, in turn, places a lower bound on the Pulse duration. Adiabatic behavior is studied at the threshold duration and two Pulses are examined: (i) a Pulse with a constant sweep rate ( cap Pulse) and (ii) a conventional sech/tanh Adiabatic Pulse. It is shown that the sech/tanh Pulse performs robust magnetization inversion although it seems to violate the Adiabatic condition. This puzzling phenomenon is solved by switching into a second-order rotating frame of reference (SORF) where it is shown that the Adiabatic condition is fulfilled. This frame coincides with the frequency frame at the beginning of the Pulse. Assuming an RF field along thex-axis of the frequency frame, the SORF then rotates about the commony-axis during the Pulse with thez-axis of the new frame aligned with the Beffvector. It is shown that Adiabatic motion may be performed in the SORF, in which the sweep rate is increased indefinitely; the Adiabatic condition is violated by this motion in the frequency frame but is fulfilled in the SORF. The lower bound on the sweep rate in the frequency frame is thereby lifted.
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Design of Selective Adiabatic Inversion Pulses Using the Adiabatic Condition
Journal of Magnetic Resonance, 1997Co-Authors: Daniel Rosenfeld, Shimon L. Panfil, Yuval ZurAbstract:Abstract Adiabatic RF Pulses play an important role in spin inversion due to their robust behavior in the presence of inhomogeneous RF fields. These Pulses are characterized by the trajectory swept by the tip of the B eff vector and the rate of motion along it. In this paper, we describe a method by which optimized modulation functions can be constructed to render insensitivity to B 1 inhomogeneity over a predetermined B 1 range and over a wide band of frequencies. This is accomplished by requiring that the optimized Pulse fulfill the Adiabatic condition over this range of B 1 inhomogeneity and over the desired frequency band for the complete duration of the Pulse. A trajectory similar to the well-known sech/tanh Adiabatic Pulse, i.e., a half-ellipse, is used. The optimization process improves the slice profile by optimizing the rate of motion along this trajectory. The optimized Pulse can be tailored to the specific design requirements; in particular, the transition sharpness may be traded off against the inverted bandwidth. Two design examples, including experimental results, demonstrate the superiority of the optimized Pulses over the conventional sech/tanh Pulse: in the first example, a large frequency band is to be inverted using a weak RF amplitude in a short time. In the second example, a Pulse with a very sharp transition is required.
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Design of Adiabatic Pulses for fat‐suppression using analytic solutions of the bloch equation
Magnetic resonance in medicine, 1997Co-Authors: Daniel Rosenfeld, Shimon L. Panfil, Yuval ZurAbstract: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.
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A new Adiabatic inversion Pulse
Magnetic resonance in medicine, 1996Co-Authors: Daniel Rosenfeld, Yuval ZurAbstract:Adiabatic Pulses play an important role in magnetization inversion in the presence of RF field inhomogeneity. In this work the authors present an efficient Adiabatic inversion Pulse that is able to selectively invert magnetization over a large frequency bandwidth in a short time. The Pulse is constructed in two steps: (i) the optimal trajectory is determined and (ii) the optimal rate of motion along that trajectory is determined. The resulting Pulse enables separately controlling and trading off the Pulse duration against the transition width. The superiority of this Pulse over the well known sech/tanh Adiabatic Pulse is demonstrated in a scenario where a large bandwidth should be inverted at a short time using limited B1 amplitude.
Michael Garwood - One of the best experts on this subject based on the ideXlab platform.
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parallel transmit optimized 3d composite Adiabatic spectral spatial Pulse for spectroscopy
Magnetic Resonance in Medicine, 2021Co-Authors: Edward J. Auerbach, Michael Garwood, Naoharu Kobayashi, Gregory J MetzgerAbstract:Purpose To develop a 3D composite Adiabatic spectral-spatial Pulse for refocusing in spin-echo spectroscopy acquisitions and to compare its performance against standard acquisition methods. Methods A 3D composite Adiabatic Pulse was designed by modulating a train of parallel transmit-optimized 2D subPulses with an Adiabatic envelope. The spatial and spectral profiles were simulated and validated by experiments to demonstrate the feasibility of the design in both single and double spin-echo spectroscopy acquisitions. Phantom and in vivo studies were performed to evaluate the Pulse performance and compared with semi-LASER with respect to localization performance, sequence timing, signal suppression, and specific absorption rate. Results Simultaneous 2D spatial localization with water and lipid suppression was achieved with the designed refocusing Pulse, allowing high-quality spectra to be acquired with shorter minimum TE/TR, reduced SAR, as well as adaptation to spatially varying B0 and B 1 + field inhomogeneities in both prostate and brain studies. Conclusion The proposed composite Pulse can serve as a more SAR efficient alternative to conventional localization methods such as semi-LASER at ultrahigh field for spin echo-based spectroscopy studies. SubPulse parallel-transmit optimization provides the flexibility to manage the tradeoff among multiple design criteria to accommodate different field strengths and applications.
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Designing 3D selective Adiabatic radiofrequency Pulses with single and parallel transmission.
Magnetic resonance in medicine, 2017Co-Authors: Albert Jang, Edward J. Auerbach, Michael GarwoodAbstract:Purpose To introduce a method of designing single and parallel transmit (pTx) 3D Adiabatic π Pulses for inverting and refocusing spins that are insensitive to transmit B1 ( B1+) inhomogeneity. Theory and Methods A 3D Adiabatic Pulse is created by replacing each piece-wise constant element (or sub-Pulse) of an Adiabatic full passage (AFP) by a 2D selective Pulse. In this study, the parent AFP is an HS1 and each sub-Pulse is a 2D Pulse derived from a jinc function designed using a spiral k-trajectory. Spatial selectivity in the third direction is achieved by blipping the slab-selective gradient between sub-Pulses, yielding a rectangular slab profile identical to that of the parent AFP. The slew-rate limited sub-Pulse can be undersampled utilizing pTx, thus shortening the overall Pulse width. Simulations and experiments demonstrate the quality of spatial selectivity and Adiabaticity achievable. Results The 3D Adiabatic Pulse inverts and refocus spins in a sharply demarcated cylindrical volume. When stepping RF amplitude, an Adiabatic threshold is observed above which the flip angle remains π. Experimental results demonstrate that pTx is an effective means to significantly improve Pulse performance. Conclusion A method of designing 3D Adiabatic Pulses insensitive to B1 inhomogeneity has been developed. pTx can shorten these Pulses while retaining their Adiabatic character. Magn Reson Med, 2017. © 2017 International Society for Magnetic Resonance in Medicine.
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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.
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Localized detection of glioma glycolysis using edited 1H MRS.
Magnetic resonance in medicine, 1993Co-Authors: Daniel G. Schupp, Hellmut Merkle, Jutta M. Ellermann, Michael GarwoodAbstract:In vivo 1H MRS can be used to detect and quantify the lactate resonance at 1.3 ppm provided that overlapping lipid resonances are eliminated. A homonuclear spectral editing method was developed to acquire uncontaminated 1H spectra of lactate with Adiabatic Pulses. An advantage of the Adiabatic Pulse sequence is the ability to induce uniform flip angles and to maximize sensitivity in applications employing surface coil transmitters which produce highly inhomogeneous B1. Glycolytic activity in an intracerebral C6 glioma in rats was monitored by using Adiabatic editing sequences to observe [3-13C]lactate produced from infused [1-13C]glucose. Acute hyperglycemia (serum glucose > 22 mM, n = 10) had no significant effect (P = 0.08) on the total ([12C] + [13C]) tumor lactate signal intensity.
Albert Macovski - One of the best experts on this subject based on the ideXlab platform.
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Two-dimensional selective Adiabatic Pulses.
Magnetic resonance in medicine, 1992Co-Authors: Steven M. Conolly, Dwight G. Nishimura, John M Pauly, Albert MacovskiAbstract:Using the technique of separable k-space excitation, we have designed a two-dimensional selective Adiabatic Pulse that inverts magnetization from a square region in the xy plane with insensitivity to RF variations. We also have designed a two-dimensional Adiabatic Pulse that inverts selectively in frequency and in one spatial dimension. The Pulses should be useful for both MR imaging and spectroscopy. We present experimental results to demonstrate that the two-dimensional Adiabatic Pulses are feasible on commercial MR imaging systems. © 1992 Academic Press, Inc.
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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.
Daniel M Spielman - One of the best experts on this subject based on the ideXlab platform.
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self refocused Adiabatic Pulse for spin echo imaging at 7 t
Magnetic Resonance in Medicine, 2012Co-Authors: Priti Balchandani, Gary H Glover, Mohammad Mehdi Khalighi, John M Pauly, Daniel M SpielmanAbstract:Spin echo Pulse sequences are used to produce clinically important T2 contrast. However, conventional 180° radiofrequency Pulses required to generate a spin echo are highly susceptible to the B1 inhomogeneity at high magnetic fields such as 7 Tesla (7 T), resulting in varying signal and contrast over the region of interest. Adiabatic 180° Pulses may be used to replace conventional 180° Pulses in spin echo sequences to provide greater immunity to the inhomogeneous B1 field at 7 T. However, because the spectral profile of an Adiabatic 180° Pulse has nonlinear phase, pairs of these Pulses are needed for proper refocusing, resulting in increased radiofrequency power deposition and long minimum echo times. We used the Adiabatic Shinnar Le-Roux method to generate a matched-phase Adiabatic 90°–180° Pulse pair to obviate the need for a second Adiabatic 180° Pulse for phase refocusing. The Pulse pair was then reformulated into a single self-refocused Pulse to minimize the echo time, and phantom and in vivo experiments were performed to validate Pulse performance. The self-refocused Adiabatic Pulse produced transmit profiles that were substantially more uniform than those achieved using a conventional spin echo sequence. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.
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Self‐refocused Adiabatic Pulse for spin echo imaging at 7 T
Magnetic resonance in medicine, 2011Co-Authors: Priti Balchandani, Gary H Glover, Mohammad Mehdi Khalighi, John M Pauly, Daniel M SpielmanAbstract:Spin echo Pulse sequences are used to produce clinically important T2 contrast. However, conventional 180° radiofrequency Pulses required to generate a spin echo are highly susceptible to the B1 inhomogeneity at high magnetic fields such as 7 Tesla (7 T), resulting in varying signal and contrast over the region of interest. Adiabatic 180° Pulses may be used to replace conventional 180° Pulses in spin echo sequences to provide greater immunity to the inhomogeneous B1 field at 7 T. However, because the spectral profile of an Adiabatic 180° Pulse has nonlinear phase, pairs of these Pulses are needed for proper refocusing, resulting in increased radiofrequency power deposition and long minimum echo times. We used the Adiabatic Shinnar Le-Roux method to generate a matched-phase Adiabatic 90°–180° Pulse pair to obviate the need for a second Adiabatic 180° Pulse for phase refocusing. The Pulse pair was then reformulated into a single self-refocused Pulse to minimize the echo time, and phantom and in vivo experiments were performed to validate Pulse performance. The self-refocused Adiabatic Pulse produced transmit profiles that were substantially more uniform than those achieved using a conventional spin echo sequence. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.