The Experts below are selected from a list of 123 Experts worldwide ranked by ideXlab platform
Michael B Smith - One of the best experts on this subject based on the ideXlab platform.
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array optimized Composite Pulse for excellent whole brain homogeneity in high field mri
Magnetic Resonance in Medicine, 2007Co-Authors: Christopher M Collins, Zhangwei Wang, Weihua Mao, Jieming Fang, Wanzhan Liu, Michael B SmithAbstract:A number of methods to improve excitation homogeneity in high-field MRI have been proposed, and some of these methods rely on separate control of radiofrequency (RF) coils in a transmit array. In this work we combine accurate RF field calculations and the Bloch equation to demonstrate that by using a sequence of Pulses with individually optimized current distributions (i.e., an array-optimized Composite Pulse), one can achieve remarkably homogeneous distributions of available signal intensity over the entire brain volume. This homogeneity is greater than that achievable using the same transmit array to produce either a single optimized (or RF shimmed) Pulse or a single RF shimmed field distribution in a standard 90x-90y Composite Pulse arrangement. Simulations indicate that with a very simple array-optimized Composite Pulse, excellent whole-brain excitation homogeneity can be achieved at up to 600 MHz.
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array optimized Composite Pulse for excellent whole brain homogeneity in high field mri
Magnetic Resonance in Medicine, 2007Co-Authors: Christopher M Collins, Zhangwei Wang, Weihua Mao, Jieming Fang, Wanzhan Liu, Michael B SmithAbstract:The advancement to higher static magnetic (B0) field strengths and concomitant higher radiofrequency (RF) magnetic (B1) field frequencies necessitates the design of magnetic resonance imaging (MRI) technologies with an increased consideration of the field-perturbation effects of human tissue. In recent years a number of methods to alleviate image artifacts related to RF wavelength effects in high-field MRI have been proposed, and some of these methods rely on separate control of RF coils in a transmit array. In this work we refer to the two most commonly discussed methods as “RF shimming” and “multicoil tailored Pulses.” With RF shimming the current magnitudes and phases of coils in a transmit array (or impedances of elements in a volume coil) are adjusted individually to produce a homogeneous RF field or RF excitation within a target region (1–3). This can be viewed as using multiple RF field sources to find an advantageous field distribution within the bounds of the Maxwell equations. In contrast, tailored Pulses are designed around the highly-shaped, coordinated simultaneous pulsing of RF and gradient coils to achieve spatially-selective excitation based primarily on the use of the Bloch equations and predefined k-space trajectories (4). The resulting excitation distribution is very flexible. It seems to be limited more by the imagination of the Pulse designer than by the Maxwell equations (5), but requires very long RF Pulses. Tailored RF Pulses have been used successfully to compensate for RF inhomogeneity (6). Additionally, multicoil tailored RF Pulses, in the specific form of transmit (SENSE) (7), have successfully been used to perform spatially-selective excitations with reduced Pulse durations (8). One previously published work is especially worthy of mention here because of its relation to the method we propose. Ledden and Cheng (9) proposed pulsing each of eight coils at separate times in quick succession. Unfortunately, in their mathematical treatment they failed to consider the vector nature of either the spin magnetization or the appropriate portion of the applied RF magnetic field, and assumed that the result of the proposed Composite Pulse would be equivalent to that obtained by merely summing the magnitudes of the fields from the eight separate coils. In our earlier investigations into using a sequence of Pulses with different field distributions (starting in 2003), we showed that by pulsing different coils sequentially it is not possible to produce the effect of merely summing the separate expected flip angle magnitudes (10), as Ledden and Cheng assumed (9). A logical progression from single RF-shimmed Pulses is to use array-optimized Composite Pulses. Composite Pulses have successfully been used with single coils to improve the homogeneity of the flip angle despite an inhomogeneous RF field (11,12). When used with a single excitation coil, they are theoretically more limited than tailored Pulses in producing homogeneous excitations with a given single inhomogeneous field distribution, but do not require the long, complex, simultaneous shaped RF and gradient Pulses. Here we combine accurate full-Maxwell field simulations and the Bloch equation to show that by adjusting individual coil currents in a sequence of Pulses through a transmit array, one can achieve remarkably homogeneous whole-brain excitations at up to 600 MHz.
Eric Y Chang - One of the best experts on this subject based on the ideXlab platform.
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fat suppression for ultrashort echo time imaging using a novel soft hard Composite radiofrequency Pulse
Magnetic Resonance in Medicine, 2019Co-Authors: Saeed Jerban, Hyungseok Jang, Eric Y ChangAbstract:Author(s): Ma, Ya-Jun; Jerban, Saeed; Jang, Hyungseok; Chang, Eric Y; Du, Jiang | Abstract: PurposeTo design a soft-hard Composite Pulse for fat suppression and water excitation in ultrashort echo time (UTE) imaging with minimal short T2 signal attenuation.MethodsThe Composite Pulse contains a narrow bandwidth soft Pulse centered on the fat peak with a small negative flip angle (-α) and a short rectangular Pulse with a small positive flip angle (α). The fat magnetization experiences both tipping-down and -back with an identical flip angle and thus returns to the equilibrium state, leaving only the excited water magnetization. Bloch simulations, as well as knee, tibia, and ankle UTE imaging studies, were performed to investigate the effectiveness of fat suppression and corresponding water signal attenuation. A conventional fat saturation (FatSat) module was used for comparison. Signal suppression ratio (SSR), defined as the ratio of signal difference between non-fat-suppression and fat-suppression images over the non-fat-suppression signal, was introduced to evaluate the efficiency of the Composite Pulse.ResultsNumerical simulations demonstrate that the soft-hard Pulse has little saturation effect on short T2 water signals. Knee, tibia, and ankle UTE imaging results suggest that comparable fat suppression can be achieved with the soft-hard Pulse and the FatSat module. However, much less water saturation is induced by the soft-hard Pulse, especially for short T2 tissues, with SSRs reduced from 71.8 ± 6.9% to 5.8 ± 4.4% for meniscus, from 68.7 ± 5.5% to 7.7 ± 7.6% for bone, and from 62.9 ± 12.0% to 4.8 ± 3.2% for the Achilles tendon.ConclusionThe soft-hard Composite Pulse can suppress fat signals in UTE imaging with little signal attenuation on short T2 tissues.
Christopher M Collins - One of the best experts on this subject based on the ideXlab platform.
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array optimized Composite Pulse for excellent whole brain homogeneity in high field mri
Magnetic Resonance in Medicine, 2007Co-Authors: Christopher M Collins, Zhangwei Wang, Weihua Mao, Jieming Fang, Wanzhan Liu, Michael B SmithAbstract:A number of methods to improve excitation homogeneity in high-field MRI have been proposed, and some of these methods rely on separate control of radiofrequency (RF) coils in a transmit array. In this work we combine accurate RF field calculations and the Bloch equation to demonstrate that by using a sequence of Pulses with individually optimized current distributions (i.e., an array-optimized Composite Pulse), one can achieve remarkably homogeneous distributions of available signal intensity over the entire brain volume. This homogeneity is greater than that achievable using the same transmit array to produce either a single optimized (or RF shimmed) Pulse or a single RF shimmed field distribution in a standard 90x-90y Composite Pulse arrangement. Simulations indicate that with a very simple array-optimized Composite Pulse, excellent whole-brain excitation homogeneity can be achieved at up to 600 MHz.
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array optimized Composite Pulse for excellent whole brain homogeneity in high field mri
Magnetic Resonance in Medicine, 2007Co-Authors: Christopher M Collins, Zhangwei Wang, Weihua Mao, Jieming Fang, Wanzhan Liu, Michael B SmithAbstract:The advancement to higher static magnetic (B0) field strengths and concomitant higher radiofrequency (RF) magnetic (B1) field frequencies necessitates the design of magnetic resonance imaging (MRI) technologies with an increased consideration of the field-perturbation effects of human tissue. In recent years a number of methods to alleviate image artifacts related to RF wavelength effects in high-field MRI have been proposed, and some of these methods rely on separate control of RF coils in a transmit array. In this work we refer to the two most commonly discussed methods as “RF shimming” and “multicoil tailored Pulses.” With RF shimming the current magnitudes and phases of coils in a transmit array (or impedances of elements in a volume coil) are adjusted individually to produce a homogeneous RF field or RF excitation within a target region (1–3). This can be viewed as using multiple RF field sources to find an advantageous field distribution within the bounds of the Maxwell equations. In contrast, tailored Pulses are designed around the highly-shaped, coordinated simultaneous pulsing of RF and gradient coils to achieve spatially-selective excitation based primarily on the use of the Bloch equations and predefined k-space trajectories (4). The resulting excitation distribution is very flexible. It seems to be limited more by the imagination of the Pulse designer than by the Maxwell equations (5), but requires very long RF Pulses. Tailored RF Pulses have been used successfully to compensate for RF inhomogeneity (6). Additionally, multicoil tailored RF Pulses, in the specific form of transmit (SENSE) (7), have successfully been used to perform spatially-selective excitations with reduced Pulse durations (8). One previously published work is especially worthy of mention here because of its relation to the method we propose. Ledden and Cheng (9) proposed pulsing each of eight coils at separate times in quick succession. Unfortunately, in their mathematical treatment they failed to consider the vector nature of either the spin magnetization or the appropriate portion of the applied RF magnetic field, and assumed that the result of the proposed Composite Pulse would be equivalent to that obtained by merely summing the magnitudes of the fields from the eight separate coils. In our earlier investigations into using a sequence of Pulses with different field distributions (starting in 2003), we showed that by pulsing different coils sequentially it is not possible to produce the effect of merely summing the separate expected flip angle magnitudes (10), as Ledden and Cheng assumed (9). A logical progression from single RF-shimmed Pulses is to use array-optimized Composite Pulses. Composite Pulses have successfully been used with single coils to improve the homogeneity of the flip angle despite an inhomogeneous RF field (11,12). When used with a single excitation coil, they are theoretically more limited than tailored Pulses in producing homogeneous excitations with a given single inhomogeneous field distribution, but do not require the long, complex, simultaneous shaped RF and gradient Pulses. Here we combine accurate full-Maxwell field simulations and the Bloch equation to show that by adjusting individual coil currents in a sequence of Pulses through a transmit array, one can achieve remarkably homogeneous whole-brain excitations at up to 600 MHz.
Nikolay V. Vitanov - One of the best experts on this subject based on the ideXlab platform.
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arbitrarily accurate passband Composite Pulses for dynamical suppression of amplitude noise
Physical Review A, 2013Co-Authors: Elica Kyoseva, Nikolay V. VitanovAbstract:We introduce flexible high-fidelity passband (PB) Composite Pulse sequences constructed by concatenation of recently derived arbitrarily large and arbitrarily accurate broadband $\mathcal{B}$ and narrowband $\mathcal{N}$ Composite sequences. Our PB sequences allow to produce flexible and tunable nearly rectangular two-state inversion profiles as a function of the individual Pulse area because the width and the rectangularity of these profiles can be adjusted at will. Moreover, these PB sequences suppress excitation around Pulse area $0$ and $2\pi$, and suppress deviations from complete population inversion around Pulse area $\pi$ to arbitrarily high orders. These features makes them a valuable tool for high-fidelity qubit operations in the presence of relatively strong amplitude noise. We construct two types of PB Pulses: $\mathcal{N}(\mathcal{B})$ in which a broadband Pulse is nested into a narrowband Pulse, and $\mathcal{B}(\mathcal{N})$ in which a narrowband Pulse is nested into a broadband Pulse; the latter sequences deliver narrower profiles. We derive exact analytic formulas for the Composite phases of the PB Pulses and exact analytic formulas for the inversion profiles. These formulas allow an easy estimation of the experimental resources needed for any desired qubit inversion profile.
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optimized control of multistate quantum systems by Composite Pulse sequences
Physical Review A, 2011Co-Authors: Genko T Genov, Boyan T Torosov, Nikolay V. VitanovAbstract:We introduce a technique for derivation of high-fidelity Composite Pulse sequences for two types of multistate quantum systems: systems with the SU(2) and Morris-Shore dynamic symmetries. For the former type, we use the Majorana decomposition to reduce the dynamics to an effective two-state system, which allows us to find the propagator analytically and use the pool of available Composite Pulses for two-state systems. For the latter type of multistate systems, we use the Morris-Shore decomposition, which reduces the multistate dynamics to a set of two-state systems. We present examples which demonstrate that the multistate Composite sequences open a variety of possibilities for coherent control of quantum systems with multiple states.
Saeed Jerban - One of the best experts on this subject based on the ideXlab platform.
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3d ute bicomponent imaging of cortical bone using a soft hard Composite Pulse for excitation
Magnetic Resonance in Medicine, 2021Co-Authors: Yanjun Chen, Saeed Jerban, Zhao Wei, Zhenyu Cai, Yunfei ZhaAbstract:PURPOSE To evaluate 3D UTE bicomponent imaging of cortical bone ex vivo and in vivo using a newly designed soft-hard Composite Pulse for excitation. METHODS Chemical shift artifacts, presenting as fat-water oscillation or combination-induced signal oscillation, significantly reduce the accuracy of quantitative UTE bicomponent analysis of cortical bone. To achieve fat suppression for more reliable bicomponent analysis, a newly developed soft-hard excitation Pulse was used with UTE imaging and compared with a single rectangular Pulse excitation without and with a conventional fat saturation (FatSat) module. These 3 sequences were applied to 8 bovine bone samples without marrow fat, 3 bovine bone samples with marrow fat, and tibial midshafts of 5 healthy human volunteers. Bicomponent analyses were performed in both ex vivo and in vivo studies. RESULTS The soft-hard Pulse provided comparable fat suppression, but much reduced bone signal attenuation compared with the FatSat module. Better bicomponent T2∗ fitting was also achieved with the soft-hard excitation Pulse because it greatly reduced chemical shift artifacts and outperformed the single rectangular Pulse without or with FatSat. Although the FatSat module reduced fat signals and related fat-water oscillation, the water signals were significantly attenuated with more than 40% reduction due to direction saturation. For the inner layer of tibial midshaft in healthy volunteers, fitting errors increased from 3.78% for the soft-hard Pulse to 11.43% and 5.16%, respectively, for the single rectangular Pulse without and with the FatSat module. CONCLUSION The 3D UTE sequence with a new soft-hard excitation Pulse allows more reliable bicomponent imaging of cortical bone.
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fat suppression for ultrashort echo time imaging using a novel soft hard Composite radiofrequency Pulse
Magnetic Resonance in Medicine, 2019Co-Authors: Saeed Jerban, Hyungseok Jang, Eric Y ChangAbstract:Author(s): Ma, Ya-Jun; Jerban, Saeed; Jang, Hyungseok; Chang, Eric Y; Du, Jiang | Abstract: PurposeTo design a soft-hard Composite Pulse for fat suppression and water excitation in ultrashort echo time (UTE) imaging with minimal short T2 signal attenuation.MethodsThe Composite Pulse contains a narrow bandwidth soft Pulse centered on the fat peak with a small negative flip angle (-α) and a short rectangular Pulse with a small positive flip angle (α). The fat magnetization experiences both tipping-down and -back with an identical flip angle and thus returns to the equilibrium state, leaving only the excited water magnetization. Bloch simulations, as well as knee, tibia, and ankle UTE imaging studies, were performed to investigate the effectiveness of fat suppression and corresponding water signal attenuation. A conventional fat saturation (FatSat) module was used for comparison. Signal suppression ratio (SSR), defined as the ratio of signal difference between non-fat-suppression and fat-suppression images over the non-fat-suppression signal, was introduced to evaluate the efficiency of the Composite Pulse.ResultsNumerical simulations demonstrate that the soft-hard Pulse has little saturation effect on short T2 water signals. Knee, tibia, and ankle UTE imaging results suggest that comparable fat suppression can be achieved with the soft-hard Pulse and the FatSat module. However, much less water saturation is induced by the soft-hard Pulse, especially for short T2 tissues, with SSRs reduced from 71.8 ± 6.9% to 5.8 ± 4.4% for meniscus, from 68.7 ± 5.5% to 7.7 ± 7.6% for bone, and from 62.9 ± 12.0% to 4.8 ± 3.2% for the Achilles tendon.ConclusionThe soft-hard Composite Pulse can suppress fat signals in UTE imaging with little signal attenuation on short T2 tissues.