Xiaoping Wu - Academia.edu (original) (raw)

Papers by Xiaoping Wu

Research paper thumbnail of High-resolution whole-brain diffusion MRI at 7T using radiofrequency parallel transmission

Magnetic resonance in medicine, Jan 30, 2018

Investigating the utility of RF parallel transmission (pTx) for Human Connectome Project (HCP)-st... more Investigating the utility of RF parallel transmission (pTx) for Human Connectome Project (HCP)-style whole-brain diffusion MRI (dMRI) data at 7 Tesla (7T). Healthy subjects were scanned in pTx and single-transmit (1Tx) modes. Multiband (MB), single-spoke pTx pulses were designed to image sagittal slices. HCP-style dMRI data (i.e., 1.05-mm resolutions, MB2, b-values = 1000/2000 s/mm , 286 images and 40-min scan) and data with higher accelerations (MB3 and MB4) were acquired with pTx. pTx significantly improved flip-angle detected signal uniformity across the brain, yielding ∼19% increase in temporal SNR (tSNR) averaged over the brain relative to 1Tx. This allowed significantly enhanced estimation of multiple fiber orientations (with ∼21% decrease in dispersion) in HCP-style 7T dMRI datasets. Additionally, pTx pulses achieved substantially lower power deposition, permitting higher accelerations, enabling collection of the same data in 2/3 and 1/2 the scan time or of more data in the s...

Research paper thumbnail of Toward imaging the body at 10.5 tesla

Magnetic resonance in medicine, 2017

To explore the potential of performing body imaging at 10.5 Tesla (T) compared with 7.0T through ... more To explore the potential of performing body imaging at 10.5 Tesla (T) compared with 7.0T through evaluating the transmit/receive performance of similarly configured dipole antenna arrays. Fractionated dipole antenna elements for 10.5T body imaging were designed and evaluated using numerical simulations. Transmit performance of antenna arrays inside the prostate, kidneys and heart were investigated and compared with those at 7.0T using both phase-only radiofrequency (RF) shimming and multi-spoke pulses. Signal-to-noise ratio (SNR) comparisons were also performed. A 10-channel antenna array was constructed to image the abdomen of a swine at 10.5T. Numerical methods were validated with phantom studies at both field strengths. Similar power efficiencies were observed inside target organs with phase-only shimming, but RF nonuniformity was significantly higher at 10.5T. Spokes RF pulses allowed similar transmit performance with accompanying local specific absorption rate increases of 25-9...

Research paper thumbnail of A generalized slab-wise framework for parallel transmit multiband RF pulse design

Magnetic Resonance in Medicine, 2015

Research paper thumbnail of Correcting for Strong Eddy Current Induced B0 Modulation Enables Two-Spoke RF Pulse Design with Parallel Transmission: Demonstration at 9.4T in the Human Brain

PLoS ONE, 2013

Successful implementation of homogeneous slice-selective RF excitation in the human brain at 9.4T... more Successful implementation of homogeneous slice-selective RF excitation in the human brain at 9.4T using 16-channel parallel transmission (pTX) is demonstrated. A novel three-step pulse design method incorporating fast real-time measurement of eddy current induced B0 variations as well as correction of resulting phase errors during excitation is described. To demonstrate the utility of the proposed method, phantom and in-vivo experiments targeting a uniform excitation in an axial slice were conducted using two-spoke pTX pulses. Even with the pre-emphasis activated, eddy current induced B0 variations with peak-to-peak values greater than 4 kHz were observed on our system during the rapid switches of slice selective gradients. This large B0 variation, when not corrected, resulted in drastically degraded excitation fidelity with the coefficient of variation (CV) of the flip angle calculated for the region of interest being large (,12% in the phantom and ,35% in the brain). By comparison, excitation fidelity was effectively restored, and satisfactory flip angle uniformity was achieved when using the proposed method, with the CV value reduced to ,3% in the phantom and ,8% in the brain. Additionally, experimental results were in good agreement with the numerical predictions obtained from Bloch simulations. Slice-selective flip angle homogenization in the human brain at 9.4T using 16-channel 3D spoke pTX pulses is achievable despite of large eddy current induced excitation phase errors; correcting for the latter was critical in this success.

Research paper thumbnail of Pushing spatial and temporal resolution for functional and diffusion MRI in the Human Connectome Project

Research paper thumbnail of Simultaneous multislice multiband parallel radiofrequency excitation with independent slice-specific transmit B1 homogenization

Magnetic Resonance in Medicine, 2013

To develop a new parallel transmit (pTx) pulse design for simultaneous multiband (MB) excitation ... more To develop a new parallel transmit (pTx) pulse design for simultaneous multiband (MB) excitation in order to tackle simultaneously the problems of transmit B1 (B1+) inhomogeneity and total radiofrequency (RF) power, so as to allow for optimal RF excitation when using MB pulses for slice acceleration for high and ultrahigh field MRI. With the proposed approach, each of the bands that are simultaneously excited is subject to a band-specific set of B1 complex shim weights. The method was validated in the human brain at 7T using a 16-channel pTx system and was compared to conventional MB pulses operating in the circularly polarized (CP) mode. Further numerical simulations based on measured B1 maps were conducted. The new method improved B1+ homogeneity by 60% when keeping the total RF power constant and reduced total RF power by 72% when keeping the excitation fidelity constant, as compared to the conventional CP mode. A new pTx pulse design formalism is introduced targeting slice-specific B1+ homogenization in MB excitation while constraining total RF power. These pulses lead to significantly improved slice-wise B1+ uniformity and/or largely reduced total RF power, as compared to the conventionally employed MB pulses applied in the CP mode. Magn Reson Med 70:630-638, 2013. © 2013 Wiley Periodicals, Inc.

Research paper thumbnail of Parallel excitation in the human brain at 9.4 T counteracting k -space errors with RF pulse design

Magnetic Resonance in Medicine, 2010

Research paper thumbnail of Pushing spatial and temporal resolution for functional and diffusion MRI in the Human Connectome Project

NeuroImage, 2013

The Human Connectome Project (HCP) relies primarily on three complementary magnetic resonance (MR... more The Human Connectome Project (HCP) relies primarily on three complementary magnetic resonance (MR) methods. These are: 1) resting state functional MR imaging (rfMRI) which uses correlations in the temporal fluctuations in an fMRI time series to deduce 'functional connectivity'; 2) diffusion imaging (dMRI), which provides the input for tractography algorithms used for the reconstruction of the complex axonal fiber architecture; and 3) task based fMRI (tfMRI), which is employed to identify functional parcellation in the human brain in order to assist analyses of data obtained with the first two methods. We describe technical improvements and optimization of these methods as well as instrumental choices that impact speed of acquisition of fMRI and dMRI images at 3T, leading to whole brain coverage with 2 mm isotropic resolution in 0.7 s for fMRI, and 1.25 mm isotropic resolution dMRI data for tractography analysis with three-fold reduction in total dMRI data acquisition time. Ongoing technical developments and optimization for acquisition of similar data at 7 T magnetic field are also presented, targeting higher spatial resolution, enhanced specificity of functional imaging signals, mitigation of the inhomogeneous radio frequency (RF) fields, and reduced power deposition. Results demonstrate that overall, these approaches represent a significant advance in MR imaging of the human brain to investigate brain function and structure.

Research paper thumbnail of High-resolution whole-brain diffusion MRI at 7T using radiofrequency parallel transmission

Magnetic resonance in medicine, Jan 30, 2018

Investigating the utility of RF parallel transmission (pTx) for Human Connectome Project (HCP)-st... more Investigating the utility of RF parallel transmission (pTx) for Human Connectome Project (HCP)-style whole-brain diffusion MRI (dMRI) data at 7 Tesla (7T). Healthy subjects were scanned in pTx and single-transmit (1Tx) modes. Multiband (MB), single-spoke pTx pulses were designed to image sagittal slices. HCP-style dMRI data (i.e., 1.05-mm resolutions, MB2, b-values = 1000/2000 s/mm , 286 images and 40-min scan) and data with higher accelerations (MB3 and MB4) were acquired with pTx. pTx significantly improved flip-angle detected signal uniformity across the brain, yielding ∼19% increase in temporal SNR (tSNR) averaged over the brain relative to 1Tx. This allowed significantly enhanced estimation of multiple fiber orientations (with ∼21% decrease in dispersion) in HCP-style 7T dMRI datasets. Additionally, pTx pulses achieved substantially lower power deposition, permitting higher accelerations, enabling collection of the same data in 2/3 and 1/2 the scan time or of more data in the s...

Research paper thumbnail of Toward imaging the body at 10.5 tesla

Magnetic resonance in medicine, 2017

To explore the potential of performing body imaging at 10.5 Tesla (T) compared with 7.0T through ... more To explore the potential of performing body imaging at 10.5 Tesla (T) compared with 7.0T through evaluating the transmit/receive performance of similarly configured dipole antenna arrays. Fractionated dipole antenna elements for 10.5T body imaging were designed and evaluated using numerical simulations. Transmit performance of antenna arrays inside the prostate, kidneys and heart were investigated and compared with those at 7.0T using both phase-only radiofrequency (RF) shimming and multi-spoke pulses. Signal-to-noise ratio (SNR) comparisons were also performed. A 10-channel antenna array was constructed to image the abdomen of a swine at 10.5T. Numerical methods were validated with phantom studies at both field strengths. Similar power efficiencies were observed inside target organs with phase-only shimming, but RF nonuniformity was significantly higher at 10.5T. Spokes RF pulses allowed similar transmit performance with accompanying local specific absorption rate increases of 25-9...

Research paper thumbnail of A generalized slab-wise framework for parallel transmit multiband RF pulse design

Magnetic Resonance in Medicine, 2015

Research paper thumbnail of Correcting for Strong Eddy Current Induced B0 Modulation Enables Two-Spoke RF Pulse Design with Parallel Transmission: Demonstration at 9.4T in the Human Brain

PLoS ONE, 2013

Successful implementation of homogeneous slice-selective RF excitation in the human brain at 9.4T... more Successful implementation of homogeneous slice-selective RF excitation in the human brain at 9.4T using 16-channel parallel transmission (pTX) is demonstrated. A novel three-step pulse design method incorporating fast real-time measurement of eddy current induced B0 variations as well as correction of resulting phase errors during excitation is described. To demonstrate the utility of the proposed method, phantom and in-vivo experiments targeting a uniform excitation in an axial slice were conducted using two-spoke pTX pulses. Even with the pre-emphasis activated, eddy current induced B0 variations with peak-to-peak values greater than 4 kHz were observed on our system during the rapid switches of slice selective gradients. This large B0 variation, when not corrected, resulted in drastically degraded excitation fidelity with the coefficient of variation (CV) of the flip angle calculated for the region of interest being large (,12% in the phantom and ,35% in the brain). By comparison, excitation fidelity was effectively restored, and satisfactory flip angle uniformity was achieved when using the proposed method, with the CV value reduced to ,3% in the phantom and ,8% in the brain. Additionally, experimental results were in good agreement with the numerical predictions obtained from Bloch simulations. Slice-selective flip angle homogenization in the human brain at 9.4T using 16-channel 3D spoke pTX pulses is achievable despite of large eddy current induced excitation phase errors; correcting for the latter was critical in this success.

Research paper thumbnail of Pushing spatial and temporal resolution for functional and diffusion MRI in the Human Connectome Project

Research paper thumbnail of Simultaneous multislice multiband parallel radiofrequency excitation with independent slice-specific transmit B1 homogenization

Magnetic Resonance in Medicine, 2013

To develop a new parallel transmit (pTx) pulse design for simultaneous multiband (MB) excitation ... more To develop a new parallel transmit (pTx) pulse design for simultaneous multiband (MB) excitation in order to tackle simultaneously the problems of transmit B1 (B1+) inhomogeneity and total radiofrequency (RF) power, so as to allow for optimal RF excitation when using MB pulses for slice acceleration for high and ultrahigh field MRI. With the proposed approach, each of the bands that are simultaneously excited is subject to a band-specific set of B1 complex shim weights. The method was validated in the human brain at 7T using a 16-channel pTx system and was compared to conventional MB pulses operating in the circularly polarized (CP) mode. Further numerical simulations based on measured B1 maps were conducted. The new method improved B1+ homogeneity by 60% when keeping the total RF power constant and reduced total RF power by 72% when keeping the excitation fidelity constant, as compared to the conventional CP mode. A new pTx pulse design formalism is introduced targeting slice-specific B1+ homogenization in MB excitation while constraining total RF power. These pulses lead to significantly improved slice-wise B1+ uniformity and/or largely reduced total RF power, as compared to the conventionally employed MB pulses applied in the CP mode. Magn Reson Med 70:630-638, 2013. © 2013 Wiley Periodicals, Inc.

Research paper thumbnail of Parallel excitation in the human brain at 9.4 T counteracting k -space errors with RF pulse design

Magnetic Resonance in Medicine, 2010

Research paper thumbnail of Pushing spatial and temporal resolution for functional and diffusion MRI in the Human Connectome Project

NeuroImage, 2013

The Human Connectome Project (HCP) relies primarily on three complementary magnetic resonance (MR... more The Human Connectome Project (HCP) relies primarily on three complementary magnetic resonance (MR) methods. These are: 1) resting state functional MR imaging (rfMRI) which uses correlations in the temporal fluctuations in an fMRI time series to deduce 'functional connectivity'; 2) diffusion imaging (dMRI), which provides the input for tractography algorithms used for the reconstruction of the complex axonal fiber architecture; and 3) task based fMRI (tfMRI), which is employed to identify functional parcellation in the human brain in order to assist analyses of data obtained with the first two methods. We describe technical improvements and optimization of these methods as well as instrumental choices that impact speed of acquisition of fMRI and dMRI images at 3T, leading to whole brain coverage with 2 mm isotropic resolution in 0.7 s for fMRI, and 1.25 mm isotropic resolution dMRI data for tractography analysis with three-fold reduction in total dMRI data acquisition time. Ongoing technical developments and optimization for acquisition of similar data at 7 T magnetic field are also presented, targeting higher spatial resolution, enhanced specificity of functional imaging signals, mitigation of the inhomogeneous radio frequency (RF) fields, and reduced power deposition. Results demonstrate that overall, these approaches represent a significant advance in MR imaging of the human brain to investigate brain function and structure.