Purpose: To develop a phantom for validating MRI pulse sequences and data processing methods to q... more Purpose: To develop a phantom for validating MRI pulse sequences and data processing methods to quantify microscopic diffusion anisotropy in the human brain. Methods: Using a liquid crystal consisting of water, detergent, and hydrocarbon, we designed a 0.5-L spherical phantom showing the theoretically highest possible degree of microscopic anisotropy. Data were acquired on the Connectome scanner using echo-planar imaging signal readout and diffusion encoding with axisymmetric b-tensors of varying magnitude, anisotropy, and orientation. The mean diffusivity, fractional anisotropy (FA), and microscopic FA (mFA) parameters were estimated. Results: The phantom was observed to have values of mean diffusivity similar to brain tissue, and relaxation times compatible with echo-planar imaging echo times on the order of 100 ms. The estimated values of mFA were at the theoretical maximum of 1.0, whereas the values of FA spanned the interval from 0.0 to 0.8 as a result of varying orientational order of the anisotropic domains within each voxel. Conclusions: The proposed phantom can be manufactured by mixing three widely available chemicals in volumes comparable to a human head. The acquired data are in excellent agreement with theoretical predictions, showing that the phantom is ideal for validating methods for measuring microscopic diffusion anisotropy on clinical MRI systems. Magn Reson
Accurate spatial alignment of MRI data acquired across multiple contrasts in the same subject is ... more Accurate spatial alignment of MRI data acquired across multiple contrasts in the same subject is often crucial for data analysis and interpretation, but can be challenging in the presence of geometric distortions that differ between acquisitions. It is well known that single-shot echo-planar imaging (EPI) acquisitions suffer from distortion in the phase-encoding direction due to B0 field inhomogeneities arising from tissue magnetic susceptibility differences and other sources, however there can be distortion in other encoding directions as well in the presence of strong field homogeneities. High-resolution ultrahigh-field MRI typically uses low bandwidth in the slice-encoding direction to acquire thin slices and, when combined with the pronounced B0 inhomogeneities, is prone to an additional geometric distortion in the slice direction as well. Here we demonstrate a presence of this slice distortion in high-resolution 7T EPI acquired with a novel pulse sequence allowing for the rever...
Objectives: We aim to develop molecular fibrin imaging as a tool to stratify high risk atheroscle... more Objectives: We aim to develop molecular fibrin imaging as a tool to stratify high risk atherosclerotic carotid plaques. Here, we describe our results with a fibrin-specific probe [ 68 Ga]CM-246 in two different experimental settings: ex vivo imaging of carotid endarterectomy specimens and in vivo / ex vivo imaging of atherosclerotic plaque rupture in rabbits. Methods: We incubated discarded endarterectomy specimens (n=11) from asymptomatic patients with [ 68 Ga] CM-246 or a scrambled peptide control probe and measured uptake by autoradiography and a tissue binding assay. In a rabbit plaque rupture model of high cholesterol diet followed by abdominal aorta balloon injury (n=13) and subsequent pharmacological plaque rupture, in vivo and ex vivo PET-MR with [ 68 Ga]CM-246 was performed. Tissues were processed for autoradiography and histology to verify presence of fibrin and plaque rupture. Results: Autoradiography (A-B) and tissue binding assays (C) with [ 68 Ga]CM-246 with human caro...
Imaging Bo Zhu, Thomas Witzel, Shan Jiang, Daniel G Anderson, Robert S Langer, Bruce R Rosen, and... more Imaging Bo Zhu, Thomas Witzel, Shan Jiang, Daniel G Anderson, Robert S Langer, Bruce R Rosen, and Lawrence L Wald Department of Radiology, Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, Harvard-MIT Health Sciences and Technololgy, Massachusetts Institute of Technology, Cambridge, MA, United States, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, United States
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
BACKGROUND AND PURPOSE: The development of new MR imaging scanners with stronger gradients and im... more BACKGROUND AND PURPOSE: The development of new MR imaging scanners with stronger gradients and improvement in coil technology, allied with emerging fast imaging techniques, has allowed a substantial reduction in MR imaging scan times. Our goal was to develop a 10-minute gadolinium-enhanced brain MR imaging protocol with accelerated sequences and to evaluate its diagnostic performance compared with the standard clinical protocol. MATERIALS AND METHODS: Fifty-three patients referred for brain MR imaging with contrast were scanned with a 3T scanner. Each MR image consisted of 5 basic fast precontrast sequences plus standard and accelerated versions of the same postcontrast T1WI sequences. Two neuroradiologists assessed the image quality and the final diagnosis for each set of postcontrast sequences and compared their performances. RESULTS: The acquisition time of the combined accelerated pre-and postcontrast sequences was 10 minutes and 15 seconds; and of the fast postcontrast sequences, 3 minutes and 36 seconds, 46% of the standard sequences. The 10-minute postcontrast axial T1WI had fewer image artifacts (P Ͻ .001) and better overall diagnostic quality (P Ͻ .001). Although the 10-minute MPRAGE sequence showed a tendency to have more artifacts than the standard sequence (P ϭ .08), the overall diagnostic quality was similar (P ϭ .66). Moreover, there was no statistically significant difference in the diagnostic performance between the protocols. The sensitivity, specificity, and accuracy values for the 10-minute protocol were 100.0%, 88.9%, and 98.1%. CONCLUSIONS: The 10-minute brain MR imaging protocol with contrast is comparable in diagnostic performance with the standard protocol in an inpatient motion-prone population, with the additional benefits of reducing acquisition times and image artifacts. ABBREVIATIONS: GRAPPA ϭ generalized autocalibrating partially parallel acquisition; GRE ϭ gradient-echo; SE ϭ spin-echo
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
PURPOSE To propose a virtual coil (VC) acquisition/reconstruction framework to improve highly acc... more PURPOSE To propose a virtual coil (VC) acquisition/reconstruction framework to improve highly accelerated single-shot EPI (SS-EPI) and generalized slice dithered enhanced resolution (gSlider) acquisition in high-resolution diffusion imaging (DI). METHODS For robust VC-GRAPPA reconstruction, a background phase correction scheme was developed to match the image phase of the reference data with the corrupted phase of the accelerated diffusion-weighted data, where the corrupted phase of the diffusion data varies from shot to shot. A Gy prewinding-blip was also added to the EPI acquisition, to create a shifted-ky sampling strategy that allows for better exploitation of VC concept in the reconstruction. To evaluate the performance of the proposed methods, 1.5 mm isotropic whole-brain SS-EPI and 860 μm isotropic whole-brain gSlider-EPI diffusion data were acquired at an acceleration of 8-9 fold. Conventional and VC-GRAPPA reconstructions were performed and compared, and corresponding g-factors were calculated. RESULTS The proposed VC reconstruction substantially improves the image quality of both SS-EPI and gSlider-EPI, with reduced g-factor noise and reconstruction artifacts when compared to the conventional method. This has enabled high-quality low-noise diffusion imaging to be performed at 8-9 fold acceleration. CONCLUSIONS The proposed VC acquisition/reconstruction framework improves the reconstruction of DI at high accelerations. The ability to now employ such high accelerations will allow DI with EPI at reduced distortion and faster scan time, which should be beneficial for many clinical and neuroscience applications.
Using simultaneous acquisition from multiple channels of a radio-frequency (RF) coil array, magne... more Using simultaneous acquisition from multiple channels of a radio-frequency (RF) coil array, magnetic resonance inverse imaging (InI) achieves functional MRI acquisitions at a rate of 100 ms per whole-brain volume [1]. InI accelerates the scan by leaving out partition encoding steps and reconstructs images by solving under-determined inverse problems using RF coil sensitivity information. Hence, the correlated spatial information available in the coil array causes spatial blurring in the InI reconstruction. Here, we propose a method that employs gradient blips in the partition encoding direction during the acquisition to provide extra spatial encoding in order to better differentiate signals from different partitions.
PURPOSE To test an integrated "AC/DC" array approach at 7T, where B0 inhomogeneity pose... more PURPOSE To test an integrated "AC/DC" array approach at 7T, where B0 inhomogeneity poses an obstacle for functional imaging, diffusion-weighted MRI, MR spectroscopy, and other applications. METHODS A close-fitting 7T 31-channel (31-ch) brain array was constructed and tested using combined Rx and ΔB0 shim channels driven by a set of rapidly switchable current amplifiers. The coil was compared to a shape-matched 31-ch reference receive-only array for RF safety, signal-to-noise ratio (SNR), and inter-element noise correlation. We characterize the coil array's ability to provide global and dynamic (slice-optimized) shimming using ΔB0 field maps and echo planar imaging (EPI) acquisitions. RESULTS The SNR and average noise correlation were similar to the 31-ch reference array. Global and slice-optimized shimming provide 11% and 40% improvements respectively compared to baseline second-order spherical harmonic shimming. Birdcage transmit coil efficiency was similar for the reference and AC/DC array setups. CONCLUSION Adding ΔB0 shim capability to a 31-ch 7T receive array can significantly boost 7T brain B0 homogeneity without sacrificing the array's rdiofrequency performance, potentially improving ultra-high field neuroimaging applications that are vulnerable to off-resonance effects.
PURPOSE: Multicoil (MC) shimming shows great promise as an alternative to spherical harmonic shim... more PURPOSE: Multicoil (MC) shimming shows great promise as an alternative to spherical harmonic shimming for compensating higherorder B0 inhomogeneity in vivo. Previous realizations [1] left space near the body for RF coils, pushing MC shim loops further away, reducing their efficiency. The shim loops also caused modest SNR loss due to their proximity. Recently it has been shown that shim currents can flow on single-turn RF coil loops without compromising the function of either subsystem [2-4]. Both RF reception and MC shimming benefit from (a.) close proximity to the body and (b.) maximal spatial degrees of freedom (large coil arrays). This suggests the most space-efficient design is to let DC and RF share the same conducting loops in a close-fitting array. Simulations show excellent shim performance with arrays of single-turn loops using < 3A of current per loop [2,4]. RF-shim integration has thus far been demonstrated in individual [2] and a pair of coils [3,4]. In this work we e...
Fig. 2. A: Signal–time plots for several contrasts in a single voxel. Min, max, and TI values are... more Fig. 2. A: Signal–time plots for several contrasts in a single voxel. Min, max, and TI values are shown, gray shading illustrates the stimulation periods. B: Data and IR model fits for two time points indicated by color. C: Baseline (3 time points preceding each simulation block) and activation (3 tp before control) signal for measured data, mean ± std. 5786 Multi-contrast inversion-recovery EPI (MI-EPI) functional MRI at 7 T Ville Renvall, Thomas Witzel, Marta Bianciardi, and Jonathan R. Polimeni Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, Department of Radiology, Harvard Medical School, Boston, MA, United States
In this HIPAA-compliant institutional review board–approved prospective study in which all subjec... more In this HIPAA-compliant institutional review board–approved prospective study in which all subjects provided written informed consent, six patients with relapsing-remitting MS and six healthy control subjects underwent diffusion-weighted imaging with a range of diffusion weightings performed with a 3-T human MR imager by using gradient strengths of up to 300 mT/m. A model of intraaxonal, extra-axonal, and free water diffusion was fitted to obtain estimates of axon diameter and density. Differences in axon diameter and density between lesions and NAWM in patients with MS were assessed by using the nonparametric Wilcoxon matched-pairs signed rank test, and differences between NAWM in subjects with MS and white matter in healthy control subjects were assessed by using the Mann-Whitney U test. Results: MS lesions showed increased mean axon diameter (10.3 vs 7.9 mm in the genu, 10.4 vs 9.3 mm in the body, and 10.6 vs 8.2 mm in the splenium; P , .05) and decreased axon density ([0.48 vs 1...
Envisioning the healthcare technology of tomorrow requires one to break from the mainstream and e... more Envisioning the healthcare technology of tomorrow requires one to break from the mainstream and explore alternative approaches to develop accessible, ubiquitous diagnostic tools. In the field of magnetic resonance imaging (MRI), despite considerable improvements in imaging quality and speed, the underlying technology remains remarkably unchanged compared to the first generation scanners that emerged on the market 30 years ago. Undeniably, one of the next revolutions in health care is cost-effectiveness. Using only simple and robust hardware technologies and state-of-the-art acquisition and processing strategies, low-cost scanners could democratize MRI, moving it away from demanding siting requirements and colossal costs, and opening up a wide range of unprecedented new applications. We believe our work will enable the realization of an inexpensive portable MRI system for use in a variety of situations where MRI systems are not traditionally available such as resource-poor environmen...
Accurate and automated reconstruction of the in vivo human cerebral cortical surface from anatomi... more Accurate and automated reconstruction of the in vivo human cerebral cortical surface from anatomical magnetic resonance (MR) images facilitates the quantitative analysis of cortical structure. Anatomical MR images with sub-millimeter isotropic spatial resolution improve the accuracy of cortical surface and thickness estimation compared to the standard 1-millimeter isotropic resolution. Nonetheless, sub-millimeter resolution acquisitions require averaging multiple repetitions to achieve sufficient signal-to-noise ratio and are therefore long and potentially vulnerable to subject motion. We address this challenge by synthesizing sub-millimeter resolution images from standard 1-millimeter isotropic resolution images using a data-driven supervised machine learning-based super-resolution approach achieved via a deep convolutional neural network. We systematically characterize our approach using a large-scale simulated dataset and demonstrate its efficacy in empirical data. The super-reso...
The first phase of the Human Connectome Project pioneered advances in MRI technology for mapping ... more The first phase of the Human Connectome Project pioneered advances in MRI technology for mapping the macroscopic structural connections of the living human brain through the engineering of a whole-body human MRI scanner equipped with maximum gradient strength of 300 mT/m, the highest ever achieved for human imaging. While this instrument has made important contributions to the understanding of macroscale connectional topology, it has also demonstrated the potential of dedicated high-gradient performance scanners to provide unparalleled in vivo assessment of neural tissue microstructure. Building on the initial groundwork laid by the original Connectome scanner, we have now embarked on an international, multi-site effort to build the next-generation human 3T Connectome scanner (Connectome 2.0) optimized for the study of neural tissue microstructure and connectional anatomy across multiple length scales. In order to maximize the resolution of this in vivo microscope for studies of the living human brain, we will push the diffusion resolution limit to unprecedented levels by (1) nearly doubling the current maximum gradient strength from 300 mT/m to 500 mT/m and tripling the maximum slew rate from 200 T/m/s to 600 T/m/s through the design of a one-of-a-kind head gradient coil optimized to minimize peripheral nerve stimulation; (2) developing high-sensitivity multi-channel radiofrequency receive coils for in vivo and ex vivo human brain imaging; (3) incorporating dynamic field monitoring to minimize image distortions and artifacts; (4) developing new pulse sequences to integrate the strongest diffusion-encoding and highest spatial-resolution ever achieved in the living human brain; and (5) calibrating the measurements obtained from this next-generation instrument through systematic validation of diffusion microstructural metrics in high-fidelity phantoms and ex vivo brain tissue at progressively finer scales with accompanying diffusion simulations in histology-based micro-geometries. We envision creating the ultimate diffusion MRI instrument capable of capturing the complex multi-scale organization of the living human brain - from the microscopic scale needed to probe cellular geometry, heterogeneity and plasticity, to the mesoscopic scale for quantifying the distinctions in cortical structure and connectivity that define cyto- and myeloarchitectonic boundaries, to improvements in estimates of macroscopic connectivity.
Purpose: To develop a phantom for validating MRI pulse sequences and data processing methods to q... more Purpose: To develop a phantom for validating MRI pulse sequences and data processing methods to quantify microscopic diffusion anisotropy in the human brain. Methods: Using a liquid crystal consisting of water, detergent, and hydrocarbon, we designed a 0.5-L spherical phantom showing the theoretically highest possible degree of microscopic anisotropy. Data were acquired on the Connectome scanner using echo-planar imaging signal readout and diffusion encoding with axisymmetric b-tensors of varying magnitude, anisotropy, and orientation. The mean diffusivity, fractional anisotropy (FA), and microscopic FA (mFA) parameters were estimated. Results: The phantom was observed to have values of mean diffusivity similar to brain tissue, and relaxation times compatible with echo-planar imaging echo times on the order of 100 ms. The estimated values of mFA were at the theoretical maximum of 1.0, whereas the values of FA spanned the interval from 0.0 to 0.8 as a result of varying orientational order of the anisotropic domains within each voxel. Conclusions: The proposed phantom can be manufactured by mixing three widely available chemicals in volumes comparable to a human head. The acquired data are in excellent agreement with theoretical predictions, showing that the phantom is ideal for validating methods for measuring microscopic diffusion anisotropy on clinical MRI systems. Magn Reson
Accurate spatial alignment of MRI data acquired across multiple contrasts in the same subject is ... more Accurate spatial alignment of MRI data acquired across multiple contrasts in the same subject is often crucial for data analysis and interpretation, but can be challenging in the presence of geometric distortions that differ between acquisitions. It is well known that single-shot echo-planar imaging (EPI) acquisitions suffer from distortion in the phase-encoding direction due to B0 field inhomogeneities arising from tissue magnetic susceptibility differences and other sources, however there can be distortion in other encoding directions as well in the presence of strong field homogeneities. High-resolution ultrahigh-field MRI typically uses low bandwidth in the slice-encoding direction to acquire thin slices and, when combined with the pronounced B0 inhomogeneities, is prone to an additional geometric distortion in the slice direction as well. Here we demonstrate a presence of this slice distortion in high-resolution 7T EPI acquired with a novel pulse sequence allowing for the rever...
Objectives: We aim to develop molecular fibrin imaging as a tool to stratify high risk atheroscle... more Objectives: We aim to develop molecular fibrin imaging as a tool to stratify high risk atherosclerotic carotid plaques. Here, we describe our results with a fibrin-specific probe [ 68 Ga]CM-246 in two different experimental settings: ex vivo imaging of carotid endarterectomy specimens and in vivo / ex vivo imaging of atherosclerotic plaque rupture in rabbits. Methods: We incubated discarded endarterectomy specimens (n=11) from asymptomatic patients with [ 68 Ga] CM-246 or a scrambled peptide control probe and measured uptake by autoradiography and a tissue binding assay. In a rabbit plaque rupture model of high cholesterol diet followed by abdominal aorta balloon injury (n=13) and subsequent pharmacological plaque rupture, in vivo and ex vivo PET-MR with [ 68 Ga]CM-246 was performed. Tissues were processed for autoradiography and histology to verify presence of fibrin and plaque rupture. Results: Autoradiography (A-B) and tissue binding assays (C) with [ 68 Ga]CM-246 with human caro...
Imaging Bo Zhu, Thomas Witzel, Shan Jiang, Daniel G Anderson, Robert S Langer, Bruce R Rosen, and... more Imaging Bo Zhu, Thomas Witzel, Shan Jiang, Daniel G Anderson, Robert S Langer, Bruce R Rosen, and Lawrence L Wald Department of Radiology, Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, Harvard-MIT Health Sciences and Technololgy, Massachusetts Institute of Technology, Cambridge, MA, United States, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, United States
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
BACKGROUND AND PURPOSE: The development of new MR imaging scanners with stronger gradients and im... more BACKGROUND AND PURPOSE: The development of new MR imaging scanners with stronger gradients and improvement in coil technology, allied with emerging fast imaging techniques, has allowed a substantial reduction in MR imaging scan times. Our goal was to develop a 10-minute gadolinium-enhanced brain MR imaging protocol with accelerated sequences and to evaluate its diagnostic performance compared with the standard clinical protocol. MATERIALS AND METHODS: Fifty-three patients referred for brain MR imaging with contrast were scanned with a 3T scanner. Each MR image consisted of 5 basic fast precontrast sequences plus standard and accelerated versions of the same postcontrast T1WI sequences. Two neuroradiologists assessed the image quality and the final diagnosis for each set of postcontrast sequences and compared their performances. RESULTS: The acquisition time of the combined accelerated pre-and postcontrast sequences was 10 minutes and 15 seconds; and of the fast postcontrast sequences, 3 minutes and 36 seconds, 46% of the standard sequences. The 10-minute postcontrast axial T1WI had fewer image artifacts (P Ͻ .001) and better overall diagnostic quality (P Ͻ .001). Although the 10-minute MPRAGE sequence showed a tendency to have more artifacts than the standard sequence (P ϭ .08), the overall diagnostic quality was similar (P ϭ .66). Moreover, there was no statistically significant difference in the diagnostic performance between the protocols. The sensitivity, specificity, and accuracy values for the 10-minute protocol were 100.0%, 88.9%, and 98.1%. CONCLUSIONS: The 10-minute brain MR imaging protocol with contrast is comparable in diagnostic performance with the standard protocol in an inpatient motion-prone population, with the additional benefits of reducing acquisition times and image artifacts. ABBREVIATIONS: GRAPPA ϭ generalized autocalibrating partially parallel acquisition; GRE ϭ gradient-echo; SE ϭ spin-echo
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
PURPOSE To propose a virtual coil (VC) acquisition/reconstruction framework to improve highly acc... more PURPOSE To propose a virtual coil (VC) acquisition/reconstruction framework to improve highly accelerated single-shot EPI (SS-EPI) and generalized slice dithered enhanced resolution (gSlider) acquisition in high-resolution diffusion imaging (DI). METHODS For robust VC-GRAPPA reconstruction, a background phase correction scheme was developed to match the image phase of the reference data with the corrupted phase of the accelerated diffusion-weighted data, where the corrupted phase of the diffusion data varies from shot to shot. A Gy prewinding-blip was also added to the EPI acquisition, to create a shifted-ky sampling strategy that allows for better exploitation of VC concept in the reconstruction. To evaluate the performance of the proposed methods, 1.5 mm isotropic whole-brain SS-EPI and 860 μm isotropic whole-brain gSlider-EPI diffusion data were acquired at an acceleration of 8-9 fold. Conventional and VC-GRAPPA reconstructions were performed and compared, and corresponding g-factors were calculated. RESULTS The proposed VC reconstruction substantially improves the image quality of both SS-EPI and gSlider-EPI, with reduced g-factor noise and reconstruction artifacts when compared to the conventional method. This has enabled high-quality low-noise diffusion imaging to be performed at 8-9 fold acceleration. CONCLUSIONS The proposed VC acquisition/reconstruction framework improves the reconstruction of DI at high accelerations. The ability to now employ such high accelerations will allow DI with EPI at reduced distortion and faster scan time, which should be beneficial for many clinical and neuroscience applications.
Using simultaneous acquisition from multiple channels of a radio-frequency (RF) coil array, magne... more Using simultaneous acquisition from multiple channels of a radio-frequency (RF) coil array, magnetic resonance inverse imaging (InI) achieves functional MRI acquisitions at a rate of 100 ms per whole-brain volume [1]. InI accelerates the scan by leaving out partition encoding steps and reconstructs images by solving under-determined inverse problems using RF coil sensitivity information. Hence, the correlated spatial information available in the coil array causes spatial blurring in the InI reconstruction. Here, we propose a method that employs gradient blips in the partition encoding direction during the acquisition to provide extra spatial encoding in order to better differentiate signals from different partitions.
PURPOSE To test an integrated "AC/DC" array approach at 7T, where B0 inhomogeneity pose... more PURPOSE To test an integrated "AC/DC" array approach at 7T, where B0 inhomogeneity poses an obstacle for functional imaging, diffusion-weighted MRI, MR spectroscopy, and other applications. METHODS A close-fitting 7T 31-channel (31-ch) brain array was constructed and tested using combined Rx and ΔB0 shim channels driven by a set of rapidly switchable current amplifiers. The coil was compared to a shape-matched 31-ch reference receive-only array for RF safety, signal-to-noise ratio (SNR), and inter-element noise correlation. We characterize the coil array's ability to provide global and dynamic (slice-optimized) shimming using ΔB0 field maps and echo planar imaging (EPI) acquisitions. RESULTS The SNR and average noise correlation were similar to the 31-ch reference array. Global and slice-optimized shimming provide 11% and 40% improvements respectively compared to baseline second-order spherical harmonic shimming. Birdcage transmit coil efficiency was similar for the reference and AC/DC array setups. CONCLUSION Adding ΔB0 shim capability to a 31-ch 7T receive array can significantly boost 7T brain B0 homogeneity without sacrificing the array's rdiofrequency performance, potentially improving ultra-high field neuroimaging applications that are vulnerable to off-resonance effects.
PURPOSE: Multicoil (MC) shimming shows great promise as an alternative to spherical harmonic shim... more PURPOSE: Multicoil (MC) shimming shows great promise as an alternative to spherical harmonic shimming for compensating higherorder B0 inhomogeneity in vivo. Previous realizations [1] left space near the body for RF coils, pushing MC shim loops further away, reducing their efficiency. The shim loops also caused modest SNR loss due to their proximity. Recently it has been shown that shim currents can flow on single-turn RF coil loops without compromising the function of either subsystem [2-4]. Both RF reception and MC shimming benefit from (a.) close proximity to the body and (b.) maximal spatial degrees of freedom (large coil arrays). This suggests the most space-efficient design is to let DC and RF share the same conducting loops in a close-fitting array. Simulations show excellent shim performance with arrays of single-turn loops using < 3A of current per loop [2,4]. RF-shim integration has thus far been demonstrated in individual [2] and a pair of coils [3,4]. In this work we e...
Fig. 2. A: Signal–time plots for several contrasts in a single voxel. Min, max, and TI values are... more Fig. 2. A: Signal–time plots for several contrasts in a single voxel. Min, max, and TI values are shown, gray shading illustrates the stimulation periods. B: Data and IR model fits for two time points indicated by color. C: Baseline (3 time points preceding each simulation block) and activation (3 tp before control) signal for measured data, mean ± std. 5786 Multi-contrast inversion-recovery EPI (MI-EPI) functional MRI at 7 T Ville Renvall, Thomas Witzel, Marta Bianciardi, and Jonathan R. Polimeni Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, Department of Radiology, Harvard Medical School, Boston, MA, United States
In this HIPAA-compliant institutional review board–approved prospective study in which all subjec... more In this HIPAA-compliant institutional review board–approved prospective study in which all subjects provided written informed consent, six patients with relapsing-remitting MS and six healthy control subjects underwent diffusion-weighted imaging with a range of diffusion weightings performed with a 3-T human MR imager by using gradient strengths of up to 300 mT/m. A model of intraaxonal, extra-axonal, and free water diffusion was fitted to obtain estimates of axon diameter and density. Differences in axon diameter and density between lesions and NAWM in patients with MS were assessed by using the nonparametric Wilcoxon matched-pairs signed rank test, and differences between NAWM in subjects with MS and white matter in healthy control subjects were assessed by using the Mann-Whitney U test. Results: MS lesions showed increased mean axon diameter (10.3 vs 7.9 mm in the genu, 10.4 vs 9.3 mm in the body, and 10.6 vs 8.2 mm in the splenium; P , .05) and decreased axon density ([0.48 vs 1...
Envisioning the healthcare technology of tomorrow requires one to break from the mainstream and e... more Envisioning the healthcare technology of tomorrow requires one to break from the mainstream and explore alternative approaches to develop accessible, ubiquitous diagnostic tools. In the field of magnetic resonance imaging (MRI), despite considerable improvements in imaging quality and speed, the underlying technology remains remarkably unchanged compared to the first generation scanners that emerged on the market 30 years ago. Undeniably, one of the next revolutions in health care is cost-effectiveness. Using only simple and robust hardware technologies and state-of-the-art acquisition and processing strategies, low-cost scanners could democratize MRI, moving it away from demanding siting requirements and colossal costs, and opening up a wide range of unprecedented new applications. We believe our work will enable the realization of an inexpensive portable MRI system for use in a variety of situations where MRI systems are not traditionally available such as resource-poor environmen...
Accurate and automated reconstruction of the in vivo human cerebral cortical surface from anatomi... more Accurate and automated reconstruction of the in vivo human cerebral cortical surface from anatomical magnetic resonance (MR) images facilitates the quantitative analysis of cortical structure. Anatomical MR images with sub-millimeter isotropic spatial resolution improve the accuracy of cortical surface and thickness estimation compared to the standard 1-millimeter isotropic resolution. Nonetheless, sub-millimeter resolution acquisitions require averaging multiple repetitions to achieve sufficient signal-to-noise ratio and are therefore long and potentially vulnerable to subject motion. We address this challenge by synthesizing sub-millimeter resolution images from standard 1-millimeter isotropic resolution images using a data-driven supervised machine learning-based super-resolution approach achieved via a deep convolutional neural network. We systematically characterize our approach using a large-scale simulated dataset and demonstrate its efficacy in empirical data. The super-reso...
The first phase of the Human Connectome Project pioneered advances in MRI technology for mapping ... more The first phase of the Human Connectome Project pioneered advances in MRI technology for mapping the macroscopic structural connections of the living human brain through the engineering of a whole-body human MRI scanner equipped with maximum gradient strength of 300 mT/m, the highest ever achieved for human imaging. While this instrument has made important contributions to the understanding of macroscale connectional topology, it has also demonstrated the potential of dedicated high-gradient performance scanners to provide unparalleled in vivo assessment of neural tissue microstructure. Building on the initial groundwork laid by the original Connectome scanner, we have now embarked on an international, multi-site effort to build the next-generation human 3T Connectome scanner (Connectome 2.0) optimized for the study of neural tissue microstructure and connectional anatomy across multiple length scales. In order to maximize the resolution of this in vivo microscope for studies of the living human brain, we will push the diffusion resolution limit to unprecedented levels by (1) nearly doubling the current maximum gradient strength from 300 mT/m to 500 mT/m and tripling the maximum slew rate from 200 T/m/s to 600 T/m/s through the design of a one-of-a-kind head gradient coil optimized to minimize peripheral nerve stimulation; (2) developing high-sensitivity multi-channel radiofrequency receive coils for in vivo and ex vivo human brain imaging; (3) incorporating dynamic field monitoring to minimize image distortions and artifacts; (4) developing new pulse sequences to integrate the strongest diffusion-encoding and highest spatial-resolution ever achieved in the living human brain; and (5) calibrating the measurements obtained from this next-generation instrument through systematic validation of diffusion microstructural metrics in high-fidelity phantoms and ex vivo brain tissue at progressively finer scales with accompanying diffusion simulations in histology-based micro-geometries. We envision creating the ultimate diffusion MRI instrument capable of capturing the complex multi-scale organization of the living human brain - from the microscopic scale needed to probe cellular geometry, heterogeneity and plasticity, to the mesoscopic scale for quantifying the distinctions in cortical structure and connectivity that define cyto- and myeloarchitectonic boundaries, to improvements in estimates of macroscopic connectivity.
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Papers by Thomas Witzel