Introduction: While post mortem serial sectioning of the brain offers exquisite spatial resolutio... more Introduction: While post mortem serial sectioning of the brain offers exquisite spatial resolution and histological detail, tearing and distortions between cryotome sections renders volumetric analysis of large volumes (such as a brain hemisphere) impractical. MRI of ex vivo samples provides 3D encoding at resolutions capable of resolving laminar and myeloarchitectonic features as small as a few hundred neurons [1]. To achieve the highest spatial resolutions (<100 μm isotropic), ex vivo imaging of brain hemispheres typically requires many hours of averaging, yet even higher resolution is desirable. Typically, however, a standard clinical in vivo RF coil such as a head array or knee coil is pressed into service. Large arrays of small surface coils are ideal for ex vivo imaging since they can be placed very near the sample, but a specifically designed array geometry and sealable sample container is needed.
Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, Jan 17, 2015
We add user-controllable direct currents (DC) to the individual elements of a 32-channel radio-fr... more We add user-controllable direct currents (DC) to the individual elements of a 32-channel radio-frequency (RF) receive array to provide B0 shimming ability while preserving the array's reception sensitivity and parallel imaging performance. Shim performance using constrained DC current (±2.5A) is simulated for brain arrays ranging from 8 to 128 elements. A 32-channel 3-tesla brain array is realized using inductive chokes to bridge the tuning capacitors on each RF loop. The RF and B0 shimming performance is assessed in bench and imaging measurements. The addition of DC currents to the 32-channel RF array is achieved with minimal disruption of the RF performance and/or negative side effects such as conductor heating or mechanical torques. The shimming results agree well with simulations and show performance superior to third-order spherical harmonic (SH) shimming. Imaging tests show the ability to reduce the standard frontal lobe susceptibility-induced fields and improve echo plana...
Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2011
In this paper we report a method to automatically segment the internal part of globus pallidus (G... more In this paper we report a method to automatically segment the internal part of globus pallidus (GPi) on the pre-operative low-resolution magnetic resonance images (MRIs) of patients affected by Parkinson's disease. Herein we used an ultra-high resolution human brain dataset as electronic atlas of reference on which we segmented the GPi. First, we registered the ultra-high resolution dataset on the low-resolution dataset using a landmarks-based rigid registration. Then an affine and a non-rigid surface-based registration guided by the structures that surround the target was applied in order to propagate the labels of the GPi on the low-resolution un-segmented dataset and to accurately outline the target. The mapping of the atlas on the low-resolution MRI provided a highly accurate anatomical detail that can be useful for localizing the target.
The macaque monkey is an important model for cognitive and sensory neuroscience that has been use... more The macaque monkey is an important model for cognitive and sensory neuroscience that has been used extensively in behavioral, electrophysiological, molecular and, more recently, neuroimaging studies. However, macaque MRI has unique technical differences relative to human MRI, such as the geometry of highly parallel receive arrays, which must be addressed to optimize imaging performance. A 22-channel receive coil array was constructed specifically for rapid high-resolution anesthetized macaque monkey MRI at 3 T. A local Helmholtz transmit coil was used for excitation. Signal-to-noise ratios (SNRs) and noise amplification for parallel imaging were compared with those of single- and four-channel receive coils routinely used for macaque MRI. The 22-channel coil yielded significant improvements in SNR throughout the brain. Using this coil, the SNR in peripheral brain was 2.4 and 1.7 times greater than that obtained with single- or four-channel coils, respectively. In the central brain, the SNR gain was 1.5 times that of both the single- and four-channel coils. Finally, the performance of the array for functional, anatomical and diffusion-weighted imaging was evaluated. For all three modalities, the use of the 22-channel array allowed for high-resolution and accelerated image acquisition.
Size-optimized 32-channel receive array coils were developed for five age groups, neonates, 6 mon... more Size-optimized 32-channel receive array coils were developed for five age groups, neonates, 6 months old, 1 year old, 4 years old, and 7 years old, and evaluated for pediatric brain imaging. The array consisted of overlapping circular surface coils laid out on a close-fitting coil-former. The two-section coil former design was obtained from surface contours of aligned three-dimensional MRI scans of each age group. Signal-to-noise ratio and noise amplification for parallel imaging were evaluated and compared to two coils routinely used for pediatric brain imaging; a commercially available 32-channel adult head coil and a pediatric-sized birdcage coil. Phantom measurements using the neonate, 6-month-old, 1-year-old, 4year-old, and 7-year-old coils showed signal-to-noise ratio increases at all locations within the brain over the comparison coils. Within the brain cortex the five dedicated pediatric arrays increased signal-to-noise ratio by up to 3.6-, 3.0-, 2.6-, 2.3-, and 1.7-fold, respectively, compared to the 32-channel adult coil, as well as improved G-factor maps for accelerated imaging. This study suggests that a size-tailored approach can provide significant sensitivity gains for accelerated and unaccelerated pediatric brain imaging.
To design and validate a radiofrequency (RF) array coil for cervical spinal cord imaging at 7T. A... more To design and validate a radiofrequency (RF) array coil for cervical spinal cord imaging at 7T. A 19-channel receive array with a four-channel transmit array was developed on a close-fitting coil former at 7T. Transmit efficiency and specific absorption rate were evaluated in a B1 (+) mapping study and an electromagnetic model. Receive signal-to-noise ratio (SNR) and noise amplification for parallel imaging were evaluated and compared with a commercial 3T 19-channel head-neck array and a 7T four-channel spine array. The performance of the array was qualitatively demonstrated in human volunteers using high-resolution imaging (down to 300 μm in-plane). The transmit and receive arrays showed good bench performance. The SNR was approximately 4.2-fold higher in the 7T receive array at the location of the cord with respect to the 3T coil. The g-factor results showed an additional acceleration was possible with the 7T array. In vivo imaging was feasible and showed high SNR and tissue contrast. The highly parallel transmit and receive arrays were demonstrated to be fit for spinal cord imaging at 7T. The high sensitivity of the receive coil combined with ultra-high field will likely improve investigations of microstructure and tissue segmentation in the healthy and pathological spinal cord.
Introduction: MR systems for whole-body at 3T and for body or head imaging at fields of 7T or hig... more Introduction: MR systems for whole-body at 3T and for body or head imaging at fields of 7T or higher suffer from significant B 1 + inhomogeneity when conventional birdcage-like volume coils are used for excitation. Through the use of transmit arrays, and techniques such as RF shimming [1] or accelerated spatially tailored RF pulses [2,3], it is possible to mitigate high field B 1 + inhomogeneity, or indeed to prescribe almost any pattern of excitation that is desired. For full control of the excitation profile through the volume of the head it is necessary to have transmit element profiles which vary along the Z-direction, as well as in the axial plane. With most multichannel transmit systems having only 8 separate channels it is a challenge to arrange coils such that there is good brain coverage and variation along the Z direction. While many transmit array designs are built on large cylindrical formers which keep the coil elements a significant distance away from the subject, in part motivated by the desire to avoid creating SAR hotspots, such designs can not be expected to have comparable receive sensitivity compared to a close fitting receive array. It is possible to create a large detunable transmit array and place inside of it a close-fitting receive array, but this entails a high degree of complexity. We propose a design with 8 loop elements tightly coupled to the head, good decoupling between elements during transmit, B 1 + variation along Z, and good receive sensitivity with whole-brain coverage.
Purpose: A technique is described for simultaneous multislice (SMS) excitation using radiofrequen... more Purpose: A technique is described for simultaneous multislice (SMS) excitation using radiofrequency (RF) parallel transmission (pTX). Methods: Spatially distinct slices are simultaneously excited by applying different RF frequencies on groups of elements of a multichannel transmit array. The localized transmit sensitivities of the coil geometry are thereby exploited to reduce RF power. The method is capable of achieving SMS-excitation using single-slice RF pulses, or multiband pulses. SMS-pTX is demonstrated using eight-channel parallel RF transmission on a dual-ring pTX coil at 3 T. The effect on B 1 þ homogeneity and specific absorption rate (SAR) is evaluated experimentally and by simulations. Slice-GRAPPA reconstruction was used for separation of the collapsed slice signals. Results: Phantom and in vivo brain data acquired with fast low-angle shot (FLASH) and blipped-controlled aliasing results in higher acceleration (CAIPIRINHA) echo-planar imaging are presented at SMS excitation factors of two, four, and six. We also show that with our pTX coil design, slice placement, and binary division of transmitters, SMS-pTX excitations can achieve the same mean flip angles excitations at $30% lower RF power than a conventional SMS approach with multiband RF pulses. Conclusion: The proposed SMS-pTX allows SMS excitations at reduced RF power by exploiting the local B 1 þ sensitivities of suitable multielement pTX arrays. Magn Reson Med 000:000-000,
Introduction: While post mortem serial sectioning of the brain offers exquisite spatial resolutio... more Introduction: While post mortem serial sectioning of the brain offers exquisite spatial resolution and histological detail, tearing and distortions between cryotome sections renders volumetric analysis of large volumes (such as a brain hemisphere) impractical. MRI of ex vivo samples provides 3D encoding at resolutions capable of resolving laminar and myeloarchitectonic features as small as a few hundred neurons [1]. To achieve the highest spatial resolutions (<100 μm isotropic), ex vivo imaging of brain hemispheres typically requires many hours of averaging, yet even higher resolution is desirable. Typically, however, a standard clinical in vivo RF coil such as a head array or knee coil is pressed into service. Large arrays of small surface coils are ideal for ex vivo imaging since they can be placed very near the sample, but a specifically designed array geometry and sealable sample container is needed.
Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, Jan 17, 2015
We add user-controllable direct currents (DC) to the individual elements of a 32-channel radio-fr... more We add user-controllable direct currents (DC) to the individual elements of a 32-channel radio-frequency (RF) receive array to provide B0 shimming ability while preserving the array's reception sensitivity and parallel imaging performance. Shim performance using constrained DC current (±2.5A) is simulated for brain arrays ranging from 8 to 128 elements. A 32-channel 3-tesla brain array is realized using inductive chokes to bridge the tuning capacitors on each RF loop. The RF and B0 shimming performance is assessed in bench and imaging measurements. The addition of DC currents to the 32-channel RF array is achieved with minimal disruption of the RF performance and/or negative side effects such as conductor heating or mechanical torques. The shimming results agree well with simulations and show performance superior to third-order spherical harmonic (SH) shimming. Imaging tests show the ability to reduce the standard frontal lobe susceptibility-induced fields and improve echo plana...
Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2011
In this paper we report a method to automatically segment the internal part of globus pallidus (G... more In this paper we report a method to automatically segment the internal part of globus pallidus (GPi) on the pre-operative low-resolution magnetic resonance images (MRIs) of patients affected by Parkinson's disease. Herein we used an ultra-high resolution human brain dataset as electronic atlas of reference on which we segmented the GPi. First, we registered the ultra-high resolution dataset on the low-resolution dataset using a landmarks-based rigid registration. Then an affine and a non-rigid surface-based registration guided by the structures that surround the target was applied in order to propagate the labels of the GPi on the low-resolution un-segmented dataset and to accurately outline the target. The mapping of the atlas on the low-resolution MRI provided a highly accurate anatomical detail that can be useful for localizing the target.
The macaque monkey is an important model for cognitive and sensory neuroscience that has been use... more The macaque monkey is an important model for cognitive and sensory neuroscience that has been used extensively in behavioral, electrophysiological, molecular and, more recently, neuroimaging studies. However, macaque MRI has unique technical differences relative to human MRI, such as the geometry of highly parallel receive arrays, which must be addressed to optimize imaging performance. A 22-channel receive coil array was constructed specifically for rapid high-resolution anesthetized macaque monkey MRI at 3 T. A local Helmholtz transmit coil was used for excitation. Signal-to-noise ratios (SNRs) and noise amplification for parallel imaging were compared with those of single- and four-channel receive coils routinely used for macaque MRI. The 22-channel coil yielded significant improvements in SNR throughout the brain. Using this coil, the SNR in peripheral brain was 2.4 and 1.7 times greater than that obtained with single- or four-channel coils, respectively. In the central brain, the SNR gain was 1.5 times that of both the single- and four-channel coils. Finally, the performance of the array for functional, anatomical and diffusion-weighted imaging was evaluated. For all three modalities, the use of the 22-channel array allowed for high-resolution and accelerated image acquisition.
Size-optimized 32-channel receive array coils were developed for five age groups, neonates, 6 mon... more Size-optimized 32-channel receive array coils were developed for five age groups, neonates, 6 months old, 1 year old, 4 years old, and 7 years old, and evaluated for pediatric brain imaging. The array consisted of overlapping circular surface coils laid out on a close-fitting coil-former. The two-section coil former design was obtained from surface contours of aligned three-dimensional MRI scans of each age group. Signal-to-noise ratio and noise amplification for parallel imaging were evaluated and compared to two coils routinely used for pediatric brain imaging; a commercially available 32-channel adult head coil and a pediatric-sized birdcage coil. Phantom measurements using the neonate, 6-month-old, 1-year-old, 4year-old, and 7-year-old coils showed signal-to-noise ratio increases at all locations within the brain over the comparison coils. Within the brain cortex the five dedicated pediatric arrays increased signal-to-noise ratio by up to 3.6-, 3.0-, 2.6-, 2.3-, and 1.7-fold, respectively, compared to the 32-channel adult coil, as well as improved G-factor maps for accelerated imaging. This study suggests that a size-tailored approach can provide significant sensitivity gains for accelerated and unaccelerated pediatric brain imaging.
To design and validate a radiofrequency (RF) array coil for cervical spinal cord imaging at 7T. A... more To design and validate a radiofrequency (RF) array coil for cervical spinal cord imaging at 7T. A 19-channel receive array with a four-channel transmit array was developed on a close-fitting coil former at 7T. Transmit efficiency and specific absorption rate were evaluated in a B1 (+) mapping study and an electromagnetic model. Receive signal-to-noise ratio (SNR) and noise amplification for parallel imaging were evaluated and compared with a commercial 3T 19-channel head-neck array and a 7T four-channel spine array. The performance of the array was qualitatively demonstrated in human volunteers using high-resolution imaging (down to 300 μm in-plane). The transmit and receive arrays showed good bench performance. The SNR was approximately 4.2-fold higher in the 7T receive array at the location of the cord with respect to the 3T coil. The g-factor results showed an additional acceleration was possible with the 7T array. In vivo imaging was feasible and showed high SNR and tissue contrast. The highly parallel transmit and receive arrays were demonstrated to be fit for spinal cord imaging at 7T. The high sensitivity of the receive coil combined with ultra-high field will likely improve investigations of microstructure and tissue segmentation in the healthy and pathological spinal cord.
Introduction: MR systems for whole-body at 3T and for body or head imaging at fields of 7T or hig... more Introduction: MR systems for whole-body at 3T and for body or head imaging at fields of 7T or higher suffer from significant B 1 + inhomogeneity when conventional birdcage-like volume coils are used for excitation. Through the use of transmit arrays, and techniques such as RF shimming [1] or accelerated spatially tailored RF pulses [2,3], it is possible to mitigate high field B 1 + inhomogeneity, or indeed to prescribe almost any pattern of excitation that is desired. For full control of the excitation profile through the volume of the head it is necessary to have transmit element profiles which vary along the Z-direction, as well as in the axial plane. With most multichannel transmit systems having only 8 separate channels it is a challenge to arrange coils such that there is good brain coverage and variation along the Z direction. While many transmit array designs are built on large cylindrical formers which keep the coil elements a significant distance away from the subject, in part motivated by the desire to avoid creating SAR hotspots, such designs can not be expected to have comparable receive sensitivity compared to a close fitting receive array. It is possible to create a large detunable transmit array and place inside of it a close-fitting receive array, but this entails a high degree of complexity. We propose a design with 8 loop elements tightly coupled to the head, good decoupling between elements during transmit, B 1 + variation along Z, and good receive sensitivity with whole-brain coverage.
Purpose: A technique is described for simultaneous multislice (SMS) excitation using radiofrequen... more Purpose: A technique is described for simultaneous multislice (SMS) excitation using radiofrequency (RF) parallel transmission (pTX). Methods: Spatially distinct slices are simultaneously excited by applying different RF frequencies on groups of elements of a multichannel transmit array. The localized transmit sensitivities of the coil geometry are thereby exploited to reduce RF power. The method is capable of achieving SMS-excitation using single-slice RF pulses, or multiband pulses. SMS-pTX is demonstrated using eight-channel parallel RF transmission on a dual-ring pTX coil at 3 T. The effect on B 1 þ homogeneity and specific absorption rate (SAR) is evaluated experimentally and by simulations. Slice-GRAPPA reconstruction was used for separation of the collapsed slice signals. Results: Phantom and in vivo brain data acquired with fast low-angle shot (FLASH) and blipped-controlled aliasing results in higher acceleration (CAIPIRINHA) echo-planar imaging are presented at SMS excitation factors of two, four, and six. We also show that with our pTX coil design, slice placement, and binary division of transmitters, SMS-pTX excitations can achieve the same mean flip angles excitations at $30% lower RF power than a conventional SMS approach with multiband RF pulses. Conclusion: The proposed SMS-pTX allows SMS excitations at reduced RF power by exploiting the local B 1 þ sensitivities of suitable multielement pTX arrays. Magn Reson Med 000:000-000,
Uploads
Papers by A. Mareyam