In MR Imaging the signals arising from the excited nuclei yield the information of the spatial sp... more In MR Imaging the signals arising from the excited nuclei yield the information of the spatial spin distribution in form of a (2d or 3d) interferogram. Therefore the Fourier transform of the sampled data directly results in an image. It will only be free of artifacts if the MR machine creates ideal raw data. In reality we cannot expect ideal behavior of the hardware, so corrections of the measured data are necessary. The effect of the following raw data nonidealities on the image are shown: 1. Offset caused by bias voltages of the data aquisition system. 2. Phase and amplitude deviations of the quadrature demodulator. 3. Nonequidistancy of the sampled data pOints caused by gradient ripple. To reduce or eliminate the resulting artifacts, appropriate preprocessing steps are introduced to the reconstruction program. The image quality improvements are demonstrated by experiment or simulation.
Ralph Kimmlingen, Eva Eberlein, Peter Dietz, Sabrina Kreher, Johann Schuster, Jörg Riegler, Volke... more Ralph Kimmlingen, Eva Eberlein, Peter Dietz, Sabrina Kreher, Johann Schuster, Jörg Riegler, Volker Matschl, Volker Schnetter, Andreas Schmidt, Helmut Lenz, Ernst Mustafa, Daniel Fischer, Andreas Potthast, Ludwig Kreischer , Michael Eberler, Franz Hebrank, Herbert Thein, Keith Heberlein, Philipp Hoecht, Thomas Witzel, Dylan Tisdall, Junqian Xu, Essa Yacoub, Gregor Adriany, Edward Auerbach, Steen Moeller, David Feinberg, Dietmar Lehne, Lawrence L. Wald, Bruce Rosen, Kamil Ugurbil, David van Essen, Van Wedeen, and Franz Schmitt Siemens Healthcare, Erlangen, Germany, Martinos Center for Biomedical Imaging, Dept. of Radiology, Massachusetts General Hospital, Boston, United States, Center for Magnetic Resonance Research, University Minnesota, Minneapolis, United States, Helen Wills Inst. of Neurosc., UC Berkeley, CA, United States, Harvard-MIT Division of Health Sciences Technology, Cambridge, United States, Dept. of Anatomy and Neurobiology, Washington U, St. Louis, United States
Page 1. CHAPTER 4 ASPECTS OF CLINICAL IMAGING AT 7 T Franz Schmitt1, Andreas Potthast1, Bernd Sto... more Page 1. CHAPTER 4 ASPECTS OF CLINICAL IMAGING AT 7 T Franz Schmitt1, Andreas Potthast1, Bernd Stoeckel3, Christina Triantafyllou2, Christopher J. Wiggins2, Graham Wiggins2, and Lawrence L. Wald2 1 Siemens Medical Solution, Dept. ...
Ultra-high-field MRI (7 Tesla (T) and above) elicits more temporary side-effects compared to 1.5 ... more Ultra-high-field MRI (7 Tesla (T) and above) elicits more temporary side-effects compared to 1.5 T and 3 T, e.g. dizziness or ''postural instability'' even after exiting the scanner. The current study aims to assess quantitatively vestibular performance before and after exposure to different MRI scenarios at 7 T, 1.5 T and 0 T. Sway path and body axis rotation (Unterberger's stepping test) were quantitatively recorded in a total of 46 volunteers before, 2 minutes after, and 15 minutes after different exposure scenarios: 7 T head MRI (n = 27), 7 T no RF (n = 22), 7 T only B 0 (n = 20), 7 T in & out B 0 (n = 20), 1.5 T no RF (n = 20), 0 T (n = 15). All exposure scenarios lasted 30 minutes except for brief one minute exposure in 7 T in & out B 0 . Both measures were documented utilizing a 3D ultrasound system. During sway path evaluation, the experiment was repeated with eyes both open and closed. Sway paths for all long-lasting 7 T scenarios (normal, no RF, only B 0 ) with eyes closed were significantly prolonged 2 minutes after exiting the scanner, normalizing after 15 minutes. Brief exposure to 7 T B 0 or 30 minutes exposure to 1.5 T or 0 T did not show significant changes. End positions after Unterberger's stepping test were significantly changed counter-clockwise after all 7 T scenarios, including the brief in & out B 0 exposure. Shorter exposure resulted in a smaller alteration angle. In contrast to sway path, reversal of changes in body axis rotation was incomplete after 15 minutes. 1.5 T caused no rotational changes. The results show that exposure to the 7 Tesla static magnetic field causes only a temporary dysfunction or ''over-compensation'' of the vestibular system not measurable at 1.5 or 0 Tesla. Radiofrequency fields, gradient switching, and orthostatic dysregulation do not seem to play a role.
Echo-planar imaging (EPI) plays a crucial role in functional MRI. Focusing especially on the peri... more Echo-planar imaging (EPI) plays a crucial role in functional MRI. Focusing especially on the period from 1988 to 1992, the authors offer personal recollections, on the development of practical means of deploying EPI, the people that participated, and its impact on MRI in general.
Perhaps more than any other "-omics" endeavor, the accuracy and level of detail obtained from map... more Perhaps more than any other "-omics" endeavor, the accuracy and level of detail obtained from mapping the major connection pathways in the living human brain with diffusion MRI depend on the capabilities of the imaging technology used. The current tools are remarkable; allowing the formation of an "image" of the water diffusion probability distribution in regions of complex crossing fibers at each of half a million voxels in the brain. Nonetheless our ability to map the connection pathways is limited by the image sensitivity and resolution, and also the contrast and resolution in encoding of the diffusion probability distribution. The goal of our Human Connectome Project (HCP) is to address these limiting factors by re-engineering the scanner from the ground up to optimize the high b-value, high angular resolution diffusion imaging needed for sensitive and accurate mapping of the brain's structural connections. Our efforts were directed based on the relative contributions of each scanner component. The gradient subsection was a major focus since gradient amplitude is central to determining the diffusion contrast, the amount of T 2 signal loss, and the blurring of the water PDF over the course of the diffusion time. By implementing a novel 4-port drive geometry and optimizing size and linearity for the brain, we demonstrate a whole-body sized scanner with G max = 300 mT/m on each axis capable of the sustained duty cycle needed for diffusion imaging. The system is capable of slewing the gradient at a rate of 200 T/m/s as needed for the EPI image encoding. In order to enhance the efficiency of the diffusion sequence we implemented a FOV shifting approach to Simultaneous MultiSlice (SMS) EPI capable of unaliasing 3 slices excited simultaneously with a modest g-factor penalty allowing us to diffusion encode whole brain volumes with low TR and TE. Finally we combine the multi-slice approach with a compressive sampling reconstruction to sufficiently undersample q-space to achieve a DSI scan in less than 5 min. To augment this accelerated imaging approach we developed a 64-channel, tight-fitting brain array coil and show its performance benefit compared to a commercial 32-channel coil at all locations in the brain for these accelerated acquisitions. The technical challenges of developing the over-all system are discussed as well as results from SNR comparisons, ODF metrics and fiber tracking comparisons. The ultra-high gradients yielded substantial and immediate gains in the sensitivity through reduction of TE and improved signal detection and increased efficiency of the DSI or HARDI acquisition, accuracy and resolution of diffusion tractography, as defined by identification of known structure and fiber crossing.
Peripheral nerve stimulation limits the use of whole-body gradient systems capable of slew rates ... more Peripheral nerve stimulation limits the use of whole-body gradient systems capable of slew rates > 80 T/m/s and gradient strengths > 25 mT/m. The stimulation threshold depends mainly on the amplitude of the induced electric field in the patient's body, and thus can be influenced by changing the total magnetic flux of the gradient coil. A gradient system was built which allows continuous variation of the field characteristics in order to permit the use of full gradient performance without stimulation (slew rate 190 -210 T/m/s, G max 32-40 mT/m). The system consists of a modular six-channel gradient coil designed with a modified target field method, two three-channel amplifiers, and a six-channel gradient controller. It is demonstrated that two coils on one gradient axis can be driven by two amplifiers in parallel, without significant changes in image quality. Scaling of the field properties and stimulation threshold according to the current polarity and ratio of both coil sets was verified in both phantom and volunteer studies. Magn Reson Med 47:800 -808, 2002.
Purpose: The aim of this study was to evaluate our preliminary experience at 3.0 T with imaging o... more Purpose: The aim of this study was to evaluate our preliminary experience at 3.0 T with imaging of the carotid bifurcation in healthy and atherosclerotic subjects. Application at 3.0 T is motivated by the signal-to-noise gain for improving spatial resolution and reducing signal averaging requirements. Materials and methods: We utilized a dual phased array coil and applied 2D, 3D time of flight (TOF) and turbo spin echo (TSE) sequences with comparison of two lumen signal suppression methods for black blood (BB) TSE imaging including double inversion preparation (DIR) and spatial presaturation pulses. The signal-to-noise ratios (SNR) of healthy carotid vessel walls were compared in 2D and 3D BB TSE acquisitions. The bright and black blood multi-contrast exam was demonstrated for a complex carotid plaque. Results: Contrast-to-noise (CNR) greater than 150 was achieved between the lumen and suppressed background for 3D TOF. For BB, both methods provided sufficient lumen signal suppression but slight residual flow artifacts remained at the bifurcation level. As expected 3D TSE images had higher SNR compared to 2D, but increased motion sensitivity is a significant issue for 3D at high field. For multi-contrast imaging of atherosclerotic plaque, fibrous, calcified and lipid components were resolved. The CNR ratio of fibrous (bright on PDW, T2W) and calcified (dark in T1W, T2W, PDW) plaque components was maximal in the T2W images. The 3D TOF angiogram indicating a 40% stenosis was complemented by 3D multi-planar reformat of BB images that displayed plaque extent. Detection of intimal thickening, the earliest change associated with atherosclerotic progression was observed in BB PDW images at 3.0 T. Conclusions: High SNR and CNR images have been demonstrated for the healthy and diseased carotid. Improvements in RF coils along with pulse sequence optimization, and evaluation of endogenous and exogenous contrast mechanisms will further enhance carotid imaging at 3.0 T.
Ultra High Field (UHF) MRI requires improved gradient and shim performance to fully realize the p... more Ultra High Field (UHF) MRI requires improved gradient and shim performance to fully realize the promised gains (SNR as well as spatial, spectral, diffusion resolution) that higher main magnetic fields offer. Both the more challenging UHF environment by itself, as well as the higher currents used in high performance coils, require a deeper understanding combined with sophisticated engineering modeling and construction, to optimize gradient and shim hardware for safe operation and for highest image quality. This review summarizes the basics of gradient and shim technologies, and outlines a number of UHF-related challenges and solutions. In particular, Lorentz forces, vibroacoustics, eddy currents, and peripheral nerve stimulation are discussed. Several promising UHF-relevant gradient concepts are described, including insertable gradient coils aimed at higher performance neuroimaging.
Starting with postwar developments in nuclear magnetic resonance (NMR) a race for stronger and st... more Starting with postwar developments in nuclear magnetic resonance (NMR) a race for stronger and stronger magnetic fields has begun in the 1950s to overcome the inherently low sensitivity of this promising method. Further challenges were larger magnet bores to accommodate small animals and eventually humans. Initially, resistive electromagnets with small pole distances, or sample volumes, and field strengths up to 2.35 T (or 100 MHz 1 H frequency) were used in applications in physics, chemistry, and material science. This was followed by stronger and more stable (Nb-Ti based) superconducting magnet technology typically implemented first for small-bore systems in analytical chemistry, biochemistry and structural biology, and eventually allowing larger horizontal-bore magnets with diameters large enough to fit small laboratory animals. By the end of the 1970s, first low-field resistive magnets big enough to accommodate humans were developed and superconducting whole-body systems followed. Currently, cutting-edge analytical NMR systems are available at proton frequencies up to 1 GHz (23.5 T) based on Nb 3 Sn at 1.9 K. A new 1.2 GHz system (28 T) at 1.9 K, operating in persistent mode but using a combination of low and high temperature multi-filament superconductors is to be released. Preclinical instruments range from small-bore animal systems with typically 600-800 MHz (14.1-18.8 T) up to 900 MHz (21 T) at 1.9 K. Human whole-body MRI systems currently operate up to 10.5 T. Hybrid combined superconducting and resistive electromagnets with even higher field strength of 45 T dc and 100 T pulsed, are available for material research, of course with smaller free bore diameters. This rather costly development toward higher and higher field strength is a consequence of the inherently low and, thus, urgently needed sensitivity in all NMR experiments. This review particularly describes and compares the developments in superconducting magnet technology and, thus, sensitivity in three Moser et al. UHF MR-Magnet Technology fields of research: analytical NMR, biomedical and preclinical research, and human MRI and MRS, highlighting important steps and innovations. In addition, we summarize our knowledge on safety issues. An outlook into even stronger magnetic fields using different superconducting materials and/or hybrid magnet designs is presented.
Magnetic Resonance Imaging Clinics of North America, 1999
Since its commercial introduction as a diagnostic tool in 1984, MR imaging has undergone dramatic... more Since its commercial introduction as a diagnostic tool in 1984, MR imaging has undergone dramatic improvements in all the features that define image quality. This article discusses what a modern MR imaging scanner should look like and how the components of an MR system, especially the magnet, gradient, and RF components must be matched with one another to ensure optimal performance.
In clinical brain MR imaging protocols, the technician collects a quick localizer and manually po... more In clinical brain MR imaging protocols, the technician collects a quick localizer and manually positions the subsequent scans using the localizer as a guide. We present a method for automatic slice positioning using a rapidly acquired 3D localizer. The localizer is automatically aligned to a statistical atlas representing 40 healthy subjects. The atlas contains the probability of a given tissue type occurring at a given location in atlas space and the conditional probability distribution of the multi-spectral MRI intensity values for a given tissue class. Accurate rigid alignment of each subject to an atlas ensures that all patients' scans are acquired in a consistent manner. A further benefit is that slices are positioned consistently over time, so that scans of patients returning for follow-up imaging can be compared side-by-side to accurately monitor the progression of illness. The procedure also helps ensure that left/right asymmetries reflect true anatomy rather than being the result of oblique slice positioning relative to the underlying anatomy. The use of an atlas-based procedure eliminates the need to refer to a database of previously scanned images of the same patient and ensures corresponding alignment across scanners and sites, without requiring fiducial markers. Since the registration method is probabilistic, the registration error tends to increase smoothly in the presence of increasing noise and unusual anatomy or pathology rather than failing catastrophically. Translations and rotations relative to the atlas can be set so that planning can be done in anatomical space, rather than scanner coordinates, and stored as part of the protocol allowing standardization of slice orientations.
Longitudinal and multi-site clinical studies create the imperative to characterize and correct te... more Longitudinal and multi-site clinical studies create the imperative to characterize and correct technological sources of variance that limit image reproducibility in high-resolution structural MRI studies, thus facilitating precise, quantitative, platform-independent, multi-site evaluation. In this work, we investigated the effects that imaging gradient non-linearity have on reproducibility of multi-site human MRI. We applied an image distortion correction method based on spherical harmonics description of the gradients and verified the accuracy of the method using phantom data. The correction method was then applied to the brain image data from a group of subjects scanned twice at multiple sites having different 1.5 T platforms. Within-site and across-site variability of the image data was assessed by evaluating voxel-based image intensity reproducibility. The image intensity reproducibility of the human brain data was significantly improved with distortion correction, suggesting that this method may offer improved reproducibility in morphometry studies. We provide the source code for the gradient distortion algorithm together with the phantom data.
In MR Imaging the signals arising from the excited nuclei yield the information of the spatial sp... more In MR Imaging the signals arising from the excited nuclei yield the information of the spatial spin distribution in form of a (2d or 3d) interferogram. Therefore the Fourier transform of the sampled data directly results in an image. It will only be free of artifacts if the MR machine creates ideal raw data. In reality we cannot expect ideal behavior of the hardware, so corrections of the measured data are necessary. The effect of the following raw data nonidealities on the image are shown: 1. Offset caused by bias voltages of the data aquisition system. 2. Phase and amplitude deviations of the quadrature demodulator. 3. Nonequidistancy of the sampled data pOints caused by gradient ripple. To reduce or eliminate the resulting artifacts, appropriate preprocessing steps are introduced to the reconstruction program. The image quality improvements are demonstrated by experiment or simulation.
Ralph Kimmlingen, Eva Eberlein, Peter Dietz, Sabrina Kreher, Johann Schuster, Jörg Riegler, Volke... more Ralph Kimmlingen, Eva Eberlein, Peter Dietz, Sabrina Kreher, Johann Schuster, Jörg Riegler, Volker Matschl, Volker Schnetter, Andreas Schmidt, Helmut Lenz, Ernst Mustafa, Daniel Fischer, Andreas Potthast, Ludwig Kreischer , Michael Eberler, Franz Hebrank, Herbert Thein, Keith Heberlein, Philipp Hoecht, Thomas Witzel, Dylan Tisdall, Junqian Xu, Essa Yacoub, Gregor Adriany, Edward Auerbach, Steen Moeller, David Feinberg, Dietmar Lehne, Lawrence L. Wald, Bruce Rosen, Kamil Ugurbil, David van Essen, Van Wedeen, and Franz Schmitt Siemens Healthcare, Erlangen, Germany, Martinos Center for Biomedical Imaging, Dept. of Radiology, Massachusetts General Hospital, Boston, United States, Center for Magnetic Resonance Research, University Minnesota, Minneapolis, United States, Helen Wills Inst. of Neurosc., UC Berkeley, CA, United States, Harvard-MIT Division of Health Sciences Technology, Cambridge, United States, Dept. of Anatomy and Neurobiology, Washington U, St. Louis, United States
Page 1. CHAPTER 4 ASPECTS OF CLINICAL IMAGING AT 7 T Franz Schmitt1, Andreas Potthast1, Bernd Sto... more Page 1. CHAPTER 4 ASPECTS OF CLINICAL IMAGING AT 7 T Franz Schmitt1, Andreas Potthast1, Bernd Stoeckel3, Christina Triantafyllou2, Christopher J. Wiggins2, Graham Wiggins2, and Lawrence L. Wald2 1 Siemens Medical Solution, Dept. ...
Ultra-high-field MRI (7 Tesla (T) and above) elicits more temporary side-effects compared to 1.5 ... more Ultra-high-field MRI (7 Tesla (T) and above) elicits more temporary side-effects compared to 1.5 T and 3 T, e.g. dizziness or ''postural instability'' even after exiting the scanner. The current study aims to assess quantitatively vestibular performance before and after exposure to different MRI scenarios at 7 T, 1.5 T and 0 T. Sway path and body axis rotation (Unterberger's stepping test) were quantitatively recorded in a total of 46 volunteers before, 2 minutes after, and 15 minutes after different exposure scenarios: 7 T head MRI (n = 27), 7 T no RF (n = 22), 7 T only B 0 (n = 20), 7 T in & out B 0 (n = 20), 1.5 T no RF (n = 20), 0 T (n = 15). All exposure scenarios lasted 30 minutes except for brief one minute exposure in 7 T in & out B 0 . Both measures were documented utilizing a 3D ultrasound system. During sway path evaluation, the experiment was repeated with eyes both open and closed. Sway paths for all long-lasting 7 T scenarios (normal, no RF, only B 0 ) with eyes closed were significantly prolonged 2 minutes after exiting the scanner, normalizing after 15 minutes. Brief exposure to 7 T B 0 or 30 minutes exposure to 1.5 T or 0 T did not show significant changes. End positions after Unterberger's stepping test were significantly changed counter-clockwise after all 7 T scenarios, including the brief in & out B 0 exposure. Shorter exposure resulted in a smaller alteration angle. In contrast to sway path, reversal of changes in body axis rotation was incomplete after 15 minutes. 1.5 T caused no rotational changes. The results show that exposure to the 7 Tesla static magnetic field causes only a temporary dysfunction or ''over-compensation'' of the vestibular system not measurable at 1.5 or 0 Tesla. Radiofrequency fields, gradient switching, and orthostatic dysregulation do not seem to play a role.
Echo-planar imaging (EPI) plays a crucial role in functional MRI. Focusing especially on the peri... more Echo-planar imaging (EPI) plays a crucial role in functional MRI. Focusing especially on the period from 1988 to 1992, the authors offer personal recollections, on the development of practical means of deploying EPI, the people that participated, and its impact on MRI in general.
Perhaps more than any other "-omics" endeavor, the accuracy and level of detail obtained from map... more Perhaps more than any other "-omics" endeavor, the accuracy and level of detail obtained from mapping the major connection pathways in the living human brain with diffusion MRI depend on the capabilities of the imaging technology used. The current tools are remarkable; allowing the formation of an "image" of the water diffusion probability distribution in regions of complex crossing fibers at each of half a million voxels in the brain. Nonetheless our ability to map the connection pathways is limited by the image sensitivity and resolution, and also the contrast and resolution in encoding of the diffusion probability distribution. The goal of our Human Connectome Project (HCP) is to address these limiting factors by re-engineering the scanner from the ground up to optimize the high b-value, high angular resolution diffusion imaging needed for sensitive and accurate mapping of the brain's structural connections. Our efforts were directed based on the relative contributions of each scanner component. The gradient subsection was a major focus since gradient amplitude is central to determining the diffusion contrast, the amount of T 2 signal loss, and the blurring of the water PDF over the course of the diffusion time. By implementing a novel 4-port drive geometry and optimizing size and linearity for the brain, we demonstrate a whole-body sized scanner with G max = 300 mT/m on each axis capable of the sustained duty cycle needed for diffusion imaging. The system is capable of slewing the gradient at a rate of 200 T/m/s as needed for the EPI image encoding. In order to enhance the efficiency of the diffusion sequence we implemented a FOV shifting approach to Simultaneous MultiSlice (SMS) EPI capable of unaliasing 3 slices excited simultaneously with a modest g-factor penalty allowing us to diffusion encode whole brain volumes with low TR and TE. Finally we combine the multi-slice approach with a compressive sampling reconstruction to sufficiently undersample q-space to achieve a DSI scan in less than 5 min. To augment this accelerated imaging approach we developed a 64-channel, tight-fitting brain array coil and show its performance benefit compared to a commercial 32-channel coil at all locations in the brain for these accelerated acquisitions. The technical challenges of developing the over-all system are discussed as well as results from SNR comparisons, ODF metrics and fiber tracking comparisons. The ultra-high gradients yielded substantial and immediate gains in the sensitivity through reduction of TE and improved signal detection and increased efficiency of the DSI or HARDI acquisition, accuracy and resolution of diffusion tractography, as defined by identification of known structure and fiber crossing.
Peripheral nerve stimulation limits the use of whole-body gradient systems capable of slew rates ... more Peripheral nerve stimulation limits the use of whole-body gradient systems capable of slew rates > 80 T/m/s and gradient strengths > 25 mT/m. The stimulation threshold depends mainly on the amplitude of the induced electric field in the patient's body, and thus can be influenced by changing the total magnetic flux of the gradient coil. A gradient system was built which allows continuous variation of the field characteristics in order to permit the use of full gradient performance without stimulation (slew rate 190 -210 T/m/s, G max 32-40 mT/m). The system consists of a modular six-channel gradient coil designed with a modified target field method, two three-channel amplifiers, and a six-channel gradient controller. It is demonstrated that two coils on one gradient axis can be driven by two amplifiers in parallel, without significant changes in image quality. Scaling of the field properties and stimulation threshold according to the current polarity and ratio of both coil sets was verified in both phantom and volunteer studies. Magn Reson Med 47:800 -808, 2002.
Purpose: The aim of this study was to evaluate our preliminary experience at 3.0 T with imaging o... more Purpose: The aim of this study was to evaluate our preliminary experience at 3.0 T with imaging of the carotid bifurcation in healthy and atherosclerotic subjects. Application at 3.0 T is motivated by the signal-to-noise gain for improving spatial resolution and reducing signal averaging requirements. Materials and methods: We utilized a dual phased array coil and applied 2D, 3D time of flight (TOF) and turbo spin echo (TSE) sequences with comparison of two lumen signal suppression methods for black blood (BB) TSE imaging including double inversion preparation (DIR) and spatial presaturation pulses. The signal-to-noise ratios (SNR) of healthy carotid vessel walls were compared in 2D and 3D BB TSE acquisitions. The bright and black blood multi-contrast exam was demonstrated for a complex carotid plaque. Results: Contrast-to-noise (CNR) greater than 150 was achieved between the lumen and suppressed background for 3D TOF. For BB, both methods provided sufficient lumen signal suppression but slight residual flow artifacts remained at the bifurcation level. As expected 3D TSE images had higher SNR compared to 2D, but increased motion sensitivity is a significant issue for 3D at high field. For multi-contrast imaging of atherosclerotic plaque, fibrous, calcified and lipid components were resolved. The CNR ratio of fibrous (bright on PDW, T2W) and calcified (dark in T1W, T2W, PDW) plaque components was maximal in the T2W images. The 3D TOF angiogram indicating a 40% stenosis was complemented by 3D multi-planar reformat of BB images that displayed plaque extent. Detection of intimal thickening, the earliest change associated with atherosclerotic progression was observed in BB PDW images at 3.0 T. Conclusions: High SNR and CNR images have been demonstrated for the healthy and diseased carotid. Improvements in RF coils along with pulse sequence optimization, and evaluation of endogenous and exogenous contrast mechanisms will further enhance carotid imaging at 3.0 T.
Ultra High Field (UHF) MRI requires improved gradient and shim performance to fully realize the p... more Ultra High Field (UHF) MRI requires improved gradient and shim performance to fully realize the promised gains (SNR as well as spatial, spectral, diffusion resolution) that higher main magnetic fields offer. Both the more challenging UHF environment by itself, as well as the higher currents used in high performance coils, require a deeper understanding combined with sophisticated engineering modeling and construction, to optimize gradient and shim hardware for safe operation and for highest image quality. This review summarizes the basics of gradient and shim technologies, and outlines a number of UHF-related challenges and solutions. In particular, Lorentz forces, vibroacoustics, eddy currents, and peripheral nerve stimulation are discussed. Several promising UHF-relevant gradient concepts are described, including insertable gradient coils aimed at higher performance neuroimaging.
Starting with postwar developments in nuclear magnetic resonance (NMR) a race for stronger and st... more Starting with postwar developments in nuclear magnetic resonance (NMR) a race for stronger and stronger magnetic fields has begun in the 1950s to overcome the inherently low sensitivity of this promising method. Further challenges were larger magnet bores to accommodate small animals and eventually humans. Initially, resistive electromagnets with small pole distances, or sample volumes, and field strengths up to 2.35 T (or 100 MHz 1 H frequency) were used in applications in physics, chemistry, and material science. This was followed by stronger and more stable (Nb-Ti based) superconducting magnet technology typically implemented first for small-bore systems in analytical chemistry, biochemistry and structural biology, and eventually allowing larger horizontal-bore magnets with diameters large enough to fit small laboratory animals. By the end of the 1970s, first low-field resistive magnets big enough to accommodate humans were developed and superconducting whole-body systems followed. Currently, cutting-edge analytical NMR systems are available at proton frequencies up to 1 GHz (23.5 T) based on Nb 3 Sn at 1.9 K. A new 1.2 GHz system (28 T) at 1.9 K, operating in persistent mode but using a combination of low and high temperature multi-filament superconductors is to be released. Preclinical instruments range from small-bore animal systems with typically 600-800 MHz (14.1-18.8 T) up to 900 MHz (21 T) at 1.9 K. Human whole-body MRI systems currently operate up to 10.5 T. Hybrid combined superconducting and resistive electromagnets with even higher field strength of 45 T dc and 100 T pulsed, are available for material research, of course with smaller free bore diameters. This rather costly development toward higher and higher field strength is a consequence of the inherently low and, thus, urgently needed sensitivity in all NMR experiments. This review particularly describes and compares the developments in superconducting magnet technology and, thus, sensitivity in three Moser et al. UHF MR-Magnet Technology fields of research: analytical NMR, biomedical and preclinical research, and human MRI and MRS, highlighting important steps and innovations. In addition, we summarize our knowledge on safety issues. An outlook into even stronger magnetic fields using different superconducting materials and/or hybrid magnet designs is presented.
Magnetic Resonance Imaging Clinics of North America, 1999
Since its commercial introduction as a diagnostic tool in 1984, MR imaging has undergone dramatic... more Since its commercial introduction as a diagnostic tool in 1984, MR imaging has undergone dramatic improvements in all the features that define image quality. This article discusses what a modern MR imaging scanner should look like and how the components of an MR system, especially the magnet, gradient, and RF components must be matched with one another to ensure optimal performance.
In clinical brain MR imaging protocols, the technician collects a quick localizer and manually po... more In clinical brain MR imaging protocols, the technician collects a quick localizer and manually positions the subsequent scans using the localizer as a guide. We present a method for automatic slice positioning using a rapidly acquired 3D localizer. The localizer is automatically aligned to a statistical atlas representing 40 healthy subjects. The atlas contains the probability of a given tissue type occurring at a given location in atlas space and the conditional probability distribution of the multi-spectral MRI intensity values for a given tissue class. Accurate rigid alignment of each subject to an atlas ensures that all patients' scans are acquired in a consistent manner. A further benefit is that slices are positioned consistently over time, so that scans of patients returning for follow-up imaging can be compared side-by-side to accurately monitor the progression of illness. The procedure also helps ensure that left/right asymmetries reflect true anatomy rather than being the result of oblique slice positioning relative to the underlying anatomy. The use of an atlas-based procedure eliminates the need to refer to a database of previously scanned images of the same patient and ensures corresponding alignment across scanners and sites, without requiring fiducial markers. Since the registration method is probabilistic, the registration error tends to increase smoothly in the presence of increasing noise and unusual anatomy or pathology rather than failing catastrophically. Translations and rotations relative to the atlas can be set so that planning can be done in anatomical space, rather than scanner coordinates, and stored as part of the protocol allowing standardization of slice orientations.
Longitudinal and multi-site clinical studies create the imperative to characterize and correct te... more Longitudinal and multi-site clinical studies create the imperative to characterize and correct technological sources of variance that limit image reproducibility in high-resolution structural MRI studies, thus facilitating precise, quantitative, platform-independent, multi-site evaluation. In this work, we investigated the effects that imaging gradient non-linearity have on reproducibility of multi-site human MRI. We applied an image distortion correction method based on spherical harmonics description of the gradients and verified the accuracy of the method using phantom data. The correction method was then applied to the brain image data from a group of subjects scanned twice at multiple sites having different 1.5 T platforms. Within-site and across-site variability of the image data was assessed by evaluating voxel-based image intensity reproducibility. The image intensity reproducibility of the human brain data was significantly improved with distortion correction, suggesting that this method may offer improved reproducibility in morphometry studies. We provide the source code for the gradient distortion algorithm together with the phantom data.
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