Whole-body Gradient Research Articles

Experimental validation of a PNS-optimized whole-body gradient coil.

Peripheral nerve stimulation (PNS) limits the usability of state-of-the-art whole-body and head-only MRI gradient coils. We used detailed electromagnetic and neurodynamic modeling to set an explicit PNS constraint during the design of a whole-body gradient coil and constructed it to compare the predicted and experimentally measured PNS thresholds to those of a matched design without PNS constraints. We designed, constructed, and tested two actively shielded whole-body Y-axis gradient coil winding patterns: YG1 is a conventional symmetric design without PNS-optimization, whereas YG2's design used an additional constraint on the allowable PNS threshold in the head-imaging landmark, yielding an asymmetric winding pattern. We measured PNS thresholds in 18 healthy subjects at five landmark positions (head, cardiac, abdominal, pelvic, and knee). The PNS-optimized design YG2 achieved 46% higher average experimental thresholds for a head-imaging landmark than YG1 while incurring a 15% inductance penalty. For cardiac, pelvic, and knee imaging landmarks, the PNS thresholds increased between +22% and +35%. For abdominal imaging, PNS thresholds did not change significantly between YG1 and YG2 (-3.6%). The agreement between predicted and experimental PNS thresholds was within 11.4% normalized root mean square error for both coils and all landmarks. The PNS model also produced plausible predictions of the stimulation sites when compared to the sites of perception reported by the subjects. The PNS-optimization improved the PNS thresholds for the target scan landmark as well as most other studied landmarks, potentially yielding a significant improvement in image encoding performance that can be safely used in humans.

The possible influence of third-order shim coils on gradient–magnet interactions: an inter-field and inter-site study

ObjectiveTo assess the possible influence of third-order shim coils on the behavior of the gradient field and in gradient–magnet interactions at 7 T and above.Materials and methodsGradient impulse response function measurements were performed at 5 sites spanning field strengths from 7 to 11.7 T, all of them sharing the same exact whole-body gradient coil design. Mechanical fixation and boundary conditions of the gradient coil were altered in several ways at one site to study the impact of mechanical coupling with the magnet on the field perturbations. Vibrations, power deposition in the He bath, and field dynamics were characterized at 11.7 T with the third-order shim coils connected and disconnected inside the Faraday cage.ResultsFor the same whole-body gradient coil design, all measurements differed greatly based on the third-order shim coil configuration (connected or not). Vibrations and gradient transfer function peaks could be affected by a factor of 2 or more, depending on the resonances. Disconnecting the third-order shim coils at 11.7 T also suppressed almost completely power deposition peaks at some frequencies.DiscussionThird-order shim coil configurations can have major impact in gradient–magnet interactions with consequences on potential hardware damage, magnet heating, and image quality going beyond EPI acquisitions.

Minimization of eddy power loss in the cryostat for a z-gradient array coil driven by an arbitrary pulse sequence: An electromagnetic approach.

This paper presents a novel computational approach to optimize gradient array performance for a given pulse sequence. Specifically, we propose an electromagnetic (EM) approach that minimizes eddy losses within the cryostat while maintaining key performance parameters such as field linearity, gradient strength, and imaging region's dimension and position. High-resolution EM simulations on the cryostat's surface are deployed to compute the net EM fields generated by each element of a gradient array coil at different frequencies. The computed fields are stored and combined for each frequency to form a quadratic vector-matrix-vector computation. The overall time-average eddy power loss within the cryostat assembly for arbitrary pulse sequences is computed using frequency domain superposition. The proposed approach estimates and regulates eddy power losses within the cryostat assembly. When compared to the stray field minimization approach, it can achieve over twice the reduction in eddy power loss. The proposed approach eliminates the need to sample the stray fields on the cryostat surface, which the number and position of the samples would be challenging when designing tunable array coils with capabilities that disrupt field symmetries. Additionally, the loss calculation considers the entire cryostat assembly rather than just the inner cylindrical surface of the warm shield. Our findings highlight the efficacy of an on-the-fly tuning method for the development of high-performance whole-body gradient array coils, effectively mitigating eddy losses within the cryostat and minimizing stray fields outside the coil assembly. This approach proves particularly advantageous for array coils with variable feeding currents.

Mapping the human connectome using diffusion MRI at 300 mT/m gradient strength: Methodological advances and scientific impact

Tremendous efforts have been made in the last decade to advance cutting-edge MRI technology in pursuit of mapping structural connectivity in the living human brain with unprecedented sensitivity and speed. The first Connectom 3T MRI scanner equipped with a 300 mT/m whole-body gradient system was installed at the Massachusetts General Hospital in 2011 and was specifically constructed as part of the Human Connectome Project. Since that time, numerous technological advances have been made to enable the broader use of the Connectom high gradient system for diffusion tractography and tissue microstructure studies and leverage its unique advantages and sensitivity to resolving macroscopic and microscopic structural information in neural tissue for clinical and neuroscientific studies. The goal of this review article is to summarize the technical developments that have emerged in the last decade to support and promote large-scale and scientific studies of the human brain using the Connectom scanner. We provide a brief historical perspective on the development of Connectom gradient technology and the efforts that led to the installation of three other Connectom 3T MRI scanners worldwide – one in the United Kingdom in Cardiff, Wales, another in continental Europe in Leipzig, Germany, and the latest in Asia in Shanghai, China. We summarize the key developments in gradient hardware and image acquisition technology that have formed the backbone of Connectom-related research efforts, including the rich array of high-sensitivity receiver coils, pulse sequences, image artifact correction strategies and data preprocessing methods needed to optimize the quality of high-gradient strength diffusion MRI data for subsequent analyses. Finally, we review the scientific impact of the Connectom MRI scanner, including advances in diffusion tractography, tissue microstructural imaging, ex vivo validation, and clinical investigations that have been enabled by Connectom technology. We conclude with brief insights into the unique value of strong gradients for diffusion MRI and where the field is headed in the coming years.

Highly efficient head-only magnetic field insert gradient coil for achieving simultaneous high gradient amplitude and slew rate at 3.0T (MAGNUS) for brain microstructure imaging.

To develop a highly efficient magnetic field gradient coil for head imaging that achieves 200 mT/m and 500 T/m/s on each axis using a standard 1 MVA gradient driver in clinical whole-body 3.0T MR magnet. A 42-cm inner diameter head-gradient used the available 89- to 91-cm warm bore space in a whole-body 3.0T magnet by increasing the radial separation between the primary and the shield coil windings to 18.6 cm. This required the removal of the standard whole-body gradient and radiofrequency coils. To achieve a coil efficiency ~4× that of whole-body gradients, a double-layer primary coil design with asymmetric x-y axes, and symmetric z-axis was used. The use of all-hollow conductor with direct fluid cooling of the gradient coil enabled ≥50 kW of total heat dissipation. This design achieved a coil efficiency of 0.32 mT/m/A, allowing 200 mT/m and 500 T/m/s for a 620 A/1500 V driver. The gradient coil yielded substantially reduced echo spacing, and minimum repetition time and echo time. In high b = 10,000 s/mm2 diffusion, echo time(TE) < 50 ms was achieved (>50% reduction compared with whole-body gradients). The gradient coil passed the American College of Radiology tests for gradient linearity and distortion, and met acoustic requirements for nonsignificant risk operation. Ultra-high gradient coil performance was achieved for head imaging without substantial increases in gradient driver power in a whole-body 3.0T magnet after removing the standard gradient coil. As such, any clinical whole-body 3.0T MR system could be upgraded with 3-4× improvement in gradient performance for brain imaging.

Parametric linear vibration response studies for longitudinal whole-body gradient coils: Theoretical predictions based on an exact linear elastodynamic model

Passive reduction of gradient coil (GC) cylinder vibration depends critically on a thorough knowledge of how all pertinent physical parameters affect the vibration response. In this paper, we employ a recently introduced linear elastodynamic Z-coil model to study how the displacement response of a whole-body GC cylinder (subject to exclusive excitation of its Z-coil windings) is affected by independent regularized variations in its: (i) length; (ii) radial thickness; (iii) mass density; (iv) Poisson ratio; and (v) Young modulus (stiffness). The results exhibit a rich variety of behaviors at different excitation frequencies, and in the parameter ranges of interest, the displacement response is found to be particularly sensitive to variations in cylinder geometry and mass density. The results also show that, with the exception of the stiffness, there are no optimal ranges of regularized values of the considered parameters that will reduce the displacement (and hence the vibration) of a GC cylinder at all frequencies of interest. For typical GC cylinder geometries and densities, and under the condition that only the Z-coil windings are excited, the model predicts that increasing the cylinder stiffness above 100 GPa will reduce vibration at all frequencies below 2000 Hz.

A Whole-Body Sensory-Motor Gradient is Revealed in the Medial Wall of the Parietal Lobe.

In 1954, Penfield and Jasper's findings based on electric stimulation of epileptic patients led them to hypothesize that a sensory representation of the body should be found in the precuneus. They termed this representation the "supplementary sensory" area and emphasized that the exact form of this homunculus could not be specified on the basis of their results. In the decades that followed, their prediction was neglected. The precuneus was found to be involved in numerous motor, cognitive and visual processes, but no work was done on its somatotopic organization. Here, we used a periodic experimental design in which 16 human subjects (eight women) moved 20 body parts to investigate the possible body part topography of the precuneus. We found an anterior-to-posterior, dorsal-to-ventral, toes-to-tongue gradient in a mirror orientation to the SMA. When inspecting body-part-specific functional connectivity, we found differential connectivity patterns for the different body parts to the primary and secondary motor areas and parietal and visual areas, and a shared connectivity to the extrastriate body area, another topographically organized area. We suggest that a whole-body gradient can be found in the precuneus and is connected to multiple brain areas with different connectivity for different body parts. Its exact role and relations to the other known functions of the precuneus such as self-processing, motor imagery, reaching, visuomotor and other body-mind functions should be investigated.SIGNIFICANCE STATEMENT Using fMRI, as well as sensitive spectral analysis, we found a new homunculus in the precuneus: an anterior-to-posterior, dorsal-to-ventral, toes-to-tongue somatotopic gradient in a mirror orientation to the SMA. When inspecting body-part-specific functional connectivity, we found differential connectivity patterns for the different body parts to the primary and secondary motor areas, parietal and visual areas, and a shared connectivity to the extrastriate body area, another topographically organized area. We suggest that a whole-body gradient can be found in the precuneus and is connected to multiple brain areas in a body-part-specific manner.

Separation of water and fat signal in whole-body gradient echo scans using convolutional neural networks.

PurposeTo perform and evaluate water–fat signal separation of whole‐body gradient echo scans using convolutional neural networks.MethodsWhole‐body gradient echo scans of 240 subjects, each consisting of 5 bipolar echoes, were used. Reference fat fraction maps were created using a conventional method. Convolutional neural networks, more specifically 2D U‐nets, were trained using 5‐fold cross‐validation with 1 or several echoes as input, using the squared difference between the output and the reference fat fraction maps as the loss function. The outputs of the networks were assessed by the loss function, measured liver fat fractions, and visually. Training was performed using a graphics processing unit (GPU). Inference was performed using the GPU as well as a central processing unit (CPU).ResultsThe loss curves indicated convergence, and the final loss of the validation data decreased when using more echoes as input. The liver fat fractions could be estimated using only 1 echo, but results were improved by use of more echoes. Visual assessment found the quality of the outputs of the networks to be similar to the reference even when using only 1 echo, with slight improvements when using more echoes. Training a network took at most 28.6 h. Inference time of a whole‐body scan took at most 3.7 s using the GPU and 5.8 min using the CPU.ConclusionIt is possible to perform water–fat signal separation of whole‐body gradient echo scans using convolutional neural networks. Separation was possible using only 1 echo, although using more echoes improved the results.

A Dichotomization Winding Scheme on a Novel Asymmetric Head Gradient Coil Design With an Improved Force and Torque Balance.

The head gradient coil is advantageous for brain imaging compared to the conventional whole-body gradient coil. It is usually asymmetrically designed for the accommodation of human shoulders. The asymmetric head coil has a specific issue associated with an unbalanced force/torque that requires minimization for imaging applications. This paper will improve the force and torque balance solution and propose a dichotomization winding scheme to augment the coil slew rate. A square force and torque optimization enables the available balanced asymmetric head gradient coil design, with a force and torque approaching the minimum level. Subsequently, two practical parallel connection winding schemes were quantitatively analyzed and evaluated. The results show that the proposed dichotomization winding scheme can increase the slew rate to almost twice that of the conventional winding counterpart, without obviously influencing the magnetic field performance.

Numerical Design of High-Efficiency Whole-Body Gradient Coils With a Hybrid Cylindrical-Planar Structure.

In this paper, a set of novel whole-body gradient coils is designed for cylindrical magnetic resonance imaging (MRI) systems. For the sake of high coil efficiency, the design scheme focuses on the imaging volume occupied by the patient, discounting the large space under the patient bed that is unused during imaging. To further improve the coil performance, the coils are designed on an unconventional, hybrid cylindrical-planar structure, where the primary coils are arranged on a chord-truncated cylindrical former while the shielding coils remain on the cylindrical assemblies. In this new coil configuration, the primary layers make the best possible use of the space closest to the imaging volume, whilst the shielding layers are positioned the furthest from the primary layers; thus, the best gradient field/current ratio can be obtained for the body coil configuration. Using a boundary element method, a full gradient set was designed to demonstrate the effectiveness of the proposed scheme. Compared with conventional designs, the new approach provides significantly improved coil performance. For the three gradient axes, the inductance was reduced by 25%-50%, the resistance was decreased by 19%-39%, and the minimum wire distance was increased by 5.2%-45.5%. In terms of shielding effect, the maximum stray fields of the X- and Y-gradient coils are reduced by 79.5% and 38.7%, respectively. It is concluded that the new design is capable of producing high-quality gradients with less eddy currents and thermal heating concerns, being suitable for MRI applications demanding a high gradient performance.

Development and implementation of an 84-channel matrix gradient coil.

Design, implement, integrate, and characterize a customized coil system that allows for generating spatial encoding magnetic fields (SEMs) in a highly-flexible fashion. A gradient coil with a high number of individual elements was designed. Dimensions of the coil were chosen to mimic a whole-body gradient system, scaled down to a head insert. Mechanical shape and wire layout of each element were optimized to increase the local gradient strength while minimizing eddy current effects and simultaneously considering manufacturing constraints. Resulting wire layout and mechanical design is presented. A prototype matrix gradient coil with 12 × 7 = 84 elements consisting of two element types was realized and characterized. Measured eddy currents are <1% of the original field. The coil is shown to be capable of creating nonlinear, and linear SEMs. In a DSV of 0.22 m gradient strengths between 24 mT∕m and 78 mT∕m could be realized locally with maximum currents of 150 A. Initial proof-of-concept imaging experiments using linear and nonlinear encoding fields are demonstrated. A shielded matrix gradient coil setup capable of generating encoding fields in a highly-flexible manner was designed and implemented. The presented setup is expected to serve as a basis for validating novel imaging techniques that rely on nonlinear spatial encoding fields. Magn Reson Med 79:1181-1191, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Gradient nonlinearity calibration and correction for a compact, asymmetric magnetic resonance imaging gradient system

Due to engineering limitations, the spatial encoding gradient fields in conventional magnetic resonance imaging cannot be perfectly linear and always contain higher-order, nonlinear components. If ignored during image reconstruction, gradient nonlinearity (GNL) manifests as image geometric distortion. Given an estimate of the GNL field, this distortion can be corrected to a degree proportional to the accuracy of the field estimate. The GNL of a gradient system is typically characterized using a spherical harmonic polynomial model with model coefficients obtained from electromagnetic simulation. Conventional whole-body gradient systems are symmetric in design; typically, only odd-order terms up to the 5th-order are required for GNL modeling. Recently, a high-performance, asymmetric gradient system was developed, which exhibits more complex GNL that requires higher-order terms including both odd- and even-orders for accurate modeling. This work characterizes the GNL of this system using an iterative calibration method and a fiducial phantom used in ADNI (Alzheimer’s Disease Neuroimaging Initiative). The phantom was scanned at different locations inside the 26 cm diameter-spherical-volume of this gradient, and the positions of fiducials in the phantom were estimated. An iterative calibration procedure was utilized to identify the model coefficients that minimize the mean-squared-error between the true fiducial positions and the positions estimated from images corrected using these coefficients. To examine the effect of higher-order and even-order terms, this calibration was performed using spherical harmonic polynomial of different orders up to the 10th-order including even- and odd-order terms, or odd-order only. The results showed that the model coefficients of this gradient can be successfully estimated. The residual root-mean-squared-error after correction using up to the 10th-order coefficients was reduced to 0.36 mm, yielding spatial accuracy comparable to conventional whole-body gradients. The even-order terms were necessary for accurate GNL modeling. In addition, the calibrated coefficients improved image geometric accuracy compared with the simulation-based coefficients.

MO-FG-CAMPUS-IeP3-02: Gradient Nonlinearity Calibration and Correction for Full-Volume Imaging of a Compact Asymmetric MRI Gradient System

Purpose: Due to engineering limitations, the spatial encoding gradient fields generated by an MRI scanner are inherently nonlinear and images must undergo geometric correction prior to display. The gradient nonlinearity (GNL) of a conventional symmetric whole-body gradient is typically characterized using a spherical harmonic polynomial (SHP) expansion of magnetic gradient fields with up to fifth-order terms. Only odd-order terms are required due to design symmetry (i.e., 3rd/5th-order). Recently, a high-performance asymmetric gradient system was developed which exhibits more complex GNL. Here, the GNL across the 26cm diameter-spherical-volume (DSV) of this system is characterized using a phantom-based iterative calibration procedure. Methods: The Alzheimer’s Disease Neuroimaging Initiative (ADNI) phantom was scanned with a 3D IR-FSPGR sequence (sagittal acquisition, Nx=Ny=256, Nz=196, Δx=Δy=1.05mm, Δz=1.3mm, BW=±125kHz) on the asymmetric gradient system. The phantom (20cm diameter with 160 spherical fiducials of 1.0/1.5cm diameter) was imaged at isocenter and at spatial offsets of ±3cm along the superior/inferior, right/left, anterior/posterior axes to cover the 26cm DSV. The spatial positions of fiducials were measured from MR images and compared with the design positions to extract distortion field, which is then used to fit the SHP model using an iterative calibration procedure. The effect of higher-order terms was tested by incrementally increasing the model order from 5 to 10. The calibrated coefficients were then used to geometrically correct the images, and for each case the residual root-mean-square-error (RMSE) was determined. For comparison, RMSE was also determined for images corrected using coefficients obtained from electromagnetic simulation. Results: The calibrated coefficients (10th-order) reduce RMSE from 0.96mm (using a 10th-order correction from electromagnetic simulation) to 0.36mm. Conclusion: The model parameters for GNL correction over the 26cm DSV of an asymmetric gradient system were successfully determined. The use of calibrated coefficients up to 10thorder coefficients including even-order terms yielded improved geometric accuracy (RMSE) comparable to conventional symmetric gradients. Funding support: NIH R01EB010065

High slew-rate head-only gradient for improving distortion in echo planar imaging: Preliminary experience.

To investigate the effects on echo planar imaging (EPI) distortion of using high gradient slew rates (SR) of up to 700 T/m/s for in vivo human brain imaging, with a dedicated, head-only gradient coil. Simulation studies were first performed to determine the expected echo spacing and distortion reduction in EPI. A head gradient of 42-cm inner diameter and with asymmetric transverse coils was then installed in a whole-body, conventional 3T magnetic resonance imaging (MRI) system. Human subject imaging was performed on five subjects to determine the effects of EPI on echo spacing and signal dropout at various gradient slew rates. The feasibility of whole-brain imaging at 1.5 mm-isotropic spatial resolution was demonstrated with gradient-echo and spin-echo diffusion-weighted EPI. As compared to a whole-body gradient coil, the EPI echo spacing in the head-only gradient coil was reduced by 48%. Simulation and in vivo results, respectively, showed up to 25-26% and 19% improvement in signal dropout. Whole-brain imaging with EPI at 1.5 mm spatial resolution provided good whole-brain coverage, spatial linearity, and low spatial distortion effects. Our results of human brain imaging with EPI using the compact head gradient coil at slew rates higher than in conventional whole-body MR systems demonstrate substantially improved image distortion, and point to a potential for benefits to non-EPI pulse sequences. J. Magn. Reson. Imaging 2016;44:653-664.

TH-CD-207-11: Gradient Nonlinearity Calibration and Correction for Head-Only Asymmetric Gradient System

Purpose: Due to engineering limitations, the gradient fields in clinical MRI inevitably contain high-order, nonlinear components. The presence of gradient nonlinearity (GNL) causes image geometrical distortion. Standard correction methods are based on parameterization of the simulated gradient fields with spherical harmonic polynomials. Conventional whole-body gradient systems typically employ symmetric designs, and the GNL for such systems usually contains only odd-order terms (up to 5th-order). Recently, a high-performance, head-only gradient system was developed. Due to asymmetric design, this new system exhibits more complex GNL profiles. Here, we demonstrate measurement-based high-order(N>5) GNL-correction on this new system using a fiducial phantom and iterative model-fitting procedure. Methods: The Alzheimer’s Disease Neuroimaging Initiative (ADNI) phantom was scanned with a 3D IR-FSPGR sequence (sagittal acquisition plane, Nx=Ny=256, Nz=196, x= y=1.05mm, z=1.3mm, BW=125kHz) on the head-only gradient system operating at 80mT/m, 500T/m/s. This phantom contains 160 fiducial spheres (diameter=1.0 or 1.5cm) that are distributed within a diameter=20cm spherical shell. The spatial positions of the fiducials were then measured from the distorted images and fit using a spherical harmonic polynomial model and iterative fitting procedure. The model order was increased from 1 to 10 step-by-step to test the effects of including high-order terms (N>5). Then, the coefficients were used to correct distortion. The residual root-mean-square-error (RMSE) was calculated from fiducial positions tracked from GNL-corrected images. Results: The RMSE analysis shows that GNL-distortion is reduced from 3.93 to 0.39mm by using correction terms up to 7th-order including both even-and odd-order terms. The addition of higher orders (N>7) provides negligible benefit. Conclusion: The GNL of a high-performance, head-only gradient system was successfully measured. The use of up to 7th-order correction terms was found to be sufficient for correcting GNL-distortion in a typical brain scan. These techniques could also improve geometric accuracy for whole-body MRI systems used for radiation therapy planning.

Unconventional gradient coil designs in magnetic resonance imaging.

In magnetic resonance imaging (MRI), the gradient coils are used to encode the spatial positions of protons by varying the magnetic field linearly across the imaging subject. With the latest development of MRI technique and new clinical and research applications, the gradient coil system requires increasingly innovative designs. In this paper, four unconventional gradient coil designs are reviewed: (1) local gradient coils; (2) new coil configurations with reduced peripheral nerve stimulation (PNS); (3) dedicated structures designed for hybrid systems (combining MRI with other medical devices); and (4) the full 3D coil designs. For the first type, the development of local gradient coils (mainly head coils) is discussed chronologically and divided into three stages: the "golden" stage in the 1990s, the "wane" stage in the 2000s, and the "revival" stage in the 2010s. For the second type, various designs for the reduction of PNS problems have been described, including local and whole-body gradient coil systems. For the third design, a dedicated gradient coil design for multi-modality combination is illustrated with an MRI-LINAC system. Finally, gradient systems with non-layered coil structure are described in the fourth design type. We hope that this review on unconventional gradient coil designs will be useful for the new development of MRI technology and emerging medical applications.

Double-wave-vector diffusion-weighting experiments with multiple concatenations at long mixing times

MR sequences where two diffusion-weighting periods are applied successively in a single acquisition seem to be a promising tool for the investigation of tissue structure on a microscopic level such as the characterization of the compartment size or eccentricity measures of pores. However, the application of such double-wave-vector (DWV) experiments on whole-body MR systems is hampered by the long gradient pulses required that have been shown to reduce the signal modulation. In this work, it is demonstrated that involving multiple concatenations of the two diffusion-weighting periods can ameliorate this problem in experiments with long mixing times between the two wave vectors. The recently presented tensor equation is extended to multiple concatenations. As confirmed by Monte-Carlo simulations, this model shows a good approximation of the signals observed for typical whole-body gradient pulse durations and the derived anisotropy measures are obtained with good accuracy. Most importantly, the signal modulation is increased with multiple concatenations because shorter gradient pulses can be used to achieve the desired diffusion-weighting. Thus, the multiple concatenation approach may help to improve the applicability and reliability of DWV measurements with long mixing times on standard whole-body MR systems.

Numerical simulation of double‐wave vector experiments investigating diffusion in randomly oriented ellipsoidal pores

Double-wave vector diffusion weighting is expected to represent a possibility to acquire information on cell size and shape. In numerical simulations, a theoretical description of the double wave vector-weighted signal is tested in a situation where the approximating assumptions (infinitely short gradient pulses, infinitely long diffusion time, infinitely long or zero delay between diffusion weightings, small gradient moment) are not strictly met. It is shown how the dependence of the signal on the angle between the diffusion gradient directions changes upon varying the delay between the second and the third gradient pulse, and how the measures of pore geometry derived from signal differences between two angles depend on the duration, temporal separation, and amplitude of the gradient pulses. The results indicate that a violation of the approximation conditions due to finite gradient pulse duration and separation generally leads to an underestimation of pore size and pore eccentricity. It is also concluded from the simulations that for pore sizes on the order of 10 microm the investigated effects are in a range that is observable even with whole-body gradient systems.

Simple anatomical measurements do not correlate significantly to individual peripheral nerve stimulation thresholds as measured in MRI gradient coils.

To examine peripheral nerve stimulation (PNS) thresholds for normal human subjects in magnetic resonance imaging (MRI) gradient coils, and determine if observed thresholds could be predicted based on gross physiologic measurements. PNS thresholds for 21 healthy normal subjects were measured using a whole-body gradient coil. Subjects were exposed to a trapezoidal echo-planar imaging (EPI) gradient waveform and the total change in gradient strength (DeltaG) required to cause PNS as a function of the duration of the gradient switching time (tau) were measured. Correlation coefficients and corresponding P values were calculated for the PNS threshold measurements against simple physiologic measurements taken of the subjects, including weight, height, girth, and average body fat percentage, in order to determine if there were any easily observable dependencies. No convincing correlations between threshold parameters and gross physiologic measurements were observed. These results suggest it is unlikely that a simple physiologic measurement of subject anatomy can be used to guide the operation of MRI scanners in a subject-specific manner in order to increase gradient system performance while avoiding PNS.

Gradient system providing continuously variable field characteristics.

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.