Segmented simultaneous multi-slice diffusion weighted imaging with generalized trajectories.

PURPOSE: The purpose of this work is to develop and evaluate a single framework for the use of Cartesian and non-Cartesian segmented trajectories for rapid and robust simultaneous multislice (SMS) diffusion weighted imaging (DWI) at 3 Telsa (T). METHODS: A generalized SMS approach with intrinsic phase navigation using Multiplexed Sensitivity Encoding (MUSE) was developed. Segmented blipped-controlled aliasing in parallel imaging echo planar imaging (EPI) and z-gradient modulated spiral trajectories were examined using SMS DWI scans at 3T with a 32-channel head coil. RESULTS: The generalized SMS MUSE reconstruction framework was successful in significantly reducing artifacts for all trajectories. A DWI brain volume with a 67.5-mm height, 1.5-mm isotropic resolution, and 90 diffusion weightings was obtained in a scan time of 6 minutes. CONCLUSION: The MUSE technique can be generalized to allow for reconstruction of both Cartesian and non-Cartesian segmented trajectories. Magn Reson Med 78:1476-1481, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Rotated stack-of-spirals partial acquisition for rapid volumetric parallel MRI.

PURPOSE: We present a volumetric sampling method that rotates the spiral interleaves of a stack of spirals (SOSP) trajectory for reduced aliasing artifacts using parallel imaging with undersampling. METHODS: The aliasing pattern in an undersampled SOSP acquisition was modified by consecutively rotating spiral interleaves in each phase-encoding plane. This allows a sampling scheme with a high reduction factor when using a volumetric multireceiver array. Phantom and in vivo brain images at a resolution of 1 × 1 × 2 mm(3) were acquired at 3T using a 32-channel coil. Images reconstructed with a reduction factor of 16 were compared for aliasing artifacts and geometry factor (g-factor). RESULTS: Phantom and in vivo brain image results revealed that the rotated SOSP acquisition with a reduction factor of 16 produces images with reduced aliasing and lower g-factors than images acquired without rotation. CONCLUSION: The proposed rotated SOSP sampling method is a highly efficient way to maximize the encoding power of volumetric receiver arrays in parallel imaging and is applicable to rapid volumetric scanning, including susceptibility-weighted imaging and functional MRI. Magn Reson Med 76:127-135, 2016. © 2015 Wiley Periodicals, Inc.

Iterative projection onto convex sets for quantitative susceptibility mapping.

PURPOSE: Quantitative susceptibility map (QSM) reconstruction is ill posed due to the zero values on the "magic angle cone" that make the maps prone to streaking artifacts. We propose projection onto convex sets (POCS) in the method of steepest descent (SD) for QSM reconstruction. METHODS: Two convex projections, an object-support projection in the image domain and a projection in k-space were used. QSM reconstruction using the proposed SD-POCS method was compared with SD and POCS alone as well as with truncated k-space division (TKD) for numerically simulated and 7 Tesla (T) human brain phase data. RESULTS: The QSM reconstruction error from noise-free simulated phase data using SD-POCS is at least two orders of magnitude lower than using SD, POCS, or TKD and has reduced streaking artifacts. Using the l1 -TV reconstructed susceptibility as a gold standard for 7T in vivo imaging, SD-POCS showed better image quality comparing to SD, POCS, or TKD from visual inspection. CONCLUSION: POCS is an alternative method for regularization that can be used in an iterative minimization method such as SD for QSM reconstruction.

Simultaneous multislice excitation by parallel transmission.

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)(+) 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)(+) sensitivities of suitable multielement pTX arrays.

Single-shot echo-planar imaging with Nyquist ghost compensation: interleaved dual echo with acceleration (IDEA) echo-planar imaging (EPI).

Echo planar imaging (EPI) is most commonly used for blood oxygen level-dependent fMRI, owing to its sensitivity and acquisition speed. A major problem with EPI is Nyquist (N/2) ghosting, most notably at high field. EPI data are acquired under an oscillating readout gradient and hence vulnerable to gradient imperfections such as eddy current delays and off-resonance effects, as these cause inconsistencies between odd and even k-space lines after time reversal. We propose a straightforward and pragmatic method herein termed "interleaved dual echo with acceleration (IDEA) EPI": two k-spaces (echoes) are acquired under the positive and negative readout lobes, respectively, by performing phase encoding blips only before alternate readout gradients. From these two k-spaces, two almost entirely ghost free images per shot can be constructed, without need for phase correction. The doubled echo train length can be compensated by parallel imaging and/or partial Fourier acquisition. The two k-spaces can either be complex averaged during reconstruction, which results in near-perfect cancellation of residual phase errors, or reconstructed into separate images. We demonstrate the efficacy of IDEA EPI and show phantom and in vivo images at both 3 T and 7 T.

Altered brain activation during visuomotor integration in chronic active cannabis users: relationship to cortisol levels.

Cannabis is the most abused illegal substance in the United States. Alterations in brain function and motor behavior have been reported in chronic cannabis users, but the results have been variable. The current study aimed to determine whether chronic active cannabis use in humans may alter psychomotor function, brain activation, and hypothalamic-pituitary-axis (HPA) function in men and women. Thirty cannabis users (16 men, 14 women, 18-45 years old) and 30 nondrug user controls (16 men, 14 women, 19-44 years old) were evaluated with neuropsychological tests designed to assess motor behavior and with fMRI using a 3 Tesla scanner during a visually paced finger-sequencing task, cued by a flashing checkerboard (at 2 or 4 Hz). Salivary cortisol was measured to assess HPA function. Male, but not female, cannabis users had significantly slower performance on psychomotor speed tests. As a group, cannabis users had greater activation in BA 6 than controls, while controls had greater activation in the visual area BA 17 than cannabis users. Cannabis users also had higher salivary cortisol levels than controls (p = 0.002). Chronic active cannabis use is associated with slower and less efficient psychomotor function, especially in male users, as indicated by a shift from regions involved with automated visually guided responses to more executive or attentional control areas. The greater but altered brain activities may be mediated by the higher cortisol levels in the cannabis users, which in turn may lead to less efficient visual-motor function.

Spectral decomposition of susceptibility artifacts for spectral-spatial radiofrequency pulse design.

Susceptibility induced signal loss is a limitation in gradient echo functional MRI. The through-plane artifact in axial slices is particularly problematic due to the inferior position of air cavities in the brain. Spectral-spatial radiofrequency pulses have recently been shown to reduce signal loss in a single excitation. The pulses were successfully demonstrated assuming a linear relationship between susceptibility gradient and frequency, however, the exact frequency and spatial distribution of the susceptibility gradient in the brain is unknown. We present a spiral spectroscopic imaging sequence with a time-shifted radiofrequency pulse that can spectrally decompose the through-plane susceptibility gradient for spectral-spatial radiofrequency pulse design. Maps of the through-plane susceptibility gradient as a function of frequency were generated for the human brain at 3T. We found that the linear relationship holds well for the whole brain with an optimal slope of -1.0 µT/m/Hz.

Accelerated multidimensional radiofrequency pulse design for parallel transmission using concurrent computation on multiple graphics processing units.

Multidimensional radiofrequency (RF) pulses are of current interest because of their promise for improving high-field imaging and for optimizing parallel transmission methods. One major drawback is that the computation time of numerically designed multidimensional RF pulses increases rapidly with their resolution and number of transmitters. This is critical because the construction of multidimensional RF pulses often needs to be in real time. The use of graphics processing units for computations is a recent approach for accelerating image reconstruction applications. We propose the use of graphics processing units for the design of multidimensional RF pulses including the utilization of parallel transmitters. Using a desktop computer with four NVIDIA Tesla C1060 computing processors, we found acceleration factors on the order of 20 for standard eight-transmitter two-dimensional spiral RF pulses with a 64 × 64 excitation resolution and a 10-µsec dwell time. We also show that even greater acceleration factors can be achieved for more complex RF pulses.

Simple analytical dual-band spectral-spatial RF pulses for B(1) + and susceptibility artifact reduction in gradient echo MRI.

Susceptibility artifacts and transmission radio frequency (RF) field (B(1) +) inhomogeneity are major limitations in high-field gradient echo MRI. Previously proposed numerical 2D spectral-spatial RF pulses have been shown to be promising for reducing the through-plane signal loss susceptibility artifact by incorporating a frequency-dependent through-plane phase correction. This method has recently been extended to 4D spectral-spatial RF pulse designs for reducing B(1) + inhomogeneity as well as the signal loss. In this manuscript, we present simple analytical pulse designs for constructing 2D and 4D spectral-spatial RF pulses as an alternative to the numerical approaches. The 2D pulse capable of exciting slices with reduced signal loss and is lipid suppressing. The 4D pulse simultaneously corrects signal loss as well as the B(1) + inhomogeneity from a body coil transmitter. The pulses are demonstrated with simulations and with gradient echo phantom and brain images at 3T using a standard RF body coil. The pulses were observed to work well for multiple slices and several volunteers.

A three-dimensional variable-density spiral spatial-spectral RF pulse with rotated gradients.

Three-dimensional spatial-spectral radiofrequency pulses using a stack-of-spirals trajectory can achieve two-dimensional spatial localization and one-dimensional spectral selection simultaneously. These pulses are useful, for example, in reduced field-of-view applications that also require frequency specificity such as lipid imaging. A limitation of the pulse design is that the length of the spiral trajectory is fixed by the frequency separation of lipid and water. This restricts the highest possible excitation resolution of the spatial profile over a given field of excitation. In this work, we examine the use of periodically rotated variable-density spirals to increase the spatial excitation resolution without changing the frequency selectivity. Variable-density spirals are used to undersample the high spatial frequencies such that higher excitation resolutions can be obtained with a small expense in increased aliasing of the slice profile. The periodic rotation of the spiral trajectories reduces the impact of the undersampling by distributing the aliasing in the frequency domain. The technique is demonstrated with simulations, phantom studies, and imaging human leg muscle at 3 T. It was found in the human study that the spatial excitation resolution could be improved from 6 x 6 to 8 x 8 (matrix size over a fixed field of view) while decreasing aliasing by approximately 40-60%.

Four-dimensional spectral-spatial RF pulses for simultaneous correction of B1+ inhomogeneity and susceptibility artifacts in T2*-weighted MRI.

Susceptibility artifacts and excitation radiofrequency field B(1)+ inhomogeneity are major limitations in high-field MRI. Parallel transmission methods are promising for reducing artifacts in high-field applications. In particular, three-dimensional RF pulses have been shown to be useful for reducing B(1)+ inhomogeneity using multiple transmitters due to their ability to spatially shape the slice profile. Recently, two-dimensional spectral-spatial pulses have been demonstrated to be effective for reducing the signal loss susceptibility artifact by incorporating a frequency-dependent through-plane phase correction. We present the use of four-dimensional spectral-spatial RF pulses for simultaneous B(1)+ and through-plane signal loss susceptibility artifact compensation. The method is demonstrated with simulations and in T(2)*-weighted human brain images at 3 T, using a four-channel parallel transmission system. Parallel transmission was used to reduce the in-plane excitation resolution to improve the slice-selection resolution between two different pulse designs. Both pulses were observed to improve B(1)+ homogeneity and reduce the signal loss artifact in multiple slice locations and several human volunteers.

Prospective motion correction for single-voxel 1H MR spectroscopy.

Head motion during (1)H MR spectroscopy acquisitions may compromise the quality and reliability of in vivo metabolite measurements. Therefore, a three-plane image-based motion-tracking module was integrated into a single-voxel (1)H MR spectroscopy (point-resolved spectroscopy) sequence. A series of three orthogonal spiral navigator images was acquired immediately prior to the MR spectroscopy water suppression module in order to estimate head motion. By applying the appropriate rotations and translations, the MR spectroscopy voxel position can be updated such that it remains stationary with respect to the brain. Frequency and phase corrections were applied during postprocessing to reduce line width and restore coherent averaging. Spectra acquired during intentional head motion in 11 volunteers demonstrate reduced lipid contamination and increased spectral reproducibility when motion correction is applied.

Simultaneous z-shim method for reducing susceptibility artifacts with multiple transmitters.

The signal loss susceptibility artifact is a major limitation in gradient-echo MRI applications. Various methods, including z-shim techniques and multidimensional tailored radio frequency (RF) pulses, have been proposed to mitigate the through-plane signal loss artifact, which is dominant in axial slices above the sinus region. Unfortunately, z-shim techniques require multiple steps and multidimensional RF methods are complex, with long pulse lengths. Parallel transmission methods were recently shown to be promising for improving B1 inhomogeneity and reducing the specific absorption rate. In this work, a novel method using time-shifted slice-select RF pulses is presented for reducing the through-plane signal loss artifact in parallel transmission applications. A simultaneous z-shim is obtained by concurrently applying unique time-shifted pulses on each transmitter. The method is shown to reduce the signal loss susceptibility artifact in gradient-echo images using a four-channel parallel transmission system at 3T.