Three dimensional radial echo planar imaging for functional MRI.

PURPOSE: To demonstrate a novel 3D radial echo planar imaging (3D REPI) sequence for flexible, rapid, and motion-robust sampling in fMRI. METHODS: The 3D REPI method expands on the recently described golden angle rotated EPI trajectory using radial batched internal navigator echoes (TURBINE) approach by exploiting the unused perpendicular direction in the EPI readout to form fast analogues of rotated stack of stars or spirals trajectories that cover all 3 dimensions of k-space. An iterative conjugate gradient algorithm with SENSE reconstruction and time-segmented non-uniform fast Fourier transform (FFT) was used for parallel imaging acceleration and to account for the effects of B0 inhomogeneity. The golden angle rotation allowed for sliding window reconstruction schemes to be applied in brain BOLD fMRI experiments. RESULTS: Combined whole brain visual and motor fMRI experiments were successfully carried out on a clinical 3T scanner at 2 mm isotropic and 1 × 1 × 2 mm3 resolutions using the 3D REPI design. Improved sampling characteristics and image quality were observed for twisted trajectories at the expense of prolonged readout times and off-resonance effects. The ability to correct for rigid motion correction was also demonstrated. CONCLUSIONS: 3D REPI presents a flexible approach for segmented volumetric fMRI with motion correction and high in-plane spatial resolutions. Improved BOLD fMRI brain activation maps were obtained using a sliding window reconstruction.

Highly accelerated EPI with wave encoding and multi-shot simultaneous multislice imaging.

PURPOSE: To introduce wave-encoded acquisition and reconstruction techniques for highly accelerated EPI with reduced g-factor penalty and image artifacts. THEORY AND METHODS: Wave-EPI involves application of sinusoidal gradients during the EPI readout, which spreads the aliasing in all spatial directions, thereby taking better advantage of 3D coil sensitivity profiles. The amount of voxel spreading that can be achieved by the wave gradients during the short EPI readout period is constrained by the slew rate of the gradient coils and peripheral nerve stimulation monitor. We propose to use a "half-cycle" sinusoidal gradient to increase the amount of voxel spreading that can be achieved while respecting the slew and stimulation constraints. Extending wave-EPI to multi-shot acquisition minimizes geometric distortion and voxel blurring at high in-plane resolutions, while structured low-rank regularization mitigates shot-to-shot phase variations. To address gradient imperfections, we propose to use different point spread functions for the k-space lines with positive and negative polarities, which are calibrated with a FLEET-based reference scan. RESULTS: Wave-EPI enabled whole-brain single-shot gradient-echo (GE) and multi-shot spin-echo (SE) EPI acquisitions at high acceleration factors at 3T and was combined with g-Slider encoding to boost the SNR level in 1 mm isotropic diffusion imaging. Relative to blipped-CAIPI, wave-EPI reduced average and maximum g-factors by up to 1.21- and 1.37-fold at Rin × Rsms  = 3 × 3, respectively. CONCLUSION: Wave-EPI allows highly accelerated single- and multi-shot EPI with reduced g-factor and artifacts and may facilitate clinical and neuroscientific applications of EPI by improving the spatial and temporal resolution in functional and diffusion imaging.

Radiofrequency phase encoded half-pulses in simultaneous multislice ultrashort echo time imaging.

PURPOSE: To describe a simultaneous multislice (SMS) ultrashort echo time (UTE) imaging method using radiofrequency phase encoded half-pulses in combination with power independent of number of slices (PINS) inversion recovery (IR) pulses to generate multiple-slice images with short T2 * contrasts in less than 3 min with close to an eightfold acceleration compared with a standard 2D approach. THEORY AND METHODS: Radiofrequency phase encoding is applied in an SMS (NSMS = 4) excitation scheme using "sinc" half-pulses. With the use of coil sensitivity encoding (SENSE) and controlled aliasing in parallel imaging (CAIPI) in combination with a gradient echo 2D spiral readout trajectory and IR PINS pulses for contrast enhancement a fast UTE sequence is developed. Images are obtained using a model-based reconstruction method. Sequence details and performance tests on phantoms as well as the heads of healthy volunteers at 3T are presented. RESULTS: An SMS UTE sequence with an undersampling factor of 4 is developed using radiofrequency phase encoded half-pulses and PINS IR pulses which enables the acquisition of 8 slices at 0.862 mm2 resolution in less than 3-min scan time. UTE images of the head are obtained showing highlighted signal of cortical bone. Image quality and T2 contrast are comparable to the one obtained by corresponding single slice acquisitions with only minor deviations. CONCLUSIONS: The method combining radiofrequency phase encoded SMS half-pulses and PINS IR pulses presents a suitable approach to SMS UTE imaging. The usage of coil sensitivity information and increased sampling density by means of interleaved slice group acquisitions allows to reduce the total scan time by a factor close to 8.

A circular echo planar sequence for fast volumetric fMRI.

PURPOSE: To demonstrate a circular EPI (CEPI) sequence as well as a generalized EPI reconstruction for fast fMRI with parallel imaging acceleration. METHODS: The CEPI acquisition was constructed using variable readout lengths and maximum ramp sampling as well as blipped-CAIPI z-gradient encoding for simultaneous multislice (SMS) and 3D volumetric imaging. A signal equation model with constant and linear phase terms was used to iteratively reconstruct images with low ghosting. Simulation, phantom, and human imaging experiments including audio/visual fMRI were performed at 3T using a 52-channel coil. RESULTS: Application of CEPI gradients with duration of 27 ms covering a 22-cm FOV at a 64 × 64 pixel resolution in SMS and 3D acquisitions resulted in images with comparable quality to those of standard Cartesian EPI. With parallel imaging techniques robust detection of BOLD fMRI activation with temporal sampling down to 275 ms was possible. The high temporal resolution enabled higher activation statistics at a penalty in increased noise and residual aliasing. The un-accelerated 3D acquisition showed large temporal instability compared with a standard 2D acquisition. CONCLUSION: Nonuniform sampling and generalized image reconstructions can be applied to EPI acquisitions including those with blipped-CAIPI z gradients. The same gradients can be used for either SMS or 3D acquisitions providing identical coverage.

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.

Low peak power multiband spokes pulses for B1 (+) inhomogeneity-compensated simultaneous multislice excitation in high field MRI.

PURPOSE: To design low peak and integrated power simultaneous multislice excitation radiofrequency pulses with transmit field inhomogeneity compensation in high field MRI. THEORY AND METHODS: The "interleaved greedy and local optimization" algorithm for small-tip-angle spokes pulses is extended to design multiband (MB) spokes pulses that simultaneously excite multiple slices, with independent spokes weight optimization for each slice. The peak power of the pulses is controlled using a slice phase optimization technique. Simulations were performed at 7T to compare the peak power of optimized MB spokes pulses to unoptimized pulses, and to compare the proposed slice-independent spokes weight optimization to a joint approach. In vivo experiments were performed at 7T to validate the pulse's function and compare them to conventional MB pulses. RESULTS: Simulations showed that the peak power-minimized pulses had lower peak power than unregularized and integrated power-regularized pulses, and that the slice-independent spokes weight optimization consistently produced lower flip angle inhomogeneity and lower peak and integrated power pulses. In the brain imaging experiments, the MB spokes pulses showed significant improvement in excitation flip angle and subsequently signal homogeneity compared to conventional MB pulses. CONCLUSION: The proposed MB spokes pulses improve flip angle homogeneity in simultaneous multislice acquisitions at ultrahigh field, with minimal increase in integrated and peak radiofrequency power.

Simultaneous multislice spectral-spatial excitations for reduced signal loss susceptibility artifact in BOLD functional MRI.

PURPOSE: Simultaneous multislice (SMS) imaging can significantly increase image acquisition rates and improve temporal resolution and contrast in gradient-echo blood oxygen level-dependent (BOLD) functional MRI (fMRI) experiments. Through-plane signal loss due to B(0) inhomogeneities at air-tissue interfaces limits fMRI of structures near the nasal cavity and ear canals. This study implemented spectral-spatial (SPSP) radiofrequency pulses for reduced through-plane signal loss across multiple simultaneously excited slices. THEORY AND METHODS: Multiband (MB) and power independent of number of slices (PINS) methods are combined with SPSP excitation for signal loss compensation in slice-accelerated human brain imaging. Nine simultaneous slices of 5-mm thickness and 20 mm apart were excited using standard MB radiofrequency pulses and the proposed SPSP-SMS pulses, yielding coverage of 36 slices in four shots with 350-ms volume pulse repetition time. The pulses were compared in breath-hold fMRI at 3T. RESULTS: The SPSP-SMS pulses recovered ∼45% of voxels with signal loss in standard SMS images. Activation in areas of signal recovery increased by 26.4% using a 12.6-ms SPSP-MB pulse and 20.3% using a 12.1-ms SPSP-PINS pulse. CONCLUSIONS: It is demonstrated that SPSP-SMS pulses can improve BOLD sensitivity in areas of signal loss across simultaneous multiple slices.

Three-dimensional Fourier encoding of simultaneously excited slices: generalized acquisition and reconstruction framework.

PURPOSE: Simultaneous multislice (SMS) acquisitions have recently received much attention as a means of increasing single-shot imaging speed. SMS acquisitions combine the advantages of single-shot sampling and acceleration along the slice dimension which was previously limited to three-dimensional (3D) volumetric acquisitions. A two-dimensional description of SMS sampling and reconstruction has become established in the literature. Here, we present a more general 3D Fourier encoding and reconstruction formalism for SMS acquisitions that can easily be applied to non-Cartesian SMS acquisitions. THEORY AND METHODS: An "SMS 3D" k-space is defined in which the field of view along the slice select direction is equal to the number of excited slices times their separation. In this picture, SMS acceleration can be viewed as an undersampling of SMS 3D k-space that can be freely distributed between the in-plane and slice directions as both are effective phase-encoding directions. RESULTS: Use of the SMS 3D k-space picture is demonstrated in phantom and in vivo brain acquisitions including data obtained with blipped-controlled aliasing in parallel imaging sampling. SMS sensitivity encoding reconstruction is demonstrated as well as non-Cartesian SMS imaging using blipped spiral trajectories. CONCLUSIONS: The full framework of reconstruction methods can be applied to SMS acquisitions by employing a 3D k-space approach. The blipped-controlled aliasing in parallel imaging method can be viewed as a special case of undersampling an SMS 3D k-space. The extension of SMS methods to non-Cartesian 3D sampling and reconstruction is straightforward.

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.

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.

Altered functioning of the executive control circuit in late-life depression: episodic and persistent phenomena.

OBJECTIVE: To characterize the functional neuroanatomy of late-life depression (LLD) by probing for both episodic and persistent alterations in the executive-control circuit of elderly adults. DESIGN: Event-related functional magnetic resonance imaging (fMRI) data were collected while participants performed an executive-control task. SETTING: Participants were recruited through a depression-treatment study within the Pittsburgh, PA, Intervention Research Center for Late-Life Mood Disorders. PARTICIPANTS: Thirteen nondepressed elderly comparison participants and 13 LLD patients. INTERVENTION: The depressed patients underwent imaging before initiating and after completing 12 weeks of paroxetine. MEASUREMENTS: Regional fMRI activity was assessed in the dorsolateral prefrontal cortex (dLPFC: BA9 and BA46 bilaterally) and the dorsal anterior cingulate cortex (dACC). Functional connectivity was assessed by correlating the fMRI time-series in the dLPFC and dACC. RESULTS: Both depressed and comparison participants performed the task as expected, with greater response latency during high versus low-load trials. The response-latency load-effect did not differ between groups. In contrast to the null findings for behavioral data, pretreatment, depressed patients showed diminished activity in the dLPFC (BA46 left, t(25)=1.9, p = 0.035) and diminished functional connectivity between the dLPFC and dACC. Moreover, right dLPFC (BA46 right, t(25)=2.17, p < 0.02) showed increased activity after treatment. CONCLUSIONS: These results support a model of both episodic and persistent neurobiologic components of LLD. The altered functional connectivity,perhaps due to vascular damage to frontal white matter, appears to be persistent. Further, at least some of the prefrontal hypoactivity (in the right dLPFC) seems to be an episodic characteristic of acute depression amenable to treatment.

Prefrontal cortex guides context-appropriate responding during language production.

Although language processing is thought to frequently require cognitive control, little is known about the cognitive and neural basis of the control of language. Here, we demonstrate that processing of context by the PFC plays an important role in the control of language comprehension and production. Using a missing letter paradigm and fMRI, we found that increased activation in the PFC (but not in posterior regions), while encoding and maintaining context information, predicted context-appropriate responses. Furthermore, greater selection demands increased activity during responding in the same regions engaged during the encoding and maintenance of context. Overall, as in other cognitive task domains, these results suggest that PFC context processing plays an important role in the control of language.