Weiran Deng

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.

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. Magn Reson Med 61:255–259, 2009.

Simultaneous multislice excitation by parallel transmission

A technique is described for simultaneous multislice (SMS) excitation using radiofrequency (RF) parallel transmission (pTX).

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

Head motion during 1H 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 1H 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. Magn Reson Med, 2010.

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 B1+ 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 B1+ 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 B1+ and through‐plane signal loss susceptibility artifact compensation. The method is demonstrated with simulations and in T2*‐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 B1+ homogeneity and reduce the signal loss artifact in multiple slice locations and several human volunteers. Magn Reson Med 64:1–8, 2010.

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. Magn Reson Med, 2013.

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. Magn Reson Med, 2011.

Simple analytical dual-band spectral-spatial RF pulses for B1+ and susceptibility artifact reduction in gradient echo MRI

Susceptibility artifacts and transmission radio frequency (RF) field (B1+) 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 B1+ 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 B1+ 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. Magn Reson Med, 2011.

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. Magn Reson Med, 2012.

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

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.