The signal loss susceptibility artifact is a major limitation in gradient-echo MRI applications. Various methods, including zshim 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 B 1 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 fourchannel parallel transmission system at 3T. Magn Reson Med 61:255-259, 2009.

Susceptibility artifacts and excitation radiofrequency field B 1 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 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 spectralspatial RF pulses for simultaneous B 1 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 1 homogeneity and reduce the signal loss artifact in multiple slice locations and several human volunteers. Magn Reson Med 64:1-8,

Susceptibility artifacts and transmission radio frequency (RF) field (B 1 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 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 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. Magn Reson Med 65:370-376, 2011. V C 2010 Wiley-Liss, Inc.

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 RF 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 RF pulse that can spectrally decompose the through-plane susceptibility gradient for spectral-spatial RF 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 −1.0μT/m/Hz.

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 3 64 excitation resolution and a 10-msec dwell time. We also show that even greater acceleration factors can be achieved for more complex RF pulses. Key words: multi-dimensional RF pulse; parallel transmission; graphics processing unit Multidimensional radiofrequency (RF) pulses are useful in a wide variety of applications including multidimensional spatial localization (1), simultaneous spectral-spatial excitation (2), B 1 þ field inhomogeneity compensation (3), and the mitigation of susceptibility artifacts (4). The use of multiple transmitters (5,6) has been proposed as means of shortening the duration of the RF pulses and reducing the specific absorption rate. Among the techniques for designing small-tip-angle multidimensional RF pulses (7), the ''spatial domain '' approach (8,9) calculates the RF pulses by numerically solving a large set of linear equations. The scale of these linear equations increases with the spatial and frequency resolution of the desired magnetization, the k-space sampling, and the number of transmitters. Numerically solving these large-scale linear equations is computationally intensive; the computation takes seconds and often minutes even for a computer equipped with a powerful central processing unit (CPU). This is a major drawback because it is impractical to design pulses in real time while patients wait in the scanner.A commodity graphics processing unit (GPU) consists of hundreds or thousands of thread processors and is capable of concurrently executing large numbers of multiple arithmetic operations. Using commodity GPUs to accelerate massively parallel applications is a recent technology that has been demonstrated to dramatically accelerate parallel imaging reconstruction applications (10,11). In the spatial domain method, RF pulses are numerically computed by solving the inverse problem of a linear system using algorithms such as singular value decomposition or conjugate gradient least squares (CGLS). Both algorithms are highly parallelizable and are therefore excellent candidates for acceleration using GPUs. Furthermore, a GPU-based computing platform has two major advantag...