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. © 2012 Wiley Periodicals, Inc.     

Broadband slab selection with B mitigation at 7T via parallel spectral‐spatial excitation

Chemical shift imaging benefits from signal‐to‐noise ratio (SNR) and chemical shift dispersion increases at stronger main field such as 7 Tesla, but the associated shorter radiofrequency (RF) wavelengths encountered require B mitigation over both the spatial field of view (FOV) and a specified spectral bandwidth. The bandwidth constraint presents a challenge for previously proposed spatially tailored B mitigation methods, which are based on a type of echovolumnar trajectory referred to as “spokes” or “fast‐k_z”. Although such pulses, in conjunction with parallel excitation methodology, can efficiently mitigate large B inhomogeneities and achieve relatively short pulse durations with slice‐selective excitations, they exhibit a narrow‐band off‐resonance response and may not be suitable for applications that require B mitigation over a large spectral bandwidth. This work outlines a design method for a general parallel spectral‐spatial excitation that achieves a target‐error minimization simultaneously over a bandwidth of frequencies and a specified spatial‐domain. The technique is demonstrated for slab‐selective excitation with in‐plane B mitigation over a 600‐Hz bandwidth. The pulse design method is validated in a water phantom at 7T using an eight‐channel transmit array system. The results show significant increases in the pulse's spectral bandwidth, with no additional pulse duration penalty and only a minor tradeoff in spatial B mitigation compared to the standard spoke‐based parallel RF design. Magn Reson Med 61:493–500, 2009. © 2009 Wiley‐Liss, Inc.     

kT‐points: Short three‐dimensional tailored RF pulses for flip‐angle homogenization over an extended volume

With Transmit SENSE, we demonstrate the feasibility of uniformly exciting a volume such as the human brain at 7T through the use of an original minimalist transmit k‐space coverage, referred to as “k_T‐points.” Radio‐frequency energy is deposited only at a limited number of k‐space locations in the vicinity of the center to counteract transmit sensitivity inhomogeneities. The resulting nonselective pulses are short and need little energy compared to adiabatic or other B‐robust pulses available in the literature, making them good candidates for short‐repetition time 3D sequences at high field. Experimental verification was performed on three human volunteers at 7T by means of an 8‐channel transmit array system. On average, whereas the standard circularly polarized excitation resulted in a 33%‐flip angle spread (standard deviation over mean) throughout the brain, and a static radio‐frequency shim showed flip angle variations of 17% and up, application of k_T‐point‐based excitations demonstrated excellent flip angle uniformity (8%) for a small target flip angle and with sub‐millisecond durations. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Spectral‐spatial pulse design for through‐plane phase precompensatory slice selection in T‐weighted functional MRI

T‐weighted functional MR images suffer from signal loss artifacts caused by the magnetic susceptibility differences between air cavities and brain tissues. We propose a novel spectral‐spatial pulse design that is slice‐selective and capable of mitigating the signal loss. The two‐dimensional spectral–spatial pulses create precompensatory phase variations that counteract through‐plane dephasing, relying on the assumption that resonance frequency offset and through‐plane field gradient are spatially correlated. The pulses can be precomputed before functional MRI experiments and used repeatedly for different slices in different subjects. Experiments with human subjects showed that the pulses were effective in slice selection and loss mitigation at different brain regions. Magn Reson Med 61:1137–1147, 2009. © 2009 Wiley‐Liss, Inc.     

Reduction of transmitter B1 inhomogeneity with transmit SENSE slice‐select pulses

Parallel transmitter techniques are a promising approach for reducing transmitter B₁ inhomogeneity due to the potential for adjusting the spatial excitation profile with independent RF pulses. These techniques may be further improved with transmit sensitivity encoding (SENSE) methods because the sensitivity information in pulse design provides an excitation that is inherently compensated for transmitter B₁ inhomogeneity. This paper presents a proof of this concept using transmit SENSE 3D tailored RF pulses designed for small flip angles. An eight‐channel receiver coil was used to mimic parallel transmission for brain imaging at 3T. The transmit SENSE pulses were based on the fast‐k_z design and produced 5‐mm‐thick slices at a flip angle of 30° with only a 4.3‐ms pulse length. It was found that the transmit SENSE pulses produced more homogeneous images than those obtained from the complex sum of images from all receivers excited with a standard RF pulse. Magn Reson Med 57:842–847, 2007. © 2007 Wiley‐Liss, Inc.     

T1 corrected B1 mapping using multi‐TR gradient echo sequences

This work presents a new approach toward a fast, simultaneous amplitude of radiofrequency field (B₁) and T₁ mapping technique. The new method is based on the “actual flip angle imaging” (AFI) sequence. However, the single pulse repetition time (TR) pair used in the standard AFI sequence is replaced by multiple pulse repetition time sets. The resulting method was called “multiple TR B₁/T₁ mapping” (MTM). In this study, MTM was investigated and compared to standard AFI in simulations and experiments. Feasibility and reliability of MTM were proven in phantom and in vivo experiments. Error propagation theory was applied to identify optimal sequence parameters and to facilitate a systematic noise comparison to standard AFI. In terms of accuracy and signal‐to‐noise ratio, the presented method outperforms standard AFI B₁ mapping over a wide range of T₁. Finally, the capability of MTM to determine T₁ was analyzed qualitatively and quantitatively, yielding good agreement with reference measurements. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fast slice-selective radio-frequency excitation pulses for mitigating B+1 inhomogeneity in the human brain at 7 Tesla.

A novel radio-frequency (RF) pulse design algorithm is presented that generates fast slice-selective excitation pulses that mitigate B+1 inhomogeneity present in the human brain at high field. The method is provided an estimate of the B+1 field in an axial slice of the brain and then optimizes the placement of sinc-like "spokes" in kz via an L1-norm penalty on candidate (kx, ky) locations; an RF pulse and gradients are then designed based on these weighted points. Mitigation pulses are designed and demonstrated at 7T in a head-shaped water phantom and the brain; in each case, the pulses mitigate a significantly nonuniform transmit profile and produce nearly uniform flip angles across the field of excitation (FOX). The main contribution of this work, the sparsity-enforced spoke placement and pulse design algorithm, is derived for conventional single-channel excitation systems and applied in the brain at 7T, but readily extends to lower field systems, nonbrain applications, and multichannel parallel excitation arrays.     

B1+ compensation in 3T cardiac imaging using short 2DRF pulses.

The purpose of this study was to determine if tailored 2DRF pulses could be used to compensate for in-plane variations of the transmitted RF field at 3T. Excitation pulse profiles were designed to approximate the reciprocal of the measured RF transmit variation where the variation over the left ventricle was approximated as unidirectional. A simple 2DRF pulse design utilizing three subpulses was used, such that profiles could be quickly and easily adapted to different regions of interest. Results are presented from phantom and in vivo cardiac imaging. Compared with conventional slice-selective excitation, the average flip angle variation over the left ventricle (measured as the standard deviation divided by the mean flip angle) was reduced with P < 0.001 and the average reduction was 41% in cardiac studies at 3T.     

Optimal design of multiple‐channel RF pulses under strict power and SAR constraints

Parallel radio frequency transmission has recently been explored as a means of tailoring the spatial response of MR excitation. In particular, parallel transmission is increasingly used to accelerate radio frequency pulses that rely on time‐varying gradient fields to achieve selectivity in multiple dimensions. The design of the underlying multiple‐channel radio frequency waveforms is mostly based on regularized least‐squares optimization in close analogy with image reconstruction in parallel imaging. However, this analogy has important limitations. Unlike image reconstruction, the design of radio frequency waveforms is subject to multiple strict constraints, which arise from technical power limits, as well as safety limits on local and global energy deposition in vivo. To optimize excitation profiles under such strict constraints, it is proposed to depart from the regularization strategy and rely on semidefinite programming instead. To render this approach fast, it is performed in a reduced search space, which is obtained by initial Lanczos iteration. The proposed algorithm is demonstrated to enable efficient pulse optimization within exactly the given constraints, including local specific absorption rate limits for multiple compartments. It is also shown that the proposed approach readily accommodates advanced forward models of the excitation process, including the effects of local off‐resonance and transverse relaxation. Magn Reson Med 63:1280–1291, 2010. © 2010 Wiley‐Liss, Inc.