Mitigate B1+ inhomogeneity using spatially selective radiofrequency excitation with generalized spatial encoding magnetic fields.

PURPOSE: High-field magnetic resonance imaging (MRI) has the challenge of inhomogeneous B(1)(+), and consequently inhomogeneous flip angle distribution, which causes spatially dependent contrast and makes clinical diagnosis difficult. METHOD: We propose a two-step pulse design procedure in which (1) a combination of linear and nonlinear spatial encoding magnetic fields (SEMs) is used to remap the B(1)(+) map in order to reduce the dimensionality of the problem, (2) the locations, amplitudes, and phases of spoke pulses are estimated in one dimension. The advantage of this B(1)(+) remapping is that when the isointensity contours of a linear combination of SEMs are similar to the isointensity contours of B(1)(+), a simple pulse sequence design using time-varying SEMs can achieve a homogenous flip-angle distribution efficiently. RESULTS: We demonstrate that spatially selective radiofrequency (RF) excitation with generalized SEMs (SAGS) using both linear and quadratic SEMs in a multi-spoke k-space trajectory can mitigate the B(1)(+) inhomogeneity at 7T efficiently. Numerical simulations based on experimental data suggest that, compared with other methods, SAGS provide a formulation allowing multiple-pulse design, a similar average flip-angle distribution with less RF power, and/or a more homogeneous flip-angle distribution. CONCLUSION: Without using multiple RF coils for parallel transmission, SAGS can be used to mitigate the B(1)(+) inhomogeneity in high-field MRI experiments.

Efficient concomitant and remanence field artifact reduction in ultra-low-field MRI using a frequency-space formulation.

PURPOSE: For ultra-low-field MRI, the spatial-encoding magnetic fields generated by gradient coils can have strong concomitant fields leading to prominent image distortion. Additionally, using superconducting magnet to pre-polarize magnetization can improve the signal-to-noise ratio of ultra-low-field MRI. Yet the spatially inhomogeneous remanence field due to the permanently trapped flux inside a superconducting pre-polarizing coil modulates magnetization and causes further image distortion. METHOD: We propose a two-stage frequency-space (f-x) formulation to accurately describe the dynamics of spatially-encoded magnetization under the influence of concomitant and remanence fields, which allows for correcting image distortion due to concomitant and remanence fields. RESULTS: Our method is computationally efficient as it uses a combination of the fast Fourier transform algorithm and a linear equation solver. With sufficiently dense discretization in solving the linear equation, the performance of this f-x method was found to be stable among different choices of the regularization parameter and the regularization matrix. CONCLUSION: We present this method together with numerical simulations and experimental data to demonstrate how concomitant and remanence field artifacts in ultra-low-field MRI can be corrected efficiently.

Mitigate B1(+) inhomogeneity by nonlinear gradients and RF shimming.

High-field MRI has the challenge of inhomogeneous B1(+) and consequently an inhomogeneous flip angle distribution. This causes spatially dependent contrast and makes clinical diagnosis difficult. Under the small flip angle approximation and using nonlinear spatial encoding magnetic fields (SEMs), we propose a method to remap the B1(+) map into a lower dimension coordinate system. Combining with RF shimming method, a simple pulse sequence design using nonlinear SEMs can achieve a homogenous flip angle distribution efficiently. Using simulations, we demonstrate that combining RF shimming and spatially selective RF excitation using generalized SEMs (SAGS) using linear and quadratic SEMs in a multi-spoke k-space trajectory can mitigate the B1(+) inhomogeneity at 7T efficiently without using parallel RF transmission.

A PERFECT MATCH CONDITION FOR POINT-SET MATCHING PROBLEMS USING THE OPTIMAL MASS TRANSPORT APPROACH.

We study the performance of optimal mass transport-based methods applied to point-set matching problems. The present study, which is based on the L2 mass transport cost, states that perfect matches always occur when the product of the point-set cardinality and the norm of the curl of the non-rigid deformation field does not exceed some constant. This analytic result is justified by a numerical study of matching two sets of pulmonary vascular tree branch points whose displacement is caused by the lung volume changes in the same human subject. The nearly perfect match performance verifies the effectiveness of this mass transport-based approach.

Enhancing image watermarking methods with/without reference images by optimization on second-order statistics.

The watermarking method has emerged as an important tool for content tracing, authentication, and data hiding in multimedia applications. We propose a watermarking strategy in which the watermark of a host is selected from the robust features of the estimated forged images of the host. The forged images are obtained from Monte Carlo simulations of potential pirate attacks on the host image. The solution of applying an optimization technique to the second-order statistics of the features of the forged images gives two orthogonal spaces. One of them characterizes most of the variations in the modifications of the host. Our watermark is embedded in the other space that most potential pirate attacks do not touch. Thus, the embedded watermark is robust. Our watermarking method uses the same framework for watermark detection with a reference and blind detection. We demonstrate the performance of our method under various levels of attacks.