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As one widely-used parallel-imaging method, Generalized Auto-calibrating Partially Parallel Acquisitions (GRAPPA) technique reconstructs the missing k-space data by a linear combination of the acquired data using a set of weights. These weights are usually derived from auto-calibration signal (ACS) lines that are acquired in parallel to the reduced lines. In this paper, a cross sampling method is proposed to acquire the ACS lines orthogonal to the reduced lines. This cross sampling method increases the amount of calibration data along the direction that the k-space is undersampled and thus improves the calibration accuracy, especially when a small number of ACS lines are acquired. Both phantom and in vivo experiments demonstrate that the proposed method, named cross-sampled GRAPPA (CS-GRAPPA), can effectively reduce the aliasing artifacts of GRAPPA when high acceleration is desired.

Design of RF pulses for parallel excitation using phased array transmit coils in MRI requires the B1+ maps that are estimated from B1+ mapping experiments. This paper characterizes the effects of B1+ mapping errors on the resulting excitation pattern using a small perturbation analysis based on linearization of the Bloch equation. The accuracy of the proposed perturbation analysis is validated by Bloch equation simulations based on experimental B1+ maps. The perturbation analysis builds a transparent connection between the B1+ mapping errors and the resulting excitation errors, and can be used to study the robustness of the designed RF pulses to B1+ mapping errors. The proposed method may also supply useful metrics for designing RF pulses that are robust to B1+ mapping errors.

Skeletal muscles are highly organized hierarchical structures characterized by an anisotropic arrangement of muscle fibers (myocytes) in fascicles. Due to its unique non-invasive microstructure probing capabilities, diffusion-weighted Magnetic Resonance Imaging (DW-MRI) constitutes a valuable non-invasive tool in the study of such fibrous biological tissues. We have implemented a DW-MRI sequence with highly sensitive directional encoding to quantify the microarchitectural properties of human calf muscles at rest. We have specifically focused on a composite model-based analysis of diffusion tensor MRI measurements to quantify in vivo the cross-sectional asymmetry of muscle fiber geometry, which is a microstructural feature well documented in prior histological studies.