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Purpose: Managing local specific absorption rate (SAR) in parallel transmission requires ensuring that the peak SAR over a large number of voxels (>105) is below the regulatory limit. The safety risk to the patient depends on cumulative (not instantaneous) SAR thus making a joint design of all RF pulses in a sequence desirable. We propose the Iterative Minimization Procedure with Uncompressed Local SAR Estimate (IMPULSE), an efficient optimization formulation and algorithm that can handle uncompressed SAR matrices and optimize pulses for all slices jointly within a practical time frame. Theory and Methods: IMPULSE optimizes parallel transmit pulses for small‐tip‐angle slice selective excitation to minimize a single cost function incorporating multiple quantities (local SAR, global SAR, and per‐channel power) averaged over the entire multislice scan subject to a strict constraint on excitation accuracy. Pulses for an 8‐channel 7T head coil were designed with IMPULSE and compared with pulses designed using generic optimization algorithms and VOPs to assess the computation time and SAR performance benefits. Results: IMPULSE achieves lower SAR and shorter computation time compared with a VOP approach. Compared with the generic sequential quadratic programming algorithm, computation time is reduced by a factor of 5‐6 by using IMPULSE. Using as many as 6 million local SAR terms, up to 120 slices can be designed jointly with IMPULSE within 45 s. Conclusions: IMPULSE can handle significantly larger number of SAR matrices and slices than conventional optimization algorithms, enabling the use of uncompressed or partially compressed SAR matrices to design pulses for a multislice scan in a practical time frame. [ABSTRACT FROM AUTHOR]

Purpose: To predict local and global specific absorption rate (SAR) in individual subjects.Materials and Methods: SAR was simulated for a head volume coil for two imaging sequences: axial T1-weighted "zero" time-of-echo (ZTE) sequence, sagittal T2-weighted fluid attenuated inversion recovery (FLAIR). Two head models (one adult, one child) were simulated inside the coil. For 19 adults and 27 children, measured B1 (+) maps were acquired, and global (head) SAR estimated by the system was recorded. We performed t-test between the B1 (+) in models and human subjects. The B1 (+) maps of individual subjects were used to scale the SAR simulated on the models, to predict local and global (head) SAR. A phantom experiment was performed to validate SAR prediction, using a fiberoptic temperature probe to measure the temperature rise due to ZTE scanning.Results: The normalized B1 (+) standard deviation in subjects was not significantly different from that of the models (P > 0.68 and P > 0.54). The rise in temperature generated in the phantom by ZTE was 0.3°C; from the heat equation it followed that the temperature-based measured SAR was 2.74 W/kg, while the predicted value was 3.1 W/kg.Conclusion: For ZTE and FLAIR, limits on maximum local and global SAR were met in all subjects, both adults and children. To enhance safety in adults and children with 7.0 Tesla MR systems, we suggest the possibility of using SAR prediction. J. MAGN. RESON. IMAGING 2016;44:1048-1055. [ABSTRACT FROM AUTHOR]

ABSTRACT High static magnetic field magnetic resonance imaging (MRI) is commonly used for preclinical studies in rodents. In this context, minimization of coil losses is mandatory to scan samples that are small compared to the radiofrequency wavelength in the medium. In this study we construct a radiofrequency (RF) birdcage probe with distributed capacitors, operating in quadrature, tailored for 7.0T 1H MRI of small animals. The design eliminates the need for extra electrical components on the probe structure and affords a high SNR, a uniform [ABSTRACT FROM AUTHOR]

Numerical methods based on solutions of Maxwell's equations are usually adopted for the electromagnetic characterization of Magnetic Resonance (MR) Radiofrequency (RF) coils. In this context, many different numerical methods can be employed, including time domain methods, e.g., the Finite-Difference Time-Domain (FDTD), and frequency domain methods, e.g., the Finite Element Methods (FEM) and the Method of Moments (MoM). We provide a quantitative comparison of performances and a detailed evaluation of advantages and limitations of the aforementioned methods in the context of RF coil design for MR applications. Specifically, we analyzed three RF coils which are representative of current geometries for clinical applications: a 1.5T proton surface coil; a 7T dual tuned surface coil; a 7T proton volume coil. The numerical simulation results have been compared with measurements, with excellent agreement in almost every case. However, the three methods differ in terms of required computing resources (memory and simulation time) as well as their ability to handle a realistic phantom model. For this reason, this work could provide "a guide to select the most suitable method for each specific research and clinical applications at low and high field". [ABSTRACT FROM AUTHOR]