IMPULSE: A scalable algorithm for design of minimum specific absorption rate parallel transmit RF pulses.

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.

SAR prediction in adults and children by combining measured B1+ maps and simulations at 7.0 Tesla.

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.

Evaluation of 3D radio-frequency electromagnetic fields for any matching and coupling conditions by the use of basis functions.

A procedure for evaluating radio-frequency electromagnetic fields in anatomical human models for any matching and coupling conditions is introduced. The procedure resorts to the extraction of basis functions: such basis functions, which represent the fields produced by each individual port without any residual coupling, are derived through an algebraic procedure which uses the S parameter matrix and the fields calculated in one (only) full-wave simulation. The basis functions are then used as building-blocks for calculating the fields for any other S parameter matrix. The proposed approach can be used both for volume coil driven in quadrature and for parallel transmission configuration.

Investigation of maximum local specific absorption rate in 7 T magnetic resonance with respect to load size by use of electromagnetic simulations.

Local specific absorption rate (SAR) evaluation in ultra high field (UHF) magnetic resonance (MR) systems is a major concern. In fact, at UHF, radiofrequency (RF) field inhomogeneity generates hot-spots that could cause localized tissue heating. Unfortunately, local SAR measurements are not available in present MR systems; thus, electromagnetic simulations must be performed for RF fields and SAR analysis. In this study, we used three-dimensional full-wave numerical electromagnetic simulations to investigate the dependence of local SAR at 7.0 T with respect to subject size in two different scenarios: surface coil loaded by adult and child calves and quadrature volume coil loaded by adult and child heads. In the surface coil scenario, maximum local SAR decreased with decreasing load size, provided that the RF magnetic fields for the different load sizes were scaled to achieve the same slice average value. On the contrary, in the volume coil scenario, maximum local SAR was up to 15% higher in children than in adults.

Tissue Border Enhancement by inversion recovery MRI at 7.0 Tesla.

INTRODUCTION: This contribution presents a magnetic resonance imaging (MRI) acquisition technique named Tissue Border Enhancement (TBE), whose purpose is to produce images with enhanced visualization of borders between two tissues of interest without any post-processing. METHODS: The technique is based on an inversion recovery sequence that employs an appropriate inversion time to produce images where the interface between two tissues of interest is hypo-intense; therefore, tissue borders are clearly represented by dark lines. This effect is achieved by setting imaging parameters such that two neighboring tissues of interest have magnetization with equal magnitude but opposite sign; therefore, the voxels containing a mixture of each tissue (that is, the tissue interface) possess minimal net signal. The technique was implemented on a 7.0 T MRI system. RESULTS: This approach can assist the definition of tissue borders, such as that between cortical gray matter and white matter; therefore, it could facilitate segmentation procedures, which are often challenging on ultra-high-field systems due to inhomogeneous radiofrequency distribution. TBE allows delineating the contours of structural abnormalities, and its capabilities were demonstrated with patients with focal cortical dysplasia, gray matter heterotopia, and polymicrogyria. CONCLUSION: This technique provides a new type of image contrast and has several possible applications in basic neuroscience, neurogenetic research, and clinical practice, as it could improve the detection power of MRI in the characterization of cortical malformations, enhance the contour of small anatomical structures of interest, and facilitate cortical segmentation.

Electromagnetic characterization of an MR volume coil with multilayered cylindrical load using a 2-D analytical approach.

We present an analytical method for the analysis of Radio Frequency (RF) volume coils for Magnetic Resonance Imaging (MRI), using a 2-D full wave solution with loading by multilayered cylinders. This allows the characterization of radio-frequency E, H, B1, B1(+) fields. Comparisons are provided with experimental data obtained at 7.0 T. The procedure permits us to clearly separate the solution to single line source problem (which we call the primordial solution) and the composite solution (i.e. full coil, i.e. the summations of primordial solutions according to the resonator drive configuration). The capability of separating the primordial solution and the composite one is fundamental for a thorough analysis of the phenomena of dielectric resonance, and of standing wave and multi-source interference. We show that dielectric resonance can be identified only by looking at the electromagnetic field from a single line source.