A formalism to investigate the optimal transmit efficiency in radiofrequency shimming

Abstract

Transmit efficiency specifies the amplitude of the magnetic resonance excitation field produced over a region of interest with respect to the radiofrequency (RF) power deposited in the sample. This metric is highly important at ultra-high field magnetic resonance imaging (≥7 T), where excitation inhomogeneities and electric field interference effects could prevent achieving the desired flip angle distribution while satisfying the power safety limits. The aim of this work was to introduce an approach to calculate a theoretical upper bound on the transmit efficiency (OPTXE) for RF shimming, independent from any particular coil design. We computed the OPTXE for head-mimicking uniform spherical samples and a realistic heterogeneous head model by maximizing the square of the net transmit field per unit power deposition. The corresponding RF shimming weights were used to combine the analytical surface current modes into ideal current patterns. OPTXE grew monotonically as the target excitation voxel approached the surface of the object, and overall decreased at higher field strengths, presenting similar trends in both the uniform sphere and heterogeneous head model. Arrays with an increasing number of loops could closely approach OPTXE in the central region of the object, but performance decreased closer to the surface and at higher magnetic field strengths. The performance of 32 loops for a two-dimensional excitation region at 7 T increased from 34% to 93% when they were arranged based on the shape of the ideal current patterns. OPTXE provides an absolute reference to evaluate coil designs and RF shimming algorithms, whereas ideal current patterns could serve as guidelines for novel coil designs at ultra-high field. The uniform sphere model enables rapid analytic simulations and provides a good approximation of the OPTXE distribution in a realistic heterogeneous head model with comparable dimensions.

32‐Channel self‐grounded bow‐tie transceiver array for cardiac MR at 7.0T

“…The performance ratio (%) of the RF array reveals the ratio of the intrinsic and the realistic and superpositions. The values derived in this way can be used to quantitatively compare expected SNR and transmit efficiency of different arrays and to assess theoretical electromagnetic upper bounds 36,37 …”

“…The values derived in this way can be used to quantitatively compare expected SNR and transmit efficiency of different arrays and to assess theoretical electromagnetic upper bounds. 36,37 EMF shaping was performed using a combined approach for Ella and Duke, where the minimum B + 1 in the ROI is maximized by using the target function with the B + 1 scaling factor (δ = 1…4) and VOPs scaling factor (λ = 0.001…5). The phase optimization was performed with a generic algorithm implemented in the global optimization toolbox of MATLAB 2019b.…”

A Simple Auxiliary Model for Field Amplitude Shaping in Complex Environments, and Application to MRI Shimming

“…Different RF shimming approaches have been proposed in literature, which can be classified into two main categories: passive and active ones [7], [15]- [22]. The first category commonly uses high-permittivity materials put close to the ROI to vary the spatial distribution of the magnetic field, improve field homogeneity and enhance signal noise ratio [16]- [19].…”

“…In [16] dielectric shimming is formulated as an electromagnetic scattering problem using integral equations. On the other hand, in the active shimming techniques, the RF field inhomogeneity can be addressed by using multielement transmit coils [20]- [22]. These techniques can be also optimized to reduce global SAR, since interferences between the electric fields from multiple transmit coils can result in amplifications of local SAR which are difficult to predict [21]- [22].…”

Maxwell parallel imaging

“…41,42 This method allows to sample the left subspace of K without actually building it, and to extract an orthogonal basis up to any prescribed accuracy. The process to numerically generate the EM basis is described in previous relevant works, [43][44][45][46][47] which make use of the scripts found in 48 and the open-source software MARIE. 49 The difference in this work is that we generate a basis only for circularly polarized magnetic fields in the absence of a realistic human body model.…”