Synopsis

Parallel transmission is a cutting-edge asset to tackle RF field inhomogeneity or to realize inner volume selection for cervical spinal cord imaging at 7T. To perform VOP-based pTx in vivo exams with a dedicated 8Tx/8Rx coil the following workflow is proposed: 1) starting from electromagnetic simulations on generic human body models, Q-matrices sets were compressed with the VOP method. 2) B₁⁺ mapping was performed on a phantom with known properties to validate these electromagnetic simulations by deriving calibration matrices, and initialize error propagations and further related safety margins. 3) MR thermometry was also performed to cross-check obtained results.

Introduction

Unlike standard MR systems, multi-transmit array (pTx) systems allow transmitting multiple radiofrequency (RF) pulses simultaneously and synchronously through independent RF channels, thus enabling RF field homogeneity optimization or multidimensional RF pulse acceleration [1]. With the spatial modulation of the RF fields in the human body, the distribution of the deposited power and therefore the local Specific Absorption Rate (SAR) change as well, depending on the amplitudes and phases of the currents applied to the individual coil elements [2]. In the sake of RF safety, local SAR in humans is subject to regulatory limitations [3]. Hence, to predict and control local SAR in a computationally efficient manner, the pTx system must be equipped with the recently introduced Virtual Observation Points (VOP [4]), which are either provided by the coil manufacturer or generated in situ.

In this work, we present a workflow which will enable to perform VOP-based pTx in vivo exams with an 8Tx/8Rx coil dedicated to spinal cord (SC) imaging at 7T. Starting from electromagnetic (EM) simulations on generic human body models (GHBMs), Q-matrices [2] sets were compressed with the VOP method. In parallel, two experiments (B₁⁺ mapping and MR thermometry) were performed on a phantom to validate EM simulations and to derive calibration matrices and error propagations, paving the way for comprehensive safety margins integration.

Methods

System: whole-body 7T Magnetom Step 2.3 (Siemens Healthineers, Erlangen, Germany).

Coil: 8Tx/8Rx transceiver array (Rapid Biomedical, Rimpar, Germany) constituted with 8 loops spread on a saddle surface cradling the neck.

EM simulations (time-domain solver, hexahedral mesh): with CST Studio Suite (CST, Darmstadt, Germany).

GHBMs: 2 models (Gustav, Laura) of the CST Voxel family.

Q-matrices generation: in-house routine.

VOP compression [4]: 10% maximum SAR overestimation.

MR phantom: SAM-V4.5BS (Speag, Zurich, Switzerland), head-shaped filled with gel with known EM and thermal properties [5].

B₁⁺ mapping: with interferometric [6] magnetization-prepared TFL sequence [7]. Simulated B₁⁺ fields are corrected through linear combinations to account for imperfect predictions of mutual coupling and losses [8] (calibration matrices obtained with least-squares minimization), and then compared to measured B₁⁺ fields.

MR thermometry: with the Proton Resonance Frequency Shift method (PRFS) [9]. RF-induced heat generated with a RF-only sequence (duration: 10min, P_tot: 20W). Phase measurements with 3D GRE sequences. Thermal simulations: integration of the heat equation.

Results

Discussion

This work presents the first steps toward VOP-based pTx in vivo imaging of the SC at 7T in our group. The proposed validations are in line with the ones recommended in [10,11]. To ensure safe local SAR management, several safety margins have also to be considered: 1) Inter-subject variability influence on SAR, which was only studied for head coil [12,13], will require a dedicated study for SC. 2) EM simulation errors on B₁⁺, which could be propagated using the methodology described in [14], are currently under integration in the process. 3) System measurement errors influence is already integrated during VOP compression [15]. The final derived safety factor is expected to encompass observed MR thermometry result differences.

Further steps include: 1) additional phantoms and B₁⁺ mapping methods to verify the consistency of the B₁⁺ correction and 2) more GHBMs simulation to create a ‘VOP database’ usable for any volunteers/patients. Future work should also include VOP consistency checking between online monitoring and offline prediction. Once enabled, parallel transmission will be a cutting-edge asset to tackle RF field inhomogeneity or to realize inner volume selection for cervical SC imaging at 7T.

Acknowledgements

References