Development of clinically applicable parallel transmit methods for Ultra-High Field MRI

Abstract

Besides many already potential and already demonstrated benefits, ultra-high field (UHF) MRI suffers from stronger spatial variations of the B1+, B0 fields and local specific absorption rate (SAR) exposure. To overcome these limitations, the concept of parallel transmission (pTx) was proposed, which uses radio-frequency (RF) coils with multiple transmit elements that can be driven with different RF pulse shapes. In combination with simultaneously applied gradient waveforms, often described with ’transmit k-space trajectories’, much more homogeneous flip angle (FA) and signal distributions can be generated under controllable SAR exposure. Despite of its promising capabilities, the complexity of dynamic pTx pulse design has impeded a clinical use of this technique up to date. In this work, a method to design pTx pulses is proposed, which is applicable in everyday clinical practice. These so-called ’Fast Online-Customized’ (FOCUS) pTx pulses combine universal or cluster-specific optimizations of gradient waveforms, RF pulse shapes and additional parameters previous to the examination (offline) and a further patient-specific customization during the examination (online). FOCUS pTx pulses have shown robust performances across 132 subjects while preserving feasible additional sequence preparation times of about one minute including B1+ , B0 mapping and pTx pulse calculation when applied with an 8Tx/32Rx head coil at 7 tesla. For practical reasons, it was distinguished between various use cases, such as low FA and high FA pulses, slice-selective and non-selective pulses, as well as pulses for gradient echo (GRE) and turbo spin echo (TSE) sequences. For all these cases, the strategies to design FOCUS pTx pulses varied in detail: For non-selective pTx pulses exciting low FAs, the optimization problem for designing RF pulse shapes can be linearized for a given transmit k-space trajectory. In this case, relatively complicated extended ’spiral non-selective’ (SPINS) and newly developed ’spherical nonselective’ (SPENS) transmit k-space trajectories, corresponding energy regularization weights and rather complicated RF pulse shapes are optimized universally using B1+, B0 maps of 12 different subjects. Thereby, the online-customization during the scan can be done by solely further optimizing the RF pulse shapes. A specifically developed algorithm was developed to adapt the regularization weight online if necessary such that the sequence’s SAR limits for the actual subject are met. By implementing this strategy into the scanner software, the online-pulse calculation was performed automatically and took a maximum of 15 seconds. The resulting FOCUS pulses reached coefficients of variation (CV) of the FA distribution of 10.8 ± 0.7% across whole heads, thereby outperforming the conventionally used circularly polarized (CP) pulses (CV = 28.2±2.4%) and Universal Pulses (UPs) (CV = 16.2±1.9%). The FOCUS pulses’ robustness regarding vastly differing head anatomies is also demonstrated by a small variance across a total of 132 different subjects. In addition, FOCUS pulses maintained robust performances when used for patients, including a brain surgery patient (Fig. 4.8) with strong anatomical anomalies. Moreover, pTx pulses for high FA excitation and inversion were designed. In this large tip angle (LTA) domain, a computationally costly, nonlinear optimization problem needs to be solved. For this reason, non-selective pTx pulses were designed using a simpler kT points trajectory, which only samples 6 locations in 3D transmit k-space and RF pulses that consist of as many rectangular shaped RF subpulses. Thereby, the calculation time for individual pulse optimization remains short, also when simultaneously optimizing those transmit k-space locations. The FOCUS pTx pulses for 180°-inversion have shown to be useful as low-SAR alternative to conventional adiabatic hyperbolic secant shaped inversion pulses, which also provide robust homogeneity for a variety of field distributions. FOCUS pTx pulses for non-selective 90° excitation were designed to generate a homogeneous phase distribution as well. This has shown to be useful in combination with pTx refocusing pulses for a TSE sequence, which were optimized with so-called ’Direct Signal Control with Variable Excitation and Refocusing’ (DiSCoVER) pulses. Also when using universal DiSCoVER refocusing pulses, stronger and more homogeneous signal was achieved throughout the head for a TSE train consisting of 10 echoes while preserving the same SAR level compared to CP pulses or the original DiSCoVER method using static pTx excitation pulses. Slice-selective pTx pulses for high FA excitation were generated with sinc-shaped RF subpulses and the spokes trajectory, which can be viewed as 2D-equivalent of the kT points. The sinc-shapes require longer pulse durations, which is why only 2 subpulses are applied on 2 respective locations in 2D transmit k-space (x,y-plane), whereas during both subpulses, slice-selection gradients are applied in z-direction. To be able to adapt to strongly different B1+ and B0 field distributions from different slices given only few degrees of freedom for the pulse design, the optimization of one UP and associated parameters was replaced by a number of cluster-specific pulses (CSPs). These clusters of different slices, given their specific B1+ and B0 field distributions, were generated based on the similarity of the performance of various pTx pulses. Subsequently, for every cluster, one neural network (NN) was trained to predict its respective CSP’s performance. Based on these predictions, the choice of the most suitable CSP for every slice to be excited can then be made quickly online. This approach has shown to provide better homogeneity at the same local SAR level than using purely CP pulses or coordinate-based clusters, which are based on the slices’ positions relative to the transmit elements. The proposed FOCUS pTx pulses have shown robust performance under local SAR constraints in a short online-calculation time (≤ 15s). Thereby it is possible to apply individually optimized pTx pulses in a clinical setting, which markedly improve the spatial FA and signal homogeneity in UHF MRI sequences. Show more