Synopsis

Ultra-high-field (UHF) provides higher SNR than conventional, clinically available field strengths. However, UHF suffers from heterogeneous transmit B₁⁺ fields. At 7 T, the shortened transmit radio-frequency (RF) wavelengths have a similar value to the dimensions of the human head/thorax which may result in signal cancellation and local signal dropouts. In this paper, we propose a novel imaging scheme based on simultaneous excitation with all transmit channels. Controlled aliasing is used to encode each transmit channel independently which we term Tx-CAIPIRINHA.

Tx-CAIPIRINHA has been demonstrated in-vivo. The concept uses the linear superposition of B₁⁺ fields via the excitation flip angle which only holds true in the low flip angle regime. When normalizing to transmit efficiency, Tx-CAIPIRINHA achieved a marginally higher SNR than B₁⁺ shimming, demonstrating the constructive combination of transmit sensitives throughout the image.

Introduction

Ultra-high-field (UHF) provides higher SNR than conventional, clinically available field strengths. However, UHF suffers from heterogeneous transmit (B₁⁺) fields [1]. At 7 T, the shortened transmit radio-frequency (RF) wavelengths have a similar value to the dimensions of the human head/thorax which may result in signal cancellation and local signal dropouts.

Radio-frequency shimming with multiple channel excitation of a parallel transmit (pTx) array is a well established method to increase the transverse magnetic field homogeneity at high magnetic field strength [2]. To perform RF shimming, relative transmit sensitivity maps are acquired. They are used to determine the combination of amplitudes and phases to either maximize the minimum B₁⁺ or minimize the coefficient of variation throughout a region of interest. However, as the field of view increases, the excitation efficiency reduces to account for larger variation until that there is no satisfactory shim solution [3]. Subject motion between adjustment mapping and image acquisition means that the optimal B₁⁺ solution can change [4]. Time interleaved acquisition of modes (TIAMO) [5] addresses the B₁⁺ heterogeneity by combining two different transmit modes. In this paper, we propose a novel imaging scheme based on simultaneous excitation with all transmit channels. Controlled aliasing is used to encode each transmit channel independently which we term Tx-CAIPIRINHA

Methods

Cardiac measurements were performed in two healthy volunteers on a whole-body research 7Tesla MR-system (Siemens Healthcare GmbH, Erlangen, Germany). Healthy volunteer measurements were acquired according to our institutions ethical practices. Data acquisition was performed with a custom built eight-channel transmit/receive cardiac transverse electro-magnetic (TEM) coil.

Images were acquired using a modified non-product, gradient-echo (GRE) sequence allowing simultaneous transmit with controlled aliasing for each transmit channel. This was achieved by introducing a different phase modulation for each transmit channel resulting in a different shift within the FOV of excitation resulting from each transmitter. The increments in phase from one line to the next ranged from 0° to 315° in steps of 45° for each transmit channel. To calibrate the receive and transmit sensitives, low resolution, low flip angle, transmit maps were acquired by transmitting on one Tx channel at a time [6]. The acquisition parameters of the GRE sequence are: TR=9.64ms,TE =1.68 ms, acquisition matrix=352x352, FOV=350x350mm². Single-slice images of 4mm thickness were acquired during four cardiac cycles in a single breath hold. For comparison purpose, an image of the same slice with identical orientation and RF pulse voltage was acquired using B₁⁺ shimming.

Aliased images formed when transmitting on all channels were reconstructed using Tx channel–GRAPPA (Tx-GRAPPA) algorithm similar to slice-GRAPPA [7]. A 7×7 GRAPPA kernel is separately fitted in k-space to each transmit channel of the calibration data. In the case of the Tx-GRAPPA method, after reconstruction of separate images for each transmit and receive channel, the images were combined. First, images from all the receivers were combined with a root sum-of-squares (rSoS) reconstruction incorporating the noise covariance matrix. In the second step, rSoS reconstruction was performed along the transmit dimension.

For comparison a conjugate gradient iterative SENSE reconstruction was also employed [8] where the product of transmit and receive sensitivities was used to form an 8x8 or 64 virtual channel SENSE reconstruction. This method provides one combined image.

Results

A simulation of the efficiency (ratio to maximum B₁⁺ possible) was performed for 100,000 random B₁⁺ fields of an 8 channel system when using Tx-CAIPIRINHA and was compared to no B₁⁺ shimming. The average attained flip angle is well behaved and it is approximately 1/√N-TxChannels i.e. 1/√8, compared to no B₁⁺ shimming. The result of this simulation is summarized in fig.1.

Figure 2 shows the reconstructed images using Tx–CAIPIRINHA with a 7×7 GRAPPA kernel (a), iterative SENSE (b) and B₁⁺ shimming (c). The efficiency of the applied B₁⁺ shim was 67% compared to an expected 35% for Tx-CAIPIRINHA (Fig 1) a ratio of 1.9:1.

Both methods Tx-GRAPPA and iterative SENSE produced similar results with high signal homogeneity in the heart. This was achieved without any prior operator adjustment of the B₁⁺ field of each transmit channel.

Discussion

Acknowledgements

References

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