Introduction

High resolution is one of the major benefits expected from 7 Tesla Cardiac MRI exams, notably high-resolution cardiac dynamic MRI (Cine). However, increasing the resolution of Cine MRI is inherently limited by the duration of a breath-hold. Moreover, 2D Cine MRI resolution is highly anisotropic, with small pixels but thick slices. Therefore, Cine MRI could benefit from higher SNR at 7T to acquire thinner slices and thus improve partial volume issues within short-axis views. Within these considerations, the problem of limited breath-hold duration is shifted to the increased number of repeated breath-hold to cover the left ventricle (LV) with thinner slices. In this work, we propose to accelerate Cine MRI at 7T using Simultaneous Multi-Slice (SMS) technique(1). Additionally, a novel cardiac-dedicated SMS technique is proposed to alleviate the issue of alignment between single-band reference data and multi-band accelerated imaging data.

Material and Methods

Acquisitions were performed on a 7T MRI scanner (Siemens Magnetom) using a 32Rx/8Tx cardiac coil (MRITOOLS). The 8Tx channels were combined as a single Tx channel using an optimized predefined phase set. Eight healthy volunteers were recruited after written consent was obtained. A 2D FLASH-Cine sequence with retrospective EKG gating was modified with CAIPIRINHA-SMS excitations(2). Instead of a separate reference acquisition, a Hadamard(3) SMS phase-shift pattern is introduced along the cardiac phase dimension to allow for slice separation (Figure1). The resulting Hadamard-decoded k-spaces can then serve for calibration of SMS-reconstruction. Cine parameters were SMS3x–GRAPPA2x, 1.8x1.8x4mm3 resolution reconstructed to 0.9x0.9x4mm3, 28ms temporal resolution, ~10s breath-hold/slice. The same acquisitions were performed without SMS acceleration (noted SS) for comparison. For both acquisitions, flip angle were maximized to fit within SAR restrictions. Image reconstruction combining Split-Slice GRAPPA(SSG) and in-plane GRAPPA was implemented online using Gadgetron(4). A second reconstruction using SSG + L1-SPIRiT was performed retrospectively. SNR and CNR were measured between the septum and the LV blood pool.

Results

Using 1ms 3-bands excitations, specific absorption rate (SAR) restrictions imposed the flip angle for SMS-3 CINE to be reduced by 37 ± 7% compared to single-band CINE, from 36-69 to 20-50 degrees. Signal-to-noise ratio (SNR) measured in the septum showed a decrease from single-slice (SS) acquisitions to SMS acquisition (SNR(SS)= 8.7 ± 3.6 vs SNR(SMS)=5.5 ± 3.7, p=0.08) although not significantly. The CNR between the LV blood pool and the septum was significantly decreased from CNR(SS)= 12.4 ± 6.7 for single-slice CINE to CNR(SMS)=5.5 ± 2.7 (p<0.001) for SMS3-CINE. Figure 2 shows the results from one volunteer of the 3 slices and the long axis reformat. The 3 slices are well separated and image contrast is sufficient for assessing cardiac function. Similarly, blood-myocardium CNR was reduced (3.1±1.3 vs 7.1±2.5, p<0.01) due to the SAR-limited SMS excitations when compared to single-slice excitations. The resliced Cine in long axis view highlight revealed few SMS slices that had poorer image quality. Exploiting the sparsity in the spatio-temporal domain, Split-Slice-GRAPPA(SSG) + L1-SPIRiT image reconstruction was able to mitigate the noise amplification from SMS acquisition. The SNR was improved by 38 ± 24% and the CNR was improved as well by 45 ± 25 %.

Conclusion and discussion

Accelerated high resolution Cine using SMS offers multiple perspectives, from simply reducing the examination time to reaching higher resolution to refine imaging of subtle pathologies such as ventricular non-compaction. The novel SMS technique detailed here shows promises for rapid high-resolution Cine which could also benefit clinical field MRI. However, the reduced flip angle imposed by SAR to account for increased power of multi-bands excitations impacts the image noise and contrast.

Acknowledgements

This work was performed by a laboratory member of France Life Imaging network (grant ANR-11-INBS-0006). This work was performed on the platform 7T-AMI, a French “Investissements d’Avenir” programme” (grant ANR-11-EQPX-0001).

References

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