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Real-time processing of image and trajectory data: Application to single-shot time series

Josip Marjanovic¹, Jonas Reber¹, Maria Engel¹, Lars Kasper¹, Benjamin E. Dietrich¹, David O. Brunner¹, and Klaas P. Pruessmann¹

¹ETH Zurich, Zurich, Switzerland

Synopsis

The steady increase in data volume is a significant problem in fMRI time series. Recently proposed signal processing architectures based on digital hardware enable data reduction in real-time before the data are stored. We demonstrate that a high degree of data savings can be achieved with few simple operations. Coil compression and field probe data processing help breaking the data bottleneck in fMRI time-series.

Introduction

Single-shot time series rely on fast acquisition of high quality images. Continuing advances in scanner hardware and imaging methods push the boundaries on achievable SNR and acquisition speed. Especially the advent of parallel imaging^1,2,3 has enabled higher spatial resolution and more robust scans with a better time resolution due to shorter readouts. More recent developments go even towards simultaneous acquisition of multiple slices utilizing multiband excitation.⁴ Those techniques rely on large receive arrays with up to 32 channels now already clinical standard with a trend to more.^5,6,7 On the other hand, this development has led to a massive increase in data to be transferred, stored, reconstructed and analyzed. The trend further accelerated as auxiliary sensor systems^{8,9,10,11,12,13,14} pave their way into MRI, which introduce advanced signal models in image reconstruction. Especially optical motion tracking cameras^11,13 and field monitoring¹⁰ generate additional data at bandwidths even beyond the imaging data. This is challenging, as they require a different processing chain. Recently, a real time processing platform^15,16 has been presented providing high-throughput digital fiber-optical inputs for modern receiver and sensor modules with in-field operation¹⁷. In this work, we apply a reconfigurable signal processing architecture based on a system-on-chip (SoC) for single-shot acquisitions supported with field monitoring. We report online data volume reduction by receiver coil compression and field probe data processing in the order of up to 80% in a setup consisting of a 32-channel array and 16 probes for magnetic field monitoring.

Methods

All experiments were performed on a 7T human scanner (Phillips Healthcare, Best, The Netherlands). Data were collected using a concurrent imaging and monitoring setup as in¹⁸. It consists of a commercially available quadrature-transmit coil surrounding a 32-channel head receive array (Nova Medical, Wilmington, USA) and in-between an integrated array of 16 ¹⁹F NMR field probes. The probes were operated using standalone versions of the direct digital synthesis (DDS) and transmit-receive (T/R) box presented in¹⁹ and acquired using an in-bore receiver module¹⁷. Additional two 16-channel in-bore receivers¹⁷ were used to digitize the coil signals. All receivers were phase-frequency locked to the processing platform through the back-channel of the optical link. The platform was used to fuse the coil data streams and compress it down to 12 virtual coils in real-time before storing using a set of embedded matrix-vector multipliers. The probe phases were calculated using the CORDIC²⁰ algorithm and fitted to a second-order spherical-harmonic field model¹⁰. After unwrapping and off-resonance correction, phases were transformed into k-space trajectories using a second matrix-vector multiplier. Since the gradient fields have a bandwidth of up to 30kHz, a decimation filter was employed for trajectory down-sampling. All operations have been performed in the field programmable gate array portion of the platform SoC (Xilinx, San Jose, USA) without additional computational resources. A picture of the used hardware without coils and probe front-end is given in Fig. 1. Two scan protocols were employed: segmented spirals¹⁸ for anatomical imaging to demonstrate the fidelity of the receive chains and single-shot spirals as used in fMRI^21,22. The imaging parameters are given in Table 1. Image reconstruction was performed by inversion of the expanded signal model^23,24,18 including static off-resonance and coil sensitivity maps obtained with a separate multi-gradient-echo spin-warp scan, as well as zero and first order k-space trajectories. The coil compression matrix was calculated from the sense maps according to²⁵.

Results

Fig. 2 shows a single slice of the acquired multi- and single-shot spirals. The multi-shot image shows a high consistency between the interleaves and data modalities. The single-shot, as commonly used in fMRI, is of high quality and provides a fine resolution. Fig. 3 shows temporal SNR (tSNR) plots for a time series acquisition of the single-shot spiral. Two images are shown - raw and compressed data. Coil compression results in a high degree of data savings (62.5% in the shown case) with 3% SNR penalty. All images were reconstructed with trajectories calculated in hardware resulting in 98.43% data savings. For a typical 30 min fMRI session 200GB raw data are reduced down to 40GB saving 80.46% in total as shown in Table 2.

Discussion

The steady increase in data volume has become a significant problem with data transfer becoming the limiting factor in reconstruction even with cutting-edge hardware. It is demonstrated that data processing already at the level of the acquisition system can drastically reduce the data volume. As demonstrated for the case of high temporal and spatial resolution single-shot data acquisition, this reduction of data volume comes at a negligible loss of image quality.

Acknowledgements

No acknowledgement found.

References

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Figures

Processing platform with high-throughput optical interfaces suitable for receiver modules and sensor systems with in-field operation and a state-of-the-art SoC based on powerfull processor and large FPGA.

In-vivo images of a healthy volunteer. a) 36-interleave spiral acquisition demonstrates the capability of high quality anatomical imaging at a resolution of 0.4 mm with a slice thickness of 2 mm. It shows a high degree of consistency between the interleaves and the three receivers used to acquire coil and probe data. b) Single-shot spiral acquisition with fourfold undersampling (R=4) with an in-plane resolution of 0.8 mm and 2 mm slice-thickness. The image shows good quality of depiction.

Temporal SNR plot for a 0.8mm single-shot spiral calculated on basis of a) 32 coils raw data and b) a set of 12 virtual coils when coil compression is applied. The image renders coil compression applicable in SNR wise very sensitive cases as time series fMRI.

Table 1. Imaging parameters of single-shot and segmented multi-shot spirals.

Table 2. Data savings achieved employing different processing steps on an exemplary 30 min fMRI session. Calculations were based on a coil with 32 channels as commonly used and moderate compression down to 12 virtual coils, a scanner with gradient dynamics up to 50 kHz and 16 probes for field monitoring.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)

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