Concentrically circular echo planar spectroscopic imaging at 3T and 7T with partial temporal interleaving
Neil Wilson1, Hari Hariharan1, M. Albert Thomas2, and Ravinder Reddy1

1Radiology, University of Pennsylvania, Philadelphia, PA, United States, 2Radiology, University of California, Los Angeles, CA, United States

Synopsis

We use concentric circular echo planar k-space readout to spectroscopic sampling at high field. At high field, higher bandwidths are required which are difficult to achieve using echo planar techniques due to gradient limitations. Often temporal interleaving is employed to mitigate this. Circular k-space sampling is unique among the echo planar trajectories in that different rings can be sampled at different rates, requiring only partial temporal interleaving.

Purpose

In echo planar spectroscopic imaging (EPSI), there is a general tradeoff between image resolution and the acquired spectral bandwidth. By acquiring multiple acquisitions with echo time shifting, separate free induction decays (fids) can be temporally interleaved to increase the spectral bandwidth. Here, we show that the unique shape of concentric circular trajectories allows for the collection of high bandwidth EPSI with limited temporal interleaving compared to other echo planar trajectories.

Introduction

CSI works by collecting an fid at each k-space location, filling it point-by-point. EPSI1,2 accelerates this by repeatedly acquiring an entire line of k-space in a single acquisition. Non-Cartesian versions have been shown that collect k-space in spirals3, circles4, or rosettes5 which can fill k-space more efficiently than lines. For each trajectory, though, as spatial resolution is increased, more k-space points need to be filled, decreasing the achievable spectral bandwidth due to the limits on gradient hardware. This is particularly problematic at ultra high field where larger bandwidths are required to sample the proton spectral range.

The achievable spectral bandwidth can be restored by acquiring additional scans with the echo time shifted by a fraction of the spectral dwell time and temporally interleaving the fids. For most EPSI trajectories, including those mentioned above, the gradient hardware requirements are identical from scan-to-scan, and any required temporal interleaving must be done for each acquisition. However, in the case of concentric circles, inner k-space circles have greatly reduced gradient hardware demands than outer k-space circles4. Therefore, inner k-space circles can be acquired at a higher spectral bandwidth, and temporal interleaving only needs to be applied to outer k-space circles. Partial temporal interleaving is illustrated in Fig 1.

Methods

The pulse sequence used a semi-LASER6 localization with sinusoidal readout gradients to encode k-space and is illustrated in Fig 2. VAPOR water suppression was applied. At 3T, parameters were: TE/TR = 35/2000 ms, FOV = 12x12 cm2, slice thickness = 1 cm, averages = 8, spectral bandwidth = 2083 Hz, total number of rings = 12 for a 24x24 image matrix with the outer 7 rings acquired at half bandwidth and interleaved twice. At 7T, parameters were: TE/TR = 45/5000ms, FOV = 20x20 cm2, slice thickness = 1 cm, averages = 4, spectral bandwidth = 2083Hz, total number of rings = 12 (24x24 image matrix) with the outer 4 rings acquired at half bandwidth and interleaved twice. Using partial interleaving saved 33% (8/24) scan time at 7T and 20% (5/24) at 3T.

Data was filtered and zero-filled before transformed to the spectral domain.

Results and Discussion

Figure 3 shows example results from a 7T scan acquired using partial temporal interleaving. Individual spectra are highlighted from the spectroscopic image and show good quality. Scan time was only around 5 minutes and could be allow for scanning in three dimensions easily.

Current choices for spectroscopic imaging at 7T are limited mainly to CSI techniques because of the strict gradient hardware demands. This has limited the spatial resolution achievable. But by using fast sequences such as SI-CONCEPT, spatial resolution can be increased in a reasonable amount of time, better utilizing the increased signal quality at 7T.

Acknowledgements

This work was supported by National Institutes of Health through Grant Number P41-EB015893 and the National Institute of Neurological Disorders and Stroke through Award Number R01NS087516.

References

[1] Posse, S., Tedeschi, G., Risinger, R., Ogg, R. and Bihan, D. L. (1995), High Speed 1H Spectroscopic Imaging in Human Brain by Echo Planar Spatial-Spectral Encoding. Magn Reson Med, 33: 34–40.

[2] Ebel, A., Soher, B. J. and Maudsley, A. A. (2001), Assessment of 3D proton MR echo-planar spectroscopic imaging using automated spectral analysis. Magn Reson Med, 46: 1072–1078.

[3] Adalsteinsson, E., Irarrazabal, P., Topp, S., Meyer, C., Macovski, A. and Spielman, D. M. (1998), Volumetric spectroscopic imaging with spiral-based k-space trajectories. Magn Reson Med, 39: 889–898.

[4] Furuyama, J. K., Wilson, N. E. and Thomas, M. A. (2012), Spectroscopic imaging using concentrically circular echo-planar trajectories in vivo. Magn Reson Med, 67: 1515–1522.

[5] Schirda, C. V., Zhao, T., Andronesi, O. C., Lee, Y., Pan, J. W., Mountz, J. M., Hetherington, H. P. and Boada, F. E. (2015), In vivo brain rosette spectroscopic imaging (RSI) with LASER excitation, constant gradient strength readout, and automated LCModel quantification for all voxels. Magn Reson Med. doi: 10.1002/mrm.25896

[6] Scheenen, T. W. J., Klomp, D. W. J., Wijnen, J. P. and Heerschap, A. (2008), Short echo time 1H-MRSI of the human brain at 3T with minimal chemical shift displacement errors using adiabatic refocusing pulses. Magn Reson Med, 59: 1–6.

Figures

Figure 1: k-space trajectories (left) and gradient waveforms (right). The blue rings are collected at high bandwidth, while red and green rings are collected at half bandwidth and interleaved temporally.

Figure 2: SI-CONCEPT with semi-LASER acquisition pulse sequence schematic.

Figure 3: Example data set showing spatial localization and highlighted spectra.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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