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
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