Evaluation of slice accelerations using multiband echo planar imaging at 3 T

Abstract

We evaluate residual aliasing among simultaneously excited and acquired slices in slice accelerated multiband (MB) echo planar imaging (EPI). No in-plane accelerations were used in order to maximize and evaluate achievable slice acceleration factors at 3 T. We propose a novel leakage (L-) factor to quantify the effects of signal leakage between simultaneously acquired slices. With a standard 32-channel receiver coil at 3 T, we demonstrate that slice acceleration factors of up to eight (MB=8) with blipped controlled aliasing in parallel imaging (CAIPI), in the absence of in-plane accelerations, can be used routinely with acceptable image quality and integrity for whole brain imaging. Spectral analyses of single-shot fMRI time series demonstrate that temporal fluctuations due to both neuronal and physiological sources were distinguishable and comparable up to slice-acceleration factors of nine (MB=9). The increased temporal efficiency could be employed to achieve, within a given acquisition period, higher spatial resolution, increased fMRI statistical power, multiple TEs, faster sampling of temporal events in a resting state fMRI time series, increased sampling of q-space in diffusion imaging, or more quiet time during a scan.

Keywords: Blipped CAIPI; Leakage (L-) factor; Residual aliasing; Single-shot fMRI time series; Spectral analysis; g-Factor.

© 2013 Elsevier Inc. All rights reserved.

Figures

Figure 1

Slice acceleration up to MB factor of 12 (upper panel) demonstrates negligible image degradation up to MB = 6 and good image quality up to MB = 8. Images were acquired with matched TR = 4.8 s at 2 mm isotropic resolution (60 or 64 slices) for comparison between MB factors. The example axial slices shown here were not from the same MB slice group. Achievable TR at a given MB factor is listed below the images to show the acceleration potential. Axial, coronal, and sagittal views of MB = 8 image from a different subject at minimum TR = 0.6 s (bottom panel) illustrate the contrast alteration and SNR compromises with TR reduction.

Figure 2

Histograms of the g-factors over the entire brain show distributions with higher peak values and widths for higher MB factors, as well as the expected increase in mean g-factor (µ in the legend). Note that the noise standard deviation for the g-factor calculation was measured from the real part of the complex MR signal.

Figure 3

Signal leakage (L-) factor maps show little difference in signal leakage for MB3 with PESHIFT = FOV/3 (A) or no PESHIFT (B), while there is significant difference in signal leakage for MB4 with PESHIFT = FOV/4 (C) and no PESHIFT (D). For high MB factors, e.g., MB = 8 (E) or MB = 12 (F), the signal leakage is most pronounced in adjacent simultaneously acquired slices, as well as those MB slices that directly overlap on top of each other (black boxes), despite the applied PESHIFT = FOV/4.

Figure 4

Coherence spectra (top five panels) from fixed volume single-shot (TR/TE = 75/30 ms) resting state fMRI time series demonstrate consistent spectral components for different MB factors. Cardiac (bottom panel, red) and respiratory (bottom panel, blue) components from simultaneous physiological monitoring are co-localized in the frequency spectrum with those non-aliased spectral peaks in the coherence spectra. Spatial coherence maps (right of the coherence spectra) at respiratory (0.3 Hz) and cardiac (1.2 Hz) frequencies, showing the percentage of the energy captured by the dominant singular value, are overlaid on anatomical images for the center slice positioned in common during different MB acquisitions. The scale for the spatial coherence color maps (from 1.5% to 5.0%) is the coherence at the respiratory or cardiac frequency (same range for all MB factors).