Mul$plexed spiral sequence for high temporal resolu$on fMRI
Benedikt A Poser, Weiran Deng, Robert James Anderson and V. Andrew Stenger University of Hawaii at Manoa, JABSOM Department of Medicine, Honolulu, Hawaii
METHODS
INTRODUCTION Simultaneous multi-slice acquisition
Sequence
simultaneous multi-slice acquisition (SMS) allows considerable TR reduction SMS has recently attracted much interest for EPI based BOLD fMRI and DTI after a long neglected study by Larkman1 in 2001, now many recent papers: Moeller2, Feinberg3, Setsompop4,
Koopmans5, and several recent abstracts
in 2D single-shot acquisitions SMS achieves very effective speed-ups by the nominal multi-slice factor where parallel imaging hardly helps
…RESULTS Resting state fMRI
a spiral-in sequence was implemented on 3T Siemens Magnetom Trio with 32 head coil using FoV 22cm and matrix size 64x64, two protocols were explored:
RSN in a run of one subject
PINS-‐RF: 36 sag. 5mm slices, no gap, TRvol=0.42s MB-‐RF: 27 axial 4mm slices, 25% gap, TRvol=0.31s
Reconstruction I
ICA shows the expected RSN …
… but mostly junk-components
2D gridding to 64x64 matrix GRAPPA approach1,2 to separate slices
respiration
cardiac
slice-by-slice
allows z-acceleration
Schematic of the GRAPPA reconstruction, shown here for multi-slice factor 2: Left: to obtain GRAPPA weights, concatenate fully sampled slices to large FoV, FT and estimate a 5x4 GRAPPA kernel 5x4 Right: apply this kernel to disentangle the aliased slices, treating them as data resulting from a larger inplane under-sampled FoV containing the simultaneous slices.
multiple slices at once
SMS is not necessarily faster than 3D EPI with zacceleration but: SMS does not suffer the bandwidth penalty from data undersampling much more effectively “freezes out” physiological noise especially resting state fMRI is expected to benefit from sub-second sampling, as physiological nuisance signal are more fully sampled and can hence be removed
We here investigate multiplexed single-shot spiral-in sequences with multi-band (MB) and PINS6 excitation pulses: - spirals are an efficient way to cover k-space - inward-spiral provides a ‘compact’ sequence without much dead time: TR ~ TE Full brain coverage at 3.5x3.5x5mm3 with a multi-slice factor of 3 yields TR= 310ms and fully samples cardiac pulsations. Example of spiral-in sequence with a SAR efficient PINS RF periodic excitation to effectively excite three 5mm slices 60mm apart simultaneously.
both
noise becomes signal respiratory fluctuations (4-6s) clearly resolved
0.25Hz 4s resp cycle
TR 310ms (194/min) shows cardiac pulsations up to 97bpm, seen as ‘vascular’ hotspots in the IC maps
Reconstruction II
CG SENSE approach using Fessler’s nuFFT7 do spiral and multiplex recon at once based on a set of separately acquired slices incorporate field maps for de-blurring / distortion correction
0.93Hz 56 bpm
Filtering increases detected BOLD task activation
fMRI
40
resting state 500 volumes, 2’ 40” visual task 500 volumes (15s on, 15s off) ICA in FSL8,9
unfiltered
RESULTS… DISCUSSION
Sample images: 2D gridding and GRAPPA recon PINS pulse SMS, TR=420ms (36 slc)
triband pulse SMS, TR=310ms (27 slc))
conventional scan, TR=1615ms
SMS typically uses RF pulses containing N frequency bands corresponding to the N desired slice locations as this is essentially a complex summation of N phase modulated pulses, SAR increases N-fold an elegant alternative is the PINS RF pulse design (‘Power Independent of Number of Slices’)6 to excite an infinite comb of slices; the effective N is hence determined by the object dimensions
2.5
filtered
Multi-slice RF excitation and SAR
perform ICA; identify and regress out all non-task components, here typically 40-80 per subject.
conventional scan, TR=1240ms
Sample image: CG recon CG without off-resonance correction
Acknowledgments UH-QMC 3T MRI Lab. DFG grant Po1576/1-1. NIH grants R01 DA019912 and K02 DA020569. Core resources supported by the NCRR (G12-RR003061, P20-RR011091), NINDS (U54-NS56883), and the ONDCP.
CG with off-resonance correction
factor-3 SMS spiral sequence allows whole-brain coverage with TR=310ms (one FatSat per TR was sufficient) GRAPPA recon is faster, but CG SENSE achieves better image quality and de-blurring; spiral disadvantage: CAIPI4 schemes for g-noise reduction not easy to incorporate short TR allows better separation of desired and undesired signals by ICA in task and resting fMRI: task activation detection was improved by prior filtering nuisance signals what temporal and spatial resolution do we need in practice? What can advances data processing achieve?
REFERENCES using GRE field map, 8 time segments along spiral
[1] Larkman D, et al JMR 2002 [3] Feinberg D, et al PlosOne 2010 [5] Koopmans PJ et al NI (in press) [7] hKp://www.eecs.umich.edu/~fessler [9] Beckmann C, IEEE TMI 2004
Contact Information University of Hawaii at Manoa JABSOM Department of Medicine 1356 Lusitana Street, UH Tower, 7th Floor, Honolulu, HI 96813
[email protected]
[2] Moeller S, et al MRM 2010 [4] Setsompop K, et al MRM 2011 [6] Norris, DG et al MRM 2011 [8] www.fmrib.ox.ac.uk/fsl
