Claudiu Schirda1, Tiejun Zhao2, Hoby Hetherington1, Victor Yushmanov1, and Jullie Pan1
1Radiology, University of Pittsburgh School of Medicine, Pittsburgh, PA, United States, 2Siemens Medical Solutions, Pittsburgh, PA, United States
Synopsis
Rosette Spectroscopic Imaging (RSI) has been shown to
provide similar or superior encoding speed and sensitivity to echo-planar
(EPSI) and spiral spectroscopic imaging (SSI), while using much lower peak
gradient and slew rates. Fully encoded k-t space 3D acquisitions with 0.4ml
voxel size in 7.2mins (20x20x12, spectral width SW=1923Hz --6.47ppm, Gmax=7.1mT/m
and Smax=86mT/m/ms), and 2D acquisitions as short as 36s (1cc) to a 9.5min dual-echo
TE=17/34ms J-refocused with 0.16ml voxel (4mm in-plane, 48x48, SW=2778Hz
--9.35ppm, Gmax=5.1mT/m and Smax=18mT/m/ms) were collected at 7Tesla in
phantoms, controls and patients with epilepsy and tumors.Purpose:
To obtain in vivo human brain high resolution (<1ml)
spectroscopic imaging data at ultra-high (7T) field, in clinically acceptable
time (<10minutes), with reduced gradient demands.
Methods:
7T promises a 2.3 fold increase in SNR compared to 3T, which
is greatly appealing when measuring small concentration metabolites in human
brain. However, for fast spectroscopic imaging (SI) techniques using gradient
readout waveforms, this also makes it 2.3 times more challenging because, even
at the same spatial resolution, for the the same spectral width (SW) in
parts-per-million (ppm), the spectral dwell time spDT=1/SW is 2.3 times as
small as at 3T. Thus, the same k-space needs to be sampled 2.3 times as fast. The
lower demands on the gradient system associated with the Rosette Spectroscopic
Imaging1,2 (RSI) technique, due
to the smoothly varying gradient waveform is, perhaps, the most important
advantage of the RSI acquisition at ultra-high field (UHF), allowing this
technique to achieve easier a greater spectral bandwidth than echo-planar SI
(EPSI) or spiral SI (SSI). For the same 1cm in-plane resolution (IPR) and SW=1923
Hz (6.47ppm) achieved by flyback EPSI with two temporal interleaves3
(40 to 48 shots total), RSI can do it using 24 shots, no temporal (one)
interleaves and a maximum gradient and slew-rate of Gmax=7.1mT/m and
Smax=86mT/m, using settings previously described1 (Figure 1). Thus, RSI can be almost twice as fast, while
using peak gradient and slew rates approximately half of the ones used by EPSI,
which results in decreased eddy currents4. Furthermore, because data
is collected and used for the entire signal decay readout, RSI does not suffer
from up to a 30% loss in sensitivity as the flyback EPSI acquisition5.
A 7T Siemens Magnetom PTX Step 2 system and an 8x2
transceiver array (2 rows, 8coils/row) was used. For transmission, the 16 coils
were driven by 8 independent RF channels (amplitude, phase determined by RF
shimming). The two rows are paired superiorly-inferiorly, with each two-coil
pair driven by a single independent RF channel using an equal amplitude 30⁰
splitter. RF shimming was used to optimize two B1 + distributions; a ring
distribution for outer volume suppression targeting the skull/skin for lipid
suppression and a homogeneous distribution for excitation6. A 38cm
ID shim insert coil (Resonance Research Inc.) was used for higher degree
shimming with a non-iterative least squares B0 mapping algorithm7
(Bolero, B0 loop encoded readout). For NAA/Cr/Cho quantification, a SW=1923Hz
(6.47ppm) and TE=40ms were used and for the dual echo J-refocused8
acquisitions SW=2778Hz (9.35ppm), with TE=17/34ms. Lipid/water/lipid inversion and
repetition time TI1/TIW/TI2/TR=180/420/780/1500 ms. Data readout/repetition time:
Tread/TR=320/1500 ms. A water reference
with TR=0.39s was also collected for channel phasing/recombination. Data without
a ring suppression was also acquired with TI1/TIW/TR=350/1200/3000ms6.
Rosette trajectories were designed and data reconstructed as previously
described1. For the 3D acquisition, the z-direction encoding was
conventional. All data was automatically processed with an LCModel9 (http://www.s-provencher.com/pages/lcmodel.shtml)
based pipeline.
Results:
Using a BRAINO MRS phantom, the Bland-Altman agreement
between RSI and phase encoded (PE) CSI for normalized NAA/Sum is within 7% of
the mean (0.25) and average (SD) correlation r=RSI*CSI for spectra in all
voxels (Schirda 2015) was 0.97(+/-0.03) (identical
spectra for r=1) . When using the ring B1 lipid suppression in vivo, an average
72% greater sensitivity was obtained compared to the spatially non-selective (global)
lipid inversion pulse.
Human data was acquired as: a) Single slice 2D, 20x20 matrix (IPR=1cm) in as
short as 0.6 minutes and up to a J-refocused 48x48 (IPR=0.4cm, 0.16mL nominal,
0.44ml effective voxel after spatial filtering, Figure 2) in 9.5 mins
and b) 3D acquisition 20x20x12 with 0.4ml nominal
resolution (1.25ml effective) in 7.2mins (Figure 3).
Discussion and Conclusion:
High resolution (<0.5ml) 2D
and 3D RSI data, with robust SNR, was collected at 7T in clinically acceptable
time (less than 10mins). The high SNR of these fully encoded k-t space RSI acquisitions
at 7T suggests that even higher spatial resolutions and/or a further decrease in
scan time through the use of parallel imaging techniques is feasible without
significant impact on data quality. The high order B0 shim insert allows for good
quality spectral data in difficult to shim regions, such as temporal lobe,
granted targeted data collection in those regions is performed. While we used an
8-channel pTx system to perform a B1 ring scalp lipid suppression, the RSI
acquisitions described here could be done using a more typical setup, with
receive only coil, by implementing a spatially non-selective lipid pulse.
However, using the scalp suppression ring provides for shorter acquisition
times and almost a two-fold increase in sensitivity.
Acknowledgements
Support: NIH R01 EB11639 , R01 NS 90417, R01 NS 081772References
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