Ya-Tien Liu1, Dian-Han Yang1, Cheng-Wen Ko1, Tzu-Chao Chuang2, and Shang-Yueh Tsai3,4
1Department of Computer Science and Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan, 2Department of Electrical Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan, 3Graduate Institute of Applied Physics, National Chengchi University, Taipei, Taiwan, 4Research Center of Mind, National Chengchi University, Taipei, Taiwan
Synopsis
In this study, we investigate the feasibility of
using MEGA-PRESS to detect Lac response to the visual stimuli in visual cortex
in fMRS experiment at 3T. The
significant raise in Lac level was found either by averaging the whole fMRS
scan or by averaging over the stimuli blocks.
Our results show consistency with the published results. More sophisticated approach to analyze fMRS
data may be necessary to facilitate reliable metabolic quantification.
Introduction
Functional magnetic resonance spectroscopy (fMRS)
can provide biochemical information of the brain by investigating metabolic
response to external stimuli. Using the same experiment design as fMRI, several
studies have reported the findings that glutamate (Glu) and lactate (Lac) can
alter during visual stimuli in primary visual cortex1,2,3 and in
motor cortex during motor task4, where 2~4% increase between stimuli
and rest sessions is reported for Glu and for Lac 9%~30% increase is found.
These studies are all performed in 7T MRI system in the consideration of the
small change of metabolic level and intrinsic low signal to noise ratio (SNR)
of MRS. As Lac can be detected using MEGA-PRESS method at 3T MRI system and is
already applied to detect the lac change during inspiratory hypoxia5,
it is possible to detect Lac change during visual stimuli at 3T system. In this
study, we investigate the feasibility of using MEGA-PRESS to detect Lac
response to the visual stimuli in visual cortex in fMRS experiment.Method
Six subjects were enrolled for the studies and
experiments were conducted in 3T MRI system (MR750, GE,
Milwaukee USA). For each subject, T1 images were acquired using SPGR
sequence. A fMRI experiment was performed using block design (6 stim and 6
rest) with checker board visual stimulation. Each on and off block consists of
10 seconds and the total scan time is 3 minutes. The experiment parameters
using EPI sequence are TR/TE=2000/33 ms, FOV=220x220 mm2,
Matrix=64x64, thickness=4mm. Online calculation was performed to localize the
primary visual cortex. MRS experiment was performed using MEGA-PRESS sequence
with VOI size in the range of 20~25 mm in AP and SI dimension and 40~59 mm in
LR dimension according to the fMRI results. The parameters of MEGA-PRESS
sequence are TR/TE=2000/142 ms, bandwidth =5000 Hz, sample point=4096. Gaussian-shaped editing pulses of 80-Hz
bandwidth were applied at 4.11 ppm and 5.29 ppm in interleaved way. The MRS
experiment is firstly conducted without visual stimuli for 25 minutes. Then
fMRS experiment was conducted with 5 blocks (3 rest and 2 stimuli) and each
block is 5 minutes. Checker board stimuli was presented during on block and the
scan time is also 25 minutes.
MEGA-PRESS data was processed using FID-A
software package6 for coil combination, frequency and phase
alignment, edit on and edit off reconstruction. The edit spectra were then
quantified using LCModel with basis sets simulated with VeSPA7.
Concentrations of Lac were reported using water scaling with relaxation and
partial volume correction. Two analysis methods were performed. First, the
averaged Lac concentrations over whole 25 minutes scan were compared between
MRS and fMRS scans. Secondly, for the fMRS scan, Lac concentrations were averaged
over the last 3.3 min of stimuli and rest blocks respectively.Results
Figure 1 shows the VOI of MRS acquisitions
(left), the averaged edit spectra of MRS (A, blue) and fMRS (A, red) scans from
one subject (Sub3), and averaged edit spectra of rest (B, blue) and stimuli (B,
red) blocks from fMRS scans. Evident
increase of Lac concentration by 19% was found in fMRS scan while relative to
MRS scan without stimuli. When comparing
the activation with rest within each fMRS scan, an elevation up to 45% of Lac
concentration was observed as shown in Fig.1.
Figure 2 illustrated the Lac concentrations of 6 subjects compared
between MRS and fMRS scans. The relative changes of Lac level range from 7% to
50% with CRLB (SD%) below 20%. Figure 3
demonstrated the changes of Lac concentrations between activation and rest
periods in fMRS scans. Relative increase
of Lac concentrations was found to be ranging from 0.61% to 45%. Discussions
The Lac increase corresponding to visual stimuli
was successfully detected on 6 subjects in our study. The Lac change found in
our study appears substantially consistent with the values published previously2,3.
Currently, the experiment setting for fMRS is similar to fMRI, which is
designed based on the well-known hemodynamic response. However, temporal
response of metabolism to physiological stimuli is still unknown so far. In our
study, larger Lac increase was found between activation and rest periods in
fMRS among 4 of 6 subjects while comparing individual differences. Notably, only the spectra acquired during the
last 3.3 min of each 5-min stimuli/rest block were averaged in our analysis to
reduce the effect of Lac regulation.
However, the less spectra to be averaged result in lower SNR and the
worse fitting reliability (higher CRLB). More sophisticated approach to analyze
fMRS data, especially for the time-course analysis, may facilitate solid
metabolic quantification. It is still recommended to perform fMRS study on 7T
system for reliable quantification on metabolites in vivo. However, at current
stage, more outcomes on fMRS are also needed to get insight into brain
metabolism during physiological stimuli. fMRS experiments conducted on current
wide-spread 3T system can be still necessary.Acknowledgements
Support from MOST grant “108-2314-B-110-002” and Mind
Research and Imaging Center (NCKU) is acknowledged.References
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