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キーワード:
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要旨:
ntroduction
Laminar-specific functional magnetic resonance imaging (fMRI) has been successfully applied to understand neuronal circuitry across the cortical layer [1-3]. FLASH based line-scanning method was proposed with high temporal (50 ms) and spatial (50 µm) resolution to better characterize the fMRI onset time by combining 2 saturation RF pulses [4]. However, the imperfect RF saturation performance led to poor boundary definition of the ROI from the cortex. Here, we propose an α-180˚SE based line-scanning (SELINE) method to solve this problem.
Methods
Line-scanning fMRI data was acquired in an anesthetized rat at Bruker 14.1T scanner using in-house transceiver surface coil with 6mm diameter. The conventional FLASH based and the proposed α-180 SE based line-scanning (SELINE) pulse sequence are used (Fig. 1c, Fig. 1d). To acquire the SELINE data, the 180˚RF pulse oriented perpendicular to the α RF pulse as moving the refocusing gradient to phase encoding gradient (Gp) in order to obtain high spatial resolution without reduced FOV aliasing problem. Functional activation was identified by performing a left forepaw stimulation task (3Hz, 4s, pulse width 300us, 2.5mA), followed by 1 second pre-stimulation, 4 second during electrical stimulation and 15 seconds post stimulation with a total 20 seconds for 10 min 40 sec.
Results/Discussion
To demonstrate the saturation performance, the line profiles of MRI signal intensity are shown in Fig. 1b. The proposed SELINE method has 2.1% mean suppressed brain region with 4 trials while the conventional FLASH based method has 26.9% (12.8 times of SELINE). Averaged time course and percentage change map have higher contrast-to-noise ratio (CNR) in the deeper layer of the SELINE method than FLASH-based method. Laminar-specific correlation coefficient matrices are shown in Fig 2a, 2b (2 times of SELINE). Layers L2/3-L4 (0.15 – 0.8 mm) show strong correlation with the SELINE method. tSNR plots with 3 trial are shown in Fig. 2c, 2d. The conventional FLASH-based method has 30.6 mean tSNR with slow slope in Fig. 3c (TR, 100ms, TE, 12.5ms). On the other hand, SELINE method has 50.4 mean tSNR with steep slope in Fig. 3d. The altered laminar SNR profile is due to the less homogeneous B1 field from the small transceiver surface coil.
Conclusion
We demonstrate the feasibility of the SELINE method for laminar-specific fMRI. The proposed scheme solves the contamination issue out of ROI. Future work will be done to improve this method. To achieve higher temporal resolution up to 100 or 200 ms by optimizing flip angle α, TR and sequence parameters, we will investigate the effect from stimulated echoes during fMRI stimulation paradigm.
