Wang Yi, Moeller Steen, Li Xiufeng, Vu An T, Krasileva Kate, Ugurbil Kamil, Yacoub Essa, Wang Danny J J
Laboratory of FMRI Technology (LOFT), Department of Neurology, University of California Los Angeles, Los Angeles, CA 90095, USA.
Center of Magnetic Resonance Research, University of Minnesota, Minneapolis, MN 55455, USA.
Neuroimage. 2015 Jun;113:279-88. doi: 10.1016/j.neuroimage.2015.03.060. Epub 2015 Mar 30.
Simultaneous multi-slice (SMS) or multiband (MB) imaging has recently been attempted for arterial spin labeled (ASL) perfusion MRI in conjunction with echo-planar imaging (EPI) readout. It was found that SMS-EPI can reduce the T1 relaxation effect of the label and improve image coverage and resolution with little penalty in signal-to-noise ratio (SNR). However, EPI still suffers from geometric distortion and signal dropout from field inhomogeneity effects especially at high and ultrahigh magnetic fields. Here we present a novel scheme for achieving high fidelity distortion-free quantitative perfusion imaging by combining pseudo-continuous ASL (pCASL) with SMS Turbo-FLASH (TFL) readout at both 3 and 7 T. Bloch equation simulation was performed to characterize and optimize the TFL-based pCASL perfusion signal. Two MB factors (3 and 5) were implemented in SMS-TFL pCASL and compared with standard 2D TFL and EPI pCASL sequences. The temporal SNR of SMS-TFL pCASL relative to that of standard TFL pCASL was 0.76 ± 0.10 and 0.74 ± 0.11 at 7 T and 0.70 ± 0.05 and 0.65 ± 0.05 at 3T for MB factor of 3 and 5, respectively. By implementing background suppression in conjunction with SMS-TFL at 3T, the relative temporal SNR improved to 0.84 ± 0.09 and 0.79 ± 0.10 for MB factor of 3 and 5, respectively. Compared to EPI pCASL, significantly increased temporal SNR (p<0.001) and improved visualization of orbitofrontal cortex were achieved using SMS-TFL pCASL. By combining SMS acceleration with TFL pCASL, we demonstrated the feasibility for whole brain distortion-free quantitative mapping of cerebral blood flow at high and ultrahigh magnetic fields.
近期，同步多切片（SMS）或多频段（MB）成像已被尝试用于动脉自旋标记（ASL）灌注磁共振成像，并结合回波平面成像（EPI）读出。研究发现，SMS-EPI可以降低标记的T1弛豫效应，提高图像覆盖范围和分辨率，且信噪比（SNR）损失很小。然而，EPI仍然存在几何失真和场不均匀性效应导致的信号丢失问题，尤其是在高场和超高场中。在此，我们提出一种新方案，通过在3T和7T时将伪连续ASL（pCASL）与SMS Turbo-FLASH（TFL）读出相结合，实现高保真无失真定量灌注成像。进行了布洛赫方程模拟，以表征和优化基于TFL的pCASL灌注信号。在SMS-TFL pCASL中实现了两个MB因子（3和5），并与标准二维TFL和EPI pCASL序列进行比较。对于MB因子为3和5的情况，SMS-TFL pCASL相对于标准TFL pCASL的时间SNR在7T时分别为0.76±0.10和0.74±0.11，在3T时分别为0.70±0.05和0.65±0.05。在3T时结合背景抑制与SMS-TFL，对于MB因子为3和5的情况，相对时间SNR分别提高到0.84±0.09和0.79±0.10。与EPI pCASL相比，使用SMS-TFL pCASL可显著提高时间SNR（p<0.001），并改善眶额皮质的可视化。通过将SMS加速与TFL pCASL相结合，我们证明了在高场和超高场下对全脑进行无失真脑血流量定量映射的可行性。
