Sarracanie Mathieu, Armstrong Brandon D, Stockmann Jason, Rosen Matthew S
Department of Physics, Harvard University, Cambridge, Massachusetts, USA; Department of Radiology, A.A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Boston, Massachusetts, USA.
Magn Reson Med. 2014 Feb;71(2):735-45. doi: 10.1002/mrm.24705.
Overhauser-enhanced MRI is a promising technique for imaging the distribution and dynamics of free radicals. A key challenge for Overhauser-enhanced MRI is attaining high spatial and temporal resolution while simultaneously limiting resonator and sample heating due to the long, high power radio-frequency pulses needed to saturate the electron resonance.
The approach presented here embeds EPR pulses within a balanced steady state free precession sequence. Unlike other Overhauser-enhanced MRI methods, no separate Overhauser prepolarization step is required. This steady-state approach also eliminates the problem of time-varying Overhauser-enhanced signal and provides constant polarization in the sample during the acquisition. A further increase in temporal resolution was achieved by incorporating undersampled k-space strategies and compressed sensing reconstruction.
We demonstrate 1 × 2 × 3.5 mm(3) resolution at 6.5 mT across a 54 × 54 × 110 mm(3) sample in 33 s while sampling 30% of k-space.
The work presented here overcomes the main limitations of Overhauser enhanced MRI as previously described in the literature, drastically improving speed and resolution, and enabling new opportunities for the measurement of free radicals in living organisms, and for the study of dynamic processes such as metabolism and flow.
奥弗豪泽增强磁共振成像（Overhauser-enhanced MRI）是一种用于对自由基分布和动力学进行成像的很有前景的技术。奥弗豪泽增强磁共振成像面临的一个关键挑战是在实现高空间和时间分辨率的同时，限制由于饱和电子共振所需的长且高功率射频脉冲而导致的谐振器和样品发热。
本文提出的方法将电子顺磁共振（EPR）脉冲嵌入到平衡稳态自由进动序列中。与其他奥弗豪泽增强磁共振成像方法不同，不需要单独的奥弗豪泽预极化步骤。这种稳态方法还消除了奥弗豪泽增强信号随时间变化的问题，并在采集过程中在样品中提供恒定的极化。通过采用欠采样k空间策略和压缩感知重建，进一步提高了时间分辨率。
我们在33秒内对一个54×54×110立方毫米的样品在6.5毫特斯拉的磁场下实现了1×2×3.5立方毫米的分辨率，同时对k空间进行30%的采样。
本文所展示的工作克服了文献中先前描述的奥弗豪泽增强磁共振成像的主要局限性，极大地提高了速度和分辨率，并为测量活生物体中的自由基以及研究代谢和流动等动态过程带来了新机遇。
