Guo Yingkun, Zhou Iris Yuwen, Chan Suk-Tak, Wang Yu, Mandeville Emiri T, Igarashi Takahiro, Lo Eng H, Ji Xunming, Sun Phillip Zhe
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, USA.
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, USA; China-America Joint Neuroscience Institute, Xuanwu Hospital, Capital Medical University, Beijing, China.
Neuroimage. 2016 Nov 1;141:242-249. doi: 10.1016/j.neuroimage.2016.07.025. Epub 2016 Jul 19.
pH-sensitive amide proton transfer (APT) MRI provides a surrogate metabolic biomarker that complements the widely-used perfusion and diffusion imaging. However, the endogenous APT MRI is often calculated using the asymmetry analysis (MTRasym), which is susceptible to an inhomogeneous shift due to concomitant semisolid magnetization transfer (MT) and nuclear overhauser (NOE) effects. Although the intact brain tissue has little pH variation, white and gray matter appears distinct in the MTRasym image. Herein we showed that the heterogeneous MTRasym shift not related to pH highly correlates with MT ratio (MTR) and longitudinal relaxation rate (R1w), which can be reasonably corrected using the multiple regression analysis. Because there are relatively small MT and R1w changes during acute stroke, we postulate that magnetization transfer and relaxation-normalized APT (MRAPT) analysis increases MRI specificity to acidosis over the routine MTRasym image, hence facilitates ischemic lesion segmentation. We found significant differences in perfusion, pH and diffusion lesion volumes (P<0.001, ANOVA). Furthermore, MRAPT MRI depicted graded ischemic acidosis, with the most severe acidosis in the diffusion lesion (-1.05±0.29%/s), moderate acidification within the pH/diffusion mismatch (i.e., metabolic penumbra, -0.67±0.27%/s) and little pH change in the perfusion/pH mismatch (i.e., benign oligemia, -0.04±0.14%/s), providing refined stratification of ischemic tissue injury.
pH敏感的酰胺质子转移（APT）磁共振成像提供了一种替代代谢生物标志物，对广泛使用的灌注和扩散成像起到补充作用。然而，内源性APT磁共振成像通常使用不对称分析（MTRasym）来计算，由于伴随的半固体磁化转移（MT）和核Overhauser（NOE）效应，这种方法容易受到不均匀位移的影响。尽管完整的脑组织pH变化很小，但在MTRasym图像中白质和灰质却表现出明显差异。在此我们表明，与pH无关的异质性MTRasym位移与MT比率（MTR）和纵向弛豫率（R1w）高度相关，通过多元回归分析可以合理校正这一现象。由于急性中风期间MT和R1w的变化相对较小，我们推测，与常规MTRasym图像相比，磁化转移和弛豫归一化的APT（MRAPT）分析提高了MRI对酸中毒的特异性，从而有助于缺血性病变的分割。我们发现灌注、pH和扩散病变体积存在显著差异（P<0.001，方差分析）。此外，MRAPT磁共振成像描绘了分级缺血性酸中毒，扩散病变处酸中毒最严重（-1.05±0.29%/s），pH/扩散不匹配区域（即代谢半暗带，-0.67±0.27%/s）存在中度酸化，灌注/pH不匹配区域（即良性低灌注，-0.04±0.14%/s）pH变化很小，这为缺血性组织损伤提供了精细的分层。
