定量动态增强MRI评估骨质疏松骨血流灌注降低及其机制

Findings in several clinical studies have indicated that bone perfusion is reduced in osteoporosis(OP), but the cause of this reduced perfusion and particularly the temporal relationship between bone perfusion and bone mineral density (BMD) are not clear. Seventy-two 6-month-old female Sprague-Dawley rats are studied. Quantitative dynamic contrast enhanced magnetic resonance imaging (DCE-MRI, measuring perfusion parameters), Micro CT (measuring bone mineral density and microstructure parameters) and synchrotron radiation microtomography (microvessel imaging and microvessel density measurement in vivo) were performed at baseline and 2, 4, 8and 12 weeks after ovariectomy (OVX group) or sham surgery (Sham group). The longitudinal relationship between bone perfusion and BMD, as well as their causal relationship in OP are dynamically evaluated by comparing perfusion parameters, BMD and bone quality between two groups. At each time point after imaging, the animals were sacri?ced and femoral bones are harvested for pathology and real time PCR examination. With bone microvessel density(MVD) in vivo and in pathology serving as the gold standard, we validate the micro foundation and pathological basis of the qquantitative DCE-MRI in the evaluation of bone perfusion. Meanwhile, the value between qquantitative parameters（Ktrans、Kep、Ve）and semiquantitative parameters（Emax、Eslope）is compared in order to provide an effective imaging technique for noninvasively dynamical evaluation of bone perfusion. By combining MVD counting, red/fatty bone marrow area ratio, number of bone marrow-derived endothelial progenitor cells，bone vascular endothelial growth factor and endothelial nitric oxide synthase expression, the possible mechanism of bone perfusion reduction in OP is investigated to provide a new target for prevention and treatment of OP.

研究发现骨质疏松（OP）骨存在血流灌注降低，但其发生原因及与骨量降低的相关性尚不清楚。本研究拟对OP动物模型（SD大鼠卵巢去除术）在不同时间点行股骨定量动态增强MRI(测定血流灌注定量和半定量参数)、Micro CT（测定骨量、骨微结构参数）和同步辐射断层（活体骨微血管成像、定量分析）扫描，与对照组比较，动态评估OP发生过程中骨髓血流灌注、骨质量演变规律和两者因果关系。以骨活体和离体免疫组化染色微血管计数为金标准，揭示定量DCE-MRI活体评价OP骨血流灌注的微观基础和病理依据，比较定量参数（Ktrans、Kep、Ve）和半定量参数（Emax、Eslope）评估价值，为OP骨髓血供的动态评估提供有效影像学技术。结合骨微血管密度、红/黄骨髓面积比、骨髓/外周血血管内皮前体细胞比值和骨髓VEGF、eNOS表达水平，探讨OVX后骨髓血流灌注减少可能机制，为OP防治提供新的靶点。

近年文献报道骨质疏松（OP）的发生与骨髓血流灌注降低有关，但绝大部分采用半定量动态增强MRI (DCE-MRI)方法，并不能客观反映骨髓血流灌注降低的真实原因。本研究采用定量DCE-MRI动态评估OP模型大鼠（去卵巢手术，OVX）骨髓血流灌注时序变化规律，并联合1H-MRS及Micro-CT检查探讨骨髓血流灌注、骨髓脂肪与骨量三者的关系以及骨髓血流灌注变化导致OP发生的可能机制。.方法：1）70只3月龄雌性SD大鼠分为OVX组（n＝35）和假手术组（n＝35），两组于基线及OVX术后2、6、10、14、18、24周(每个时间点5只)行腰5椎体定量DCE-MRI、1H-MRS以及Micro-CT扫描，分别检测定量灌注参数（Ktrans、Ve、Kep）、骨髓脂肪分数（FF）及骨密度（BMD）。同时，测定血清相关标志物。CD34标记骨髓微血管内皮细胞测定微血管密度（MVD）。透射电镜观察脱钙后腰4椎体骨髓微血管内皮细胞（VEC）以及骨髓纤维化的变化。2）50只3月龄雌性SD大鼠分为OVX组（n＝25）和假手术组（n＝25），两组于基线及OVX术后3、6、9、12周检测腰5椎体半定量灌注参数（Emax）、定量灌注参数（Ktrans、Ve、Kep）以及BMD。同时，测定骨髓MVD。.结果：与假手术组比较，1）OVX组出现Ktrans下降（2周，P=0.036）、BMD下降（2周，P=0.014）以及FF增加（6周，P=0.036），血清血管内皮生长因子（18-24周，P=0.005）和骨髓MVD（14-24周，P=0.018）下降。透射电镜显示从2周起OVX组骨髓VEC连接更紧密伴线粒体肿胀，且骨髓纤维化更明显。2）OVX组出现Emax下降（6周，P=0.003；9周，P=0.004）、Ktrans下降（3-12周，P<0.05）、Ve下降（9周，P=0.032）、Kep无显著变化（P>0.05）以及BMD下降（3-12周，P<0.05），骨髓MVD仅在12周出现下降（P=0.032），Emax、Ktrans与MVD分别呈弱、强相关。.结论：骨髓血流灌注降低是导致OP的重要机制之一，定量灌注参数有望用于早期发现OP病理生理变化。VEC功能障碍可能是OP早期骨髓血流灌注下降的原因，而骨髓脂肪增多、MVD降低以及纤维化增多可能在晚期加重骨髓缺血。