磁共振成像弛豫机理简介

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                    <section><section powered-by="gulangu"><section><p><img src="image/20201019/7b80d51707f11d32f2d180a12e81983b_1.gif" /></p></section></section><section powered-by="gulangu"><section><section><p>MRI成像原理</p></section><p><img src="image/20201019/a5462a6bbe3550d673d6a125bff2c105_2.png" /></p> <section></section></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp; MRI通过对静磁场中的人体施加某种特定频率的射频脉冲，使人体中的氢质子受到激励而发生磁共振现象。停止脉冲后，质子在弛豫过程中产生MR信号。通过对MR信号的接收、空间编码和图像重建等处理过程，即产生MR图像。</p></section></section></section><section powered-by="gulangu"><section><section><p>磁共振现象</p></section><p><img src="image/20201019/a5462a6bbe3550d673d6a125bff2c105_2.png" /></p> <section></section></section></section><section powered-by="gulangu"><section><section><p>1、原子核的组成<span>&nbsp; &nbsp;&nbsp;</span></p><p>&nbsp; &nbsp; &nbsp; &nbsp; 原子核由一定数量的质子和中子组成，<span>质子和中子都会绕其中心轴自转，称为自旋。</span><span>质子表现出来这种磁矩，叫固有磁矩或本征磁矩。</span></p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/189926ffb2ba0ab28cb6b1e58d2ed39a_4.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp;人体大约含有78%的水分。<span>在没有外加磁场的情况下，氢原子的磁矩随机指向任意方向，对外表现出的宏观磁矩为零。</span><span>如果把人体放入一个静磁场B0中，人体内的氢原子就会在外界磁场的作用下重新排列，比较多的沿着磁力线方向对齐。有点像我们用磁铁靠近铁屑时候的情况。</span></p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/98687dc4781454ba1434d17c76d64c86_5.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp;质子被分成两种组态：<span>一部分是以一定夹角绕B0旋转，叫平行态；</span><span>另一部分则是反转过来，朝向B0的反方向旋转，叫反平行态。</span></p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/b7c6fc069a2c45516ee0d87661c320d9_6.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p>2、拉莫尔进动</p></section></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp; 当经受外部磁场时，每个质子绕磁场的旋转轨迹是一个锥体，如图重力场中旋转一般，不过质子画出的是在其质心处顶点相连的双锥体。</p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/6436cf58934c8cd9ade0fea4d0b35db8_7.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp; 在平衡状态下，质子沿磁场方向排齐，同时以一定夹角绕磁场进动，这就显现出了纵向分量，然而基本横向磁分量在各个方向是四散分布的，因而没有合成的横向分量。</p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/0a142eaec553b3e795d60bfd1162350e_8.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;其实，质子的真实运动（双重进动的合成）通过螺旋状从半球的北极端降低到赤道区（此时纵向分量消失，横向分量出现）的向量的顶端轨迹来描述。如果运行继续进行，向量顶端会沿着第2个螺旋转到南极（纵向分量的反转）。</p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/e106950bc5eabd3b480e9d0dba67961b_9.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p>3、驰豫现象<span>&nbsp;</span></p><p>&nbsp; &nbsp; &nbsp; &nbsp; 从非平衡态逐渐恢复到平衡态的过程称为弛豫过程。</p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/e1c1cfeb8fd462535f04581fcbea5f33_10.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp;&nbsp;<span>纵向弛豫 T1（自旋-晶格弛豫）</span><span>&nbsp;</span></p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;90度脉冲过后，纵向磁性Mz的恢复过程，表现为递增的指数函数，常数T1（ms）对应着恢复完成63%时所需的时间。</p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/08bbc77fdafd8db001de54d52ecf8d3e_11.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp; T1弛豫，主要是分子将吸收的射频能量，通过分子间的碰撞（布朗运动）传递出去。所以又叫自旋-晶格弛豫。分子之间的碰撞频率越接近共振频率，碰撞把能量传递给附近的分子概率就越大，能量释放越快，T1越短。<span>&nbsp;</span></p><p>&nbsp; &nbsp; &nbsp; &nbsp; 利用不同组织的T1差异，可以使不同组织在图像中呈现不同的信号强度。</p><p>&nbsp; &nbsp; &nbsp; &nbsp; T1弛豫，Mz总是平行于B0方向，由于Mz&lt;&lt;&lt;&lt;&lt;&lt;B0，要检测到Mz的变化，是极其困难的事情。</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;不同组织的T1弛豫状况如下图，图中可知，<span>脂肪的T1时间短，脑脊液的T1时间长。</span></p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/d997c16c5aeba13ab3f06a10dda71a62_12.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp; 横向弛豫 T2（自旋-自旋弛豫）<span>&nbsp; &nbsp; &nbsp;</span></p><p>&nbsp; &nbsp; &nbsp; &nbsp; 90度脉冲过后，<span>在垂直于B0方向会出现一个相位相干的横向磁性Mxy。</span><span>这个横向磁性随着90度脉冲的消失，迅速按照指数型函数衰减。</span></p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/08bbc77fdafd8db001de54d52ecf8d3e_11.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp; T2弛豫，主要是在Mxy平面内，原本聚相的质子，由于分子环境的微小差异，迅速散相，所以又叫自旋-自旋弛豫。</p><p>&nbsp; &nbsp; &nbsp; &nbsp; T2弛豫，Mxy总是垂直于B0方向，没有B0的干扰，可以在xy平面，通过线圈，检测到Mxy的微小变化，这就为磁共振成像提供了可行的信号采集方法。</p><p>&nbsp; &nbsp; &nbsp; &nbsp; 利用不同组织的T2差异，可以使不同组织在图像中呈现不同的信号强度<span>产生T2加权像。</span></p><p>&nbsp; &nbsp; &nbsp; &nbsp;不同组织的T2弛豫状况如下图，图中可知，脂肪的T2时间短，脑脊液的T2时间长。T2时间只有T1时间的十分之一左右。</p></section></section></section><section powered-by="gulangu"><section><p><img src="image/20201019/94e3be57e49ea736679ad129069137d1_14.jpg" /></p></section></section><section powered-by="gulangu"><section><section><p><span></span></p></section></section></section><section powered-by="gulangu"><section><section><section powered-by="gulangu"><section><section><p>欢</p></section></section></section></section><section><section powered-by="gulangu"><section><section><p>迎</p></section></section></section></section><section><section powered-by="gulangu"><section><section><p>关</p></section></section></section></section><section><section powered-by="gulangu"><section><section><p>注</p></section></section></section></section></section></section><section powered-by="gulangu"><section><section><p><br  /></p></section></section></section><section powered-by="gulangu"><section><section><section powered-by="gulangu"><section><p><img src="image/20201019/191b06dcea97430c058297c019798249_15.jpg" /></p></section></section></section><section><section powered-by="gulangu"><section><section></section></section></section></section><section><section powered-by="gulangu"><section><section><p>关注我们</p><p>一起涨姿势！</p></section></section></section></section></section></section></section>