《MRI技术》PPT课件.ppt

Imaging Techniques of Application,回顾,核磁共振成像方法 人体中三分之二是水。水分子由两个氢原子和一个氧原子组成。氢原子的核子，即质子，可以比做一个磁性陀螺仪。磁共振系统就是利用这种物理特性制成的。在一个强磁场中通常比地球磁场强30,000倍，任意旋转的质子通常顺着磁场方向平行排列或反磁场方面平行排列。根据磁场的强度，所有的质子摆成一行，这就产生了一个可以测量的磁矩。,排列的质子在电磁频率的干扰下，队列会发生变化。磁矩开始在磁场的场线附近旋转。当电磁干扰结束的时候，质子释放从磁场获得的能量，这些能量被线圈接收。越多核子按同一方面排列，说明信号强度越高，磁场强度也越大。通过从空间上改变磁场，人们可以确定磁共振信号的发源点，从而利用计算机程序重建图像，这种方法类似于CT成像。磁共振图像的对比度依赖于人体器官中水的空间分布。人体在放松休息的时候，就等同于电磁频率被切断而进行图像重建的时刻。,MRI技术应用,MR Subsystems,RF Coils Body Coil Head Coil Surface Coils Phased Array Coils,MR Subsystems,Five Types RF Gradient Shimming Magnet Computer,MR Subsystems,Body Coil large cylinder (volume coil) sending RF pulses into the patient that provide “flip” for protons also can be used as a receiver for large FOV scans (abdominal, full spine, etc),MR Subsystems,Head Coil smaller cylinder usually volume coil with quadrature or phased array design many different designs exist high field low field,Quadrature Head,Low field Head,Head & Neck Phased Array,MR Subsystems,Surface Coils specialized coils, varying in size receive signal from localized regions and have higher SNR quadrature or linear design,Extremity & Breast,TMJ & Neck,MR Subsystems,Phased Array Coils allow large FOV scanning providing the SNR of a surface coil e.g. CTL, Pelvis, TMJ, Head/Neck, etc.,Head & Neck,Body & Spine,MR Subsystems,Gradient Coils consists of three sets of X, Y, and Z Coils located within the magnet provides a means to spatially encode the image and locate the slice produces small changes in the main magnetic field,X Coils,Y Coils,Z Coils,Summary,Five Types Radio Frequency Gradient Shimming Magnet Computer,There are three general types of MRA：,Time-of-Flight Angiography； Phase Contrast Angiography； Contrast Enhanced Angiography。,In the presence of flow and the correct TE time, blood from the 90o plane flows into the 180o plane and produces an echo。,TOF之SE法,TOF之GRE法,TOF的基本概念是增加流动自旋质子的磁化量，降低静态自旋质子的磁化量。磁化饱和会造成了固体组织和血液信号都减弱，但是未饱和的流动血液不断地流入成像层面，它们的净磁化量比静止组织大，会产生一个强的信号，增大了流动液体与静止组织的对比。,流入相关增强,当TR较短时，成像体积内静止的质子被饱和，显示低或无信号。,流动的质子未被先前的激发脉冲饱和，产生高信号。,层面定位： TOF 依赖于“新鲜”质子产生对比度，因此必须使用薄层并垂直于血管扫描，减少层内饱和。,血液流出扫描块，质子保持新鲜,SAT脉冲 ：在扫描容积和不需要的血管源之间放