磁共振图像质量参数.doc

MR 基本原理图像质量1SNRContrastIQResolutionTIME2扫描时间Time3扫描时间-Time采集一个相位信息所需要的时间 TR高频高频相位编码梯度层面编码梯度低频频率编码梯度低频高频低频4扫描时间-TimeGE Medical SystemsTraining in Partnership采集一个层面信息所需要的时间 TR*Phase高频梯度相位编码低频频率编码梯度高频低频高频梯度相位编码频率编码梯度低频高频低频高频梯度相位编码频率编码梯度低频低频高频5扫描时间-Time采集一个最佳层面信息所需要的时间 TR*Phase*NEXGE Medical SystemsTraining in Partnership1 NEX100%SNR100%时间2 NEX141%SNR200%时间3 NEX173%SNR300%时间4 NEX200%SNR400%时间9 NEX300%SNR900%时间6扫描时间-Time二维采集 连续全部层面信息所需要的时间 TR*Phase*NEX*层数GE Medical SystemsTraining in Partnership1 2 34 5多层面采集连续采集.7扫描时间-Time二维采集 间隔全部层面信息所需要的时间 TR*Phase*NEX多层面采集1234GE Medical SystemsTraining in Partnership一个TR内采集四个层面第1层第2层第3层第4层8扫描时间-Time三维采集 GE Medical SystemsTraining in Partnership宽大的射频脉冲覆盖整个扫描容积。空间定位依靠两个相位编码，一个频率编码梯度来确定扫描时间为 TR*NPE*NSL*NEX3D 容积相位编码相位编码3D Slab频率编码 9GE Medical SystemsTraining in Partnership信噪比SNR10信噪比-SNR定义 信号是扫描的区域的平均信号强度值，噪声是扫描范围内背景的噪声强度值的标准差SNR即 上面的信号与噪声的比值GE Medical SystemsTraining in Partnership信号来源噪声来源11信噪比-SNR影响因素 信号：体素的大小FOV层厚层间隔GE Medical SystemsTraining in Partnership噪声：患者因素线圈因素患者及线圈的相互影响激发次数TR时间12信噪比-SNRGE Medical SystemsTraining in PartnershipSNR TIMETRNEX ResolutionFOVMatrixSlice Thick.1318信噪比-SNRGE Medical SystemsTraining in Partnership 不改变扫描的时间及分辨率单纯提高信噪 比的方法改变频率编码梯度的频率+ 32 kHz 采样频率 = 4ms+ 16 kHz 采样频率 = 8ms噪声信号14接收带宽变化的其他影响GE Medical SystemsTraining in PartnershipRBWSNR化学位移TE运动伪影1521GE Medical SystemsTraining in Partnership空间分辨率Space Resolution16空间分辨率-Res影响空间分辨率的因素:FOV矩阵层厚体素GE Medical SystemsTraining in Partnership1712空间分辨率-Res当空间分辨率增加时会影响信噪比扫描时间体素GE Medical SystemsTraining in Partnership1812GE Medical SystemsTraining in PartnershipChemical Shift Frequency difference between protons bound in fat versus protons bound in water Separated by 3.5 ppm, independent of field strength Chemical shift artefact is dependent on field strength and bandwidthH 20Fat(+)(-)approx. 220 Hz (1.5 T )approx. 143 Hz (1.0 T )approx. 104 Hz (0.7 T )approx. 74 Hz (0.5 T )approx. 35 Hz (0.2 T )1921GE Medical SystemsTraining in Partnership对比度CNR20组织对比度的起源GE Medical SystemsTraining in Partnership 在扫描出来的图像上之所以会区别处不同的组织，是因为组织内在的因素决定的，反映在图像上表现为信号强度的明暗对比不同MR成像的目的是选择一个恰当参数的脉冲序列使图像内在的三种对比度的一个突出显示21组织内在对比度GE Medical SystemsTraining in PartnershipT1-WI ：磁化矢量纵向恢复的变化引起横向磁化矢量改变，这种改变对图像的明暗对比起主要作用T2-WI ：原子核在XY平面失相位的快慢对图像的明暗对比起主要作用Proton density-WI ：当组织内质子的含量对图像的明暗对比起主要作用22T1 恢复 T1驰豫 - T1 relaxation 纵向恢复 自旋-晶格能量交换GE Medical SystemsTraining in Partnership2328T1 恢复影响因素磁场强度低场强 = 快速恢复高场强 = 较慢恢复GE Medical SystemsTraining in Partnership组织环境状态固态、致密 =快速恢复液态、输送 =较慢恢复2429GE Medical SystemsTraining in PartnershipT1 RecoveryTR and flip angle control saturation which controls T1 recovery effectsfatcsfMZT1 is the time it takes for themagnetization to recover to 63%of its original equilibrium valueTime (TR)25T2 衰减 T2 驰豫-T2 Relaxation 横向衰减 自旋-自旋效应GE Medical SystemsTraining in Partnership2631GE Medical SystemsTraining in PartnershipT2 DecayTE controls decay which controls T2 effectsSignalT2 is the time it takes forthe magnetization to decayto 37% of its original valuecsffatTime (TE)27质子密度 Proton Density与磁场强度有关反映局部组织质子含量T1和T2的作用不明显图像的明暗取决于局部质子含量的多少Tissue AGE Medical SystemsTraining in PartnershipTissue B2834组织对比度GE Medical SystemsTraining in PartnershipTRTE短T1-WeightedNot Used长PD- WeightedT2-Weighted2935短长GE Medical SystemsTraining in PartnershipPlenty of room left!3019