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<p><p><img src="image/20201019/1bd08b695d5fd805cb63ee05a2610294_1.jpg" /></p></p><p><br /></p><p><span> </span><span> MRI有多种伪影 ,图像处理有关的伪影、与<span>RF</span>场有关的伪影、与主磁场有关的伪影、磁化伪影、与梯度场有关的伪影、与病人有关的伪影、数据错误伪影等。</span></p><p><strong><span><span></span></span></strong><span><strong><span>一、</span></strong><strong><span>图像处理伪影</span></strong></span></p><p><span><strong><span>1.1 <span>混淆伪影</span></span></strong></span></p><p><span><span>由于计算机只能识别带宽内的频率</span>,<span>所以不能识别高于最大频率或低于最小频率的频率。如果有超出频带的频率</span><span>,</span><span>那么在允许的带宽内可能把高频当作低频</span><span>,</span><span>低频当作高频</span><span>,</span><span>这样会出现频率混乱</span><span>,</span><span>表现在图像上则出现混淆伪影。例如</span><span>,</span><span>如果高频比</span><span>fmax</span><span>高</span><span>2kHz,</span><span>它被看做是比</span><span>-fmax</span><span>高</span><span>2kHz,</span><span>这样在图像相反一边产生信息混淆。可以采取以下方法消除或减少混淆伪影。</span></span></p><p><span><strong><span>1.1.1</span></strong><strong><span>表面线圈</span></strong></span></p><p><span><span>由于大的传输</span>/<span>接收线圈覆盖了大部分身体</span><span>,</span><span>线圈内接收的信号来自许多组织</span><span>,FOV</span><span>外的组织在图像上产生混淆。如果使用的线圈只覆盖设定的</span><span>FOV</span><span>区域</span><span>,</span><span>得到的信号处于带宽范围内</span><span>,</span><span>图像不会出现混淆伪影。此种方法使用的线圈叫表面线圈</span><span>,</span><span>使用表面线圈可以提高信噪比</span><span>,</span><span>但是它的缺点是图像均匀性比较差。</span></span></p><p><span><strong><span>1.1.2</span></strong><strong><span><span>增加</span>FOV</span></strong></span></p><p><span><span>如果加大</span>FOV<span>使其包括整个研究区域</span><span>,</span><span>也可消除混淆伪影</span><span>,</span><span>但是要求使用弱梯度场。最大最小频率范围覆盖了较大的区域</span><span>,</span><span>使</span><span>FOV</span><span>所包含的成像部位均在频率范围内</span><span>,</span><span>就不会产生混淆。</span></span></p><p><span><strong><span>1.1.3</span></strong><strong><span>过采样</span></strong></span></p><p><span> <span>有两种过采样</span>,<span>一种是频率过采样</span><span>(NFW),</span><span>一种是相位过采样</span><span>(NPW)</span><span>。</span></span></p><p><span>a <span>频率过采样。频率过采样能消除混淆伪影的原因是在满足</span><span>Nyquist</span><span>采样频率时在频率编码方向提高采样频率。必要时设备会自动选择频率过采样。</span></span></p><p><span>b <span>相位过采样。加大</span><span>FOV</span><span>避免混淆</span><span>,</span><span>在显示图像时消除不需要的部分。这种方法在采集数据时实际采集的信号比需要的</span><span>FOV</span><span>范围大。在最后显示图像时</span><span>,</span><span>只显示需要的</span><span>FOV</span><span>内的图像</span><span>,</span><span>其它部分舍弃。因为相位编码步数</span><span>(Ny)</span><span>增加</span><span>,</span><span>为保持同样的扫描时间采集次数</span><span>(NEX)</span><span>要减少。在这种情况下</span><span>,</span><span>信噪比没有改变</span><span>,</span><span>扫描时间有轻微增加</span><span>,</span><span>这是因为过扫描的时间比</span><span>1/2NEX</span><span>稍微大一些。</span></span></p><p><strong><span>1.1.4</span></strong><strong><span>饱和脉冲</span></strong></p><p><span><span>饱和脉冲使</span>FOV<span>外的组织饱和</span><span>,</span><span>几乎不产生信号。这样线圈在接收信号时</span><span>,</span><span>几乎接收不到</span><span>FOV</span><span>外组织的信号</span><span>,</span><span>从而减少图像的混淆伪影。饱和脉冲法只是减弱混淆伪影</span><span>,</span><span>完全消除比较困难</span><span>,</span><span>因为使</span><span>FOV</span><span>外的组织完全饱和而成像区域的组织不受影响比较困难。</span></span></p><p><span><strong><span>1</span></strong><strong><span>.2 <span>化学位移伪影</span></span></strong></span></p><p><span><span>产生化学位移伪影的原因是不同分子内的质子进动频率有轻微差别。例如</span>,<span>脂肪和水内的氢质子的进动频率存在轻微差别</span><span>,</span><span>水内质子比脂肪内质子进动快一些</span><span>,</span><span>约有</span><span>3.5ppm</span><span>。例如</span><span>:1.5T</span><span>场强下</span><span>,</span><span>进动频率的相位差约为</span><span>220Hz;0.3T</span><span>场强下</span><span>,</span><span>相差约为</span><span>73Hz</span><span>。由此可见</span><span>,</span><span>使用弱磁场可减少化学位移伪影。</span></span></p><p><span><span>如果</span>X<span>方向的</span><span>FOV</span><span>分成</span><span>256</span><span>个像素</span><span>,</span><span>像素的带宽是</span><span>125Hz</span><span>。在</span><span>1.5T,</span><span>脂肪与水内质子的进动频率差异为</span><span>220Hz,</span><span>对应像素约为</span><span>2</span><span>个像素。这表明脂肪和水质子有</span><span>2</span><span>个像素的混淆。如果像素尺寸是</span><span>1mm,</span><span>则在图像里脂肪有</span><span>2mm</span><span>的错误定位。降低带宽可增加化学位移伪影</span><span>,</span><span>这是选择低带宽的副作用。所以高场低带宽技术的化学位移伪影最严重。化学位移伪影的解决方法有</span><span>:</span></span></p><p><span>(1)<span>使用脂肪抑制方法去除脂肪信号。如果没有脂肪信号</span><span>,</span><span>就没有化学位移伪影。</span></span></p><p><span>(2)<span>保持</span><span>FOV</span><span>不变增加像素尺寸</span><span>,</span><span>并降低频率编码步数</span><span>,</span><span>可减少化学位移伪影。</span></span></p><p><span>(3)<span>降低磁场强度。低场强化学位移伪影少</span><span>,</span><span>但降低场强不切合实际。</span></span></p><p><span>(4)<span>增加带宽减少伪影</span><span>,</span><span>但使用这种方法信噪比会降低。</span></span></p><p><span>(5)<span>改变相位编码和频率编码的方向。这种方法只是改变化学位移伪影的方向。在诊断中改变伪影的方向可判断是解剖结构还是伪影。</span></span></p><p><span>(6)<span>使用长</span><span>TE,</span><span>长</span><span>TE</span><span>使脂肪信号产生更多的失相</span><span>,</span><span>使脂肪信号降低从而减少伪影。</span></span></p><p><span><strong><span>1</span></strong><strong><span>.3 <span>第二种化学位移伪影</span></span></strong></span></p><p><span><span>在梯度回波技术中会出现第二类化学位移伪影。由于水内质子进动快</span>,<span>经过一段时间后</span><span>,</span><span>可能超前脂肪内质子</span><span>360</span><span>°。这样有些时刻脂肪和水的自旋同相</span><span>,</span><span>有些时刻它们有</span><span>180</span><span>°失相。在</span><span>1.5T</span><span>场强中</span><span>,</span><span>脂肪和水内质子自旋每</span><span>4.5ms</span><span>同相一次。在</span><span>1.5T,</span><span>脂肪和水每</span><span>2.25ms</span><span>失相一次。脂肪和水内质子的同相、失相形成第二种类型的化学位移伪影。</span></span></p><p><span><span>如果</span>TE<span>是</span><span>2.25,6.75,11.25,15.75ms</span><span>等依此类推</span><span>,</span><span>脂肪和水内质子自旋失相</span><span>,</span><span>可以在脂肪周围的器官周边看到黑色边缘</span><span>(</span><span>边缘效应</span><span>),</span><span>边缘效应伪影是第二种化学位移导致的。这种伪影不只发生在频率编码轴</span><span>(</span><span>第一类化学位移伪影主要发生在频率编码轴</span><span>),</span><span>因为它是脂肪和水质子在各个方向相位消减的结果。常规</span><span>SE</span><span>扫描序列图像中没有边缘伪影</span><span>,</span><span>因为</span><span>180</span><span>°重聚脉冲消除了这种伪影</span><span>,</span><span>而在梯度回波中没有使用</span><span>180</span><span>°重聚脉冲。解决第二种化学位移伪影的方法</span><span>:</span></span></p><p><span>(1)<span>使用合适的</span><span>TE</span><span>使脂肪和水的自旋同相。</span><span>(2)</span><span>变换相位编码和频率编码方向</span><span>,</span><span>使伪影变换方向。</span></span></p><p><span>(3)<span>增加带宽</span><span>(</span><span>信噪比会降低</span><span>)</span><span>减少化学位移伪影。</span></span></p><p><span>(4)<span>使用脂肪抑制方法</span><span>,</span><span>抑制脂肪信号减弱化学位移伪影。</span></span></p><p><span><strong><span>1.4 <span>截尾伪影</span></span></strong></span></p><p><span><span>在高对比度界面会出现此种伪影</span>,<span>产生黑白交替</span><span>,</span><span>有时可能会被误认为病灶。产生截尾伪影的原因是由于采样数目和采样时间有限</span><span>,</span><span>使信号强度来回交换所致。这种伪影在相位编码方向常可看到</span><span>(</span><span>因为相位编码方向的空间分辨力比频率编码方向的低</span><span>)</span><span>。减少截尾伪影的方法有</span><span>:</span></span></p><p><span>(1)<span>增加采样减少波纹</span><span>(</span><span>时间域的宽信号对应频率域的窄信号</span><span>)</span><span>。</span></span></p><p><span>(2)<span>减少像素尺寸</span><span>,</span><span>可通过增加相位编码步数或减小</span><span>FOV</span><span>减小像素尺寸。</span></span></p><p><span><strong><span>1.5 <span>部分容积伪影</span></span></strong></span></p><p><span><span>部分容积伪影的产生原理与</span>CT<span>的容积伪影类似</span><span>,</span><span>由于层厚大</span><span>,</span><span>使得三维体积内的物体在二维平面上显示</span><span>,</span><span>降低了分辨力。降低层厚可减少部分容积伪影。</span></span></p><p><span> </span></p><p><strong><span>二、<span>与</span>RF<span>场有关的伪影</span></span></strong></p><p><span><strong><span>2.1 <span>串扰伪影</span><span>(Crosstalk)</span></span></strong></span></p><p><span><span>串扰伪影是因为</span>RF<span>脉冲不是真正的</span><span>sinc</span><span>函数</span><span>,</span><span>经付里叶变换</span><span>(F.T.)</span><span>后出现波瓣</span><span>,</span><span>两个相邻层面的</span><span>RF</span><span>脉冲经</span><span>F.T.</span><span>变换后可能有交叉。串扰使每个层面的有效</span><span>TR</span><span>减小</span><span>(</span><span>由于相邻层面的</span><span>RF</span><span>信号产生饱和</span><span>),</span><span>产生更多的</span><span>T1</span><span>加权。串扰导致</span><span>T1</span><span>加权增加</span><span>,</span><span>信噪比降低。解决串扰伪影的方法有</span><span>:</span></span></p><p><span>(1)<span>使相邻层面有间隔</span><span>,</span><span>可减少串扰程度。但增加层间隔会导致病灶漏检概率增加。</span></span></p><p><span>(2)<span>两次采集间隔</span><span>100%</span><span>间隔带</span><span>,</span><span>即层厚与层间隔一样</span><span>,</span><span>扫描时采用交叉形式。例如首先扫描</span><span>1</span><span>、</span><span>3</span><span>、</span><span>5</span><span>、</span><span>7</span><span>层</span><span>,</span><span>然后扫描</span><span>2</span><span>、</span><span>4</span><span>、</span><span>6</span><span>、</span><span>8</span><span>层。</span></span></p><p><span>(3)<span>改善</span><span>RF</span><span>脉冲波形</span><span>,</span><span>使</span><span>RF</span><span>脉冲经</span><span>F.T.</span><span>变换后产生的波瓣减少。</span></span></p><p><span><strong><span>2.2 RF<span>拉链伪影</span><span>(Zipper)</span></span></strong></span></p><p><span><span>在图像频率编码轴方向的中央条带</span>(<span>零相位</span><span>)</span><span>上出现交错的亮点和黑点形成拉链伪影。产生拉链伪影的原因有以下几个方面</span><span>:</span></span></p><p><span><strong><span>2.2.1</span></strong><strong><span><span>自由感应衰减信号</span>(FID)</span></strong></span></p><p><span><span>在</span>FID<span>没有完全衰减前</span><span>,180</span><span>°</span><span>RF</span><span>脉冲的边瓣与</span><span>FID</span><span>信号重叠形成频率编码方向的拉链伪影。其解</span></span></p><p><span><span>决方法主要有两种</span>:<span>一是增加</span><span>TE,</span><span>增加</span><span>TE</span><span>使得</span><span>FID</span></span></p><p><span><span>信号与</span>180<span>°</span><span>RF</span><span>脉冲的间隔增加</span><span>,</span><span>减少了重叠程度</span><span>,</span><span>从而减少拉链伪影。二是增加层厚</span><span>,</span><span>较大的层厚需要使用宽</span><span>RF</span><span>脉冲</span><span>,</span><span>减少时间域的</span><span>RF</span><span>信号从而减少重叠机会。</span></span></p><p><span><strong><span>2.2.2</span></strong><strong><span>激发回波</span></strong></span></p><p><span><span>这种伪影在图像上表现为沿着频率编码轴的中央出现或窄或宽的噪声带。其形成机理与</span>FID<span>信号类似。相邻层面</span><span>RF</span><span>脉冲有缺陷或双回波序列的</span><span>90</span><span>°—</span><span>180</span><span>°—</span><span>180</span><span>°脉冲有缺陷形成的激发回波可能没</span></span></p><p><span><span>有相位编码</span>,<span>这样在频率编码轴的中央线出现伪影。其解决方法主要是使用梯度破坏脉冲</span><span>,</span><span>破坏激发回波的形成</span><span>,</span><span>或者调节传输器减少伪影。</span></span></p><p><span><strong><span>2.3 RF<span>馈穿拉链伪影</span></span></strong></span></p><p><span><span>当激发</span>RF<span>脉冲在数据采集期间没有完全关闭</span><span>,</span><span>能量会反馈进入接收线圈</span><span>,</span><span>在相位编码方向零频处出现拉链带。如使连续采集</span><span>RF</span><span>激发脉冲的相位改变</span><span>180</span><span>°</span><span>,</span><span>可在很大程度上减少</span><span>RF</span><span>馈穿。</span></span></p><p><span><strong><span>2.4 RF<span>噪声</span></span></strong></span></p><p><span>RF<span>噪声</span><span>(</span><span>如电视台</span><span>,</span><span>电台</span><span>,</span><span>荧光灯等</span><span>)</span><span>使图像出现明显的雪花斑点。与</span><span>RF</span><span>馈穿伪影类似</span><span>,</span><span>除了零频外</span><span>,</span><span>某些特定频率也会产生</span><span>RF</span><span>噪声。解决</span><span>RF</span><span>噪声的方法有</span><span>:</span></span></p><p><span>(1)<span>提高屏蔽室</span><span>RF</span><span>屏蔽性能</span><span>,</span><span>减少外来</span><span>RF</span><span>干扰。</span></span></p><p><span>(2)<span>尽可能移走电子设备</span><span>,</span><span>减少周围电子设备对</span><span>MRI</span><span>设备的影响。</span></span></p><p><span>(3)<span>关闭屏蔽室门</span><span>,</span><span>否则设备受外界干扰严重。</span></span></p><p><span><span>在安放好病人后</span>,<span>检查屏蔽室内各种注意事项是否达到要求。</span></span></p><p><span> </span></p><p><span><strong><span>三、</span></strong><strong><span>外部磁场伪影</span></strong></span></p><p><span><span>与</span>BO<span>场有关的伪影常常由磁场非均匀性引起。磁场非均匀性一般是由于屏蔽不好或环境因素导致图像畸变。在</span><span>SE</span><span>和</span><span>FSE</span><span>脉冲序列中可以使用</span><span>180</span><span>°重聚脉冲减少此种伪影。在</span><span>GRE</span><span>中</span><span>,</span><span>小的空间非均匀性就能产生很多边缘伪影</span><span>(</span><span>斑马纹状</span><span>)</span><span>。可使用合适的屏蔽线圈</span><span>(</span><span>自动屏蔽</span><span>)</span><span>解决磁场非均匀性产生的伪影。</span></span></p><p><span> </span></p><p><span><strong><span>四、</span></strong><strong><span>磁化伪影</span></strong></span></p><p><span><span>任何物质在磁场内都有某种程度的磁化。磁化率是描述磁化程度的一个参数。自然界有顺磁性、抗磁性和铁磁性三类物质</span>,<span>每种物质有不同的磁化率特性。这些物质的主要特性如下</span><span>:</span></span></p><p><span>(1)<span>抗磁性物质有成对电子</span><span>,</span><span>具有负磁化率</span><span>,</span><span>一般是非磁性。</span></span></p><p><span>(2)<span>顺磁性物质包括不成对电子</span><span>,</span><span>具有小的正磁化率</span><span>,</span><span>受外界磁场吸引较小。</span></span></p><p><span>(3)<span>铁磁性物质有很大的磁化率</span><span>,</span><span>受磁场吸引很强。</span></span></p><p><span><span>在面临不同磁化率界面时会产生</span>MRI<span>磁化伪影。</span></span></p><p><span> </span></p><p><span><strong><span>五、</span></strong><strong><span> <span>与梯度场有关的伪影</span></span></strong></span></p><p><span><strong><span>5.1 <span>涡流</span></span></strong></span></p><p><span><span>当梯度场快速切换时产生小的波动电流</span>(<span>产生突然高突然低的电流</span><span>)</span><span>。这些电流会导致梯度剖面线的畸形从而产生图像伪影。</span></span></p><p><span><strong><span>5.2 <span>梯度场非线性</span></span></strong></span></p><p><span><span>理想梯度场是不存在的</span>,<span>梯度场非线性使局部磁场的线性遭到破坏产生图像伪影。此类伪影与</span><span>BO</span><span>的非均匀性引起伪影类似。</span></span></p><p><span><strong><span>5.3 <span>几何畸变</span></span></strong></span></p><p><span><span>几何畸变是梯度场非线性或梯度场能量下降的结果。实际的梯度场有减幅峰</span>,<span>产生图像畸变</span><span>(</span><span>如圆形变椭圆形</span><span>)</span><span>。</span></span></p><p><span> </span></p><p><span><strong><span>六、</span></strong><strong><span>与病人有关的伪影</span></strong></span></p><p><span><span>病人的运动或血管的脉动会产生运动伪影。运动伪影发生在相位编码方向。这是因为</span>:</span></p><p><span>(1)<span>梯度场方向的运动导致相位积聚</span><span>,</span><span>使相位编码梯度场的信号不匹配</span><span>,</span><span>产生伪影。</span></span></p><p><span>(2)<span>频率编码方向采样</span><span>(</span><span>毫秒级</span><span>)</span><span>比一个相位编码步</span><span>(</span><span>秒级</span><span>)</span><span>时间少的多。这样大多数运动与频率编码方向的采样相比速度慢</span><span>,</span><span>运动伪影主要沿相位编码方向传播。在频率编码方向也有可能发生与病人有关的伪影</span><span>,</span><span>但不明显。</span></span></p><p><span><strong><span>6.1 <span>周期性运动</span></span></strong></span></p><p><span><span>血管脉动、心脏或</span>CSF<span>周期性运动产生周期性运动伪影</span><span>(</span><span>例如</span><span>Ghost</span><span>伪影</span><span>)</span><span>。</span><span>Ghost</span><span>伪影间距</span><span>(SEP)</span><span>为</span><span>:</span></span></p><p><span>SEP=(TR)(Ny)(NEX)/T(<span>运动周期</span><span>)</span><span>或</span><span>SEP=</span><span>采集时间</span><span>/T(</span><span>运动周期</span><span>)</span></span></p><p><span>TR<span>是重复时间</span><span>,T</span><span>是运动周期。增加</span><span>TR</span><span>、</span><span>Ny</span><span>或</span><span>NEX,</span><span>可增加</span><span>Ghost</span><span>伪影的间距</span><span>,</span><span>从而减少图像上伪影数目。解决周期性运动伪影的方法有</span><span>:</span></span></p><p><span>(1)<span>使用空间预饱和脉冲使流动质饱和从而减少伪影。</span></span></p><p><span>(2)<span>通过增加</span><span>TR,Ny</span><span>或</span><span>NEX</span><span>增加</span><span>Ghost</span><span>的间距</span><span>(</span><span>等同于增加扫描时间</span><span>),</span><span>减少图像中伪影个数。</span></span></p><p><span>(3)<span>改变相位编码和频率编码方向</span><span>,</span><span>使伪影方向改变。</span></span></p><p><span>(4)<span>使用心动门控</span><span>,</span><span>减少心脏周期性运动伪影。</span><span>(5)</span><span>使用流动补偿</span><span>,</span><span>减少血液流动产生的伪影。</span></span></p><p><span><strong><span>6.2 <span>随机运动</span></span></strong></span></p><p><span><span>随机运动是由病人随意或不随意的运动造成的。随机运动产生模糊伪影。在相位编码方向可观察到平行条带。这种随机运动产生的伪影与截尾伪影类似</span>,<span>但有差异</span><span>,</span><span>截尾伪影产生衰减条带</span><span>,</span><span>随机运动产生的条带均匀。解决这种类型伪影。解决这种类型伪影的方法有</span>:</span></p><p><span>(1)<span>使病人在扫描过程中保持镇静以使身体稳定。</span></span></p><p><span>(2)<span>采用呼吸补偿。</span></span></p><p><span>(3)<span>使用腹部扫描模式减少肠蠕动伪影。</span><span>(4)</span><span>必要时使用镇痛剂</span><span>,</span><span>减少病人的运动。</span></span></p><p><span>(5)<span>使用快速扫描序列</span><span>(</span><span>如</span><span>FSE,GRE,EPI</span><span>序列等</span><span>),</span><span>减少扫描时间。</span></span></p><p><span><strong><span>6.3 Magic<span>角伪影</span></span></strong></span></p><p><span><span>在关节成像中</span>,<span>如果腱与主磁场的夹角接近某个角度</span><span>(55</span><span>°</span><span>)</span><span>在</span><span>T1</span><span>像和质子密度像中腱是亮的</span><span>,</span><span>但在</span><span>T2</span><span>像中是正常的。信号强度的改变可能会在图像上与病理组织的图像相混</span><span>,</span><span>难以区分。产生</span><span>Magic</span><span>角伪影的原因是由于腱的主要成分是胶原质</span><span>,</span><span>是各向异性结构。各向异性的特性是随方向变化的</span><span>,</span><span>这样腱的</span><span>T2</span><span>与方向有关。在满足</span><span>Magic</span><span>角的情况下</span><span>,</span><span>腱的</span><span>T2</span><span>轻微增加。</span></span></p><p><span> </span></p><p><span><strong><span>七、</span></strong><strong><span>数据误差</span></strong></span></p><p><span><span>在数据采集过程中</span>,<span>与单一层面</span><span>k</span><span>空间数据有关的计算可能产生随机误差</span><span>,</span><span>产生十字交叉条纹伪影</span><span>,</span><span>这种伪影一般在单一图像中出现</span><span>,</span><span>不会出现在其它图像上。解决数据误差引起伪影的方法</span><span>:</span></span></p><p><span>(1)<span>除去数据的离散误差</span><span>,</span><span>与相邻数据平均</span><span>,</span><span>减少伪影。</span></span></p><p><span>(2)<span>简单地重复序列</span><span>,</span><span>观察图像是否还有伪影以解决问题</span></span></p><p><p><img src="image/20201019/f5d8f191a770204752cff019c1184c28_2.png" /></p></p><p><br /></p><p><br /></p><p><br /></p><p><p><img src="image/20201019/7822a4e2249e86a45760fbbfc1675e38_3.gif" /></p></p><p><br /></p>
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发表于 2020-10-19 09:24:27