CT图像环形伪影分析

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                    <p style=";text-autospace:ideograph-other"><span style=""><img src="http://mmbiz.qpic.cn/mmbiz/A1DicCH9Vl6NyPYRdpyamWZASxaxvdqhrrx2z3kBCdiajEibcmQHdAOBxHIAvqsicMahHKOWc2ddsIA7mBSSjibKVgg/0" data-ratio="0.25" data-w="400"  /></span></p><p style=";text-autospace:ideograph-other"><span style=""> </span><span style="">CT<span style="font-family:宋体">图像产生环形伪影是第三代</span><span style="font-family:Times New Roman">CT</span><span style="font-family:宋体">机</span><span style="font-family:Times New Roman">(</span><span style="font-family:宋体">旋转</span><span style="font-family:Times New Roman">-</span><span style="font-family:宋体">旋转扫描方式</span><span style="font-family:Times New Roman">)</span><span style="font-family:宋体">的常见故障之一，造成环形伪影的因素很多，主要包括：</span></span><span style="">球管输出量不足</span><span style="">，</span><span style="">球管位置安装偏差，准直器内掉进异物，模型校准数据不准，数据采集系统<span style="font-family:Times New Roman">(DAS)</span><span style="font-family:宋体">故障等</span></span><span style="">。其中</span><span style="">DAS<span style="font-family:宋体">部分引起的环形伪影最为常见</span></span><span style="">，也最为复杂。其它几种因素分别采取重新做模型校准或更换球管、适当调整球管位置、清除准直器内异物等措施即可。下面就以日本东芝公司生产的第三代机<span style="font-family:Times New Roman">TCT-300S</span><span style="font-family:宋体">为例，讨论由</span><span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">造成的环形伪影问题 资料来源 </span><span style="font-family:Times New Roman">:</span><span style="font-family:宋体">医 学 教 育网 。</span></span><span style=""><br  /></span><span style="">  1 </span><span style="">数据采集系统的工作原理</span><span style=""><br  /></span><span style=""> </span><span style="">  </span><span style=""> TCT<span style="font-family:宋体">一</span><span style="font-family:Times New Roman">300S</span><span style="font-family:宋体">的</span><span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">包括</span><span style="font-family:Times New Roman">512</span><span style="font-family:宋体">个探测器，</span><span style="font-family:Times New Roman">32</span><span style="font-family:宋体">块</span><span style="font-family:Times New Roman">Q-MUX</span><span style="font-family:宋体">集成电路</span><span style="font-family:Times New Roman">(IC)</span><span style="font-family:宋体">，</span><span style="font-family:Times New Roman">Q</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">V</span><span style="font-family:宋体">、</span><span style="font-family:Times New Roman">FPA</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">ADC</span><span style="font-family:宋体">、</span><span style="font-family:Times New Roman">TIMING I</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">O</span><span style="font-family:宋体">印刷电路板</span><span style="font-family:Times New Roman">(PWB)</span><span style="font-family:宋体">各</span><span style="font-family:Times New Roman">l</span><span style="font-family:宋体">块以及探测器电源和</span><span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">电源。</span></span><span style=""><br  /></span><span style="">  </span><span style="">  </span><span style="">探测器由<span style="font-family:Times New Roman">512</span><span style="font-family:宋体">个氙气探测器组成，内充高压氙气，极间加</span><span style="font-family:Times New Roman">200VDC</span><span style="font-family:宋体">电压。氙气接受</span><span style="font-family:Times New Roman">X</span><span style="font-family:宋体">线入射后电离，产生电荷，在极间高压作用下移动，形成电流信号，向</span><span style="font-family:Times New Roman">Q-MUX IC</span><span style="font-family:宋体">输出。</span></span><span style=""><br  /></span><span style="">  </span><span style="">  </span><span style="">Q-MUX IC<span style="font-family:宋体">是</span><span style="font-family:Times New Roman">16</span><span style="font-family:宋体">路积分电路，通过电容的积分作用把输入的电流信号转换成电荷信号，每一块</span><span style="font-family:Times New Roman">Q-MUX IC</span><span style="font-family:宋体">负责</span><span style="font-family:Times New Roman">16</span><span style="font-family:宋体">个通道，一共</span><span style="font-family:Times New Roman">32</span><span style="font-family:宋体">块</span><span style="font-family:Times New Roman">ICs</span><span style="font-family:宋体">完成全部探测器组成的</span><span style="font-family:Times New Roman">512</span><span style="font-family:宋体">个通道的转换工作。所有通道按照一定的组合规律通过</span><span style="font-family:Times New Roman">4</span><span style="font-family:宋体">根模拟总线</span><span style="font-family:Times New Roman">(QMBUS)</span><span style="font-family:宋体">与</span><span style="font-family:Times New Roman">Q</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">V PWB</span><span style="font-family:宋体">相连接。</span></span><span style=""><br  /></span><span style="">  </span><span style="">  </span><span style="">因此，每一根<span style="font-family:Times New Roman">QMBUS</span><span style="font-family:宋体">传送</span><span style="font-family:Times New Roman">8</span><span style="font-family:宋体">块</span><span style="font-family:Times New Roman">Q-MUX ICs</span><span style="font-family:宋体">、</span><span style="font-family:Times New Roman">128</span><span style="font-family:宋体">个通道的信号，并且各个相邻信号之间间隔</span><span style="font-family:Times New Roman">4</span><span style="font-family:宋体">个通道。</span><span style="font-family:Times New Roman">QMBUS</span><span style="font-family:宋体">为同轴缋孪摺</span><span style="font-family:Times New Roman">?br&gt;   Q</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">V PWB</span><span style="font-family:宋体">有</span><span style="font-family:Times New Roman">4</span><span style="font-family:宋体">个</span><span style="font-family:Times New Roman">Q</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">V</span><span style="font-family:宋体">放大器对应于</span><span style="font-family:Times New Roman">4</span><span style="font-family:宋体">根模拟总线，</span><span style="font-family:Times New Roman">Q</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">V</span><span style="font-family:宋体">放大器是电荷／电压转换器，把</span><span style="font-family:Times New Roman">Q-MUX</span><span style="font-family:宋体">输出的电荷信号转换成电压信号，输入到</span><span style="font-family:Times New Roman">FPA</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">ADCPWB</span><span style="font-family:宋体">。</span></span><span style=""><br  /></span><span style="">  </span><span style="">  </span><span style="">FPA<span style="font-family:宋体">／</span><span style="font-family:Times New Roman">ADC PWB</span><span style="font-family:宋体">包括浮点放大和模数转换</span><span style="font-family:Times New Roman">2</span><span style="font-family:宋体">个功能块。浮点放大器提供</span><span style="font-family:Times New Roman">Xl</span><span style="font-family:宋体">、</span><span style="font-family:Times New Roman">X8</span><span style="font-family:宋体">、</span><span style="font-family:Times New Roman">x64 3</span><span style="font-family:宋体">种增益，根据信号水平自动选择适当增益，使输出电压信号保持在</span><span style="font-family:Times New Roman">10V</span><span style="font-family:宋体">级，所选取增益用</span><span style="font-family:Times New Roman">2</span><span style="font-family:宋体">位数字信号表示，</span></span><span style="">模拟信号经<span style="font-family:Times New Roman">ADC</span><span style="font-family:宋体">转换成</span><span style="font-family:Times New Roman">14</span><span style="font-family:宋体">位数字信号</span></span><span style="">，并与<span style="font-family:Times New Roman">2</span><span style="font-family:宋体">位增益位合成</span><span style="font-family:Times New Roman">16</span><span style="font-family:宋体">位数字信号，输出给</span><span style="font-family:Times New Roman">FRU</span><span style="font-family:宋体">。至此，</span><span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">的工作完成。  </span></span><span style="">  </span><span style="">TIMING I<span style="font-family:宋体">／</span><span style="font-family:Times New Roman">O PWB</span><span style="font-family:宋体">提供</span><span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">的时序控制，包括</span><span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">与</span><span style="font-family:Times New Roman">FRU</span><span style="font-family:宋体">的接口功能和</span><span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">采样序列控制。</span></span><span style=""><br  /></span><span style="">  </span><span style="">  </span><span style="">通过对<span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">的分析，可以明确</span><span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">各部分与探测器通道的对应关系，因为</span><span style="font-family:Times New Roman">CT</span><span style="font-family:宋体">图像环形伪影正是反映了相应某个或某些通道的故障，明确它们之间的内在联系之后，就能根据各种伪影的特征，分析其性质，判别原因，加以检修。</span></span><span style=""><br  /></span><span style=""><br  /></span><span style="">资料来源 <span style="font-family:Times New Roman">:</span><span style="font-family:宋体">医 学 教 育网</span></span><span style=""><br  /></span><span style="">  2 CT<span style="font-family:宋体">环形伪影故障分析及检修</span></span><span style=""><br  /></span><span style="">  CT<span style="font-family:宋体">图像产生的环形伪影可分为单环和多环，其故障原因也各不相同，下面就一一举例讨论。</span></span><span style=""><br  /></span><span style=""><br  /></span><span style=""><br  /></span><span style="">  2.1 <span style="font-family:宋体">单环伪影</span></span><span style=""><br  /></span><span style="">  </span><span style="">  </span><span style="">造成单环伪影的故障相对简单，通过<span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">原理分析，基本上可以确定是某个探测器通道的问题。对于此类故障，</span><span style="font-family:Times New Roman">TCT-300S</span><span style="font-family:宋体">可以</span></span><span style="">通过系统设置关闭故障通道来解决</span><span style="">。现在的关键是如何定位故障通道。在<span style="font-family:Times New Roman">TCT-300S</span><span style="font-family:宋体">的维修界面下执行</span><span style="font-family:Times New Roman">DCA</span><span style="font-family:宋体">程序，检查各通道的</span><span style="font-family:Times New Roman">DCA</span><span style="font-family:宋体">数据，其</span><span style="font-family:Times New Roman">Mean</span><span style="font-family:宋体">值的正常范围是</span><span style="font-family:Times New Roman">100-500</span><span style="font-family:宋体">，结果发现第</span><span style="font-family:Times New Roman">174</span><span style="font-family:宋体">通道</span><span style="font-family:Times New Roman">Mean</span><span style="font-family:宋体">值不在该数量级上，且远远低于正常范围，确定是第</span><span style="font-family:Times New Roman">174</span><span style="font-family:宋体">通道故障。退出</span><span style="font-family:Times New Roman">DCA</span><span style="font-family:宋体">程序，进入</span><span style="font-family:Times New Roman">OPT</span><span style="font-family:宋体">程序的</span><span style="font-family:Times New Roman">System Mode</span><span style="font-family:宋体">，关闭第</span><span style="font-family:Times New Roman">174</span><span style="font-family:宋体">通道，此时第</span><span style="font-family:Times New Roman">174</span><span style="font-family:宋体">通道的数据取第</span><span style="font-family:Times New Roman">173</span><span style="font-family:宋体">和</span><span style="font-family:Times New Roman">175</span><span style="font-family:宋体">通道的平均值。对系统进行初始化，并重新做水模校准，环形伪影消失。</span></span><span style=""><br  /></span><span style="">  </span><span style="">  </span><span style="">另外，由若干个相互独立、没有一定规律的单环组成的多环伪影，也可用以上方法解决。<span style="font-family:Times New Roman">TCT-300S</span><span style="font-family:宋体">系统设置最多可关闭</span><span style="font-family:Times New Roman">4</span><span style="font-family:宋体">个通道。  </span></span><span style=""><br  /></span><span style=""><br  /></span><span style=""><br  /></span><span style="">  2.2 <span style="font-family:宋体">多环伪影</span></span><span style=""><br  /></span><span style=""> </span><span style="">  </span><span style=""> 通过对<span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">的分析可知，引起</span><span style="font-family:Times New Roman">CT</span><span style="font-family:宋体">图像多环伪影的原因较多，也较为复杂。</span><span style="font-family:Times New Roman">4</span><span style="font-family:宋体">根模拟总线、</span><span style="font-family:Times New Roman">32</span><span style="font-family:宋体">块</span><span style="font-family:Times New Roman">Q-MUX ICs</span><span style="font-family:宋体">及</span><span style="font-family:Times New Roman">Q</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">V PWB</span><span style="font-family:宋体">、</span><span style="font-family:Times New Roman">FPA</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">ADC PWB</span><span style="font-family:宋体">故障均可造成多环伪影，且多环之间存在一定规律，分析其中规律可以鉴别故障原因，加以检修。</span></span><span style=""><br  /></span><span style=""><br  /></span><span style=""><br  /></span><span style="">  2.2.1 <span style="font-family:宋体">全幅图像环形伪影</span><span style="font-family:Times New Roman">(</span><span style="font-family:宋体">表现为白图</span><span style="font-family:Times New Roman">)</span></span><span style=""><br  /></span><span style="">  </span><span style="">  </span><span style="">检查<span style="font-family:Times New Roman">DCA</span><span style="font-family:宋体">数据，发现</span><span style="font-family:Times New Roman">512</span><span style="font-family:宋体">个通道的</span><span style="font-family:Times New Roman">Mean</span><span style="font-family:宋体">值全部低于正常值</span><span style="font-family:Times New Roman">(</span><span style="font-family:宋体">甚至全部为</span><span style="font-family:Times New Roman">0)</span><span style="font-family:宋体">。由于数据采集过程中，只有</span><span style="font-family:Times New Roman">FPA</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">ADC PWB</span><span style="font-family:宋体">参与全部</span><span style="font-family:Times New Roman">512</span><span style="font-family:宋体">个通道的数据处理，因此判断该板故障。更换新板后，重做水模校准，故障消失。</span></span></p><p><span style="">                 ADC<span style="font-family:宋体">板为模拟信号转数字信号，进行全部通道数据处理</span></span><span style=""><br  /></span><span style=""><br  /></span><span style=""><br  /></span><span style="">  2.2.2 <span style="font-family:宋体">全幅图像间隔均匀的环形伪影</span></span><span style=""><br  /></span><span style=""> </span><span style="">  </span><span style=""> 检查<span style="font-family:Times New Roman">DCA</span><span style="font-family:宋体">数据，结果显示</span><span style="font-family:Times New Roman">512</span><span style="font-family:宋体">个通道的</span><span style="font-family:Times New Roman">Mean</span><span style="font-family:宋体">值中，每个</span><span style="font-family:Times New Roman">4</span><span style="font-family:宋体">个通道出现</span><span style="font-family:Times New Roman">1</span><span style="font-family:宋体">个异常值，其规律为所有</span><span style="font-family:Times New Roman">(3.7</span><span style="font-family:宋体">，</span><span style="font-family:Times New Roman">…</span><span style="font-family:宋体">，</span><span style="font-family:Times New Roman">4(n-1)+3</span><span style="font-family:宋体">，</span><span style="font-family:Times New Roman">…</span><span style="font-family:宋体">，</span><span style="font-family:Times New Roman">511)</span><span style="font-family:宋体">通道的</span><span style="font-family:Times New Roman">Mean</span><span style="font-family:宋体">值低于正常范围，其中</span><span style="font-family:Times New Roman">n=1,2…</span><span style="font-family:宋体">，</span><span style="font-family:Times New Roman">128</span><span style="font-family:宋体">，一共有</span><span style="font-family:Times New Roman">128</span><span style="font-family:宋体">个通道异常。根据</span><span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">原理分析可知，第</span><span style="font-family:Times New Roman">3</span><span style="font-family:宋体">根模拟总线或</span><span style="font-family:Times New Roman">Q</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">V PWB</span><span style="font-family:宋体">中第</span><span style="font-family:Times New Roman">3</span><span style="font-family:宋体">个</span><span style="font-family:Times New Roman">Q</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">V</span><span style="font-family:宋体">放大器故障。为了鉴别故障原因，可做以下试验：交换第</span><span style="font-family:Times New Roman">3</span><span style="font-family:宋体">根和第</span><span style="font-family:Times New Roman">2</span><span style="font-family:宋体">根模拟总线，再检查</span><span style="font-family:Times New Roman">DCA</span><span style="font-family:宋体">数据。如果异常通道没有发生改变，则</span><span style="font-family:Times New Roman">Q</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">V PWB</span><span style="font-family:宋体">故障；如果异常通道随之转移到</span><span style="font-family:Times New Roman">(4(n-1)+2)</span><span style="font-family:宋体">通道，则模拟总线故障。在实例中为模拟总线接触不良，重接后伪影消失。</span></span><span style=""><br  /></span><span style=""><br  /></span><span style=""><br  /></span><span style="">  2.2.3 <span style="font-family:宋体">部分环形区域内产生间隔均匀的环形伪影</span></span><span style=""><br  /></span><span style=""> </span><span style="">  </span><span style=""> 检查<span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">数据，发现部分通道的</span><span style="font-family:Times New Roman">Mean</span><span style="font-family:宋体">值偏低，也是每隔</span><span style="font-family:Times New Roman">4</span><span style="font-family:宋体">个通道出现</span><span style="font-family:Times New Roman">1</span><span style="font-family:宋体">个异常值，一共</span><span style="font-family:Times New Roman">16</span><span style="font-family:宋体">个通道。由此判断是某块</span><span style="font-family:Times New Roman">Q-MUX IC</span><span style="font-family:宋体">故障。通过检查异常通道的序号与</span><span style="font-family:Times New Roman">Q-MUX IC</span><span style="font-family:宋体">的对应关系，可以确定故障</span><span style="font-family:Times New Roman">IC</span><span style="font-family:宋体">的序号：同时也可以做</span><span style="font-family:Times New Roman">ICs</span><span style="font-family:宋体">之间的交换试验，此时发现异常通道随之转移到相应位置，从而最终确定</span><span style="font-family:Times New Roman">Q-MUXIC</span><span style="font-family:宋体">故障。更换该板，重新做水模校准，环形伪影消失。</span></span><span style=""><br  /></span><span style="">  </span><span style="">  </span><span style="">通过对<span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">原理及其故障的讨论，基本上可以明确</span><span style="font-family:Times New Roman">CT</span><span style="font-family:宋体">图像产生环形伪影的原因及其检修的思路和方法。当然，以上所举的实例比较典型，在日常工作中遇到的问题可能更加复杂，需要不断积累实践经验， 比如通过</span><span style="font-family:Times New Roman">Raw Data</span><span style="font-family:宋体">重建鉴别</span><span style="font-family:Times New Roman">DAS</span><span style="font-family:宋体">和</span><span style="font-family:Times New Roman">FRU</span><span style="font-family:宋体">故障，通过改变</span><span style="font-family:Times New Roman">mAs</span><span style="font-family:宋体">鉴别</span><span style="font-family:Times New Roman">Q</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">V</span><span style="font-family:宋体">和</span><span style="font-family:Times New Roman">FPA</span><span style="font-family:宋体">／</span><span style="font-family:Times New Roman">ADC</span><span style="font-family:宋体">故障等。另外，</span></span><span style="">DAS<span style="font-family:宋体">经过调整、维修或更换后，必须重新采集校准数据。</span></span><span style=""><br  /></span></p><p><img data-s="300,640" src="http://mmbiz.qpic.cn/mmbiz/A1DicCH9Vl6NyPYRdpyamWZASxaxvdqhrVMH1I0clUJJ0rIjYhVzHo5ibNriaxFScCVssI837D39diajicT8CMAyv5A/0" data-ratio="0.6106719367588933" data-w=""  /></p><p class="p0" style="margin-bottom:0pt; margin-top:0pt; "><span style="color: rgb(62, 62, 62); font-size: 14pt; font-family: 仿宋; background-color: rgb(255, 255, 255); background-position: 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