CT 机的基本结构——X 射线探测部分

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                    <p style="max-width: 100%; min-height: 1em; color: rgb(62, 62, 62); font-family: &#39;Hiragino Sans GB&#39;, &#39;Microsoft YaHei&#39;, Arial, sans-serif; font-size: 16px; line-height: 25.6px; white-space: normal; widows: 1; background-color: rgb(255, 255, 255); box-sizing: border-box !important; word-wrap: break-word !important; text-align: center;"><span style="max-width: 100%; font-family: 黑体; box-sizing: border-box !important; word-wrap: break-word !important;">这部分包括探测器、模数转换器和后准直器，其作用是探测穿过人体后的X 射线量，并转化为数字信号。</span></p><p style="max-width: 100%; min-height: 1em; color: rgb(62, 62, 62); font-family: &#39;Hiragino Sans GB&#39;, &#39;Microsoft YaHei&#39;, Arial, sans-serif; font-size: 16px; line-height: 25.6px; white-space: normal; widows: 1; background-color: rgb(255, 255, 255); box-sizing: border-box !important; word-wrap: break-word !important; text-align: center;"><span style="max-width: 100%; font-family: 黑体; box-sizing: border-box !important; word-wrap: break-word !important;"><br  /></span></p><p style="max-width: 100%; min-height: 1em; color: rgb(62, 62, 62); font-family: &#39;Hiragino Sans GB&#39;, &#39;Microsoft YaHei&#39;, Arial, sans-serif; font-size: 16px; line-height: 25.6px; white-space: normal; widows: 1; background-color: rgb(255, 255, 255); box-sizing: border-box !important; word-wrap: break-word !important; text-align: center;"><strong><span style="max-width: 100%; font-family: 黑体; box-sizing: border-box !important; word-wrap: break-word !important;">（一）探测器</span></strong></p><p style="max-width: 100%; min-height: 1em; color: rgb(62, 62, 62); font-family: &#39;Hiragino Sans GB&#39;, &#39;Microsoft YaHei&#39;, Arial, sans-serif; font-size: 16px; line-height: 25.6px; white-space: normal; widows: 1; background-color: rgb(255, 255, 255); box-sizing: border-box !important; word-wrap: break-word !important; text-align: center;"><span style="max-width: 100%; font-family: 黑体; box-sizing: border-box !important; word-wrap: break-word !important;">探测器可分为气体和固体两大类。前者采用气体电离的原理，探测器内多充有惰性气体，如氙气（xenon）或氪气（krypton），当X 射线射入后，使氙气产生成对的光电子， 由收集电极集中后，便产生与X 射线强度成比例的电流。后者常用碘化钠（Nal）、碘化铯（Csl）、钨酸镉（CDW04）、锗酸铋（BGO）作为荧光体，它们当受到X 射线照射后发出荧光，然后再经过光电转换和信号放大。目前，碘化钠已极少使用。气体探测器和固体探测器各有优缺点，气体探测器灵敏度差一些，但利用率高，受温度湿度影响小一些，性能也比较稳定。固体探测器灵敏度高，容易受温度湿度影响，使用一段时间后其性能指标会有些变化，由于其余辉时间（激发荧光后恢复到原状态的时间）长，使利用率降低。固体探测器的分辨率要比气体探测器分辨率高。对固体探测器来说，使用一段时间后有些探测器性能指标可能有些变化，会造成CT 图像上的伪影，以至扫描出来的图像无法诊断。这时，可以用软件将性能和曲线不太好的探测器隔离起来，即不运用这些探测器所采集的数据，将其放置一段时间不用，当经过一段时间放置的探测器如果再重新检查，则有一些还可以恢复使用，有一些是不可以恢复使用的。在探测器的排列和数量方面有许多不同，第1、2 代CT 机探测器的数目是从单个至数十个， 目前第3、4 代CT 机的探测器大都在数百个，甚至千余个，第3 代CT 机探测器的排列是沿X 射线球管的对应扇形面呈弧形排列，是与X 射线球管同时旋转，相对运动的；第4 代CT 机则是固定在扫描机架圆周内的360°，一般是以每半度一个探测器或多达一千多个安装的，是不随X 射线球管旋转的。当然，探测器个数越多，空间分辨率就越高。在目前的螺旋形CT 机中还常使用气体探测器，以第3 代CT 机型的结构形式，其探测器数量大多在数百个。</span></p><p style="max-width: 100%; min-height: 1em; color: rgb(62, 62, 62); font-family: &#39;Hiragino Sans GB&#39;, &#39;Microsoft YaHei&#39;, Arial, sans-serif; font-size: 16px; line-height: 25.6px; white-space: normal; widows: 1; background-color: rgb(255, 255, 255); box-sizing: border-box !important; word-wrap: break-word !important; text-align: center;"><span style="max-width: 100%; font-family: 黑体; box-sizing: border-box !important; word-wrap: break-word !important;"><br  /></span></p><p style="max-width: 100%; min-height: 1em; color: rgb(62, 62, 62); font-family: &#39;Hiragino Sans GB&#39;, &#39;Microsoft YaHei&#39;, Arial, sans-serif; font-size: 16px; line-height: 25.6px; white-space: normal; widows: 1; background-color: rgb(255, 255, 255); box-sizing: border-box !important; word-wrap: break-word !important; text-align: center;"><strong><span style="max-width: 100%; font-family: 黑体; box-sizing: border-box !important; word-wrap: break-word !important;">（二）模数转换器</span></strong></p><p style="max-width: 100%; min-height: 1em; color: rgb(62, 62, 62); font-family: &#39;Hiragino Sans GB&#39;, &#39;Microsoft YaHei&#39;, Arial, sans-serif; font-size: 16px; line-height: 25.6px; white-space: normal; widows: 1; background-color: rgb(255, 255, 255); box-sizing: border-box !important; word-wrap: break-word !important; text-align: center;"><span style="max-width: 100%; font-family: 黑体; box-sizing: border-box !important; word-wrap: break-word !important;">模数转换器的作用是将探测器所形成的光电信号转化为数字信号，并输送到阵列处理机，主要是A/D 的转化过程。</span></p><p style="max-width: 100%; min-height: 1em; color: rgb(62, 62, 62); font-family: &#39;Hiragino Sans GB&#39;, &#39;Microsoft YaHei&#39;, Arial, sans-serif; font-size: 16px; line-height: 25.6px; white-space: normal; widows: 1; background-color: rgb(255, 255, 255); box-sizing: border-box !important; word-wrap: break-word !important; text-align: center;"><span style="max-width: 100%; font-family: 黑体; box-sizing: border-box !important; word-wrap: break-word !important;"><br  /></span></p><p style="max-width: 100%; min-height: 1em; color: rgb(62, 62, 62); font-family: &#39;Hiragino Sans GB&#39;, &#39;Microsoft YaHei&#39;, Arial, sans-serif; font-size: 16px; line-height: 25.6px; white-space: normal; widows: 1; background-color: rgb(255, 255, 255); box-sizing: border-box !important; word-wrap: break-word !important; text-align: center;"><strong><span style="max-width: 100%; font-family: 黑体; box-sizing: border-box !important; word-wrap: break-word !important;">（三）后准直器</span></strong></p><p style="max-width: 100%; min-height: 1em; color: rgb(62, 62, 62); font-family: &#39;Hiragino Sans GB&#39;, &#39;Microsoft YaHei&#39;, Arial, sans-serif; font-size: 16px; line-height: 25.6px; white-space: normal; widows: 1; background-color: rgb(255, 255, 255); box-sizing: border-box !important; word-wrap: break-word !important; text-align: center;"><span style="max-width: 100%; font-family: 黑体; box-sizing: border-box !important; word-wrap: break-word !important;">后准直器位于探测器前面，其结构与探测器做在一个整体上，在被扫描物体后方，主要的作用是除去通过X 射线被照射物体后产生的散射线，结构比较简单。</span></p><p><br  /></p><p style="white-space: normal; 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