Knox H H, Gagne R M
Center for Devices and Radiological Health/FDA, Rockville, MD 20857, USA.
Health Phys. 1996 Aug;71(2):219-24. doi: 10.1097/00004032-199608000-00015.
The most direct way of getting the value of the multiple scan average dose (MSAD) in computed tomography is to employ a pencil chamber for integration of a single scan dose profile. Because the active length of the pencil chamber is fixed, the measurement can represent the value of the MSAD from a different number of contiguous scans depending on the slice thickness. This characteristic makes it difficult to compare the value of MSAD using the pencil chamber to the information required by Federal regulations on the computed tomography dose index (CTDI). The CTDI, which is the MAD at the center of a set of 14 contiguous scans, is the dose descriptor used in the Federal Performance Standard. Two alternative methods were developed to make the CTDI measurements at the center of a CT dosimetry phantom. These alternative methods were compared to the results of thermoluminescent dosimeter (TLD) measurements from more than 20 different CT scanners. One alternative method involved the use of radio-opaque sleeves with the pencil chamber to limit the length of the single scan dose profile incident on the pencil chamber. In addition, the TLD data were also used to obtain a set of conversion factors for converting the results of a measurement with the pencil chamber without a radio-opaque sleeve to a value of the CTDI. The alternative methods of obtaining the CTDI agree on the average to better than 10% for all values of slice thickness on the different CT scanners.
在计算机断层扫描中获取多次扫描平均剂量（MSAD）值的最直接方法是使用笔形电离室对单次扫描剂量分布进行积分。由于笔形电离室的有效长度是固定的，根据切片厚度的不同，该测量可以代表来自不同数量连续扫描的MSAD值。这一特性使得使用笔形电离室得到的MSAD值难以与联邦法规对计算机断层扫描剂量指数（CTDI）所要求的信息进行比较。CTDI是一组14次连续扫描中心处的平均剂量（MAD），是联邦性能标准中使用的剂量描述符。开发了两种替代方法来在CT剂量学体模中心进行CTDI测量。将这些替代方法与来自20多种不同CT扫描仪的热释光剂量计（TLD）测量结果进行了比较。一种替代方法是在笔形电离室上使用不透射线的套管，以限制入射到笔形电离室上的单次扫描剂量分布的长度。此外，TLD数据还用于获得一组转换因子，用于将使用没有不透射线套管的笔形电离室的测量结果转换为CTDI值。对于不同CT扫描仪上的所有切片厚度值，获取CTDI的替代方法平均一致性优于10%。
