Medical Physics Department, Agios Savvas Hospital, Athens, Greece. j_tsalas2hotmail.com
Phys Med. 2011 Apr;27(2):69-74. doi: 10.1016/j.ejmp.2010.03.001. Epub 2010 Apr 24.
Helical scanning requires the irradiation of larger lengths than those planned. This is referred to as overscan and results to an increase of patient dose. Its impact on patient dose was investigated for three first generation multislice CT scanners; a six-, a quad- and a dual-slice. The amount of overscan was determined using the scanners' dose-length product (DLP) indications and films positioned on the CT table. With the preset protocol for the chest examination selected in all CT scanners, the overscan length calculated from the DLP indications was 6.3, 3.5 and 2cm respectively, whereas the corresponding figures derived from the films were 6.6, 4.8 and 2.5 to 3.2cm. For a 30cm scan length, the respective contributions of overscan to the DLP values were 17, 10 and 6%, whereas for a scan length of 20cm the respective values increased to 24, 15 and 9%. For the smallest scan lengths allowed in helical mode, the respective contributions reached 53, 88 and 67% because for the six-slice scanner the smallest scan length was limited to twice the collimation, whereas in the quad and dual scanners no limitation existed. For small scan lengths the presence of overscan cancels out any dose reduction offered by helical scanning with pitch factor values larger than one and therefore the axial mode should be preferred, when this is not prohibited by the diagnostic task in question.
螺旋扫描需要照射比计划长度更大的长度。这被称为过扫描，并导致患者剂量增加。为此，对三台第一代多层 CT 扫描仪（一台六排、一台四排和一台双排 CT 扫描仪）进行了过扫描对患者剂量影响的研究。使用扫描仪的剂量长度乘积（DLP）指示和放置在 CT 台上的胶片来确定过扫描量。在所有 CT 扫描仪中选择胸部检查的预设协议，从 DLP 指示计算的过扫描长度分别为 6.3、3.5 和 2cm，而从胶片得出的相应长度分别为 6.6、4.8 和 2.5 至 3.2cm。对于 30cm 的扫描长度，过扫描对 DLP 值的各自贡献分别为 17、10 和 6%，而对于 20cm 的扫描长度，各自的贡献值增加到 24、15 和 9%。对于螺旋模式允许的最小扫描长度，各自的贡献达到 53、88 和 67%，因为对于六排 CT 扫描仪，最小扫描长度限制为准直宽度的两倍，而在四排和双排 CT 扫描仪中则没有限制。对于小的扫描长度，过扫描的存在抵消了任何螺旋扫描提供的剂量减少，其螺距因子值大于 1，因此，当不受所涉及的诊断任务限制时，应优先选择轴向模式。
