The impact of overscan on patient dose with first generation multislice CT scanners

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.