Sandra Hopkins Final Report.pdf - University of Surrey

2.3.3 Equipment configurationFigure 8 shows a schematic of the Siemens Axiom Artis dFC in the AP direction. The distance ofthe x-ray tube from the floor is fixed, however the height of the detector can vary so that theFigure 8 : Schematic of the Axiom Artis dfCrange of distance for the source to detector (SID) can be from 120 cm to 90cm. In clinical use thecouch top is typically positioned 15cm below the iso-centre (centre point of rotation). This is so thatthe patient’s heart is at the iso centre and the c-arm rotates around this central point. In this positionthe couch top is at 60cm from the focus. This is defined as the point of reference for calculatingpatient entrance skin dose. The detector is then moved as close to the patient as possible, typically5cm from the patient’s surface. Typically the SID will be 95-105cm depending on patient size andtube orientation.2.4 Patient Radiation Dose2.4.1 Stochastic EffectsIf a cell is damaged by radiation the repair process may result in mutations and are evident as cancerand hereditary effects. There may be numerical alterations such that the cell carries more or lessthan the normal number of chromosomes (eg Down’s syndrome) or there may be structuralalterations where a chromosome has been broken and a segment has either been lost from the cell(deletion) or attached to another chromosome (translocation). These types of effects are known asStochastic effects. There is assumed to be no threshold dose below which an effect will not occurand as the exposure increases so the risk of stochastic disease increases. At low doses, it is assumed7