Sandra Hopkins Final Report.pdf - University of Surrey

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3. Method3.1 Verification of Patient Dose Indicators.3.1.1 Dose area productSince the Dose Area Product data is used to assess the dose to the patient and the values are used foraudit it is necessary to ensure that the meter is correctly calibrated. The manufacturer’s work to atolerance of +/- 30% but ideally better accuracy is preferred.The C-arm is positioned in a Lateral positionas shown in figure 11. An MDH 9020 meterand 6cc ionisation chamber is placed in thecentre of the image field at a known distancefrom the x-ray tube focus. For different setkV values the dose area product is recordedfrom the system and the total output from theionisation chamber. In order to calculate thedose area product it is necessary to obtain thefield area at the ionisation chamber distance.Figure 11 : Standard set up for measuring skin dose removing table attenuation and backscatter.This is done by replacing the chamber with an imaging plate at exactly the same distance. This isexposed and the field area is measured. Typically this is carried out during annual performancetesting of the equipment with all automatic Copper filters removed from the beam. In order toincrease exposure factors during testing it is necessary to place large area copper filters in front ofthe detector. The increased attenuation drives up the exposure factors and allows the calibration tobe checked across the full clinical kilovoltage range. Accuracy of the MDH chamber is ensured bysending it annually to a national calibration test house and applying any correction factors provided.3.1.2 Skin Dose IndicatorMethod 1 – Ion chamber free in airSince it is known that the skin dose indicator is either calculated from the Dose Area Product dataor directly from exposure factors the set up above in figure 11 can be used since this eliminatedboth table attenuation and back scatter. This is a free in air exposure. Some manufacturers calibratetheir dose meters free in air whilst others include the table within their measurement. This is usuallycarried out with automatic filters removed from the beam. However, on this occasion somemeasurements were also made with Copper attenuation added to confirm that the calibration took15
this into account. In order to increase exposure factors during testing large area copper filters areagain placed in front of the image detector.Method 2 – Ion chamber with scatterIt is also possible to make a skin dose calculation under simulated clinical conditions as shown infigure 12. This will include attenuation due to the table and back scattered radiation from thepatient. This method is a standard method used for measuring skin entrance dose 38 .X-raydetectorPerspexIonisationchamberThe two different methods were compared in orderto determine the difference in skin dose due to thesefactors. The information could then be used to makeadjustments to any differences between indicatedand calculated skin dose.Both methods consider each exposure separately butwill not take into consideration any additional dosedue to backscatter from nearby exposures on theskin.Figure 12 : Measuring skin dose under simulated clinical conditionsMethod 3 – Thermo Luminescent DosimetersThermo Luminescent dosimeters were placed individually into light proof sachets and positioned onthe film in a grid across the whole area (figure 13). TLDs were positioned 4cm apart. The top of thecouch top was positioned at 60cm so that the indicated/calculated skin dose should correspond tothe actual dose at the couch top. 25cm Perspex was positioned above the Gafchromic film toprovide typical attenuation for a large patient.In order to be clear about the x-ray fielddistribution, the following orientations wereused for separate acquisition runs.1) AP2) RAO 303) CAUD 304) LAO20CRAN30Figure 13 : TLDs positioned on Gafchromic film16
Page 1 and 2: Calculation, verification and monit
Page 3 and 4: AcknowledgementsI would like to exp
Page 5 and 6: 4.3.2 Simulated Patient Skin Absorb
Page 7: These types of interventional proce
Page 12 and 13: 2.3.3 Equipment configurationFigure
Page 14 and 15: Skin injuries can occur when skin d
Page 16 and 17: couch height position when the cent
Page 18 and 19: This formula provides the idealised
Page 22 and 23: The field size was set to Mag 1 (20
Page 24 and 25: Further combined orientations were
Page 26 and 27: 70 183 67 60.7 +10.3%102 205 175 15
Page 28 and 29: Method 3 - Thermo Luminescent Dosim
Page 30: 13172125293337414549535761656973778
Page 33 and 34: 4.3.2 Simulated patient Skin Absorb
Page 35 and 36: However, a second set of orientatio
Page 37 and 38: further analysis work. An excerpt o
Page 39 and 40: Figure 29 : Intervention coronary e
Page 41 and 42: maximum skin dose to average dose 3
Page 43 and 44: of 29% 42 . Image quality assessmen
Page 45 and 46: always resulted in a reduction in d
Page 47 and 48: References1. NRPB W4 - Radiation ex
Page 49 and 50: 30. Theodorakou C and Horrocks J A,

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