Sandra Hopkins Final Report.pdf - University of Surrey

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5.3 Practical verification and adaptation of skin dose distribution softwareA Matlab programme prepared to assess skin dose distribution on a different system was adaptedfor use with the Siemens Axiom Artis. Comparisons of dose distribution using Gafchromic filmdemonstrated that rotations in the RAO/LAO and CRAN/CAUD are not symmetric. It was notpossible to make the programme replicate the dose distribution exactly but it was deemedsufficiently accurate to be used to determine the approximate maximum skin dose region. Runningthe programme on a set of routine diagnostic examinations (consisting of no more than 12 runs)determined that it was highly unlikely that 2Gy would ever be reached. Routine diagnostic coronaryangiograms therefore do not pose any concern with regard to deterministic effects. The mainconcern for routine exams is with respect to stochastic effects and this is where optimisation ofequipment configuration and technique to minimise patient dose whilst still providing satisfactoryimage quality is required. Since routine exams within one hospital are likely to be very similar itwould be possible to determine a maximum skin dose based on a fraction of the total dose as arough guide to the expected maximum skin dose without any further measurement.The programme was used to determine the maximum skin dose for a therapeutic examination. Theimage of the skin dose distribution is broadly similar to that for a routine diagnostic examination,however in this case one of the orientations displays a significantly higher dose compared to theother orientations. This is the orientation that was used most for optimum visualisation of the regionof interest. For the case used where the total skin dose was 2.6 Gy this led to a maximum skin doseof 0.6mGy which would not be of concern with regard to deterministic effects. However, maximumskin doses for therapeutic examinations can be much greater than this, particularly for complicatedexaminations and there will be cases where the maximum skin dose is likely to reach or exceed the2Gy limit. The programme will adequately indicate for each examination, the value and region ofthe maximum skin dose. The calculated maximum skin dose will be an under-estimate, since it willnot include the dose contribution from scattered radiation from nearby projections.Other errors using this programme would be that there is no record of any translational movementof the table. Observations of typical Cardiac Angiography examinations demonstrate that there issome table movement and the table height may not be positioned at the iso-centre. These errors willaffect the absolute skin dose value. However, the skin dose distribution software should besufficiently accurate to act as a device to notify staff of the potential for skin damage for a particularexam.5.4 Equipment considerations and dose reduction techniquesThe transition from image intensifier detectors to digital detectors has always been suggested as ameans of dose reduction . Whilst developments in image processing and general equipmentdevelopment will result in patient dose reduction, the introduction of digital detectors has not39
always resulted in a reduction in dose. Indeed, the field size factors used on Siemens flat platedetectors for the smaller field sizes are higher than those previously given for image intensifiersystems. However, some studies have also found the doses have reduced as a result of theintroduction of flat plate detector technology. One comparative study found that a 30% dosereduction was possible compared with a conventional system when used for interventionalcardiology 46 . Another study had similar findings 47 , however a separate study determined higherdoses on a digital system and this was attributed to the use of higher exposure factors 48 . It is likelythat the introduction of digital technology ‘should’ reduce patient doses and this needs to be carriedout by optimising specific imaging techniques. Medical Physics, Applications specialists andRadiology staff all have a part to play in reducing patient doses by careful optimisation of techniqueand exposure factors. Examples of equipment options that are routinely employed in imagingsystems now are virtual collimation, wedge filtering, auto positioning and last image hold. Staffshould be trained to routinely take advantage of these options where they exist. Systems withvariable filtration are likely to provide the lowest patient doses 40 . Adjusting operational parameters,such as using variable pulsed fluoroscopy, has also been shown to reduce radiation exposure 49 .Another study adjusted projection, filtration, field size and collimation in order to reduce dose andmonitored image quality using a contrast-detail phantom. Effective dose was reduced from 13 to4.6mSv for a standardized PCI procedure 50 .As further developments take place other techniques are being introduced that may well reducepatient dose or possibly change the way in which these examinations are carried out. For example,Rotational Angiography can give lower patient dose and uses less contrast agent than a standardseries of 5 to 6 views to get the same clinical information 51 . For standard angiography each separaterun takes 5-6 seconds and is static while contrast media is injected. For Rotational Angiography thex-ray tube moves automatically whilst contrast media is being introduced and consequently thevessels can be viewed from more than one angle from this single run. The single run will takelonger than a conventional run but since less runs are required this should lead to a reduction inoverall dose. Magnetic Catheter guidance is in the development stage but has potential. Finally,there appears to be a trend to move from the standard fluoroscopy technique to using a ComputedTomography technique to obtain the same diagnostic information. Most CT scanners on the marketwill include a Cardiac package option if required and as more departments make the transition toCT Cardiac angiography manufacturers are further developing their scanners to make them moreuser friendly and dose efficient.40
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 20 and 21: 3. Method3.1 Verification of Patien
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: of 29% 42 . Image quality assessmen
Page 47 and 48: References1. NRPB W4 - Radiation ex
Page 49 and 50: 30. Theodorakou C and Horrocks J A,

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