Sandra Hopkins Final Report.pdf - University of Surrey

Calculation, verification and monitoring of patient dose inDiagnostic and Interventional CardiologyBySandra Ann HopkinsA dissertation submitted to the Department of Physics,University of Surrey, in partial fulfilment of the degree ofMaster of Science in Radiation and Environmental ProtectionDepartment of PhysicsFaculty of Electronics and Physical SciencesUniversity of SurreyAugust 2010© Sandra Ann Hopkins 2010

AbstractRoutine Coronary Angiography is a relatively high dose diagnostic procedure. The results from adiagnostic procedure may indicate the need for an interventional examination, which has thepotential for even higher patient dose. Doses for these procedures are monitored via an integrateddose area product meter (DAP). Calibrated dose area product meters were used to measure DAP fora group of standard sized patients and compared against recommended reference levels. This wascarried out on two virtually identical systems that were installed approximately 5 years apart.Results indicated that patient doses were higher on the new lab despite cardiologist techniqueappearing to have improved (25.1Gy.cm 2 for the old lab and 32.2Gy.cm 2 for the new lab). The mainreason for this increase was due to dose saving filters not being automatically inserted by the systemduring acquisition runs in the new lab. Medical Physics are working with applications specialistsand users with the aim of reducing doses in the new cath lab below the 29Gy.cm 2 reference level.The equipment also has a skin dose indicator, which can be used to monitor skin dose to warn usersif dose levels might reach the deterministic 2Gy threshold. This skin dose value assumes allradiation is directed at the same region of the skin. However, the change in x-ray tube orientationthroughout a procedure means that the dose is distributed around the patient’s back. A Matlabprogram was previously developed for a different piece of imaging equipment to replicate the dosedistribution and determine the region of maximum dose on the patient’s back. This was adapted forthe local equipment. Gafchromic film was used to produce a visual indication of how the dosewould be distributed in order to compare with the local Matlab programme. An exact match was notfound but the programme was felt to be sufficient to warn users whether skin effects were apossibility. Measurements using TLDs highlighted the fact that skin dose values are an underestimate as they do not include any contribution from scattered radiation from nearby acquisitionruns. The programme was used on a sample of routine diagnostic coronary examinations. Thisdemonstrated that there should not be any risk of skin effects for this standard procedure. Theprogramme was also used to demonstrate skin dose distribution in a more complex interventionalcase. These complex procedures need to be analysed on a case by case basis as it is difficult togeneralise on the dose distribution and the possibility of skin effects. However, it is possible for the2Gy limit to be reached during a complex interventional examination and the local programmewould be able to determine which regions of the patient’s back would be of most concern. It isexpected that doses should be lower with digital flat plate detectors but evidence demonstrates thatthis is not always the case. Optimisation work must be carried out on new equipment to ensure thatdoses are always as low as reasonably practical in line with current legislative requirements.I

AcknowledgementsI would like to express my gratitude to the following people for their assistance and advice. In thecardiac catheterisation laboratories at Portsmouth Hospitals NHS Trust: Clare Smith for her supportwith the collection of patient dose data. To all my colleagues in the Radiological Sciences Groupfor covering for me and encouraging me whilst I worked on this project. A special thank you toAntonio De Stefano for working through the programming aspects of the project. Finally, manythanks to my supervisors: Anne Davis from Portsmouth Medical Physics and Professor PatrickRegan of the University of Surrey for their support and advice.II

Contents1. Introduction2. Theory2.1 Cardiac Angiography2.2 Interventional Cardiography2.3 Imaging2.3.1 X-ray Generator and Tube2.3.2 Flat Plate Detector2.3.3 Equipment Configuration2.4 Patient Radiation Dose2.4.1 Stochastic Effects2.4.2 Deterministic Effects2.4.3 Dose Reference Levels2.4.4 Guidance for High Skin Dose Procedures2.5 Measuring Patient Dose –Dose Area Product Meter and Skin Dose Indicator2.6 Patient Dose Record2.7 Gafchromic Film2.8 Thermoluminescence and Thermo Luminescent Dosimeters2.9 Skin Dose Distribution Software3. Method3.1 Verification of Patient Dose Indicators3.1.1 Dose Area product3.1.2 Skin Dose Indicator3.2 Measuring Patient DAP and Comparison with Previous System3.3 Comparison of Equipment3.3.1 Detector Dose Rates3.3.2 Simulated Patient Skin Absorbed Dose Rates (SADRs)3.4 Practical Verification and adaptation of Skin Dose Distribution Software3.5 Analysis of Skin Dose Data using in house Distribution Software4. Results4.1.1 Verification of Dose Area Product Calibration4.1.2 Verification of Skin Dose Indicator Calibration4.2 Measuring Patient Doses and Comparison with Previous System4.3 Comparison of Equipment4.3.1 Detector Dose RatesIII

4.3.2 Simulated Patient Skin Absorbed Dose Rates (SADRs)4.4 Practical Verification and adaptation of Skin dose Distribution Software4.5 Analysis of Skin Dose Data using in House Skin Dose Distribution Software4.5.1 Diagnostic Cardiac Angiography4.5.2 Therapeutic Examinations5. Discussion5.1 Verification of Dose Area Product and Skin Dose Indicator Calibration5.2 Measuring Patient DAP doses5.3 Practical Verification and Adaptation of Skin Dose Distribution Software5.4 Equipment Considerations and Dose Reduction Techniques6. Conclusion7. References8. AppendicesA. Example of a Typical Routine Coronary Angiogram Study ReportB. Mathematical modelling and equations of original Matlab dosedistribution modelC. Dose Report for Thermoluminescent Dosimeter irradiation of GafchromicfilmD. Patient Dose Area Product data for the Old Cardiac Cath LabE. Patient Dose Area Product data for the New Cardiac Cath LabF. Excerpt from final Matlab programme using definitions and equationsdefined in Appendix BG. Maximum Skin dose values for a set of Routine Coronary AngiogramsH. Output from Matlab Programme – key elements for Routine DiagnosticCoronary AngiogramI. Example Study Report for a therapeutic examination of 39 runs.J. Output from Matlab Programme – key elements for TherapeuticCoronary examinationIV

1. IntroductionThe use of ionising radiation for clinical diagnosis is a long established method for the examinationof a variety of medical conditions. Different imaging modalities and complexities of examinationsmean that some procedures contribute more to patient dose than others. Figure 1 shows thecontribution of the collective dose from the 15 diagnostic examinations which provided the largestcontribution to the collective dose. It can be seen that, with the exception of Computed Tomography(CT), routine diagnostic coronary angiography provides the largest contribution to the collectivedose in the UK. Although the frequency of the examination is low (0.4%), the dose for eachexamination can be high. UK estimates calculate that this exam produces 5.6% of the totalcollective dose 1 . A more recent study for the European population estimates that this contributionhas increased 2 and this is attributed to the fact that this procedure can be performed on an outpatient basis and causes less trauma than surgical procedures. There have also been technicaldevelopments which have led to an increase in frequency such as the introduction of digitaldetectors and the evolution of stents. Coronary angiography is the standard technique for imagingthe left ventricle and coronary arteries. It is a routine diagnostic procedure with a clearly establishedmethod for performing it. Although patient doses are considered to be relatively high for adiagnostic procedure, using current imaging techniques it is unlikely to ever reach a level wheredeterministic effects could be considered a possibility. However, the results of a diagnosticexamination may indicate that treatment is required. This treatment could involve a more complexinterventional procedure such as a graft or insertion of a stent.Figure 1. Contribution to UK collective dose and frequency from the 15 exams making the biggest contribution to thecollective dose 1 .181614Collective DoseFrequency% contribution121086420CT abdomenBa EnemaCT chestCT pelvisCT HeadCoronary AngiographyAbdomenLumbar SpinePelvisMammographyIVUPTCAArteriographyHipCT Spine1

These types of interventional procedures can be very complex and there is the potential to achieveskin entrance doses that could potentially lead to deterministic effects such as skin damage.The FDA (Food and Drug Adminstration) publication gives advice on precautions to be taken toreduce skin dose 3 . The biological effects of ionising radiation on the skin have been analysed by theInternational Commission on Radiological Protection (ICRP) who have published advice andguidance 4,5,6 .With increased user awareness and developments in imaging techniques there have been far lessincidence of skin damage. However, it is still possible that skin damage could occur in complexcases and some imaging equipment designers have included a skin dose indicator on theirequipment. This indicator provides the total accumulated skin dose for a single examination.However, this value assumes that the x-ray beam orientation has been continually focussed in onedirection so that the same area of skin has been irradiated throughout the entire procedure. Inreality, this is rarely the case and the x-ray beam will be oriented in a variety of ways to improvevisualisation of particular coronary vessels. This will mean that the accumulated skin dose is anover estimate of the actual maximum skin entrance dose to the patient and can be misleading andthis has been acknowledged in other studies 7 . Although, the indicator is a good warning device tolet users know that the total patient skin dose is high it does not give them a true indication of thedistribution of the dose on the patient’s skin. Users would benefit from knowing when it is likelythat the skin damage threshold has been reached so that in this case the patient can be followed upand appropriate treatment and consultation be provided as required. Some manufacturers havepreviously developed computer packages to monitor skin dose distribution but these are no longercommercially available. Attempts have since been made to develop dose distribution software 8,9 .This current project includes adaptation of existing software to provide skin dose distributioninformation for local equipment.2. Theory2.1 Cardiac AngiographyThis is one of the most commonly performed imaging tests for evaluating the heart and its majorvessels. Coronary angiography can show the exact site and severity of the narrowing of arteries.After the introduction of a catheter into a peripheral vessel, usually the femoral or axilliary vein orartery, the cardiologist, under direct fluoroscopic visualisation, guides the catheter intravascularly tothe region of interest, injects contrast media to confirm the location of the catheter then injects2

are more complex and can require more extensive imaging. As only one area of the heart may betreated the x-ray beam may be orientated toward the same area of the patient’s skin for longerperiods of time and has an increased likelihood of developing skin damage.2.3 Imaging2.3.1 X-ray generator and tubeThe imaging equipment used in this case is a Siemens Axiom Artis which is powered by aPolydoros 100 kW high frequency multipulse generator. The X-ray tube is a Megalix Cat 125/35/80and is dual focus (0.4 and 0.8 mm with 8 degree target angle). The imaging chain contains a flatpanel amorphous silicon (CsI scintillator) digital detector. Nominal image field sizes are 25, 20 and16 cm. Source to image distance ranges from 90 to 120cm. All fluoroscopy is pulsed with optionalpulse rates varying from 0.5 to 30 pulses per second. The digital radiography mode has anacquisition rate of 15 or 30 frames per second. A 1024 pixel matrix is used. All modes useautomatic dose control with no manual exposure control available to users. There are copper dosesaving filters available on this system with filter thicknesses which vary from 0.1 to 0.9 mm Cu.The use of these filters is automatically linked to the programme selected by the user and is basedon the X-ray absorption by the patient. Filtration is available for both fluoroscopy and acquisition.2.3.2 Flat plate detectorTechnical developments in imaging have meant that many systems that previously had an imageintensifier as the detecting medium now have a flat plate detector. In this case the caesium phosphorconverts x-ray photons to light photons. The amorphous silicon is sensitive to light and converts thelight to an electrical signal. The light is captured by a ‘pixel’ in the photodiode/thin film transistor(TFT) array and converted into an electronic signal. Contrast resolution is improved by reducing thetemperature as this results in less noise. A key difference resulting from using a digital detectorinstead of an image intensifier is that digital detectors result in very little image distortion.Additionally high contrast resolution improves with magnification using an image intensifierwhereas resolution remains constant with a digital detector since the resolution is determined bypixel size.6

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

that the dose response relationship is linear 5 . For all coronary angiograms there will be a risk to thepatient of inducing cancer. However, the benefit to the patient for having the examination exceedsthe risk to the patient of contracting cancer. Typical effective dose for a standard coronaryangiogram is ~ 7mSv and this equates approximately to a risk of inducing cancer of 1 in 2600.(based on current risk factors for fatal and non fatal cancers of 5.5% per sievert 13 ). Forinterventional examinations, where the dose may be higher, the stochastic risk will also beproportionally higher.2.4.2 Deterministic EffectsIf sufficient numbers of cells are damaged there will be temporary or permanent loss of organfunction. This is a termed a deterministic effect. For this type of damage there is a threshold dosebelow which no effect will be seen. The severity of the effect depends on the level of exposurereceived.Skin EffectApprox threshold Time of Descriptiondose (Gy) onsetEarly Transient erythema 2 2 – 24 Temporary skin reddeninghoursMain erythema reaction 6 ~1.5 weeks Reddening and oedema ofskin (burning and itching)Temporary epilation 3 ~ 3 weeks Temporary hair lossPermanent epilation 7 ~ 3 weeks Permanent hair lossDry desquamation 14 ~ 4 weeks Flaky skinMoist desquamation 18 ~ 4 weeks Blistering of superficialskin. Possible exposure toinfectionSecondary ulceration 24 > 6 weeks Delayed healing.Late erythema 15 8-10 weeks Mauve skin discolourationIshaemic dermal necrosis 18 > 10 weeks Vascular damage which caneffect dermis functionDermal atrophy 10 > 52 weeks Epiermis reduced to a fewlayers of cells.Telangiectasis 10 > 52 weeks Dilation of superficialdermal capillariesTable 1 : potential skin effects from radiation exposure 6 .8

Skin injuries can occur when skin dose levels exceed the 2Gy threshold for deterministic effects.With protracted examinations there is a risk to the patient of a deterministic effect occurring. Asummary of the potential effects from cardiac exposures on the reaction of the skin is given in table1. Skin effects have been discussed in the literature and retrospective assessments of skin dosemade 14, 15, 16, 17, 18, 19 .2.4.3 Dose reference LevelsA joint document by the Royal College of Radiologists (RCR) and the National RadiologicalProtection Board (NRPB) produced a document in 1990 suggesting the concept of a reference dosein order to indicate ‘abnormally high doses’ 20 . In 1992 a National Protocol was developedsuggesting methods on how local departments could compare their doses with these NationalReference Doses 21 . The suggestion was to take a representative sample of close to standard sizedpatients and compare the mean with the reference dose. The reference dose was the third quartilevalue derived from a national database of dose data from many contributing hospitals. If the meandose exceeded this reference dose then an investigation was required in order to take correctiveaction or to justify the high dose on clinical grounds. This same concept was adopted by theInternational Commission on Radiological Protection (ICRP) 5, 22 and the Diagnostic ReferenceLevel was introduced. In 1997 this concept was taken up as a European Medical ExposureDirective 23 and the requirements of this were implemented in the UK by the Ionising Radiation(Medical Exposure) Regulations 2000 24 . In IRMER, Diagnostic Reference levels (DRL) weredefined as ‘dose levels for typical examinations for groups of standard-sized patients or standardphantoms and for broadly defined types of equipment’. It is important to note that this relates to‘diagnostic examinations’ so will apply to routine diagnostic coronary angiography examinationsbut does not apply to therapeutic coronary exams such as angioplasties. DRLs are based on anational database which is extended and revised every five years. Recommended DRLs for thecurrent database are determined from this data 25, 26 . For Coronary Angiograms the current andpreviously recommended DRL is given in the table 2.9

NRPB W-14 25(June 02)HPA-RPD-029 26(August 07)Recommended DRL DAP per exam (Gy.cm 2 ) 36 29Number of hospitals 7 38Fluoroscopy time per examination (seconds) 294 243Mean patient range 75 – 85 kg 75 – 85 kgNumber of rooms 17 110Number of patients 8000 34236Fluoroscopy time range (seconds) 185 to 385 93 to 606Third quartile time (seconds) 337 270Mean DAP (Gy.cm 2 ) 30.4 25.7DAP range (Gy.cm 2 ) 11.8 to 60.7 11.7 to 72.5First quartile DAP (Gy.cm 2 ) 22.3 18.9Third quartile DAP (Gy.cm 2 ) 36.3 29.0Patient age (years) 60(16-97) 62 (16-99)Patient weight (kg) 78(35-172) 79(29-183)Number of images per exam Not available 737(6-2200)Table 2 : Summary of data used to derive Diagnostic Reference Levels2.4.4 Guidance for high skin dose proceduresAlthough guidance documents 25, 26 do give some data on Coronary angioplasties, strictly speakingDiagnostic Reference Levels do not apply. However there have been studies of doses frominterventional cardiac examinations where reference levels have been proposed 27, 28 . Moreemphasis is placed, however, on making interventionists more aware of the potential injury fromthese procedures and methods for decreasing their incidence by using dose control strategies 6 .2.5 Measuring Patient Dose - Dose Area Product Meter and Skin dose indicatorThe dose area product (DAP) meter is a measure of dose in Gray multiplied by the area irradiated.This gives an indication of patient dose that can be used to compare with the same parameter for thesame exam. DAP is an easily available measurement which is commonly used to monitor patientdoses for many routine diagnostic imaging procedures. Some imaging systems calculate the DAPfrom known exposure factors but it is understood that the Siemens Axiom Artis uses an integralionisation chamber to determine the DAP. In addition to this value, the system also gives a value ofskin entrance dose in mGy. This is calculated at a reference point of 60cm from the focal spot. Thisis equivalent to a distance of 15cm in front of the isocentre which is taken to be the most typical10

couch height position when the centre of the heart is positioned at the isocentre. Siemens apply atolerance of +/-30% to this but it is known that greater accuracy can be achieved.2.6 Patient Dose RecordFor every examination, on the Siemens Axiom Artis, there is a patient dose record. This providesinformation on every acquisition run including the field size, skin dose in mGy, Dose Area Product,amount of copper in the beam, frame rate and tube orientation. Fluoroscopy runs are not recordedindividually. However at the end of the dose record the total dose is recorded and from this theproportion of dose as a result of fluoroscopy can be calculated by subtracting the total accumulateddose from the individual acquisition runs. Appendix A shows an example patient dose record.2.7 Gafchromic filmIt is possible to get a visible indication of skin dose distribution by using film. Large films withslow x-ray response are required (doses are too high to use conventional diagnostic imaging film forthis technique) 29, 30, 31 although common resin coated photographic paper has also been consideredas an alternative 32 . Kodak EDR2 film, which was originally used for portal imaging andradiotherapy has been used for skin dosimetry 33, 34, 35 . There are limitations to using this film since itsaturates at 1 to 1.5Gy which means it may not be adequate for high dose interventional cardiology.In addition, the requirement for wet processing can be time consuming and cumbersome.Gafchromic film does not saturate at higher radiation doses and does not require processing.Currently Gafchromic film is too expensive to use routinely for patient skin dose mapping but it is auseful tool to provide an immediate visual indication of the dose distribution 36, . The films developby changing colour from their original orange to a grey level which becomes progressively darkerin proportion to absorbed dose. The active component in radiochromic dosimetry film is a longchain fatty acid, which when exposed to radiation, active diacetylenes are polymerised to producepolydiacetylene which result in the distinctive colour change. Variations in construction provide fordifferent sensitivities of Gafchromic film. That used in this case is XR Type R dosimetry film. Theresponse of the film from 80kVp to 120 kVp x-rays is energy independent and is also dose rateindependent. XR Type R dosimetry film can be handled in normal room light for several dayswithout noticeable effects. The quoted acceptable dose range for this film is from 0.1 Gy to 1.5 Gy.2.8 Thermoluminescence and Thermo Luminescent DosimetersThere are many theoretical models to describe thermoluminescence but none can completelydescribe the complex mechanism in specific substances. However, a general theoretical mechanismhas been described with reference to the crystal structure of the alkali halides. All crystals contain11

lattice defects which are an important part of the thermoluminescence process. When the crystalabsorbs ionising radiation free electrons are produced. These may become trapped in the latticedefects. Additionally, the holes that are produced in conjunction with the free electron, may alsobecome trapped. Many hole centres are thermally unstable and may decay quickly at roomtemperature. The electrons will remain trapped provided they do not gain sufficient energy withwhich to escape. The energy required depends on the depth of the trap and the temperature of thematerial. Released electrons may recombine with holes at luminescence centres with the excessenergy being radiated as visible or ultraviolet photons. This electron capture and delayedrecombination with a hole at a luminescence centre is what makes up the process ofthermoluminescence. The complete process is displayed in figure 9Figure 9 : A simple energy band model for thermoluminescenceA glow curve is a plot of thermoluminescence intensity against temperature and can be derivedfrom the electron release formula. The equation for the glow curve intensity from electrons at asingle trapping level E is given as;I = n o Cexp – [ 1/R .s.exp(-E/kT)dT].s.exp(-E/kT)37Where E is the trapping level, R is the heating rate, n o is the number of electrons present at time t oand temperature T o , C is a constant related to luminescence efficiency, k is the Boltzmann constant,T is the temperature and s is a frequency factor associated with the particular lattice defect.12

This formula provides the idealised case for a single trapping level. At low T the curve risesexponentially and after reaching a maximum ‘the glow peak’ it falls to zero. In reality there will bemore than one trapping level each of which will give rise to a glow peak maximum. The area andpeak height of each glow peak depends on the number of lattice defects and the amount of impurityatoms present.The TLDs used are known as TLD-100 (LiF : Mg : Ti). This is Lithium Fluoride doped withmagnesium and titanium. This is the most understood and commonly used phosphor.Although the exact process is unclear, it is suggested that the magnesium ions form electron traps incombination with certain defect centre in the lattice. The effect of titanium is thought to be in theformation of luminescence recombination centres.The reader used is a Harshaw model 5500 automatic TLD reader. Figure 10 shows an example ofthis TLD reader.The reader uses nitrogen gas during the read process mainly to suppress the chemiluminescencesignals from the dosimeter and the reader. In addition it purges the Photomultiplier assembly,keeping moisture out of this area. The dosemeters are lifted into the read chamber by a vacuumpick. The TLD is heated as a function of time according to a time temperature profile (TTP). Thisprofile is divided into three parts; the preheat, acquisition and anneal. The preheat ensures that alldosemeters in a group have a common starting point and can also be used to eliminate the fasterfadinglow temperature peaks. The dosimetric data is collected and the glow curve generated duringthe acquisition stage. The light detected by the photomultiplier tube at this stage will generate acurrent proportional to the amount of light detected. The anneal stage is used to hold thetemperature at a sufficiently high temperature to ensure that all the signal is removed. A calibrationis required to be carried out to produce a reader calibration factor which converts the raw data innano coulombs into the desired dose units, in this case mGy.13

Figure 10 : The model 5500 Harshaw TLD reader2.9 Skin dose distribution softwareThe basis for the skin dose distribution software was a previously reported Matlab programmedesigned for imaging equipment designed for cardiac work, but by a different manufacturer 8, 9 . Theaim of the mathematical model was to calculate the dose distribution in the plane of the couch top,from exposure and projection data stored in the patient dose record. Appendix B describes themathematical processes used to develop this original model and includes the main equations used inthe original programme. It is of note that this skin dose software was developed for a system thathad exposure factors available but did not have skin dose available so the Matlab programme alsohad to calculate skin dose based on a measured entrance surface dose using 25cm Perspex and thennormalising for the tube potential, current, pulse time and number of frames given for eachacquisition run. This aspect of the programme was not required. The initial programme developeddid not include the contribution due to fluoroscopy and different suggestions were made as to howto include this. This aspect of the programme was also not required as data from the Siemenssystem made it possible to determine the total fluoroscopy skin dose which could again be utilisedto improve the accuracy of an adapted programme.14

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

The field size was set to Mag 1 (20cm field) to simulate what is typically used clinically andacquisition runs were made to be long enough to be above the minimum quoted sensitivity of theGafchromic film (100mGy). The dose record for these runs was saved to CD for future reference.4 TLDs were not dosed to act as a reference for background dose and 8 other TLDs were dosed to aknown dose in order to determine the dose to the TLDs placed on the Gafchromic film. Themeasured dose from the TLDs was compared to the dose indicated in the final dose record.Since the Gafchromic film was being placed on top of the table, the attenuation of the table neededto be taken into account. The attenuation of the table is given in technical details as less than 1.2mmAluminium at 100kV.3.2 Measuring Patient DAP Doses and Comparison with Previous x-ray systemScreening log books were designed to ensure that all necessary information required for the analysiswas recorded routinely. Data required for each examination was as follows;a) Screening timeb) Fluoroscopy dosec) Acquisition dosed) Number of acquisition runse) Number of imagesf) Patient weightFigure 14 shows an example of part of a screening log book for coronary angiograms. It isimportant that local data is compared with reference data for a similar mean weight and that it is notskewed by extremely small or large patients. The mean patient weight for the current reference datais 79kg. This is higher than the mean weight typically used for standard radiographic dose audits.This is because cardiac patients are typically heavier on average than patients requiring standardradiographic exams. For this reason only patients within 60kg and 100kg were considered in theanalysis and the mean weight was required to be within 5kg of 79kg.Data was collected until there was approximately 80 patient dose records available. The data wasWeight (kg)Procedure(s)(Please tick all that apply)Degree of difficulty of procedure(Please tick one)Dose (µGy.m2)Height (cm)Left CoronaryStraightforwardPlease attach patient label hereSexRadiographer (initials)M / FRight CoronaryLeft VentricleAngiosealDifficultExam CompletedSatisfactorily?Fluoroscopy DoseLevel?Y / NPercentage (of 2Gy)Low / Normal / HighOperator (initials)Booked/TrackedComments?Archiving checkedDeleted?Registrar or Consultant?Figure 14 : Example of Cardiac Angiography screening log book17

then analysed and compared with data taken from the older cardiac catheterisation laboratory whichwas essentially the same equipment. The mean of this data is also compared with the NationalDiagnostic Reference Level given in section 2.4.3.3.3 Comparison of X-ray EquipmentMeasurements were taken on both x-ray systems to determine the performance of each so that anydifferences between the two systems could be considered when comparing patient doses.3.3.1 Detector Dose RatesFluoroscopy imaging equipment is set up by engineers at commissioning and checked routinelyduring annual visits to ensure that the detector dose is within specification. For Siemens systemsthis is checked using 2mm of Copper on the x-ray tube, removing the anti scatter grid andpositioning a 60cc flat ionisation chamber at the centre of the detector face with the focus todetector at maximum distance. Ideally detector dose rates should be within 20% of manufacturer’sspecification. All dose levels are checked and the variation of dose with field size is also checked.3.3.2 Simulated patient Skin Absorbed Dose Rates (SADRs)In order to interpret patient dose results in each lab it is necessary to compare the SADRs in eachlab under standard conditions and using fluoroscopy and aquisition curves typically used duringclinical examinations. A standard phantom of 19cm Perspex is used and set up as already describedin section 3.1.2 (method 2) for skin dose indicator accuracy measurements.3.4 Practical Verification and adaptation of Skin Dose Distribution softwareMeasurements of the field size at the detector face were established as this was required for input tothe programme. This was done by placing a radio opaque ruler directly onto the detector face andmeasuring the length of the visible ruler on the display monitor.Gafchomic film (dimensions 35cm x 43cm) was used to confirm the position and orientation of thex-ray field as the x-ray tube is rotated in various positions. Measurements were taken from theGafchromic film and correlated with images produced by the Matlab programme.The original equations were used to produce skin dose distributions using Gafchromic film and theMatlab programme for the following parameters:0RAO, 35CRAN0RAO, 35CAUD35RAO, 0CRAN35LAO, 0CRANThis only looked at separate RAO/LAO and CRAN/CAUD rotations and no combinations of both.18

Further combined orientations were trialled in order to assess how closely the Model recreated theactual distribution.It was also possible to manipulate patient dose records to amend the tube angle in order to simulateparticular orientations in order to carry out further analysis of the image display for the Matlabprogramme.3.5 Analysis of Skin dose data using in house dose distribution softwareThe adapted programme was used to calculate the peak skin dose for a number of routine diagnosticexams in order to determine whether deterministic effects are of concern for this type of exam andwhat would be a typical maximum skin dose value based on the indicated Dose Area Product value.Additionally the programme was trialled on a therapeutic exam to ascertain how it might be used inpractise in a clinical environment to determine when a patient requires monitoring for skin effects.19

4. Results4.1.1 Verification of Dose Area Product CalibrationTable 3 shows the results of measurements carried out to determine the accuracy of the Dose AreaProduct meter in the new lab. The field area was measured to be (0.013 +/- 0.002)m 2 . This wasmeasured at the same distance as the ionisation chamber.Set kV DisplayedmA+/- 2%Measured dosemGyCalculated DoseArea ProductµGy.m 2Displayed DoseArea ProductµGy.m 2Difference betweenindicated andactual %50 762.1 18.3 +/- 0.7 342 +/- 7 336.2 -1.5%66 329.9 12.6 +/- 0.5 235 +/- 5 234.8 +0.1%81 74.5 6.5 +/- 0.3 121 +/- 2 122.6 +1.1%81 74.5 6.5 +/- 0.3 121 +/- 2 122.4 +1.0%96 39.4 3.9 +/- 0.2 74 +/- 1 78.2 +6.1%109 44.3 5.5 +/-0.2 103 +/- 2 107.1 +3.8%Table 3 : Measurements to determine the accuracy of the Dose Area product meterThus, for the entire kilovoltage range the indicated Dose Area Product was within 7% of the actualvalue. This is considered to be good accuracy. For the old lab the Dose Area Product was measuredto be within 10%4.1.2 Verification of Skin Dose Indicator CalibrationMethod 1 - Ion chamber free in airDisplayedkVDisplayedmA+/- 2%IndicatedmGy/min+/-5%MeasuredmGy/min+/- 4%Differencebetween indicatedand actual70 187 72 63.5 +13.4%60 138 35 31.4 +11.5%50 752 165 152.4 +8.3%81 166 82 73.8 +11.1%90 312 221 196.1 +12.7%90 555 450 404 +11.4%70 181 68 61.7 +10.5%20

70 183 67 60.7 +10.3%102 205 175 159.3 +9.9 %90 566 464 403.5 +15.0 %90 312 222 198.4 +11.9 %81 167 85 78.8 +7.9 %70 182 67 61.7 +8.6 %Table 4 : Measurements to determine the accuracy of the Skin Dose Indicator with no added Copper filters in the beamWith no additional Copper filtration in the beam the indicated skin dose is between 8% and 15%higher than the actual skin dose (table 4). Additional measurements carried out at the same kV werefor consistency or were with different amounts of attenuation to change the mA and therefore theindicated mGy/min.With various amounts of additional copper automatically added into the beam the accuracy of theskin dose indicator over reads by 15% to 22% (table 5)DisplayedkVDisplayedmA+/-2%AddedCu bysystemIndicatedmGy/min+/-5%MeasuredmGy/min+/- 4%Differencebetween indicatedand actual83.6 813 0.1 mm 226 191.7 +17.9 %87.2 817 0.1 mm 409 341.6 +19.7 %81.8 809 0.1 mm 338 286.2 +18.1 %86.7 807 0.1 mm 291 255.3 +14.0 %75.6 690 0.2 mm 189 167.2 +13.0 %83.4 681 0.2 mm 180 155.6 +15.7 %64.5 532 0.3 mm 44 38.2 +15.2 %73.5 740 0.3 mm 74 60.5 +22.3 %73.5 471 0.3 mm 60 50.3 +19.3 %64.5 492 0.3 mm 28 22.9 +22.2 %64.5 537 0.3 mm 44 38.4 +14.6 %81 492 0.3 mm 73 60.7 +20.3 %76.1 452 0.6 mm 30 25.5 +17.6 %67.4 646 0.9 mm 18 15.2 +18.4 %73.6 311 0.9 mm 9 7.5 +20 %Table 5 : Measurements to determine the accuracy of the Skin Dose Indicator with various Copper filters in the beam21

Method 2 – Ion chamber with scatterTable 6 shows the difference between indicated and measured skin dose for different exposureconditions and different scattering conditions. All measurements were carried out with the table setto –16 (16cm below the iso-centre) which would provide a focus to couch top distance of 60cm.This would mean that the detector would be positioned at the same place that the indicated skindose indicator calculates its dose. All measurements were taken using the 20cm field which is usedfor the majority of acquisition runs during a coronary angiography procedure.SetkVSource toImagingPlatedistanceThicknessof Perspex(cm) +/0.5DisplayedkVDisplayedmA+/- 2%AddedCu bysystemIndicatedmGy/min+/- 5%MeasuredmGy/min+/- 4%Differencebetweenindicated andmeasured %(cm) +/- 0.570 91 16 70 158 0 57 50.8 +12.2%81 91 16 81 93 0 42 37.9 +10.8%60 91 16 60 299 0 87 74.3 +17%60 94 21 60.8 800 0 302 274.6 +10.0%70 94 21 70 404 0 188 157 +19.7%81 94 21 81 227 0 126 117.6 +7.1%96 94 21 96 128 0 90 79.7 +12.9%96 99 25 96 277 0 216 190.6 +13.3%109 99 25 109 184 0 182 153.6 +18.5%81 99 25 81 497 0 323 295.2 +9.4%70 99 25 72.7 800 0 462 439.7 +5.1%70 99 25 75.2 800 0.1 275 249.4 +10.3%109 99 25 78.3 660 0.1 250 223.4 +11.9%102 93 21 66 775 0.2 112 112 0%109 99 25 81 753 0.3 159 144 +10.4%109 93 21 76.1 631 0.6 61 55.44 +10.0%109 93 21 80.5 493 0.6 46 44.01 +4.5%102 93 21 82.5 600 0.9 40 35.46 +12.8%Table 6 : Difference in indicated and measured dose for different scattering conditionsIt is of note that the indicated dose again over reads with respect to actual measured dose.22

Method 3 – Thermo Luminescent DosimetersThe diagram below shows the darkened areas of the Gafchromic film for each of the acquisitionruns. The Gafchromic film would have been turned over on the couch top so that the patient lay ontheir back on the back of the film. Consequently the left hand side of the image below (figure 15)equates to the left hand side of the patient.Patient’s HeadCaudalPatient’sback LHSPatient’sback RHSCranialRAOLAOFigure 15 : Dose distribution for TLD workTable 7 gives the dose values of the TLDs (in mGy). The grid corresponds directly to theGafchromic image above such that A1 corresponds to the top right of the image above. Those dosesin red were within the darkened areas of the Gafchromic film and so correspond directly to patientskin dose. The TLDs were from a batch that had a calibration of +/- 10% for the entire batch.23

I H G F E D C B A1 12.8 21.2 32.7 59.5 445.7 54 29.8 16.7 11.52 18.9 30.4 47.4 234.3 411.9 73.6 37.8 24.8 16.83 31.1 44.8 76.2 102.4 107.6 87 50.9 28.8 19.64 55.5 549.6 583 351.8 399 281.2 66.8 36.1 21.15 64.7 572.3 640.6 350.0 449.6 268.9 84.5 44.1 25.56 46.7 77.7 53.0 113.2 149.8 201.6 110.5 55.7 28.87 25.4 36.5 98.4 65.6 120.6 929.5 1067.8 63.5 27.78 15.2 21.0 34.2 24.9 68.5 964.3 1154.3 56.2 26.29 12.0 14.4 20.4 25.9 43.8 64.3 64.4 39.4 20.2Table 7 : Thermoluminescent dosimeter doses in mGy and their location on the Gafchromic filmTaking those TLDs that were closest to the centre of each field to determine the skin dose thefollowing skin doses for each orientation were determined. These doses were compared with theindicated doses given by the X-ray system (entire dose report available in appendix C).Orientation TLD dose(mGy)+/10%System indicateddose (mGy) +/- 5%Difference betweenmeasured and indicatedAP 450 298 -33%RAO 30 641 474 -26%CAUD 40 446 310 -30%LAO 20 CRAN 40 1154 785 -32%The skin dose measured by the TLD is 26% to 33% higher than the skin dose indicated by thesystem. This is significantly different to comparisons made using free in air and scatter conditionsusing an ionisation chamber. In this case the indicated dose under reads whereas for previousmeasurements it was shown to over read. These differences need to be considered when deciding onthe accuracy of the skin dose indicator. It is considered that the TLD dose is likely to be a moreaccurate indicator of skin dose than the ionisation chamber as it includes contributions from scatterfrom other acquisition runs in the neighbouring region.4.2 Measuring patient doses and comparison between old and new cath labsAppendix D and E give the detailed dose data for the old and new cath laboratories respectively.Both systems are from the same manufacturer and have the same options available so that they canbe programmed in the same way.24

13172125293337414549535761656973778185This data is summarised schematically in figures 16 and 17 and a summary of the data is shown intable 8Patient Dose Area Product Data in old LabDose Area product (Gy.cm2)90.080.070.060.050.040.030.020.0Fluoro DoseAcquisition DoseMeanCurrent DRLOld DRL10.00.0159Figure 16 : Patient dose area product data in the old cath lab (results using DAP accurate to within +/-10%)Patient Dose Area Product Data in New LabDose Area product (Gy.cm2)100.0090.0080.0070.0060.0050.0040.0030.00Fluoro DoseAcquisition DoseMeanCurrent DRL20.0010.000.0014710131619222528313437404346495255586164677073Figure 17 : Patient dose area product in the new cath lab (results using DAP accurate to within +/-7%)25

The most commonly used field size for fluoroscopy during routine coronary angiography is thenormal (25cm) field. From Table 9 it can be seen that a larger field size factor is required for thenew lab. Since the nominal specification doses are similar this will mean that the detector dose forthe new lab will be higher. Table 10 gives the values of the detector dose determined at the defaultpulse rate of 7.5 pulses per second.25cm field kV mA Added Measured Specification % differenceCopper nGy/pulse nGy/pulseOld Lab 61.5 146.9 0.6 mm 18.8 +/- 0.8 17.3 + 8.7 %New Lab 65 107.6 0.6 mm 27.6 +/- 1.1 24.1 + 14.5%20cm fieldOld Lab 63.1 155.2 0.6mm 28.2+/- 1.1 27.2 + 3.7%New Lab 65 141.3 0.6mm 34.5 +/- 1.4 31.9 + 8.2%Table 10 : Detector dose rates for fluoroscopyMeasurements are also made on acquisition. Typically the Mag 1 (20cm) field size is used whendigital acquisition is being used. Table 11 gives the measured values for both the 25cm and 20cmfield. Typically the pulse rate used for aquiring images during coronary angiography is 15 pulsesper second. The measurements below were made using this pulse rate and a default specificationdose of 0.17 µGy/pulse.25cm field kV mA Added Measured Specification % difference+/- 2% Copper µGy/pulse µGy/pulseOld Lab 64.5 383 0.3 mm 0.09 0.098 - 8.2%New Lab 81 70 0 0.14 0.16 - 12.5%20cm fieldOld Lab 64.5 485 0.3 mm 0.145 0.146 - 0.7%New Lab 81 94 0 0.20 0.19 +5.3%Table 11 : Detector dose rates for aquisitionThere are obvious differences which will be discussed in more detail later. For fluoroscopy thedetector dose for the new lab is higher than for the old lab due to the fact that the field size factor ishigher. Apart from this the exposure factors are fairly similar. For the acquisition dose, the new labdoses are higher than the old lab. Additionally, there are some significant differences in exposurefactors. These differences will have an effect on patient dose and will again be discussed later.27

4.3.2 Simulated patient Skin Absorbed Dose Rates (SADRs)Table 12 and 13 gives the measured SADRs for each lab for fluoroscopy and acquisition. Thedifferences in exposure factors between the two labs are again particularly noticeable onacquisition.25cm field kV mA+/-2%AddedCopperMeasuredmGy/minOld Lab 64.7 145 0.6 mm 4.1 +/- 0.2New Lab 68.4 102 0.3mm 4.0 +/- 0.220cm fieldOld Lab 66 164 0.6 mm 5.9 +/- 0.2New Lab 68.4 102 0.3 mm 5.3 +/- 0.2Table 12 : Patient SADRs for 19cm Perspex for fluoroscopy (7.5 pps)25cm field kV mA+/-2%AddedCopperMeasureduGy/exposureMeasuredmGy/minOld Lab 64.5 561 0.3 mm 34 +/- 1 31 +/- 1New Lab 81 171 0 78 +/- 2 70 +/- 220cm fieldOld Lab 66 615 0.2mm 59 +/- 1 52 +/- 1New Lab 81 225 0 98 +/- 3 88 +/- 2Table 13 : Patient SADRs for 19cm Perspex for aquisition (15 pps)SADRs for fluoroscopy in each lab are broadly similar with the new lab having slightly lower dosesthan the old lab. However, for acquisition doses the SADRs in the new lab are noticeably higher.The reasons for this will be discussed later.4.4 Practical verification and adaptation of skin dose distribution softwareMeasured field sizes are given in table 14:Field size X (cm) Y (cm)25cm 17 +/- 0.5 17 +/- 0.520cm (Mag 1) 13.5 +/- 0.5 13.5 +/- 0.516cm (Mag 2) 11 +/- 0.5 11 +/- 0.5Table 14 : Image field size at detector.28

For the purpose of this work a field area of 13.5cm was used as this tied in with the acquisition partof the examination, which represents approximately 85% of the total skin dose.Figure 18 shows gafchromic film imaged with the following orientations;0RAO, 35CRAN0RAO, 35CAUD35RAO, 0CRAN35LAO, 0CRANThis demonstrated good symmetry.This was recreated in the originalMatlab programme and is shown infigure 19. The original programmedemonstrated some non symmetrysuch that the field areas in theRAO/LAO directions were narrowerthan the CRAN/CAUD directions.Figure 18 : Gafchromic image of symmetric rotationThe programme was modified to make it symmetric. The image from the symmetric programme isalso shown in figure 20. For this particular set of parameters the new symmetric programme was abetter representation of the true situation.Figure 19 : Original programme for 35 degree rotationsFigure 20 : Symmetric programme for 35 degree rotations29

However, a second set of orientationswas checked which was still expected tobe symmetric but included orientationsthat combined the RAO/LAO with theCRAN/CAUD rotation as follows:RAO35 CAUD35RA035 CRAN35LAO35 CAUD35LAO CRAN35Figure 21 shows Gafchromic film imagedwith the above parameters. The resultsclearly show non symmetry in theRAO/LAO rotation compared with theFigure 21 : Gafchromic film imaging 35 degree combined rotationsCRAN/CAUD rotation. Figure 22 shows the image created with the symmetric programme. TheGafchromic image shows that when rotated in the RAO/LAO direction and combined withCRAN/CAUD rotation the left hand and right hand edges do not change their angulation very muchcompared to the CRAN/CAUD direction. This is recreated in the symmetric version of the MatLabprogramme. Figure 23 also shows the image displayed for the above orientations using the originalprogramme which appeared to provide a better match.Figure 22 : Symmetric programme for combined anglesFigure 23 : Original programme for combined angles30

The reason for this non symmetry is due to the rotation ofthe C-arm within the gantry in the cranial caudal directionwhich subsequently affects the centre of rotation for theRAO/LAO direction. Figure 24 shows an example of acombined LAO/Caudal rotation.Figure 24 : Combined rotation of the C-armA final set of orientations were imaged with Gafchromic film as follows;RAO0 CRAN 40 ; RAO0 CAUD40RAO40 CRAN0 ; LAO40 CRAN0RAO40 CRAN40 ; RAO40 CAUD40LAO40 CRAN40 ; LAO40 CAUD40Figure 25 : Gafchromic film imaged with a mixture of combined and single rotationsThe Gafchromic film (figure 25) was compared with images produced using the revised symmetricand original programmes (see figures 26 and 27). Neither programme was a perfect match,however, it was felt that the original programme was a closer representation and was used for all31

further analysis work. An excerpt of the Matlab programme used to define the equations using theoriginal definitions from appendix B is given in appendix F.Figure 26 : Symmetric programmeFigure 27 : Original programme4.5 Analysis of skin dose data using in house skin dose distribution software4.5.1 Diagnostic Cardiac AngiographyThe maximum skin dose for 194 diagnostic Coronary Angiograms was checked using the skin dosedistribution software. These were checked by a separate aspect of the programme that did not plotthe distribution pattern for each study but instead produced an Excel document listing the maximumskin dose for each study on a CD together with the total DAP and the total skin dose. The individualdata is given in appendix G.From this data the maximum skin dose value was 715.1mGy and the average was 140.3mGy. Allvalues are well below the 2Gy threshold value for deterministic effects.This data did not include the fluoroscopy element so results will be an underestimate but will besufficient to show whether there is likely to be any skin effects seen for routine diagnosticcardiography exams.The fluoroscopy element was added by dividing the total fluoroscopy dose, proportionally, betweenthe individual acquisition runs. This was included in the part of the programme which analysedindividual exams in more detail and included the dose distribution plot.32

An example of the skin dose distribution for a typical Coronary Angiogram consisting of 10 runs isgiven in figure 28. The different colours relate to the different amount of dose at each position withred being the highest dose. The total skin dose for this exam was 457.3 mGy and the maximumdose was 122.4 mGy. The study report that relates to this particular exam is given in appendix Aand the skin dose analysis (including the additional fluoroscopy element) in appendix H.Figure 28 : Routine standard Coronary Angiogram dose distribution map4.5.2 Therapeutic ExaminationsAs the nature of therapeutic examinations are extremely varied it would not be appropriate todetermine an average value. Instead it would be expected that the programme would be used on acase by case basis. Figure 29 shows an example of a therapeutic exam consisting of 39 runs. Thestudy report relating to this procedure is given in appendix I and the skin dose analysis in appendixJ. It is of note that the position of the imaged areas are very similar to the routine Coronaryangiogram except there is one region where the dose is significantly higher than all the rest and thisexplains the different distribution of colour for this exam. The total skin dose for this exam was2578 mGy which is above the value for skin deterministic effects. However, the maximum skindose was 648.3mGy33

Figure 29 : Intervention coronary exam dose distribution map5. Discussion5.1 Verification of Dose Area Product and Skin Dose Indicator CalibrationIt is understood that for the Siemens Axiom Artis there is an integral ionisation chamber which canbe calibrated by the service engineer if required. Siemens have a specified criteria of +/- 30%. Theskin dose indicator is derived from the same output data so it is expected that on one system the twocalibrations would be fairly similar. The Dose Area Product calibration was found to be within 10%so this was much better than that specified by manufacturers and gave assurance as to the accuracyof the patient dose audit carried out using this equipment. Some systems have been found to be lessaccurate. This can be due to differences in the method of calibration carried out by the manufacturerto that shown here. It is known that Siemens carry out a free in air measurement and that is themethod employed here. However, some manufacturers keep the patient table in the beam path whencarrying out this calibration and this needs to be taken into account.The skin dose indicator calibration was checked using more than one method.34

Firstly a free in air measurement with no additional copper filters in the beam. The calibration wasfound to be within 8% to 15%. The accuracy of this measurement is affected by the fact that highexposure factors are needed to get a high enough dose rate to provide sufficient accuracy. It wasalso found that the indicated dose rate was not always stable despite the exposure factors remainingstable. There was a larger difference between measured and indicated skin dose with Copper in thebeam path. However, the calibration was still inside the manufacturers tolerance and wasconsidered sufficiently accurate for the current work.Additional measurements of skin dose with Perspex in the beam to simulate the patientdemonstrated that the calibration was still satisfactory. It was expected that the radiation backscattered from the Perspex would increase the dose to the ion chamber and lead to a closercorrelation. However, this increase appears to be offset by the reduction in dose to the ion chamberdue to the table attenuation. This implies that calibration of the skin dose indicator can be checkedwith sufficient confidence with either the free in air method or with scattering media on the table.The final method for measuring skin dose was with the use of thermoluminescent dosimeters(TLDs). Whereas the skin dose indicator appeared to over read when compared to the ion chamber,the indicated skin dose indicator now appeared to under read by 26% to 33%. This is a highlysignificant difference. TLD measurements positioned outside the direct radiation path, receivedscattered radiation equal to approximately 5% of the direct dose. This value would be verydependent on position beyond the primary beam but demonstrates that for a coronary angiographyexam with a minimum of 9 runs there will be additional radiation to the skin from scatteredradiation from nearby runs.The results given here where only four runs were involved demonstrated that the actual dose couldbe higher by up to 15%. The skin dose indicator does not take this into account. Since the skin doseindicator provides a total and does not give any indication of dose distribution it is not essential.However, it demonstrates that for any skin dose distribution software the additional contributionfrom scattered radiation from adjacent runs needs to be considered.There have been a number of different studies using both film and/or TLDs to measure the skinentrance dose to the patient. One study used pre-packaged therapy verification film held in aspecially made gown with TLDs placed in specific locations on the film as a cross check betweenfilm and TLD exposure. TLDs were, on average, within 9% of the film estimate 39 . By wrappingfilm completely around a patient this would ensure that doses could be measured for lateralexaminations which was not possible for film and TLDs lying flat on the patient couch top. TLDshave also been arranged in a grid and a concentration factor determined, which is the ratio of the35

maximum skin dose to average dose 30 . In another study, results from a TLD array wrapped aroundthe patient were used to measure the maximum skin dose and action levels for DAP were proposed.A first DAP action level of 125Gy.cm 2 corresponded to the 2Gy dose threshold for optionalradiopathological follow up and a second DAP action level of 250Gy.cm 2 corresponded to the 3Gyskin dose which would require systematic follow up 40 . These values have not been compared in thisstudy but is worthy of further work. Indeed, another study demonstrated the need for both operatorand procedure specific DAP values for action levels 41 .5.2 Measuring patient DAP dosesThe old and new labs are from the same manufacturer and any differences are mainly mechanicaland software related so it is expected that with the same x-ray tube and flat plate detector and thesame staff group, patient doses should be very similar between the two labs. However, the datademonstrates that there has been an increase in patient DAP dose by 28%. This has been as a resultof a 35% increase in fluoroscopy dose and a 27% increase in acquisition dose. The mean dose nowexceeds the current Diagnostic Reference level of 29Gy.cm 2 and requires investigation and effortsto reduce this dose or justify the need for this higher dose. The mean weight of the sample grouphas reduced from 81.3kg to 80.4kg so it is clear that patient weight is not a contributory factor.There may have been a change in technique, however, the data demonstrates that the fluoroscopyscreening time has reduced from 236 seconds to 162 seconds and the mean number of acquisitionimages has reduced from 879 to 674. If there has been any change in cardiologist technique this hasresulted in improved technique resulting in reduced fluoroscopy and acquisition and should haveresulted in a patient dose reduction. This demonstrates that the increase in patient dose does notrelate to the examination technique and is related to equipment performance.Both systems have the same detector, however the manufacturer’s specification dose rates haveincreased for the newer system. The older system had field size factors based on image intensifiertechnology with the increase in dose increasing in accordance with the square of the field size. Forflat plate detectors a linear relationship has now been employed by manufacturers and the field sizefactor at 25cm and 20cm fields is higher than what had previously been recommended for imageintensifier systems. Detector dose measurements found the new system to have higher detector dosefor both fluoroscopy and acquisition. Additionally, for acquisition there were differences in theamount of copper pre-filtration that is automatically added by the system and differences inexposure factors. The significance of this to patient dose was assessed by the use of a 19cm Perspexphantom to simulate the patient. For fluoroscopy patient doses for the two systems were found to bebroadly similar. However, the new lab had less copper pre-filtration in the beam. Copper filtrationhardens the beam. The x-rays are therefore more penetrating and will result in reduced patient doseat the expense of image contrast. The improved latitude and image processing performance of36

digital detectors means that the systems should cope with this reduced contrast. The disadvantage ofadditional copper is a reduction in the tube output and it is necessary to increase the output from thex-ray unit to compensate for the absorption in the copper filter. This increases tube loading andreduces x-ray tube life.Measured fluoroscopy dose using a standard Perspex phantom were broadly similar on the twosystems whereas the patient audit for fluoroscopy dose was higher on the new system. This may bedue to the fact that dose is very dependent on patient thickness when pre-filters are employed.Figure 30 shows the variation of kV with mA for the Siemens Axiom Artis. At small patientthicknesses 0.9mm of copper is placed in the beam. As patient thickness increases so the mAincreases until it comes to a maximum allowable value for the x-ray tube. At this point the amountof copper reduces from 0.9mm to 0.6mm, the kV increases to improve penetration and the mAvalues reduce back to minimum values. As the patient thicknessVoltage v's Current (uA)11500010500095000850007500065000550004500014000 19000 24000 29000 34000 39000 44000 49000 54000Figure 30: kV/mA operating curve for the Siemens Axiom Artiscontinues to increase the mA again increases to maximum allowable values and at this point thecopper reduces to 0.3mm, 0.2mm and then later to 0.1mm copper. When the level of Copper isreduced, the kilovoltage increases, mA reduces and the patient dose is increased. This means thatfor measuring patient dose for the Siemens system the point on the kV/mA curve criticallydetermines the patient dose. A previous study using a standard phantom, determined that copperfiltration of 0.35mm would reduce the entrance dose by 58% with a mean increase in tube loading37

of 29% 42 . Image quality assessment by cardiologists considered there to be insignificant detrimentto image quality in the procedure being investigated.Measured patient doses for Acquisition using 19cm Perspex using standard clinical settings for thenew cath lab were found to be 126% higher for the 25cm field and 67% higher for the 20cm field(which is the field size typically employed for acquisition) when compared to the older cath lab.Part of the reason for this increase in dose is due to the higher specification detector doses describedearlier. Additionally it can be seen that there is no copper pre-filtration employed in the new roomwhereas 0.2mm of copper has been inserted in the old room. Manufacturers have made the decisionto remove the copper filtration in order to improve image quality and prolong tube life. This wouldresult in a significant increase in dose. However, it can be seen from the exposure factors that thekilovoltage has been increased and the mA reduced in an effort to reduce the patient dose. Theseadjustments, however, do not completely compensate for the increase in dose due to the removal ofcopper pre-filtration.Since users were happy with their old lab where pre-filtration was employed it would seemappropriate to re-programme the new room to have pre-filtration and exposure factors more in linewith the old room.Current Local DAP values are just above the Diagnostic Reference Level. A previous surveycarried out by the National Radiological Protection board 25 recommended a reference level forCardiac Angiography procedures of 36Gy.cm 2 . Current local values would have been below thisvalue. This demonstrates the fact that nationally, doses for Cardiac angiography are generallyreducing. Data collected in a study from 15 years ago 43 determined a mean DAP of 67Gy.cm 2 and a3 rd quartile value of 69Gy.cm 2 . Overweight or very thin patients were excluded from the study in asimilar manner to the current study so that they did not skew the data. However, this mean value isconsiderably higher than that obtained in the current study. This is partly attributable to the fact thatolder fluoroscopy systems would generally operate at a higher dose and also that the data comesfrom examinations performed in other countries where the standard technique may be verydifferent. A larger and more recent study included England in the data analysis and this providedDAP values more in line with what is currently achieved locally 27 . A study on two different systemsin the UK in 1997 44 gave mean DAP values for Cardiac Angiography of 47.7Gy.cm 2 and 23.4Gy.cm 2 . The difference in DAP can be explained by the fact that one room has a biplane imageintensifier system which will give greater imaging capability but will also increase the dose. Thisstudy did not remove large or small patients. Instead size correction factors were used that had beenobtained in a previous study 45 .38

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

6. ConclusionPatient dose was measured in two cardiac catheterisation labs using calibrated dose area product(DAP) meters. These systems were virtually identical from a technical perspective but wereinstalled approximately five years apart. Measured DAP determined that the newer lab had higherpatient doses compared with the old lab and they were also above the Diagnostic Reference Level.This was despite a reduction in the screening time and the number of acquisition runs. The reasonfor this higher dose was attributed to manufacturer’s setting higher detector doses in the new laband also dose saving copper pre-filtration not being automatically utilised for acquisition runs.Users need to work with Medical Physics and applications specialists to lower doses whilstmaintaining adequate image quality to ensure that doses are as low as reasonable practicable.A Matlab programme was adapted for the local cardiac systems to record the skin dose distributionfor coronary angiography procedures and to provide a value and position for the region ofmaximum skin dose for each procedure. Skin dose measurements using thermoluminescentdosimeters (TLDs) determined that indicated skin dose did not include any contribution fromscattered radiation from nearby runs so would always be an under estimate. This is worthy of moredetailed study but the current programme was considered sufficient to determine whether a patientneeded to be followed up for any signs of skin damage. Tests using the Matlab programmedetermined that skin effects are not likely to be seen for routine coronary angiography proceduresbut complex interventional procedures should be analysed on a case by case basis to determinewhether follow up is required. The accuracy of the programme to display the correct region ofirradiation requires further refinement. Additionally, the programme does not take into account anytranslational movement of the patient table. This information is not currently available in thepatient’s dose record. This should be investigated further as the information is available in the x-rayroom on the TV monitor.41

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