Characterising CT automatic exposure control systems using a CeLT phantom
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are broad stroke in nature therefore not sufficient for local DRL’s in a paediatric context. Paediatric patients are all sizes and shapes with different physiology and metabolic systems influencing the effects of radiation and therefore dose 1,2 . Along with the nature of the patient the specific scanner plays an important role in determining the dose received by the patient. With the introduction of multi-slice CT scanner technology dose has been on the increase though lately with the worldwide emphasis on radiation dose reduction in CT 3 that has been reversed, iterative reconstruction is also now being marketed as a major dose reduction (<1 mSv) technique. Following the outcome of the CT dose survey of 2010 the Medical Exposures Radiation Unit now requires CT DRL’s to be established and audited annually. This is in line with the European Medical Exposures Directive (MED), introduced into Irish law by SI 478 (2002). Mandatory paediatric protocols which are age and size based are now required along with the recording of a dose parameter such as the DLP and CTDI, caveat these are usually based on adult phantoms. Following analysis of our system prior to completing the national audit we have established our own local DRL’s which are age and size based. I will present our data over the last two years and compare them the national audit findings and international data 4 . The challenge for paediatric imaging is to establish meaningful local DRL’s which are scanner and centre based. References 1. Report of the AAPM task group 204, (2011), Size-specific dose estimate (SSDE) in pediatric and adult body CT examinations 2. Alessio et al. (2010), A pediatric CT dose and risk estimator. Pediatr Radiol, Online. 3. Image Gently, The alliance for radiation safety in pediatric imaging, http://www.pedrad.org/associations/5364/ig/. 4. Verdun et al. (2008), CT radiation dose in children: a survey to establish age-based diagnostic reference levels in Switzerland, Eur Radiol 18: 1980e1986. Keywords: Paediatric, CT, dose reference levels, DRL’s Characterising CT automatic exposure control systems using a CeLT phantom DAVID COSTELLO 1 and SUSAN MAGUIRE 2 1 Mater Misericordiae University Hospital, Dublin, Ireland, 2 Mater Private Hospital, Dublin, Ireland Introduction: CT automatic exposure control (AEC) systems have significant impact on patient dosimetry in modern CT systems. However a reliable method for assessment of AEC systems has not been established as part of quality assurance protocols. Methods: Six systems were assessed from three different vendors (1 GE, 4 Siemens, 1 Philips) using the CeLT phantom. The CeLT phantom (Medical Physics, BCUHB, Wales) consists of four water filled elliptical sections of varying diameters with four CT number inserts in each section. There is also an internal channel allowing for dose measurement. Standard clinical protocols were used to asses the different systems. A number of exposure parameters such as kVp and effective mAs were varied to assess their impact on AEC functionality. Results: The CeLT phantom facilitated the assessment of the different AEC technologies used by each vendors and highlighted the role of the AEC system in both image and dose optimisation. Conclusion: The CeLT phantom is a useful tool for assessing CT AEC systems. Keywords: CT, AEC, CeLT phantom, Dose optimisation, Image optimisation RADIOTHERAPY PHYSICS SESSION 1 Introduction of IPEM code of practice for determination of reference air kerma rate for HDR 192 Ir brachytherapy sources based on the NPL air kerma standard EAMONN HAYES Cork University Hospital, Ireland Introduction: BIR/IPSM 1992 guidelines suggested a traceable calibration method using a Farmer chamber calibrated in a 280 kV x- ray beam. Traceability of measurement is indirect involving external beam primary standards. It is now possible to calibrate ionisation chambers directly traceable to an air kerma standard using an 192 Ir primary standard at the National Physical Laboratory (NPL) in the UK. Methods: IPEM Code of Practice recommends using well chambers as positional uncertainty is greatly improved compared to farmer chamber and jig arrangement. Secondary standard calibrations at NPL involves two steps. Firstly, the reference air kerma rate (RAKR) of the 192 Ir source is determined using the NPL primary standard. The calibrated source is then used to calibrate the secondary standard well chamber. The calibrated source is first used to find dwell position of maximum response and a suitable calibrated electrometer measures the well chamber’s current at the position of maximum response. The secondary standard calibration coefficient, N K R , is defined as N K R ¼ primary standard measurement (Gy/s)/ secondary standard measurement (A). Details are presented of how RAKR determination of a hospital source is performed using a calibrated secondary standard well chamber and a calibrated electrometer as an independent check against manufacturer’s source certificate RAKR. If the measured RAKR is in agreement with the source certificate RAKR within 3% then the measured RAKR is used in the brachytherapy planning system. A second independent check on the hospital RAKR is also required of which details are presented. Results: RAKR measurements of hospital 192 Ir sources during the period of August 2011 and April 2012 are presented and show close agreement with source certificate RAKR values (0.996 to 1.003) and also with our previous method of measuring RAKR of 192 Ir sources. Conclusion: RAKR of 192 Ir HDR brachytherapy sources at CUH are now routinely calibrated using the IPEM code of practice. We now have traceability of measurements to the NPL primary standard using a robust ionisation well chamber. The use of a tertiary ionisation well chamber for the second independent measurement check also builds in redundancy to the measurement system. 336 IAPM 3 rd Annual Scientific Meeting 2012
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Transcript of Characterising CT automatic exposure control systems using a CeLT phantom
336 IAPM 3rd Annual Scientific Meeting 2012
are broad stroke in nature therefore not sufficient for local DRL’s ina paediatric context. Paediatric patients are all sizes and shapeswith different physiology and metabolic systems influencing theeffects of radiation and therefore dose1,2. Along with the nature ofthe patient the specific scanner plays an important role indetermining the dose received by the patient. With theintroduction of multi-slice CT scanner technology dose has been onthe increase though lately with the worldwide emphasis onradiation dose reduction in CT3 that has been reversed, iterativereconstruction is also now being marketed as a major dosereduction (<1 mSv) technique. Following the outcome of the CTdose survey of 2010 the Medical Exposures Radiation Unit nowrequires CT DRL’s to be established and audited annually. This is inline with the European Medical Exposures Directive (MED),introduced into Irish law by SI 478 (2002). Mandatory paediatricprotocols which are age and size based are now required alongwith the recording of a dose parameter such as the DLP and CTDI,caveat these are usually based on adult phantoms. Followinganalysis of our system prior to completing the national audit we
have established our own local DRL’s which are age and size based.I will present our data over the last two years and compare themthe national audit findings and international data4. The challengefor paediatric imaging is to establish meaningful local DRL’s whichare scanner and centre based.References
1. Report of the AAPM task group 204, (2011), Size-specific doseestimate (SSDE) in pediatric and adult body CT examinations
2. Alessio et al. (2010), A pediatric CT dose and risk estimator.Pediatr Radiol, Online.
3. Image Gently, The alliance for radiation safety in pediatricimaging, http://www.pedrad.org/associations/5364/ig/.
4. Verdun et al. (2008), CT radiation dose in children: a survey toestablish age-based diagnostic reference levels in Switzerland,Eur Radiol 18: 1980e1986.
Keywords: Paediatric, CT, dose reference levels, DRL’s
Characterising CT automatic exposure control systems using a CeLT phantom
DAVID COSTELLO1 and SUSAN MAGUIRE21Mater Misericordiae University Hospital, Dublin, Ireland, 2Mater Private Hospital, Dublin, Ireland
Introduction: CT automatic exposure control (AEC) systems havesignificant impact on patient dosimetry in modern CT systems.However a reliable method for assessment of AEC systems has notbeen established as part of quality assurance protocols.Methods: Six systems were assessed from three different vendors (1GE, 4 Siemens, 1 Philips) using the CeLT phantom. The CeLT phantom(Medical Physics, BCUHB, Wales) consists of four water filledelliptical sections of varying diameters with four CT number insertsin each section. There is also an internal channel allowing for dosemeasurement. Standard clinical protocols were used to asses the
different systems. A number of exposure parameters such as kVpand effective mAs were varied to assess their impact on AECfunctionality.Results: The CeLT phantom facilitated the assessment of thedifferent AEC technologies used by each vendors and highlightedthe role of the AEC system in both image and dose optimisation.Conclusion: The CeLT phantom is a useful tool for assessing CT AECsystems.Keywords: CT, AEC, CeLT phantom, Dose optimisation, Imageoptimisation
RADIOTHERAPY PHYSICS SESSION 1
Introduction of IPEM code of practice for determination of reference air kerma rate for HDR 192Ir brachytherapysources based on the NPL air kerma standard
EAMONN HAYESCork University Hospital, Ireland
Introduction: BIR/IPSM 1992 guidelines suggested a traceablecalibration method using a Farmer chamber calibrated in a 280 kV x-ray beam. Traceability of measurement is indirect involving externalbeam primary standards. It is now possible to calibrate ionisationchambers directly traceable to an air kerma standard using an 192Irprimary standard at the National Physical Laboratory (NPL) in the UK.Methods: IPEM Code of Practice recommends using well chambers aspositional uncertainty is greatly improved compared to farmerchamber and jig arrangement. Secondary standard calibrations atNPL involves two steps. Firstly, the reference air kerma rate(RAKR) of the 192Ir source is determined using the NPL primarystandard. The calibrated source is then used to calibrate thesecondary standard well chamber. The calibrated source is firstused to find dwell position of maximum response and a suitablecalibrated electrometer measures the well chamber’s current atthe position of maximum response. The secondary standardcalibration coefficient, NKR
, is defined as NKR¼ primary standard
measurement (Gy/s)/ secondary standard measurement (A).Details are presented of how RAKR determination of a hospital
source is performed using a calibrated secondary standard wellchamber and a calibrated electrometer as an independent checkagainst manufacturer’s source certificate RAKR. If the measuredRAKR is in agreement with the source certificate RAKR within 3%then the measured RAKR is used in the brachytherapy planningsystem. A second independent check on the hospital RAKR is alsorequired of which details are presented.Results: RAKR measurements of hospital 192Ir sources during theperiod of August 2011 and April 2012 are presented and show closeagreement with source certificate RAKR values (0.996 to 1.003)and also with our previous method of measuring RAKR of 192Irsources.Conclusion: RAKR of 192Ir HDR brachytherapy sources at CUHare now routinely calibrated using the IPEM code of practice. Wenow have traceability of measurements to the NPL primarystandard using a robust ionisation well chamber. The use ofa tertiary ionisation well chamber for the second independentmeasurement check also builds in redundancy to themeasurement system.
