ImPACT Performance Agency Evaluation Group Assessment of CT - PowerPoint PPT Presentation

Imaging A Medical Devices ImPACT Performance Agency Evaluation Group Assessment of CT Scanners CT dosimetry and a data base for CTDI values EFOMP Workshop at ECR 2002 S. Edyvean, ImPACT St George’s Hospital, London

CT scanner dose distribution higher lower 2 ImPACT EFOMP 2002

CT scanner dosimetry • Computed Tomography Dose Index (CTDI) – CTDI FDA – CTDI 100 – CTDI w + 50 1 ∫ CTDI 100 = D(z) dz T − 50 CTDI w =1/3CTDI c +2/3CTDI p c = centre positon, p= periphery position 3 ImPACT EFOMP 2002

CT scanner dosimetry • Monte Carlo Calculations • NRPB (1991,1993) • GSF (1991) • WinDose 4 ImPACT EFOMP 2002

NRPB CT organ dose datasets • NRPB SR250, 1993 • 23 Organ dose data sets generated using MC calculations → Effective Dose • Organ dose and ED expressed relative to CTDI in air → n D mGy / mGy, n ED mSv/ mGy • Numerical phantom NRPB R248 , 249, 250, SR250 5 ImPACT EFOMP 2002

NRPB CT organ dose datasets • Relevant information from manufacturers for 27 scanner models • Condensed to 13 scanner groups based on scanners with similar dosimetry characteristics • Different kVs � 23 Monte Carlo organ dose data sets 6 ImPACT EFOMP 2002

Purpose of ImPACT survey • In 1996 80 scanner models with different names in use in the UK • NRPB SR250 MC data sets not applicable to ~65% of scanners in UK • increasing due to number of newer scanner models • How to estimate patient doses on these scanners ? 7 ImPACT EFOMP 2002

Three choices 1. Generate new MC datasets – no relevant expertise – difficult to acquire accurate information on ~ 80 scanner models 8 ImPACT EFOMP 2002

Three choices 2. Data set from an ‘average’ scanner – does not address differences between models 3. Use dose distribution characteristics to match new scanner models to those used in NRPB SR250 Then use appropriate MC data set – Based on assumption that old and new scanners fell within similar ranges – method chosen 9 ImPACT EFOMP 2002

Scanner Characteristics Survey • Measurement based approach – easy to carry out on existing and future scanners • Standard protocol – how to do the measurements – accuracy and tolerances of set up – data sheets x-ray tube • 30 centres, 74 scanners y-axis x-ray slice • UK + Holland,France,Spain ion chamber scanner z-axis tilt Scanner couch detectors 10 ImPACT EFOMP 2002

Scanner models • 80 models � ~ 40 groups – at first by partial knowledge – then refined by review of measurement data 11 ImPACT EFOMP 2002

CTDI air CTDI phantom X-ray source Perspex Ion Phantom chamber Detectors HVL Off-axis variation Aluminium

ImPACT survey data • Separated scanners into – those with NRPB SR250 MC data sets – newer models without 13 ImPACT EFOMP 2002

Scanners with NRPB SR250 MC data sets • Survey measurement data versus calculated ED • Establish which combination of data gave best correlation CTDI air CTDI ph HVL 14 ImPACT EFOMP 2002

Scanners with NRPB SR250 MC data sets • Survey measurement data versus calculated ED • Establish which combination of data gave best correlation CTDI air CTDI ph HVL 15 ImPACT EFOMP 2002

Scanners with NRPB SR250 MC data sets • Survey measurement data versus calculated ED • Establish which combination of data gave best correlation with ED CTDI air CTDI ph HVL 16 ImPACT EFOMP 2002

CTDI data : phantom factor CTDI air CTDI ph HVL Phantom Factor = CTDI w / CTDI air Gelijns, K. Patient Dosimetry in Diagnostic Radiology, Chest Examinations and Computed Tomography, PhD Thesis, Leiden 17 ImPACT EFOMP 2002

‘ImPACT’ factor CTDI air CTDI ph CTDI air CTDI c ,CTDI p HVL • Multi-variate analysis – for combination which gave best correlation � ‘ ImPACT’ Factor ImF = a. (CTDIc / CTDIair)+ b. (CTDIp / CTDIair)+ constant 18 ImPACT EFOMP 2002

HVL vs normalised ED 12 y = 7.6119x + 0.0736 R 2 = 0.9131 10 8 HVL (mm) 6 4 2 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 (mSv/mGy) Normalised Effective Dose ( ED / CTDI air ) 19 ImPACT EFOMP 2002

HVL vs normalised ED 12 y = 7.6119x + 0.0736 R 2 = 0.9131 10 8 HVL (mm) 6 4 2 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 (mSv/mGy) Normalised Effective Dose ( ED / CTDI air ) 20 ImPACT EFOMP 2002

Phantom factor vs normalised ED 1 y = 0.7089x + 0.0012 0.9 Phantom Factor (CTDI w / CTDI air ) 2 = 0.9369 R 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 N o rm alised E ffective D o se ( ED / CTDI air ) 21 ImPACT EFOMP 2002

ImF vs Effective Dose 1.6 y = 0.977x + 0.0203 1.4 2 = 0.977 R 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Normalised Effective Dose (Abdomen) ( ED / CTDI air ) 22 ImPACT EFOMP 2002

(r 2 ) Correlations for data combinations Correlation co-efficient (r^2) 1 0.975 0.95 head 0.925 neck 0.9 chest abdo 0.875 pelvis 0.85 0.825 0.8 HVL CTDI CTDI HVL, HVL CTDI CDTI HVL & CTDI CTDI (PF) (ImF) 23 ImPACT EFOMP 2002

Newer scanner models • ImF calculated • Compare to ImFs for SR250 scanners • Find closest match • Use that MC dataset Data Set SR250 ImF New ImF Scanner Scanner MC 1 xxx 0.60 Philips AV 0.62 MC 2 yyy 0.75 24 ImPACT EFOMP 2002

CTDI data 25 ImPACT EFOMP 2002

ImPACT Factor - scanner matching 26 ImPACT EFOMP 2002

Organ and Effective Dose 28 ImPACT EFOMP 2002

Spiral scanning 29 ImPACT EFOMP 2002

CTDI and collimation 2.50 Single slice plus4 2.00 relative CTDI Multi-slice vol zoom 1.50 1.00 0.50 0.00 0 5 10 15 20 collimation 30 ImPACT EFOMP 2002

CTDI and slice width 31 ImPACT EFOMP 2002

Multi-slice scanners 32 ImPACT EFOMP 2002

ImPACT 1996-1997 survey • Enabled NRPB datasets to be used for 99% of installed scanners in the UK • New scanners are measured routinely by ImPACT and published CTDI in air and in phantom � ImF 33 ImPACT EFOMP 2002

EUR16262 Quality Criteria for CT • Dose Reference Levels CTDI w , DLP CTDI w =1/3CTDI c +2/3CTDI p c = centre positon, p= periphery position 34 ImPACT EFOMP 2002

Scanner models • Name can be misleading – Siemens Plus, Plus 4 – Philips LX, AV Expander • Filtration can change on existing scanner models – name stays the same – need to identify by model name & date 35 ImPACT EFOMP 2002

ImPACT data • Survey data tried to minimise errors – chambers cross checked – extensive work checking consistency of data • data > 10% from mean of a group looked at • newer scanners use our own data • checked against manufacturers data • Data for 10 mm slices only • used for matching • look-up tables simplified • need focal spot with narrow slices 36 ImPACT EFOMP 2002

ImPACT survey • Goal achieved – matching new scanners to NRPB MC datasets • Additional use for phantom CTDI – EUR16262 (10 mm data) • Data continuously updated – Need to keep aware of manufacturers’ changes CTDI air CTDI Px 37 ImPACT EFOMP 2002

Acknowledgements • N. Keat, M.A. Lewis, H. Kiremidjian, Dr. S. Sassi, Dr. A.J Britten, • S E Jackson (DoH) • Department of Health Grant Ref RPP 26 • Dr. P. Shrimpton (nrpb) • Participating centres A Medical Devices Imaging Performance ImPACT Agency Evaluation Group Assessment of CT Scanners www.impactscan.org 38 ImPACT EFOMP 2002