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

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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 Georges Hospital, London CT scanner dose

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SLIDE 1

EFOMP Workshop at ECR 2002

CT dosimetry and a data base for CTDI values

ImPACT

  • S. Edyvean, ImPACT

St George’s Hospital, London Imaging Performance Assessment of CT Scanners

A Medical Devices Agency Evaluation Group

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SLIDE 2

ImPACT EFOMP 2002 2

CT scanner dose distribution

higher lower

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SLIDE 3

ImPACT EFOMP 2002 3

  • Computed Tomography Dose Index (CTDI)

– CTDIFDA – CTDI100 – CTDIw

CTDIw =1/3CTDIc +2/3CTDIp

c = centre positon, p= periphery position

CT scanner dosimetry

dz D(z) T 1 = CTDI100

∫

+ − 50 50

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SLIDE 4

ImPACT EFOMP 2002 4

  • Monte Carlo Calculations
  • NRPB (1991,1993)
  • GSF (1991)
  • WinDose

CT scanner dosimetry

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SLIDE 5

ImPACT EFOMP 2002 5

  • NRPB SR250, 1993
  • 23 Organ dose data sets generated using MC calculations

→ Effective Dose

  • Organ dose and ED expressed relative to CTDI in air

→ nD mGy / mGy, nED mSv/ mGy

  • Numerical phantom

NRPB CT organ dose datasets

NRPB R248 , 249, 250, SR250

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SLIDE 6

ImPACT EFOMP 2002 6

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

  • rgan dose data sets
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SLIDE 7

ImPACT EFOMP 2002 7

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 ?
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SLIDE 8

ImPACT EFOMP 2002 8

Three choices

  • 1. Generate new MC datasets

– no relevant expertise – difficult to acquire accurate information

  • n ~ 80 scanner models
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SLIDE 9

ImPACT EFOMP 2002 9

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

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SLIDE 10

ImPACT EFOMP 2002 10

scanner z-axis detectors Scanner couch x-ray tube ion chamber x-ray slice y-axis tilt

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

  • 30 centres, 74 scanners
  • UK + Holland,France,Spain
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SLIDE 11

ImPACT EFOMP 2002 11

  • 80 models ~ 40 groups

– at first by partial knowledge – then refined by review of measurement data

Scanner models

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SLIDE 12

CTDIphantom

Perspex Phantom

Off-axis variation HVL

Aluminium

CTDIair

Detectors Ion chamber X-ray source

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SLIDE 13

ImPACT EFOMP 2002 13

  • Separated scanners into

– those with NRPB SR250 MC data sets – newer models without

ImPACT survey data

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SLIDE 14

ImPACT EFOMP 2002 14

Scanners with NRPB SR250 MC data sets

CTDIph HVL CTDIair

  • Survey measurement data versus calculated ED
  • Establish which combination of data gave best

correlation

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SLIDE 15

ImPACT EFOMP 2002 15

Scanners with NRPB SR250 MC data sets

CTDIph HVL CTDIair

  • Survey measurement data versus calculated ED
  • Establish which combination of data gave best

correlation

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SLIDE 16

ImPACT EFOMP 2002 16

Scanners with NRPB SR250 MC data sets

CTDIph HVL CTDIair

  • Survey measurement data versus calculated ED
  • Establish which combination of data gave best

correlation with ED

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SLIDE 17

ImPACT EFOMP 2002 17

CTDI data : phantom factor

Phantom Factor = CTDIw / CTDIair

Gelijns, K. Patient Dosimetry in Diagnostic Radiology, Chest Examinations and Computed Tomography, PhD Thesis, Leiden

CTDIph HVL CTDIair

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SLIDE 18

ImPACT EFOMP 2002 18

‘ImPACT’ factor

  • Multi-variate analysis

– for combination which gave best correlation

‘ImPACT’ Factor

ImF = a. (CTDIc / CTDIair)+ b. (CTDIp / CTDIair)+ constant

CTDIair CTDIc,CTDIp

CTDIph HVL CTDIair

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SLIDE 19

ImPACT EFOMP 2002 19

HVL vs normalised ED

y = 7.6119x + 0.0736 R2 = 0.9131

2 4 6 8 10 12 0.2 0.4 0.6 0.8 1 1.2 1.4 Normalised Effective Dose HVL (mm)

(mSv/mGy)

( ED / CTDIair )

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SLIDE 20

ImPACT EFOMP 2002 20

HVL vs normalised ED

y = 7.6119x + 0.0736 R2 = 0.9131

2 4 6 8 10 12 0.2 0.4 0.6 0.8 1 1.2 1.4 Normalised Effective Dose HVL (mm)

(mSv/mGy)

( ED / CTDIair )

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SLIDE 21

ImPACT EFOMP 2002 21

y = 0.7089x + 0.0012 R

2 = 0.9369

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 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

Phantom Factor (CTDIw / CTDIair )

Phantom factor vs normalised ED

( ED / CTDIair )

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SLIDE 22

ImPACT EFOMP 2002 22

y = 0.977x + 0.0203 R

2 = 0.977

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Normalised Effective Dose (Abdomen)

ImF vs Effective Dose

( ED / CTDIair )

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SLIDE 23

ImPACT EFOMP 2002 23

Correlations for data combinations

0.8 0.825 0.85 0.875 0.9 0.925 0.95 0.975 1

HVL CTDI CTDI HVL, CTDI

Correlation co-efficient (r^2)

head neck chest abdo pelvis

(PF) (ImF) HVL CTDI CDTI HVL & CTDI

(r2 )

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SLIDE 24

ImPACT EFOMP 2002 24

Newer scanner models

  • ImF calculated
  • Compare to ImFs for SR250 scanners
  • Find closest match
  • Use that MC dataset

Data Set MC 1 MC 2 SR250 ImF Scanner xxx 0.60 yyy 0.75 New ImF Scanner Philips AV 0.62

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SLIDE 25

ImPACT EFOMP 2002 25

CTDI data

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SLIDE 26

ImPACT EFOMP 2002 26

ImPACT Factor - scanner matching

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SLIDE 27
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SLIDE 28

ImPACT EFOMP 2002 28

Organ and Effective Dose

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SLIDE 29

ImPACT EFOMP 2002 29

Spiral scanning

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ImPACT EFOMP 2002 30

CTDI and collimation

0.00 0.50 1.00 1.50 2.00 2.50 5 10 15 20 collimation relative CTDI

plus4 vol zoom

Single slice Multi-slice

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SLIDE 31

ImPACT EFOMP 2002 31

CTDI and slice width

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SLIDE 32

ImPACT EFOMP 2002 32

Multi-slice scanners

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ImPACT EFOMP 2002 33

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

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SLIDE 34

ImPACT EFOMP 2002 34

  • Dose Reference Levels CTDIw, DLP

EUR16262 Quality Criteria for CT

CTDIw =1/3CTDIc +2/3CTDIp

c = centre positon, p= periphery position

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SLIDE 35

ImPACT EFOMP 2002 35

  • 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

Scanner models

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SLIDE 36

ImPACT EFOMP 2002 36

  • 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

ImPACT data

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SLIDE 37

ImPACT EFOMP 2002 37

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

CTDIPx CTDIair

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SLIDE 38

ImPACT EFOMP 2002 38

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

ImPACT

Imaging Performance Assessment of CT Scanners

A Medical Devices Agency Evaluation Group

www.impactscan.org