Imaging A Medical Devices ImPACT Agency Evaluation … · used in NRPB SR250 Then use appropriate...

EFOMP Workshop at ECR 2002

CT dosimetry and a data base for CTDI values

ImPACT

S. Edyvean, ImPACTSt George’s Hospital,

London

Imaging Performance

Assessment of CT Scanners

A Medical Devices Agency Evaluation Group

ImPACT EFOMP 2002 2

CT scanner dose distribution

higher

lower

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

dzD(z)T1 = CTDI100 ∫

+

−

50

50

ImPACT EFOMP 2002 4

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

CT scanner dosimetry

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

ImPACT EFOMP 2002 6

NRPB CT organ dose datasets

• Relevant information from manufacturersfor 27 scanner models

• Condensed to 13 scanner groupsbased on scanners withsimilar dosimetry characteristics

• Different kVs23 Monte Carlo

organ dose data sets

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 ?

ImPACT EFOMP 2002 8

Three choices

1. Generate new MC datasets– no relevant expertise – difficult to acquire accurate information

on ~ 80 scanner models

ImPACT EFOMP 2002 9

Three choices2. Data set from an ‘average’ scanner

– does not address differences between models

3. Use dose distribution characteristics tomatch new scanner models to thoseused in NRPB SR250Then use appropriate MC data set– Based on assumption that old and

new scanners fell within similar ranges– method chosen

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


• 80 models ~ 40 groups– at first by partial knowledge– then refined by review of measurement data

Scanner models

CTDIphantom

Perspex Phantom

Off-axis variationHVL

Aluminium

CTDIair

Detectors

Ion chamber

X-ray source


• Separated scanners into – those with NRPB SR250 MC data sets– newer models without

ImPACT survey data


Scanners with NRPB SR250 MC data sets

CTDIph

HVL

CTDIair

• Survey measurement data versus calculated ED

• Establish which combination of data gave bestcorrelation



CTDIph

HVL

CTDIair


• Establish which combination of data gave bestcorrelation



CTDIph

HVL

CTDIair


• Establish which combination of data gave bestcorrelation with ED


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


‘ImPACT’ factor

• Multi-variate analysis – for combination which gave best correlation

‘ImPACT’ Factor

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

CTDIair CTDIc,CTDIpCTDIph

HVL

CTDIair


HVL vs normalised EDy = 7.6119x + 0.0736

R2 = 0.9131

0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Normalised Effective Dose

HVL

(mm

)

(mSv/mGy)( ED / CTDIair )


y = 0 .7 0 8 9 x + 0 .0 0 1 2R 2 = 0 .9 3 6 9

0

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 a lis e d E ffe c tiv e D o s e

Phan

tom

Fac

tor (

CTD

I w/ C

TDI ai

r)Phantom factor vs normalised ED

( ED / CTDIair )


y = 0.977x + 0.0203R2 = 0.977

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 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 )


Correlations for data combinations

0.80.8250.85

0.8750.9

0.9250.95

0.9751

HVL CTDI CTDI HVL,CTDI

Cor

rela

tion

co-e

ffici

ent (

r^2)

headneckchestabdopelvis

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

(r2

)


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 ImFScanner

xxx 0.60

yyy 0.75

New ImFScanner

Philips AV 0.62


CTDI data


ImPACT Factor - scanner matching


Organ and Effective Dose


Spiral scanning


CTDI and collimation

0.00

0.50

1.00

1.50

2.00

2.50

0 5 10 15 20

collimation

rela

tive

CTD

I plus4vol zoom

Single slice

Multi-slice


CTDI and slice width


Multi-slice scanners


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


• Dose Reference Levels CTDIw, DLP

EUR16262 Quality Criteria for CT

CTDIw =1/3CTDIc +2/3CTDIpc = centre positon, p= periphery position


• Name can be misleading– Siemens Plus, Plus 4 – Philips LX, AV Expander

• Filtration can change on existingscanner models – name stays the same– need to identify by model name & date

Scanner models


• 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


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’ changesCTDIPxCTDIair


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

Imaging A Medical Devices ImPACT Agency Evaluation … · used in NRPB SR250 Then use appropriate...

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