5/3/2011 - AAPM

5/3/2011

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John M. Boone, Ph.D., FAAPM, FSBI, FACR

Professor and Vice Chair of Radiology

Professor of Biomedical EngineeringUniversity of California Davis Medical Center

Updating Image Quality

and Dose Metrics in CT

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John M. Boone, Ph.D., FAAPM, FSBI, FACR

Chair, AAPM Science CouncilMember TG-111

Member TG-200

Co-chair TG-204

Chair, ICRU Committee on CT



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Introduction

CTDI100-based metrics

Image Quality and CT Dosimetry Phantom

CT Dose versus Scan Length

Correction for Patient Size

CT Scanner Output

Summary



AAPM Report-96

ICRU & AAPM TG-200

ICRU & AAPM TG-111

ICRU & AAPM TG-204

ICRU

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Corporate Disclosures (required by UC Davis):

• Varian Imaging Systems, Consultant

• Artemis, Consultant

• Varian Imaging Systems, Research Funding

• Hologic Corporation, Research Funding

• Fuji Medical Systems, Research Funding

• Stanford Research Institute, Research Funding (R21 subcontract)

• Creativ Microtech, Research Funding (R21 subcontract)

California BCRP 7EB-0075

California BCRP 11I-0114

R21 CA89260

R01 EB002138- (BRP)

R01 CA089260-

R01 CA012955-

Susan G. Komen Foundation

University of Pittsburgh

Acknowledgements:

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Introduction





CT Scanner Output

Summary



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Use of CT: USA & UC Davis Trends

Boone, J M et al J Am Col Radiology, 2008;5(2): 132–138

Brenner, D J et New Eng J Med, 2007;357: 2277-2284

1990: Helical CT

1998: Multi-Slice CT

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Evolution of CT Scanners and Dosimetry

19901970 1980 2000 2010

1994: mA modulation

2006: Dual

Source CT

1989: Helical/Spiral CT

1972: First

clinical CT brain

scan

1974: 4th

generation CT

1974: First whole-

body CT scanner

1992: Dual Slice CT

1997: 4 Slice CT

2000: 8-40 Slice CT

2000: 64 Slice CT

2007: Adaptive

Dose Shield

2004: Flying

Focal Spot

1981: CTDI

1984:

CTDIFDA

1995:

CTDI100

1995: CTDIw

1999:

CTDIvol

2010: TG111

2011: TG200

2011: TG204

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Introduction





CT Scanner Output

Summary



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CTDI100 Dose Metrics and Its Derivatives

CTDI100

(peripheral)

CTDI100 (center)

16 and 32 cm diameter PMMA

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CTDIw = CTDI100,center + CTDI100,periphery

CTDIvol = CTDIw / pitch

Dose Length Product (DLP) = L × CTDIvol

Effective Dose ≈ DLP × k

scan location k

Head

Chest

Body

Abd-Pelvis

Pelvis

0.0021

0.014

0.015

0.015

0.015

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32 cm

r = 1.19

47”

119 cmwaistline

28 cm≈34”

86 cmwaistline

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Introduction





CT Scanner Output

Summary



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CTDI100 Dose Metrics and Its Derivatives

10 cm15 cm

CT dosimetry phantom

CT dosimetry probe

40 cm

CT scatter tails

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20 cm

60 cm

13.4 kg29.5 lb

30 cm

TG-200

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An Integrated CT Image Quality / Dosimetry Phantom

modulation transfer function (MTF)

noise power spectrum (NPS)

dosimetry

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three sections of the TG-200/ICRU phantom

section A section B section C

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MTF insert

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NPS Section

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Previous Era of CT

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Modulation Transfer Function Assessment in CT

oversampled slit LSF MTF

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Modulation Transfer Function Assessment in CT

effect of kernel

effect of slice thickness

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Traditional method for noise / low contrast

detectability assessment in CT

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120 kV

400 mAs

Pitch ≈ 1.0

step 1:

120 kV

xxx mAs

Pitch ≈ 1.0

step 3: step 4:

Measure

NPS(f)

Read out dose

adjust mAs

20 mGy

step 2:

Noise Power Spectra Assessment of Noise in CT

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2D NPS 3D NPS


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2

2

2

1

[ ( , )1( , )

2

Ni iD

D

i x y

DFT DI x y DI x yNNPS u v

N N N

2D Noise Power Spectrum

CT image of phantom

2D Fourier Transform

region of interest

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sagittal

RAMP

apodizing filter

f=0 f=fNf=fN


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2D NPS

1D NPS


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effect of technique

effect of kernel

effect of slice thickness


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Noise Power Spectra (3D)

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Noise Power Spectra (3D)

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CT image quality evaluation

Old Era New Era

phantom

analysis

results

simple more sophisticated

2( )

( )

( )

ifxLSF x e dx

M TF f

LSF x dx

complicated basic

useful & quantitativeperfunctory

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Introduction





CT Scanner Output

Summary



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TG-111

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ac

x-ray beam profile along z

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DL(z)

L

b

-L/2 +L/2f(z)

Axial CT Scanning

39f(z) D(z)

L

-L/2 +L/2

Helical CT Scanning

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Dose profiles as a function of Scan Length

scan length

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Equilibrium Dose as a function of Scan Length

Deq

D(L

)

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TG-111 Method

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TG-111 Method

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TG-111 Method

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TG-111 Method

Extensions to TG-111 Concepts

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weighted bi-exponentialdose spread functions

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h = scatter / primary

PP S

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Gd2O2S scintillator

fiber optic bundle

photodiode

electronics

Real Time X-ray Probe

time

volt

age

0.2 – 1.0 ms

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Dose Probe Isotropy

• Projection irradiation @ 120 kVp/7 mA• τ = 5 s • N = 4• Average % error = 0.80%

x-ray tube

x-ray detector

dose probe

isocenterprobe rotation

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Dose Probe Linearity

• Projection irradiation @ 80 kVp and 120 kVp• Varied tube current at a constant tube potential

r2=0.999

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ICRU Method

55beam profile 56

Co

rre

ctio

n F

acto

r

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Introduction





CT Scanner Output

Summary



58effective diameter

Rel

ativ

e d

ose

CTDIvol

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• Pediatric patient scanned initially with a Siemens scanner in outpatient clinic

• CareDose 4D used

• CTDIvol = 4.78

30.4 cm

20.5 cm

Example Case of Size Specific Dose Estimation

32 cm PMMA Dose Reference Phantom 60

Example Case of Size Specific Dose Estimation

• Post-surgery, patient scanned in-patient GE scanner using “Auto mA”

• GE auto mA used

• CTDIvol = 17.7

16 cm PMMA Dose Reference Phantom

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TG-204 Size conversion factors for CTDIvol

30.4 cm

20.5 cm

30.4 cm

20.5 cm

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Uncorrected data from scanners:

17.7 mGy-cm / 4.78 mGy-cm ≈ 3.7× difference in CTDIvol

TG-204 SSDE Corrections:

17.7 mGy-cm (16 cm PMMA reference) x 0.71 ≈ 12.5 mGy-cm

4.8 mGy-cm (32 cm PMMA reference) x 1.47 ≈ 7.1 mGy-cm

12.5 / 7.1 ≈ 1.7× difference in CTDIvol

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Introduction





CT Scanner Output

Summary



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F(q)

F(z)

CT Scanner Output Measures along z-axis

Influence of beam width / collimation / penumbra

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D(z

)

z

CT Scanner Output Measures along z-axis

Influence of beam width / collimation / penumbra

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F(q)

F(z)

CT Scanner Output Measures versus Fan Angle

Influence of Bow Tie Filter

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sign

al



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time

sign

al



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F(q)

F(z)

CT Scanner Output Measure

ICRU CT REPORT CHAPTER 4F(z)

F(q)70

Introduction





CT Scanner Output

Summary



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organ dosesCT scan & patient parameters

Monte Carlo modeling should be

the basis for patient CT dosimetry

Monte Carlo

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Real time air kerma probe

CT beam profile

f(z) f(q)

useful beam characterization

data needed for MC simulation

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practical methods to correct dosimetry estimates for CT scan length

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practical methods to correct dosimetry estimates for patient size

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Introduction





CT Scanner Output

Summary



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5/3/2011 - AAPM