New Advances in CT Dosimetry - Human Health Campus

New Advances in CT Dosimetry

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

Professor and Vice Chairman of RadiologyProfessor of Biomedical EngineeringUniversity of California, Davis

Chairman, AAPM Science Council

Chairman, ICRU committee on CT Image Quality and Patient Dosimetry


IntroductionCTDI100

-based metricsImage Quality and CT Dosimetry PhantomCT Dose versus Scan LengthCorrection for Patient SizeCT Scanner OutputSummary

ICRU perspectives

ICRU Committee on Image Quality and Patient Dose in CT

Jacob Geleijns(Netherlands) Mike McNitt‐Gray (US) John Boone (US)

Walter Huda (US)Cynthia McCollough (US) Sue Edyvean

(UK) Wolfram Leitz

(Sweden)

Jim Brink (US)

Use of CT: USA & UC Davis Trends

Boone, J M et al J Am Col Radiology, 2008;5(2): 132–138Brenner, D J et New Eng J Med, 2007;357: 2277-2284

1990: Helical CT

1998: Multi-Slice CT

Evolution of CT Scanners and Dosimetry

19901970 1980 2000 2010

1994: mA

modulation1994: mA

modulation

2006: Dual Source CT

2006: Dual Source CT

1989: Helical/Spiral CT1989: Helical/Spiral CT

1972: First clinical CT brain scan 1972: First clinical

CT brain scan

1974: 4th

generation CT

1974: 4th

generation CT

1974: First whole-

body CT scanner 1974: First whole-

body CT scanner

1992: Dual Slice CT1992: Dual Slice CT

1997: 4 Slice CT1997: 4 Slice CT

2000: 8-40 Slice CT2000: 8-40 Slice CT

2000: 64 Slice CT2000: 64 Slice CT

2007: Adaptive Dose Shield

2007: Adaptive Dose Shield

2004: Flying Focal Spot

2004: Flying Focal Spot

1981: CTDI1981: CTDI

1984: CTDIFDA

1984: CTDIFDA

1995: CTDI100

1995: CTDI100

1995: CTDIw

1995: CTDIw

1999: CTDIvol

1999: CTDIvol 2010: TG1112010: TG111


IntroductionCTDI100


CTDI100

(peripheral)

CTDI100

Dose Metrics and Its Derivatives

CTDI100

(center)

16 and 32 cm diameter PMMA

CTDIw

= 1/3 CTDI100,center

+ 2/3 CTDI100,periphery

CTDIvol

= CTDIw

/ pitch

Dose Length Product (DLP) = L × CTDIvol

Effective Dose ≈

DLP ×

k

scan location k

Head

Chest

Body

Abd-Pelvis

Pelvis

0.0023

0.017

0.015

0.017

0.019

32 cm

ρ

= 1.19

47”

119 cmwaistline

28 cm≈

CTDIvol


IntroductionCTDI100


Monte Carlo modeling is the most accurate

method for determining organ doses in CT IntroductionCTDI100


Proposed methods are designed to support and promote MC methods, and to facilitate the integration of MC‐based dosimetry

into clinical patient CT dosimetry

ICRU CT phantom

spatialresolution

dosimetry

contrast resolution

contrast

polyethylene phantom30 cm diameter30 cm length~14 kg

Modulation Transfer Function Assessment in CT

effect of kernel

effect of slice thickness

LSF(x)

Position (mm)

Noise Assessment

noise power spectrum (NPS)

120 kVp200 mA, 0.5 s, 100 mAsPitch = 1.0

13.33 mGy

120 kVp200 mA, 0.5 s, 150 mAsPitch = 1.0

20.0 mGy

Noise Power Spectra Assessment of Noise in CT

effect of technique

effect of kerneleffect of slice thickness

2

22

1

[ ( , )1( , )2

N i iDD

i x y

DFT DI x y DI x yNNPS u vN N N=

− Δ Δ= ∑

spatial frequency (1/mm)

NPS(f) [HU2mm

2 ]

@ 20 mGy to phantom

Scanner 1

Scanner 2

low contrast test objects

$15,000 USD

$1,500 USD

Noise Power Spectra Assessment of Noise in CT

CT image quality evaluationOld Era New Era

phantom

analysis

results

simple more sophisticated

2( )( )

( )

ifxLSF x e dxMTF f

LSF x dx

π∞

−

−∞∞

−∞

=∫

∫

complicated basic

useful & quantitativeperfunctory


IntroductionCTDI100


dose

z-axis position

primary beam

total dose

total dose

dose

z-axis position

Dose profiles as a function of Scan Length

scan length

Equilibrium Dose as a function of Scan Length

weighted bi‐exponential

dose spread functions:

η = scatter / primary

Gd2

O2

S scintillator

fiber optic bundle

photodiode

electronics

Real Time X‐ray Probe

time

volta

ge

ICRU Method

beam profile

Correction

Factor


IntroductionCTDI100


effective diameter

Relativ

e do

se

patient size

dose

CTDIvol

32 cm PMMA

normalization point

correctio

n factor

CTDIvol

32 cm

after normalization

1.0

patient size

Family of physical phantomsCynthia McCollough, Mayo Clinic

standard phantomsTom Toth & Keith Strauss

Monte Carlo phantoms (1 – 50 cm)John M. Boone, UC Davis

Anthropomorphic Monte Carlo phantomsMike McNitt‐Gray, UCLA

AAPM Task Group 204 –

Pediatric CT Dose

120 kVp

lateral dimension

AP dimension

equivalentdiameter

same area

Scout

Practical Implementation during Patient Scanning

Relationship between effective diameter & lateral width

Lateral Dimension

PA Dimension

Dose Index value (CTDIvol

) is on most scanners…..

Correctio

n Factor

Patient’s Lateral Width (cm)0 10 20 30 40 50 60 70

80 90 100

Dose ≈

Cf

×

CTDIvol

Cf

= 2.25


IntroductionCTDI100


F(θ)

F(z)

CT Scanner Output Measures along z‐axis

Influence of beam width / collimation / penumbra

dose

z-axis position

CT Scanner Output Measures

(in phantom with scatter tails)

32 cm diameter • 15 cm long14.3 kg •

31.6 pounds

16 cm diameter • 15 cm long3.6 kg • 7.8 pounds

proposed ICRU phantoms30 cm long

25 cm dia14.0 kg

30.8 pounds

30 cm dia20.1 kg

44.3 pounds

30 cm dia10 kg

22.2 pounds

D(z)

z

CT Scanner Output Measures along z‐axis

Influence of beam width / collimation / penumbra

dose

(z)

CT Scanner Output Measures

z

with phantom

in air

QC / QA recommendations acceptance testing

acceptance testing& periodically

z

F(θ)

F(z)

CT Scanner Output Measures versus Fan Angle

Influence of Bow Tie Filter

time

signal



time

signal



F(θ)

F(z)

F(θ)

F(z)

CT Scanner Output Measure


IntroductionCTDI100


organ dosesCT scan & patient parameters

Monte Carlo modeling should be

the basis for patient CT dosimetry

Monte Carlo

Real time air kerma probe

CT beam profile

f(z) f(θ)

useful beam characterization

data needed for MC simulation

practical methods to correct dosimetry estimates for CT scan length

practical methods to correct dosimetry estimates for patient size

ICRU phantom Real time air kerma probe

spatial frequency (1/mm)

NPS(f) [HU2mm

2 ]

@ 20 mGy to phantom

Scanner 1

Scanner 2

Phantom combining IQ and dosimetry allows better comparisons


IntroductionCTDI100


Documents

New Advances in CT Dosimetry - Human Health Campus