CT Dose Estimation

1

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

Professor and Vice Chair of Radiology

University of California Davis Medical Center

CT Dose Estimation

2

Introduction

The CTDI Family of Metrics

Dose Length Product

Effective Dose

Dose Reporting

Summary

CT Dose Estimation

3

Introduction

4

Introduction

5

The radiation from CT has caught the eye of the public

6

California SB-1237, SB-58, AB-510

Dose reporting required as of July 1, 2012

Mad River Hospital

Cedars-Sinai

Alex Padilla

7

Introduction

The CTDI Family of Metrics

Dose Length Product

Effective Dose

Dose Reporting

Summary

CT Dose Estimation

8

CTDI:

Computed Tomography Dose Index

where it all began….

Tom Shope

9

CTDIL =

-L/2

+L/2

1

nT ∫ D(z) dz

100 mm

CTDI100 =

-50mm

+50mm

1

nT ∫ D(z) dz

10

Phantoms

100 mm long ion

chamber

CTDI: Measurement tools

11

32 cm body PMMA phantom

16 cm head PMMA phantom

also, pediatric body phantom

100 mm pencil chamber

PMMA plug

peripheral hole

center hole

12 Measuring CTDI100 in the real world

13

32.1 mGy

14

100 mm pencil chamber

and thin CT x-ray beam

100 mm CT x-ray beam

and small radiation meter

100 mm

15

center CTDI100 periphery CTDI100

Weighted CTDI:

CTDIw = ⅓ × CTDI100 + ⅔ × CTDI100

center periphery

16

helical CT acquisition

low pitch high pitch

Dose 1

pitch

18

Weighted CTDI:

Volume CTDI:

CTDIvol =

CTDIw

pitch

CTDIw = ⅓ × CTDI100 + ⅔ × CTDI100

center periphery

19

CTDIvol is a useful parameter

because it is required to be

displayable on all modern CT

scanners due to IEC

regulations. It is also stored in

the header of the CT images.

Siemens

General Electric

20

CTDI is a good measure of CT x-ray tube radiation output,

but is not a stand-alone metric for patient dose

etcetera….

r = 1.19

47”

119 cm waistline

28 cm

34”

86 cm waistline

32 cm

typical patient

CTDI body phantom

CTDI is a good measure of dose to

a 32 cm diameter, 1.19 g/cm3 piece

of plastic. Most patients will be

smaller, and have higher doses.

Larger patients will have lower

doses (at the same techniques)

AAPM Report 204

5.40 mGy = CTDIvol

(32 cm phantom)

water equivalent

diameter

circle of equal area

(weighted)

24

Size-specific dose estimate (SSDE)

To perform patient size-specific

dose estimation, the conversion

factor above is multiplied by the

value of CTDIvol.

Dosepatient CTDIvol × f

SSDE

5.40 mGy = CTDIvol (32 cm phantom)

SSDE = CTDIvol × f

13.0 mGy = 5.40 × 2.4

26

Introduction

The CTDI Family of Metrics

Dose Length Product

Effective Dose

Dose Reporting

Summary

CT Dose Estimation

27

L

Which scan has more “dose”?

L

…to first order, the dose is the same

D = De

Dm

28

L

DLP = CTDIvol × L (mGy • cm)

Dose Length Product (DLP):

DLP is related to the total energy deposited in the patient

a c

x-ray beam profile along z

f(z) DL(z)

L

b

-L/2 +L/2

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

Dose profiles as a function of Scan Length

scan length DL(z

)

Equilibrium Dose as a function of Scan Length

Deq

D(L

)

TG-111 Method

TG-111 Method

TG-111 Method

L

AAP Task Group 200

& ICRU Phantom

39

ICRU Method: Real Time X-ray Meter

40 beam profile [ × RECT ]

41

Introduction

The CTDI Family of Metrics

Dose Length Product

Effective Dose

Dose Reporting

Summary

CT Dose Estimation

42

L

DLP = CTDIvol × L (mGy • cm)

Dose Length Product (DLP):

DLP is related to the total energy deposited in the patient

43

Most modern dosimetry is Monte Carlo based

=

person mathematical model

of person

44

CT geometry is modeled and organ doses are estimated

45

Gonads

Bone marrow

Colon

Lung

Stomach

Bladder

Breast

Liver

Esophagus

Thyroid

Skin

Bone surface

Brain

Salivary Glands

remainder

0.08

0.12

0.12

0.12

0.12

0.04

0.12

0.04

0.04

0.04

0.01

0.01

0.01

0.01

0.12

Organ dose wt E

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

YY mSv

absorbed

effective dose

organ doses computed

tissue weighting factors from ICRP 103

× =

46

other minor details

equivalent dose effective dose

mSv mSv

absorbed dose

mGy

D D × wr = H H × wt = E

kerma

mGy

K

47

other minor details

equivalent dose effective dose

mSv mSv

absorbed dose

mGy

D D × wr = H H × wt = E

units

quantities

kerma

mGy

K

48

other minor details

CTDIvol is the air kerma

measured inside the phantom

equivalent dose effective dose

mSv mSv

absorbed dose

mGy

D D × wr = H H × wt = E

kerma

mGy

K

49

other minor details

Incident kerma leads to absorbed dose

estimate through the Monte Carlo process

equivalent dose effective dose

mSv mSv

absorbed dose

mGy

D D × wr = H H × wt = E

kerma

mGy

K

50

other minor details

correction for different types of radiation

such as a, b, n: wr for x-rays is 1.0

equivalent dose effective dose

mSv mSv

absorbed dose

mGy

D D × wr = H H × wt = E

kerma

mGy

K

51

other minor details

corrects for organ radiosensitivity using

tissue weighting factors, wt.

equivalent dose effective dose

mSv mSv

absorbed dose

mGy

D D × wr = H H × wt = E

kerma

mGy

K

abdomen

E = 0.015 × DLP

Effective dose (mSv) and dose length product (mGy.cm) for abdomen CT and the clinical

indication: rule out abscess.

The conversion factor is (1/59.1 =) 0.017 mSv / (mGy.cm)

Included, data from 46 scanners, 21 single slice, 25 multislice

Effective dose calculated with the ImPACT CT Dosimetry calculator

The short-cut calculation of E (abdomen)

53

Effective Dose per DLP

(AAPM TG-96)

Region of Body

Head and neck

Head

Neck

Chest

Abdomen / pelvis

trunk

k (mSv/[mGy-cm])

0.0031

0.0021

0.0059

0.014

0.015

0.015

DLP × k E

54

Introduction

The CTDI Family of Metrics

Dose Length Product

Effective Dose

Dose Reporting

Summary

CT Dose Estimation

55

WORKFLOW in RADIOLOGY

All images are digital (on PACS)

Radiologists look at images on computers

As they look at the images, they dictate their

“findings” using voice recognition software

In California, CTDIvol & DLP need to be included

into the Radiologist’s Report

EXAM DATE [5/24/2012 12:35 pm]

INDICATION:

[]

DOSE:

[Dose information for this CT examination:

Series 1: (no contrast)

CTDIvol = 12.4 mGy

DLP = 496 mGy-cm

Diameter = 28.2 cm

SSDE = 16.1 mGy

Series 2: (with contrast)

CTDIvol = 13.2 mGy

DLP = 577 mGy-cm

Diameter = 28.2 cm

SSDE = 17.2

UC Davis Health System CT scanners ……..low dose CT practice.]

FINDINGS:

[]

IMPRESSION:

[]

Report: Gaga, Lady – MRN: 1234567

text and

format setup

by institution

The Structured Dose Report with Radimetrics Feed:

57

Institutional CT dose assessment

UC DOSE: Dose Monioring Software

58

Introduction

The CTDI Family of Metrics

Dose Length Product

Effective Dose

Dose Reporting

Summary

CT Dose Estimation

CTDIvol: (mGy)

Is a measurement of “dose” to a large plastic phantom, and for

normal size patients CTDIvol underestimates their dose. Should

not be considered as the “patient dose”

Uses two reference phantoms, 16 cm and 32 cm, depending on

scan protocol. The 16 cm reference phantom is used for all

heads and for some pediatric bodies (depends on scanner)

DLP: (mGy cm)

Includes CTDIvol and scan length

Scales linearly with effective dose

Summary

60

Size Specific Dose Estimate (SSDE)

Is an automatic way in which to correct for patient

body size, to produce a more accurate dose

estimate.

Effective Dose (E)

Is not a physical dose metric, but rather embodies

organ radio-sensitivities

Is controversial to use for specific patients

Summary

61

Introduction

The CTDI Family of Metrics

Dose Length Product

Effective Dose

Dose Reporting

Summary

CT Dose Estimation