Practical exercise: Effective dose estimate in CT

(1)

Practical exercise:

Effective dose estimate

in CT

TRAINING COURCE PROGRAM

19 – 20 May 2011, Sofia, Bulgaria

Virginia Tsapaki

Medical Physics Dpt

Konstantopoulio General Hospital

email: [email protected]

(2)

Dose DataMed1 project

“

In order to assess population exposures from

medical radiology in terms of the collective or per

caput effective dose it is necessary to estimate

representative mean effective doses (E), for each

type of x-ray examination that makes a significant

contribution to the collective dose in a country”

(3)

3

CT exams in the

TOP20 list of the

report 154

CT head

CT neck

CT chest

CT spine

CT abdomen

CT pelvis

CT trunk

(4)

(5)

5

(6)

Summary of Dose Metrics



Air dose



CTDI – CT Dose Index



DLP



Organ Dose



Effective Dose

Slice Plane

Rotation Axis

(7)

CT Dose Index in multislice CT



CTDI

vol

= CTDI

w

/ pitch



CTDI

vol

was introduced to allow for variations

in exposure in the z direction when the pitch

was not equal to one

(8)

)

_

(

Exposure

length

CTDI

DLP

=

vol

×

100 mGyCm = 10 mGy CTDI

×

10 cm

×

30 cm

Dose Length Product (DLP)

Dose in one rotation x exposure length (mGycm)

DLP : A convenient index for total dose

(9)

9

E =

Σ

w

T

.H

T



E : effective dose



w

T

: weighting factor for organ or tissue T



H

T

: equivalent dose in organ or tissue T



The

E

, is defined in ICRP 60 [ICRP, 1991], ICRP 103 [ICRP,

2007b] and ICRU 51 [ICRU, 1993].



It is the sum over all the organs and tissues of the body of

the product of the equivalent dose,

H

T

, to the organ or

tissue and a tissue weighting factor,

w

T

, for that organ or

tissue

Effective dose (E)

(10)

Effective dose



Calculation from organ doses



Calculation from DLP using conversion

factors.

(European Guidelines)

(11)

CTDI pencil chamber method

(12)

(13)

13

)

_

(

Exposure

length

CTDI

DLP

=

vol

×

10 cm

300 mGyCm = 10 mGy CTDI

×

30 cm

Dose Length Product (DLP)

Dose in one rotation x exposure length (mGycm)

DLP : A convenient index for total dose

(14)

A first order effective dose estimate

(

for mean population dose purposes

)

DLP

E

=

DLP

×

EUR16262 Guidelines on Quality Criteria for CT

Region of Body E/DLP Conversion Factor

mSv.mGy

-1.

cm

-1

Head

0.0023

Neck

0.0054

Chest

0.017

Abdomen

0.015

Pelvis

0.019

(15)

European Guidelines for Multislice Computed

Tomography

ttp://www.msct.info/CT_Quality_Criteria.htm

.

15

The document includes both adult and paediatric

conversions coefficients

(16)

Important for paediatric exams



When using conversion coefficients for children

one must be aware that these coefficients have

been obtained for a 16 cm CT dose phantom,

whereas the CT console indicator will provide DLP

or CTDI assuming the use of the 32-cm diameter

body phantom.



In pediatric examinations, the figures displayed in

the CT console should be multiplied by a factor of

2 for children and of 3 for infants in order to give

a realistic estimate of the patient's dose.

(17)

Organ doses



NRPB



GSF

Jones DG and Shrimpton P Nomalised Organ

Doses for x-ray computed tomography calculated

using Monte Carlo techniques NRPB SR 250 NRPB

Chilton Oxon 1993

Zankl M, Panzer W and Drexler G

The Calculation

of Dose from External Photon Exposures using

Reference Human Phantoms and Monte Carlo

Methods Part IV: Organ Doses from CT

Examinations

Bericht 30/91

GSF-Forschungszentrum fur Umwelt und Gesundheit,

Institut fur Strahlenschutz, Neuherberg, Germany

1991

(18)

Anthropomorphic phantoms (Rando phantom):

TLDs placed in specific positions in the phantom for organ dose

measurement

(19)

Software methods



CT-Expo [www.sascrad.com]



ImPACT CT Patient Dosimetry

Calculator (www.impactscan.org)

(20)

Introduction Provides an introduction and instructions for use

Scan Calculation The data entry and results sheet

Paediatric Information on relative doses to adult and paediatric patients

Phantom _{Allows interactive selection of the scan range used for dose calculation using a diagram}

of the phantom used to generate SR250

Scanners _{Provides data on CT scanner models, including CTDI in air and phantom, as well as the}

scanner matching data

MatchData Gives data required to perform the scanner matchings in the Scanners worksheet

Collimation Lists relative CTDI values at different collimations for a range of CT scanners. These values are more useful for multi-slice scanners, as the CTDI can vary considerably over the range of available collimations

MonteCarloData Contains the unformatted SR250 data set.

Introduction

This spreadsheet is a tool for calculating patient organ and effective doses from CT scanner examinations. It makes use of the NRPB Monte Carlo dose data sets produced in report SR250 (link at bottom of page). SR250 provides normalised organ dose data for irradiation of a mathematical phantom by a range of CT scanners.

As SR250 was produced in 1993, it does not include data for more modern scanners. To overcome this problem, the ImPACT CT scanner dose survey was carried out by physicists in the UK and Europe.

This work, provides a method for 'matching' the dose distribution of newer scanners to scanners included in SR250. The matching results are included in this spreadsheet. As new scanners are introduced, their matches will be included in updates to this spreadsheet. More details can be found on the dose survey page on the ImPACT website (link below)

Insta lla tion

Installation is fairly simple, and only requires the SR250 data sets (MCSET01.DAT to MCSET23.DAT) to be present in the same directory as this spreadsheet. (SR250 is sold by the NRPB - see link below)

Macros are used on this spreadsheet for a variety of purposes. Depending on your version of Excel , and macro options, the security level may have to be switched to 'medium' (select 'Tools' -> 'Options' ->

'Security' -> 'Macro Security'), and/or ImPACT added to your trusted Macro sources.

The results produced by the CTDosimetry spreadsheets have been checked against those produced by CTDOSE, produced by John Le Heron, the standard software used to calculate doses from the NRPB SR250 datasets. The two methods produce identical results for a range of scans and scanners, with the exception of small differences between the doses calculated for muscle and for the 'remainder' organs. These differences are present despite an apparent similarity in calculation method, and are typically 1-2%.

This spreadsheet has been checked for macro viruses, and the logic and calculations been tested

extensively, however ImPACT accept no responsibility for loss or damage incurred as a result of its use.

Workshe e ts

CTDosimetry.xls consists of 12 worksheets

I mPACT CT Patient Dosimetry Calculator

Ve rsion 1.0.3 24/08/2010

The system should work on any PC with Microsoft Excel 2000 or above. It has not yet been tested on a Apple computer, but it is anticipated that it should work on a Mac.

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21

Manufacturer

Select the scanner manufacturer from the drop down list

Scanner

Select the scanner model or scanner model group for the drop down list.

k V

Choose the appropriate scan kV.

Scan Region

Choose head or body.

Tube current

The x-ray tube current. Note that this should be the actual scanner mA, and not the

'effective mAs' displayed on some multi-slice scanners

Rotation time

The scanner tube rotation time

Spiral pitch

The scanning pitch (table travel per rotation/total collimated slice width). For axial

scanning, (couch increment)/(collimated slice width) should be used

mAs/rotation

The total mAs per gantry rotation. Do not enter data in this box - it is calculated

automatically.

Effective mAs

The mAs/per rotation divided by the spiral pitch. This is a calculated value that provides

a basis for comparison of spiral protocols with different pitches

Collimation

The total nominal x-ray beam width along the z-axis, selected from a range of possible

values in the drop down box. This determines the relative CTDI compared to the

reference (usually 10 mm) collimaiton.

The Monte Carlo data set that is used for this combination of scanner, kV and body part is displayed in the

cell marked 'Data Set'. The data set that is currently loaded is displayed below. If these do not match, no

dose is calculated. To load the appropriate data set, and enable dose calculation, press the 'Update Data

Set' button.

Scan and patient data is entered in the box on the top right of the ScanCalculations worksheet.

The following four selections, made in the top left box on the ScanCalculations worksheet define the Monte

Carlo data set that is used:

To calculate doses using CTDosimetry.xls, the user must enter a number of parameters relating to the

scanner and the scan series

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42,5

-1

-10

64

-1

-10

90 0 10 80 70 60 50 40 30 20

Zoom In

Zoom Out

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23

Scanner Model:

Acquisition Parameters:

Manufacturer:

mA

340

mA

Scanner:

Rotation time

0.8

s

kV:

mAs / Rotation

272

mAs

Scan Region:

Collimation

mm

Data Set

MCSET19

Slice Width

10

mm

Current Data MCSET19

Pitch

1.35

Scan range

Rel. CTDI

1.26

at selected collimation

Start Position -5

cm

CTDI (air)

34.5

mGy/100mAs

End Position 45

cm

CTDI (soft tissue)

36.9

mGy/100mAs

Patient Sex: f

n

CTDI

w

12.8

mGy/100mAs

Organ

w

T

H

T

w

T

.H

T

Remainder Organs

H

T

Gonads

0.2

33.434

6.687

Adrenals

30.370

Bone Marrow (red)

0.12

15.854

1.902

Brain

0.006

Colon

0.12

32.619

3.914

Upper Large Intestine

35.786

Lung

0.12

7.010

0.841

Small Intestine

34.884

Stomach

0.12

35.678

4.281

Kidney

38.987

Bladder

0.05

39.058

1.953

Pancreas

30.545

Breast

0.05

1.498

0.075

Spleen

33.313

Liver

0.05

33.378

1.669

Thymus

1.209

Oesophagus (Thymus)

0.05

1.209

0.060

Uterus

35.652

Thyroid

0.05

0.100

0.005

Muscle

17.640

Skin

0.01

13.381

0.134

Bone Surface

0.01

22.972

0.230

CTDI

w

(mGy)

34.7

Remainder1

0.025

17.709

0.443

CDTI

v ol

(mGy)

25.7

Remainder 2

0.025

17.709

0.443

DLP (mGy.cm)

1284.5

Total Effective Dose (mSv)

22.637

I mPACT CT Patient Dosimetry Calculator

version 0.99m, 1/07/2002

Update Data Set

GE

GE QX/i, LightSpeed, LightSpeed Plus

120

Body

Look up

Get From Phantom

Diagram

5

(24)

2. Scan Range

Scan Range Data (Slice Positions)

1.

Age Group

Gender

Scan Range z

L

from z-

to z+

[cm]

0

3.

Scanner Data for Scan Region "Body"

Manufacturer

_n

CTDI

_w

U

_ref

P

_B,H

k

_CT

k

_OB

∆

L

Scanner

[mGy/mAs]

[kV]

[cm]

0,150

130

0,42

0,80

1,00

0,0

-0,0070

3,35

0,00

4. Select mode

5. Scan Parameters

Please Enter Actual Settings:

U

I

t

Q

el

Q

N * h

col

TF

h

rec

p

Ser.

[kV]

[mA]

[s]

[mAs]

[mm]

0

1,0

6. Results

Dose Values per Scan or per Series*

Tissue or H

T

per Series Remainder H

T

per Series

CTDI

w

CTDI

vol

DLP

w*

E

*

D

uterus*

Organ

[mSv]

Organs

[mSv]

[mGy]

[mGy*cm]

[mSv]

Thyroid

0,0

Brain

0,0

hrec?

n.a.

Breasts

0,0

Thymus

0,0

Child/Baby: all CTDI and DLP values refer to 16cm head phantom !

Oesophagus

0,0

Spleen

0,0

Lungs

0,0

Pancreas

0,0

Dose Values per Examination

Liver

0,0

Adrenals

0,0

DLP

w

E

D

uterus

Stomach

0,0

Kidneys

0,0

[mGy*cm]

[mSv]

Colon

0,0

Small intest.

0,0

Ser?

n.a.

Testicles

0,0

Upp. large int.

0,0

Effective dose E refers to ICRP 60

Ovaries

0,0

Uterus

0,0

Scanner Model

Calculate

Adult Demo Spiral Scanner male female

Body mode for head/neck region Spiral mode

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25

Dose Reference Level (DRL) values are

reported by various national or international

organizations for abdomen CT

DRL

CTDI

DLP

Abdomen EUR 16262 SC 1999

35

900

Abdomen EC MDCT 2004

25

524

Abdomen UK SC 2003

20

460

Abdomen UK MDCT 2003

20

470

Abdomen Germany MDCT 2003

24

1500

Abdomen IAEA 2006

10.9

696

Abdomen USA NEXT Survey

30

*

740

*

Data from USA are not DRL but 75th percent of data distribution.

They are put in this table for comparison purposes.

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Table 1. Effective dose in various exams.

Examination

Effective dose (mSv)

Abdomen radiography

0.7

Chest or Abdomen CT

8.0

3 phase CT liver study

15.0

CT guided biopsy

23.0

CT guided RF ablation

35.0

Repeated CT guided RF ablation

112.0

CT coronary angiography

10.0

Coronary angiography

8

Thallium heart scan

35

CT urography

14.8

CT colonography

5.1

Neonatal abdomen CT

13.1

1 year old abdomen CT

11.1

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Thank you for

your attention

..