Method of Multislice CT Effective Doses Estimation on the Basis of

Dose Distribution Curves

Sergey Kruchinin and Mikhail Zelikman

Research and Practical Centre of Medical Radiology, Srednyaya Kalitnikovskaya, 28/1, Moscow, Russia

Keywords: CT, Effective Dose, Scattered Radiation.

Abstract: Conventional dose-length product method for CT effective dose evaluation in case of multislice scanners

with comparatively large X-ray beam collimation leads to underestimation of effective doses due to partly

neglecting of scattered in patient body radiation. It is possible to avoid this problem taking into account

“tails” of dose distribution on the length exceeds zone ±50 mm relative to the beam centre. Method of DLP

evaluation on the basis of dose distribution curves with “tails” estimated on full length (±200 mm or more)

is presented.

1 PURPOSE

Modern multislice CT scanners use X-ray beam

collimation of 30 – 40 mm or more and this leads to

increasing of scattered in patient body radiation and

its contribution to the effective dose as a result.

Traditionally effective doses evaluation for

various computed tomography exams is performed

on the basis of dose-length product (DLP) parameter

and conversion factors, specified for study type and

patient age by European Commission: as a rule adult

or child but sometimes child conversion factors are

presented more detailed

 (European Commission’s

Study Group, 1999); (Deak et al., 2010). DLP in this

case is calculated using CT dose index (CTDI)

measurements on the length of 100 mm but when the

value of scattered radiation is significant this length

of dose integration becomes insufficient and results

in effective dose underestimation. Taking into

account dose distribution along z-axis on the length

of approximately ±200 mm (or more) relative to the

center of X-ray beam, provides an opportunity to

estimate CTDI and respectively DLP more correctly.

Features of method concerning DLP evaluation

on the basis of dose distribution curves with “tails”

estimated on the length of 200 mm or more (on each

side from X-ray beam centre) are described in the

paper.

2 METHODS AND MATERIALS

All the experiments (chest study parameters setting)

were conducted on the basis of two 64-slice CT

scanners: Aquilion 64 (Toshiba Medical Systems)

and Light Speed VCT (GE Healthcare). X-ray beam

collimation for CT units has been chosen in the

range from 32 to 40 mm.

Measuring of absorbed dose distribution along z-

axis has been performed utilizing CTDI cylindrical

phantoms (PMMA, “body” – 32 cm in diameter,

about 15 cm in length) and specially designed

cylindrical containers into each of which a set of

thermoluminescent dosimeters (TLDs) has been

placed. For experimental evaluation of absorbed

dose distribution on both sides from X-ray beam

centre (and plotting of corresponding curves) two

phantoms have been positioned close to each other

and this made possible to estimate in two steps

“tails” of absorbed dose distribution at the interval

of approximately ±200 mm along z-axis relative to

the beam centre. To plot a complete curve it is

necessary to combine the data obtained for the right

and the left sides of distribution.

Attention must be given for TLDs calibration

because utilized TLDs have a sensible dependence

of measured values on X-ray energy. It must be

taken into account that energy spectrum of scattered

radiation differs from that of the main beam. For this

reason the traditional calibration in air using

standard irradiation source (for example

137

Cs) or

Kruchinin S. and Zelikman M..

Method of Multislice CT Effective Doses Estimation on the Basis of Dose Distribution Curves.

DOI: 10.5220/0004184400910094

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2013), pages 91-94

ISBN: 978-989-8565-34-1

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

CT scanner X-ray beam leads to substantial

distortions in dose estimates despite the fact that the

last method takes into account beam rotation

.TLDs

(LiF:Mg, Ti; 4.5±0.12 mm in diameter, 0.9±0.1 mm

thickness) from chosen group were placed inside a

container in which 22 – 24 sensors are housed

simultaneously. As a reference dosimeter Unfors Xi

with pencil type ionization chamber has been used.

Container with TLDs as well as Unfors Xi detector

were installed at the central and periphery holes of

CTDI phantom located in the centre of scanner

gantry and after this several exposures for one tube

rotation and with fixed table position have been

made. Multiple exposures are needed in order to

reliably exceed the sensitive threshold of

thermoluminescent dosimeters. Considering

reference device (Unfors Xi) readings and TLDs

measurements (estimates of an integral under

absorbed dose distribution curve on ±50 mm

interval) two values of the weighted CTDI have

been calculated and their ratio gives the calibration

factor for testing TLDs group.

Effective doses on DLP basis have been

calculated in respect that conversion factor for adult

chest studies in accordance with appropriate

document is equal to 0.017 mSv·mGy

-1

·cm

-1

(European Commission’s Study Group, 1999).

An adult (hermaphrodite) anthropomorphic body

phantom consists of 25 mm elements simulating

human body organ sections with different densities.

This phantom with TLDs (about 100 sensors) placed

inside was chosen for reference measurements. One

phantom section with thermoluminescent sensors

housed in is shown on Fig.1, as an example. TLDs

distribution in phantom was the following: red bone

marrow – 7; colon – 5; lungs – 24; stomach – 4;

breast – 2; gonads – 2; bladder – 2; liver – 5;

esophagus – 8; thyroid – 6; skin – 11; bone surface –

10; remind organs – 13. Using an anthropomorphic

phantom+TLDs, effective dose evaluation has been

made on the basis of weighting factors for different

organs and tissue presented in International

Commission on Radiological Protection (ICRP)

documents: ICRP Publication 60 and ICRP

Publication 103 recommendations (ICRP, 1991);

(ICRP, 2007).

3 RESULTS AND DISCUSSION

The right parts of absorbed dose distribution curves

received at the central and periphery holes of

doubled CTDI “body” phantom are presented on

Fig.2 and Fig.3 respectively. Fig.4 shows the right

parts of weighted distribution curves. Full length

weighted curves have been used for weighted CTDI

calculation on the basis of integrals under the curves

on the interval from –200 to +200 mm). Weighting

has been made using weighted factors 1/3 and 2/3

for centre and periphery zones respectively. It

follows from the received data that the integrals

under the absorbed dose distribution curves at the

central hole of doubled CTDI phantom (in the range

of ±200 mm) differ from the integrals on the interval

from –50 to +50 mm (traditional CTDI method)

approximately by (53 – 65)% for both scanners. For

periphery holes this difference is not so significant

and is about (11 – 13)%. Mentioned above means

that ignoring of scattered radiation outside ±50 mm

zone along z-axis leads to underestimation of the

effective dose up to approximately 20% in average

when using the traditional method on the basis of

single CTDI "body" phantom and 100 mm

ionization chamber (weighted CTDI) in case of

collimation 32 – 40 mm.

Figure 1: Anthropomorphic phantom section with TLDs

housed in.

Figure 2: The right part of absorbed dose distribution at

the centre hole of doubled CTDI phantom.

BIODEVICES2013-InternationalConferenceonBiomedicalElectronicsandDevices

Table 1: Effective dose estimates obtained on the basis of various methods.

CT scanner type

X-ray beam

collimation, mm

Effective dose estimates, mSv

Anthropomorphic body phantom + TLDs

DLP parameter

method

Dose

distribution

curves

method

60 ICRP

Recommendations

103 ICRP

Recommendations

Aquilion 64

32 6,98 7,98 6,47 7,57

Light Speed VCT

40 6,53 7,61 5,54 6,66

After the experimental receiving of weighted

CTDI for one tube rotation and with fixed patient

table position (taking into account dose distribution

outside ±50 mm zone along z-axis), knowing pitch

and scanning length, it is possible to calculate DLP

parameter and patient effective dose as a result.

Effective dose estimates achieved on the basis of

conventional DLP method, anthropomorphic body

phantom+TLDs (60 and 103 ICRP Publications

recommendations) and proposed method utilizing

dose distribution curves are presented in Table 1.

Electrical parameters settings for both devices have

been chosen as close as possible (X-ray tube voltage,

current and time of one tube rotation).

Figure 3: The right part of absorbed dose distribution at

the periphery hole of doubled CTDI phantom.

The results obtained using presenting method are

very close to reference estimations based on

anthropomorphic phantoms+TLDs measurements.

Difference for chest exams on both CT scanners

does not exceed 8%. When compared it must be

taken into account that doses evaluated using

weighted factors from ICRP 103 Publication

recommendations exceed those based on ICRP 60

Publication recommendations by approximately 14 –

16% (15% in average) and that effective dose

conversion factors recommended by European

Commission have been calculated using Monte-

Carlo simulation method in 1999 when there were

ICRP 60 Publication recommendations only. Up to

now for ICRP 103 Publication recommendations

appropriate conversion factors are calculated for CT

scanner Somatom Sensation 64 (Siemens

Healthcare) only (Deak et al., 2010).

Figure 4: The right part of weighted dose distribution in

doubled CTDI phantom.

4 CONCLUSIONS

The presented method allows estimating of CT

effective doses more accurately in comparison with

traditional DLP procedure by taking into account of

scattered in patient body radiation outside ±50 mm

scanning zone along z-axis.

In this case patient effective dose evaluation

accuracy increases by approximately 20% (chest

exams) for considered models of 64-slice CT

scanners.

Since the use of traditional DLP method for

multislice CT scanners with large beam collimation

(30 – 40 mm and more) leads to underestimation of

patient effective dose, it seems necessary to utilize

presented method or, as an alternative, to calculate a

set of correction factors for widely used now

weighted CTDI values which are displayed on

scanner’s console for various types of CT studies.

Additionally these correction factors should take

into account differences in dose evaluation using

ICRP 103 Publication and ICRP 60 Publication

MethodofMultisliceCTEffectiveDosesEstimationontheBasisofDoseDistributionCurves

recommendations.

REFERENCES

Deak, P., Smal, Y. and Kalender, W. 2010, ‘Multisection

CT protocols: sex- and age- specific conversion

factors used to determine effective dose from dose-

length product’, Radiology, vol. 257, no.1, pp. 158 –

66.

European Commission’s Study Group 1999, European

Guidelines on Quality Criteria for Computed

Tomography (EUR 16262 EN), Luxembourg,

European Communities.

ICRP 1991, 1990 Recommendations of the International

Commission on Radiological Protection (Publication

60), ann. ICRP 21, no. 1 – 3.

ICRP 2007, 2007 Recommendations of the International

Commission on Radiological Protection (Publication

103), ann. ICRP 37, no. 2 – 4.

BIODEVICES2013-InternationalConferenceonBiomedicalElectronicsandDevices