Patient Eksposure Monitoring

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Patient eposure monitorinand radiation qualities

in to-dimensional

diital -ra imainToroi P

STUK-A239 / OCTOBER 2009

STUK STEILYTURVAKESKUSSTRLSKERHETSCENTRALEN

RADIATION AND NUCLEAR SAFETY AUTHORITY

Osoite / Address Laippatie 4, 00880 Helsinki

Postiosoite / Postal address PL / P.O.Box 14, FI-00881 Helsinki, FINLAND

Puh. /Tel. +358 9 759 881 Fax +358 9 759 88 500 www.stuk.

Radiation and Nuclear Safet Authorit STUK

Department of Phsics, Facult of Science, Uniersit of Helsinki

ACADEMIC DISSERTATION

To e presented ith the permission of the Facult of Science of the Uniersit

of Helsinki, for pulic criticism, in the Lecture Room CK 112 of Eactum,

Kumpula on Noemer 13th, 2009, at 12 oclock noon.


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Superisors:

Docent Sauli Saolainen

HUS Helsinki Medical Imain CenterUniersit of Helsinki, Finland

Ph.D. Antti Kosunen

Dosimetr Laorator

STUKRadiation and Nuclear Safet Authorit, Helsinki, Finland

Reieers:

Docent Antero Koiula

Department of Oncolo and RadiotherapUniersit of Oulu, Finland

Professor Hannu Aronen

Department of Radiolo

Uniersit of Turku, Finland

Opponent:

Docent Aaro Kiuru

Department of RadioloUniersit of Turku, Finland

The conclusions presented in the STUK report series are those of

the authors and do not necessarily represent the official position of

STUK.

ISBN 978-952-478-480-1 (print)

ISBN 978-952-478-481-8 (pdf)

ISSN 0781-1705

Edita Prima Oy, Helsinki/Finland, 2009

Sold by:

STUK Radiation and Nuclear Safety Authority

P.O.Box 14, FI-00881 Helsinki, Finland

Phone: +358 9 759 881

Fax: +358 9 7598 8500


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TOROI Paula. Patient exposure monitoring and radiation qualities in

two-dimensional digital x-ray imaging. STUK-A239. Helsinki 2009. 53 pp. +

apps. 74 pp.

Key words: dosimetry, radiation qualities, exposure monitorin, diital imain,

dianostic radiolo, mammoraph, caliration, dosemeter, -ra ener

spectra

Abtract

The methods for estimatin patient eposure in -ra imain are ased on the

measurement of radiation incident on the patient. In diital imain, the usefuldose rane of the detector is lare and ecessie doses ma remain undetected.

Therefore, real-time monitorin of radiation exposure is important. Accordin to

international recommendations, the measurement uncertaint should e loer

than 7% (confidence leel 95%).

The kerma-area product (KAP) is a measurement quantit used for monitorin

patient exposure to radiation. A field KAP meter is typically attached to an x-ray

deice, and it is important to reconize the effect of this measurement eometr

on the response of the meter. In a tandem caliration method, introduced inthis stud, a field KAP meter is used in its clinical position and caliration is

performed ith a reference KAP meter. This method proides a practical a

to calirate field KAP meters. Hoeer, the reference KAP meters require

comprehensie caliration.

In the caliration laorator it is recommended to use standard radiation

qualities. These qualities do not entirely correspond to the lare rane of clinical

radiation qualities. In this ork, the enery dependence of the response of different

KAP meter tpes as eamined. Accordin to our findins, the recommended

accurac inKAP measurements is difficult to achiee ith conentional KAP

meters ecause of their stron enery dependence. The enery dependence of the

response of a noel lare KAP meter as found out to e much loer than ith

a conentional KAP meter. The accurac of the tandem method can e improed

usin this meter tpe as a reference meter.

A KAP meter cannot e used to determine the radiation eposure of patients

in mammoraph, in hich part of the radiation eam is alas aimed directl

at the detector ithout attenuation produced the tissue. This ork assessed

hether piel alues from this detector area could e used to monitor the


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radiation eam incident on the patient. The results ere conruent ith the

tue output calculation, hich is the method enerall used for this purpose.

The recommended accurac can e achieed ith the studied method.

Ne optimization of radiation qualities and dose leel is needed hen other

detector types are introduced. In this ork, the optimal selections ere examined

ith one direct diital detector type. For this deice, the use of radiation qualities

ith hiher eneries as recommended and appropriate imae qualit as

achieed increasin the lo dose leel of the sstem.


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TOROI Paula. Potilaan steilyaltistuksen monitorointi ja steilylaadut

kaksiulotteisessa digitaalisessa rntgenkuvantamisessa. STUK-A239. Helsinki

2009, 53 s. + liitteet 74 s.

Avainsanat: dosimetria, steillaadut, steilaltistuksen monitorointi,

diitaalinen kuantaminen, dianostinen radioloia, mammorafia, kalirointi,

annosmittari, rnten eneriaspektri

Tiiitelm

Menetelmt potilaan rntenkuauksesta saaman steilaltistuksen

ariointiin perustuat potilaaseen kohdistuan steiln mittaamiseen.Diitaalisessa kuantamisessa ilmaisimelle sopia annosalue on laaja ja liian

suuret annokset saattaat jd huomaamatta. Sen uoksi reaaliaikainen

steilaltistuksen monitorointi on trke. Kansainlisten suositusten

mukaisesti mittauseparmuuden pitisi olla alle 7% (luottamustaso 95%).

Mittaussuuretta kerman ja pinta-alan tulo (KAP) ktetn potilaan

steilyaltistuksen monitorointiin. KAP-kenttmittari on tyypillisesti kiinnitettyn

rntenlaitteeseen ja on trke huomioida mittauseometrian aikutus mittarin

asteeseen. Tss tss esitetss tandem-kalirointi -menetelmss, KAP-kenttmittaria ktetn sen omalla paikallaan ja kalirointi suoritetaan

KAP-ertailumittarin aulla. Tm menetelm tarjoaa ktnnllisen taan

kaliroida KAP-kenttmittareita. Kuitenkin, KAP-ertailumittarilla pit olla

kattaa kalirointi.

Kalirointilaoratoriossa suositellaan ktettn standardin mukaisia

steillaatuja. Nm laadut eit tsin astaa kliinisten steillaatujen

laajaa alikoimaa. Tss tss tutkittiin eri KAP-mittaritppien asteiden

eneriariippuuutta. Tutkimuksemme perusteella perinteisill KAP-mittareilla

suositeltu tarkkuus on aikea saauttaa KAP-mittauksissa johtuen niiden

ahasta eneriariippuuudesta. Uudentppisen ison KAP-mittarin

eneriariippuuuden todettiin olean paljon pienempi kuin taallisilla

KAP-mittareilla. Tandem-menetelmn tarkkuutta oidaan parantaa kyttmll

tmn tppist mittaria ertailumittarina.

KAP-mittaria ei oida ktt potilaan steilaltistuksen ariointiin

mammorafiassa. Mammorafiassa osa steilykeilasta osuu suoraan ilmaisimelle

ilman kudoksen aiheuttamaa aimennusta. Tss tyss tutkittiin oisiko tlt

alueelta ilmasimelta saatua pikselilukemaa ktt potilaaseen kohdistuan


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steilkeilan monitorointiin. Tulokset oliat htenei leisesti ktetn

steiltuottolaskennan kanssa ja suositeltu tarkkuus oidaan saauttaa

tutkitulla menetelmll.

Steilylaatujen ja annostason uusi optimointi on tarpeen, kun otetaan kyttn

uusia ilmaisintyyppej. Tss tyss optimaalisia aihtoehtoja tutkittiin yhdell

taulukuailmaisinperusteisella diitaalisella mammorafialaitteella. Tll

laitteella suurempieneristen steillaatujen ktt suositeltiin ja riitt

kuanlaatu saautettiin nostamalla laitteen matalaa annostasoa.


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Cotet

AbSTRACT 3

TIIvISTELM 5

ORIgINAL PUbLICATIONS 9

LIST OF AbbREvIATIONS 10

AIMS OF THE STUDy 11

1 INTRODUCTION 15

2 PATIENT ExPOSURE IN x-RAy IMAgINg 19

2.1 Quantities 19

2.1.1 Measurement quantities 19

2.1.2 Patient doses 20

2.2 Dosemeters and measurements 21

2.2.1 Dosemeter tpes 212.2.3 Estimation of patient eposure 22

2.3 Caliration of dosemeters 23

2.4 Radiation qualities 23

2.4.1 Radiation qualities in calirations 24

2.4.2 Optimal radiation qualities in the clinical situation 25

3 METHODS 28

3.1 Monitorin patient eposure 28

3.1.1 Kerma-area product meter 28

3.1.2 Diital detector 28


3.2.1 From calirations to clinical use 29

3.2.2 From film-screen to diital mammoraph 29


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4 RESULTS 31


4.1.1 Kerma-area product meter 314.1.2 Diital detector 32


4.2.1 From calirations to clinical use 33

4.2.2 From film-screen to diital mammoraph 36

5 DISCUSSION 38



6 CONCLUSIONS 41

7 AKNOwLEDgEMENTS 43

REFERENCES 45


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Oriial pblicatio

This thesis is ased on the folloin papers and the ill e referred to in thetet their Roman numerals.

I Toroi P, Komppa T, Kosunen A. A tandem caliration method for kerma-

area product meters. Phs. Med. biol. 2008; 53: 49414958.

II Toroi P, Komppa T, Kosunen A, Tapioaara M. Effects of radiation qualit

on the caliration of kerma-area product meters in x-ray eams. Phys. Med.

biol. 2008; 53: 52075221.

III Toroi P, Kosunen A. The ener dependence of the response of a patient

dose calirator, Phs. Med. biol. 2009; 54: N151N156.

Iv Toroi P, Nieminen M, Tenkanen-Rautakoski P, varjonen M. Determinin air

kerma from piel alues in diital mammoraph. Phs. Med. biol. 2009;

54: 38653879.

v Toroi P, Zanca F, youn KC, an Oneal C, Marchal g, bosmans H.

Eperimental inestiation on the choice of the tunsten/rhodium anode/filter comination for an amorphous selenium-ased diital mammoraphy

sstem. European Radiolo 2007; 17 (9): 23682375.

The author took part in plannin all the studies and mainl carried out the

measurements and analsed the results. The literature reie and ritin of

the articles as also primaril performed the author. The results of these

studies hae not een used in other Ph.D. theses.


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Lit of abbreiatio

AAPM American Association of Phsicists in MedicineALARA As lo as reasonal achieale

CEC Commission of the European Communities

CNR Contrast-to-noise ratio

CR Computed radioraph

DAP Dose-area product

DR Direct diital sstem

DRL Dianostic reference leel

EC European Commission

EI Eposure indeHVL Half-alue laer

IAEA International Atomic Ener Aenc

ICRP International Commission on Radiation Protection

ICRU International Commission on Radiation Units and

Measurements

IEC International Electrotechnical Commission

ISO International Oranization for Standardization

KAP, PKA

Kerma-area product

Kerma,K Kinetic ener released per unit massMED Medical Eposure Directie

MgD Mean landular dose

NRPb National Radiation Protection board (UK)

PDC Patient dose calirator (Radcal)

PMMA Polmethl methacrlate

PSDL Primar Standard Dosimetr Laorator

Pv Piel alue

RQR Name of the set of standard radiation qualities (IEC 2005)

SDNR Sinal difference-to-noise ratio = CNR

SENTINEL Safet and efficac for ne techniques and imain usin ne

equipment to support European leislation (an EU project)

STUK Radiation and Nuclear Safet Authorit in Finland

SSDL Secondar Standard Dosimetr Laorator

STM Ministr of Social Affairs and Health in Finland

TLD Thermoluminescent dosimeter


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Aim of te tdy

The main purpose of the ork presented in this thesis as to assess thepossiilities to improe the accuracy and reliaility of patient exposure monitorin

in to-dimensional -ra imain. Measurement uncertainties ere ealuated

to conclude hether the internationall recommended accurac leels can e

achieed in eposure measurements ith monitorin dosemeters. Improed

caliration methods for the dosemeters ere introduced, ith particular attention

paid to the differences in radiation qualities used in caliration and in the actual

measurement. The effect of the transition from film-screen to diital imain

on radiation qualities as studied in relation to eposure leels in diital

mammoraph.

The specific aims of the research descried in this thesis ere to:

1) present a ne method for cross caliration of kerma-area product (KAP)

meters, here another KAP meter is used as a reference instrument (studies

I, II, III);

2) eamine the ener dependence of the response of conentional and noel

tpe KAP meters and ealuate the inaccuracies due to differences in radiationqualities used in a caliration laorator and in the clinical situation (studies

II, III); and

3) stud the effect of usin unconentional radiation qualities on eposure

leels and introduce an eposure monitorin method for the direct diital

mammoraph sstem (studies Iv, v).


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Stud I

Toroi P, Komppa T, Kosunen A.

A tandem caliration method for kerma-area product metersPhs. Med. biol. 2008

The kerma-area product (KAP) is used in medical x-ray examinations to monitor

the radiation eposure of patients. As an effort to improe the caliration

procedures for KAP meters, a tandem caliration method as deeloped. In this

method, the field KAP meter is used in its clinical position and the reference alue

is measured simultaneousl at a larer distance in the eam ith a reference

KAP meter. In this ork the tandem method as descried, demonstrated and

compared ith other methods. Uncertainties ere estimated and compared iththe recommended accurac.

Stud II

Toroi P, Komppa T, Kosunen A, Tapioaara M.

Effects of radiation qualit on the caliration of kerma-area product meters in

-ra eams

Phs. Med. biol. 2008

The caliration coefficients of KAP meters depend on the ener spectrum

(radiation quality) of the x-ray eam. This dependence as examined y measurin

the caliration coefficients for seeral radiation qualities in the rane of filtrations

and tue oltaes enerall used in medical -ra imain and in caliration

laoratories. The accurac ofKAP measurements as inestiated ith respect

to caliration coefficients and procedures needed in clinical practice.

Stud III

Toroi P, Kosunen A.

The ener dependence of the response of a patient dose calirator

Note in Phs. Med. biol. 2009

The ener dependence of response of a noel lare KAP meter tpe (patient

dose calirator, PDC) as eamined measurin the caliration coefficients

for seeral standard and clinical radiation qualities. The uncertaint related to

enery dependence as estimated and compared ith the response of traditional

meters.


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Stud Iv

Toroi P, Nieminen M, Tenkanen-Rautakoski P, varjonen M.

Determinin air kerma from piel alues in diital mammoraphPhs. Med. biol. 2009

The use of direct diital detector elements for monitorin patient eposure in

diital mammoraph as eamined. The response of piel alues from the

detector elements in the unattenuated primar eam area as inestiated

ithin a lare eposure and ener rane for to detector tpes. Usin these

caliration results, air kerma as measured from clinical patient imaes and

compared ith the tue output calculation, hich is the eneral method for

eposure estimation.

Stud v

Toroi P, Zanca F, youn KC, an Oneal C, Marchal g, bosmans H.

Eperimental inestiation on the choice of the tunsten/rhodium anode/filter

comination for an amorphous selenium-ased diital mammoraph sstem

Eur. Rad. 2007

The use of unconentional radiation qualities ith hiher eneries for a direct

diital mammoraph sstem as inestiated. An optimization stud as

performed ith a lare rane of radiation qualities and reast thicknesses.

The use of the tunsten/rhodium anode/filter comination as under specific

interest.


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1 Itrodctio

The radiation dose from indiidual medical -ra imain sstem needs toe measured for different purposes, includin qualit control, optimization

and the estimation of patient eposure. The importance of patient eposure

estimation as hihlihted the Medical Eposure Directie (MED) 97/43/

Euratom (EC 1997) and taken into Finnish leislation in 2000 (STM 2000).

based on the recommendations of the European Commission (EC, 1997), patient

eposures should e compared to an achieale leel, such as the dianostic

reference leel (DRL). Patient exposure leels hae een recorded and compared

in international studies (e.. vano et al. 2008a) and in Finland (e.. Rannikko et

al. 1997, Karppinen and Jrinen 2006, Kiljunen 2008). The risk for the patientcan e estimated from the measurale quantities usin simulation models (e..

Lampinen 2000, Tapioaara and Siiskonen 2008) or usin calculated conersion

factors (ICRU 2005, ICRP 2007). Hoeer, the asis for comparale and accurate

results is a uniform and accurate measurement method and the use of properl

calirated dosemeters.

Standardized caliration and measurement methods are important to

achiee accurate and comparale results. For dosimetr in dianostic radiolo,

the International Atomic Ener Aenc (IAEA) has proided uidelines in the

International Code of Practice (2007). Other uidelines hae een ien, for

eample, the ICRU (2005) and STUK (2004). In the dianostic use of -ras,

uncertainties of 7% (confidence leel of 95%, coerae factor k = 2) or loer are

recommended for exposure measurements (waner et al. 1992, ICRU 2005, IAEA

2007). Een thouh there are lare uncertainties in estimatin the health effects

of radiation, it is enerall knon that een a 10% reduction in the dose leel

is essential for optimization purposes, hich is h the oal for measurement

accurac is set so hih (IAEA 2007).

In past, the main interest in medical -ra imain dosimetr as in

estimatin the radiation eposure of the staff. Susequentl, the determination

of patient exposure has ecome eneral and oliatory, ut traditions for patient

dosimetr are still under deelopment. The dosemeters used for eposure

measurements are tpicall adjusted the manufacturer and set to displa a

proper alue ith one measurement eometry and radiation spectrum. To achiee

comparale results, the caliration of these dosemeters should e traceale to

international standards. The electrical adjustments made manufacturers

ma lack this requirement if the are not authorized to proide such a serice

the national metrolo od.

Standard radiation qualities are recommended and enerall used in

caliration laoratories for caliratin dianostic dosemeters (IEC 2005, ICRU


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2005, IAEA 2007). Hoeer, these do not completel coer the lare rane of

radiation qualities used in clinical situations (e.. Lin 2007, vano et al. 2008c). A

proper radiation qualit specification is needed for the step eteen calirationand clinical radiation qualities. The -ra ener spectrum is continuous and

it is difficult to descrie it properl ith one parameter. The half-alue laer

(HVL) is enerally used for this purpose, ut for dosemeters ith a stron enery

dependence it ma e inadequate. For eample, the response of a kerma-area

product (KAP) meter is knon to hae stron ener dependence (Larsson et al.

1996, 1998, 2006, bednarek and Rudin 2000, Malusek et al. 2007).

In film-screen radioraph, oereposure is detected ecessiel dark

film. In diital imain, the useful dose rane is expanded due to the lare dynamic

rane of diital detectors, and hih exposures may thus remain undetected. Thisemphasizes the need for real-time eposure monitorin durin eaminations.

Especiall ith fluoroscop and interentional sstems, the eposures are lare

and the duration of irradiation is determined the user, and it is important

that a real-time estimation of the patient eposure is aailale (Cusma et al.

1999). Modern -ra units are often equipped ith a fied or remoale KAP

meter or a displa of the computedKAP alue ased on the -ra tue output

and field size settins. KAP meters are commonl used for monitorin patient

exposure in eneral radioloy and especially for interentional radioloy, in hich

lon eposures are performed. The need for KAP meter calirations is eident(Larsson et al. 1996, 1998, 2006, Jankoski 2008, Hetland 2009). KAP meters

are often calirated and adjusted y the manufacturer and the caliration is not

traceale to international standards. It is enerall recommended that a KAP

meter should e calirated usin the same -ra unit and irradiation eometr

as used ith patients, mainly ecause of the equipment-specific characteristics of

etra-focal and stra radiation (Shrimpton and wall 1982, ICRU 2005). In some

cases, meters are attached to the sstem and it ma een e impossile for the

user to send it for caliration.

TheKAP of an -ra eam is the surface interal of air kerma oer the

area of the entire eam in a plane perpendicular to the eam ais (ICRU 2005).

generall, field calirations of KAP meters are performed ith the eam-area

method;KAP reference alues for caliration are determined approimatin

the surface interal the product of the nominal area of the -ra field and

the air kerma measured at the centre of the field (Shrimpton and wall1982,

NRPb 1992, Larsson et al. 1996 and 1998, ICRU 2005, IAEA 2007). This method

has the limitation of the dependence of resultin caliration coefficients on the

non-uniformities of the -ra eam. Other sources of uncertaint include the

measurements of field size and the location of the planes of air kerma and field

size determination (Larsson et al. 1996, 1998, 2006, Malusek et al. 2007). The


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accuracy of caliration could e improed y measurin the reference KAP alue

directl accordin to the definition, thus aoidin the prolems arisin from the

non-uniformities of the -ra field. Larsson et al. (1996) used TLDs to measurethe surface interal, ut this method is quite laorious.

because the difference eteen conentional and diital imain is on

the detector side, not in the production of radiation, the methods for eposure

measurement ill not necessar chane. For eample, KAP meters can e used

in the same a in oth sstems. Hoeer, KAP meters cannot e used in

mammoraph ecause of their hiher useful ener rane and the attenuation

and filtration effect of the KAP chamer (IEC 2000). Mammoraph is used in

screenin a lare roup of health omen and it is er important to capture

possile inappropriate eposure leels or malfunctions of the sstem at an earlstae. The diital output ma offer ne possiilities for usin piel alues from

a diital detector in dose calculation and monitorin (Flod et al. 1990, Tucker

and Rezentes 1997, Aria et al. 2007, Kauppinen 2008, vano et al. 2008). In

mammoraphy, the imae detector area is commonly only partly coered y tissue

and the incident air kerma could e monitored usin piel alues from the

detector elements in the unattenuated primar eam area. An eposure inde

(EI) or other relationship eteen piel alues and the dose on the detector

is usuall eamined ith one radiation qualit and under some additional

attenuation (IEC 2007, 2008, EC 2006). Hoeer, if piel alues are used foreposure estimation, the response function of the detector should e studied as

a function of ener.

The radiation qualities and eposure leels for film-screen imain hae

een optimized durin man ears of eperience. with ne diital sstems, the

detector types and their responses are different and ne optimization is therefore

needed. The manufacturer should perform the primar adjustment of optimal

radiation qualities and eposure leels. In man cases, ne technoloies hae

een rapidly taken in use and exposure parameters hae een adopted from film-

screen sstems. This is commonl the case ith computed radioraph sstems

(CR), here the detector system is proided y a different manufacturer from the

actual -ra sstem. It is not so clear ho is responsile for the optimization of

this type of system. It is important to ensure the quality of clinical imaes. These

methods alas hae the prolem of sujectiit, ut one difficult, specific to

diital imaes, is that the appearance and presentation of imaes differs from

film-screen imaes. Radioloists ma complain aout poor imae qualit een

thouh the information content of the imae is adequate for dianosis. In addition

to oseration of the clinical imae quality, ojectie physical optimization studies

are needed.


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with film-screen sstems, imae qualit as related to the specific dose

on the detector. In diital imain the contrast can e adjusted and the optimal

contrast-to-noise ratio (CNR) is the quantit of interest (bosmans et al. 2005,Tapioaara 2005, 2006, Doyle et al. 2006, EC 2006). It may proide the possiility

to chane to spectra ith a loer dose ithout affectin imae qualit or een

proidin etter imae qualit. when ne optimization is performed for a

specific system ith a set eometry, the adjustale parameters are the radiation

qualit and the dose leel. based on simulation studies, hiher ener spectra

could e more optimal for diital mammoraph (Dance et al. 2000, Fahri

and yaffe 1994a, 1994, Fahri et al. 1996). Hoeer, these studies need to e

eperimentall erified.

This thesis concentrates on methods for monitorin patient eposure andcaliration issues in medical -ra imain, ecludin computed tomoraph.

Caliration methods for KAP meters here under inspection and a ne

monitorin method for mammoraph as introduced. Differences in radiation

spectra eteen caliration and clinical use ere hihlihted, especially for KAP

meters ith a stron enery dependence. Chanes in radiation qualities eteen

film-screen and diital imain ere also eamined for diital mammoraph.


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2 Patiet expore i x-ray imai

2.1 Qatitie

2.1.1 Mearemet qatitieQuantities for patient eposure measurement hae een introduced in man

pulications (ICRU 2005, IAEA 2007). In this chapter, those of most importance

ith respect to this thesis are introduced. To e ale to estimate the effects of

radiation, the enery released to a patient is the quantity of interest. The asoreddoseD is the mean radiation ener d transmitted to a mass unit diided

the mass of the unit dm.

(1)

Kerma (K) is the kinetic ener dEtr

released unchared particles to chared

particles in a mass unit diided mass of the unit dm.

(2)

D andKhae the same unit, J/k, and the specific unit in the SI sstem is the

ra (g). Tpicall, in the dosimetr of medical -ra imain the medium is

air and the quantities used are the dose asored in the air (Dair

) and air kerma

(Kair

). In the enery rane of x-ray dianostics there is an equilirium of chared

particles andDair

Kair

(IAEA 2007). The quantit air kerma has een used in

this ork as the asis of all directl measured application-specific quantities in

accordance ith the ICRU (2005) and IAEA (2007).

The most commonl used quantities for patient eposure measurement

are the incident air kerma (Ki), entrance surface air kerma (K

e) and kerma-area

product (KAP,PKA

). The specific term Ki

is used for the Kair

from an incident

x-ra eam measured on the central eam ais at the position of the patient or

phantom surface, and this is calledKe

hen the ackscatterin from the patient

or phantom is included.Ke

is used to proide a etter estimate of the patient

skin dose. The kerma-area product of an -ra eam is the surface interal of

air kermaKair

oer the areaA of the entire eam in a plane perpendicular to the

eam ais (ICRU 2005, IAEA 2007):

KdE

dmtr=

dD

dm

=


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(3)

This surface interal is often approimated the product of the nominalareaA of the -ra field and the air kerma measured at the centre of the field.

Hoeer, this approimation is onl accurate and reliale in uniform, sharp-

eded -ra fields. Otherise, non-uniformit of the field ma cause inaccurac

in the quantit.Kair

,KiandK

eare all quantities for one point. Hoeer, the risk

to the patient is related to the total eposure and also to size of the radiation

field. This is h theKAP quantit is more closel related to radiation ener

transmitted to the patient and the total dose of the patient (Shrimpton et al.

1984, Le Heron 1992).

2.1.2 Patiet doeThe eneric term patient eposure has een used forK

air,K

i,K

eandKAP. These

quantities are the ones used in measurements and can e compared to DRLs

(EC 1997). None of these is directl proportional to the radiation risk needed for

optimization studies and the comparison of different eaminations. Therefore,

some quantities more closel related to the risk hae een ealuated and the

can e calculated from these measured quantities (ICRU 2005, ICRP 2007). If the

measured quantit is for one point, the radiation field size should e ealuatedseparatel hen the total dose or risk to the patient is to e estimated.The

effectie dose is commonl used for comparisons and for cancer risk estimation.

Hoeer, the equialent dose in an oran or tissue ould e more accurate for

eneral risk estimation. These quantities are presented in detail elsehere

(ICRP 2007).

In normal mammoraph (not manification), the radiation field is of

a standard size and it is alas larer than the reast. Therefore, it is not of

interest to estimate theKAP quantit for mammoraph and calculations are

ased on air kerma measurements. because the landular tissue is the most

radiation-sensitie part of a reast, the mean landular dose (MGD) is used for

risk estimation in mammoraph. This is also the quantit recommended for

use as a DRL in mammoraph (EC 1997). The most enerall used factors for

calculatin theMGD are those descried Dance et al. (2000a). The MGD is

calculated from the measured air kerma usin taulated conersion factors

g and correction factors for the spectra (s) and landularit (c).

MGD = K g s c (4)

( , )airA

KAP K x y dx dy=


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2.2 Doemeter ad mearemetThe asis of all patient dose estimations is the patient eposure quantit to

e measured. If taulated alues are used for conersion from a measured toa risk related quantit, the accurac of the final result can onl e adjusted

the accurac of the measured quantit. Uncertainties can e ealuated on the

asis of the guide to the epression of uncertaint in measurement (ISO 1995).

All of the estimated uncertainties in this thesis are reported as epanded total

relatie uncertainties, otained multiplin the comined relatie standard

uncertaint ith the coerae factor, k = 2 (confidence leel 95%). In dianostic

-ra dosimetr, an uncertaint of 7% or loer is recommended for eposure

measurements (waner et al. 1992, ICRU 2005, IAEA 2007). This sets hih

requirements for eposure measurement methods and for the caliration ofdosemeters. These specific meters are used to detect and measure the amount

of radiation. Interaction eteen the radiation and material is needed to e ale

to measure the ener released the radiation.

2.2.1 Doemeter typeIonization chamers, thermoluminescent (TLD) and semiconductor dosemeters

are enerally used for radiation detection in x-ray imain. The eneral properties

of detectors are presented in the literature (e.. Knoll 2000), international

requirements are ien in IEC standards (1997, 2000) and the required

performance has een addressed the American Association of Phsicists in

Medicine (waner et al. 1992). The properties of these meters are discussed in

man pulications (Mc Keeer et al. 1994, brenier and Lisona 1998, Aschan

1999, Dewerd and waner 1999, Meyer et al. 2001, Zoetelief et al. 2000, warren-

Forard and Duan 2004, Martin 2007). Hih qualit ionization chamers

are er stale, hae ood repeatailit and the response has a small ener

dependence, and they are therefore recommended to e used as reference meters

in caliration laoratories (IAEA 2007).

Ionization chamers consist of electrodes ith a as cait eteen.

gas particles are ionized the radiation and chared particles moin in the

electrical field are collected the electrodes, alloin the cumulated chare to

e measured. The kerma-area product is usuall measured, closel accordin

to the definition (equation 3), ith a plane-parallel transmission ionization

chamer. The sinal from a chamer of this tpe is proportional to the surface

interal oer the sensitie area of the chamer. It is presupposed in the concept

of the kerma-area product that the area of interation in the definition and the

sensitie area of the chamer in the measurement are lare enouh to coer the

entire -ra eam, includin the penumra reions. A KAP chamer consists


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of polmethl metharcrlate (PMMA) alls coered ith conductin material.

The clinical requirement for liht transparenc limits the possiilities for use

as a all material. because of the use of materials ith a hih atomic numer,the ener dependence is stroner than for chamers ith a raphite coatin

(Larsson et al. 1996, bednarek and Rudin 2000).

2.2.3 Etimatio of patiet exporePatient eposure can e estimated in three as:

1. Phantom measurement

2. Tue output calculation

3. Eposure monitorin

Measurements performed ith a standard phantom, simulatin an aerae

patient, are ood for controllin technical parameters, comparin different

sstems and optimization. Hoeer, these measurements do not ie eposure

data for indiidual patients or take in account the ood or poor optimization of

eposure leels for patients of different size and composition.

The tue output (air kerma/tue loadin) is separatel measured for

different radiation qualities and patient exposure is calculated usin the display

of tue current and eposure time (e.. STUK, 2004). Actual patient eposureleels can e estimated ith the tue output calculation. Hoeer, the eposure

is not actuall measured ith an dosemeter and possile malfunctions of the

sstem ma remain undetected.

In eposure monitorin, actual measurement of the patient eposure is

performed durin an eperiment ith a suitale dosemeter (e.. KAP, TLD).

Eposure can e measured for a specific patient usin a TLD, ut this method

has seeral limitations. For example, the measurement is laour intensie, cannot

e done for all patients and the dosemeter ma hide some clinicall interestin

tarets. In eneral and interentional radioraph, KAP meters are often usedin the -ra eam to monitor patient eposure durin the eamination. The

actual eposure leel is measured durin the eamination and feedack on the

eposure can e etracted directl. Tue oltaes oer 50 kv are recommended

for KAP meters and therefore the are not suited for mammoraph ith loer

tue oltaes (under 35 kv).


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2.3 Calibratio of doemeterIn order to otain comparale and reliale results, the caliration of the meter

should e traceale to national or international standards. In a primary standarddosimetr laorator (PSDL), the standard alues are measured ith an open

air ionization chamer, and the dosemeters of a secondar standard dosimetr

laoratory (SSDL) are calirated aainst this standard. The dosemeter type used

in SSDL should e of a reference class, and these requirements are typically only

fulfilled ionization chamers (IAEA 2007).

In the caliration, a caliration coefficientNis calculated. The reference

alueMref

is measured ith a reference dosemeter and is diided the displa

of the dosemeter under calirationMcal

.

(5)

In some cases, the dosemeter is part of the -ra sstem and cannot e sent

to a standard laorator for caliration. In these cross-caliration cases, field

caliration needs to e performed. In this case, the reference dosemeter is used

in field measurements and it should hae caliration traceale to international

measurement standards.

2.4 Radiatio qalitieThe -ra spectrum is continuous and includes a lare rane of eneries. This

makes it difficult to exactly descrie the spectrum and reproduce it ith different

sstems. The term radiation qualit is used for the spectrum of radiant ener

produced a ien radiation source ith respect to its penetration or its

suitailit for a specific application. The radiation qualit of an -ra eam can

e specified, for instance, the tue oltae and total filtration, toether ith

the anode anle and anode material. The half-alue laer (HVL) is the most

enerall used radiation qualit specifier hen attemptin to descrie spectra

ith one parameter. Hoeer, real spectra ma e er different, een thouh

the hae the sameHVL (Fiure 1).

NM

M

ref

cal

=


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Figure 1. X-ray spectra for two radiation qualities (total filtration and tube voltage) withthe same half-value layer of 3 mm Al. The spectra were produced using the computer

program Spektripaja (Tapiovaara and Tapiovaara 2008).

2.4.1 Radiatio qalitie i calibratioThe response of an ideal dosemeter does not hae ener dependence and same

caliration coefficient can e used for different spectra. An international standard

defines the alloed ener dependence for dosemeters (IEC 1997, 2000). The

limits of deiation from the reference alue (at 100 kv) hen the total filtration

is 2.5 mm Al and the -ra tue oltae is eteen 50 kv and 150 kv are 8% for

KAP meters and 5% for others. For other filtrations, no requirements are stated

in the standards.

In practice, the ener dependence should e studied in calirations

usin different radiation qualities. Separate radiation quality specific caliration

coefficients can e ien for different qualities. Another approach is to ie one

caliration coefficient Nfor the reference radiation qualit Q and a correction

factor kqfor the other radiation qualit q:

(6)

Standard RQR radiation qualities (IEC 2005, Tale 1), intended to represent

the -ra eam incident on the patient in radioraphic, fluoroscopic and dental

0

200

400

600

800

1000

1200

0 20 40 60 80 100Energy (keV)

Photons/channel

3 mmAl 80 kV

5 mmAl 60 kV

kN q

N Qq =

( )

( )


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examinations, are enerally used for caliration in laoratories (ICRU 2005, IAEA

2007). In mammoraphy, RQR-M qualities are recommended for calirations (IEC

2005, IAEA 2007). They are ased on molydenum anode-filter cominations andspecified tue oltaes (25, 28, 30 and 35 kv). Standard radiation qualities do not

coer the hole rane of radiation qualities used in medical x-ray examinations.

The caliration coefficients need to e conerted to the actual clinical radiation

qualities interpolation usin appropriate specifiers of the ener spectrum.

For reference class ionization chamers, the response depends rather smoothl

on the radiation ener, and the half-alue laer (HVL) is often sufficient for

specifin the radiation qualit for these dosemeter tpes. Hoeer, this ma

not e the case for dosemeters ith a stron ener dependence.

Table 1. RQR standard radiation qualities (IEC 2005) realized at STUK.

CodeTubevoltage(kV)

STUK:

Filtration(mm Al)

STUK:

HVL(mm Al)

IEC:HVL(mm Al)

RQR 2 40 2.59 1.42 1.42

RQR 3 50 2.59 1.77 1.78

RQR 4 60 2.78 2.16 2.19

RQR 5 70 3.10 2.62 2.58RQR 6 80 3.02 2.98 3.01

RQR 7 90 3.25 3.48 3.48

RQR 8 100 3.37 3.94 3.97

RQR 9 120 3.82 5.03 5.00

RQR 10 150 4.45 6.60 6.57

2.4.2 Optimal radiatio qalitie i te cliical itatioClinical radiation qualities are selected ased on man ears of eperience

and optimization. In eneral, the anode material is tunsten (w) and tue

oltaes rane from 50 kv to 150 kv. Aluminium (Al) is tpicall used as a filter

material to remoe the lo-ener part of the spectrum, hich ould simpl

add to the patient eposure ithout contriutin to the imae. Other materials,

such as copper (Cu), are sometimes used to further harden the spectrum. In

mammoraph, much loer eneries are used ecause of the thinner imain

taret and to achiee etter contrast. Tue oltaes tpicall rane from 22 kv

to 40 kv (IAEA 2007). In film-screen mammoraph, anodes and filtrations of

moldenum (Mo) hae traditionall een the most commonl used, ut other


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choices such as rhodium (Rh) hae also een introduced, especiall for imain

thick reasts (ginold et al. 1995, Desponds et al. 1991, Jennins et al. 1981,

Dance et al. 2000). by usin K-ede filtrations, hiher eneries are also filteredand the radiation spectrum can e narroed to approach the optimal ener

spectrum (busher et al. 2002).

Optimization is alays a prolem ith to dimensions. The imae quality

should e ood enouh for dianosis ut the patient dose should e kept as lo

as reasonal achieale (ALARA, ICRP 2007). good imae qualit in clinical

patient imaes is the primar task of optimization, and radioloists iein

imaes hae an important role in the optimization process (Tapioaara 2006).

Optimization is difficult ith patient imaes ecause the taret ill chane in

different exposures and extreme alues cannot e used. Post-processin of diitalimaes is carried out for patient imaes and this ma induce some prolems if

it is used for unconentional tarets. Therefore, oriinal imaes (for processin)

should e used hen phantom imaes are examined and post-processin should

e optimized separately. Methods for optimizin post-processin are presented, for

example, y Carton et al. (2003) and Zanca et al. (2006). Technical measurement

can e used as a primar method for the optimization of imae acquisition.

The isiility of the oject in the imae is dependent on the contrast, noise

and sharpness. The radiation qualit determines the intrinsic -ra contrast

enerated the tissues and the -ra contrast decreases for hiher eam

eneries. The quantum noise primaril depends on the numer of asored

quanta, and is then also affected the eam qualit. The eam qualit does

not hae a sinificant effect on the sharpness of the diital detector. with film-

screen systems, the film-specific air kerma leel is needed for ood imae quality.

Therefore, optimization is mainl ased on the contrast determined the

radiation qualit. In diital sstems, the optimal air kerma leel can e chosen

and the contrast can e adjusted at the epense of affectin the noise. Since the

effects of contrast, imae sharpness and noise are interrelated and dependent

on some characteristics specific to the imain sstem, ne optimization is

needed for ne detectors. because the resolution is not epected to chane

sinificantl ith the eposure parameters, measurements of the contrast-to-

noise ratio (CNR)*) are considered a aluale tool in imae qualit optimization

for a particular sstem (e.. bosmans et al. 2005, Tapioaara 2005, 2006, Dole

et al. 2006, EC 2006). In CNR the difference eteen the sinal from the taret

*) In stud v the term sinal difference-to-noise ratio (SDNR) as used for this same quantit.


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(S) and ackround (B) is diided y the root-mean-squared noise from the sinal

S

and ackround B

.

(7)

To e ale to optimise not only the technical imae quality ut the actual clinical

imae qualit, the choice of taret and ackround for CNR measurement is

important, and the should simulate the clinical tarets and ackrounds

(Tapioaara 2006).

Radiation qualities other than conentional ones used ith film-screen

sstems ma e optimal for ne diital sstems. Especiall in mammoraph,loer eneries hae een used to achiee sufficient contrast. with diital systems,

the contrast is adjustale and radiation qualities ith hiher eneries could

e used, at least ith thicker reasts (Fahri and yaffe 1994a and , Fahri

et al. 1996, venkatakrishnan et al. 1999, Dance et al. 2000, berns et al. 2003,

Oenauer et al. 2003, youn et al. 2006).

2 2( )

2

s B

S BCNR

=

+


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3 Metod

3.1 Moitori patiet expore

3.1.1 Kerma-area prodct meterKAP meters are enerally used to monitor the eam of an x-ray system and patient

eposure. To achiee adequate accurac in KAP measurements, appropriate

caliration of the KAP meter is needed. In stud I, a ne caliration method

for field KAP meters, a tandem method, as introduced and eamined. In thismethod, the field KAP chamer is positioned similarl to measurements ith

patients and the reference KAP meter is simultaneousl used in the eam. For

this purpose, a Diamentor M4 (PTw, german) KAP meter as calirated in

the SSDL laorator for the incident eam and as used as a reference meter

on a clinical site. The appropriate caliration eometr (measurement distance

and field size) for the tandem method as studied.

Other caliration methods, namely the eam area and laoratory method,

ere also used for caliration and the results and uncertainties ere compared

ith the tandem method. In the eam area method, the KAP quantit as

approimated the product of the measured air kerma and the radiation

field size. In the laorator method, the field KAP meter as calirated in the

laorator for transmitted radiation. The effects of unit-specific radiation ere

assessed to demonstrate the uncertainty of the uncorrected laoratory caliration.

The field KAP chamer as used in its conentional position d0

, and also at a

loner distance, d = d0

+ 30 cm. The unit-specific correction factors k(d,d0) ere

calculated as the quotient of the KAP meter readinsM(d) andM(d0):

(8)

Multiplication this correction factor conerts the laorator-proided

caliration coefficientNand theKAP alue to distance d, hile the KAP meter

is used at d0.

3.1.2 Diital detectorKAP meters cannot e used in mammoraph and other methods are therefore

needed for eposure monitorin. In stud Iv, the use of piel alues from the

detector elements in the unattenuated primary eam area to monitor the incident

k d d M d M d( , ) ( ) / ( )0 0=


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air kerma (Pv method) as inestiated for to direct diital mammoraph

systems. The detector of system 1, Nuance Excel (Planmed Oy, Helsinki, Finland),

is ased on amorphous selenium direct conersion technolo and sstem 2,Senoraphe Essential (gE Medical systems, buc, France), on indirect conersion

ith a CsI scintillator and amorphous silicon technolo. The repeatailit and

response of the to direct diital detector tpes ere assessed ith a lare

exposure rane and different radiation qualities. Usin these caliration results,

air kerma as measured from clinical imaes and compared to the tue output

calculation.

3.2 Radiatio qalitie

3.2.1 From calibratio to cliical eThe difference eteen standard and clinical radiation qualities as analzed.

The effect of differences as eamined more carefull for KAP meters, hich

are knon to hae a stron ener dependence. Caliration coefficients of KAP

meters ere determined in studies II and III usin the RQR standard radiation

qualities (Tale 1) and seeral other qualities that are tpicall used in medical

-ra units. For these clinical radiation qualities, the tue oltaes raned from40 kv to 150 kv and aluminium filters of different thickness and toether ith

copper ere used. In stud II, caliration coefficients ere eamined for 11

traditional KAP meters manufactured for clinical purposes and in stud III, for

a ne lare non-transparent kerma-area product meter (patient dose calirator,

PDC, Radcal).

The results ere examined ith respect to radiation enery usin different

specifiers for the ener distriution of the -ra eam. The accurac ofKAP

measurements at different radiation qualities as inestiated, especiall ith

respect to the interpolations eteen the radiation qualities used in caliration

procedures hen determinin the caliration coefficients needed in clinical

practice.

3.2.2 From film-cree to diital mammorapyIn routine practice, the direct diital mammoraph sstem Noation DR

(Siemens, Erlanen, german), eamined in stud v, uses a tunsten (w) anode

in comination ith a 50 m thick rhodium (Rh) filter, and for thin reasts

a moldenum (Mo) anode toether ith a 25 m Rh filter, instead of the

conentional comination of an Mo anode ith an Mo filter (30 m). Stud v


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inestiated hether occasionall poor imae qualit as related to the use

of these anode/filter cominations or some other factor. An optimization stud

of eposure parameters (radiation qualit and dose leel) as performed forthis sstem. For this purpose, the SDNR (another term for CNR) as used to

estimate imae quality and mean landular doses (MgD) to estimate the risk for

a patient. An optimization study as performed ith different reast thicknesses

(from 2 cm to 7 cm), dose leels and radiation qualities. All aailale anode/filter

cominations (Mo/Mo, Mo/Rh and w/Rh) and a tue oltae rane from 23 kv

to 35 kv ere used.


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4 Relt

4.1 Moitori patiet expore

4.1.1 Kerma-area prodct meterbased on stud I, the tandem method proides a feasile and practical a

to calirate field KAP meters of an tpe in their clinical position. Accurate

measurements of the irradiation eometr are not required, ut comprehensie

caliration for the reference KAP meter is needed. The results for the field KAPmeter calirated ith three methods are presented in Fiure 2. The larer

difference eteen the tandem method and laorator method as circa 5%.

After unit-specific correction of the laoratory method, the difference as reduced

to 1%.

Figure 2. Calibration coefficients for three different calibration methods: the tandem,beam area and laboratory method (see study I for a full description of the methods). The

total filtration was 4.8 mm Al in the tandem and beam area methods and 5 mm in the

laboratory method. In measurements of the unit-specific correction for the laboratory

method the total filtration was 4.8 mm Al. The field size was 6 cm x 6 cm at the reference

measurement distance. The total uncertainties of the calibration coefficients for the

tandem method are 5.4% for 60 kV and 6.7% for lower tube voltages, for the beamarea method 7.5% and for the corrected laboratory method 6.4 7.8% (confidence level

95%). (Study I)

0.90

0.95

1.00

1.05

1.10

1.15

30 50 70 90 110 130 150

Tube voltage (kV)

Calibrationcoefficient

Tandem method

Beam area method

Laboratory method

Corrected laboratory method


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In the tandem caliration method, the estimated total relatie uncertaint of

caliration coefficients for the field KAP meter as 5.4 6.7%, dependin on

the radiation qualit. This estimation is onl for the caliration coefficient andthe part of the uncertaint from careful measurement and calculation of the

KAP alue for a patient is tpicall 34% under ood conditions. It means that

the uncertaint of the caliration coefficient should not eceed 6% to achiee

the total relatie uncertainty of 7%. with these uncertainties, the recommended

accuracy in KAPmeasurement can only e achieed in a limited rane of radiation

qualities.

4.1.2 Diital detectorAccordin to stud Iv, the air kerma estimation ased on the piel alues from

direct diital detectors can e used for patient eposure monitorin in diital

mammoraph. The main adantae of the method is that it is ased on real

measurement from an actual eposure, and chanes in eposure that ould not

e captured tue output calculation can therefore e detected. In the repeat

measurements of pixel alues the drop in alues for the ery first imae osered

y Pyry et al. (2006c) as found not to e related to a drop in the air kerma leel.

Hoeer, other chanes and the trend in piel alues durin the measurement

session ere related to chanes in air kerma.For the studied sstem, the difference in air kerma can e oer 40% for

different radiation qualities ut ith the same piel alue. This hihlihts the

issue that an eposure or similar inde cannot e used for accurate eposure

estimation ithout separate conersion factors for different radiation qualities.

In stud Iv, the conersion factors ere measured and used for clinical imaes.

A comparison of the piel alue method and the tue output calculation ith

clinical imaes is presented in Fiure 3. The differences eteen the to methods

ere typically under 2%. The estimated total uncertainty of the method (6.9%) is

comparale to the accuracy of the tue output calculation, and the recommended

accurac can e achieed ith the piel alue method.


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Figure 3. Comparison of air kerma at 0.5 m distance from the focal point in patientexaminations for systems 1 and 2. Air kerma was calculated based on pixel values (PV)

and tube output measurement (TOP). The equation for the linear trend line (continuous)

fitted to the points is presented in the figure (thinner line for system 1 and thicker for

system 2). The broken line indicates unity (y = x). The total uncertainty of air kerma is

6.9% (confidence level 95%) for both methods. (Study IV)

4.2 Radiatio qalitie

4.2.1 From calibratio to cliical eStandard radiation qualities (e.. RQR, Tale 1) differ from the ones used in

patient imain and the do not coer the totalHVL rane of clinical use. For

eample, if a filter ith a hiher atomic numer (e.. copper) is used toether

ith hiher tue oltaes (>100 kv), theHVL is hiher than ith larest RQR

qualit (IEC 2005), and etrapolation is needed. This situation is also the same

for mammoraph, in hich the hihestHVL of standard radiation qualities is

0.36 mmAl (RQR-M 4, 35 kv) and for radiation qualities used in clinical imain,

HVL ma e >0.5 mm Al. Etrapolation of caliration coefficients cannot e

recommended for an dosemeter. Interpolation of caliration coefficients in

the rane of standard radiation qualities can e performed if the response of a

dosemeter has a small ener dependence. For eample, the standard radiation

qualities recommended for mammoraph are ased on an Mo/Mo anode/filter


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comination, althouh other cominations such as Mo/Rh, Rh/Rh and w/Rh

are also used in clinical imain. Hoeer, hih qualit ionization chamers

recommended for mammoraph hae a small ener dependence (Dewerdet al. 2002, witzani et al. 2004), and HVL can therefore e used to specif the

radiation spectra and for interpolation of the caliration coefficient. This is not

the case for a dosemeter ith a stron ener dependence.

Figure 4. Calibration coefficients of the Diamentor M4 KAP meter for an x-ray beamincident on the chamber, for different total filtrations and RQR standard radiation

qualities (IEC 1994 and 2005), presented as a function of the half-value layer (HVL).

Calibration coefficients for the tube voltages of 90, 100, 120 and 150 kV are connected

by dotted lines. (Study II)

1.0

1.1

1.2

1.3

1.4

1.5

0 2 4 6 8 10

HVL (mm Al)

Calibratio

ncoefficient

RQR (IEC 1994) RQR (IEC 2005)

2.5 mmAl 3 mmA l

4 mmA l 5 mmA l

4 mmA l +0.1mmCu 4 mmA l +0.2 mmCu

90 kV 100 kV


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The ener dependence as sinificant for all eamined clinical KAP meters

and the shape of caliration cure aried eteen different KAP meter tpes.

Caliration coefficients for one tpical KAP meter are presented as a function oftheHVL in Fiure 4. The difference eteen the hihest and loest caliration

coefficients is 40%.

To achiee the 7% uncertaint leel for clinicalKAP measurements, the

field KAP meter should e calirated for an appropriate numer of clinical

radiation qualities. To otain the correct manitude of estimation for eposure

in the clinical situation, the field KAP meter could e adjusted ith a selected

aerae radiation qualit from the rane of radiation qualities to e used in

practice. Radiation qualit specific coefficients can e used for the calculation

hen more accurate results are needed.If a KAP meter is calirated in the laorator ith standard radiation

qualities, the adequate specification for the spectrum is needed to e ale to

interpolate the caliration coefficients for clinical radiation qualities. For accurate

measurements, it is not sufficient to make the interpolation ased on a sinle

parameter, such as the -ra tue oltae, total filtration or HVL: at least to

parameters are needed to specif the -ra spectrum. If possile, the radiation

qualities that are used in clinical measurements should e simulated in the

caliration. If this is not possile, etensie caliration coerin the clinical

rane is necessar. The easiest approach ould e to perform the calirationith fied filtration and a certain tue oltae rane, and to then determine the

correct caliration coefficient interpolation eteen these data usin to

specifiers. Caliration of KAP meters usin the IEC RQR radiation qualities

results in unacceptale errors if the actual filtration of the clinical -ra eam

markedl differs from that used in realizin the RQR spectrum.

The ener dependence of a ne PDC kerma-area product meter as also

studied and caliration coefficients are presented in Fiure 5. This tpe of KAP

meter has a loer enery dependence than typical transparent KAP meters, and

HVL can e used as a radiation qualit specifier ith an uncertaint loer than

2%. The uncertaint of caliration coefficients of a field KAP meter calirated

ith the tandem method can e reduced to 4.9% usin this meter tpe as a

reference meter.


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Figure 5. Calibration coefficients of the PDC-1 meter (Radcal) for an x-ray beam incident

on the chamber, for different total filtrations and RQR standard radiation qualities (IEC

2005), presented as a function of the half-value layer (HVL). The logarithmic trend line

is fitted to the points with fixed filtrations. (Study III)

4.2.2 From film-cree to diital mammorapyMany diital mammoraphy systems still use a conentional Mo/Mo anode/filter

comination, althouh ased on simulation studies some other comination

could e more optimal for diital sstems. Hoeer, in the studied sstem, other

cominations ere used. An optimization stud as performed and in Fiure

6, the calculated SDNR is plotted as a function of the MgD for three different

PMMA thicknesses (2 cm, 5 cm and 7 cm) and three anode/filter cominations.

The SDNR of the w/Rh anode/filter comination is alas hihest for a ien

dose, and the same trend appeared for all thicknesses and tue oltaes. b

usin a tunsten anode in comination ith a rhodium filter, the same SDNR

can e achieed ith a sinificantl loer MgD than usin a moldenum

anode in comination ith an Mo or Rh filter. This difference is larest for the

thickest reasts, ut it applies for all reast thicknesses, and the use of the w/

Rh comination can therefore e recommended for all reast thicknesses.

For some thicknesses, the dose leels proided the clinicall used AEC

mode achieed SDNR alues that ere proal too lo for ood imae qualit.

0.88

0.90

0.92

0.94

0.96

0.98

0 2 4 6 8 10HVL (mmAl)

Calibrationcoefficient

3 mmAl

5 mmAl

4 mmAl+0.1 mmCu

4 mmAl+0.2 mmCu

RQR


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Hoeer, the dose leel of the sstem as er lo for the w/Rh comination

compared to most film-screen sstems. This lo dose settin as attriuted as

the main reason for the occasionally poor imae quality in clinical mammorams.As a result of our stud, the AEC settin as reset for a hiher dose leel, hich

as still loer than enerall used ith film-screen sstems. based on clinical

estimation, the imae qualit improed to an appropriate leel.

Figure 6. The signal difference to noise ratio (SDNR) as a function of mean glandulardose (MGD) for a peak tube voltage of 27 kV and for three PMMA thicknesses and

anode/filter materials. The three points in each curve correspond to three different

dose levels (the middle point chosen to be near to the value that automatic exposure

control would select). Uncertainties of 4.3% for the SDNR and 4.1% for the MGD (95%confidence level) are shown with error bars. SDNR levels of 5 (continuous line) and 7

(dashed line) are plotted. (Study V)

0

2

4

6

8

10

12

14

0 2 4 6 8 10MGD (mGy)

SDNR

Mo/Mo 2 cm Mo/Mo 5 cm

Mo/Mo 7 cm Mo/Rh 2 cm

Mo/Rh 5 cm Mo/Rh 7 cm

W/Rh 2 cm W/Rh 5 cm

W/Rh 7 cm


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5 Dicio

5.1 Moitori patiet exporeFeedack on patient exposure is important for a user to e ale to optimize the use

of radiation. The alue can e ased on calculations, ut direct measurement has

some adantaes compared to indirect exposure estimation. Chanes in air kerma

that ould not e noted from eposure parameters, such as the effect of a short

exposure or incorrect filtration, can e reealed. Especially ith diital systems,

some malfunctions of the system can only e detected y measurement, ecause a

hih air kerma ill not oerexpose the film. A monitorin deice can also e usedto check the output stailit of the -ra sstem repeatin the eposure ith

the same settins (radiation qualit, tue loadin and eam area). Comparison

of results from direct measurement and the tue output calculation ill proide

additional information on the reliailit of oth dosimetric approaches.

KAP meters are eas to use and can proide useful alues for patient

eposure estimation. Hoeer, it is important to properl calirate a field KAP

meter at the clinical site. If the field KAP meter is calirated in the laorator,

clinical correction measurements should e performed. The tandem caliration

method, introduced in stud I, is straihtforard to perform in the clinical

enironment, ecause accurate adjustment of the reference meter or field size

measurement is not required. Hoeer, a reference KAP meter ith etensie

caliration is needed. This is hy this method is most useful for situations here

the same reference meter can e used for man calirations and for the testin

of field KAP meters. The tandem method as reported toether ith other

methods in a Finnish pulication (Toroi et al. 2008) that proided standardized

methods for the caliration of KAP meter for users of -ra sstems in Finland.

Our preliminary studies ith KAP meters (Pyry et al. 2005, 2006 a, ) ere also

reconized y IAEA in the International Code of Practice for dianostic radioloy

(2007) and the tandem method as included in cross-caliration methods for

clinical KAP meters. The implementation of this uideline as studied y a roup

of experts athered y the IAEA and a technical document aout the ork of this

roup is epected to e pulished in 2009. Our ork ith KAP meters (studies

I, II and III) also has an important role in this pulication.

Independentl of the caliration method, the ener dependence of the

response of commercial clinical KAP meters has such an influence that ith

one electrical adjustment of meter, the recommended accurac (uncertaint


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al. 2008c). Caliration and electrical adjustment of a field KAP meter should

e properl performed ith at least one radiation qualit, and in this a the

meter can e used to estimate the manitude of exposure in the clinical situation.More caliration coefficients can e used hen accurate eposure estimation is

needed, for eample, if eposure leels for DRLs are collected or an optimization

stud is performed.

KAP meters cannot e used for exposure monitorin in mammoraphy, and

a ne method for this purpose as proposed in stud v. The use of piel alues

for eposure monitorin is an appropriate method for estimatin air kerma in

patient eaminations. Hoeer, the response of this dosemeter also has ener

dependence, and radiation qualit correction factors are needed hen accurate

measurements are to e performed.In the future, pixel alues could also e usedfor measurin the patient dose and risk-related quantities. The usefulness of

the piel alue method could also e studied for other modalities. To e ale to

use the piel alue for eposure estimation or for other phsical measurements,

oriinal data ithout imae-specific processin are needed. with help from the

manufacturer, the oriinal piel alues from a selected reference ROI could e

added in the Dicom header.

It is not releant to compare the results from indiidual examinations, ut

instead to compare the eneral leels. In diital imain, not only the imae data

ut also eposure parameters are tpicall stored in a diital format. In directdiital sstems, eposure information and sometimes alsoKAP measurement

results are stored in the Dicom header. This propert of ne sstems proides

a possiilit to automaticall collect patient data for a lare patient olume

(vano et al. 2002, vano and Fernandez 2007). Automation ould free users from

collectin patient eamination data.

5.2 Radiatio qalitieThe difference eteen standard and clinical radiation qualities causes a prolem

in situations here etrapolation is needed or the response of a dosemeter has

a stron ener dependence. The response of clinical KAP meters has stron

a ener dependence and the HVL cannot e used alone to specif radiation

spectra for interpolations. The radiation qualit dependence of KAP meters,

seen in stud II, is the main draack of the tandem method, ut the ne PDC

tpe KAP meter eamined in stud III could e a solution to this prolem. b

usin this meter tpe as a reference meter in the tandem caliration method,

the accurac of the caliration coefficient can e improed and an uncertaint

of 7% can e achieed inKAP measurements.


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To our knolede, stud III as the first scientific pulication concernin

the PDC meter tpe, and more studies should e performed to specif other

characteristics of this meter tpe. For eample, the field size dependence of theresponse should e assessed. Hoeer, this PDC meter tpe cannot e used as

a field instrument for patient eposure monitorin, and it ill not resole the

prolem of the ener dependence of clinical KAP meters. Manufacturers ill

hae an important role in the future to proide meters ith smaller ener

dependences. Another possiilit ith diital sstems is to include some

automated calculation proram to hich caliration coefficients for different

radiation qualities could e entered to proide corrected results.

Chanin oer to diital imain also makes it important to proide ne

optimization of radiation qualities and eposure leels for the ne detectors.In our stud v, the use of the w/Rh anode/filter comination as inestiated

for one direct diital mammoraph sstem. It as concluded that w/Rh ould

actuall proide the est results for all reast thicknesses, not onl for thicker

reasts, as suested y a simulation study (Dance et al. 2000). The selection of

the tue oltae did not hae such a lare influence. Hoeer, in this case, dose

leels ere set to e optimal for the old Mo/Mo comination and the leel as

too lo for this sstem. Adjustment of the sstem improed the imae qualit

to an appropriate leel. Since this stud, similar results hae also een reported

other researchers (Muhoora 2008, williams 2008).


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6 Coclio

Accurate and reliale estimation of patient eposure can e achieed usina properl calirated eposure monitorin dosemeter. KAP meters hae een

used for this purpose in eneral -ra imain, and for mammoraph a ne

possiility is the use of pixel alues from diital detectors. with these monitorin

dosemeters, internationall recommended accurac leels can onl e achieed

in eposure measurements if radiation qualit correction factors are used in

the measurements. If clinical radiation qualities differ from those used in

the caliration, the HVL cannot alone e used for the interpolation. when

sitchin from film-screen to diital sstems, clinicall used radiation qualities

and eposure leels should e re-optimized for the ne detectors. At least forsome mammoraph sstems, it is possile to reduce the eposure leel ith an

appropriate selection of radiation qualities.

In the tandem method, a field KAP meter is calirated usin a reference

KAP meter, and some disadantaes of traditional methods can e aoided. The

tandem method is eas to perform in clinical -ra units, and is not sensitie

to minor chanes in caliration eometr. This method as also adopted in

international recommendations. The main draack of the method is the

uncertaint arisin from the dependence of the response of the reference KAP

meter on the -ra ener spectrum. If the clinical KAP meter tpe is used as areference meter, the achieed accuracy of the caliration coefficient is only slihtly

etter than ith traditional eam area method or laorator method.

The ener dependence of the response of clinical KAP meters has the

effect that, independentl of the caliration method, radiation qualit specific

correction factors are enerall needed in clinical measurements to achiee

the recommended accurac leel. Another issue is that if different radiation

qualities are used in caliration and in measurements, care should e taken

in the interpolation of caliration coefficients. The HVL cannot e used alone

to specif the radiation spectra, and at least to specifin parameters from

amon the tue oltae, filtration orHVL, should e used. For current standard

radiation qualities, none of these parameters is fixed and interpolation is difficult.

In addition, the do not coer the rane of clinical need and the user ma hae

to etrapolate the caliration coefficient. This is emphasized ith conentional

KAP meters that hae a stron ener dependence. The noel PDC KAP meter

tpe meter has a much smaller ener dependence. The accurac of the tandem

method is improed if this meter tpe is used as a reference meter, and the

recommended accurac can e achieed inKAP measurements.

Pixel alues from a diital detector proide a real exposure measurement

result for optimization studies and eposure monitorin. The information is in


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diital format and can easil e used for the automatic collection of data from

a lare roup of imaes. Oriinal imaes (for processin) are needed to e ale

to use piel alues for optimization or eposure monitorin. This ra data issometimes not easil aailale to the user and help from the manufacturer is

needed.An actual exposure measurement usin pixel alues ill reeal chanes

in the -ra tue output that ould not e detected from eposure parameters.

An uncertaint leel of loer than 7% can e achieed if radiation qualit

correction factors are used. Piel alues can also e used to measure the imae

qualit in optimization studies. For the direct diital sstem, an unconentional

radiation qualit ith the w/Rh anode/filter comination ae the est results

for all reast thicknesses, and the dose leel could e reduced elo the leel

enerall used in film-screen imain. Hoeer, it is important to ensure thatthe dose leel is hih enouh for dianosis.


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7 Akowledemet

The main ork for this thesis as carried out durin the ears 2004 and 2009at the Dosimetr Laorator of the Radiation and Nuclear Safet Authorit

(STUK). Part of stud Iv as performed in the Department of Radiolo at the

Uniersit Hospital of Leuen, belium, durin the ears 2005 and 2006. I am

rateful to Eero Kettunen, M.Sc., director of the Radiation Practices Reulation

Department of STUK, and Professor Hilde bosmans, Ph.D., Uniersity Hospital

of Leuen for the opportunit to ork in these projects. From the Uniersit of

Helsinki I ish to epress m ratitude to Professor Juhani Keinonen, Ph.D.,

Head of the Department of Phsics.

I am most rateful to my superisors, Docent Sauli Saolainen, Ph.D., andAntti Kosunen, Ph.D., for their help ith the thesis and for introducin me to

the field of medical phsics and dosimetr. I ish to thank the official reieers

of the thesis, Professor Hannu Aronen, M.D., and Docent Antero Koiula, Ph.D.,

for their constructie comments and Ro Siddall for reisin the lanuae of

the manuscript.

I receied help from three reat mentors durin the preparation of this

thesis and the are ratefull acknoleded for this. Tuomo Komppa, Lic. Sc.,

tauht me the importance of the lanuae and spellin; he as alas there for

me if I needed help, reardless of the time, especially ithKAP issues. Professor

Hilde bosmans, Ph.D., introduced me to the field of diital mammoraph and

is thanked for his forearin uidance durin the time in Leuen. From Markku

Tapioaara, M.Sc., I could ask anthin concernin the topics of this thesis and

I alas ot an anser that as understandale.

I am also rateful to other colleaues at STUK for their interest and

participation in m ork. I alas felt that I eloned there. I ish to epress

m armest appreciation to other for co-orkers of the Uniersit Hospital of

Leuen, ho arml elcomed me to belium and introduced me to the field of

diital mammoraph. Special thanks to m ood friend Federica Zanca, M.Sc. I

oe my thanks to all other co-authors of the pulications included in this thesis:

Docent Miika Nieminen, Ph.D., Mari varjonen, Ph.D., Petra Tenkanen, M.Sc.,

Professor Kenneth youn, Ph.D., Chantal van Oneal, M.D., and gu Marchal,

M.D., Ph.D.

I epress m reat appreciation to m parents, Leila and Raine Pr, for

eerthin and especiall for the help ith childcare. M deepest thanks to m

est friend and eloed husand, Mika, for his loe and support and ein the

est dad for our children. Leo and Meri ere the ones ho droe me to finish

this thesis quickl to e ale to dedicate m free time to e ith them in the

future.


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Studies I, II, and III hae een contriutions to the IAEA collaoratie

research project E2.10.06, the ojectie of hich is to test the implementation

of the Code of Practice for dosimetr in -ra dianostic radiolo. Stud Iv asperformed in the frameork of a multi-centre project ithin the actiities of the

SENTINEL project. The SENTINEL project, contract FP6-012909, as partially

supported and has receied fundin from the EC-Euratom Sith Frameork

Proramme. A research rant from the Finnish Radiolo Association as used

to finalize this snopsis and is ratefull acknoleded.


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Patient Eksposure Monitoring

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