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Transcript of 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 Radiation qualities 29
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 Monitorin patient eposure 31
4.1.1 Kerma-area product meter 314.1.2 Diital detector 32
4.2 Radiation qualities 33
4.2.1 From calirations to clinical use 33
4.2.2 From film-screen to diital mammoraph 36
5 DISCUSSION 38
5.1 Monitorin patient eposure 38
5.2 Radiation qualities 39
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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