Establishment of diagnostic radiology calibrations in an SSDL lecture on...• Third step: 1 st and 2 nd HVL are determined by interpolation between 3 data points very close to the

Establishment of diagnostic radiology calibrations in an SSDL

Prepared by Miloš Živanović

Background

• Diagnostic radiology is the largest contributor to the total population dose from artificial sources of radiation

• Doses from radiological procedures are usually small, but in some cases, such as interventional procedures, they can even cause deterministic effects

Establishment of diagnostic radiology calibrations in an SSDLPrepared by Miloš Živanović

Background

• Quality Control (QC) checks in diagnostic radiology are performed to ensure the image quality on one side and low dose on the other side

• Testing equipment must be regularly calibrated to ensure accurate measurements


International measurement system


IAEA TRS 457:2007 , pp. 10

IAEA/WHO SSDL network


• Established in 1976 to ensure traceability of measurements

• Objectives: improve the accuracy of measurements, establish traceability, achieve consistency, foster cooperation

• Membership composed of SSDLs, PSDLs, international organizations

CIPM Mutual Recognition Arrangement


• National Metrology Institutes and Designated Institutes are signatories of CIPM MRA

• BIPM maintains a database of all signatories and their Calibration and Measurement Capabilities

• Some SSDLs are members of CIPM MRA, but some are not

Abbreviations• SSDL and PSDL – Secondary (Primary) Standards Dosimetry

Laboratory

• NMI – National Metrology Institute

• DI – Designated Institute

• CMC – Calibration and Measurement Capabilities


Important literature• IEC 61267:2005: Medical diagnostic X-ray equipment - Radiation

conditions for use in the determination of characteristics– Definition and establishment of reference radiation qualities

• IAEA TRS 457:2007: Dosimetry in diagnostic radiology : an international code of practice– Code of practice for calibrations of diagnostic radiology equipmenthttps://www.iaea.org/publications/7638/dosimetry-in-diagnostic-radiology-an-international-code-of-practice


https://www.iaea.org/publications/7638/dosimetry-in-diagnostic-radiology-an-international-code-of-practice

Important literature• IEC 61674:2012: Medical electrical equipment - Dosimeters

with ionization chambers and/or semiconductor detectors as used in X-ray diagnostic imaging

• IEC 61676:2002: Medical electrical equipment - Dosimetricinstruments used for non-invasive measurement of X-ray tube voltage in diagnostic radiology


Important literature• BIPM guide to the expression of uncertainty in measurements

(JCGM 100:2008)

• IAEA TECDOC 1585: Measurement Uncertainty, A Practical Guide for Secondary Standards Dosimetry Laboratories

https://www.iaea.org/publications/7913/measurement-uncertainty


https://www.iaea.org/publications/7913/measurement-uncertainty

Which services should an SSDL provide?

• Air kerma free-in-air is used for most QC measurements; this quantity is commonly disseminated by PSDLs

𝐾𝐾 =d𝐸𝐸trd𝑚𝑚

• This quantity is the most important quantity for diagnostic radiology calibrations

• Unit: Gy



• Kerma area product (PKA) and kerma length product (PKL) are usually established in SSDLs, based on the traceable kerma measurements and length measurements.

• Only 5 laboratories have CMCs in BIPM KCDB for PKAand only 4 for PKL



• (Non-invasive) tube voltage measurement

• Practical Peak Voltage is adopted by IEC standards and IAEA TRS 457

• Different quantities are in use, Average Peak Voltage, Average Voltage etc.



• Exposure time• Current time product• Total filtration?• HVL measurement?• Beam profile measurement?

• Many manufacturers offer calibrations or testing in the mentioned quantities


Checklist – what will you need to establish calibrations

• Irradiation room and control room– Restricted access– Appropriate shielding

• X-ray unit with calibration bench and auxiliary equipment– Filters and filter wheels– Lasers for positioning


Checklist – what will you need to establish calibrations

• Secondary standards traceable to SI• Measurement equipment for ambient conditions,

length, time etc.• Quality system (according to ISO/IEC 17025)• Competent staff• UsersEstablishment of diagnostic radiology calibrations in an SSDLPrepared by Miloš Živanović

Finding users• Establishing calibration service in diagnostic

radiology is expensive (the costs can exceed 500000 euro)

• Needs for calibrations in terms of different quantities and in different radiation qualities should be evaluated


Existing equipment and facilities

• Equipment for DR calibrations can be used for different types of calibrations (e.g. radiation protection calibrations) and vice versa


Facilities


Facilities


• At least one irradiation room and one control room should be available

• Room size should be appropriate to avoid scatter from the walls and furniture

Facilities


• Walls, door and windows should be appropriately shielded (usually not a big issue for X-ray facilities)

• Access to irradiation room and control room should be restricted

Facilities


• Environmental conditions should be monitored and controlled (temperature between 18 °C and 24 °C)

• Temperature change not greater than 1 °C/h

• X-ray unit generator and cooling system might cause significant heating of the room

X-ray unit• F – focus• K1, K2, K3 –

collimators• F1 – additional

filtration• F2 – filter wheel• M – Monitor

chamber• D – detector• B – calibration

bench


F

B

X-ray unit• Anode material depends on the application; tungsten is used

for most applications• Anode angle should not be larger than 27° (IAEA TRS 457:

2007) or lower than 9° (IEC 61267:2005)• Liquid cooling of the anode is preferable• Inherent filtration should be lower than the equivalent of 2.5

mm Al. Otherwise, some radiation qualities will not be achievable (IEC 61267:2005, IAEA TRS 457: 2007)


X-ray unit• Filters should be of 99.9 % purity

• Filter thickness should be known within 0.01 mm (filters for HVL measurements)

• Ripple should be below 10 % (4 % for mammography) (IEC 61267:2005)


X-ray unit• Apertures should be constructed with appropriate opening

angle (IAEA TRS 457: 2007)


Radiation qualities• Introduction of standard radiation qualities is necessary

because response of every dosimeter (including standards) is dependent on photon energy

• Standard radiation quality is radiation beam with standardized and known properties: target material, PPV, total filtration, first and second HVL


Radiation qualities• Most complete specification of radiation quality would

include spectral distribution of the photon fluence

• Practically, radiation quality is defined by anode material, total filtration, tube voltage (PPV!), first and second HVL (for mammography qualities, only first HVL is determined)

• Term homogeneity coefficient is often used – ratio of first and second HVL


Radiation qualities• RQR series – beams emerging from X-ray generator (W target

material, Al filtration)

• RQT series – CT applications (W target material, Al+Cufiltration)

• RQR-M series – mammography applications (Mo target material and filtration)


Determination of HVLs• First step: air kerma is measured without any additional filtration

• Second step: air kerma is measured in the same place, but with added filtration between 0 mm Al and filtration corresponding to air kerma reduction by factor 6 (thicknesses should be concentrated around the values of 1st and 2nd HVL)

• Third step: 1st and 2nd HVL are determined by interpolation between 3 data points very close to the HVL values (not the whole range!)


Determination of HVLs – general radiography

• Homogeneity coefficient should be within ±0.03 of the standard value

• If you position additional filters corresponding to the standard HVL value, air kerma should be within (0.500 ± 0.015) Ka,0

• More information in IAEA TRS 457:2007 appendix 5 or IEC 61267:2005 annex b


Determination of HVLs -mammography

• In case of RQR-M series, only first HVL is measured

• First HVL should be within 0.02 mm Al of the standard value


Secondary standards• Reference class ionization chambers • Reference class electrometers• Usually chambers are calibrated together with

electrometer in dosimetry laboratories• Alternatively, ionization chamber can be calibrated

separately in dosimetry laboratory and electrometer in laboratory for electrical quantities


Secondary standards• Reference class ionization chambers requirements are not well

defined• Clause 4.2 of IEC 61267: 2005 gives the following requirements:

– Dosimeter shall comply with IEC 61674: 2012– Energy dependence shall be within ±3% over the range of the radiation

qualities N-15 to N-200– Dimensions of detector shall be such that it is completely irradiated by the

beam– Sensitivity should be appropriate for measurements with phantoms and

added filters for HVL determination


Secondary standards• IAEA TRS 457 gives additional requirements, e.g. stability within 0.3%• No clear definition of reference class dosimeter


IAEA TRS 457:2007 , pp. 52

QMS• QMS should be established in accordance with ISO/IEC

17025:2017

• QA/QC procedures should be established (e.g. ionization chamber stability checks)

• Many procedures are similar to equivalent procedures used for calibrations of radiation protection and radiation therapy equipment


Uncertainty budget• IAEA TECDOC 1585 is an excellent starting point (together

with IAEA TRS 457)

• At least uncertainty components over 0.1% should be included in the budget

• Uncertainty components below 0.1% should be evaluated and care should be taken that they remain under 0.1%


Uncertainty budget• Reference chamber calibration factor is usually the largest

contribution to the uncertainty

• Other components likely to be over 0.1%: reference chamber stability, radiation quality, beam uniformity, chamber positioning, electrometer calibration factor (if calibrated separately)


Uncertainty budget• Other components that should be evaluated: ambient

temperature and pressure, repeatability of standard and user dosimeter indication, resolution of user dosimeter, leakage, saturation, electrometer linearity…

• Combined uncertainty below 3% (k=2, p=0.95) should be achievable


Proving competence• Audits• Intercomparisons (in particular key and

supplementary comparisons)• Accreditation• CMCs in BIPM KCDB database


Intercomparisons• Key and supplementary comparisons are usually

required to publish CMCs in KCDB• These comparisons are available to NMIs and DIs

only• The frequency of these comparisons is once in 5 or

10 years


Intercomparisons• IAEA organizes bilateral comparisons for SSDL

network members (these comparisons can be used to support CMCs!)

• Bilateral or multilateral comparisons can be organized between SSDLs


Basic calibration procedure


• Calibration is usually performed in one of the three ways:• calibration by substitution• calibration by substitution with monitor• calibration using reference monitor chamber

Basic calibration procedure


• Usually a distance of 1 meter is used• Field size should be large enough to irradiate the wholedetector, but small enough to minimize scatter

• Dosimeter reference point and orientation are usuallystated by the manufacturers

• Some user dosimeters are sensitive to change in ambientconditions

User dosimeter settings


• Settings should be stated in calibration certificate (e.g.ionization chamber voltage and polarity)

• Some semiconductor detectors’ indications are sensitiveto device settings (e.g. anode material, high voltage etc.)

• Special care should be taken to match detector settings tocalibration conditions

User dosimeter settings


• Software for multimeters can be very complicated• Some settings can be ambiguous• Some software applications are (partially) black box,causing different indications when using differentapplications

• In some cases, user should be consulted on theappropriate settings

Monitor chamber


• Unsealed transmission monitorchambers should be used

• The chamber should be largerthan the largest beam size

• Monitor chamber should add aslittle filtration as possible

• Leakage current should be under2% of the maximum indication ofthe most sensitive current range

Monitor chamber


Substitution method with monitor• monitor chamber is used to correct for variations of X-ray output• this is the method of choice when high accuracy and low measurement uncertainty are required• use of two thermometers is recommended (if you use one, you don’t need to perform correction for temperature and pressure!) … = …

𝑄𝑄ref𝑇𝑇𝑃𝑃0𝑇𝑇0𝑃𝑃

𝑄𝑄T𝑇𝑇𝑃𝑃0𝑇𝑇0𝑃𝑃

Monitor chamber


Reference monitor chamber

• Monitor chamber can be calibrated in terms of air kerma and used as a reference chamber

IAEA TRS 457: 2007

Monitor chamber


Reference monitor chamber• monitor chambers are very sensitive to changes in

radiation energy, so this influence should be estimated• monitor chambers are basically operating as KAP-meters,

so increasing the beam size increases the signal• calibration factor for monitor chamber is valid only for

one specific distance

Monitor chamber


Reference monitor chamber

• reproducibility of the setup for user instrument calibration (moving the filter wheel, moving the collimators etc.) introduces additional uncertainty

• dose rate can be different then during monitor chamber calibration (additional uncertainty is introduced)

Monitor chamber


Other uses of monitor chambers

• Irradiation of passive dosimeters• Indication of beam on-off status• Indication of the shutter position• Quality control

KAP-meter calibration


• Kerma area product is defined as the air-kermaintegral over the irradiated area in X-ray beam in theplane perpendicular to the beam axis

• In homogenous beam, PKA = KaA

𝑃𝑃KA = �𝐴𝐴

𝐾𝐾(𝑥𝑥,𝑦𝑦)dxdy



• Additional collimator is required for calibrations of KAP-meters

• Circular or square collimator with diameter between 40 mm and 60 mm should be used

• Suitable distance from the focus for collimator positioning is 950 mm



• Air kerma is measured in the reference point WITH collimator 4 in place

• Field size is calculated based on the collimator 4 size and collimator and detector distances from the focus



• KAP-meters can be calibrated for transmitted radiation, in which case KAP-meter is positioned in position 1 during reference measurements

• If a KAP-meter is calibrated for incident radiation, reference measurements are done without KAP-meter in position 1

• In both cases, measurements with user KAP-meter are performed in position 2



• How to know when to calibrate KAP-meter for transmitted and when for incident radiation?

• KAP-meters permanently installed on X-ray units should be calibrated for transmitted radiation

• KAP-meters used for in-situ calibrations of other KAP-meters should be calibrated for incident radiation

• KAP-meters used for patient dosimetry and QA/QC – depending on the mode of use



• Calibration factor is practically calculated according to the following formula (example for circular collimator):

𝑁𝑁P =𝐾𝐾a𝑟𝑟2π

𝑑𝑑r𝑑𝑑c

2

𝑀𝑀𝑘𝑘d𝑘𝑘T

where Ka is reference air kerma value, r is the radius of the collimator, drand dc are distances of reference point and collimator (usually 100 cm and 95 cm), M is indication of the KAP-meter, kd is correction for air density and kT correction for indication of transmission monitor chamber.



• Further uncertainty components should beconsidered:

• collimator distance• collimator dimensions• beam uniformity (more important than in airkerma calibration)

CT-chamber calibration


• Kerma length product is defined as the air-kerma integral overthe line of length L:

𝑃𝑃KL = �𝐿𝐿

𝐾𝐾(𝑧𝑧)dz

• In homogenous beam, PKL = KaL



• Additional collimator K4 is positioned, usually at 950 mm

• Collimator width should be approximately 2 CT-chamber diameters, and length between 20 and 50 mm

• Field size is calculated based on the K4 length and collimator and detector distances from the focus



• Reference measurements are performed WITHOUT K4

• Collimator is positioned only for CT-chamber measurements



• Calibration factor is practically calculated according to the following formula:

𝑁𝑁P =𝐾𝐾a𝑙𝑙

𝑑𝑑r𝑑𝑑c

𝑀𝑀𝑘𝑘d𝑘𝑘T

where Ka is reference air kerma value, l is the length of the collimator along the CT-chamber axis, dr and dc are distances of reference point and collimator (usually 100 cm and 95 cm), M is indication of the CT-chamber and kT correction for indication of transmission monitor chamber.

X-ray tube high voltage• High voltage generator

generates a large potential difference between cathode (C) and anode (A)

• Electrons accelerate on their way to the anode, gaining kinetic energy of eU [keV]


http://commons.wikimedia.org/wiki/File:Xraytubeinhousing_commons.png

X-ray tube high voltage• High voltage is used for

beam characterization… but high voltage is usually not constant

• Ideally, a single number could be used for high voltage, and this number would be meaningful for purposes of beam characterization


http://commons.wikimedia.org/wiki/File:Xraytubeinhousing_commons.png

X-ray tube high voltage• X-ray generators

produce different waveforms

• How to compare the output of different X-ray machines? How does the high voltage translate into dose rate or radiological image?


IAEA TRS 457:2007 , pp. 257

X-ray tube high voltage• Voltage ripple can be as high as 100 %• If voltage ripple is less than 2 %, the high voltage generator

is considered constant potential

• According to IAEA TRS 457, voltage ripple should be less than 10 % for conventional applications and less than 4 % for mammography


PPV definition• Based on the contrast produced by 1 mm thick aluminum on 10 cm

PMMA phantom irradiated by “reference” X-ray tube

• Contrast decreases monotonously with constant potential voltage, so relation between contrast and PPV is unambiguous

• PPV does not depend strongly on total filtration, anode angle and contrast configuration

• PPV is the value of the high voltage of constant potential X-ray tube that would produce the same contrast as the tube under consideration


PPV measurement


• Û – practical peak voltage

• p(Ui) is the probability of occurence of the voltage Ui(Ui - ΔU/2, Ui + ΔU/2)

• w(Ui) is the weighting function

PPV measurement


• w(Ui) is approximated by the following formulas:

a) for Ui < 20 kV, w(Ui) = 0

b) for 20 kV ≤ Ui < 36 kV, w(Ui) = exp(aUi2 + bUi + c)

c) for 36 kV ≤ Ui , w(Ui) = dUi4 + eUi3 + fUi2 + gUi + h

IEC 61676: 2002

PPV measurement


• for mammography beams, w(Ui) is approximated by the following formula:

w(Ui) = exp(kUi4 + lUi3 + mUi2 + nUi + o)

IEC 61676: 2002

PPV measurement


• Û can be measured in three ways:

• invasively

• non-invasively by analyzing wave function

• non-invasively with a calibrated PPV meter

0

10

20

30

40

50

60

70

80

0 5 10 15 20

U (k

V)

Which is the preferred method?


• Û can be measured in three ways:

• invasively

• non-invasively by analyzing wave function

• non-invasively with a calibrated PPV meter

0

10

20

30

40

50

60

70

80

0 5 10 15 20

U (k

V)

PPV calibration


• Reference value divided by user instrument indication• Usually no corrections• How to treat measurement uncertainty?• When non invasive devices are used, usually only the uncertainty of reference instrument calibration factor is considered

Conclusions


• Consider the needs of your (potential) users before you start establishing calibrations of DR dosimeters• Only air kerma calibrations are necessary, but many users will request calibrations in other quantities• Use any equipment that you already have

Conclusions


• Monitor chambers can help you in many ways and their use is recommended• Calibration procedure is very similar to procedures in radiation protection and radiotherapy• Radiological multimeters based on semiconductor detectors can make you problems

Where to find more information?


• IAEA TRS 457: 2007

• IEC standards

Thank you for your attention!

Establishment of diagnostic radiology calibrations in an SSDLBackgroundBackgroundInternational measurement systemIAEA/WHO SSDL networkCIPM Mutual Recognition ArrangementAbbreviationsImportant literatureImportant literatureImportant literatureWhich services should an SSDL provide?Which services should an SSDL provide?Which services should an SSDL provide?Which services should an SSDL provide?Checklist – what will you need to establish calibrationsChecklist – what will you need to establish calibrationsFinding usersExisting equipment and facilitiesFacilitiesFacilitiesFacilitiesFacilitiesX-ray unitX-ray unitX-ray unitX-ray unitRadiation qualitiesRadiation qualitiesRadiation qualitiesDetermination of HVLsDetermination of HVLs – general radiographyDetermination of HVLs - mammographySecondary standardsSecondary standardsSecondary standardsQMSUncertainty budgetUncertainty budgetUncertainty budgetProving competenceIntercomparisonsIntercomparisonsBasic calibration procedureBasic calibration procedureUser dosimeter settingsUser dosimeter settingsMonitor chamberMonitor chamberMonitor chamberMonitor chamberMonitor chamberMonitor chamberKAP-meter calibrationKAP-meter calibrationKAP-meter calibrationKAP-meter calibrationKAP-meter calibrationKAP-meter calibrationKAP-meter calibrationCT-chamber calibrationCT-chamber calibrationCT-chamber calibrationCT-chamber calibrationX-ray tube high voltageX-ray tube high voltageX-ray tube high voltageX-ray tube high voltagePPV definitionPPV measurementPPV measurementPPV measurementPPV measurementWhich is the preferred method?PPV calibrationConclusionsConclusionsWhere to find more information?Thank you for your attention!

Documents

Establishment of diagnostic radiology calibrations in an SSDL lecture on...• Third step: 1 st and 2 nd HVL are determined by interpolation between 3 data points very close to the