DOSIMETRY FOR MEDICAL APPLICATION OF IONIZING RADIATIONS: Calibration requirements and clinical applications

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DOSIMETRY FOR MEDICAL APPLICATION
OF IONIZING RADIATIONS:

Calibration requirements and clinical
applications

Olivera Ciraj
-
Bjelac
, Milojko Kovacevic, Danijela Arandjic, Djordje Lazarevic

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Department

Laboratory for Radiation Measurements

Belgrade, Serbia

ociraj@vinca.rs



Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Content


Global trends in medical exposures


Dosimetric quantities and units


Dosimetry in diagnostic radiology


Metrology and calibration requirements


Clinical application


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs


Medical exposure
contributes 99% of
man
-
made
radiation exposure
to humans


The concept of risk
is used to quantify
possible
detrimental effects


Medical exposure to ionizing radiation

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Total dose from man
-
made sources of
radiation>
0.61
mSv

Medical:
0.6
mSv


(>
99.97%)

Source:
United Nations Scientific Committee for Effect of Atomic Radiation (UNSCEAR), 2010


Dose?


Medical exposure to ionizing radiation

The role of dosimetry is to determine the
amount of radiation received by a person
from the radiological examination

Dosimetry in diagnostic radiology


Patient dose assessment


Establishment of Diagnostic Reference Levels
(DRL), optimisation of protection


Assessment of x
-
ray equipment performance



Standards of good practice


Assessment of radiation detriment


Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Global trend


3,6 billion radiological examinations in the
period 1997
-
2007


Increase of 50% compared to previous decade


Significant increase of CT practice:


Examination frequency


Dose per examination


Interventional procedures





Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs




Depends on the examination type


Variations for the same type of procedure

0.02
-

0.05
mSv


2
mSv




100CxR

5
-
20
mSv

400
-

1000
CxR


Dose to patient

50
chest radiographies
=
annual natural background radiation dose

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Proble
ms


Dose for the same
examination type varies up
to 2 orders of magnitude


Increased utilization of
high
-
dose procedures


CT


Interventional
procedures


Increase of probability for
stochastic effects, in
particular in the case of the
repeated examinations


Possible radiation injuries
in high
-
dose procedures


Ef
fects

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Radiation injuries

10

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

ICRP 85

Basic metrology elements


International Measurements
System (IMS)


Framework for dosimetry in
diagnostic radiology


Consistency in radiation
dosimetry


Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

International Measurements System
(IMS)



Bureau International des
Poids

et
Mesures

(BIPM)


National Primary Standard Dosimetry
Laboratories (PSDL)


Secondary Standards Dosimetry Laboratories
(SSDL)


Users performing measurements

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Traceability chain

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Metrology and traceability


Dosimeters used to determine doses received
by individuals


Measurements need to be traceable though an
unbroken chain of comparisons
to national
and international standards


Traceability is needed to ensure accuracy and
reliability


Legal and economic implications


Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Role of the SSDL


The prime function:
to provide a service in
metrology


Designated by the competent national
authorities


SSDL
-
Secondary standards, calibrated against
the primary standards of laboratories
participating in the IMS

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Journal of the ICRU
Vol

5

No 2 (2005) Report 74


Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Dosimetric quantities in units in
diagnostic radiology


Basic dosimetric quantity: Air
kerma



Easy to measure


Calibration:


Dosimeters calibrated in terms of air
kerma


Clinical application:


Quantities derived from air
kerma

for different
imaging modalities



Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Dosimetric quantities


Basic dosimetric quantities


Application specific dosimetric quantities



Quantities for risk assessment



Conversion coefficient for tissue and organ
dose assessment

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs



Energy
fluence


Unit:J
/m
2



Kerma


Unit:J
/kg,
Gy



A
bsorbed

dose


Unit:J
/kg,
Gy


Basic dosimetric quantities

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Basic dosimetric quantities




Charged
-
particle equilibrium


Absence of
bremsstrahlung

losses

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Application specific dosimetric
quantities

Quantity

Symbol

Unit

Equation

Incident air
kerma

Ki

Gy

Entrance
-
surface
air
kerma

Ke

Gy

Air
-
kerma

area
product

P
KA

Gym
2

Air
-
kerma

length
product

P
KL

Gym

X
-
ray

tube output

Y(d)

Gy
/As

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Application specific dosimetric
quantities: computed tomography

Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Quantity

Symbol

Unit

Equation

CT air
-
kerma

index
(free

in air)

Ca,100

Gy

CT air
-
kerma

index
(in standard
phantom
)

C
PMMA
,

100

Gy

Weighted CT air
kerma

index

Cw

Gy

Normalized
weighted CT air
kerma

index


nCw

Gy
/As

Air
-
kerma

length
product

P
KL

Gym

Application specific dosimetric
quantities: computed tomography

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Quantities describing risk


Organ and tissue dose


Equivalent dose


Effective dose





Dose
-
conversion
coefficients for
assessment of organ
and tissue doses

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

The Use of Effective Dose (E)


E

is a risk
-
related quantity and should only be used in the
low
-
dose
range


Primary
use
:


to demonstrate compliance with dose limits


in regulation, for prospective planning of radioprotection


Not
for:


detailed retrospective dose and risk assessments after exposure
of individuals


epidemiological studies, neither in accidents.



In the last cases:
organ doses are needed
!



ICRP 103, ICRP 105

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Effective Dose in Medical Exposure


The relevant quantity for planning the exposure of patients and
risk
-
benefit assessments is the
equivalent dose or the absorbed
dose to irradiated tissues.


The assessment and interpretation of
E

is very problematic
when organs and tissues receive only partial exposure or a very
heterogeneous exposure (x
-
ray diagnostics
)


E
can be of value for comparing doses from


different diagnostic procedures


similar procedures in different hospitals and countries


different technologies for the same medical examination.


ICRP 103, ICRP 105

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Dosimeters in diagnostic radiology

Tube voltage 20
-
150
keV
, various A/F combinations,
various modalities

Ionization chambers

Accurate

Good energy dependence

Design for different
application (cylindrical,
parallel
-
plate, different
volumes..)

Semiconductor dosimeters

Compact

Energy dependant




Others

TLD

OSL

Film (
radiochromic
)

Scintillation

(
kVp

meters)


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

PSDL/SSDL

user

Dosimetry standards in diagnostic
radiology


IEC 61674: Dosimeters with
ionization chambers and/or
semi
-
conductor detectors as
used in X
-
ray diagnostic imaging


Diagnostic dosimeter:
detector and measuring
assembly


IEC 60580: Dose area product
meters

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Requirements for
dosemeters

User


IEC 61674


Ionization chambers


Semiconductor
detectors

SSDL


Ionization chamber of
reference class

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Calibrations in diagnostic radiology


Air
kerma
:


Radiography and mammography


Kerma
-
length product


Dosimeters in CT


Kerma
-
area product


Radiography and fluoroscopy


PPV:
kVp

m
eters



Frequency: according to national
regulations

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Calibration in diagnostic radiology

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs


SSDL
with relevant measurement capabilities


General requirements
:

beam qualities, tube
voltage and filtration measurements


Dosimeter of reference class (with
electrometer)


Calibrated


Quality control


Traceability for all beam qualities


Auxiliary equipment: electrometers,
thermometers, barometers…


Environmental conditions









Equipment

Dosimetry


Ionization chambers


Position system


HV supply for monitor and
reference class ionization
chamber


Electrometer


Radiation source


X
-
ray generator,
50
-
150
kVp
, 20
-
40
kVp


Ripple less than
10%
for
radiography and less than
4%
for mammography


Beam qualities according
IEC 61267



Shutter

mechanism


Filters and attenuators


Tube voltage meter
(
ppv
,
±
1.
5%)


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Application

Type of

chamber

Range tube

voltage (kV)

Intrinsic

uncertainty

(k=2)

Maximum variation

of response (%)

Range of air

kerma

rate

Unatte
-

nuated


beam

Attenuated

beam

General

radiography

cylindrical or

plane parallel

60
-
150

3.2

±
2.6

1
mGy
/s
-

500
mGy
/s

10
μGy
/s
-

5
mGy
/s

Fluoroscopy

cylindrical or

plane parallel

50
-
100

3.2

±
2.6

0.1
μGy
/s
-


100
μGy
/s

Mammogra
-

phy

plane parallel

22
-
40

3.2

±
2.6

10
μGy
/s
-

10
mGy
/s

CT

cylindrical

100
-
150

3.2

±
2.6

0.1
mGy
/s
-

50
mGy
/s

Dental

radiography

cylindrical or

plane parallel

50
-
90

3.2

±
2.6

1
μGy
/s
-

10
mGy
/s

Reference class dosemeter

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Specification of the x
-
ray beam


Spectrum


X
-
ray beam quality
:


First half
-
value layer
(HVL
1
)


Second half
-
value layer
(HVL
2
)


Homogeneity coefficient
:




Tube voltage


Total filtration




Vinca Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Radiation beam qualities (IEC 61267)

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Radiation quality

Radiation origin

Phantom material

Application

RQR

Unfiltered beam
emerging from x
-
ray
assembly

No phantom

General radiography,
fluoroscopy, dental
radiology

RQA

Radiation beam from
an added filter

Aluminium

Measurements
behind the patient
(on the image
intensifier)

RQT

Radiation beam from
an added filter

Copper

CT applications (free
in air)

RQR
-
M

Unfiltered beam
emerging from x
-
ray
assembly

No phantom

Mammography (free
in air)

RQA
-
M

Radiation beam from
an added filter

Aluminium

Measurements
behind the
patient

Typical calibration set up

X
-
ray tube
window

Focal

spot

S
hutter

A
pertures

Additional
filtration

Monitor
chamber

Test point

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Calibration procedures


Procedures before calibration (acclimatization, positioning,
stabilization…)


Calibration procedures (methods, number of
measurements, interval between
measurements…corrections…


Procedures following calibration (uncertainty budget,
certificate…)

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Dosimetry formalism


Air
kerma
:



Reference conditions
:

set of influencing quantities




Influencing condition
:
quantities that are not subject of
mesusremst but have an impact on the result


Air density correction
:


Beam quality correction
:

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Calibrations of
dosemeters

for CT


Traditionally, irradiation of the whole volume


Contras
:


Information on chamber response only


Size on active volume only assumed



Far from real situation


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Calibration for CT
:
air
kerma

length
product


Cylindrical chamber
, 100 mm


Non
-
uniform irradiation


Uniform response


RQT 9

(120
kVp
, HVL: 8.5 mm Al)



Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Fo
cal
spot

M
onitor

chamber

A
pertur
e

Ionization
chamber

w

d
a

d
r


In laboratory (SSDL)


F
ield

calibration


Film



10 cm


10 cm

Ref.
chamber

Calibration for fluoroscopy
:
air
kerma

area product

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Cekerevac

at al, Poster B3

Calibration in terms of practical peak
voltage


X
-
ray tube voltage
measurements


Practical

Peak

Voltage

(
ppv
):





Property of the whole
exposure cycle


Related to image contrast






Invasive or non
-
invasive
measurements


Voltage divider


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs


Uncertainty of the reference standard


Uncertainty of user’s instrument


Uncertainty due to calibration set up


Uncertainty of the evaluation procedure

Uncertainty budget

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Uncertainty budget


Air
kerma
:


±

2.7 %


Air
kerma

length product:
±

3.0 %


Air
kerma

area product:
±

15 %


Non
-
invasive tube voltage measuring devices:
2.5 %


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs


Goal: minimal uncertainty


Assurance and control of traceability


Quality manual: technical details, methods, traceability,
uncertainty budget, QC, safety….


Continuous improvements and reviews


External peer review/audit


Quality Management System

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Patient dose assessment


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Clinical dosimetry


Direct measurement on
patients or phantoms


Indirect measurements
on patients or phantoms


Output of the X
-
ray tube,
scaled for exposure and
geometry

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Dosimetric
quantites

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

BSF


KAP


Ke

Quantities

a)
incident air
kerma
, entrance
surface air
kerma

and
kerma
-
area product (radiography);

b)
kerma
-
area product and
entrance surface air
kerma

rate (fluoroscopy);

c)
incident and entrance surface
air
kerma

(mammography);
and

d)
kerma
-
length product
(computed tomography)

Patients and phantoms

Patient


Real situation

Phantoms


Objects that simulate real
patients in terms of
interaction of radiation
with matter


Easy to perform


Standardized


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Radiography

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Fluoroscopy

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Mammography

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Computed tomography

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Patient dose levels

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Uncertainty of clinical dose
assessment


Radiographers taking the
x
-
ray images


Determining tube output


Calculation of individual
patient doses


Determining dose to an
average patient






Use of k=2 for expression
of uncertainty of dose
assessment


Typically >10% and close
to 25%*


*if correction for beam quality and for individual patient is not applied

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Form measurements towards risk
assessment


Conversion coefficients


Conversion of measured quantity into organ doses
and effective dose


Ratio of the dose to a specified tissue or effective
dose divided by the normalization quantity


Measured using phantoms or calculated using
computer models


Voxel

phantoms based on images of human
anatomy

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Organ dose
assesment

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

ICRU 74


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Optimization of protection

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Application of DRLs


Values

of

measured

quantities

above

which

some

specified

action

or

decision

should

be

taken



Values

must

be

specified


Action

must

be

specified






DRLs

will

be

intended

for

use

as

a

convenient

test

for

identifying

situations

where

the

levels

of

patient


dose

are

unusually

high
.


Quantities

that

are

easily

measured!

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Diagnostic Reference Levels

Doses to patients from radiographic and fluoroscopic X
-
ray imaging procedures
in the UK

2005 review. HPA RPD
-
029, HPA; 2007.

Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs

Re
-
cap


Diagnostic radiology is major contribution to total dose
from man
-
made sources of radiation


Dose measurements: population dose assessment,
optimization of practice


Application
-
specific dosimetric quantities (patients,
phantoms)


Calibration of dosimeters in the conditions that are similar
to the clinical environment, in terms of


air
kerma


kerma
-
area product


kerma
-
length product


Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs


Olivera
Ciraj
-
Bjelac
,
PhD, research associate, dosimetry and radiation physics


Milojko Kovacevic,
MSc
, Head of MDL, radiation physicist


Danijela Arandjic,
MSc
, PhD student, dosimetry and radiation physics


Djordje Lazarevic,
MSc
, PhD student, dosimetry and radiation physics


Dragana
Divnic
, technician


Milos

Jovanovic
, technician


Nikola
Blagojevic

, technician



Vinca

Institute of Nuclear Sciences

Radiation and Environmental Protection Laboratory

www.vinca.rs