Transcript Quality control and patient dosimetry in diagnostic

DOSIMETRY FOR MEDICAL APPLICATION
OF IONIZING RADIATIONS:
Calibration requirements and clinical
applications
Vinca Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
www.vinca.rs
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
[email protected]
Content
 Metrology and calibration requirements
 Clinical application
Vinca Institute of Nuclear Sciences
Radiation and Environmental Protection Laboratory
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 Global trends in medical exposures
 Dosimetric quantities and units
 Dosimetry in diagnostic radiology
Medical exposure to ionizing radiation

Medical exposure
contributes 99% of
man-made
radiation exposure
to humans
The concept of risk
is used to quantify
possible
detrimental effects
0.005
0.002
0.002
0.005
mSv
Medical
Nuclear weapons
Occupational
Chernobyl
0.61
Atmospheric nuclear tests
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
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
Medical exposure to ionizing radiation
The role of dosimetry is to determine the
amount of radiation received by a person
from the radiological examination
Dose?
 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
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Dosimetry in diagnostic radiology
Global trend
 3,6 billion radiological examinations in the
period 1997-2007
 Significant increase of CT practice:
 Examination frequency
 Dose per examination
 Interventional procedures
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 Increase of 50% compared to previous decade
Dose to patient
0.02- 0.05 mSv
2 mSv
 100CxR
5-20 mSv
400- 1000 CxR
50 chest radiographies= annual natural background radiation dose
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 Depends on the examination type
 Variations for the same type of procedure
 Dose for the same
examination type varies up
to 2 orders of magnitude
 Increased utilization of
high-dose procedures
 CT
 Interventional
procedures
Effects
 Increase of probability for
stochastic effects, in
particular in the case of the
repeated examinations
 Possible radiation injuries
in high-dose procedures
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Problems
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Radiation injuries
ICRP 85
10
 International Measurements
System (IMS)
 Framework for dosimetry in
diagnostic radiology
 Consistency in radiation
dosimetry
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Basic metrology elements
 Bureau International des Poids et Mesures
(BIPM)
 National Primary Standard Dosimetry
Laboratories (PSDL)
 Secondary Standards Dosimetry Laboratories
(SSDL)
 Users performing measurements
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International Measurements System
(IMS)
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Traceability chain
 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
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Metrology and traceability
 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
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Role of the SSDL
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Journal of the ICRU Vol 5
No 2 (2005) Report 74
Dosimetric quantities in units in
diagnostic radiology
 Basic dosimetric quantity: Air kerma
 Easy to measure
 Calibration:
 Clinical application:
 Quantities derived from air kerma for different
imaging modalities
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 Dosimeters calibrated in terms of air kerma
Dosimetric quantities
 Quantities for risk assessment
 Conversion coefficient for tissue and organ
dose assessment
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 Basic dosimetric quantities
 Application specific dosimetric quantities
 Energy fluence
Unit:J/m2
dR

da
 Kerma
Unit:J/kg, Gy
 tr 
dEtr
K
  
dm
  
 Absorbed dose
Unit:J/kg, Gy
 en 
d

D
 
dm
  
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Basic dosimetric quantities
Basic dosimetric quantities
 en
K  D  
 

 tr
  

 



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 Charged-particle equilibrium
 Absence of bremsstrahlung losses
Application specific dosimetric
quantities
Symbol
Unit
Equation
Incident air kerma
Ki
Gy
Entrance -surface
air kerma
Ke
Gy
Ke  Ki  B
Air-kerma area
product
PKA
Gym2
PKA   K(x , y)dxdy
A
Air-kerma length
product
PKL
X-ray tube output
Y(d)
Gym
PKL   Kair (z)dz
L
Gy/As
Y (d)  Ka (d) / PIt
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Quantity
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Application specific dosimetric
quantities: computed tomography
Application specific dosimetric
quantities: computed tomography
Symbol
Unit
50
CT air-kerma index
Ca,100
(free in air)
Gy
CT air-kerma index
(in standard
CPMMA, 100
phantom)
Gy
Weighted CT air
kerma index
Normalized
weighted CT air
kerma index
Air-kerma length
product
Cw
Equation
Gy
C a ,100
1

K (z)dz
T 50
1
CW  C PMMA ,100 ,c  2C PMMA ,100 ,p 
3
CVOL  CW
nCw
Gy/As
n C VOL 
PKL
Gym
NT
C
 W
I
p
CVOL
PI t
PKL   n CVOLj l j PItj
j
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Quantity
Quantities describing risk
 Dose-conversion
coefficients for
assessment of organ
and tissue doses
c
dosimetric quantity
normalisat ion quantity
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 Organ and tissue dose
 Equivalent dose
 Effective dose
The Use of Effective Dose (E)
 E is a risk-related quantity and should only be used in the
low-dose range
 Primary use:
 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
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 to demonstrate compliance with dose limits
 in regulation, for prospective planning of radioprotection
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.
 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
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 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)
Dosimeters in diagnostic radiology
Ionization chambers
Semiconductor dosimeters Others
Accurate
Compact
TLD
Good energy dependence
Energy dependant
OSL
Design for different
application (cylindrical,
parallel-plate, different
volumes..)
PSDL/SSDL
Film (radiochromic)
Scintillation
(kVp meters)
user
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Tube voltage 20-150 keV, various A/F combinations,
various modalities
 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
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Dosimetry standards in diagnostic
radiology
User
 IEC 61674
 Ionization chambers
 Semiconductor
detectors
SSDL
 Ionization chamber of
reference class
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Requirements for dosemeters
 Air kerma:
 Radiography and mammography
 Kerma-length product
 Dosimeters in CT
 Kerma-area product
 Radiography and fluoroscopy
 PPV: kVp meters
 Frequency: according to national
regulations
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Calibrations in diagnostic radiology
Calibration in diagnostic radiology
 Calibrated
 Quality control
 Traceability for all beam qualities
 Auxiliary equipment: electrometers,
thermometers, barometers…
 Environmental conditions
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 SSDL with relevant measurement capabilities
 General requirements: beam qualities, tube
voltage and filtration measurements
 Dosimeter of reference class (with
electrometer)
Dosimetry
Radiation source
 Ionization chambers
 Position system
 HV supply for monitor and
reference class ionization
chamber
 Electrometer
 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%)
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Equipment
Reference class dosemeter
Range tube
voltage (kV)
Intrinsic
uncertainty
(k=2)
Maximum variation
of response (%)
Unattenuated
beam
General
radiography
cylindrical or
plane parallel
60-150
3.2
±2.6
Fluoroscopy
cylindrical or
plane parallel
50-100
3.2
±2.6
Mammography
plane parallel
22-40
3.2
±2.6
10 μGy/s10 mGy/s
CT
cylindrical
100-150
3.2
±2.6
0.1 mGy/s50 mGy/s
Dental
radiography
cylindrical or
plane parallel
50-90
3.2
±2.6
1 μGy/s10 mGy/s
1 mGy/s500 mGy/s
Attenuated
beam
10 μGy/s5 mGy/s
0.1 μGy/s100 μGy/s
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Application
Type of
chamber
Range of air
kerma rate
 Spectrum
 X-ray beam quality:
 First half-value layer (HVL1)
 Second half-value layer
(HVL2)
 Homogeneity coefficient:
h
 Tube voltage
 Total filtration
HVL1
HVL2
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Specification of the x-ray beam
Radiation quality
RQR
Radiation origin
Unfiltered beam
emerging from x-ray
assembly
Phantom material
No phantom
Application
General radiography,
fluoroscopy, dental
radiology
Measurements
behind the patient
(on the image
intensifier)
RQA
Radiation beam from
Aluminium
an added filter
RQT
Radiation beam from
Copper
an added filter
CT applications (free
in air)
RQR-M
Unfiltered beam
emerging from x-ray
assembly
Mammography (free
in air)
RQA-M
Radiation beam from
Aluminium
an added filter
No phantom
Measurements
behind the patient
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Radiation beam qualities (IEC 61267)
Typical calibration set up
Apertures
Focal
spot
Test point
Shutter
Additional
filtration
Monitor
chamber
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X-ray tube
window
 Procedures before calibration (acclimatization, positioning,
stabilization…)
 Calibration procedures (methods, number of
measurements, interval between
measurements…corrections…
 Procedures following calibration (uncertainty budget,
certificate…)
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Calibration procedures
Dosimetry formalism
 Air kerma:
K  M Q  M 0 N K ,Q0
 Reference conditions: set of influencing quantities
K  M Q  M 0 N K ,Q0  ki
 Influencing condition: quantities that are not subject of
mesusremst but have an impact on the result
P 273.15  T
 Air density correction:
kTP  0
P 273.15  T0
 Beam quality correction:
KQ  M Q  N K ,Q0  kQ,Qo
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i
Calibrations of dosemeters for CT
 Information on chamber response only
 Size on active volume only assumed
 Far from real situation
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 Traditionally, irradiation of the whole volume
 Contras:
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)
Focal
spot
Aperture
Monitor
chamber
Ionization
chamber
w
da
dr
N PKL ,Q  K  w / M 
dr
da
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Calibration for fluoroscopy: air
kerma area product
N PKA ,Q 
M KAP
Ref.
chamber
Film
10 cm
Cekerevac at al, Poster B3
10 cm
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 In laboratory (SSDL)
 Field calibration
M ref N Kref,Qo kQ Anom
Calibration in terms of practical peak
voltage
 X-ray tube voltage
measurements
 Practical Peak Voltage
(ppv):
 Invasive or non-invasive
measurements
 Voltage divider
Uˆ 
 pU U wW 
i 1
n
i
i
i
 pU wU 
i 1
i
i
 Property of the whole
exposure cycle
 Related to image contrast
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n
Uncertainty budget
Uncertainty of the reference standard
Uncertainty of user’s instrument
Uncertainty due to calibration set up
Uncertainty of the evaluation procedure
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Air kerma: ± 2.7 %
Air kerma length product: ± 3.0 %
Air kerma area product: ± 15 %
Non-invasive tube voltage measuring devices:
2.5 %
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Uncertainty budget
 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
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Quality Management System
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Patient dose assessment
Clinical dosimetry
 Output of the X-ray tube,
scaled for exposure and
geometry
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 Direct measurement on
patients or phantoms
 Indirect measurements
on patients or phantoms
Quantities
a) incident air kerma, entrance
surface air kerma and kermaarea 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)
KAP
BSF
Ke
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Dosimetric quantites
Patient
 Real situation
Phantoms
 Objects that simulate real
patients in terms of
interaction of radiation
with matter
 Easy to perform
 Standardized
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Patients and phantoms
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Radiography
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Fluoroscopy
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Mammography
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Computed tomography
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Patient dose levels
Uncertainty of clinical dose
assessment
 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
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 Radiographers taking the
x-ray images
 Determining tube output
 Calculation of individual
patient doses
 Determining dose to an
average patient
Form measurements towards risk
assessment
 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
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 Conversion coefficients
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Organ dose assesment
ICRU 74
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Optimization of protection
Application of DRLs
 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!
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 Values of measured quantities above which some specified
action or decision should be taken
 Values must be specified
 Action must be specified
Doses to patients from radiographic and fluoroscopic X-ray imaging procedures
in the UK—2005 review. HPA RPD-029, HPA; 2007.
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Diagnostic Reference Levels
 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
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Re-cap
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
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