RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

1
RADIATION PROTECTION INDIAGNOSTIC
ANDINTERVENTIONAL RADIOLOGY
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• L10 Patient dose assessment

2
Introduction

• A review is made of
• The different parameters influencing the patient
  exposure
• The problems related to instrument calibration
• The existing dosimetric methods applicable to
  diagnostic radiology

3
Topics

• Parameters influencing patient exposure
• Dosimetry methods
• Instrument calibration
• Dose measurements

4
Overview

• To become familiar with the patient dose
  assessment and dosimetry instrument
  characteristics.

5
Part 10 Patient dose assessment
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 1 Parameters influencing the patient
  exposure

6
Essential parameters influencing patient exposure

Tube voltage Tube current Effective filtration
Kerma rate mGy/min

Kerma ?Gy

Exposure time
min
Area exposure product ?Gy m2
Field size
m2
7
Factors in conventional radiography beam,
collimation

• Beam energy
• Depending on peak kV and filtration
• Regulations require minimum total filtration to
  absorb lower energy photons
• Added filtration reduces dose
• Goal should be use of highest kV resulting in
  acceptable image contrast
• Collimation
• Area exposed should be limited to area of
  CLINICAL interest to lower dose
• Additional benefit is less scatter, netter
  contrast

8
Factors in conventional radiography grid,patient
size

• Grids
• Reduce the amount of scatter reaching image
  receptor
• But at the cost of increased patient dose
• Typically 2-5 times Bucky factor or grid ratio
• Patient size
• Thickness, volume irradiatedand dose increases
  with patient size
• Except for breast (compression) no control
• Technique charts with suggested exposure factor
  for various examinations and patient thickness
  helpful to avoid retakes

9
Factors affecting dose in fluoroscopy

• Beam energy and filtration
• Collimation
• Source-to-skin distance
• Inverse square law maintain max distance from
  patient
• Patient-to-image intensifier
• Minimizing patient-to- I I will lower dose
• But slightly decrease image quality by increased
  scatter
• Image magnification
• Geometric and electronic magnification increase
  dose
• Grid
• If small sized patient (les scatter) perhaps
  without grid
• Beam-on time!

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Factors affecting dose in CT

• Beam energy and filtration
• 120-140 kV shaped filters
• Collimation or section thickness
• Post-patient collimator will reduce slice
  thickness imaged but not the irradiated thickness
• Number and spacing of adjacent sections
• Image quality and noise
• Like all modalities dose increasegtnoise
  decreases

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Factors affecting dose in spiral CT

• Factors for conventional CT also valid
• Scan pitch
• Ratio of couch travel in 1 rotation dived by
  slice thickness
• If pitch 1, doses are comparable to
  conventional CT
• Dose proportional to 1/pitch

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Part 10 Patient dose assessment
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 2 Patient dosimetry methods

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How to measure doses
Calorimetry
Chemical (Fricke dosimeter)
They need to know a characteristic parameter
Absolute methods
Ionometry (ionization chamber)
Photography
Scintillation
Relative methods
TL
Ionometry
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Patient dosimetry

• Radiography entrance surface dose ESD
• By TLD
• Output factor
• Fluoroscopy Dose Area Product (DAP)
• CT
• Computed Tomography Dose Index (CTDI)

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From ESD to organ and effective dose

• Except for invasive methods, no organ doses can
  be measured
• The only way in radiography measure the Entrance
  Surface Dose (ESD)
• Use mathematical models to estimate internal
  dose.
• The physical methods similar to those used in
  radiotherapy can be used but not accurate
• Mathematical models based on Monte Carlo
  simulations the history of thousands of photons
  is calculated
• Dose to the organ tabulated as a fraction of the
  entrance dose for different projections
• Since filtration, field size and orientation play
  a role long lists of tables (See NRPB R262 and
  NRPB SR262)

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From DAP to organ and effective dose

• In fluoroscopy moving field, measurement of
  Dose-Area Product (DAP)
• In similar way organ doses calculated by Monte
  Carlo modelling
• Based on mathematical model
• Conversion coefficients were estimated as organ
  doses per unit dose-area product
• Again numerous factors are to be taken into
  account as projection, filtration,
• Once organ doses are obtained, effective dose is
  calculated following ICRP60

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Part 10 Patient dose assessment
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 3 Instrument calibration

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Calibration of an instrument

• Establish Calibration Reference Conditions (CRC)
  type and energy of radiation, SDD, rate, ...
• Compare response of your instrument with that of
  another instrument (absolute or calibrated)
• Get the calibration factor

f the reference instrument
Response o
appropriate unit

F
Response of the instrument to be calibrated
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Range of use
Hypothesis the instrument reading is a known
monotonic function of the measured quantity
(usually linear within a specified range)
Instrument Reading
1/F tg ?
Response at calibration
?
MeasuredQuantity
Calibration Value
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Use of a calibrated instrument

• Under the same conditions as the CRC
• Within the range of use

Q (dosimetric quantity) F x R (reading of the
instrument)
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Correction factors for use other than under the
CRC
A. Energy correction factor
Correction
Factor
1.06
1.04
1.02
1
0.98
0.96
0.94
0.92
1
2
3
4
HVL(mm Al)
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Correction factors for use other than under the
CRC
B. Directional correction factor
23
Correction factors for use other than under the
CRC
C. Air density correction factor (for ionization
chambers)

t
p
)
273
(

0
K
D

t
p
)
273
(
0
p
t
,
calibration values
0
0
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Accuracy and precision of a calibrated instrument
(1)
A
C
Readings
B
True value
Curve A Instrument both accurate and
precise Curve B Instrument accurate but not
precise Curve C Instrument precise but not
accurate
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Accuracy and precision of a calibrated instrument
(2)
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Requirements on Diagnostic dosimeters
Traceability
Well defined reference X Ray spectra not
available
Accuracy
At least 10 - 30
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Limits of error in the response of diagnostic
dosimeters
Parameter
Range of values
Reference condition
Deviation ()
Radiation quality
According to manufacturer
70 kV
5-8
Dose rate
According to manufacturer
--
4
Direction of radiation incidence
5
Preference direction
3
Atmospheric pressure
80-106 hPa
101.3 hPa
3
Ambient temperature
15-30
20 C
3
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Part 10 Patient dose assessment
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 4 Dose measurements how to measure dose
  indicators ESD, DAP,CTDI

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What we want to measure

• The radiation output of X Ray tubes
• The dose-area product
• The computed-tomography dose index (CTDI)
• Entrance surface dose

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Measurements of Radiation Output
X Ray tube
Filter
SDD
Ion. chamber
Lead slab
Table top
Phantom (PEP)
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Measurements of Radiation Output

• Operating conditions
• Consistency check
• The output as a function of kVp
• The output as a function of mA
• The output as a function of exposure time

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Dose Area Product (DAP)
Transmission ionization chamber
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Dose Area Product (DAP)
0.5 m
1 m
2 m
Air Kerma Area Areaexposure product
2.5103 ?Gy 4010-3 m2 100 ?Gy m2
40103 ?Gy 2.510-3 m2 100 ?Gy m2
10103 ?Gy 1010-3 m2 100 ?Gy m2
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Calibration of a Dose Area Product (DAP)
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Computed Tomography Dose Index (CTDI)
TLD dose (mGy)
50
Nominal slice width
3 mm
40
30
CTDI 41.4
20
10
0
1
2
3
4
5
6
7
8
9
10
11
12
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Computed Tomography Dose Index (CTDI)
CTDI
Dose
Dose profile
Nominal slice width
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TLD arrangement for CTDI measurements
X Ray beam
Gantry
Support jig
X Ray beam
Capsule
Axis of rotation
axis of rotation
Capsule
Couch
Gantry
LiF -TLD
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Measurement of entrance surface dose
TLD
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Summary

• In this lesson we learned the factors influencing
  patient dose, and how to have access to an
  estimation of the detriment through measurement
  of entrance dose, dose area product or specific
  CT dosimetry methods.

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Where to Get More Information

• Equipment for diagnostic radiology, E. Forster,
  MTP Press, 1993
• The Essential Physics of Medical Imaging,
  Williams and Wilkins. Baltimore1994
• Leitz, W., Axiesson, B., Szendro, G. Computed
  tomography dose assessment - a practical
  approach. Nuclear Technology 37 1-4 (1993) 377-80