RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY - PowerPoint PPT Presentation

1 / 40
About This Presentation
Title:

RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

Description:

... conventional radiography: beam, collimation. Beam energy ... Collimation. Area exposed should be limited to area of CLINICAL interest to ... Collimation or ... – PowerPoint PPT presentation

Number of Views:537
Avg rating:3.0/5.0
Slides: 41
Provided by: iaea4
Category:

less

Transcript and Presenter's Notes

Title: 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!

10
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

11
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

12
Part 10 Patient dose assessment
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology
  • Topic 2 Patient dosimetry methods

13
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
14
Patient dosimetry
  • Radiography entrance surface dose ESD
  • By TLD
  • Output factor
  • Fluoroscopy Dose Area Product (DAP)
  • CT
  • Computed Tomography Dose Index (CTDI)

15
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)

16
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

17
Part 10 Patient dose assessment
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology
  • Topic 3 Instrument calibration

18
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
19
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
20
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)
21
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)
22
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
24
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
25



Accuracy and precision of a calibrated instrument
(2)
26
Requirements on Diagnostic dosimeters
Traceability
Well defined reference X Ray spectra not
available
Accuracy
At least 10 - 30
27
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
28
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

29
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

30
Measurements of Radiation Output
X Ray tube
Filter
SDD
Ion. chamber
Lead slab
Table top
Phantom (PEP)
31
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

32
Dose Area Product (DAP)
Transmission ionization chamber
33
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
34
Calibration of a Dose Area Product (DAP)
35
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
36
Computed Tomography Dose Index (CTDI)
CTDI
Dose
Dose profile
Nominal slice width
37
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
38
Measurement of entrance surface dose
TLD
39
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.

40
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
Write a Comment
User Comments (0)
About PowerShow.com