Radiation protection in diagnostic and interventional radiology
Download
1 / 44

RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY - PowerPoint PPT Presentation


  • 71 Views
  • Uploaded on

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology. RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY. L10: Patient dose assessment. Introduction. A review is made of: The different parameters influencing the patient dose

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY' - hedya


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Radiation protection in diagnostic and interventional radiology

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY

L10: Patient dose assessment


Introduction
Introduction and Interventional Radiology

  • A review is made of:

  • The different parameters influencing the patient dose

  • The problems related to instrument calibration

  • The existing dosimetric methods applicable to diagnostic radiology

10: Patient dose assessment


Topics
Topics and Interventional Radiology

  • Parameters influencing patient exposure

  • Dosimetry methods

  • Instrument calibration

  • Dose measurements

10: Patient dose assessment


Overview
Overview and Interventional Radiology

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

10: Patient dose assessment


Part 10 patient dose assessment

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 10: Patient dose assessment

Topic 1: Parameters influencing patient dose


Essential parameters influencing patient exposure
Essential parameters influencing patient exposure and Interventional Radiology

}

Tube voltage

Tube current

Effective filtration

Kerma rate

[mGy/min]

}

Kerma

[Gy]

}

Exposure time

[min]

Area exposure

product

[Gy m2 ]

Field size

[m2]

10: Patient dose assessment


Factors in conventional radiography beam collimation
Factors in conventional radiography: beam, collimation and Interventional Radiology

  • 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, better contrast

10: Patient dose assessment


Factors in conventional radiography grid patient size
Factors in conventional radiography: grid,patient size and Interventional Radiology

  • Grids

    • Reduce the amount of scatter reaching image receptor

    • But at the cost of increased patient dose

    • Improves image contrast significantly

    • Typically 2-5 times: “Bucky factor”

    • Patient size

    • Thickness, volume irradiated…and dose increases with patient size

    • Except for breast (compression): no control

    • Technique charts with technique factors for various examinations and patient thickness essential to avoid retakes

    • Also, patient thickness must be measured accurately to use technique charts properly

10: Patient dose assessment


Factors affecting dose in fluoroscopy
Factors affecting dose in fluoroscopy and Interventional Radiology

  • Beam energy and filtration

  • Collimation

  • Source-to-skin distance

    • Inverse square law: maintain max distance from patient

  • Patient-to-image intensifier

    • Minimizing patient-to-image intensifier distance will lower dose and improve image sharpness

10: Patient dose assessment


Factors affecting dose in fluoroscopy1
Factors affecting dose in fluoroscopy and Interventional Radiology

  • Image magnification

    • Geometric and electronic magnification increase dose

  • Grid

    • If small sized patient (less scatter) probably not needed

      • No need for grids on pediatric patients

      • Grids not necessary for high contrast studies, e.g., barium contrast studies

  • Beam-on time!

10: Patient dose assessment


Factors affecting dose in ct
Factors affecting dose in CT and Interventional Radiology

  • Beam energy and filtration

    • 80-100 kV reduces dose for pediatric patients

    • 120-140 kV with additional filtration reduces adult doses (HVL can be increased to reduce dose)

  • 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 increase=>noise decreases

10: Patient dose assessment


Factors affecting dose in spiral ct
Factors affecting dose in spiral CT and Interventional Radiology

  • 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

10: Patient dose assessment


Part 10 patient dose assessment1

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 10: Patient dose assessment

Topic 2: Patient dosimetry methods


Radiation dose measurement
Radiation Dose Measurement and Interventional Radiology

Ionization chamber measurements

Thermoluminescent dosimeters (TLDs)

Optically stimulated luminescent (OSL) dosimeters

Solid state dosimeters

Film (silver halide or radiochromic)


Patient dosimetry
Patient dosimetry and Interventional Radiology

  • Radiography: entrance surface dose ESD

    • By TLD or OSL

    • Output factor

  • Fluoroscopy: Dose Area Product (DAP) or using film

  • CT:

    • Computed Tomography Dose Index (CTDI)

    • Using pencil ion chamber, OSL, or TLD

10: Patient dose assessment


From esd to organ and effective dose
From ESD to organ and effective dose and Interventional Radiology

  • Except for invasive methods, no organ doses can be measured

  • The only way in radiography: measure the Entrance Surface Dose (ESD)

  • Use 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 projection play a role: long lists of tables (See NRPB R262 and NRPB SR262)

10: Patient dose assessment


From dap to organ and effective dose
From DAP to organ and effective dose and Interventional Radiology

  • In fluoroscopy: moving field, measurement of Dose-Area Product (DAP)

  • In similar way organ doses calculated by Monte Carlo modelling

  • 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 ICRP 103

10: Patient dose assessment


Part 10 patient dose assessment2

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 10: Patient dose assessment

Topic 3: Instrument calibration


Calibration of an instrument
Calibration of an instrument and Interventional Radiology

  • 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)

  • Determine the calibration factor

Response o

f the reference instrument

[appropriate unit]

=

F

Response of the instrument to be calibrated

10: Patient dose assessment


Range of use
Range of use and Interventional Radiology

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

10: Patient dose assessment


Use of a calibrated instrument
Use of a calibrated instrument and Interventional Radiology

  • Under the same conditions as the CRC

  • Within the range of use

Q (dosimetric quantity)= F x R (reading of the instrument)

10: Patient dose assessment


Correction factors for use other than under the crc
Correction factors for use other than under the CRC and Interventional Radiology

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)

10: Patient dose assessment


Correction factors for use other than under the crc1
Correction factors for use other than under the CRC and Interventional Radiology

B. Directional correction factor

10: Patient dose assessment


Correction factors for use other than under the crc2
Correction factors for use other than under the CRC and Interventional Radiology

C. Air density correction factor

(for ionization chambers)

+

p

t

(

273

)

=

K

0

D

+

p

t

(

273

)

0

p

,

t

calibration values

0

0

10: Patient dose assessment


Accuracy and precision of a calibrated instrument 1
Accuracy and precision of a calibrated instrument (1) and Interventional Radiology

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

10: Patient dose assessment


Accuracy and precision of a calibrated instrument (2) and Interventional Radiology

Calibration

Calibration

Primary standard

(absolute measurement)

Secondary standard

Field instrument

Traceability

decreases

Accuracy

Relative uncertainty associated to the dosimetric quantity Q:

rQ2 ≥ rC2 + rR2

Where:

rC is the relative uncertainty of the reading of

the calibrated instrument

rR is the relative uncertainty of the reading of

the reading instrument

10: Patient dose assessment


Requirements on diagnostic dosimeters
Requirements on Diagnostic dosimeters and Interventional Radiology

Traceability

Well defined reference X Ray spectra

not available

Accuracy

At least 10 - 30 %

10: Patient dose assessment


Limits of error in the response of diagnostic dosimeters
Limits of error in the response of diagnostic dosimeters and Interventional Radiology

Parameter

Range of values

Reference condition

Deviation (%)

Radiation quality

According to manufacturer

70 kV

5-8

Doserate

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

10: Patient dose assessment


Part 10 patient dose assessment3

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 10: Patient dose assessment

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


What we want to measure
What we want to measure and Interventional Radiology

  • The radiation output of X Ray tubes

  • The dose-area product

  • The computed-tomography dose index (CTDI)

  • Entrance surface dose

10: Patient dose assessment


Measurements of radiation output
Measurements of Radiation Output and Interventional Radiology

X Ray tube

Filter

SDD

Ion. chamber

Lead slab

Table top

Phantom (PEP)

10: Patient dose assessment


Measurements of radiation output1
Measurements of Radiation Output and Interventional Radiology

  • 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

10: Patient dose assessment


Dose area product dap
Dose Area Product (DAP) and Interventional Radiology

Transmission ionization chamber

10: Patient dose assessment


Dose area product dap1
Dose Area Product (DAP) and Interventional Radiology

0.5 m

1 m

2 m

Air Kerma:

Area:

Areaexposure product

2.5*103Gy

40*10-3 m2

100Gy m2

40*103Gy

2.5*10-3m2

100Gy m2

10*103Gy

10*10-3 m2

100 Gy m2

10: Patient dose assessment


Calibration of a dose area product dap

Ionization and Interventional Radiology

chamber

Film cassette

10 cm

10 cm

Calibration of a Dose Area Product (DAP)

10: Patient dose assessment


Computed tomography dose index ctdi

and Interventional Radiology (ei di)

CTDI=

En

En: nominal slice width

ei : TLD thickness

Normalized CTDI:

CTDI

CTDIn=

mAs

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

10: Patient dose assessment


Computed tomography dose index ctdi1
Computed Tomography Dose Index (CTDI) and Interventional Radiology

CTDI

Dose

Dose profile

Nominal slice width

10: Patient dose assessment


Tld arrangement for ctdi measurements
TLD arrangement for CTDI measurements and Interventional Radiology

X Ray beam

Gantry

Support jig

X Ray beam

Capsule

Axis of

rotation

axis

of

rotation

Capsule

Couch

Gantry

LiF -TLD

10: Patient dose assessment


Ctdi in air with pencil type ionization chamber
CTDI in air with pencil-type ionization chamber and Interventional Radiology

  • The Computed Tomography Dose Index (CTDI) in air can be measured using a 10cm pencil ionization chamber, bisected by the scan plane at the isocentre, supported from the patient table

  • The ion chamber can be supported using a retort stand and clamp, if a dedicated holder is not available

18: Optimization of protection in CT scanner


Ctdi in air with pencil type ionization chamber1
CTDI in air with pencil-type ionization chamber and Interventional Radiology

Ionization

chamber

Table

18: Optimization of protection in CT scanner


CTDI in air with pencil-type ionization chamber and Interventional Radiology

Axial slice positions

Helical scan (pitch 1)

18: Optimization of protection in CT scanner


Measurement of entrance surface dose
Measurement of entrance surface dose and Interventional Radiology

TLD or OSL

10: Patient dose assessment


Summary
Summary and Interventional Radiology

  • In this lesson we learned the factors influencing patient dose, and how to determine the entrance dose, dose area product, and CT dose.

10: Patient dose assessment


Where to get more information
Where to Get More Information and Interventional Radiology

  • The Essential Physics of Medical Imaging. JT Bushberg, JA Seibert, EM Leidholdt, JM Boone. Lippincott Williams & Wilkins, Philadelphia, 2011

  • The 2007 Recommendations of the International Commission on Radiological Protection, ICRP 103, Annals of the ICRP 37(2-4):1-332 (2007)

10: Patient dose assessment


ad