RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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

• L 22 Optimization of Protection in Dental
  Radiology

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Introduction

• Dental radiology makes use of specific types of
  equipment, needed for different purposes.
  Frequent exposures though each with low dose
  involve a risk for the practitioner and for the
  patient
• Background general principles of x-ray
  diagnostic imaging

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Topics

• Dental X-ray equipment
• Radiation protection in dental radiology
• Quality control for dental equipment

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Overview

• To be able to apply the principle of radiation
  protection to dental radiology system including
  design and Quality Control.

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IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology
Part 22 Optimization of protection in dental
radiology
Topic 1 Dental x-ray equipment
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Types of units

• Intra-Oral units
• Standard dental tube
• uses an intra-oral image receptor
• has extra-oral x-ray tube
• Panoramic (OPG)
• Cephalometric ( Ceph)

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Intra-Oral Dental X-Ray Equipment
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Modern Dental X-Ray Unit
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Panoramic X-Ray Equipment
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Cephalometric X-Ray Equipment
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X-Ray Tube

• stationary Anode
• avoid overheating
• tube duty cycle
• typical 130 intaroral
• 110 OPG
• 420 mAs/hr intraoral

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Tube Head
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Generator Circuit
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Generators Pre-Heat

• Medium frequency - stable waveform
• Single phase (SP) - pulsed
• Pre-Heat separate circuit for heating filament
• Single Phase units without a pre-heat circuit
• initial pulses of variable kV

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Collimator

• 1. Lead Collimator
• with central hole
• 2. Spacer Tube

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Applicator Cones
Good Bad
Bad
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Cephalometric Holder
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Intra-Oral Dental X-Ray Equipment (technical data)

• Exposure time from 60 ms to 2.5 s
• Tube Min. 50 kV, 7mA
• Focal spot size ?1 mm
• Inherent filtration 2 mm Al equivalent
• Focus-skin distance 20 cm
• Irradiated field 28 cm2 with round section, 6
  cm diameter collimator

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Panoramic X-Ray Equipment (technical data)

• Focal spot 0.5 mm
• kV 60 - 80 kV in 2 kV steps
• mA 4 - 10 mA steps 4, 5, 6, 8, 10
• Exposure time 12 s (standard projections) 0.16 -
  3.2 s (cephalometric projections)
• Flat panoramic cassette 15x30 cm (Lanex Regular
  screens))

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Image Receptors in Dental Radiology
Intraoral Radiology

• Small films (2 x 3 or 3 x 4 cm) in light-tight
  envelopes (no screen)
• Digital intraoral sensors - compared with
  category E film, the radiation dose is reduced by
  60.

Panoramic Radiology and Cephalometry

• Film-screen combination
• Digital sensors - compared with film-screen
  sensitivity class 200, the radiation dose is
  reduced by 50-70.

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Dental Radiology Film Types

• Sensitivity class D
• Very good spatial resolution
• Typical delivered dose about 0.5 mGy
• Typical exposure times 0.3 - 0.7 s

• Sensitivity class E
• Good spatial resolution
• Typical delivered dose about 0.25 mGy
• Typical exposure times 0.1 - 0.3 s

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Part 22 Optimization of Protectionin Dental
Radiology
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 2 Radiation Protection in Dental Radiology

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Radiation Protection in Dental Radiology

• Facts
• Very frequent examination (about 25 of all the
  radiological examinations)
• Delivered doses may differ of a factor 2 or 3.
  (entrance doses between 0.5 and 150 mGy)
• Image Quality often very low
• Organs at risk parathyroid, thyroid, larynx,
  parotid glands

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Radiation Protection in Dental Radiology
Technical hints to reduce patient doses Quality
Control of Film Processing

• Keep under control time and temperature of the
  developing process.
• Do not use oxydized chemicals
• Do not adjust development time by viewing the film

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RP in Dental Radiology
Technical hints to reduce patient doses Privilege
the use of sensitive films

• Sensitivity class D
• Very good spatial resolution
• Typical delivered dose about 0.5 mGy
• Typical exposure times 0.3 - 0.7 s

• Sensitivity class E
• Good spatial resolution
• Typical delivered dose about 0.25 mGy
• Typical exposure times 0.1 - 0.3 s

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Radiation Protection in Dental Radiology
Technical hints to reduce patient doses Lead
apron and collar
Useful when the path of primary beam intercepts
the protected organs (downward bite-twin
projection).
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Radiation Protection in Dental Radiology
Panoramic examination

• Image quality not as good as in intra-oral films
• Important global information
• Relatively low dose(one panoramic examination ?
  3?5 intra-oral films)

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Part 22 Optimization of Protection in Dental
Radiology
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 3 Quality Control for Dental Equipment

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Why Dental QC ?

• Widespread use of dental units
• Lack of QC history on most units
• Dental practitioners working in the primary
  health care sector do not have the continuous
  medical physics support available in a
  hospital-based diagnostic imaging department

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What Tests ?

• Collimation
• Dose Evaluation
• Exposure Time
• Half Value Layer

• Kilovoltage (kVp)
• Leakage Radiation

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Quality Control for Dental Equipment

• The recommended tests are consequently divided
  into
• those simple tests which can be performed by
  dental practice staff
• those more complex tests which can be carried
  out by medical physicists.

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Quality Control for Dental Equipment
Tests which can be performed bydental practice
staff
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Quality Control for Dental Equipment
Tests performed by medical physicists
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Dental QC Methods

• Unit
• Intra-Oral Receptors
• (I/O)
• Cephalometric
• (Ceph)
• Panoramic
• (OPG )

• Test Method
• as for Radiology QC
• as for Radiology QC
• where possible
• immobilise unit
• remove slit collimator

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Test Equipment

• kVp meter
• measure kVp average
• Programmable delay 100 ms
• Range 50 to 120 kV
• Aluminum filters
• 4 x 1mm
• Grade 1100

• Dosimeter
• small large volume chambers
• 2mm wide detector for OPG
• Timer
• triggering at 75 peak kV

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Collimation

• Expose film or fluorescent screen
• Measure x-ray field image

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Collimator Light Intensity

• Ceph units
• Place external detector 1m from focus
• Measure illuminance in lux ( lumen/m2)
• Read each quadrant
• Limit gt100 lux at 1m

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Dead man Switch

• timer at 50 cm from focus
• set low kV, mA, long time
• start exposure
• release switch during exposure
• Require exposure cut-out when switch is
  released. Check exposure time is less than set
  time

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Kilovoltage

• Accuracy
• Set kVp meter to 100 msec delay
• Observe kVp waveform at 70 kV if poss.
• Limit measured kVp within 5 of set value
• Reproducibility
• Take 5 repeat exposures
• Limit coefficient of variation 2

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Dose Evaluation

• Skin dose from I/O units
• place cone 10 mm from dosimeter
• set maxillary molar/ bitewing setting
• Should be (65-70 kVp)
• 2-3 mGy for molar view
• lt 5 mGy for any view

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Output Reproducibility (1)

• Standard I/O units Ceph units
• Dosimeter position
• I/O units 10 mm from cone
• Ceph units 75 cm from focus
• or other recommended distance
• Five repeat exposures
• Limit coefficient of variation 5

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Output Reproducibility (2)

• Optional Method for OPG units
• Align detector on film cassette slit
• Measure dose rate
• Take 5 repeat exposures
• Limit coefficient of variation 5

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Exposure Time Accuracy

• Standard I/O units Ceph units
• Set timer to trigger at 75 peak kV
• Test times in the normal working range
• Limit 10 error for I/O units
• 5 error for all other units

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Timer Reproducibility

• Standard I/O units Ceph units
• Place timer in beam
• 5 repeat exposures
• Limit coefficient of variation 5

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Half Value Layer (HVL)

• Standard I/O units
• Position cone facing down
• Place dosimeter at 40 cm from focus
• Position Al filters near end of cone
• Measure dose
• measure with no added filters
• with 2,3,4 mm Al added, then again with no
  filters
• Plot on semi-log paper and find HVL

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HVL Set-up
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HVL Ceph OPG Units

• Position Al filters on collimator
• Cephalometric units
• Position dosimeter at 75 cm from focus
• OPG Units
• Position dosimeter on film cassette slit
• Measure dose rate, dose for fixed exposure time,
  or dose for full scan
• NB Test kVp accuracy before measuring HVL

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OPG Quality Control (kVp/HVL measurement)
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HVL- Minimum Values

• kVp HVL (mm Al)
• Intraoral Ceph/OPG
• 60 1.5 1.8
• 70 1.5 2.1
• 80 2.3 2.3
• 90 2.5 2.5

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Leakage Radiation

• Cover collimator with 2 mm lead
• Set large ionization chamber (180 cc) or solid
  state detector at known distance d cm from focus
• Expose on maximum kVp for 0.5 - 1 sec (but be
  careful!)
• Scale up the reading to 1 hour

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Leakage Radiation

• Choose the most likely points for leakage -
• where collimator joins to housing, lateral to the
  tube, joints in housing etc.
• Normalise measurement to 1 m from focus with
  inverse square law
• Dose1m dosed x (distanced/100)2

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Leakage Measurement

• In normal radiography, leakage measurements are
  normalised to the maximum continuous rated tube
  current R
• Leakage Dose (for P mAs at 100 cm) x (R x
  3600/P)
• BUT...a dental unit has no continuous operation
  rating

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Leakage Measurement

• Instead we normalize the dose reading to tube
  duty cycle
• Leakage Limit at 1 m from focus
• 0.25 mGy/hour - Standard I/O units
• 1.0 mGy/hour - Ceph OPG units

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Calculation of Leakage

• Measured leakage at 100 cm 2mGy/hr
• Duty cycle 1 20 (i.e. must wait 20 times
  the exposure time between exposures to allow tube
  cooling)
• Thus actual leakage 2 / 20 mGy/hr
• 0.1 mGy/hr
• i.e. acceptable

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Radiation Protection in Dental Radiology
Quality Control of Film Processing

• Keep under control time and temperature of the
  developing process.
• Do not use oxidized chemicals
• Regularly check processing with phantom

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Dental Phantom
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Dental Phantom
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Where to Get More Information

• Code of practice for protection of persons
  against ionising radiations arising from medical
  and dental use, DHSS, HMSO, London, 1964
• Guidance notes for the protection of persons
  against ionising radiations arising from medical
  and dental use, DHSS, HMSO, London, 1988
• Radiation protection and quality assurance in
  dental radiology. Radiation protection 81.
  European Commission.(1995) CG-89-95-971-EN-C

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Summary

• General and specific types of x-ray equipment
  (and image receptors) are reviewed (panoramic
  and cephalometric), with technical data about
  operating conditions
• Although doses are generally low, the high
  frequency of examinations requires radiation
  protection (for the practitioner) in dental
  radiology
• Some tests are detailed for Quality control of
  dental equipment.