Radiation Protection

IAEAInternational Atomic Energy Agency

RADIATION PROTECTION INDIAGNOSTIC AND

INTERVENTIONAL RADIOLOGY

L 22: Optimization of Protection in Dental Radiology

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

IAEA 22: Optimization of Protection in Dental Radiology 2

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


Dental X-ray equipment

Radiation protection in dental radiology

Quality control for dental equipment

Topics


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

Overview


Part 22: Optimization of protection in dental radiology

Topic 1: Dental x-ray equipment



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)


Intra-Oral Dental X-Ray Equipment


Modern Dental X-Ray Unit


Panoramic X-Ray Equipment


Cephalometric X-Ray Equipment


X-Ray Tube

• stationary Anode

• avoid overheating

• tube duty cycle:• typical: 1:30 intaroral

• 1:10 OPG

• 420 mAs/hr intraoral


Tube Head


Generator Circuit


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


Collimator

1. Lead Collimator

with central hole

2. Spacer Tube


Applicator Cones

Good Bad Bad


Cephalometric Holder


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


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))


Image Receptors in Dental 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%.

Intraoral Radiology

Panoramic Radiology and Cephalometry• Film-screen combination• Digital sensors - compared with film-

screen sensitivity class 200, the radiation dose is reduced by 50-70%.


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


Part 22: Optimization of Protectionin Dental Radiology

Topic 2: 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

Radiation Protection in Dental Radiology


Keep under control time and temperature of the developing process.

Do not use oxydized chemicals

Do not adjust development time by viewing the film

Technical hints to reduce patient dosesQuality Control of Film Processing



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

Technical hints to reduce patient dosesPrivilege the use of sensitive films

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

RP in Dental Radiology


Technical hints to reduce patient dosesLead apron and collar

Useful when the path of primary beam intercepts the protected organs (downward bite-twin projection).



Panoramic examination

• Image quality not as good as in intra-oral films

• Important global information

• Relatively low dose(one panoramic examination 35 intra-oral films)



Part 22: Optimization of Protection in Dental Radiology

Topic 3: Quality Control for Dental Equipment



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


What Tests ?

• Collimation

• Dose Evaluation

• Exposure Time

• Half Value Layer

• Kilovoltage (kVp)

• Leakage Radiation


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.

Quality Control for Dental Equipment



Tests which can be performed bydental practice staff

Physical parameter Tolerances Frequency

Image Quality ±10% reference values

Quarterly

Developer temperature and condition of processing solutions

Specified by the film manufacturer

Every time processing solutions are used

Processing Base+Fog: >0.2 ODSpeed and Contrast >0.15 OD about baseline

Every time processing solutions are changed



Tests performed by medical physicists

Physical parameter Tolerances Frequency

Tube voltage >50 kV and error <10%

3 yearly

Beam size/collimation <60 mm diameter (intra-oral) <150 x 10 mm at cassette (panoramic)

3 yearly

Dose at cone tip 50 kV: <5.0 mGy70 kV: <2.5 mGy(E speed film)

1-3 yearly

Dose-width product for panoramic film

<75 mGy mm 1-3 yearly


Dental QC Methods

UnitIntra-Oral Receptors (I/O)

Cephalometric (Ceph)

Panoramic (OPG )

Test Methodas for Radiology QC

as for Radiology QC

where possible:• immobilise unit• remove slit collimator


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


• Expose film or fluorescent screen

• Measure x-ray field image

Collimation

Unit Film position Limits of X Ray field

Standard End of Cone 60 mm diameter

OPG Slit, in film cassette (2 films)

Equal slit images

Vertical image ± film height

Ceph At cassette ±1 % of FFD on all margins


Collimator Light Intensity

Ceph units:

• Place external detector 1m from focus

• Measure illuminance in lux ( lumen/m2)

• Read each quadrant

• Limit: >100 lux at 1m


“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


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%


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

< 5 mGy for any view


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%


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%


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


Timer Reproducibility

Standard I/O units & Ceph units

• Place timer in beam

• 5 repeat exposures

Limit: coefficient of variation ≤5%


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


HVL Set-up


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


OPG Quality Control (kVp/HVL measurement)


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

HVL- Minimum Values


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


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


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


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


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



• Keep under control time and temperature of the developing process.

• Do not use oxidized chemicals

• Regularly check processing with phantom

Quality Control of Film Processing


Dental Phantom


Dental Phantom


• 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

Where to Get More Information


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.

Radiation Protection

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