Lecture 14 Optimization Dental radiology.ppt

IAEA
International Atomic Energy Agency
IAEA Regional Training Course on Radiation
Protection of patients for Radiographers,
Accra, Ghana, 11-15 July 2011
Optimization of Protection in Dental 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

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

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 (orthopantomography (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 intraoral
• 1:10 OPG
• 420 mAs/hr intraoral

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

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
Quality Control of Film Processing
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).
Technical hints to reduce patient doses

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)

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 by
dental 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 OD
Speed 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 mGy
70 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
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

“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

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

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

• 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

Summary
• 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.

IAEA
THANK YOU!
22: Optimization of Protection in Dental Radiology 35

Lecture 14 Optimization Dental radiology.ppt

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