Applying Radiation Principles In Practice

Question 1

International commission on radiation protection

Core principles

Answer 1

Justification
Optimisation
Limitation

Question 2

Justification

Answer 2

• net benefit for patient
• exposure will normally provide new information to aid the patients management
• use of evidence based selection criteria

Question 3

Optimisation

Answer 3

• technique - getting it right the first time
• keeping doses ALARP = as low as reasonably practicable
• reducing retakes

Question 4

ALARP principle
- principle of radiation protection is to keep radiation doses to patients as low as reasonably possible

Answer 4

• dose optimisation by choice of technique
• use of selection criteria to justify all exposures
• production of optimal quality images

Question 5

Limitation

Answer 5

• dose limits should not be exceeded for workers / public
• no dose limits to individual patients, justification instead protects from clinically in-warranted examinations

Question 6

Optimisation - equipment factors

IO radiography

Answer 6

IO radiography
- method of X-ray generation
- kilo voltage
- filtration
- collimation
- cone length
- image receptor

Question 7

Efficient X-ray generation

Answer 7

• constant potential or DC waveform
• keeps the kV at its peak throughout the exposure
• X-ray production more efficient
• more high energy useful xray photons produced
• fewer low energy moir harmful X-rays

Question 8

Direct current machine

Answer 8

• majority of modern X-ray sets are DC, older equipment may be AC

DC:
- eliminates the filament warm up time from exposure time
- more efficient production of photons which contribute to the image
- a necessity if using digital equipment

Question 9

Operating kilovoltage

Answer 9

• new week-meant should operate at 60-70kV
• at lower kV the photoelectric effect predominates which is pure absorption
• by changing from 50kVp to 65kVp, all else being equal, the effective dose can be halved

Higher kV:
- lower dose
- lower contrast

(More X-ray photons pass through the pt and are instant on image receptor but results in lower contrast)

Question 10

Exposure time -

Influences pt dose

Answer 10

• guideline exposure settings
• standard projections each X-ray set
• child / adult / large adult
• available range should be sufficient to enable even the fastest digital imaging systems to be correctly exposed

Question 11

Filtration

Also equipment factors that influences dose on pt

Answer 11

• filtration removes low energy photons which contribute most to dose
• up to and including 70kV - 1.5mm Al
• > 70kV-2.5mm Al

Question 12

Rectangular collimation

Answer 12

• shapes and limits the beam size
• can reduce dose by 50% compared to circular collimation
• size determined by the image receptor size
• should always be used for PA, BW, occlusal RG

Question 13

Spacer cone

Answer 13

Longer
- improvement image quality
- reduced pt dose

Should give fsd (focal spot to skin distance) at least 20cm

Where interchangeable cones provided, the longer should be used (correct exposure factors for longer cones should be selected)

End of the spacer cones as close to the pt skin as possible without touching it

Question 14

Spacer cone: length

Answer 14

• inverse square law
• beam intensity reduces by the increase in distance squared
• doubling the distance reduces intensity by factor of 4

Question 15

Image receptor

Answer 15

Image receptor which produces a diagnostic image at the lowest possible dose should be used

Dose advantages
DR > CR > Film

Film
- fastest available should be used
- F speed 20% faster than E speed
- D speed should no longer be used (dose req is too great)

Question 16

Dental digital imaging systems

Answer 16

Phosphor plates

Solid state (CCD/CMOS)

Question 17

Digital

Answer 17

• offers dose reduction advantage for IO radiography over film systems (20-70% but probably <25% in general practice)
• direct current machine vital for digital systems
• equipment must allow adequately short exposure time - digital overexposure not apparent in image
• dose advantage for panoramic radiography very variable

Question 18

Thyroid collar

Answer 18

Used only when the thyroid gland is unavoidably in teh primary beam

Discuss with MPE:
- cephalometric
- CBCT / panoramic examinations where FOV extends below mandible
- other situations where thyroid in primary beam

Question 19

Viewing conditions
Digital - display screens

Answer 19

Subject to quality assurance
Brightness and contrast adjusted using suitable test pattern

Down to increase probability of greater diagnostic image

Digital - ambient light (reduce it to maximise contrast)
- decrease glare
- reduce distracting reflections

Question 20

Film

Answer 20

• light box
• adjustable ambient light
• film masking
• magnification (x2)

Question 21

Technique factors which influence pt dose

Answer 21

• Tooth of interest and image receptor need to be parallel
• short tooth to image receptor distance
• X-ray beam perpendicular to tooth and image receptor

Question 22

Panoramic technique

Answer 22

• light beam diaphragms
• use sectional options
• use children’s setting when required

Question 23

Light beam diaphragms for panoramic radiography

Answer 23

• occlusal plane parallel to floor
• appropriate antero-posterior position
• no rotation

Question 24

Practical dose limitation

Staff exposure comes from 3 sources:

Answer 24

• the primary beam
• radiation leakage from tube head
• scatter from the pt

Question 25

Practical dose limitations

Answer 25

- appropriate maintenance of equipment - apply local rules / enforcement of controlled areas Never holding - image receptor in pt mount during exposure - X-ray tube head during exposure