This document discusses the risks of radiation from medical imaging. It begins with a brief history of x-rays and their discovery. It then explains how x-rays and other ionizing radiation are produced, types of radiation, and the difference between ionizing and non-ionizing radiation. The biological effects of radiation exposure are described as either somatic (impacting exposed cells) or genetic (impacting future generations). While low levels of radiation from diagnostic exams pose uncertain risks, current models suggest that any exposure increases cancer risk. The document provides radiation doses from various medical imaging tests and their equivalent natural background radiation doses. It stresses justifying exams, using the lowest possible doses, and alternative non-ionizing tests when possible.

1. RADIATION RISKS

2. HISTORY
• X-rays accidentally discovered by
Wilhelm Conrad Roentgen in
1895
• Why the name?
• But he knew they could penetrate
the body

3. HISTORY
• Roentgen’s discovery was a
bomb shell
• News papers and magazines
provided interesting stories.
• Advertisers associated their
products with the term “x-ray”
• Headache medicines, stove
polish, golf balls, razors….

4. AD FROM 1896

5. PRODUCTION OF X-RAYS
• X-rays are produced when rapidly moving
electrons that have been accelerated through a
potential difference of order 1 kV to 1 MV strikes
a metal target.
Evacuated
glass tube
Target
Filament

6. NATURAL BACKGROUND RADIATION
•
•
•
•
•
Average 2.4msv per year
Cosmic radiation from sun
Terrestrial: soil, buildings, granite
Body activity: potassium
Radon gas is naturally occuring
decay product of uranium and
thorium
• Skiing at high altitudes and airline
flights
• Return flight from Sydney to LA:
dose 0.16msv

7. The Electromagnetic Spectrum
Waveform of Radiation
NONIONIZING
IONIZING
Radio
Infrared
Microwaves
Ultraviolet
Visible light
Gamma rays
X-rays
7

8. Difference between ionizing and
nonionizing radiation
• Energy levels:
– Ionizing radiation has enough energy to break
apart (ionize) material with which it comes in
contact (knock off e-)
– Non ionizing radiation does not
8

9. Types of Ionizing Radiation
• Important in healthcare:
• Diagnosing - X-Rays, PET
Scans, Nuclear Medicine
• Therapy - Radiation
Treatment, Nuclear
Medicine
9

10. Biological Effects of Radiation
• Somatic
– Affects cells originally
exposed (cancer)
– Affects blood, tissues,
organs, possibly entire
body
– Effects range from
slight skin reddening to
death (acute radiation
poisoning)
• Genetic
– Affects cells of future
generations
– Reproductive cells
most sensitive
10

11. EFFECTS
• Effects of low level radiation in
diagnostic imaging is
presumptive and continue to
be debated
• No large clinical trials
• Data collected from the atomic
bomb survivors

12. RISKS
• Based on linear/no lower threshold model and
extrapolated
• Indicates that no dose, however small, is entirely without
risk
• Average lifetime risk of induction of fatal cancer from
exposure to 5msv is 1 in 4000 and to 20msv is 1 in 1000
• Risk is considerably great in children and young adults
• Hence, all imaging procedures should be justified

13. TISSUE WEIGHTING FACTOR
Gonads
Red Bone Marrow
Colon
Lung
Stomach
Bladder
Breast
Liver
Oesophagus
Thyroid
Skin
Bone Surface
0.20
0.12
0.12
0.12
0.12
0.05
0.05
0.05
0.05
0.05
0.01
0.01

14. PLAIN RADIOGRAPHY
INVESTIGATION
DOSE(msv)
EQUIVALENT NATURAL
BACKGROUND RADIATION
Extremities
0.01
1.5 days
Chest
0.02
3 days
Skull
0.07
11 days
C spine
0.1
15 days
T spine
0.7
4 months
L spine
1.3
7 months
Hip
0.3
7 weeks
Pelvis
0.7
4 months
Abdomen
1.0
6 months

15. CT and NM
Head
2.3
1 year
C spine
1.5
8 months
T spine
6.0
2.5 years
Chest
8.0
3.6 years
L spine
3.3
1.4 years
Abdomen
10.0
4.5 years
Pelvis
10.0
4.5 years
Bone scan
4.0
1.6 years
V/Q
1.3
7 months

16. RISKS
• Risk is a defered risk that may
occur 5 to 15 years after
exposure
• Use of medical imaging is rising
• Population exposure to ionizing
radiation is increasing and
majority of them is from CT scans
• Risk is cumulative

17. • Radium was discovered in Marie
Curie and Pierre Curie in 1898
• Highly radioactive and its decay
product radon gas is also
radioactive
• Marie Curie died secondary to
aplastic anaemia, believed to be
induced by the long term
exposure to radiation
• Carried test tubes with
radioactive isotopes in her pocket
and stored them in her desk
drawer

18. APPROPRIATE REQUESTING
• Cannot ignore the increasing use of
ionizing radiation
• All requests should be subject to
Justification and Optimisation
• Radiologist has the knowledge and
experience to determine the radiation
risks and consider alternative
investigations
• Referring doctor who has seen the
patient can best assess the potential
benefits
• Justification should be a joint
responsibility
• Optimisation is to achieve diagnostic
quality images by using low radiation
dose

19. RESPONSIBILITIES OF REFERRING DOCTOR
• Avoid unnecessary duplication of
tests
• Ensure that test could potentially
change management
• Provide adequate clinical details
• Be aware that many imaging
tests have risks
• Consult with imaging colleagues if
appropriate
• Consider the use of US or MRI
(non ionizing) when appropriate

20. PREGNANCY AND LACTATION
• Raising the awareness of patients
for the need to inform the
possibility of pregnancy
• 28 day rule. In a young patient who
has missed a period, pregnancy
should be excluded
• If appropriate employ tests that do
not use ionizing radiation
• Foetus more sensitive during 3 to 8
weeks and 1st trimester

21. EFFECTS ON FOETUS
•
•
•
•
Miscarriage and foetal death in the first few weeks
Malformations within first 8 weeks of implantation
CNS abnormalities during weeks 8 to 25
Carcinogenesis
• IV contrast can be used in exceptional circumstances
• Contrast induced foetal hypothyroidism is not validated
• Current guidelines recommend neonatal thyroid function
testing

22. PE IN PREGNANCY
• Risks of missing PE in pregnancy far outweighs
the risks of exposing the foetus to radiation
• CXR is useful to exclude conditions that mimic
PE
• D-Dimer is less useful in pregnancy, however
low probability and negative D-Dimer excludes
PE
• Doppler of both lower limbs should be
considered first and treatment commenced if
positive

23. PE IN PREGNANCY
• CTPA and V/Q scan have equal diagnostic
quality
• CTPA recommended in first 2 trimesters
• Phantom studies have shown less foetal dose
with CTPA in first two trimesters
• MRI is a promising new option but availability
and expertise are the main limitations