2. X- ray Dose Measurements
X- ray Dose Units
30 January 2015 2Dr Saad Wahby Al Bayatti
3. Radiation Absorbed Dose
• This is a measure of the amount of energy
absorbed from the radiation beam per unit mass of
tissue
• Unit of measurement:
• SI unit : Gray, (Gy) measured in joules/kg
• original unit : rad, measured in ergs/gm
• 1 Gray = 100 rads
30 January 2015 3Dr Saad Wahby Al Bayatti
4. Equivalent Dose
• A measure which allows comparison between
different types of radiation in regard to their
absorbed doses in the body (RBE radiobiological
effectiveness)
• Unit of measurement (SI unit) : Sievert (Sv)
• subunits : millisievert (mSv) ( ×1/1 000)
• : microsievert ( µSv) ( ×1/1 000 000)
• original unit : rem
• 1 Sievert = 100 rems
30 January 2015 4Dr Saad Wahby Al Bayatti
5. Quality Factor (Q)
• Represents the biological effects of each
type of radiation:
• X- rays, gamma rays and beta particles
Q = 1
• Fast neutrons protons Q = 10
• Alpha particles Q = 20
30 January 2015 5Dr Saad Wahby Al Bayatti
6. Equivalent Dose
• equivalent dose = radiation absorbed dose (Gy)×
Q
• Since Q for x -ray = 1, then
equivalent dose = radiation absorbed dose
(Sv) = (Gy)
30 January 2015 6Dr Saad Wahby Al Bayatti
7. Effective Dose
• Each body tissue is affected differently by
radiation
• Effective dose: This is a measure that allows doses
from different investigations of different parts of
the body to be compared , by converting all doses
to an equivalent whole body dose
30 January 2015 7Dr Saad Wahby Al Bayatti
8. Weighing Factor (W)
• Measures the radiosensitivity, i.e.the risk of
the tissue being damaged by radiation.
• The higher the damage, the higher is W
30 January 2015 8Dr Saad Wahby Al Bayatti
9. Weighing factors for different body tissues
Tissue Weighing factor
Testes and ovaries 0.20
Red bone marrow, colon, lung,
stomach
0.12
Breast, bladder, liver, thyroid 0.05
Bone surfaces , skin 0.01
Remainder 0.01
30 January 2015 9Dr Saad Wahby Al Bayatti
10. Effective Dose
• Effective dose = equivalent dose × weighing factor
= radiation absorbed dose(Gy)×Q ×W
• SI unit : Sievert (Sv)
• subunit : millisievert (mSv)
• Effective dose (whole body) = sum of W
30 January 2015 10Dr Saad Wahby Al Bayatti
11. Absorbed dose
Multiplied by a factor to reflect harm by a specific radiation
Equivalent dose
Multiplied by a factor to reflect sensitivity of a specific tissue
Effective dose, commonly called “dose”
30 January 2015 11Dr Saad Wahby Al Bayatti
12. Effective doses
X -ray examination
X- ray exam. Effective dose (mSv)
• CT cest 8.0
• barium meal 7.7
• lumbar spine 2.2
• CT head 2.0
• skull 0.1
• chest 0.04
• OPG 0.007
• 2 intra oral films (E speed) 0.002
30 January 2015 12Dr Saad Wahby Al Bayatti
13. The frequency of taking films is based on
the following factors:
1. Patient’s oral hygiene
2. Caries activity
3. Dental history
4. Reliability of patient
5. ADA Guidelines
17. Biological Effects of X-ray
• X-Rays interact with living tissues and can cause
biological changes.
• These changes are mediated directly by excitation
or ionization of atoms or indirectly as a result of
chemical changes occurring near the cells.
• Affected cells may be damaged or killed.
30 January 2015 17Dr Saad Wahby Al Bayatti
18. Biological Effects of X-ray (cont’d)
• Genetic effects involve chromosomal damage or
mutation in the reproductive cells and will affect
future generations.
• Somatic effects involve damage to other tissues and
result in changes within the individual’s lifetime
(e.g. radiation burns, leukemia).
• Radiation is a particular hazard because its effects
are painless, latent and cumulative
30 January 2015 18Dr Saad Wahby Al Bayatti
21. Ionization
The process of removing an electron from an electrically
neutral atom to produce an ion. An ion is an atom or
subatomic particle with a positive or negative charge.
23. Attenuation
Reduction of x-ray beam intensity (that
reaches film) by interaction with matter.
1. Coherent scattering
2. Compton scattering
3. Photoelectric absorption
24. Coherent Scattering
Low-energy x-ray interacts with outer-
shell electron and causes it to vibrate
briefly. Scattered x-ray of same energy
as primary x-ray is then emitted, going
in a different direction than primary x-
ray. Electron not ejected from atom. (No
ionization).
26. Compton Scattering
Outer shell electron ejected
(Ionization)
Scatter radiation results
Occurs majority of the time
30% of scatter exits head
27. recoil electron
scattered x-ray
Compton Scattering
primary x-ray
The primary x-ray strikes an outer-shell electron,
knocking it out of its orbit (ionization). The primary x-
ray loses some of its energy and continues in a different
direction as a scattered x-ray.
35. DNA altered; cell function
altered or development
changed. It is unclear which
critical lesion/s in DNA may
lead to cancer.
Mutation
Normal
Mutation
37. Cellular Repair
1. Damage to biologic molecules
(single-strand break of DNA)
2. Removal of damaged section by
cell enzymes
3. Placement of new material by
other cell enzymes
42. Biologic Effects Of Ionizing Radiation
Stochastic
• The probability of occurrence
of the change, rather than its
severity, is dose dependant
• All or non, the person either has
the condition, or not
• No threshold
e.g.
• Radiation induced cancer,
greater exposure of population
to radiation increases cancer
probability, but not its severity
Deterministic
• The severity of response is
proportional to the dose
• Occur in all people when the
dose is large enough
• There is a dose threshold below
which the response is not seen
e.g.
• Oral effects after radiation
therapy
• Radiation sickness after whole
body radiation
30 January 2015 42Dr Saad Wahby Al Bayatti
46. Radiation Hazards
Indirect Direct
Somatic
Affects individual
No effect on offspring
Genetic
Do not affect individuial
Offspring is affected
Radiation damage
30 January 2015 46Dr Saad Wahby Al Bayatti
47. Radiation Hazards
Direct Damage
RH +Radiation R
+
+ H
+
+ e
-
R
+
Dissociation
R
+
X + Y
Cross-linking
R
+
+ S RS
30 January 2015 47Dr Saad Wahby Al Bayatti
48. Radiation Hazards
Direct Damage
Radiation
• DNA /RNA molecule nuclear acid
breakdown
• Nuclear acid breakdown
Somatic cells radiation induced
malignancy
Genetic cells radiation induced
congenital abnormality
30 January 2015 48Dr Saad Wahby Al Bayatti
49. Radiation Hazards (indirect damage)
Water Hydrolysis
• H 2 O H
+
+ OH
-
• H
+
+ H
+
H2
• OH
-
+ OH
-
H 2 O 2
Radiation
30 January 2015 49Dr Saad Wahby Al Bayatti
50. • H2O2 + DNA Molecular
breakdown
• H2O2 + Proteins
• Molecular breakdown Cell damage
Radiation Hazards (indirect damage)
Water Hydrolysis
30 January 2015 50Dr Saad Wahby Al Bayatti
51. Radiation Hazards
Direct
DNA /RNA hit
Radiation induced malignancy
Indirect
H2O2 formation
TOXIC
breakdown of large molecules(protiens/DNA)
Somatic
Radiation damage
30 January 2015 51Dr Saad Wahby Al Bayatti
52. Radiation Hazards
Direct
DNA/RNA hit
Radiation induced congenital
abnormality
Indirect
H2O2 formation
TOXIC
breakdown large molecules ( proteins/ DNA)
Genetic
Radiation damage
30 January 2015 52Dr Saad Wahby Al Bayatti
55. Factors Affecting Radiosensitvity
• Dose: the amount of radiation
received. The higher the dose,
the greater is the effect
(consider the threshold)
• Dose rate: the rate of exposure.
e.g. a total dose of 5Gy can be
given as
- 5Gy/min (single dose) is more
destructive
- 5mGy/min(fractionized), less
destructive, injured cells can
recover
• Oxygen: the higher the O2 level
in irradiated cells, the greater is
the damage. (H2O2 formation)
• Linear Energy Transfer
(LET): the rate of loss of
energy from a particle as it
moves in its track through
matter (tissue)
e.g. alpha particles vs. X-ray
30 January 2015 55Dr Saad Wahby Al Bayatti
59. X- ray Protection
Patient
• Radiographs are only taken when necessary
• Number, frequency and type of radiographs is the
responsibility of the dentist
• Use high output DC x- ray generators
• Minimum kilovoltage should be 60kV
• Minimum milliamperage should be 8 mA
• Minimum filtration should be 1.5 mm Al
30 January 2015 59Dr Saad Wahby Al Bayatti
60. X- ray Protection
Patient
• Maximum Beam diameter should be 7 cm
(circular beams)
• Use rectangular collimation for intra oral films
• Minimum target – skin distance should be 20 cm
• Accurate timer
• Use open- ended lead lined cylindrical cones
(PID)
• Do not use close ended pointed plastic cones
30 January 2015 60Dr Saad Wahby Al Bayatti
61. X- ray Protection
Patient
• Use high speed films (D or E )
• Use film holders and beam-aiming devices
• Avoid retakes, master radiographic
techniques
• Avoid retakes, master film processing
techniques
• Use lead aprons
• Use thyroid collars30 January 2015 61Dr Saad Wahby Al Bayatti
72. X- ray Protection
Staff
• Never hold the x- ray head during exposure
• Never hold the film during exposure
• Never stand in the path of the primary beam
• Stand behind a lead barrier (2 mm thickness)
• Watch the patient through leaded glass during
exposure
• Stand minimum 2 M from the x-ray beam behind
the patient’s head
30 January 2015 72Dr Saad Wahby Al Bayatti
73. X- ray Protection
Public
• Warning/Informative signals to indicate
hazardous x- radiation
• Patients should be seated away from x- ray
rooms
• Patients are not allowed to wait in corridors
next to x- ray rooms
30 January 2015 73Dr Saad Wahby Al Bayatti
81. Long cylindrical Open-end cone
Short cylindrical Open-end cone
Long rectangular cone
Short rectangular cone
30 January 2015 81Dr Saad Wahby Al Bayatti
86. 1 M
2 M
A
B
Source
Inverse square law
I 1/D
Intensity of radiation at B =1/4 at A
2
30 January 2015 86Dr Saad Wahby Al Bayatti
87. Inverse-square law states that the
intensity (quantity) of X-ray is inversely
proportional to the square of the
distance from the source of radiation
30 January 2015 87Dr Saad Wahby Al Bayatti
88. 2 M
minimum .
Operator `
Patient
Danger
X -ray
Safe operator’s position
30 January 2015 88Dr Saad Wahby Al Bayatti
89. Operator `
Lead barrier
Using a lead barrier allows less than
2 M distance
X- ray
Patient
Danger
30 January 2015 89Dr Saad Wahby Al Bayatti
91. Patients’
Waiting rooms
X- ray room
X- ray room
Restricted areas
Restrictedareas
Safe patients waiting rooms
30 January 2015 91Dr Saad Wahby Al Bayatti
