Radiation Protection Course For Radiologists Lecture 2 of 8 Radiation Units And Dose Quantities

1. Radiation Protection Course For Radiologists
Lecture 2 of 8
Radiation Units And Dose Quantities
Prof Amin E A Amin
Dean of the Higher Institute of Optics Technology
&
Prof of Medical Physics
Radiation Oncology Department
Faculty of Medicine, Ain Shams University

2. Objective
To become familiar with dosimetric quantities and units, and to
perform related calculations.

3. Physical Quantities
• Physical quantity is the
numerical value of a measurable
property.
• A physical quantity consists of a
magnitude and unit.
Measuring length

4. Physical Quantities Are classified into two types:
• Base quantities
• Derived quantities
Physical Quantities
Base quantity
is like the brick – the
basic building block of a
house
Derived quantity
is like the house that was build up
from a collection of bricks (basic
quantity)

5. Base Units
The units for the basic physical quantities are known
as base units.

6. Derived Units
A derived quantity is a combination of various
basic quantities. Units for derived quantities are
known as derived units.

7. Base Quantities
• There are three base quantities.
– Mass
– Length
– Time.

8. How Units Are Defined
• A unit has to have a special name and symbol.
• The usefulness of a unit is as a means of
communicating to everyone who does science
how it was particularly defined.

9. Systems Of Units
• Four systems of units
– MKS (meters, kilograms, seconds)
– CGS ( centimeters, grams, seconds)
– British (Foot, Pound, Seconds)
– International (SI) (Meter, Kilogram, Seconds)

10. SI Units
❖The SI units (adopted in 1960) represent a set of basic
physical quantities.
❖Second(s). The second is defined as a number of the period
of vibration of radiation from the cesium-133 atom.
❖ Metre (m). The distance travelled by light in vacuum during
a time of
1
299 792 458
second.
❖ Kilogramme (kg). Defined as the mass of a specific
platinum-iridium alloy cylinder kept at the International
Bureau of Weights and Measures in France.

11. This Platinum Iridium
cylinder is the
standard kilogram.
SI Units

12. Standard Prefixes
Very large or very small numbers, using prefixes
Fraction Prefix Symbol Fraction Prefix Symbol
10-1 deci d 10 deka da
10-2 centi c 102 hecto h
10-3 milli m 103 kilo k
10-6 micro  106 mega M
10-9 nano n 109 giga G
10-12 pico p 1012 tera T
10-15 femto f 1015 peta P

13. Basic Dosimetric Quantities
• Several quantities and units are needed in the field of diagnostic
radiology and related dosimetry
• Some can be measured directly while others can only be
calculated

14. Background
• Radiation studies began in 1895 with the discovery of x-rays
• Early physicist and therapist eventually knew that ionizing
radiation was hazardous, however, there was no definite way
to quantify the dose or damage.
• No suitable unit.
• Many injuries and deaths.
• Early risk associated with use of ionizing radiation.

15. Background
• Skin erythema dose: it is in early radiotherapy. It is the
amount of radiation which, applied to the skin, makes it turn
temporarily red (erythematous).
• 1928 - ROENTGEN introduced by International Committee
of Radiation Protection (ICRP).

16. Special Quantities For Radiology
• For radiology there are
special quantities.
– Exposure
– Dose
– Dose equivalent
– Activity

17. Basic Dosimetric Quantities
• Exposure and exposure rate
• Absorbed dose and KERMA
• Mean Absorbed Dose in a tissue
• Equivalent dose H
• Effective Dose
• Related dosimetry quantities (surface and depth dose,
backscatter factor…..)
• Specific dosimetry quantities (Mammography, CT,…)

18. air
medium
(tissue)
exposure
C/kg102.58R1
(mass)
(charge)
4-
=


=
m
Q
X
kerma
absorbed dose
(dose)
rad100J/kg1Gy1
(mass)
ansferred)(energy trtr
==


=
m
E
K
rad100J/kg1Gy1
(mass)
absorbed)(energyab
==


=
m
E
D
Physical Dose
Radiation Dosimetry

19. Exposure
• Exposure is a dosimetric quantity for ionizing radiation, based
on the ability of the radiation to produce ionization in air.
• This quantity is only defined for photons producing interactions
in air.

20. Exposure
❖Before interacting with the patient
(direct beam) or with the staff
(scattered radiation), X Rays interact
with air
❖The quantity “exposure” gives an
indication of the capacity of X Rays to
produce a certain effect in air
❖The effect in tissue will be, in
general, proportional to this effect in
air

21. Exposure
• Exposure - the quantity that expresses the ionization
produced in a volume element in air by X-rays or gamma
radiation
• The SI unit of exposure is coulomb per kilogram (C.kq-1)

22. X = dQ/dm
Exposure
• The exposure is the absolute value of the total
charge of the ions of one sign produced in air
when all the electrons liberated by photons per
unit mass of air are completely stopped in air.
d Q is the absolute value of the total charge of
the ions of one sign produced in air when all
the electrons liberated by photons in air of
mass dm are completely stopped in air

23. Exposure: X
• The SI unit of exposure is Coulomb per kilogram [C kg-1]
• The former special unit of exposure was Roentgen [R]
• 1 R = 2.58 x 10-4 C kg-1
• C kg-1 = 3876 R

24. Free-Air Ionization Chamber

25. Roentgen
• Limitations
– only applies to photons
– only applies in air
– only applies to energies less than 3 MeV

26. Exposure Rate: X/T
• Exposure rate (and later, dose rate) is the exposure produced
per unit of time.
• The SI unit of exposure rate is the C/kg per second or R/s.
• In radiation protection it is common to indicate these rate
values
“per hour” (e.g. R/h).

27. Patient Dosimetry Quantities

28. Absorbed Dose, D
• The absorbed dose D, is the energy absorbed per unit
mass.
• This quantity is defined for all ionizing radiation (not
only for electromagnetic radiation, as in the case of the
“exposure”), and for any material.

29. Absorbed Dose
Energy imparted by ionising radiation to the matter
Dd - Absorbed dose
de - the mean energy imparted in a volume element
dm - the mass of the volume element
The unit in SI - joule per kilogram - (J.kg-1) - gray (Gy)
• 1 Gy = 1 J/kg.
• The former unit was the “rad”. 1 Gy = 100 rad.
dm
d
Dd

=

30. Absorbed Dose, D And KERMA
• The KERMA (kinetic energy released in a mass)
K = dEtrans/dm
– where dEtrans is the sum of the initial kinetic energies of all
charged ionizing particles liberated by uncharged ionizing
particles in a material of mass dm
• The SI unit of kerma is the joule per kilogram (J/kg),
termed Gray (Gy).
• In diagnostic radiology, Kerma and D are equal.

31. Exposure And Absorbed Dose Or
KERMA
• Exposure can be linked to air dose or kerma by suitable
conversion coefficients.
• For example, 100 kV X Rays that produce an exposure of 1 R
at a point will also give an air kerma of about 8.7 mGy (0.87
rad) and a tissue kerma of about 9.5 mGy (0.95 rad) at that
point.

32. Relation Between Absorbed Dose And
Exposure
• It is possible to calculate the absorbed dose in a material if the
exposure is known
• D [Gy]. = f . X [C kg-1]
❖ f = conversion coefficient depending on medium
• The absorbed energy in a quantity of air exposed to 1 [C kg-1]
of X Rays is 0.869 [Gy]
❖f(air) = 0.869

33. Ratio Of Absorbed Dose In Soft
Tissue To That In Air
• Values of absorbed dose to tissue will vary by a few percent
depending on the exact composition of the medium that is
taken to represent soft tissue.
• The following value is usually used for 80 kV and 2.5 mm
Al:
Dose in soft tissue = 1.06 Dose in air

34. Example Of Conversion Coefficient: f
f values ([Gy] / Ckg-1])
Photon energy Water Bone Muscle
10 keV 0.91 3.5 0.93
100 keV 0.95 1.5 0.95

35. Mean Absorbed Dose In A
Tissue Or Organ
The mean absorbed dose in a tissue or organ DT is the energy
deposited in the organ divided by the mass of that organ.

36. Organ Dose
The average absorbed dose in an organ or tissue - DT
T - the total energy imparted in a tissue or organ
mT - the mass of that tissue or organ
The unit in SI of the organ dose is J.kg-1, termed the gray (Gy)
D
m
T
T
T
=


37. Units Of Radiation Dose
• Gray (Gy) is the SI unit of absorbed dose.
• One gray is equal to an absorbed dose of 1 Joule/kilogram (100
rads).

38. Units Of Radiation Dose
• Rad is the special unit of absorbed dose.
• One rad is equal to an absorbed dose of 100
ergs/gram or 0.01 joule/kilogram (0.01 gray).

39. Dose Equivalent
• Dose is a physical quantity
• We need a quantity to express stochastic radiation damage
• Dose equivalent (HT) is the quantity

40. Sievert - Unit of Dose Equivalent (HT)
• Dose equivalent is the quantity that expresses stochastic radiation
damage by relating the physical dose absorbed (D) to the LET through
the use of a radiation quality factor wR.
• The reference radiation is 250 kVp x-rays.

41. Equivalent Dose: H
• The equivalent dose H is the absorbed dose multiplied by a
dimensionless radiation weighting factor, wR which expresses
the biological effectiveness of a given type of radiation
• HT =WR x DTR
where DTR is the dose in gray averaged over the tissue (T) for a
specific radiation (R).
• To avoid confusion with the absorbed dose, the SI unit of
equivalent dose is called the sievert (Sv).
• The unit of equivalent dose Sv is equal to J.kg-1.
• The old unit was the “rem”
• 1 Sv = 100 rem

42. Units Of Dose Equivalent
• Sievert is the SI unit of any of the quantities expressed as dose
equivalent.
• The dose equivalent in sieverts is equal to the absorbed dose in
grays multiplied by the quality factor (1 Sv=100 rems).

43. Units Of Dose Equivalent
• Roentgen Equivalent Man
• The old unit of dose equivalent for any type of ionizing
radiation absorbed by body tissue in terms of estimated
biological effect .
• The dose equivalent in rems is equal to the absorbed dose in
rads multiplied by the quality factor (1 rem=0.01 sievert).

44. Radiation weighting factor
• The specific value that accounts for the ability of different types
of ionizing radiation to cause varying degrees of biological
damage
• It is a function of LET.
• Higher LET - Higher Wr

45. Linear Energy Transfer (LET)
• The linear rate of energy absorption in a medium is measured
by the Linear energy Transfer (LET)
• LET = dEL / dl (units: keV/micron)
• dEL is the average energy transferred to the absorbing medium
when charged particle radiation traverses a distance l

46. Linear Energy Transfer (LET)
In Water (keV/micron)
Radiation LET
X-ray, Gamma < 3.5
Electrons (Betas) < 3.5
Alpha approx 175
Neutrons
- thermal approx 5
- 0.01 MeV > 53
- 0.1 MeV > 175
- >0.1 - 2 MeV > 53
- > 2 MeV - 20 MeV approx 23

47. Radiation Weighting Factor - wR
wR- modifying factor based on the type and
quality of the radiation (affecting externally or
internally) used for radiation protection purposes
to account for the relative effectiveness of
different types of radiation in inducing health
effects

48. Radiation Weighting Factor, Wr
• For most of the radiation used in medicine (X Rays, , e-) wR is
= 1, so the absorbed dose and the equivalent dose are
numerically equal
• The exceptions are:
– alpha particles (wR = 20)
– neutrons (wR = 5 - 20).

49. Relative Biological Effectiveness:
RBE
• RBE compares the effect of a test radiation
with that of a reference radiation to produce
the same biological effect.
RBE =
Dose of 250 kVp X-rays for Observed Biological Effect
Dose of Test Radiation for the Same Biological Effect
Quality Factor - represents same idea but in qualitative form
Radiation Weighting Factor: ICRP Term wR

50. Equivalent Dose HT,R
A quantity derived from the organ dose-DT,R (the absorbed
dose averaged over a tissue or organ):
wR- the radiation weighting factor
When the radiation field is composed of various types or
energies of radiation:
H w DT R R T R, ,.=
H w DT
R
R T R=  . ,

51. Radiation Weighting Factor
Type of radiation and energy
range
Radiation weighting factor
Photons, all energies 1
Electrons, all energies 1
Alpha particles 20
Protons, energy >2 MeV 5
Neutrons.
energy <10 keV
5
Neutrons.
energy 10 to 100 keV
10
Neutrons. energy
>100 keV to 2MeV
20
Neutrons, energy
>2MeV to 20MeV
10
Neutrons, energy
> 20MeV
5

52. Example - Alpha Radiation
A “pure” alpha emitter (Am - 241) is inhaled
and irradiates the lungs with a dose of 100 mGy.
What is the dose equivalent?
Halpha = Wa x Da = 20 x100 Gy = 2000 mSv
= 2 sieverts

53. Detriment
• Radiation exposure of the different organs and tissues in the
body results in different probabilities of harm and different
severity
• The combination of probability and severity of harm is called
“detriment”.

54. Detriment
• Detriment as a measure of the harmful health effects to
individuals or descendants of the exposed individuals that could
occur as a result of radiation exposure at low doses.
• Concept of detriment combines probability, severity and time of
exposure to radiation

55. Detriment
• While the risk factors adopted in ICRP-26 were based principally
on the probability of radiation induced cancer death, the present
recommendations of ICRP-60 uses the concept of radiation
detriment which takes in to account not only the life time
attributable probability of death but also factors such as reduction
of life expectancy and morbidity due to non- fatal cancers and
hereditary effects.

56. Detriment
• Detriment takes in to account:
– Morbidity due to fatal cancers
– Morbidity due to non- fatal cancers
– Reduction of life expectancy, and
– Hereditary effects

57. Detriment
• Detriment is found to vary with the rate at which the dose
is received and, therefore, it is found appropriate to
include a dose rate effectiveness factor in assessing the
overall risk.
• Quantitative number for health detriment is essential to
form a definition of the effective dose, to derive a dose
limit for radiation protection and to provide a basis for
justification and optimization in radiation protection.

58. Tissue Weighting Factor
• To reflect the combined detriment from stochastic effects due to
the equivalent doses in all the organs and tissues of the body,
the equivalent dose in each organ and tissue is multiplied by a
tissue weighting factor, WT, and the results are summed over
the whole body to give the effective dose E.

59. Tissue Weighting Factor
Multipliers used for radiation protection purposes to
account for the different sensitivities of the organs and
tissues to the induction of stochastic effects of
radiation.
The sum of the tissue weighting factors is equal to 1.

60. Tissue Weighting Factor
Tissue or Organ Tissue weighing factor - wT
Gonads 0.20
Bone marrow – red 0.12
Colon 0.12
Lung 0.12
Stomach 0.12
Bladder 0.05
Breast 0.05
Liver 0.05
Oesophagus 0.05
Thyroid 0.05
Skin 0.01
Bone surface 0.01
Remainder 0.05

61. Effective Dose, E
• For a single organ
• E = T wT.HT
• E: effective dose
• wT: weighting factor for organ or tissue T
• HT: equivalent dose in organ or tissue T

62. Effective Dose - E
The sum of the equivalent doses in all the tissues of
the body multiplied by the tissue weighting factors
wT- tissue weighting factor
The unit of effective dose is J.kg-1, termed the
sievert (Sv)
E w H w w DT T T R T R
RTT
= =  . . . ,

63. Effective Dose (E = wT H)
• If a patient undergoes a specific organ imaging nuclear
medicine procedure, how do we assess the risk?
• Situation: We measure the dose (physical quantity) to the
patient (0.25 Gy). We know the radiation weighting factor for
gamma radiation (I-131)
• Problem: A limited body-region of the patient is exposed.
What does “risk” mean when a “single” tissue is irradiated?
• Resolution: The “Effective dose” (E) assesses risk by
modifying the dose equivalent using a tissue weighting factor
wT provided in ICRP - 60.

64. Effective Dose:
Thyroid Irradiation
Diagnostic: Nuclear Medicine Procedure Iodine - 131 Imaging.
Dose 0.25 Gy
Dose Equivalent: HTh = WR x 0.25 Gy
= 0.25 sieverts.
Effective Dose (E or HE)
E = HE = WT x HT = 0.05 x 0.25
= 0.125 Sv = 12.5 mSv

65. Committed Equivalent Dose HT(t)
• Irradiation from incorporated radionuclides is spread out in the
time after ingestion or inhalation of radioactive material, it
depends on its physico-chemical form and biokinetic.
• HT(t) is the time integral over the time t of the equivalent dose
rate in particular tissue that will be received by an individual
following an intake of radioactive material
• Integration period ô for adults is 50 years and for children 70 years
H H t dtT T
t
t
( ) ( )
.
t
t
=
+

0
0

66. Committed Effective Dose E(t)
E(t)- is the sum of weighted committed tissue or
organ doses from an intake of radioactive material
The unit of committed effective dose and committed
equivalent dose is J.kg-1, termed the sievert (Sv)
E w HT T
T
( ) . ( )t t=

67. Collective Dose
• This is used to measure the total impact of a radiation practice
or source on all the exposed persons
• For example diagnostic radiology
• Measured in man-sievert (man-Sv)

68. Entrance Surface Dose (ESD)
• The ESD is measured on the surface of a patient or a phantom
• It can also be calculated by using the tube output and geometrical
description of the procedure
• It can be used as a metric to estimate risk of deterministic skin effects
TLD or film

69. Entrance Surface Dose (ESD)
• Absorbed dose is a property of the absorbing medium as well
as the radiation field, and the exact composition of the
medium should be clearly stated.
• Usually ESD refers to soft tissue (muscle) or water
• Absorbed dose in muscle is related to absorbed dose in air by
the ratio of the mass energy coefficients

70. Entrance Surface Dose (ESD)
❖ The obtained value for all typical diagnostic X Ray
qualities can be assumed to be 1.06 (± 1%)
❖ where (µen/) are the mass energy coefficients of water and
air, respectively.
06.1



















=
air
en
Water
en
F





71. Entrance Surface Dose (ESD)
• On the other hand, the ESD measured on the surface of the
patient or phantom includes a contribution from photons
scattered back from deeper tissues, which is not present for
free air measurements
• For this reason, correction factor (backscatter factor) must be
introduced
• If measurements are made at other distances than the true
focus-to-skin distance, doses must be corrected by the inverse
square law

72. Backscatter Factors (Water)
HVL Field size (cm x cm)
mm Al 10 x 10 15 x 15 20 x 20 25 x 25 30 x 30
2.0 1.26 1.28 1.29 1.30 1.30
2.5 1.28 1.31 1.32 1.33 1.34
3.0 1.30 1.33 1.35 1.36 1.37
4.0 1.32 1.37 1.39 1.40 1.41

73. Dose Area Product
• The dose-area product (DAP) quantity is defined as the dose in
air in a plane, integrated over the area of interest
• The DAP (cGy·cm2) is constant with distance since the cross
section of the beam is a quadratic function which cancels the
inverse quadratic dependence on dose
• This is true neglecting absorption and scattering of radiation in
air and even for X Ray housing near the couch table

74. Inverse Square Law
D
2D
3D
1
3
2
4 1
2
3
4
5
6
7
8
9

75. Dose-Area Product Meter

76. Dap-Meter (Diamentor ®)

77. Dose Area Product
• It is always necessary to calibrate and to check the
transmission chamber for the X Ray installation in use
• In some European countries, it is compulsory that new
equipment is equipped with an integrated ionization
transmission chamber or with automatic calculation
methods
• It is convenient, in this case, also to check the read-out as
some systems overestimate the real DAP value

78. • The Mean Glandular Dose (MGD) is the special dose quantity
used in mammography.
• It is defined as the mean, or average, dose to the glandular tissue
within the breast.
The Average Glandular Dose (AGD)
(Mammography)

79. The Average Glandular Dose (AGD)
(Mammography)
• The Average Glandular Dose (AGD) is the dosimetry quantity
generally recommended for risk assessment
• The use of AGD is recommended by the ICRP, the British
Institute of Physical Sciences in Medicine, the NCRP, the BSS
and the Netherlands Commission on Radiation Dosimetry
(NCS)

80. • The assumption is that the glandular tissue, and not the fat, is
the tissue at risk from radiation exposure.
• Obviously, it is just about impossible to determine the actual
dose to the glandular tissue during a specific mammographic
procedure because of variations in breast size and
distribution of glandular tissue within the breast.
• The MGD is based on some standard breast parameters.
The Average Glandular Dose (AGD)
(Mammography)

81. The Average Glandular Dose AGD
(Mammography)
• The AGD cannot be measured directly but it is derived from
measurements with the standard phantom for the actual
technique set-up of the mammographic equipment
• The Entrance Surface Air Kerma (ESAK) free-in-air (i.e.,
without backscatter) has become the most frequently used
quantity for patient dosimetry in mammography
• For other purposes (compliance with reference dose level) one
may refer to ESD which includes backscatter

82. The ESAK (Mammography)
• ESAK can be determined by:
– a TLD dosimeter calibrated in terms of air kerma free-in-air at a
HVL as close as possible to 0.4 mm Al with a standard phantom
– a TLD dosimeter calibrated in terms of air kerma free-in-air at a
HVL as close as possible to 0.4 mm Al stuck to the patient skin
(appropriate backscatter factor should be applied to Entrance
Surface Dose measured with the TLD to express ESAK)
– Note: due to low kV used the TLD is seen on the image
– a radiation dosimeter with a dynamic range covering at least 0.5
to 100 mGy (better than  10% accuracy)

83. Dosimetric Quantity For C.T.
• CTDI (Computed Tomography Dose Index)
• DLP (Dose-Length Product)
• MSAD (Multiple Scan Average Dose)

84. Computed Tomography Dose Index
(CTDI)
• The CTDI is the integral along a line parallel to the axis of
rotation (z) of the dose profile (D(z)) for a single slice, divided
by the nominal slice thickness T
• In practice, a convenient assessment of CTDI can be made using
a pencil ionization chamber with an active length of 100 mm so
as to provide a measurement of CTDI100 expressed in terms of
absorbed dose to air (mGy).
D(z)dz
T
1
=
+
-
CTDI 



85. Computed Tomography Dose Index
(CTDI)
• measurements of CTDI may be carried
out free-in-air in parallel with the axis of
rotation of the scanner (CTDI100, air)
• or at the centre (CTDI100, c)
• and 10 mm below the surface (CTDI100,
p) of standard CT dosimetry phantoms
• the subscript ‘n’ (nCTDI) is used to
denote when these measurements have
been normalised to unit mAs.
= dxxDCTDI s )(1

86. Computed Tomography Dose Index
(CTDI)• On the assumption that dose in a particular phantom decreases
linearly with radial position from the surface to the centre, then the
normalised average dose to the slice is approximated by the
(normalised) weighted CTDI: [mGy(mAs)-1]
• where:
– C is the tube current x the exposure time (mAs)
– CTDI100,p represents an average of measurements at four
different locations around the periphery of the phantom
)( CTDI
3
2
+CTDI
3
1
C
1
=CTDI p100,c100,wn

87. Reference Dose Quantities
• Two reference dose quantities are proposed for CT in order to promote the
use of good technique:
– CTDIw in the standard head or body CT dosimetry phantom for a single
slice in serial scanning or per rotation in helical scanning: [mGy]
where:
– nCTDIw is the normalised weighted CTDI in the head or body phantom
for the settings of nominal slice thickness and applied potential used for
an examination
– C is the tube current x the exposure time (mAs) for a single slice in
serial scanning or per rotation in helical scanning.
CCTDI=CTDI wnw

88. Reference Dose Quantities
• DLP Dose-length product for a complete examination: [mGy.cm]
where:
– i represents each serial scan sequence forming part of an
examination
– N is the number of slices, each of thickness T (cm) and
radiographic exposure C (mAs), in a particular sequence.
N.B.: Any variations in applied potential setting during the
examination will require corresponding changes in the value of
nCTDIw used.
CNTCTDI=DLP wn
i


89. Reference Dose Quantities
• In the case of helical (spiral) scanning [mGy • cm]:
• where, for each of i helical sequences forming part of an
examination:
– T is the nominal irradiated slice thickness (cm)
– A is the tube current (mA)
– t is the total acquisition time (s) for the sequence.
• N.B.: nCTDIw is determined for a single slice as in serial
scanning.
tATCTDI=DLP wn
i


90. • Multiple Scan Average Dose (MSAD): The average dose across
the central slice from a series of N slices (each of thickness T)
when there is a constant increment between successive slices:
• where:
• DN,I(z) is the multiple scan dose profile along a line parallel to
the axis of rotation (z).
Reference Dose Quantities
(z)dzD=MSAD IN,
2
I
+
2
I
-
1
I