Describes different units of radiation dose and the dose limits in diagnostic radiology imaging. Discuses different radiation units described by ICRU. Describes different radiation dose limits given by different organizations like ICRP, NCRP, AERB.

2. Introduction
 Exposure of the general public, patients, and radiation workers to ionizing radiation must be
limited to minimize the risk of harmful biologic effects.
 To this end, scientists have developed occupational and non-occupational effective dose (EfD)
limits and equivalent dose (EqD) limits for tissues and organs such as the lens of the eye, skin,
hands, and feet.

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4. Equivalent Dose
The equivalent dose in tissue T is given by the expression:
HT =  WR DT,R
where DT,R is the absorbed dose averaged over the tissue or organ T, due to radiation R, organ and
weighted by the radiation weighting factor (WR)
 In radiological protection, it is the absorbed dose averaged over a tissue or organ
 It is weighted for the radiation quality of interest
 The weighting factor is called the radiation weighting factor, WR

5. Equivalent Dose
 WR is selected for the type and energy of the radiation incident on the body
 This weighted absorbed dose, called the equivalent dose, is strictly a dose
 The unit of equivalent dose is the joule per kilogram with the special name of sievert (Sv)

6. Radiation Weighting Factors
Type and Energy Range wR
Photons: all energies 1
Electrons and muons: all energies 1
Alpha particles, Fission fragments, heavy
ions
20
Protons and charged pions 2
neutrons function depending on
neutron energy, En

7. Radiation Weighting Factors
Radiation Type and Energy Range wR
Neutrons
A continuous function
of neutron energy

8. Effective Dose
The effective dose is the sum of the weighted equivalent doses in all the tissues and organs of the body.
It is given by:
E = wT HT
where HT is the equivalent dose in tissue or organ T and wT is the weighting factor for tissue T.

9. Tissue Weighting Factors
 Account for fact that the probability of stochastic effects depends on the organ or tissue
irradiated
 Represent the relative contribution of irradiation of each organ or tissue to the total
detriment due to the effects resulting from uniform irradiation of the whole body
 Achieved by normalizing the sum of the tissue weighting factors to one
 This factor “indicates the ratio of the risk of stochastic effects attributable to
irradiation of a given organ or tissue (T) to the total risk when the whole body is
uniformly irradiated.”

10. Tissue Weighting Factors - Used in calculating Effective Dose ( ICRP 103)
 In 2007, the International Commission on
Radiological Protection (ICRP) published a new
set of tissue weighting factors as below:
 WT = 0.12 (for each of 6): stomach, colon, lung,
bone marrow (red), breast & remainder tissues*
 WT = 0.08: gonads
 WT = 0.04: urinary bladder, esophagus, liver,
thyroid
 WT = 0.01: bone surface, skin, brain, salivary
glands
 *Remaining tissues, collectively (13 organs):
adrenals, extra-thoracic region, gallbladder, heart,
kidney, lymph nodes, muscle, oral mucosa,
pancreas, small intestine, spleen, thymus, and
uterus/cervix (♀), prostate(♂).
 Note: Changes in respective WT from last ICRP
publication-60 of 1991 to ⟶ current ICRP
publication-103 of 2007 are:
 gonads: 0.20 ⟶ 0.08
 breast and reminder tissue: 0.05 ⟶ 0.12
 bladder, esophagus, liver, thyroid: 0.05 ⟶ 0.04
 addition of brain and salivary glands to 0.01
category

11. QUANTITIES USED IN RADIOLOGICAL
PROTECTION
 For a monitoring programme to be simple and effective, individual dosimeters and survey meters must
be calibrated using a quantity that approximates effective or equivalent dose
 Effective dose represents the uniform whole body dose that would result in the same radiation risk as
the non-uniform equivalent dose, which for X rays is numerically equivalent to absorbed dose.
 In concept at least, it is directly related to stochastic radiation risk and provides an easy to understand
link between radiation dose and the detriment associated with that dose.
 However, it is an abstract quantity that is difficult to assess and impossible to measure directly.
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12. QUANTITIES USED IN RADIOLOGICAL
PROTECTION
 The need for readily measurable quantities that can be related to effective dose and equivalent dose
has led to the development of operational quantities for the assessment of external exposure.
 Defined by the International Commission on Radiation Units and Measurements, the operational
quantities provide an estimate of effective or equivalent dose that avoids both underestimation and
excessive overestimation in most radiation fields encountered in practice.
 The operational quantities are defined for practical measurements in both area and individual
monitoring.
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13. Operational Quantities
 For radiation measurement purposes, the following operational quantities are defined:
 Ambient dose equivalent
 Directional dose equivalent
 Personal dose equivalent
 Where doses are estimated from area monitoring results, the relevant operational quantities are
ambient dose equivalent and directional dose equivalent
 For individual monitoring, the use of the personal dose equivalent is recommended

14. Area Monitoring
 The quantities recommended for area monitoring refer to a phantom termed the ICRU sphere.
 The ICRU sphere (ICRU, 1980) is a
 30 cm diameter,
 tissue-equivalent sphere with a density of 1 g cm-3 and
 a mass composition of 76.2% oxygen, 11.1% carbon, 10.1% hydrogen, and 2.6% nitrogen

15. ICRU Reference Sphere
Radiation field
This slide shows the radiation field
impinging on the ICRU reference sphere
of radius 15 cm, diameter 30 cm.
d is the reference depth at which the dose
quantity is calculated.

16. Ambient Dose Equivalent
 The ambient dose equivalent, H*(d), at a point, is the dose equivalent that would be produced by
the corresponding expanded and aligned field, in the ICRU sphere at a depth d in millimeters on
the radius opposing the direction of the aligned field.
 For measurement of strongly penetrating radiations, the reference depth is 10 mm and the
quantity denoted as H*(10).
 The unit is J kg-1
 The special name for the unit of ambient dose equivalent is sievert (Sv)

17. Expanded Field
 An expanded radiation field is defined as a hypothetical radiation field in which the
fluence, and its angular and energy distributions, have the same value throughout the
volume of interest as the actual field at the point of reference.
 The expanded radiation field ensures that the whole ICRU sphere is exposed to a
homogeneous radiation field with the same fluence, energy distribution and direction
distribution as the real radiation field at the point of interest.

18. Expanded fields
Field at point, P
Expanded field
P P P
P

This illustrates the field concepts described
in the definitions of ambient and
directional dose equivalent. In this slide,
the field is expanded, but not aligned.
That is, the sphere is uniformly illuminated
by the radiation is multi-directional.

19. Expanded and Aligned Field
An expanded and aligned field is a hypothetical field in which the fluence and its energy
distribution are the same as in the expanded field but the fluence is unidirectional

20. Expanded and aligned field
P
In this figure, the field is expanded and aligned.

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22. Directional Dose Equivalent
 The directional dose equivalent, H‘(d,), at a point, is the dose equivalent that would be produced
by the corresponding expanded field in the ICRU sphere at a depth d on a radius in a specified
direction .
 Directional dose equivalent is of particular use in the assessment of dose to the skin or eye lens
 The unit is J kg-1
 The special name for the unit of directional dose equivalent is sievert (Sv)

23. Personal Dose Equivalent
 The personal dose equivalent, Hp(d), is the dose equivalent in soft tissue, at an appropriate
depth d, below a specified point on the body,
 Hp(d) can be measured with a dosimeter which is worn at the surface of the body and
covered with an appropriate thickness of tissue-equivalent material.
 Since Hp(d) is defined in the body, it cannot be measured directly and will vary from
person to person and also according to the location on the body where it is measured.
 However, practically speaking, personal dose equivalent can be determined using a detector
covered with an appropriate thickness of tissue equivalent material and worn on the body.

24. Personal Dose Equivalent
 The unit is J kg-1
 The special name for the unit of personal dose equivalent is sievert (Sv)
 Hp(10), measured at a depth of 10 mm in soft tissue, is the operational
surrogate for the effective dose, E

25. The dose to the lens of the eye
is measured at a depth of 3mm Hp(3)
The shallow (skin) dose is
measured at a depth of 0.07mm Hp(0.07)
The whole body dose is
measured at a depth of 10mm Hp(10)

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27. CURRENT RADIATION PROTECTION PHILOSOPHY
 Current radiation protection philosophy is based on the
assumption that a linear non-threshold relationship exists
between radiation dose and biologic response.
 Thus, even the most minuscule dose of radiation has a non-
zero potential to cause some harm.
 It proposes that, when employed in the healing arts for the
welfare of the patient:
 The potential benefits of exposing the patient to
ionizing radiation must far outweigh any potential
risk
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28. Effective Dose Limiting System
 The effective dose limit concerns the upper boundary dose of
ionizing radiation that results in a negligible risk of:
 Bodily injury
 Hereditary damage
 These limits may be expressed for whole-body exposure,
partial-body exposure, and exposure of individual organs.
 Separate limits are set for occupationally exposed individuals
and for the general public.
 The sum of both the external and internal whole-body
exposures is considered when EfD limits are established.
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29. Effective Dose Limiting System
 These upper limits are designed to minimize the risk to humans
in terms of deterministic and stochastic effects, and they do not
include natural background and medical exposure.
 The potential for terminal cancer, hereditary imperfections
induced by reproductive cell mutations, shortening of life span
because of the induction of cancer, or other abnormalities and
the overall poorer quality of life are taken into account.
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30. Revised Concepts of Radiation Exposure and Risk
 Revised concepts of radiation exposure and risk have brought about more recent
changes in NCRP recommendations for limits on exposure to ionizing radiation.
 Because many conflicting views exist on assessing the risk of cancer induction from
low-level radiation exposure, the trend has been to:
 Create more rigorous radiation protection standards
 The adoption of the EfD limiting system is a direct consequence of this conservatism.
 The benefit obtained from any diagnostic imaging procedure must always be
weighed against the risk that is taken.
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31. BASIS OF EFFECTIVE DOSE LIMITING SYSTEM
 The concept of radiation exposure and of the
associated risk of radiation-induced malignancy
is the basis of the effective dose limiting system.
 A summary and review of information on the
biological effects of ionizing radiation are
included in ICRP Publication 103 and, on the basis
of the review, quantitative estimates are made of
the consequences of radiation exposure.
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32. BASIS OF EFFECTIVE DOSE LIMITING SYSTEM
 The ICRP system of radiological protection aims to protect human health by
preventing detrimental tissue reactions and reducing the risk of stochastic effects to
an acceptable level.
 This is achieved by setting dose limits at levels that are sufficiently low to ensure
that no threshold dose is reached, even following exposure for the whole of an
individual’s lifetime – prevention of harmful tissue reactions;
 And keeping all justifiable exposures as low as is reasonably achievable, economic
and social factors being taken into account, subject always to the boundary
condition that the appropriate dose limits shall not be exceeded – reducing the
risk of stochastic effects.
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33. Occupational Risk
 The risk to a radiographer from radiation exposure may be equated with occupational
risk in other industries that are generally considered reasonably safe.
 That risk is generally estimated to be a 2.5% chance of fatal accident over an entire
career.
 To ensure that the hazard to radiation workers is no greater than the hazard to the
general working public, the NCRP proposes that radiation protection programs for
radiation workers be designed to prevent individual workers from having a total
external plus internal cumulative EfD in excess of their age in years times 10 mSv.
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34. Cumulative Effective Dose (CumEfD) Limit
 A radiation worker's lifetime effective dose must be limited
to his or her age in years times 10 mSv. This is called the
cumulative effective dose (CumEfD) limit and pertains to
the whole body. Adhering to this limit ensures that the
lifetime risk for these workers remains acceptable. CumEfD
limits, however, do not include:
 Radiation exposure from natural background radiation
 Exposure acquired as a consequence of a worker's
undergoing medical imaging procedures
 The limits do include the possibility of both:
 Internal exposure & External exposure
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35. Cumulative Effective Dose (CumEfD) Limit
 Consider the following situation:
 A worker at age 40 years has been employed at a nuclear power plant for 10 years.
 He had previously been employed as a radiation worker in another industry, during the
course of which he received a cumulative EfD of 100 mSv.
 Therefore the radiation protection program for his current position should have
ensured that he has not accumulated a total EfD greater than 300 mSv during his 10
years of employment.
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36. International Commission on Radiological Protection
Recommendation for Downward Revision of the Annual
Effective Dose Limit
 In 1991 the ICRP recommended that the annual EfD limit for occupationally exposed persons be
reduced from 50 mSv to 20 mSv as a result of
 Newer information obtained regarding the Japanese atomic bomb survivors in whom the risk
of radiation from the atomic bomb detonations was estimated to be approximately three to
four times greater (more damaging) than previously estimated.
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37. International Commission on Radiological Protection
Recommendation for Downward Revision of the Annual
Effective Dose Limit
 The NCRP has not acted yet but very well might recommend lower limits on exposure because of
the:
 Revised risk estimates derived from the more recent reevaluations of dosimetric studies on the
atomic bomb survivors of Hiroshima and Nagasaki
 Appearance, as a result of longer follow-up time, of increased numbers of solid tumors in the
survivor population
 Therefore, in the future, the annual whole-body EfD limit for occupationally exposed persons in
the United States may be limited to 10 to 20 mSv per year.
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38. Limits for Non-occupationally Exposed Individuals.
 In addition to limits for occupationally exposed individuals, the NCRP sets limits for non-
occupationally exposed individuals who are not undergoing medical imaging procedures. An
example would be a person accompanying a patient to the imaging department such as a:
 Spouse
 Parent
 Guardian
 A limit also has been set for individual members of the general public not occupationally exposed.
The NCRP-recommended annual EDL is 1 mSv for continuous or frequent exposures from
artificial sources other than medical irradiation and natural background and a limit of 5 mSv
annually for infrequent exposure.
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39. Vulnerability of the Embryo-Fetus to Radiation Exposure
 Epidemiologic studies of atomic bomb survivors exposed in utero provided conclusive
evidence of a dose-dependent increase in the incidence of severe intellectual disability for
fetal doses greater than approximately 0.4 Sv.
 The greatest risk for radiation-induced intellectual disability occurred when the embryo-
fetus was exposed 8 to 15 weeks after conception.
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40. Limits for Pregnant Radiation Workers.
 To reduce exposure for pregnant radiation workers and control the exposure to the unborn during
potentially sensitive periods of gestation,
 the NCRP now “recommends” a monthly EqD limit not exceeding 0.5 mSv per month to the
embryo-fetus and a limit during the entire pregnancy not to exceed 5.0 mSv after declaration of
the pregnancy.
 Both limits are proposed to reflect the fact that not all pregnant workers are monitored monthly and
that personnel dosimetry does not result in exact measures of EqD, just approximations based on the
personnel dosimeter readings.
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41. Limits for Pregnant Radiation Workers.
 This 9- month EqD value excludes both medical and natural background radiation.
 It is designed to restrict significantly the total lifetime risk of leukemia and other malignancies in
persons exposed in utero.
 The occurrence of tissue reactions is expected to be statistically negligible if the EqD remains at or
below the recommended limit. These reactions include:
 Small head size
 Intellectual disability
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42. Limits for Pregnant Radiation Workers.
 ICRP: A female worker should, on becoming aware that she is pregnant, notify the employer in
order that her working conditions may be modified if necessary
 The employer shall adapt the working conditions in respect of occupational exposure so as to ensure
that the embryo or foetus is afforded the same broad level of protection as required for members of
the public, that is, the dose to the embryo or foetus should not normally exceed 1 mSv
 In general, in diagnostic radiology it will be safe to assume that provided the dose to the employee’s
abdomen is less than 2 mSv, then the doses to the foetus will be lower than 1 mSv

43. Limits for Education and Training Purposes
 For education and training purposes, the same dose limits should apply to students of radiography
in general and to those individuals under 18 years of age.
 The dose limit is the same for kindergarten through twelfth-grade students attending science
demonstrations involving ionizing radiation as it is for student radiologic technologists who begin
their education before they are 18 years old.
 The limit for any education and training exposures of individuals under the age of 18 years is an
EfD of 1 mSv annually.
 Occasional exposure for the purpose of education and training is permitted, provided special care
is taken to ensure that the annual EfD limit of 1 mSv is not exceeded.
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44. Limits for Education and Training Purposes
 The BSS also adds stronger restrictions on occupational doses for “apprentices” and “students”
aged 16 to 18 – namely dose limits of an:
 effective dose of 6 mSv in a year
 equivalent dose to the lens of the eye of 20 mSv in a year
 equivalent dose to the extremities or the skin of 150 mSv in a year
These stronger dose limits would apply, for example, to any 16-18 year old student radiographers
10/17/2022 45

45. Negligible Individual Dose.
 To provide a low-exposure cutoff level so that regulatory agencies may consider a level of effective
dose as being of negligible risk, an annual negligible individual dose (NID) of 0.01 mSv/year per
source or practice has been set.
 This means that at this EfD level, a reduction of individual exposure is unnecessary.
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46. Action Limits
 Health care institutions go to great lengths to avoid having personnel even approach EfD limits.
 In a well-designed and well-run facility, radiologic technologists' personnel dosimeter readings should
be significantly below a tenth of the maximum EfD limits, even for those technologists who receive
the most exposure.
 Hospitals normally establish their own internal action limits.
 The purpose of these action limits is to trigger an investigation when they are exceeded that should
uncover the reason for any abnormal exposure.
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47. Action Limits
 A prime reason for an unusual reading can result from a personnel dosimeter accidently remaining in an
x-ray room while exposures were being made because it had unknowingly fallen off the uniform of a
technologist.
 Sometimes work habits, such as where the technologist stands during interventional radiography, can
lead to personnel readings that exceed the action limit.
 Corrective actions can then be initiated by the RSO.
 In general, the RSO must be an active participant along with the imaging department manager in an
ongoing program that is designed to prevent personnel from receiving anywhere near the maximum
allowed exposures
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48. Special Limits for Selected Areas
 Because the tissue weighting factors used for calculating EfD are so small for some organs, an
organ that is associated with a low weighting factor may receive an unreasonably large dose,
whereas the EfD remains within the allowable total limit.
 Therefore, special limits are set for the crystalline lens of the eye and localized areas of the skin,
hands, and feet to prevent tissue reactions.
 At the present time, international radiation protection advisory groups such as the ICRP and most
governmental regulatory agencies in other countries limit exposure of the lens of the eye to less
than 20 mSv per year, in view of recent data that show a lower threshold for cataracts than
was originally thought.
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49. Special Limits for Selected Areas
 In the United States, the NCRP is continuing to study the mater but is unlikely to change its
guidance in the near future.
 Although this seems to be a very large discrepancy, the NCRP, in addition to its special limit for
the lens of the eye, maintains a cumulative limit of 10 mSv times the worker's age, which
indicates a de facto lower limit to the lens.
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10/17/2022 52
Diagnostic Radiology Physics: a Handbook for Teachers and Students – chapter 24, 52
Recommended dose limits in planned exposure situationsa (ICRP 103, 2007)
Type of limit Occupational Public
Effective dose 20 mSv per year, averaged over
defined periods of 5 yearse
1 mSv in a yearf
Annual equivalent dose in:
Lens of the eyeb 20 mSv 15 mSv
Skinc,d 500 mSv 50 mSv
Hands and feet 500 mSv –
b this limit is a 2011 ICRP recommendation
c The limitation on effective dose provides sufficient protection for the skin against stochastic effects
d Averaged over 1 cm2 area of skin regardless of the area exposed
e With the further provision that the effective dose should not exceed 50 mSv in any single year Additional restrictions apply to
the occupational exposure of pregnant women
f In special circumstances, a higher value of effective dose could be allowed in a single year, provided that the average over 5
years does not exceed 1 mSv per year

52. AERB Directive No. 01/2011
[Under Rule 15 of the Atomic Energy (Radiation Protection) Rules 2004]
Ref.No. No.CH/AERB/ITSD/125/2011/1507 dated April 27, 2011
 The occupational exposures of any worker shall be so controlled that the following limits are
not exceeded:
an effective dose of 20 mSv/yr averaged over five consecutive years (calculated on
a sliding scale of five years);
an effective dose of 30 mSv in any year;
an equivalent dose to the lens of the eye of 150 mSv in a year;
an equivalent dose to the extremities (hands and feet) of 500 mSv in a year and
an equivalent dose to the skin of 500 mSv in a year;
limits given above apply to female workers also. However, once pregnancy is
declared the equivalent dose limit to embryo/fetus shall be 1 mSv for the
remainder of the pregnancy.
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53. Apprentices and Trainees
 The occupational exposure of apprentices and trainees between 16 and 18 years of age
shall be so controlled that the following limits are not exceeded:
an effective dose of 6 mSv in a year;
an equivalent dose to the lens of the eye of 50 mSv in a year;
an equivalent dose to the extremities (hands and feet) of 150 mSv in a year and
an equivalent dose to the skin of 150 mSv in a year.
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AERB Directive No. 01/2011
[Under Rule 15 of the Atomic Energy (Radiation Protection) Rules 2004]
Ref.No. No.CH/AERB/ITSD/125/2011/1507 dated April 27, 2011

54.  Dose Limits for Members of the Public
 The estimated average doses to the relevant members of the public shall not exceed the
following limits:
 an effective dose of 1 mSv in a year;
 an equivalent dose to the lens of the eye of 15 mSv in a year; and
 an equivalent dose to the skin of 50 mSv in a year.
Signed By Shri S.S. Bajaj, Chairman, AERB
10/17/2022 55
AERB Directive No. 01/2011
[Under Rule 15 of the Atomic Energy (Radiation Protection) Rules 2004]
Ref.No. No.CH/AERB/ITSD/125/2011/1507 dated April 27, 2011

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57. Summary
 Adherence to occupational and non-occupational EfD limits helps prevent harmful
biologic effects of radiation exposure.
 The concept of radiation exposure and the associated risk of radiation-induced
malignancy is the basis of the EfD limiting system.
 Internal action limits are established by health care facilities to trigger an
investigation to uncover the reasons for any unusual high exposures received by
individual staff members.
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58. References
 Bushberg J. The essential physics of medical imaging. 3rd ed. Philadelphia, PA: Wolters Kluwer / Lippincott
Williams & Wilkins; 2012.
 National Council on Radiation Protection and Measurements (NCRP): Ionizing radiation exposure of the
population of the United States, Report No. 160, Bethesda, MD, 2009, NCRP.
 Kelsey C. Radiation biology of medical imaging. Hoboken, NJ: Wiley Blackwell; 2014.
 Forshier S. Essentials of radiation biology and protection. 2nd ed Clifton Park, NY: Delmar; 2009.
 ACR Practice guideline for imaging pregnant or potentially pregnant adolescents and women with ionizing
radiation.
 ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP
Publication 103. Ann. ICRP 37 (2-4)
 ICRP, 2012 ICRP Statement on Tissue Reactions / Early and Late Effects of Radiation in Normal Tissues and
Organs – Threshold Doses for Tissue Reactions in a Radiation Protection Context. ICRP Publication 118. Ann.
ICRP 41(1/2).
 Sherer MAS, Visconti PJ, Russell Ritenour E, Haynes K. Radiation Protection in Medical Radiography. 8th ed.
London, England: Mosby; 2017. 10/17/2022 59

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