Radiation Units and protection

Radiation Units

Radiation Protection

Introduction

Determination of the energy imparted to matter by radiation is the main subject of Dosimetry.

The imparted energy is responsible for the effects that radiation causes in matter, for instance, a rise in temperature, or chemical or physical changes in the material properties.

Several of the changes produced in matter by radiation are proportional to the absorbed dose, giving rise to the possibility of using the dosimeter material sensitive part to measure absorbed dose.

Introduction

Radiation discussion involves the measure of two phenomena which are activity and exposure.

Activity is basically just how much radiation is coming out of the material, whether it's particles or waves i.e. The rate of disintegration of a radioactive material.

Exposure measures the effect of radiation on substances that absorb it.

Radiation Activity

Radiation activity is measured in an international (SI) unit called a Becquerel (Bq).

The Becquerel counts how many particles or photons (in the case of wave radiation) are emitted per second by a source.

Radiation exposure

Radiation exposure is expressed in several ways to account for the different levels of harm caused by different forms of radiation and the different sensitivity of body tissues.

Measurement of the ionization produced by radiation is the first choice used to quantify the passage of radiation through matter.

Radiation exposure

Radiation exposure is a measure of the ionization of air due to ionizing radiation from high-energy photons (i.e. X-rays and gamma rays).

Radiation exposure is defined as the sum of electrical charges (∆q) on all the ions of one sign produced in air when all the electrons, liberated by photons in a volume of air whose mass is ∆m, are completely stopped in air.

Radiation exposure

Radiation exposure is given the symbol X. The SI unit of radiation exposure is the coulomb per kilogram (C/kg), but in practice, the roentgen is used.

The roentgen, abbreviated R, is the unit of radiation exposure. In the original definition 1 R means the amount of X-rays or γ-radiation that is required to liberate positive and negative charges of one electrostatic unit of charge in 1 cm³ of dry air at (STP)

Radiation exposure

one roentgen corresponds to 2.58 x 10-4 coulomb per kg of ions generated in air.

The calculation of radiation dose (in Gy) from a radiation exposure of 1 R depends on the energy of the X-rays or γ-rays and the composition of the irradiated material. For example, if soft tissue is exposed to γ-rays of 1 R, the radiation dose will be approximately 9.3 milligray (mGy)

Absorbed dose

Dose is defined as the amount of energy deposited by ionizing radiation in a substance.

For a given radiation field, the absorbed dose will depend on the type of matter which absorbs the radiation.

Although a large number of possible interactions are known, there are three key interaction mechanisms of gamma rays with matter Photoelectric effect

• Compton scattering

Pair production

Absorbed dose

Absorbed dose is the radiation energy deposited per unit mass of the material.

The definition of absorbed dose is the quotient dE/dm. where dE is the mean energy imparted by ionizing radiation to material of mass dm.

The quantity absorbed dose has been defined to describe the quantity of radiation for all types of ionizing radiation, including charged and uncharged particles; all material; and all energy.

Absorbed dose

Absorbed dose is a measure of the biologically significant effects produced by ionizing radiation.

The old unit of absorbed dose is RAD (an acronym for “radiation absorbed dose”) and represent the absorption of 100ergs of energy per gram of absorbing material.

1 rad = 100 ergs/g = 10-2J/Kg

Absorbed dose

• The SI unit for absorbed dose is Gray (Gy) and is defined as

1Gy = 1 J/Kg

• Thus, the relationship between Gray, cent gray and rad is

1Gy = 100rad = 100cGy.

Equivalent dose

Because the biologic effects of radiation depend not only on dose, but also on the type of radiation, the dosimetry quantity relevant to radiation protection is the dose equivalent (H).

• It is defined as

H = D.Q

Where D is the absorbed dose and Q is the quality factor for the radiation.

Equivalent dose

Equivalent dose = absorbed Dose multiplied the appropriate radiation weighting factor.

The radiation weighting factors are needed because different types of radiation (like alpha, beta, gamma, and neutrons) can have different effects even if the absorbed dose is the same.

Weighting factors

Equivalent dose

Equivalent dose is expressed in sieverts (Sv), or, more frequently, millisieverts (mSv) which are 1/1000th of a sievert, and the organ should always be specified (for example "25 mSv to the skin").

In the simplest cases, for gamma (photon) and beta (electron) radiation, the radiation weighting factor is 1, and therefore, for example, an absorbed dose of 1 mGy in an organ equals an equivalent dose of 1 mSv to that organ.

Equivalent dose

The SI unit for both dose and dose equivalent is Joules per kilogram, but the special name for the SI unit of dose equivalent is SIEVERT (Sv).

1Sv = 1 J/Kg.

• The older unit of dose equivalent is REM

1rem = 10-2Sv

Equivalent dose

Equivalent dose is calculated for individual organs.

It is based on the absorbed dose to an organ, adjusted to account for the effectiveness of the type of radiation.

Equivalent dose is expressed in millisieverts (mSv) to an organ.

The use of quality factor in radiation protection is analogous to the use of relative biologic effectiveness (RBE) in radiation biology.

Effective dose

Whole-body exposures are rarely uniform For a given exposure received, internally or externally, dose equivalents for various tissues may differ markedly.

Also, tissue vary in sensitivity to radiation-induced effects.

To take into account these non-uniform irradiation situation the concept of effective dose has been adopted by ICRP (internal commission for radiation protection)

Effective dose

Effective dose = sum for all organs of (equivalent dose to the organ times the appropriate tissue weighting factor).

The tissue weighting factors are needed because different organs have different levels of sensitivity to radiation, even if the equivalent dose is the same Effective dose is expressed in sieverts (Sv), or, more frequently, millisieverts (mSv) which are 1/1000th of a sievert. This is the most frequently used dose in radiological protection.

Effective dose

The effective dose (HE) is defined as “the sum of the weighted dose equivalents for irradiated tissues or organs.

HE = ƩWT.HT

Where WT is the weighting factors of tissue T and HT is the mean dose equivalent received by tissue T.

Radiation Protection

Radiation protection is defined by International Atomic Energy Agency (IAEA) as the process of protection of people and environment from the harmful effects of exposure to ionizing radiation.

The purpose of radiation protection is to provide an appropriate level of protection for humans without limiting the beneficial actions.

Radiation protection is to prevent the occurrence of harmful deterministic effects and to reduce the probability of occurrence of stochastic effects (e.g. cancer and hereditary effects).

Radiation Safety

Radiation safety principles aim to limit exposure to ionizing radiation for radiation workers, people exposed to radiation for medical reasons (diagnostic and therapeutic) and the general public.

Wherever there is known risk of exposure to ionizing radiation, health professionals must be guided by the ALARA (as low as reasonably achievable) principles of radiation safety for time, distance and shielding.

Radiation Safety

• Time

The less time spent near a radiation source, the less radiation absorbed (less exposure) This is especially important for personnel such as radiation therapists diagnostic radiographers, physicists preparing radioactive sources, and for nursing staff when caring for individuals who have a radioactive source in a body tissue or cavity.

For inpatients, the nurse should restrict direct contact to 30 minutes per eight-hour shift

Radiation safety

• Distance

The inverse-square law states that radiation exposure and distance are inversely related.

That means that as the distance from the source increases, the intensity of radiation decreases.

To calculate exposure, the rule to use is that the amount of radiation exposure at one meter from the radioactive source equals the amount of radiation exposure at any distance from the source times the distance squared

Radiation Safety

Shielding

The type of shielding device used depends on the range of emission of the radioactive source. The common material used in radiation shielding is LEAD.

Standard shielding devices include lead aprons, thyroid shields, and eye shields.

Rooms that house x-ray generating equipment are shielded using specified materials. Radioactive sources need to be transported by licensed personnel in lead containers.

Radiation Safety

Radiation Protection

• In a radiology facility, consideration needs to be given to the:
• patient
• staff involved in performing the radiological procedures

members of the public

other staff that may be in the radiology facility, carers and comforters of patients undergoing procedures, and persons who may be undergoing a radiological procedure as part of a biomedical research project

Radiation Protection

The means to achieve the objectives of radiation protection have been put forward by the International Commission of Radiological Protection (ICRP) The ICRP Publication 103 divides all possible situations where radiological exposure can occur into three types:

• planned exposure situations
• emergency exposure situations

existing exposure situations

Radiation Protection

Normal exposures: occur in the daily operation of a radiology facility with reasonably predictable magnitudes Potential exposures: are unintended exposures or accidents. These exposures remain part of the planned exposure situation as their possible occurrence is considered in the granting of an authorization The ICRP then divides exposure of individuals (both normal and potential) into three categories :

• occupational exposure
• public exposure

medical exposure

Radiation Protection

Radiation exposures of workers incurred as a result of their work, in situations which can reasonably be regarded as within the responsibility of the employing or operating management is regarded as Occupational exposure Public exposure includes all exposure other than occupational or medical exposures, and covers a wide range of sources of which natural sources are by far the largest

Radiation Protection

Public exposure in a radiology facility would include exposure:

to persons who may happen to be close to or within the facility and potentially subject to radiation penetrating the walls of an X ray room of the embryo and foetus or pregnant workers Medical exposure is divided into three components:

• patient exposure
• biomedical research exposure
• carers and comforters exposure

An individual person may be subject to one or more of these categories of exposure.

Radiation Protection

• The ICRP system of radiation protection has 3 fundamental principles:
• Dose limitation
• Justification of practice

Optimization

Radiation protection

• Justification

“Any decision that alters the radiation exposure situation should do more good than harm.

Medical exposures shall be justified by weighing the expected diagnostic or therapeutic benefits against the potential radiation detriment, with account taken of the benefits and the risks of available alternative techniques that do not involve exposure to radiation. The procedure should be judged to do more good than harm.

Radiation Protection

• Dose limitation

“The total dose to any individual should not exceed the appropriate limits” Different limit values apply for the general population and for persons occupationally exposed to radiation.

Radiation Protection

Optimization of practice

Optimization of protection should ensure the selection of the best protection option under the prevailing circumstances i.e maximizing the margin of good over harm.

Thus,optimization involves keeping exposures AS LOW AS RESONABLY ACHIEVABLE(ALARA).

It means adjusting the quality and quantity of the radiation to the body habitus of the patient to use only the dose necessary for producing a study from which a diagnosis can be made with confidence.

ALARA Vs ALADA

ALARA as reffered to As Low As Resonably Achievable refers to the continual application of the optimization principle in the day to day practice.

Because of some risk however small ,exists from any radiation dose .All doses should be kept ALARA.

ALADA(As Low As Diagnostically Acceptable ) concerns that the dose rather than the diagnostic utility of the image may become the overriding metric of quality.

Radiation Units and protection

This is not the intent of ALARA and it certainly can be applied to medicine if correctly interpreted ,some see the distinction useful to keep the right balance on the elements that most affect patient care.

Therefore ,reducing the examination dose to the point where diagnostic information is lost or that results in the exam needing a repetition is counterproductive and increases rather than decreases the overall risk to the patient.

Radiation Protection

• Diagnostic Reference Levels (DRLs)

This gives the guideline of the dose to be received by a person having a diagnostic test.

This reference are not legal limits for the patient since patients have no legal limit but the dose should be kept as low as reasonable achievable.

Diagnostic reference levels

Dose Limits

Radiation Protection

Classified workers

This is anyone who is likely to receive effective dose of 6 mSv in a year (3/10 of dose limit).

• Classified workers must:
• Must have a medical examination before being designated
• Must have periodic review of health at least once a year Must be at least 18 years old

Records of doses received by classified workers must be kept for at least 50 years.

Designation of special areas

Controlled areas

A controlled area is a limited access area in which the occupational exposure of personnel to radiation is under the supervision of an individual in charge of radiation protection.

In facilities that use x-rays for medical imaging, these areas are usually in the immediate areas where X-ray equipment is used, such as x-ray procedure rooms and x-ray control booths or other areas that require control of access.

Designation of special Areas

The workers in these areas are primarily radiologists and radiographers who are specifically trained in the use of ionizing radiation and whose radiation exposure is usually individually monitored.

Controlled area of the radiation therapy, diagnostic radiology and nuclear medicine department should be directed by appropriate signs and posters for safety purposes. If possible, controlled area should be guarded by people in the department.

Designation of special areas

Examples of controlled areas are the treatment control room and other areas near to the treatment room may also be designated as controlled areas.

Designation of special areas

Supervised areas

Supervised area is defined as any area not designated as a controlled area, but which occupational exposure conditions are kept under review even though specific protective measures and safety provisions are not normally needed.

Example of supervised areas includes patients waiting areas.

Designation of special areas

Controlled areas

Any person working in the area is likely to receive an effective dose of > 6 mSv or equivalent dose of > 3/10 of any relevant dose limit Any person who enters or works in area must follow special procedures to restrict significant exposure.

Designation of special areas

Supervised area

Required if anyone working in the area is likely to receive a dose > 1 mSv/year or an equivalent dose of > 1/10 of any relevant dose limit (i.e. more than the dose limits for the general public).