Radiation Monitoring

RADIATION MONITORING

Objective

At the end of this lecture you should understand the concept of Personnel and community radiation monitoring , monitoring devices and permissible dose limits.

outline

• Introduction
• Principle of radiation protection
• Personnel and Radiation area monitoring devices
• Permissible dose limits
• Classified person and Non-designated person
• Summary

References

Introduction

Ionizing radiation can’t be seen, felt or sensed by human body in any way but excessive exposure to them may have adverse health effect To avoid the excessive exposure, appropriate and efficient radiation monitoring is needed.

Radiation exposure must be monitored for both personal safety and regulatory purpose

A record of exposure should be part of the employment record of all radiation workers.

The radiation dosimetry report will provide average annual effective dose[EfD] to the whole body.

Principle of radiation protection

RADIATION PROTECTION

• When to protect ?
• Why to protect ?
• Whom to protect ?

How to protect ?

Protection against radiation

The purpose of the radiation protection standard settings define by the International Commission on Radiological Protection (ICRP) are.

• 1.Prevent the occurrence of deterministic effects
• of Radiation.
• 2.Limit the radiation to acceptable level that the
• occurrence of stochastic effect can be

prevented

Framework of Radiation Protection(System of Radiation Protection)

System of Radiation Protection” is the name given by the ICRP to the application of the 3 basic principles of Radiation Protection (no part should be taken in isolation):

• Justification of practice
• Optimization of protection
• Application of individual Dose Limits

Minimum risk, maximum benefit

How do we protect?

Patient Protection

• Cardinal principles
• Technique and
• exposure parameter
• selection
• Filtration
• Collimation
• Shielding
• Immobilization
• Equipment of low
• tube current
• Public protection
• Information boards
• Restricted entry
• inside radiation
• area
• Regular radiation
• survey
• X-ray room design
• Radiation warning
• lamps and signs

Professional Protection

• Patient protection
• Cardinalprinciples
• Use protective
• apparels
• Minimum
• fluoroscopy time
• Unnecessary
• holding of patients
• Personnel
• monitoring

ALARA

Radiation monitoring

• The aim of external exposure monitoring is the measurement of:
• Radiation levels in and around work areas (needs an area monitor)

Levels around radiation therapy equipment or source containers (needs an area monitor) Dose equivalents received by individuals working with radiation (needs a personal monitor).

The results of external exposure monitoring is used:

• To assess workplace conditions and individual exposures;

To ensure acceptably safe and satisfactory radiological conditions in the workplace;

To keep records of monitoring over a long period of time, for the purposes of regulation or as good practice.

Personnel monitoring

Personnel Monitoring is the monitoring of individuals who are exposed to radiation during the course of their work.

Radiologists, radiology technologists, medical physicists, radiographers,nurses and other frequent users of x-ray systems such as endoscopists,anaesthetists, cardiologists, surgeons etc as well as ancillary workers who frequently work in controlled areas should also be monitored.

Personnel monitoring

Personal dosimeters are used for individual monitoring Records equivalents/effective radiation doses received by individuals working with radiation.

All instruments/dosimeters must be calibrated in terms of appropriate quantities used in radiation protection

Personal monitoring device provides:

• Occupational absorbed dose and cumulative life time dose

Assurance that dose is within permissible limit Individual monitoring is also used to verify the effectiveness of radiation control practices in the workplace It is useful for detecting changes in radiation levels in the workplace and provide information in case of accidental exposures

Ideal monitoring device

• Characteristics
• Small, light weight, inexpensive and easy to use

Made of materials durable enough to tolerate normal daily use.

Able to detect and record both small and large exposures in a consistent and reliable manner.

• Unaffected by environmental condition(heat, humidity pressure)

Unaffected by non ionizing radiation

Optically stimulated luminescence[OSL]

• Film badges
• Thermoluminescent dosimeter[TLDs]
• Pocket dosimeter
• Extremity dosimeter[TLD ring badges] are used for monitoring of the hands only
• Types of personnel monitoring devices

TLD- thermo-luminescent dosimeter,

Film badge

Disassembled film badge, demonstrating badge components: plastic holder, metal filters, and film packet.

Film budge

• Advantage
• It gives permanent records
• Type of radiation and energy can be evaluated
• Least expensive
• Small, light, easy to handle
• Disadvantages
• Cant give instantaneous reading
• Its film fades at high temperature and humidity
• High sensitivity to light, pressure and chemicals
• Limited shelf life[one month].

Cant measure exposure less than 10mR (100μGy).

Thermoluminescence dosimeter

(TLD) badge

It is based on phenomenon of thermo luminescence, the emission of light when certain material are heated after radiation exposure In early 1960s, Cameron and co-workers from University of Wisconsin developed the TLD badge, used to measure individual dose from x ray , beta particles and gamma radiation.

• Response is directly proportional to the amount of radiation absorbed.

Use thermoluminescent phosphors like lithium fluoride

Types of TLD badges

Chest badge wrist badge Finger dosimeter

Guideline for using TLD badge

• Are to be used only by person directly working in radiation
• TLD badge issued to a person should not be used by any other person
• Should be worn compulsory at the chest level
• If lead apron is used , it should be worn under the lead apron

Institution advised to keep one TLD badge as control to asses background radiation level.

Worker should ensure their badges are not left in radiation field or near hot ovens, burners etc.

All used or unused badges should be returned after service period (3months).

Practical consideration :

TLD must be calibrated before it can be used.

Since the response of the TLD material is affected by their previous radiation history and thermal history, the material must be annealed to removed residual effect.

• TLD should be worn at chest position.
• TLD is changed in every 3 months.

The TLD badge should be stored away from light, radiation and dust when it is not used.

Thermo- luminescent dosimeter [TLD]

• Advantages
• Unaffected by visible light, moisture and mechanical vibration

It is reusable, one TLD can be used 100 times so one card can be used 300 months More sensitive and more accurate than film budge and give reliable results, no fading observed due to changes in environmental condition Can be worn for interval up to 3months at a time Can measure exposure as low as 10 microsivert.

• Disadvantages
• Doesn’t provide permanent record like film budge
• Doesn’t give instantaneous results
• It is costful but due to reuse it is cost effective

Instrument of reading TLD badge is expensive

Optically stimulated Luminescence(OSL)

Working mechanism is similar to the TLDs except the light emission is stimulated by laser light Measure radiation using a thin layer of aluminum oxide as the detecting mediam

OSL

• Advantages
• Lightweight, durable, and easy to carry.

OSL's tamperproof blister packet is not affected by heat, moisture and pressure High sensitivity, providing accurate readings as low as 10 μSv for x-ray and gamma ray photons with energies from 5 ke V to 40 Me V.

• Can be worn for longer periods of time (up to 1 y ear) to record occupational exposure
• Color-coding, graphic formats, and body location icons provide easy identification
• More accurate and sensitive than TLD
• Disadvantage

Most expensive

POCKET DOSIMETER

• It can detect x-ray and gamma ray.

Named as they are commonly worn in the pocket.

Advantages of pocket dosimeter :

• It gives immediate reading
• Reusable
• Small, compact, Easy to use
• Can be used for procedures that last a short time

Disadvantages :

Dosimeter must be calibrated to zero or its initial reading must be noted each day it is used Mechanical shock can cause false high readings No permanent, legal record of exposure/Doesn't provide permanent record Records only exposure received in body area where worn

RESTRICTED AREA/RADIATION AREA MONITORING

RADIATION SURVEY INSTRUMENTS FOR AREA MONITORING

• Types of instruments:
• Ionization chamber-type survey meter.
• Proportional counter
• GM detector

Gas filled radiation detectors.

Requirements

• Radiation survey instruments for area monitoring
• should meet the following requirements:
• Must be portable, so that one person can

carry and operate the device in an efficient manner for a period of time.

• Must be durable enough to withstand normal use, including routine handling that occurs during standard operating procedures.
• Must be reliable; only in such a case can radiation exposure or exposure rate in a given area be accurately assessed.

Radiation Monitoring

• Should interact with ionizing radiation similarly to the way human tissue reacts. This permits dose to be determined more accurately.
• Should be able to detect all common types of ionizing radiation. Such a capability increases their usefulness.
• The energy of the radiation should not significantly affect the response of the detector, and the direction of the incident radiation should not affect the performance of the unit. Such characteristics ensure consistency in unit operation among individual users.
• Should be cost-effective. The initial cost and subsequent maintenance charges should be as low as possible.
• Should be calibrated annually to ensure accurate operation.

Ionization Chamber-Type Survey Meter

Measures x-radiation and gamma radiation, and, if equipped with a suitable window, can also record beta radiation Both a rate meter device (for exposure rate) used for area surveys and an accurate integrating or cumulative exposure instrument

Useful in measuring radiation output from both radiographic and fluoroscopic x-ray equipment.

• Used for radiation protection surveys.

Useful in assessing the exposure rates in radioisotope storage facilities.

Instrument of choice when determining exposure rates from patients containing therapeutic doses of radioactive materials

Advantages

Ability to measure a wide range of radiation exposures within a few seconds while over a broad expanse of radiation energies Disadvantages

• Delicate detector of the unit

Without adequate warm-up time, its meter drifts and produces an inaccurate reading.

• Measure radiation intensities ranging from 10 to several thousand micrograys per hour (1 mR/hr to several thousand milliroentgens per hour)
• In the integratemode, it can sum exposures from as little as10 μGy_a to several Gy_a (1 mR to several R).

Proportional Counter.

Serves no useful purpose in diagnostic imaging.

Generally used in a laboratory setting to detect alpha and beta radiation and small amounts of other types of low-level radioactive contamination.

The Geiger-Muller (GM)

Primary portable radiation survey instrument for area monitoring in nuclear medicine facilities With the exception of alpha particle emission, it is sensitive to detect individual particles (e.g., electrons emitted from certain radioactive nuclei), hence can easily detect any area contaminated by radioactive material As its detector allows rapid monitoring, it can be used to locate a lost radioactive source or low-level radioactive contamination By using its audio mode, can be employed to scan radiation barriers for any shielding defects.

Disadvantages.

Likely to saturate or jam when placed in a very high-intensity radiation area thereby giving a false reading.

(e.g., that associated with a linear accelerator used in radiation therapy).

PERMISSIBLE DOSE LIMITS

Definitions:

A dose limit is defined as “The value of the effective dose or the equivalent dose to individuals from controlled practices that shall not be exceeded.” The limits apply to the sum of the relevant doses from external exposure in the specified period.

Why have dose limits?

To protect workers and members of the public from the effects of ionizing radiation.

They are set at a level that balances the risk from exposure with the benefits of using ionizing radiation.

Dose limits are a fundamental component of radiation protection.

Radiation Monitoring

Absorbed dose: The amount of energy deposited in a medium per unit mass of the medium by ionizing radiation. It is measured in units of gray (Gy), Equivalent dose: The dose of radiation exposure received by each organ or tissue. It is measured in units of Sievert (Sv)

Radiation Monitoring

Effective Dose: The dose converted assuming that the entire body has been exposed to the same amount as each organ or tissue. It is measured in units of Sievert (Sv). Effective dose is calculated for the whole body.

Individual dose limit of radiation exposure[2017 ICRP recommendation]

• Category
• Radiation worker
• General public
• Effective dose
• Whole body dose
• 20mSv/Year
• Not exceeding 50mSv in any single year
• 1mSv/Year
• Equivalent dose
• Lens of the eye
• 20mSv/year[previously was 150mSv/year updated in 2013]
• 15mSv/year
• Skin
• 500/year
• 50mSv/year
• Hands and feet
• 500/year

–

Categorization of people likely to be exposed to radiation

• Classified person
• [Workers engaged in radiation work]
• Non-classified person[Non-designated person]
• Those who might be exposed to radiations because their work places.

Members of the public.

Classified workers: Those who are expected to receive high levels of radiation exposure thus dose monitoring is compulsory.

Radiation Monitoring

Employee designate as classified persons: are those who are likely to receive an effective dose greater than 6 mSv per year or an equivalent dose greater than 15 mSv per year for the lens of the eye or greater than 150 mSv per year for the skin or the extremities .

• Employee must not be designate as a classified person unless:

That employee is aged 18 years or over certified in the health record that that is fit for the work with ionising radiation which that employee is to carry out.

Summary

Cardinal principles must be used for protection of patient and personnel.

Personnel monitoring ensures that occupational radiation exposure levels are kept well below the annual effective dose (EfD) limit The working habits and conditions of diagnostic imaging personnel can be assessed over a designated period of time through the use of the personnel dosimeter A radiation worker should wear a personnel monitoring device at collar level during routine computed radiography, digital radiography, or conventional radiographic procedures to approximate the maximum radiation dose

Radiation Monitoring

Health care facilities must maintain a record of exposure recorded by personnel dosimeters as part of each radiation worker's employment record.

Four types of personnel monitoring devices exist: optically stimulated luminescence (OSL) dosimeters, TLDs , film badges, and pocket ionization chambers.

Area monitoring can be accomplished through the use of radiation survey instruments.

Dose limits are a fundamental component of radiation protection so as to:

To protect workers and members of the public from the effects of ionizing radiation.

References

• Radiation protection in medical radiography 7edition.
• Radiologic science for technologists(bushong)

International commission for radiation protection[ICRP] guideline.

Introduction

• Radiation exposure to humans can be broadly classified as:
• internal exposure
• external exposure

This presentation will only discuss monitoring of external exposure.

Introduction

• The aim of external exposure monitoring is the measurement of:

Radiation levels in and around work areas (needs an area monitoring) Levels around radiation therapy and diagnostic radiology department or source containers (needs an area monitoring) Dose equivalents received by individuals working with radiation (needs a personal monitoring).

Introduction

• The results of external exposure monitoring is used:
• To assess workplace conditions and individual exposures;

To ensure acceptably safe and satisfactory radiological conditions in the workplace;

To keep records of monitoring over a long period of time, for the purposes of regulation or as good practice.

Introduction

Radiation exposure standards were introduced as early as the start of the 20th century when the potential hazards of radiation were realized.

This was achieved by introducing the organizations which organize the standards guidelines for radiation protection and monitoring.

Radiation protection organizations

• International Commission on Radiological Protection (ICRP)
• International Atomic Energy Agency (IAEA)
• International Commission on Radiological Units and Measurements (ICRU)
• Tanzania Atomic Energy Commission (TAEC)

Medical Radiology and Imaging Professional Council (MRIPC)

Introduction

Recommendations regarding dosimetric quantities and units in radiation protection dosimetry are set forth by the International Commission on Radiation Units and Measurements (ICRU).

The recommendations on the practical application of these quantities in radiation protection are established by the International Commission on Radiological Protection (ICRP).

Radiation Monitoring

• Radiation monitoring instruments are classifield into:
• Area survey meters ( or area monitors)

Personal dosimeters ( or individual dosimeters)

Radiation Monitoring

All these instruments must be calibrated in terms of appropriate quantities for radiation protection.

• Two issues must be addressed:
• Which quantities are used in radiation protection?
• Which quantities are in particular appropriate for
• area monitoring ?

individual monitoring ?

Radiation Monitoring

• Brief introduction of radiation protection quantities:

The absorbed dose is the basic physical dosimetry quantity.

However, it is not entirely satisfactory for radiation protection purposes because the effectiveness in damaging human tissue differs for different types of ionizing radiation.

To account additionally also for biological effects of radiation upon tissues, specific quantities were introduced in radiation protection.

Radiation Monitoring

• The basic quantity in radiation protection is the
• equivalent dose H
• It’s definition requires two steps:

The assessment of the organ dose DT

The introduction of radiation-weighting factors to account for the biological effectiveness of the given radiation in inducing health effects.

Radiation Monitoring

1st step: Definition of Organ dose DT The organ dose is defined as the mean absorbed dose DT("physical" dose) in a specified tissue or organ T of the human body given by DT= eT/mT

• where
• mT is the mass of the organ or tissue under consideration

εT is the total energy imparted by radiation to that tissue or organ.

Radiation Monitoring

2nd step: Introduction of radiation-weighting factors The organ dose is multiplied by a radiation-weighting factor wR to account for the biological effectiveness of the given radiation in inducing health effects.

HT = WR .DT

where DT,R is the absorbed dose delivered by radiation type R averaged over a tissue or organ T.

The resulting quantity is called the equivalent dose HT Unit: J/kg or sievert (Sv)

Radiation monitoring

Radiation monitoring involves the measurement of radiation dose or radionuclide contamination for reasons related to the assessment or control of exposure to radiation or radioactive substances and the interpretation of the results.

Some means of monitoring personnel exposure must be employed to ensure that occupational radiation exposure level are kept well below the annual effective dose limit.

The radiographers and other occupationally exposed persons must be aware of the various personnel and area radiation exposure monitoring devices and their functions.

Personnel monitoring

The monitoring of radiation exposure to any person occupationally exposed regularly to ionizing radiation.

Exposure monitoring of personnel is required whenever radiation workers are likely to risk receiving 10% or more of annual occupational effective dose limit of 50mSv. (5rem) The exposure monitoring is accomplished through the wearing of personnel dosimeter.

Personnel monitoring

Individual monitoring is used for those who regularly work in controlled areas or those who work full time in supervised areas: ‰ To have their doses monitored on a regular basis; ‰ To verify the effectiveness of radiation control practices in the workplace; ‰ For detecting changes in radiation levels in the workplace; ‰ To provide information in case of accidental exposures.

Personal dosimeter

• The characteristics of personal dosimeters
• It must be lightweight and easy to carry.

It should be made of materials durable enough to tolerate normal daily use.

The dosimeter must be able to detect and record both small and large exposure in a consistent and reliable manner.

It should not be affected its performance by a very warm weather, humidity and ordinary mechanical shock.

It should be reasonably inexpensive to purchase and maintain.

Types of personal dosimeters

There are four main type of personnel dosimeter used to measure individual exposure of the body to ionizing radiation.

• Film badges
• Optically stimulated luminescence (OSL)
• Pocket ionization Chamber

Thermal-luminescence dosimeter (TLD)

Film badge

• Film badge is composed of three parts
• A durable, lightweight plastic film holder
• An assortment of metal filter

A film packet

The film holder made of plastic material of a low atomic number to filter low-energy radiation.

Inside the plastic holder are metal filters of aluminum or copper that are secured in a permanent position.

Film badge

This filters allow the measurement of the approximate energy of the radiation reaching the dosimeter.

The radiation-dosimetry film contained in the radiographic film packet is similar to dental film.

This film is sensitive to dose ranging from as low as 0.1mSv to as high as 5000mSv.

Control badge

The control badge serves as a basis for comparison with the remaining film badge after they have been returned to the monitoring company for processing.

The control badge is supposed to be kept in a radiation free area within an imaging facility.

Its optical density reading should be zero.

If the control badge reading above zero is indicated, then the batch of badge may have been exposed to radiation while in transit.

Film badge

Optical stimulated luminescence

• The OSL dosimeter contains an aluminium oxide (AL2O3) detector.
• Three different filter are incorporated into the detector packet of the OSL dosimeter.
• The filter are respectively made of aluminium, tin and copper.
• Each filter blocks a portion of radiation.

Al offer the least absorption and Cu offer the most absorption.

OSL

The dosimeter is “read out” by using laser light at selected frequencies.

When laser light is incident upon the sensing material, it becomes luminescent in proportion to the amount of radiation exposure received.

Although the OSL dosimeter can be worn for up to 1yrs, it is common practice to wear it for a period of 2months.

OSL

The OSL dosimeter provide a sensitivity by giving an accurate reading as low as 1mrem for x-ray and gamma ray photons with energies ranging from 5keV to greater than 40MeV.

The OSL dosimeter is more sensitive than a TLD.

In diagnostic imaging the increased sensitivity of the OSL dosimeter makes it ideal for monitoring employees working in low-radiation environment and for pregnant worker.

OSL

Pocket Ionization Chamber

• Is the most sensitive type of personnel dosimeter.

The use of these monitors in diagnostic imaging is uncommon.

Externally the pocket dosimeter resembles an ordinary fountain pen, but it contains a thimble ionizing chamber that measures radiation exposure.

Types of Pocket Ionization Chamber

• There are two type of pocket ionization chamber
• The self-reading type.
• Contains a built-in electrometer (a device measuring electrical charge)
• The non-self reading type.

This requires a special accessory electrometer to read the device.

Special Charging Unit

Each dosimeter must be charged to predetermined voltage before use so that the quartz fiber indicator shows a zero(0) reading.

As the dosimeter exposed to ionizing radiation, it discharges and the fiber indicator advances along the scale in a linear fashion, thereby showing the net exposure in milliroentgens.

Pocket chambers generally used in medical imaging are sensitive to exposure from 0 to200mR (0 to 5.2 × 10-5 C/Kg).

Advantages of Pocket Ionization Chamber

Provide an immediate exposure readout for radiation workers who work in high- exposure area.

Are compact, easy to carry, and convenient to use They are reasonably accurate and sensitive, they are ideal monitoring device for procedures of relatively short duration.

Disadvantages of Pocket Ionization Chamber

Are fairly expensive

If not read each day, the dosimeter may give an inaccurate reading because the electric charge tend to escape.

Can be discharged if subjected to mechanical shock thus giving result in false high reading.

Pocket Ionization Chamber

Thermo Luminescence Dosimeter (TLD)

• The exterior of a TLD badge may look similar to that of film badge.

However the interior of this monitor mechanism differ completely.

The device contain a crystalline form of lithium fluoride (LiF) which functions as the sensing material of the TLD.

Ionizing radiation causes the LiF crystals in the TLD to undergo changes in some of their physical properties.

When irradiated, some of the electrons in the crystalline lattice structure of the LiF molecule absorb energy and are “excited” to higher energy level or band.

The presence of impurities in the crystal causes electrons to become trapped within these bands.

TLD

TLD

When the LiF crystals are passed through a special heating process, these trapped electrons receive enough energy to rise above their present locations into a region called conduction band.

From there electrons can return to their original or normal state with emission of energy in form of visible light.

The intensity of light is proportional to the amount of radiation that interacted with the crystal.

TLD

The LiF crystal interact with ionizing radiation as human tissue, hence this monitor determines dose more accurately.

• After the TLD reading has been obtained, the crystal can be reused.
• Exposures as low as 3mR can be measured precisely.

Humidity , pressure and normal temperature changes do not affect the TLD.

TLD

Radiation Meter for Area Survey

• Radiation survey instruments for area monitoring should meet the following requirement.
• They must be portable so that one person can carry and operate the device.
• They must be durable enough to withstand normal use.
• They must be reliable (measure accurately the exposure)
• They should interact with ionizing radiation in a manner similar to human tissue.
• They should be able to detect all common types of ionizing radiation.
• They should be cost effective.

They should be calibrated annually to ensure accurate operation.

There are two main kind of dosimeter used to measure radiation exposure in area.

• Geiger Muller counter
• Ionization chamber

Types of Area Survey Meters

Geiger Muller Counter

Geiger Muller counter is an instrument used for measuring area ionizing radiation exposure.

It detect ionizing radiation such as alpha particles, beta particle and gamma ray, using the ionization effect produced in a Geiger Muller tube.

Geiger Muller Counter

• Geiger Muller counter consists of two main part which are;
• Geiger Muller tube, the sensing element which detects the radiation.

The processing electronics, which displays the results.

The Geiger Muller tube is filled with an inert gas such as helium, neon or Argon at low pressure, to which a high voltage is applied.

The tube briefly conducts electrical charges when a particle or photon of incident radiation makes the gas conductive by ionization.

Geiger Muller tube read out

• There are two main type of read out
• Counts

Radiation dose

The counts display is the simples and is the number of ionizing events displayed either as a count rate (count per second) or as a total over a set time period.

The counts read out is normally used when alpha or beta particles are being detected.

Geiger Muller Counter read out

The radiation dose display is more complex to achieve is displayed as dose rate in a unit such as the Sievert which is normally used for measuring gamma or x-ray dose rate.

A G-M tube can detect the presence of radiation but not its energy which influence the radiation’s ionizing effect.

G-M counter limitation

There two main limitation of the Geiger counter.

Because the output pulse from a Geiger muller tube is always the same magnitude regardless of the energy of the incident radiation, the tube cannot differentiate between radiation type.

Inability to measure high radiation rate due to the “dead time” of the tube.

Dead time is an insensitive period after each ionization of the gas during which any further incident radiation will not result in a count, and the indicated rate is therefore lower than actual.

Geiger Muller Counter

Ionization chamber

Ionization chambers measure exposure by detecting liberated electron charge when x-ray photons ionize the gas within the chamber.

The chambers need a high positive voltage applied at the collecting anode to attract the liberated electrons. The electron charge is collected and used to determine the radiographic exposure expressed in coulombs per kilogram (C/kg), the preferred SI unit, or the legacy unit, roentgens (R).

Ionization chamber