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Radiographic Science

Dosimetry & detection of radiation

Recap

Alpha () and Beta () particles

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Do not travel very far Travel further than Alpha or Beta particles

Gamma () and X-rays

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Ionising radiation is potentially damaging to cells / tissue

Units of measurement(1)

Exposure

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A measure of the ionisation produced in air by X-radiation. Unit = coulombs per kilogram Base unit of radiation dose Unit = Grays (Gy) (mGy used in practice) One gray deposits one joule of energy in a kilogram of irradiated tissue **Does not make any distinction between different types of radiation, or organs irradiated**

Absorbed dose

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Units of measurement(2) Dose equivalent

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Unit = Sieverts (Sv) (mSv used in practice) Permits damage capability of a quantity of radiation to be expressed Provides a more accurate guide to risk Quality Factor utilised

1 Gy of alpha particles gives a dose equivalent of 20 Sv 1 Gy of X-rays gives a dose equivalent of 1 Sv

Linear Energy Transfer

Linear energy transfer (LET)

A measure of how the energy of a photon / particle is distributed along its path Total energy deposited in an absorber per unit length Alpha particles and neutrons are both heavy and travel slowly ­ High LET Beta particles, X-rays and gamma rays have low or zero mass ­ Low LET

Weighting factors (Wt) for different tissues of the body

ICRP 60 (1990) document introduced tissue weighting factors

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Testes & ovaries Wt = 0.20 Liver Wt = 0.05 Hereditary damage and malignancy taken into consideration

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Effective Dose (Equivalent) & Collective effective dose (Equivalent) Summation of Wt factors for individual organs Takes into account the differing radiosensitivities of the body tissues Collective effective dose used to measure the total dose of radiation to a population

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Multiply the average annual effective dose (1.9 mSv) by population (57 million)

Personal monitoring- Why?

To ensure dose limits are not exceeded (IRR 99) ­ 20 mSv per year (whole body) ­ 3/10th =Classified worker

To check that doses are ALARP Provides documentation in case of ionisation incident / emergency

Who is monitored?

Radiographers Radiography Assistants Radiologists Cardiologists Theatre technicians Nurses Carers

What is monitored?

Whole body

TLD / Film badge Measured quantity = mSv

Eyes, Thyroid

Collar / Head badge Measured quantity = mSv

Fingers

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Finger tips ­ NM / Interventionalists Measured quantity = mSv

Types of monitoring - TLD

Body depth (WB) dose Skin dose Lithium Fluoride (Teflon) discs

Serial Barcode - ID

TLD's

Certain crystalline powders such as Lithium Fluoride (LiF) possess the property of thermoluminescence, ie they can store energy during exposure to ionising radiation and subsequently release it as light when heated.

LiF atomic number = 8.1, which is near to the atomic number of soft tissue (7.4), than silver (z=47)

How does this work?

Electron "traps" are present in the LiF material Initial expsoure to radiation "lifts" electrons into conduction band and may eventually fall into an electron "trap" The number of electrons caught in these "traps" is proportional to the radiation dose to which the material has been subjected However, if these electrons are subjected to heat, these electrons are able to jump out of the traps and into the conduction band Luminescence takes place and the light output is recorded The TLD plate is finally subjected to an even higher temperature and allowed to cool slowly This process "empties" all the electron traps and allowing the dosemeter to be reused

Thermoluminescence

Glow curve produced by heating LiF TLD

Dose recording

Doses usually monitored every three months Expired TLD's sent to NRPB for evaluation Records of doses received sent back to RPS Doses are then displayed on notice board Should be no greater than 0.5 mSv **Per month** Maximum annual dose limit = 20 mSv (IRR99) (WB) Annual classification level = 3/10th Annual dose (6 mSv) (WB)

Types of TLD's

1) Finger monitor Approximately 20mg of lithium fluoride in a thin plastic holder which can be worn on the hands when handling radioactive sources, for example Technetium 99m, or taped close to a critical organ during radiotherapy - or monitoring departments (survey). 2) The TLD monitor This type of body dosemeter has two discs of lithium fluoride under an open window and a thick dome of plastic. The open window is used to estimate the dose received by the skin of the wearer and the disc under the thick (plastic dome) is used to estimate the whole body dose of the wearer.

Advantages of TLD monitoring (1)

- Very good accuracy for detection of photons

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- Wide range of doses can be monitored - Good accuracy for detection of Beta photons - Relatively cheap - Powder / Disc reusable - Easy to set up as a computerised record of dose levels - Longer dose range

Advantages of TLD monitoring (2)

Lithium Fluoride is almost tissue equivalent (Low photon energy dependency) - No darkroom processing / chemicals required to obtain result - Indefinite user interval (usually worn in clinical practice for up to three months before being exchanged for a new TLD) - Excellent resistance to environment (i.e. temperature, humidity) -Very good ruggedness (Not easily damaged) - Can be placed anywhere on the body of the user

Disadvantages of TLD monitoring

Dose reading is retrospective If the reading is lost or missing, then there is no source of reference (unlike a film badge method, whereby the developed film may be kept and re-read) Cost - TLD method of recording radiation dose can be expensive in comparison to other methods. However, TLD monitors are usually only replaced every three months, compared to each month for conventional film badge methods

Photographic Film badges

Photographic film used as a measure of exposure to ionising radiation Film becomes more optically dense as the wearer is exposed to ionising radiation Use of the plastic, tin, lead & aluminium filters enables the distinction to be made whether the dose was a result of: ­ particles ­ High or low energy X-rays ­ rays

Example of film badge holder

Thin Plastic = 1mm Low energy B particles Thick plastic = 3m High energy B particles Dural filter (Al & Cu) Low energy X & rays

Cadmium (0.7mm &Pb (0.3mm) Tin (0.7mm) & Pb(0.3mm) (Est. of neutron dose) Pb edge shielding (0.3mm)

Method of monitoring ­ Film badges

Films from a batch are issued and control films kept Issued films are usually worn for a four week period Films returned to radiation monitoring and a new film issued from the next batch

The response of the film emulsion to incident radiation varies with the type of radiation (low energy radiation gives greater film blackening). All films are processed plus one exposed film under identical conditions. Graphs are generated using the densities produced on the films exposed to known radiation doses.

Photographic film

Film contained in a light proof envelope Has two emulsions

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Slow emulsion

Doses 10 mSv - 1 Sv

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Fast emulsion

0.2 mSv - 10 mSv

Has identification number & dot for orientation

Calculating exposure

Advantages and disadvantages of film badges

Advantages of film badge compared with TLD's - Sensitive

Disadvantages of film badge compared with TLD's

- Darkroom and equipment expensive Energy and dose rate discrimination - Wide dose latitude - Batch handling - Permanent record - Relatively cheap -Poor resistance to the environment (affected by temperature and humidity) - Films are not reusable - Need to be replaced by user every four weeks - High photon energy dependence

Digital Dosemeters

Pocket dosemeters provide an instantaneous readout Expensive Audible / Visible warning High sensitivity High accuracy Non-permanent records Usage includes:

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Industrial workers Radiotherapists

Types of monitors

Film Badge Digital dosemeter

TLD Finger/thyroid/eye monitors

TLD WB monitor

Patient Dose & Diagnostic Reference levels

There are no legal limits for patients undergoing diagnostic radiological examinations DRL's introduced in IR(ME)R 2000 ­ Purpose?

"DRL's not to be exceeded for standard procedures when good and normal practice regarding diagnostic & technical performance is applied"

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Each department must establish DRL's for common procedures. These can be national or local DRL's are in terms of Entrance Surface Dose and measured in mGy using TLD's / DAP meter Expsoure factors can also be recorded (+FFD)

Calculating ESD

Example DRL's

Plain film (mGy): ­ Skull (PA)

4

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Chest

- 0.2

Fluoroscopy (Gy x cm2 / time) ­ Barium meal ­ 17 ­ Barium swallow

- 12

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Thoracic spine (lat)

­ 16

Barium enema

- 35

Lumbar spine (LSJ)

- 35

Barium follow through

-17

Abdomen (AP)

-7

Public doses vs Medical doses

FLIGHT: Jet (10 km) return Concorde (15 km) return Spain return BACKGROUND: average Cornwall MEDICAL EXPOSURES: AP L.spine AP Pelvis Chest 0.07 mSv 0.14 mSv 0.02 mSv 2.6 mSv year-1 7.8 mSv year-1 2.15 mSv 1.22 mSv 0.05 mSv

Dose Area Product (Gy x cm2)

DAP meters are particularly useful with dynamic beams

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Fluoroscopy

Now commonplace in general radiography Fits onto tube housing noninvasively Doses normally recorded for each examination DAP meter is an ionisation chamber

Ionisation of air

Because air (in its normal state) does not contain any conduction electrons, it is considered to be a good electrical insulator If air is exposed to X-or gamma rays, some of the photons of radiation release electrons from the atoms in the air, ionising it and enabling it to conduct electricity. The more radiation the air is exposed to, the better able it is to conduct electric current. By measuring the electrical properties of the air the quantity of radiation causing the ionisation may be calculated / estimated.

Exposure

The measure of strength of an X- or gamma ray beam x the quantity of charge on the ions produced per unit mass = Exposure

E=Q m

Free air ionisation chamber

This instrument is not used in hospitals, but as mentioned above, forms the basis of calibration of hospital dosemeters The free air chamber is used to measure radiation exposure. This chamber basically consists of a box of air and a known mass is exposed to a beam of X - or gamma rays and the negative ions produced in the air are collected on a positively charged metal plate The total charge collected is measured with an electrometer.

Free air ionisation chamber

Look at the information on learntech (Blackboard) Section on detection of radiation

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Ionisation chamber

Thimble ionisation chamber

Using some of the principles demonstrated in the free air ionisation chamber, a much smaller ionisation chamber, called a thimble chamber, has been designed Suitable for use in x-ray and radiotherapy departments This instrument uses a smaller volume of air than the free air chamber This reduces the size of the chamber and for this reason air equivalent materials are employed in the manufacture of the walls of the chamber

Thimble chamber

The walls are constructed from a mixture of bakelite and graphite or plastic coated with a layer of graphite These materials have a similar effective atomic number to air but are much more dense. This allows the chamber to "behave" as if it had a much greater volume of air than is actually present.

Geiger-Muller counters

Designed to detect the arrival of individual photons of ionising radiation Can also be used to detect individual particles These instruments are detectors rather than dosemeters Useful for detecting the presence of radioactive contamination (NM departments)

GM Counters ­ Principle of operation

The ionisation chamber is filled with a gas The cylindrical metal wall of the tube and a metal rod form the electrodes High voltage between these electrodes creates strong electric field Ionising photon/particle enters tube ­ ionising atom of gas Collision with further atoms - more ions ­ "Avalanche effect"

Geiger-Muller counter

References

ICRP 60, 1990, Recommendations of the international Commission on Radiological Protection ­ Annals of the ICRP, Vol 21, No 1-3, Oxford, Pergamon Plaut S. (1993) Radiation protection in the X-ray department, London, Butterworth Heineman Ball & Moore (1997) Essential Physics for Radiographers, 3rd Edition, London, Blackwell Scientific Publications Graham (2003) Principles of Radiologic Physics, 4th Edition, London, Churchill Livingstone

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