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UNITS FOR MEASURING RADIATION

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Learning objective: ď Ž

At the end of this session, student should be able to: - explain the quantity involved in ionizing radiation - define and state the value of the annual dose limit involved. - apply the unit conversion 2


ď Ž

Several forms of ionizing radiation are used in medical imaging.

ď Ž

Even though the risk is low, or if there is a risk at all, it is appropriate to manage the radiation delivered to patients being imaged and to use only sufficient radiation to produce the necessary image quality.

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How much radiation is delivered to a patient's body?

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Physical Quantity

SI Unit

Non-SI Unit

Relationship

Activity

Becquerel (Bq)

Curie (Ci)

1Ci = 3.7 x 1010 Bq

Exposure

Coulomb per kg (C/kg)

Roentgen (R)

1R = 2.58 x 10-4 C/kg

Absorbed Dose

Gray (Gy)

Rad (rad)

1 rad = 0.01 Gy 1 Gy = 100 rad

Dose Equivalent

Sievert (Sv)

Rem (rem)

1 rem = 0.01 Sv 1 Sv = 100 rem

Sievert (Sv)

Rem (rem)

1 rem = 0.01 Sv 1 Sv = 100 rem

Effective Dose

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Activity (Bq)  Rate of decay. no of atoms that decay and emit radiation in one second.

Exposure (C/kg)  ionization produced in air by photons.  Radiation quantity that expresses the concentration of radiation delivered to a specific point, such as the surface of the human body 6


Absorbed dose (Gy)  energy imparted by ionizing radiation per unit mass of irradiated material.  This is the quantity that is most directly related to biological effects.

Equivalent dose (Sv)  the product of the absorbed dose and effective quality factor (Q)  equivalent dose = absorbed dose x Q

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Type of ionising radiation X-rays or gamma rays Electrons or beta-particles Thermal neutrons Fast neutrons (or neutrons of unknown energy) Protons Alpha-particles Recoil nuclei (eg. In alpha-decay) Fission fragment

Quality factor (Q) 1 1 2.3 10 10 20 20 20 8


Effective dose-equivalent (Sv)  the sum of the weighted dose-equivalent of all organs or tissues of interest.  It is used to compare radiation doses on different body parts on an equivalent basis because radiation does not affect different parts in the same way

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 The effective dose (H) to an individual is found by calculating a weighted average of the equivalent dose (E) to different body tissues, with the weighting factors (W) designed to reflect the different radiosensitivities of the tissues:  H = ∑ i E i Wi

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DOSE LIMIT

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ANNUAL DOSE LIMIT   

Annual dose – the dose received over a period of 1 calendar year. Annual dose limit – the value of the annual dose that must not be exceeded. Every licensee shall ensure that no worker or member of the public receives exposure exceeding the annual dose limits. The annual dose limits shall not apply to medical exposure or exposure due to natural background radiation.

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DOSE LIMIT IN OCCUPATIONAL EXPOSURE 

Effective dose limit (had dose berkesan) for whole body radiation  20 mSv per calendar year  averaged over a defined periods of 5 years, and  with the provision that the effective dose shall not exceed 50 mSv in any single year.  Additional restriction (pengehadan) applies to the occupational exposure for pregnant women.

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Occupational dose limit ď Ž

Limit on equivalent dose: Eye 150 mSv

Skin 500 mSv

Hand & feet 500 mSv In a calendar year 16


ď Ž

Dose limits are similar for men and women, except for pregnant women. =

ď Ž

When a female worker is confirmed to be pregnant the foetus shall, from the date of confirmation, be afforded the level of protection so that the dose to the foetus shall not exceed 1 mSv for the remaining period of the pregnancy.

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DOSE LIMIT FOR MEMBERS OF THE PUBLIC ď Ž

There are dose limits allowed for members of the public. BSS regulation recommend 1 mSv per year. Effective dose limit for whole body exposure

1 mSv

Equivalent effective dose limit for the lens shall not exceed

15 mSv

Average equivalent effective dose limit to the skin (averaged over any 1 cm2)

50 mSv

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Effective dose limit for a person who knowingly assists in the support of a patient during the period of diagnostic examination or treatment of the patient.

<5 mSv

Effective dose limit for a person below the age of 16 years visiting patient undergoing treatment or diagnostic examination involving radioactive material during the period of the treatment or examination of the patient.

< 1 mSv

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PLANNED SPECIAL EXPOSURE ď Ž

Also known as dose limit in special circumstances (keadaan).

ď Ž

Means voluntary (sukarela) exposure during normal operation whereby 1 or more of the annual dose limits for a worker are likely to be exceeded, permitted (dibenarkan) only in situations when alternative techniques which do not involve normal exposure cannot be used.

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ď Ž

Shall only carried out when approved by the appropriate authorities (AELB @ Ministry of Health of Malaysia).

ď Ž

The dose received over a period of 1 calendar year does not exceed twice the annual dose limits, and in a lifetime, five times those limits.

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In choosing volunteers, the following shall be taken into account: Previous experience;  State of health;  Special skill;  Social and economic responsibilities.

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Person who is forbidden to participate in planned special exposure:

If during the previous 12 months, such worker has received an exposure giving rise to the annual dose limit (whole body and partial body exposures).

If such worker has previously received accidental or emergency exposures which exceeds 5 times the annual dose limit (whole body and partial body exposures).

If such worker is a female of reproductive capacity.

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ANNUAL LIMIT ON INTAKE (ALI)

The derived limit for the amount of radioactive material taken into the body of an adult worker by inhalation or ingestion in a year.

Ways of intake – oral (ingestion), inhalation, injection or skin absorption

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ď Ž

The ALI of any radionuclide depends on all of the following factors: - The type of radiation emitted. - Energy of the radiation and that of any radioactive progeny. - The selective deposition in specific body tissue. - The effective half-life .

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7 8 units for measuring radiation