An toàn bức xạ trong Xquang (Radiation protection)

Sources of Exposure to Ionizing Radiation Naturally Occurring Radiation Sources ¬ Annual average total effective dose from exposure to ionizing radiation in USA is approximately 3.6 mSv

Trang 1

Radiation Protection – Chapter 23, Bushberg

Kalpana Kanal, Ph.D., DABR Lecturer, Diagnostic Physics Dept of Radiology UWMC, HMC, SCCA

a copy of this lecture may be found at:

http://courses.washington.edu/radxphys/PhysicsCourse04-05.html

Kalpana M Kanal, Ph.D., DABR

2

1 Sources of Exposure to Ionizing Radiation Naturally Occurring Radiation Sources

¬ Annual average total effective dose from exposure to ionizing radiation in USA is approximately 3.6 mSv or 360 mrem[National Council on Radiation Protection and Measurement (NCRP)]

¬ 3 mSv or 300 mrem(80%) is from naturally occurring sources

¬ Radon

¬ Internal radiation

¬ Terrestrial radioactivity

¬ Cosmic radiation

c.f Bushberg, et al The Essential Physics of Medical

Imaging, 2 nd ed., p 748.

3

¬ Radon

¬ Biggest contributor to natural background (2 mSv or 200 mrem/year)

¬ Radon (Rn-222) is a radioactive gas formed during the decay of radium

¬ Radium is a decay product of uranium found in the soil and has

a half-life of 1620 years

¬ Radon is an alpha emitter with a half-life of approx 4 days

4

¬ Radon

¬ The progeny of radon are also radioactive, attach to aerosols and are deposited in the lungs

¬Bronchial mucosa is irradiated inducing bronchogenic cancer

¬ Average concentration of radon outdoors is 4-8 Bq/m3(0.1-0.2 pCi/L)

¬ Indoors is 40 Bq/m3(1 pCi/L)

¬Remedial action recommended

in excess of 160 Bq/m3(4 pCi/L)

Trang 2

5

¬ Internal Radiation

¬ Second largest source of natural background radiation (0.4 mSv or

40mrem/year)

¬ Ingestion of food and water containing primordial radionuclides

¬ K-40 is most significant

¬ Skeletal muscle has the highest concentration of potassium in the body

6

¬ Terrestrial or External Radiation

¬ Terrestrial radioactive materials that have been present on earth since its formation are called primordial radionuclides

¬ External radiation exposure, inhalation, ingestion

¬ 0.28 mSv or 28 mrem/year(∼0.3 mSv or 30 mrem/year)

7

¬ Cosmic Radiation

¬ Cosmic rays are energetic protons and alpha particles which originate

in galaxies

¬ Most cosmic rays interact with the atmosphere, with fewer than 0.05%

reaching sea level

¬ 0.27 mSv or 27 mrem/year(∼0.3 mSv or 30 mrem/year)

8

¬ Cosmic Radiation

¬ Exposures increase with altitude approx doubling every 1500 m as there is less atmosphere to attenuate the cosmic radiation

¬ Leadville, Colorado at 3200 m, 1.25 mSv/year

¬ More at poles than equator

Trang 3

9

¬ Cosmic Radiation

¬ Air travel can add to individual’s cosmic exposure

¬ Airline crews and frequent fliers receive an additional ∼1 mSv

¬ 5 hour transcontinental flight will result in an equivalent dose of ∼25 µSv or 2.5 mrem

¬ Apollo astronauts –2.75 mSv or

275 mrem during a lunar mission

10

1 Sources of Exposure to Ionizing Radiation

Technology Based Radiation Sources

0.6 mSv

fluoroscopy are highest contributors

to medical x-rays

c.f Bushberg, et al

The Essential Physics of Medical Imaging, 2 nd ed., p

744.

11

¬1 mSv for diagnostic radiology is lower than expected because it includes personnel who receive very small occupational exposures

¬15 mSv or more are typical of special procedures utilizing fluoroscopy and cine

→

1 Occupational Exposures

12

1 Collective Effective Dose Equivalent

¬ The product of the average effective dose equivalent and the size of the exposed population is the collective effective dose equivalent

¬ Expressed in person-sieverts (person-Sv or person-rem)

Trang 4

13

1 Genetically Significant Dose (GSD)

¬ The genetically significant equivalent dose (GSD)is a dose parameter that

is an index of potential genetic damage

¬ The GSD is defined as that equivalent dose that, if received by every member of the population, would be expected to produce the same genetic injury to the population as do the actual doses received by the irradiated individuals

¬ GSD is determined by taking the equivalent dose to the gonads ofeach exposed individual and estimating the number of children expected for a person of that age and sex

14

1 Genetically Significant Dose (GSD)

15

1 Summary

from all radiation sources is 3.6 mSv/year or 360 mrem/year

16

1 Summary

of the exposed population

source of radiation exposure

Trang 5

17

Raphex 2002 General Question

approximately mrem

18

Question

nuclear medicine in the US is:

19

2 Personnel Dosimetry Film Badges

¬ A film pack (A) consists of a black envelope (B) containing film(C) placed inside a special plastic film holder (D)

¬ Using metal filters typically lead (G), copper (H) and aluminum (I), the relative optical densities of the film underneath the filters can be usedto identify the general energy range of the radiation and allow for the conversion of the film dose to tissue dose

¬ Open window (J) where film is not covered by a filter or plasticand is used to detect medium and high-energy beta radiation

c.f Bushberg, et al The Essential Physics of Medical Imaging, 2 nd

ed., p 749.

20

2 Personnel Dosimetry Film Badges

¬ Most film badges can record doses from about 100 µGy to 15 Gy (10 mrad to

1500 rad) for photons and from 500 µGy to 10 Gy (50 mrad to 1,000 rad) for beta radiation

¬ The dosimetry report lists the “shallow” equivalent dose, corresponding to the skin dose, and the “deep” equivalent dose, corresponding to penetrating radiation

¬ Generally placed at waist level or shirt-pocket level

¬ For fluoroscopy, placed at collar level outside the lead apron to measure radiation dose to thyroid and lens of eye

¬ Pregnant radiation workers typically wear a second badge at waist level (behind the lead apron, if used) to assess the fetal dose

¬ Excessive moisture or heat will damage film inside badge

Trang 6

21

¬ TLD is a dosimeter in which consists of a scintillator in which electrons become trapped in excited states after interactions with ionizing radiation

¬ If the scintillator is later heated, the electrons can then fallto their ground state with the emission of light

¬ Thermoluminescent (TL) means emitting light when heated

¬ The amount of light emitted by the TLD is proportional to the amount of energy absorbed by the TLD

¬ After TLD has been read, it may be baked in an oven and reused

2 Personnel Dosimetry Thermoluminescent (TLD) Dosimeters

22

2 Personnel Dosimetry Thermoluminescent (TLD) Dosimeters

¬ Lithium Fluoride (LiF) is one of the most useful TLD materials

¬ LiF TLDs have a wide dose response range of 10 µSv to 103mSv (1 mrem

to 105rem)

¬ Used in nuclear medicine to record extremity exposures

23

2 Personnel Dosimetry Optically Stimulated Luminescent (OSL) Dosimeters

¬ The principle of OSL is similar to TLDs except that the light emission is stimulated by a laser light instead of heat

¬ Crystalline aluminum oxide activated with carbon (Al2O3:C) is commonly used

¬ Broad dose response range like TLDs

¬ They can be reread several times

24

2 Personnel Dosimetry Pocket Dosimeters

¬ Major disadvantage to film and TLD dosimeters is that the accumulated exposure is not immediately indicated

¬ Pocket dosimeters measure radiation exposure, which can be read instantaneously

¬ Can measure exposures from 0 to

200 mR or 0 to 5 R

¬ Analog or digital

Trang 7

25

2 Summary

Imaging, 2 nd ed., p 753 Kalpana M Kanal, Ph.D., DABR

26

Raphex 2002 General Questions

distinguish between low and high energy x-rays

27

3 Radiation Detection Equipment In Radiation

Safety

¬ Geiger-Mueller Survey Instruments

¬ Measurements are in counts per minute (cpm)

¬ Surveys radioactive contamination in nuclear medicine

¬ Are extremely sensitive to charged particulate radiations with sufficient energy to penetrate the survey meter window

¬ Are relatively insensitive to x-and gamma radiations

¬ Portable Ionization Chambers

¬ Used when accurate measurements of radiation exposure are required, measurement of x-ray machine outputs

¬ Measure 1 mR/hr to 500 R/hr

28

4 Radiation Protection and Exposure Control

¬ There are four principal methods by which radiation exposures topersons can be minimized: time, distance, shielding and contamination control

¬ Time

¬reducing time spend near a radiation source

¬ Distance

¬inverse square law

¬For diagnostic x-rays, a good rule of thumb is that at 1 m from a patient at 90 degrees to the incident beam, the radiation intensity is 0.1% to 0.15% (0.001 to 0.0015) of the intensity of the beam incident upon the patientfor a 400 cm2area field area

¬The NCRP recommends that personnel should stand at least 2 m from the x-ray tubeand the patient and behind a shielded barrier or out of the room, whenever possible

Trang 8

29

¬ Shielding is used to reduce exposure to patients, staff and the public

¬ Shielding against primary (focal spot), scattered (patient) and leakage (x-ray tube housing, limited to 100 mR/hr at 1 m from housing) radiation

Shielding

30

¬ Shielding calculations depend on:

¬ radiation exposure level(mR/week)depends on techniques and patient load

¬ workload (amount of x-rays produced per week), W (mA.min/week)

¬ use factor, U,indicates the fraction of time during which the radiation under consideration is directed at a particular barrier

¬a wall that intercepts the primary beam is called a primary barrier and is assigned a use factor according to typical room use

¬U ranges between 0 and 1, secondary barriers have a use factor of 1

Shielding

31

¬ Shielding calculations depend on:

¬ occupancy factor, T,indicates the fraction of time during a week that a single individual might spend in an adjacent area

¬T = 1 for full occupancy (work areas, offices etc.)

¬T = 1/4 for partial occupancy (corridors, rest rooms etc.)

¬T = 1/16 for occasional occupancy (waiting rooms, toilets, etc.)

¬ Distance, d,measured from source of radiation to the area to be protected

Shielding

32

¬ Shielding calculations determine the thickness of an attenuatingmaterial required to reduce radiation exposure to acceptable levels

¬ 1 mSv/year or 100 mrem/year (2 mR/week) for non-occupational personnel (members of public and non-radiation workers)

¬ 0.1 or 10 mR/week for controlled areas

Shielding

Trang 9

33

¬ Lead usually used for shielding and specified as weight per square foot (lb/ft2) Typically 2 lb/ft2(0.8 mm or 1/32thinch) or 4 lb/ft2(1.6 mm or 1/16th

inch) is sufficient for diagnostic radiology

¬ Calculated using HVL and TVL of the material[(1/2)n –reduction in beam intensity, n is HVL]

¬ Shielding material used from base of floor to a height of 7 feet

¬ Acrylic, leaded glass, gypsum drywall, steel are other materialsused besides lead for shielding

Shielding

34

etc., pg 771 of Bushberg)

35

¬ Tube Voltage and Beam Filtration

¬ Achieve an optimal balance between image quality and dose to the patient

¬ Patient exposure can be reduced by using a higher kVp ad lower mAs

¬Increasing kVp increases transmission (less absorption) of x-rays through the patient

¬Even though mR/mAs increases as kVp increases, an accompanying reduction in mAs will decrease the incident exposure

to the patient

¬Contrast will decrease due to higher effective energy of the x-ray beam

4 Radiation Protection and Exposure Control Protection of the Patient in Medical X-ray Imaging

36

¬ Tube Voltage and Beam Filtration

¬ Filtration of the polychromatic x-ray energy spectrum can significantly reduce exposure by selectively attenuating the low-energy x-rays in the beam

¬ As the tube filtration increases, the beam becomes hardened (effective energy increases) and dose to patient decreases because fewer low-energy photons are in the incident beam

¬ The amount of filtration that can be added is limited by the increased demands on tube loading to offset reduction in tube output, and the decreased contrast due to excessive beam hardening

¬ Quality of x-ray beam is assessed by measuring the HVL

Trang 10

37

¬ Field Area, Organ Shielding and Geometry

¬ Reducing field size limits the patient volume exposed to primarybeam, reduces the amount of scatter and thus radiation dose to adjacent organs (scatter being reduced improves image contrast)

¬ Gonadal shielding can be used to protect the gonads from primary radiation when the shadow of the shield does not interfere with the anatomy under investigation

¬ Increasing source-to-object distance (SOD) and source-to-image distance (SID) helps reduce dose (patient volume exposed decreased due to reduced beam divergence)

¬ For fixed SID (C-arm fluoro system), patient dose is reduced by increasing the SOD as much as possible

¬A minimum patient to focal spot distance of 20 cm is required

38

¬ X-Ray Image Receptors

¬ The speed of the image receptor determines the number of x-ray photons and thus the patient dose necessary to achieve an appropriate signal level

¬ Higher speed system requires less exposure to produce the same optical density and thus reduces dose to patient

¬ Either a faster screen (reduced spatial resolution) or faster film (increased quantum mottle) will reduce the incident exposure to the patient

39

¬ X-Ray Image Receptors

¬ Computed Radiography (CR) devices have a wide dynamic range so they compensate to some degree for under-and overexposure and can reduce retakes

¬ CR roughly equivalent to 200 speed screen-film systems

¬ Techniques for extremities with CR devices should be used at higher exposure levels while exposures for pediatric patients should beused at increased speed (e.g 400 speed) to reduce dose

40

¬ Computed Tomography (CT)

¬ Reduce mAs and perhaps kVp for thinner and pediatric patients

¬ Modern MSCT scanners –dose modulation, mA changes with patient size

c.f Bushberg, et al The Essential Physics of Medical Imaging, 2 nd ed., p 779.

Định dạng
Số trang	14
Dung lượng	486,01 KB