RA 221- RADIOBIOLOGY UNIT 3

CH 35,36,37 Bushong

Makes recommended dose limits for occupational, gestational, and public areas

National Council on Radiation Protection (NCRP)

Annual effective dose for occupational exposures

50 mSV

Cumulative effective dose for occupational exposures

10 mSv * Age

Equivalent annual dose (tissues and organs) for lens of the eye of the occupational worker

150 mSv

Annual equivalent dose (tissues and organs) for thyroid, skin, hands, and feet of the occupational worker

500 mSv

Annual effective dose for the frequent general public exposure

1 mSv/year

Annual effective dose for the infrequent general public exposure

5 mSv/year

Annual equivalent dose (tissues and organs) for skin, hands, and feet of the general public

50 mSV

Annual equivalent dose (tissues and organs) for the lens of the eye of the general public

15 mSv

Annual effective dose for education and training exposures

1 mSv

Annual effective dose for education and training exposures for students over the age of 18

50 mSv

Annual equivalent dose (tissues and organs) for the lens of the eye of the educational/training exposures (students)

15 mSv

Annual equivalent dose (tissues and organs) for the skin, hands, and feet of the educational/training exposures (students)

50 mSv

Total equivalent dose for embryo-fetus exposures

5 mSv

Total equivalent dose in 1 month for embryo-fetus exposures

0.5 mSv

Annual negligible individual dose

0.01 mSv

Providing radiation protection for workers and the public

Health physics

Dose limits and our practice of ALARA are based on what radiation dose-response relationship

Linear, nonthreshold

radiation dose to patients is measured in

mGyt

radiation dose of exposure personnel is measured in

mGya

Of the cardinal principles of radiation protection, which is most effective. What is overall better

Distance; no repeat exposures

Formula used for time (cardinal principle)

Exposure= Exposure rate * exposure time

formula used for distance (cardinal principle)

Inverse Square Law (I1/I2 = (D2/D1)²)

Formula used for shielding (cardinal principle)

Tenth Value Layer (TVL)

TVL definition

thickness of absorber that reduces the radiation intensity to one-tenth of its original value

What represents areas of equal radiation exposure in the fluoro room

Isoexposure Lines

The inverse square law does not apply in fluoro via the isoexposure lines because

the inverse square law takes the primary beam while the fluoro exposure takes into account scatter radiation

unit for exposure

mGy

unit for exposure rate

mGy/hour

Unit for exposure time

hr, sec, min

The 5 minute timer (cumulative timer)

alerts the radiologist that 5 minutes of radiation was utelized

When only part of the body is exposed, the risk of stochastic radiation is

not proportional to tissue dose, but proportional to effective dose

Effective dose is the

equivalent whole-body dose from only the part of the body that was exposed

weighted average of the radiation dose to the various organs and tissues

equivalent whole body dose

Effective dose formula

Dt *Wt (d= dose to specific tissue(equivalent dose); w= tissue weighting factor) -or absorbed dose (Gy)* tissue weighting factor* radiation weighting factor -

The actual occupational effective dose is what percentage of the occupational monitored radiation dose

10 %

The radiation monitor placed at the collar estimated the dose to the

head and neck (body receives essentially zero dose)

Radiation weighting factor for xray and gamma rays is

1

0.20 tissue weighting factors for what part of the body

gonads

0.12 is the tissue weighting factors for what part of the body

bone marrow, colon, lung, stomach

0.05 tissue weighting factors for what part of the body

liver, breast, thyroid

0.01 tissue weighting factors for what part of the body

Skin

Leakage radiation from protective tube housing must be

less than 1 mGya/hr at a distance 1m from the protective housing

xray beam on must be

clearly indicated when tube is energized

the SID indicator must be accurate within what percentage of indicated SID

2%

the x-ray beam and the light beam collimation must coincide to what percentage

2 %

The Positive Beam Limitation PBL must be accurate within what percentage of the SID

2%

Automatically provides an x-ray beam equal to the IR size used (does not exceed limits)

PBL

The xray beam and the IR should be

aligned (beam alignment)

Total filtration (added + inherent) should equal to atleast what when operated above 70 kVp

2.5 mm Al or Pb

Total filtration (added + inherent) should equal to atleast what when operated between 50- 70 kVp

1.5 mm Al or Pb

Total filtration (added + inherent) should equal to atleast what when operated below 50 kVp

0.5 mm Al or Pb

For any given radiographic technique, the output intensity should not exceed

5%

What is the feature that is checked by making repeated xray exposures at the same techniques

reproducibility

radiation intensity is expressed in units of

mGya/mAs

The max variation in linearity is

10% from one station to another

When adjacent mA stations are used, and the exposure time is adjusted accordingly, the tube should produce a constant mAs

Linearity

The exposure cord should be fixed to the operating console

Operator shield

The exposure switch must allow min. of 2m from the x-ray and a lead apron available at all times

Mobile x-ray imaging system

the intensity of the X-ray beam at tabletop should not exceed what at each mA at 80 kVp

20 mGya/min

the intensity of the X-ray beam at tabletop should not exceed what during fluoroscopy if there is no optional high-level control

100 mGya/min

the intensity of the X-ray beam at tabletop should not exceed what during fluoroscopy if there is an optional high-level control

200 mGya/min

Entrance skin exposure and Source-to-skin Distance is

inversely related (as one goes up the other goes down)

the min. SSD of mobile fluoroscopy is

30 cm (12 in)

the min. SSD of stationary fluoroscopy is

38 cm (15 in)

Coupled w/ the xray tube and interlocked so that the tube is not energized when the IR is parked

Primary Protective Barrier

the primary protective barrier must be

2mm of Pb or equivalent

when filtration is unknown, what is measured

the half-value layer

What is adjusted when the IR is 35 cm above tabletop w/ collimators fully open and an unexposed border is visible on the image monitor

Collimation

The fluoro exposure control is termed

dead-man switch

The bucky slot cover that covers the 5 cm wide opening at gonadal level is at least

0.25 Pb or equivalent

The protective curtain between personnel and the pt is at least

0.25 Pb or equivalent

The useful beam

primary radiation

any wall to which the primary beam can be directed

Primary Protective Barrier

What is the most common primary protective barrier

Lead

Types of scatter radiation

leakage and compton scatter

the most important source of scatter radiation is the

patient

Radiation emited from the x-ray tube housing in all directions other than that of the useful beam is termed

leakage radiation

Protective barriers designed to shield areas from secondary radiatradiationin are termed

secondary protective barriers

Area primarily occupied by radiology personnel and patients, personnel wear dosimeters and radiation safety is applied based on annual occupational dose limit of 50 mSv

Controlled Area

Area occupied by anyone and follows the dose limit for the public of 1 mSv/year

uncontrolled area

Barrier thickness factor expressed in units of mA-min. per week, and considers the volume and types of exams performed in the room

Workload

Relationship between workload and shielding thickness

direct (more exposures= more lead)

Percentage of time that the x-ray beam is on and directed toward a particular protective barrier

Use factor

The use factor for secondary radiation is

always 1

For a secondary barrier, leakage and scatter radiation are present

100% of the time

All barriers are secondary because

Useful beam always is intercepted by the pt and the IR

A high level of accuracy in radiation detection and measurement means

instruments can detect and precisely measure the intensity of a radiation field

in Sensitivity, a high level can

measure very low radiation intensities

the measure of radiation intensity

Radiation Dosimetry

the devices used to measure radiation

radiation dosimeters

As radiation passes, gas is ionized electrons released are detected as an electrical signal that is proportional to the radiation intensity

Gas filled detectors

three types of gas filled detectors

ionization chamber, proportional counter, Geiger counter

Emit light in response to absorption of an x-ray and is proportional to the amount of energy absorbed by the material

scintillation detector

Thermoluminescence dosimetry (TLD) is made up of

Lithium Fluoride (LiF)

Features of TLD

Measure as low as 50 microgray, released as light via heat, only readible once, worn for 3 months

Optically Stimulable Luminescence (OSL) is made up of

Aluminum Oxide Al2O3

Features of OSL

Measure as low as 10 microgray, released via laser light from an electron trap, can be reanalyzed multiple times, worn for three month

Used to detect radioactive particles in nuc. med facilities

Geiger-Mueller Detector

Used to measure radiation in an area, storage area for radioisotopes, pts with radioactive sources, and can measure as low as 10 micrograys

Handheld Ionization Chambers