Basic Principles of Radiation Protection

Basic Principles of Radiation Protection

Responsibility for Radiation Protection

A. Radiographer is primarily responsible for protecting the

patient from unnecessary exposure

1. Best accomplished by avoiding repeat exposures

2. Should use smallest amount of radiation that produces

a diagnostic radiographic image

B. Radiologist and referring physician assume shared

responsibility for radiation safety of the patient

1. Best accomplished by consultation

2. Should not order unnecessary exams

C. Safe use of radiation in diagnostic imaging to determine

the extent of disease or injury should outweigh the risk

involved from the exposure

Patient Exposure and Protection

The heart of radiation protection for the patient lies in the concept

of ALARA. It is primarily the radiographer’s responsibility

to see that ALARA is in practice so that patients are properly

protected. Taking adequate histories, communicating clearly,

using proper immobilization, and conducting radiographic and

fluoroscopic exams calmly and professionally add to the level of

safety your patients will encounter while under your care. This

section reviews the many technical aspects of the radiographer’s

practice that contribute to ALARA for the patient.

Beam Limitation

Beam limitation protects the patient by limiting the area of

the body and the volume of tissue being irradiated

A.Collimator

1. Variable aperture device

2. Contains two sets of lead shutters placed at right

angles to each other

3. Higher set of lead shutters is placed near the x-ray

tube window to absorb off-stem (off-focus) radiation

4. Lower set of lead shutters is placed near the bottom of

the collimator box to restrict the beam further as it exits

5. Accuracy of the collimator is subject to strict quality

control standards

6. Collimation should be no larger than the size of the

image receptor being used

7. Collimators that automatically restrict the beam to

the size of the image receptor have a feature called

positive beam limitation (PBL), also called automatic

collimation

8. PBL responds when an image receptor is placed in

the tray containing sensors that measure its size

B. Cylinder cones

1. Metal cylinders that attach to the bottom of the collimator

2. Used to restrict the beam tightly to a small circle

3. Diameter of the far end of the cone determines field

size

4. Cones may be extended an additional 10 to 12 inches

by a telescoping action for even tighter restriction of

the beam

5. Cones may be used for exam of the os calcis, various

skull projections, and cone-down views of vertebral

bodies

6. Use of cones results in a restriction of the x-ray beam

by cutting out a major portion of the beam

7. When cones are used, mAs must always be increased

to make up for the rays attenuated by the cone

8. Cylinder cones do not work by focusing the x-ray

beam down the cone; x-rays cannot be focused

C. Aperture diaphragm

1. Flat piece of lead with a circle or square opening in

the middle

2. Placed as close to the x-ray tube window as possible

3. Has no moving parts

Filtration

A filter is placed in the x-ray beam to remove long-wavelength

(low-energy) x-rays. Low-energy x-rays contribute

nothing to the diagnostic image but increase patient

dose through the photoelectric effect. As low-energy rays

are removed, the beam becomes “harder” (predominantly

short-wavelength, high-energy). This process of removing

low-energy rays results in a lower patient dose.

A. Two types of filtration: inherent and added

B. Inherent filtration

1. Glass envelope of the x-ray tube

2. Insulating oil around the tube

3. Diagonal mirror used for positioning light

C. Added filtration

1. Aluminum sheets placed in the path of the beam near

the x-ray tube window

2. Mirror placed in the collimator head

D. Total filtration

1. Equals inherent plus added filtration

2. Must equal at least 2.5-mm aluminum equivalent for

x-ray tubes operating at greater than 70 kVp

E. Half-value layer: Amount of filtration that reduces

the intensity of the x-ray beam to half of its original

value—measured at least annually by a qualified radiation

physicist

F. The radiographer never adjusts added tube filtration; if

it is suspected that the filtration has been altered, the

x-ray tube must not be used until checked by a radiation

physicist

Gonadal Shields

Gonadal shields are used to protect gonads from unnecessary

radiation exposure. They should be used whenever they

do not obstruct the area of clinical interest.

A. Gonadal shielding may reduce female gonad dose by up

to 50%

B. Gonadal shielding may reduce male gonad dose by up to

95%

C. Proper collimation may also greatly reduce gonadal dose

and should be used in conjunction with gonadal shields

D. Most commonly used gonadal shields

1. Flat contact shield: Flat piece of lead or a lead apron

placed over the gonads

2. Shadow shield: Suspended from the x-ray tube housing

and placed in the x-ray beam light field; requires

no contact with the patient; especially useful during

procedures requiring sterile technique

Exposure Factors

Exposure technique determines the quantity and quality of

x-rays striking the patient

A. Use optimal kVp for the part being radiographed

B. Use the lowest possible mAs to reduce the amount of

radiation striking the patient

C. The part being imaged should be measured with calipers

D. A reliable technique chart should be consulted to determine

the proper exposure factors to use

E. Use of automatic exposure controls (AEC) reduces the

number of repeat exposures

Grids

A. Result in an increase in patient dose because increased

mAs are required

B. Use appropriate type and ratio of grid for part being

imaged and exam being performed

Repeat Exposures

A. Always result in an increase in radiation dose to the

patient

B. Must be kept to a minimum

C. Should be tracked via a departmental repeat exposure

analysis

D. Reasons for repeat exposures should be documented

E. In-service education for areas of frequent repeat exposures

should be conducted by qualified radiographers or

radiologists

Technical Standards for Patient Protection

A. Minimum source-to-skin distance for portable radiography:

at least 12 inches

B. Fluoroscopy

1. Use of intermittent fluoroscopy (as opposed to a

constant beam-on condition)

2. Tight collimation of the beam

3. High kVp

4. Source-to-tabletop distance for fixed fluoroscopes:

not less than 15 inches

5. Source-to-tabletop distance for portable fluoroscopes:

not less than 12 inches

6. Proper filtration of the beam

7. Fluoroscopy timer that sounds alarm after 5 minutes

(300 seconds) of beam-on time

8. Fluoroscopy timer should not be reset before alarm

goes off; fluoroscopist must be made aware of time

that the patient and other persons in the room were

exposed

9. Exposure switch must be dead-man type

10. Limit dose at the tabletop to no more than 100 mGya

per minute

11. Limit use of high-level-control fluoroscopy (HLCF)

during interventional procedures to no more than

200 mGya per minute

12. Long exposure times (>30 minutes) can lead to skin

effects; the patient should be informed of the possibility

of skin injury (e.g., erythema, epilation)

13. Fluoroscopy times should be recorded

14. Pulsed fluoroscopy or low-dose modes should be

used when possible to achieve ALARA objectives

15. Personnel during fluoroscopy should stand on the

image intensifier side of the C-arm during lateral or

oblique projections

16. When possible, use the C-arm with the x-ray tube

below the patient for anteroposterior and posteroanterior

projections

17. During fluoroscopy, the radiographer should always

wear the radiation dosimeter outside of the lead

apron at the level of the collar

18. Avoid use of electronic magnification modes on

the image intensifier when possible because this

increases the dose to the patient and other persons

in the room

19. Air kerma

a. Provides an approximate skin dose where the

x-ray beam is entering the patient

b. Is displayed during a fluoroscopic procedure

c. Monitoring air kerma is a way to keep dose to

the patient lower, especially during prolonged

fluoroscopic procedures

d. Expressed as Gya (a indicates energy deposited in

a mass of air)

20. Dose area product (DAP)—the total of air kerma

striking the surface of the patient

a. May be read on the DAP meter on the fluoroscopic

monitor

b. Expressed as mGy-cm2

21. Last image hold (LIH)

a. Provides for keeping the most recently acquired fluoroscopic

image displayed on the monitor without

continued radiation exposure to the patient

b. Using last image hold reduces patient exposure

by allowing the fluoroscopist to evaluate an

image without continuing to expose the patient

22. Automatic brightness control (ABC) or automatic

exposure rate control (AERC)

a. kVp and mA are automatically adjusted during

fluoroscopy, which keeps image brightness level

b. The ABC or AERC delivers to the image receptor

only the exposure that is required to maintain

the necessary image quality

Cardinal Principles of Radiation Protection

A. Time: Amount of exposure is directly proportional to

duration of exposure

B. Distance: Most effective protection from ionizing radiation

1. Dose is governed by the inverse square law

2. The greater the distance from the radiation, the lower

the dose

3. Dose varies inversely according to the square of the

distance; for example, if the exposure is 5 mGya at a

distance of 3 feet, stepping back to a distance of 6 feet

causes the exposure to decrease to 1.25 mGya

4. The inverse square law should always be used during

fluoroscopy in which close contact with the patient

is not required and during mobile radiography and

fluoroscopy

C. Shielding: Lead-equivalent shielding absorbs most of the

energy of the scatter radiation

1. A lead apron of at least 0.25-mm lead equivalent must

be worn (0.5-mm lead equivalent should be worn)

during exposure to scatter radiation; a thyroid shield

of at least 0.5-mm lead equivalent should be worn for

fluoroscopy

2. The radiographer should never be exposed to the primary

beam

3. If exposure of the radiographer to the primary beam

is unavoidable, the exam should not be performed

4. Family members of the patient, nonradiology employees,

or radiology personnel not routinely exposed

should be the first choices to assist with immobilization

of the patient for an exam when all other types

of immobilization have proved inadequate

5. The radiographer should be the last person chosen to

assist with immobilization during an exposure

6. Radiographers and student radiographers should not

be viewed as quick and easy-to-use immobilization devices

7. Thickness recommendations for lead devices have

been determined by NCRP Report #102

Source of Radiation Exposure to Radiographer

A. Source of radiation exposure to the radiographer is scatter

radiation produced by Compton interactions in the

patient

B. Greatest exposure to the radiographer occurs during

fluoroscopy, portable radiography, and surgical radiography

C. It is vital that lead aprons be worn when fluoroscopy and

mobile radiography or fluoroscopy is performed

D. Photons lose considerable energy after scattering

E. Scattered beam intensity is about 1⁄1000 the intensity of

the primary beam at a 90-degree angle at a distance of 1

m from the patient

F. Beam collimation helps reduce the incidence of Compton

interactions, resulting in decreased scatter from the

patient

G. The use of high-speed image receptors may further

reduce the amount of scatter produced because of

decreased quantity of radiation needed for exposure