2721 Management of Imaging Personnel Dose

RT Protection

• Scatter poses the greatest occupational hazard to the RT

• The patient is the source of scatter in rad exams (fluoro, mobile, diagnostic)

• At a 90 degree angle, at a distance of 3 feet from the patient, the scattered x ray intensity is approximately 0.1% of the original intensity of the primary beam

• Never stand in the primary beam

ALARA Concept

• Apply cardinal principles

• Collimate the beam

• In 1954 the National Committee on Radiation Protection (Later known as the National Council on Radiation Protection and Measurements) put forth the principle that radiation exposures should be kept “as low as reasonably achievable” with consideration for economic and societal factors

• The ALARA concept is accepted by all regulatory agencies

• The ALARA concept adopts an extremely conservative model with respect to the relationship between ionizing radiation and potential risk

Occupational Risk

• Keep effective dose limits as low as possible (do not keep dose in upper range of limit, ex: annual dose right below or at 50 mSv)

• The risk to a radiographer from radiation exposure may be equated with occupational risk in other industries that are generally considered reasonable safe

• NCRP proposed that radiation protection programs for radiation workers be designed to prevent individual workers from having a total external plus internal cumulative EfD in excess of their age in years times 10 mSv

Dose Reduction Methods

• Avoid repeat exams

• The patient as a source of scattered radiation

• Scattered radiation occupational hazard

• Filtration of the diagnostic beam

• Protective apparel

• Technical exposure factors

Patient Restraint

• RT should never hold

• First choice - Use mechanical restraining devices

• Last resort - Use family member or friend of pt, provide shielding, instructions for position to hold; other hosp personnel should not be used to hold routinely

Distance: Inverse Square Law

• (d²/d¹)²

• The ISL also implies that if a radiographer moves closer to a source of radiation, the radiation exposure to the radiographer drastically increases

• Ex: if the radiographer stands 2 feet away from an x ray source instead of 6 feet away the radiographer’s radiation exposure increases by a factor of 9

Shielding

• Structural Protective Barriers

  • Most common materials; lead and concrete

• Accessory protective devices: Lead impregnated vinyl

  • Ex: Apron, gloves, protective eye wear

Protective Structural Barriers

• Doors to X-ray room

  • Must be closed during exposures; attenuation equivalent to 1/32” lead

• Floors and ceiling

  • Usually dose not require shielding if made of concrete (Provides adequate shielding from scatter radiation)

Primary Protective Barrier

• Prevents direct, un-scattered radiation

• Located perpendicular to the line of travel to the x-ray beam

• 1/16” lead and extends 7 feet upward from floor

Secondary Protective Barrier (Leakage and Scatter)

• Ceiling is secondary protective barrier

• Should overlap the primary barrier by 1/2”

• Consist of 1/32” lead

• Control-booth barrier - Protection of radiographer

  • Must extend 7 feet upward from floor

  • Lead glass window - 1.5mm lead equivalent

• Clear lead-plastic secondary protective barrier

  • 7 feet upward from the floor, lead equivalencies from 0.3- 2mm

  • Ex: special procedures open view window

  • Typically 0.5mm lead equivalencies

Accessory Protective Devices: Lead Aprons and Gloves

• At 100 kVp a protective lead apron must be equivalent to at least 0.25mm thickness lead equivalent

• A lead apron of 0.5 or 1 mm lead equivalent offer much more protection

• 0.25 mm lead would be appropriate for mammography

• 0.5mm lead equivalent is most widely used and recommended for diagnostic imaging

• Wrap around is recommended for interventional studies

Accessory Protective Devices

• Neck and Thyroid Shields

  • Minimum of 0.5mm lead equivalent

• Lead gloves

  • 0.25mm lead equivalent is recommended

• Protective eyeglasses

  • Minimal lead equivalent of 0.35mm

• Care of Protective Garments

  • Inspect annually for cracks

    • Inspect visually and fluoroscope or take radiographs using high kVp

    • Store on properly designed racks (hanging) or lay flat on table (not folded on x-ray unit)

    • Remove from service if crack exists

RT Protection During Fluoro

• Protective Curtain

  • A protective curtain of at least 0.25 mm lead equivalent or retractable panel mounted on side of table of at least 0.5mm lead equivalent should be positioned between the fluoroscopist and patient to minimize scatter to personnel during fluoroscopy

• Standing position

  • Standing at right angles to the patient decreases radiologist and RT dose

Mobile Fluoro and RT Protection During Mobile Procedures

• C-arm Fluoro (Mobile)

  • Scatter radiation is more intense with x-ray tube over patient

• Portable X-ray Machine

  • Length of Exposure Cord

    • The exposure switch must allow the RT to remain at least 6ft from the x-ray tube during the exposure

    • RT must stand at maximum distance from pt, tube and useful beam

    • RT should attempt to stand at a right angle to the x-ray beam

Diagnostic X-ray Suite Protection Design

• Radiation Shielding Categories

  • Primary Radiation

  • Secondary Radiation

    • Scatter

    • Leakage

• Over Shielding: It is better to spend extra money up front and over shield than to not meet requirements later, if regulatory standards change. Follows ALARA principle

Radiation Absorbent Barrier Design Considerations

• The mean energy of the x-rays that will strike the barrier

• Whether the barrier is of primary or secondary nature

• The distance from the x ray source to a position of occupancy 0.3m from the barrier

• The workload of the unit

• The use factor of the unit

• The occupancy factor behind the barrier

• The intrinsic shielding (Tube housing attenuation) of the x ray unit

• Whether the area beyond the barrier is “controlled” or “uncontrolled”

Calculation Considerations

• Workload (W)

• Inverse Square Law

• Use Factor (U) (beam direction factor)

• Occupancy Factor (T)

Workload (W)

• Describes weekly radiation use of a diagnostic x ray unit

• Radiation “on time”

  • Indicates the level of radiation activity in a room

  • Amount of use of the x ray unit; the time the x ray tube is energized/week

  • Includes the voltage & mA of the generator, daily patient load (# of exams/wk) measured in milliampere-minutes per week (mA-min/wk)

  • Busy room = 1500-1000 mA-min/wk

  • Private office = 100 mA-min/wk

  • Formula: W = mAs/patient x # of patients/hr x # hrs/wk/ 60 min

  
  

Use Factor (U) or Beam Direction Factor

• Quantity that indicates the proportional amount of time during which the x ray beam is energized and directed toward a particular barrier (wall, floor, ceilings)

• Full Use (U = 1)

  • Floors of radiation rooms except dental installations, doors, walls, and ceilings of radiation rooms exposed routinely to the primary beam

• Partial Use (U = 1/4)

  • Doors and walls of radiation rooms not exposed routinely to the primary beam; also floors of dental installations

• Occasional Use (U = 1/16)

  • Ceilings of radiation rooms not exposed routinely to the primary beam; because of the low use factor, shielding requirements for a ceiling usually determined by secondary rather than primary beam considerations

Time of Occupancy Factor (T)

• Used to indicate the fraction of time that an area of the hospital (outside the exposed area) may be occupied by a given individual

• Designed to overestimate the fraction of time that a given individual might actually spend in an area; more than ensures that exposure is kept low

• Two Types:

  • Uncontrolled areas

    • Areas occupied by non rad workers (general public), those subject to a dose limit of 10 mrem/week; shielding lowers the dose equiv limit to 2mrem/wk

    • The walls must have 1-2 tenth value layers more than controlled areas

    • Included hallways, waiting rooms, stairways, exterior of hosp, toilets, shops, nurses stations, offices, kids play areas

    • Takes on different values depending on purpose of area

  • Controlled areas

    • Areas occupied by radiation workers; those who wear dosimeters

    • Must reduce the exposure rate/ dose equiv limit to < 100 mrem/wk

    • T value = 1

New Approaches to Shielding

• More rigorous analysis of workload using kVp actually used

• Abandon use of adding one HVL if leakage and scatter barrier requirement are similar - use exact calculations

• More accurate estimate of use factor to reflect true percentage of time the beam is directed at various barriers

• More realistic occupancy (T) values for low occupancy areas

• Workload: More realistic use of contemporary survey data

• Leakage and scatter: Explicit barrier calculations

• Use factor: Adjusted for beam direction data reflecting actual usage patterns

• Occupancy factor: Realistic assumptions of occupancy of low occupancy areas

Pregnant/Potentially Pregnant RT

• Declaration of pregnancy

• Provide written notification of pregnancy or suspected pregnancy to supervisor

• Termination or involuntary leave of absence or altered work schedule not allowed

• Counseling (during pregnancy while continuously employed)

• Review of monitoring records (previous rad exposure hx)

• Review of dose limit during pregnancy (for fetus)

  • DL for period of preg 0.5 rem (5 mSv) DL/mo 50 mrem (0.5 mSv)

• Personnel Monitoring Devices

  • 2nd dosimeter worn at waist level under apron to monitor fetal exp

  • Must maintain exposure level on a separate record from mother and identify as fetal exp

• Review of proper radiation protection principles and practices

  • Utilize ALARA

  • Cardinal principles of radiation

• Protective apparel ( 0.5mm lead apron; wrap around type can be used)

• Read and sign form acknowledging rad risk