Management of Imaging Personnel Radiation Dose

What is the annual limit for occupationally exposed personnel?

Federal government follows NCRP recommendations, permitting diagnostic imaging personnel to receive an annual occupational effective dose of 50 millisievert (mSv) for whole body exposure

What is lifetime effective dose?

cumulative effective dose (CumEfD) limit measured in mSv

age x 10 mSv

What is the EfD limit for the general public?

frequent exposures: 1 mSv

infrequent exposure: 5 mSv

Why are occupational personnel allowed more radiation exposure than the general public?

they will not significantly cause an increase in disease in the population as a whole (won’t alter the GSD)

What are some areas that increase a radiographer’s risk of exposure?

fluoroscopy, IR procedures with HLCF, mobile exams, C-arm fluoroscopy

How can you apply ALARA to radiation control procedures?

utilizing time, distance, shielding

adequately collimating the beam

At a 90^o angle to the primary beam, at a distance of 1 meter, the scattered x-ray intensity is ___

1/1000 the intensity of the primary beam (0.1%)

How does collimation affect occupational dose?

by reducing the number of x-ray photons available to undergo Compton scatter

Filtration as a method of dose-reduction primarily benefits ___

the patient

Lead aprons are usually ___ mm lead equivalent (but may vary)

0.5

NCRP recommends lead aprons be ___ mm lead, but the minimum they are made is ___ mm lead

0.5; 0.25

Lead aprons protect personnel from ___ and ___ radiation

scatter and leakage

How can you alter technical exposure factors to reduce exposure?

use higher kVp (increases avg energy of photons, decreasing large angle scatter) and lower mAs

If a patient needs to be restrained/immobilized, how can you do this while receiving minimal dose?

• never stand in the primary beam

• use restraints when possible

• use a non-occupational person (preferably male or non-childbearing age person) to hold patient

What is imaging department protocol for pregnant personnel?

• worker should declare pregnancy

• RSO will provide a fetal radiation dosimeter

• monthly EqD to embryo-fetus should not exceed 0.5 mSv

How do work schedule alterations affect personnel dose?

ALARA guidelines have all workers rotating equally to distribute radiation exposure risk evenly to all employees

What are the 3 basic principles of radiation protection?

1. time

2. distance

3. shielding

Explain time (as a basic principle of radiation protection)

• the amount of time a radiation worker receives at a particular location is directly proportional to the length of time the individual is in the path of ionizing radiation

• especially important in fluoroscopy

  • this is why units are equipped with 5-minute timer alerts

Explain distance (as a basic principle of radiation protection)

• most effective means of protection from ionizing radiation

• personnel will receive significantly less radiation exposure by standing farther away from a source of radiation

• to reduce exposure, follow the Inverse Square Law

Define the Inverse Square Law

expresses the relationship between distance and intensity of radiation and is a tool to be used in governing the dose received by personnel

Explain the Inverse Square Law (ISL) and the formula

the intensity of radiation is inversely proportional to the square of the distance from the source

I1/I2 = (D2)²/(D1)²

If you double your distance from the radiation source, you are getting ___ of the radiation intensity

¼

doubled, so 2 times

2² = 4 → ¼ times

Use the Inverse Square Law to solve:

if a radiographer stands 1 meter away from an x-ray tube and is subject to exposure of 2 mGy_a/hr, what will the exposure be when standing 2 meters from the x-ray tube?

I1 = 2 mGy

I2 = ?

D1 = 1

D2 = 2

2/? = 2²/1²

I2 = 0.5 mGy_a/hr

Use the Inverse Square Law to solve:

if exposure rate 7 ft from tube is 1.5 mGy/hr, what will the dose be at 8 ft from tube in 30 minutes?

I1 = 1.5 mGy

I2 = ?

D1 = 7 ft

D2 = 8 ft

1.5/? = 8²/7²

I2 = 1.148 mGy/hr = 0.01913 mGy/min

0.01913 × 30 minutes = 0.57 mGy

Explain shielding (as a basic principle of radiation protection)

• when time and distance are not achievable, shielding may be used to provide protection from radiation

• structural barriers: lead and concrete

• accessory protective devices: aprons, gloves, thyroid shields, eyeglasses

Explain the use of protective structural shielding

• used to protect imaging personnel and general public

• lead sheets of appropriate thickness are placed in the walls of radiography and fluoroscopy rooms

• physicist determines the exact protection requirements needed

What are primary protective barriers?

• prevent direct, or unscattered radiation from reaching personnel or general public on the other side of the barrier

• located perpendicular to the undeflected line of travel of the x-ray

• consists of 1.6 mm (1/16 in) lead

• extends 2.1 m upward from the floor

What are secondary protective barriers?

• protect against leakage and scatter radiation

• any wall or barrier that is never struck by the primary beam

• overlap the primary barrier by ½ in

• consists of 0.8 mm (1/32 in) lead

Explain how the control booth barrier protects the radiographer

• extend 2.1 m upward from floor and is permanently secured to the floor

• x-ray photons should scatter at least 2 times before reaching any area behind the barrier

• a type of secondary protective barrier

• glass is 1.5 mm lead

The control booth will not allow exposure to exceed ___

1 mSv per week

Leads are inspected for holes/cracks every ___

2 years

Thyroid shield should be a minimum of ___ mm lead (per the NCRP)

0.5

Gloves should be worn when ___, and should be ___ mm lead equivalant

worn when hands are near the fluoroscopic field; 0.25 mm lead equivalant

Protective eye glasses are made of clear lenses that contain ___ mm lead equivalent

0.35

How can you reduce dose from scatter during a fluoroscopic exam?

• stand far away from the patient

• wear a 0.5 mm lead apron (and gloves if hands are near the beam)

• stand behind the radiologist when possible

What are some dose reduction techniques in fluoroscopic procedures?

• adequate beam collimation

• adequate filtration

• control of technical factors

• appropriate source-to-skin distance

• use of timing device

• protective tube housing

Explain the fluoroscopic protective curtain

minimum of 0.25 mm lead should be positioned between patient and fluoroscopist to intercept scatter above tabletop

Explain the fluoroscopy bucky slot cover

bucky slot shield of at least 0.25 mm lead must automatically cover the bucky slot opening in the side of the x-ray table during standing fluoroscopic exams when bucky tray is at the foot of the table (protects at the gonadal level)

How do mobile x-ray units allow for radiation safety?

• some have a remote exposure device, allowing for more distance

• cord should be long enough to allow radiographer to stand > 2 m away

Ideally, where should you stand during mobile procedures?

at a right angle to the x-ray beam scattering line (the patient)

Explain the protection measures in place for c-arm fluoroscopic units

• position of the c-arm can change the scatter in the room

• tube over the table and image intensifier under the table results in higher patient exposure and increased scatter

• keep image intensifier as close to the patient as possible

• collimate, use last image hold, use foot pedal or handheld switch, limit use of boost/magnification

The exposure rate caused by scatter near the entrance surface of the patient (tube side) is ___ than the exposure rate caused by scatter near the exit surface of the patient (image intensifier side)

higher (by a factor of 2-3)

What does HLCF stand for?

high level control fluoroscopy

Explain the protection measures for HLCF

• limit boost mode

• use low dose or pulsed mode operations

• manually collimate

• time intervals

• use last image hold

• road mapping - vessel contrast studies

What are the maximum allowed entrance exposure rates for HLCF?

regular: 8.8 cGy/min (10 R/min)

boost mode: 20-40 cGy/min

NCRP recommends an annual EqD limit to localized areas of the skin and hands of ___

500 mSv

What are some radiation-absorbent barrier design considerations?

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

• whether the barrier is of a primary or secondary nature

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

• the workload of the unit

• the use factor of the unit

• the occupancy factor behind the barrier

• the intrinsic shielding

• whether the area beyond the barrier is controlled or uncontrolled

What are the 3 radiation sources generated in an x-ray room?

1. primary radiation: emerges directly from the x-ray tube collimator and moves without deflection toward a wall, door, etc. (requires the most protective shielding)

2. scatter radiation: occurs when x-ray beam passes through matter, undergoes Compton interactions, and emerge from the object in all directions

3. leakage radiation: rotation generated in the x-ray tube that does not exit from the collimator opening but penetrates through the protective tube housing

What are some calculation considerations for x-ray suite protection?

• workload

• kVp

• ISL (inverse square law)

• use factor

• occupancy factor

• controlled vs. uncontrolled areas

Explain workload (W) (as a calculation consideration)

• reflects the unit’s radiation on time

• radiation output - weighted time that the unit is actually delivering radiation during the week

• units of mAs per week

Explain the Inverse Square Law (as a calculation consideration)

• helps calculate the intensity of the beam which is important in barrier design

• helps determine primary and secondary barrier thickness values

Explain use factor (U) (as a calculation consideration)

• quantity that is used to select fractional contact time

• time that a wall or structure is struck by radiation to some degree for some fraction of weekly beam-on time, AKA beam direction factor

• U (primary) - time the primary beam radiation is directed at primary barrier during the week

• U (secondary) - time scatter and leakage radiation emerged and struck surfaces

U (secondary) = ___ for all radiation-accessible structures

1

Explain occupancy factor (T) (as a calculation consideration)

• used to modify shielding requirements for a particular barrier by taking into account the fraction of the work week during which the space beyond the barrier is occupied

• (are there people on the other side of the wall?)

Explain controlled and uncontrolled areas (as a calculation consideration)

controlled area: when the region adjacent to a wall of an x-ray room is used only by occupationally exposed personnel

uncontrolled area: nearby hall or corridor that is frequented by the general public

What does MPED stand for?

maximum permitted equivalent dose

What is the maximum permitted equivalent dose (MPED) for controlled and uncontrolled areas?

controlled areas: 1000 μSv/wk or 1 mSv/wk

uncontrolled areas: 20 μSv/wk

What are requirements that need to be taken into consideration for calculating barrier shielding?

• (mA-minutes) x (U) x (T)

• planner must know whether the area beyond the barrier is controlled or not

• NCRP states the calculation determines the thickness of a barrier sufficient to reduce the air kerma in a full or partially occupied area to a value that is less than or at most equal to P/T

What are requirements that need to be taken into consideration for calculating primary barrier shielding?

• x-ray transmission

• air kerma values

  • kVp can help determine air kerma

• distance from x-ray source to representative locations

• number of patients examined in room per week

• use factor and occupancy factor

• uncontrolled vs. controlled area

What are the U.S. shielding requirements for primary and secondary shields?

primary: 1/16 in (1.6 mm)

secondary: 1/32 in (0.8 mm)

What are requirements that need to be taken into consideration for calculating secondary barrier shielding?

• must intercept both scatter and leakage radiation

  • scatter and leakage emerge in all directions, so use factor is always 1

Intensity of radiation scattered at a 90 degree angle at 1 meter from the source is reduced by a factor of ___ relative to the primary radiation

1000

Regulatory standards mandate that the maximum permissible leakage exposure rate at 1 meter from the target in all directions can not exceed ___ when it is operated continuously at its maximal permitted kVp and mA combinations

100 mR/hr or 0.88 mGy_a/hr

___ may be used at barriers to reduce leakage radiation levels to permissible values

HVLs

Explain the use of radiation warning signage

• important for safety in radiology departments

• some states require warning lights near the door to the exam room from any corridor (they should self-illuminate when x-ray equipment is energized)

• radiation warning signs are posted within controlled areas of the hospital or facility and are for on the door to CT, IR, and radioactive storage areas