Chapter 6 Lecture Notes

Rad Bio

Chapter 6 Lecture Notes

Protection of Personnel

Review of terms:

1.  early effects of radiation – occur relatively quickly after a high dose of radiation is received

2.  late effects of radiation – occur months or years after exposure to intermittent doses of smaller amounts. The main effects being cancer and genetic effects

Absorbed dose limits have been set to reduce possibility of occurrence of early or late effects of radiation for occupational workers.

Radiation Monitoring (p. 102) – personnel must be monitored when expected to receive 10% of the annual effective dose equivalent.

Dosemitry – measurement of ionizing radiation doses to personnel

Personal dosimeters – record external radiation doses

1.  film badges – most popular, contains special radiation-dosimetry film similar to dental film, enclosed in a plastic holder.

a.  Must be changed monthly, worn at collar level outside lead aprons. Level of the sternum receives highest level of radiation for the employee.

b.  Control badge must be kept in a radiation-free area, and exposure will be measured by amount of darkening on the film after processing.

c.  Keep away from radiation sources, excessive heat and high humidity. Should only be worn by the person assigned the badge.

d.  This type of badge is only accurate over 10 mrem or above.

2.  thermoluminescent dosimeters – contain lithium fluoride or calcium fluoride crystals instead of film.

a.  crystals store radiant energy when heated

b.  heat causes crystals to emit light that gets measured by a machine to determine how much radiation the crystal has received

c.  commonly worn as finger rings

d.  superior over film badges because they are nearly tissue equivalent, can be worn for 3 months, are reusable, are extremely accurate, and are more sensitive than film badges (to 5mrem)

3.  pocket dosimeters – most sensitive of film badges.

a.  resembles a fountain pen

b.  contains an ionization chamber that collects a charge

c.  may give false readings if subjected to trauma or high humidity

Dosimetry report (p. 105) – should be posted by employer for all personnel to view, and contain the following information:

1.  personnel ID #

2.  monitor type (TLD or badge)

3.  name and soc. Sec #

4.  birthdate and sex

5.  exposure period

6.  current exposure

7.  cumulative quarterly exposure

8.  cumulative annual exposure

9.  cumulative total (life) exposure

10.  unused part of permissible accumulated dose

Should be listed in millirems, and an “M” on the reading means you’ve received a minimum exposure, not enough to read on the film badge.

Dose-limiting Recommendations (p. 106):

·  Keep radiation exposures as low as reasonably achievable (ALARA).

·  Federal government regulations state diagnostic radiology personnel have an annual dose limit of 5 rem for whole-body occupational exposure.

·  The public has an annual effective dose limit of .5 rem (about 1/10 the allowed dose for occupational exposure).

·  Annual dose limit for the lens of the eye is 15 rem. Skin/extremities = 50 rem, and whole body = 5 rem.

·  Dose limits during pregnancy = 0.5 rem for one year, and 0.05 rem per month. A pregnant radiography must voluntarily tell employer of pregnancy in writing, and give the approximate conception date.

·  Cumulative whole body dose is calculated by multiplying 1 rem x age

·  Radiography students may not receive more than .1 rem annually

Calculation of exposure for patients – done by a physicist by three means

1.  skin dose (or ESE – entrance skin exposure) – formula p. 110 which the radiographer is not qualified to report to the patient… must be done by physicist:

2.  gonadal dose

3.  bone marrow dose

Formulas:

1.  ESE

2.  Direct Square law for intensity (for kVp and mAs)

Personnel exposure principles – bottom p. 111

1.  time – reducing time spent in the vicinity of the radiation source during operation, and remain behind protective barriers when not required to be in the room

2.  distance – keep at maximum distance from the source while in the room

3.  shielding – wearing lead protective apparel while in the room

3 cardinal rules of radiation protection

Structural Shielding:

1.  Primary barriers – protect from primary radiation (directly from the x-ray tube), and include lead-lined walls and bucky slot covers.shield effectively up to 140 kV, consist of 1/16 inch of lead equivalency, and must extend at least 7 feet from the floor.

2.  Secondary barriers – provide protection from secondary radiation (that which has been scattered primarily from the patient or leakage from the tube), must be 1/32 inch lead equivalent. Includes control booth because the beam should never be directed toward it.

3.  Glass windows of shielded control booth have avg. lead equivalency of 1.5 mm

4.  Lead housing of the tube should be 1.5mm Pb. It functions solely to reduce leakage radiation that shall not exceed 100mR/hr at 3 feet (1 meter)

Protective Garments:

1.  lead commonly used for its high atomic #

2.  shields evaluated by HVL (half-value-layer) and TVL (tenth-value layer) – or the amount of lead thickness that will reduce radiation intensity by 50% or 10%.

3.  all radiographers performing fluoroscopic or mobile procedures should wear lead shielding

4.  according to NCRP report #102 (bio book wrong), lead aprons must be a minimum of 0.25mm Pb equivalent, although most are 0.5mm thick, and 1.0mm thickness will protect up to 99% of the beam.

5.  thyroid shields are available, which must be .25mm equiv

6.  lead gloves must be .25mm equiv

7.  eyewear is not required, but available

Mobile exams:

1.  instruct others to leave the area

2.  inform surrounding people that an x-ray exposure is about to take place

3.  patients and people assisting must wear lead aprons

4.  always instruct people when they may re-enter the area

5.  stand at right angles to the patient

6.  radiographer is to wear lead apron during all mobile exams

7.  stand at least six feet from the tube, the patient, and useful beam

Fluoroscopic exams:

1.  wear a wrap-around apron if available

2.  never turn your back to the useful beam if not wearing a wrap-around

3.  highest radiation intensity for fluoro is 90 degrees from the incident beam (at the gonads for the operator)

4.  less radiation during portable fluoroscopic exams if the image intensifier is on top

5.  always move the bucky tray to head or foot to prevent unnecessary scatter

6.  position yourself behind the radiologist when possible

7.  fluoro switch must be a deadman type

8.  mobile fluoro source-to-tabletop distance must not be less than 30cm (12in)

9.  fixed fluoro source-to-tabletop distance must not be less than 38cm (15in)

10.  fluoroscopic equipment must have at least 2.5mm Al equivalency filtration

11.  image intensifier itself can act as a primary protective barrier, and must be 2mm lead equivalent

12.  cumulative timing devices must alarm every 5 minutes of fluoro

13.  fluoro x-ray intensity must not exceed 10R/min at the tabletop

14.  use of intermittent fluoro is recommended

15.  protective curtain between patient and person operating fluoro must be .25mm lead equivalent

16.  use inverse square law to calculate radiation intensity, and keep in mind when standing in the x-ray room during an exam (p. 121 example)

Patient Immobilization:

1.  use if there is voluntary motion

2.  use low time setting to eliminate involuntary motion

3.  combine immobilization with proper communication

4.  you should not routinely hold patients for exposures

5.  if you do hold, use lead gloves if hands will be in the primary beam