Unit 6 (Dose Limits)
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concerned with providing occupational radiation protection and minimizing radiation dose to the public:
how did the term "health physics" originate?
this term originated with the Manhattan Project which was a secret wartime effort to develop the atomic bomb. Physicist and physicians were responsible for those developing the bomb. They were considered the first health physicists.
name the three cardinal principles of
- time to be minimized
- distance to be maximized
- shielding to be maximixed
how is the cardinal principal of time calculated?
exposure = exposure rate x exposure time
the dose of an individual is directly related to the ____________________. If time of exposure doubles, then dose will ___________.
- duration of exposure
depress ___________________ when at all possible to minimize exposure to patient and personnel.
a radiation worker is exposed to 230 mR/hr from a radiation source. if the worker remains at that position for 36 minutes, what will be the total occupational exposure?
occupational exposure = 230mR/h (36min) / 60 min
= (230 x 36) / 60
a fluoroscope emits 4.2 R/min at the tabletop for every milliampere of operation. what is the patient exposure in a BE exam that is conducted at 1.8 mA and requires 2.5 minutes of fluoroscopic time?
patient exposure = (4.2R)/mAmin (1.8mA) (2.5min)
as the distance between the source of radiation and a person increases, the radiation exposure:
when it comes to distance, how is the decrease in dose calculated?
the inverse square law
the inverse square law states the intensity of the radiation at a location is inversely proportional to the square of its distance from the source of radiation
where should the radiologic technologist be during fluoroscopy?
as far from the patient as practicable
in isoexposure lines for radiation doses, the normal position for radiologist or radiologic technologist is approximately:
in isoexposure lines for radiation doses, two steps back from normal position is approximately:
shielding items usually consist of:
in the cardinal principle of shielding, the measurement of decrease of intensity can be achieved by:
thickness of absorber necessary to reduce the x-ray beam to half of its original intensity:
thickness of absorber that reduces the radiation intensity to one-tenth its original value:
the operating kVp of a radiographic imaging system rarely exceeds 100 kVp. the output intensity is 4.6 mR/mAs at 100 cm SID. the distance to a desk on the other side of the wall to which the x-ray beam is directed is 200 cm. the wall contains 0.96 mm Pb, and 300 mAs is anticipated daily. the exposure is to be restricted to 2 mR/wk, how long each day may the desk be occupied?
- (4.6mR/mAs)(300mAs) = 1380mR
- .96mm or 4HVL = 22mR
- 2mR / 110mR/wk = 0.018 week
- time allowed = 43 minutes
the dose of radiation that would be expected to produce no significant radiation effects:
maximum permissible dose (MPD)
- below: no response should occur
- at: small response should occur
the concept of MPD is no longer used and has been replaced by:
DL (dose limits)
dose limits imply that if received annually, the risk of death would be:
less than 1 in 10,000
organization that has assessed risks based on data obtained to establish dose limits:
national council of radiation protection (NCRP)
NCRP has assessed risks based on data obtained from:
- national academy of sciences
- biologic effects of ionizing radiation committee (BEIR)
- national safety council
NCRP dose limits for occupational exposures when it comes to effective dose: annual and cumulative
- annual: 50mSv (5000mrem)
- cumulative: 10mSv x age or 1000mrem x age
NCRP dose limits for occupational exposures when it comes to equivalent annual dose for tissues and organs; lens of eye and thyroid, skin, hands, feet:
- lens of eye: 150mSv
- thyroid, skin, hands, feet: 500mSv
how do you convert Sv to Rem and vice vera?
- Sv to Rem: divided by .01
- Rem to Sv: multiply by .01
NCRP dose limits for public exposures when it comes to effective dose, frequent exposure:
NCRP dose limits for public exposures when it comes to equivalent dose for tissues and organs; lens of eye and skin, hands, feet:
- lens of eye: 15mSV
- skin, hands, feet: 50mSv
NCRP dose limits for public exposures when it comes to effective dose, infrequent exposure:
dose limit for nonoccupational workers is _____ of that for a radiation worker.
NCRP dose limits for educational and training exposures when it comes to annual effective dose:
NCRP dose limits for educational and training exposures when it comes to equivalent dose for tissues and organs; lens of eye and skin, hands, feet:
- lens of eye: 15mSv
- skin, hands, feet: 50mSv
NCRP dose limits for embryo/fetus exposures when it comes to total equivalent dose and equivalent dose in one month:
- total equivalent dose: 5mSv
- equivalent dose in one month: .5mSv
NCRP dose limits for a negligible individual when it comes to annual dose:
current dose limits are based on ________________ relationship and they are considered to be an acceptable level of occupational radiation exposure.
linear, nonthreshold dose-response
in diagnostic radiology , it is seldom necessary to exceed __________ the appropriate dose limit.
the _________ issued a number of recommendations, including an annual whole-body dose limit of ______________.
- international commission on radiological protection (IRCP)
- 20mSv (2000mrem)
occupational exposure is described in _____________ and is measured in units of ___________.
- dose equivalent
- mSv or mrem
concept that accounts for different types of radiation because of their varying relative biologic effectiveness (RBE):
effective dose (E)
effective dose (E) considers ____________________, so effective dose (E) is ___________ than that recorded by a collar-positioned radiation monitor.
- the relative radiosensitivity of various tissues and organs
- much less
NRC occupational dose limits:
whole body (TEDE)
any organ (TODE)
lens of eye (LDE)
embryo/fetus of DPW
member of the public
- whole body: 5000 mrem/yr
- any organ: 50,000 mrem/yr
- skin: 50,000 mrem/yr
- extremity: 50,000 mrem/yr
- lens of eye: 15,000 mrem/yr
- embryo/fetus: 500 mrem/yr
- member of public: 100 mrem/yr
- **note: 1000 mrem = 1 rem
how do you calculate effective dose?
effective dose (E) = radiation weighting factor (Wr) x Tissue weighting factor (Wt) x absorbed dose
what determines the radiation weighting factor (Wr) in the effective dose formula?
(E) = (Wr) x (Wt)
- Wr is equal to 1 for the types of radiation we use in medicine
- Wr for other types of radiation is determined by the LET of the radiation
what determines the tissue weighting factor (Wt) in the effective dose formula?
(E) = (Wr) x (Wt)
- Wt accounts for the relative radiosensitivity of various tissues and organs
- higher value Wt denotes more radiosensitive tissue or organs
fill in the chart:
weighting factors for various tissues:
active bone marrow esophagus
stomach bone surface
- gonads: 0.20
- active bone marrow: 0.12
- colon: 0.12
- lung: 0.12
- stomach: 0.12
- bladder: 0.05
- breast: 0.05
- esophagus: 0.05
- liver: 0.05
- thyroid: 0.05
- bone surface: 0.01
- skin: 0.01
- **dependent on RBE of tissues**
a grave misconception concerning the most critical time for irradiation to an embryo/fetus:
that the most critical time for irradiation is during the first two weeks (when it is most unlikely that the expectant mother knows of her condition) when actually this is the time during pregnancy that the effect of the radiation exposure is seldom even realized
the most likely biologic response to irradiation during the first two weeks of pregnancy is:
destruction of the embryo and therefore no pregnancy
the time from approximately the 2nd week to
the 10th week of pregnancy:
During early organogenesis, the most likely congenital abnormalities are associated with:
During later organogenesis, the most likely congenital abnormalities are associated with:
during the second and third trimester of pregnancy, skeletal deformity and neurologic deficiency responses are ___________, but the principle response would be _______________.
- malignancy during childhood
the congenital responses of radiation during pregnancy require a dose of ______________, which is highly unlikely unless the pt is receiving ________________________.
- more than 25 rads
- multiple exams of the abdomen and/or pelvis
for a technologist, the congenital responses of radiation during pregnancy:
are essentially impossible because the required dose of more than 25 rads will not be reached due to the low levels of radiation exposure
At least _______ of exposure is needed to possibly cause a spontaneous abortion during the first two weeks of pregnancy.
spontaneous abortion in the absence of radiation exposure is in the __________ range.
there is an estimated ____ increase in congenital abnormalities with a _______ fetal dose.
congenital abnormalities exist in approximately ______ of all live births without radiation exposure.
the risk of childhood malignancies post utero
irradiation is a _______ increase over naturally occurring malignancies.
often the ______________ is used to help assess the occurrence of malignancies of children irradiated in-utero.
relative risk estimate
when pregnancy is declared by a technologist, the dose limit becomes:
0.5 mSv per month (50 mrem/mo)
when pregnancy is declared by a technologist, the fetus’s dose limit is __________ for the period of pregnancy.
5 mSv (500 mrem)
the majority of pregnant radiologic technologists receive less than 1 mSv/yr (100 mrem/yr), and this dose is at the collar. the badge at waist usually receives only ____ of this value.
- less than 1 mSv/yr (100 mrem/yr)
describe a pregnant technologist's badge location and protection:
- fetal badge should always be worn at waist under lead apron
- lead aprons are usually 0.5 mm lead equivalent and capable of attenuated 90% of the beam at 75 kVp, which is sufficient
involving the pregnant technologist, know the human responses to low-level xray exposure for the following:
death from all causes
- life-span shortening: 10 days/rad
- cataracts: none below 200 rads
- leukemia: 10 cases/10 mill per rad per year
- genetic effects: doubling dose = 50 rads
- death from all causes: 2 deaths per 100,000 per rad
know the effects of irradiation in utero for the pregnant technologist for the following time frames:
- 0-14 days: spontaneous abortion is 25% naturally occurring with a 0.1% increase per 10 rad
- 2-10 days: congenital abnormalities are 5% naturally occurring with a 1% increase per 10 rad
- 2nd-3rd trimester: cell depletion has no effect at less than 50 rad/latent malignancy at 4:10,000 natural coincidence and 6:10,000/rad
- 0-9 months: genetic effects are 10% naturally occurring with a 0.0000005 mutation/rad
explain protective measures for the pregnant technologist:
- two occupational radiation monitors
- dose limit of 5mSv/9 months
- dose limit of .5mSv/month
let it be noted that harmful effects after occupational exposure is:
currently, for how long has there has been no biologic response in radiologic personnel?
for the past 50 years or so
what three steps are to be considered by the director of radiology to insure radiation protection?
- new employee training
- periodic inservice training
- counseling during pregnancy
name three things that should be involved with new employee training:
- employee should be provided a copy of the facility's radiation protection manual and other appropriate materials which may include a summary of doses, responses, and proper radiation control work habits
- the employee should read and sign a form indicating that he/she has been instructed in the area of radiation protection
- employee should be encouraged to notify supervisor when she becomes pregnant or suspects she is pregnant
how often should in-service training occur?
- usually conducted in monthly intervals
- at least twice a year, the training should be devoted to radiation protection (with a portion directed to pregnancy)
- emphasize RDL as 50 mSv/yr (5 rem/yr) environmental background radiation is approximately 1mSv/yr (0.1 rem/yr)
- review radiation monitoring reports
describe the routine procedure objectives of technologist counseling during pregnancy:
- the director of radiology takes the next action when an employee declares pregnancy
- director counsels by reviewing the previous doses and any modifications to her schedule that may be appropriate
- under no circumstance should termination or involuntary leave of absence occur as consequence of pregnancy
- review dose level during pregnancy: 5 mSv (.5 rem)/month, refers to fetus dose
- explain alterations in work schedules are normally not required
- finally, the radiologic technologist should be required to sign a form following counsel
After __________, the risk to fetus dramatically decreases
requires the clinician, radiologist, or radiologic technologist determine the time of the patient’s previous menstrual cycle:
xray examinations in which the fetus is not in or near the primary beam:
may be allowed, but should be accompanied by pelvic shielding
ideally, the _______________ should be responsible for determining menstrual cycle and for withholding the examination request if there is any question about its necessity.
what is another name for the female patient questionnaire and some questions it may ask?
- aka: X-ray Consent for Women of Childbearing Age
- are you or could you be pregnant?
- what was the date of your last menstrual cycle?
- have you had a hysterectomy?
- do you use an IUD?
often signs are posted throughout the radiography department that may read as follows:
- are you pregnant or could you be?
- warning - special precautions are necessary if you are pregnant
- caution - if there is any possibility that you are pregnant, it is very important that you inform the radiologic technologist before you have an x-ray examination
it is estimated that ____________ of all women referred for a x-ray examination are potentially pregnant
fewer than 1%
if a pregnant patient escapes detection and is irradiated the following steps need to be made:
- contact the medical physicist
- have them to estimate fetal dose
- referring physician and radiologist should determine stage of gestation the exposure occurred
- may apply 10 to 25 rads rule: below 10 rads a therapeutic abortion is not recommended; above 25 rads the risk of latent injury may justify a therapeutic abortion per textbook; between 10-25 rad many factors may be considered before deciding recommendation
fill in the chart:
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