Radiation Protection Flashcards
Which of the following projections is most likely to deliver the largest dose to the ovaries?
A. AP lumbar spine, 7 x 17 in. cassette, 80 kVp
B. AP lumbar spine, 14 x 17 in. cassette, 80 kVp
C. AP abdomen, 80 kVp
D. AP abdomen, 70 kVp
The answer is B.
EXPLANATION: Exposure dose to patients can be expressed as entrance skin exposure (ESE), sometimes referred to as skin entrance exposure (SEE). Exposure can also be expressed in terms of organ dose. Organ doses to the gonads, bone marrow, breast, thyroid, lens, and lung can be determined. Patient position and beam restriction often make a significant difference in patient dose. Examinations performed PA rather than AP often decrease exposure to sensitive organs. This is so because the lower energy x-ray photons will be absorbed by the anatomic structures closer to the x-ray source, and the higher energy photons will penetrate and exit the part (penetrating the sensitive part rather than being absorbed by it). PA abdomen radiographs deliver less quantity dose to the reproductive organs than AP abdomen radiographs do. An AP lumbar spine radiograph, 7 x 17 in. cassette, 80 kVp delivers about 74 mrad to the ovaries, whereas the same projection using a 14 x 17 in. cassette delivers 92 mrad. An AP abdomen radiograph with 70 kVp delivers 80 mrad, whereas at 80 kVp, the ovarian dose is 68 mrad.
All the following statements regarding mobile radiographic equipment are true except
A. the exposure cord must permit the operator to stand at least 6 ft from the patient, x-ray tube, and useful beam
B. exposure switches must be the two-stage type
C. a lead apron should be carried with the unit and worn by the radiographer during exposure
D. the radiographer must alert individuals in the area before making the exposure
The answer is B.
EXPLANATION: NCRP Report No. 102 states that the exposure switch on mobile radiographic units shall be so arranged that the operator can stand at least 2 m (6 ft) from the patient, the x-ray tube, and the useful beam. An appropriately long exposure cord accomplishes this requirement. The fluoroscopic and/or radiographic exposure switch or switches must be of the “dead man” type; that is, the exposure will terminate should the switch be released. A lead apron should be carried with every mobile x-ray unit for the operator to wear during the exposure. Lastly, the radiographer must be certain to alert individuals in the area, enabling unnecessary occupants to move away, before making the exposure.
What is the annual TEDE limit for radiation workers?
50 mSv
EXPLANATION: Whenever a radiation worker could receive 10% or more of the annual TEDE limit, that person must be provided with a radiation monitor. The annual TEDE limit for radiation workers is 50 mSv (5 rem, 5,000 mrem), but it is the responsibility of the radiographer to practice the ALARA principle, that is, to keep radiation dose as low as reasonably achievable.
According to the NCRP, the annual occupational dose-equivalent limit (50 rem) to the thyroid, skin, and extremities is
500 mSv
EXPLANATION: According to the NCRP, the annual occupational whole-body dose-equivalent limit is 50 mSv (5 rem or 5,000 mrem). The annual occupational whole-body dose-equivalent limit for students under the age of 18 years is 1 mSv (100 mrem or 0.1 rem). The annual occupational dose-equivalent limit for the lens of the eye, a particularly radiosensitive organ, is 150 mSv (15 rem). The annual occupational dose-equivalent limit for the thyroid, skin, and extremities is 500 mSv (50 rem). The total gestational dose-equivalent limit for embryo/fetus of a pregnant radiographer is 5 mSv (500 mrem), not to exceed 0.5 mSv in 1 month.
The late effects of radiation are considered to
- have no threshold dose.
- be directly related to dose.
- occur within hours of exposure.
1 and 2 only
EXPLANATION: Exposure to high doses of radiation results in early effects. Examples of early effects are blood changes and erythema. If the exposed individual survives, then late, or long-term, effects must be considered. Individuals who receive small amounts of low-level radiation (such as those who are occupationally exposed) are concerned with the late effects of radiation exposure—effects that can occur many years after the initial exposure. Late effects of radiation exposure, such as carcinogenesis, are considered to be related to the linear nonthreshold dose–response curve. That is, there is no safe dose; theoretically, even one x-ray photon can induce a later response.
Which of the following tissues or organs is the most radiosensitive?
A. Rectum
B. Esophagus
C. Small bowel
D. Central nervous system (CNS)
Small bowel
EXPLANATION: The most radiosensitive portion of the GI tract is the small bowel. Projecting from the lining of the small bowel are villi, from the crypts of Lieberkühich are responsible for the absorption of nutrients into the bloodstream. Because the cells of the villi are continually being cast off, new cells must continually arise from the crypts of Lieberküeing highly mitotic, undifferentiated stem cells, they are very radiosensitive. Thus, the small bowel is the most radiosensitive portion of the GI tract. In the adult, the CNS is the most radioresistant system.
Which of the following is (are) included in whole-body dose equivalents?
- Gonads
- Lens
- Extremities
1 and 2 only
EXPLANATION: Whole-body dose is calculated to include all the especially radiosensitive organs. The gonads, the lens of the eye, and the blood-forming organs are particularly radiosensitive. The annual dose limit to the less sensitive skin, hands, and feet (extremities) is 50 rem/year.
The rad is the unit of
radiation dose.
EXPLANATION: There are several radiation units that are used to express quantity and effects of radiation. Rad (radiation absorbed dose) expresses energy deposited (as a result of ionizations) in any kind of absorber. The unit of exposure, the roentgen, is used to express the quantity of ionization in air. The unit of dose equivalent is the rem (radiation equivalent man), which expresses dose to biologic material.
Which interaction between x-ray photons and matter involves partial transfer of the incident photon energy to the involved atom?
Compton scattering
EXPLANATION: The photoelectric effect and Compton scattering are the two predominant interactions between x-ray photons and matter in diagnostic x-ray. In Compton scatter, the high-energy incident photon uses only part of its energy to eject an outer-shell electron. It retains most of its original energy in the form of a scattered x-ray. The outer-shell electron leaves the atom and is called a recoil electron. Compton scatter is the interaction between x-ray photons and matter that occurs most frequently in diagnostic x-ray and is the major contributor of scattered radiation fog. In the photoelectric effect, the low-energy incident photon uses all its energy to eject an atom’s inner-shell electron. When photon ceases to exist, it means it has used all its energy to ionize the atom. The part has absorbed the x-ray photon. This interaction contributes to patient dose and produces short-scale contrast.
Which of the following cells are the most radiosensitive?
A. Myelocytes
B. Erythroblasts
C. Megakaryocytes
D. Myocytes
Erythroblasts
EXPLANATION: Bergonié and Tribondeau theorized in 1906 that all precursor cells are particularly radiosensitive (e.g., stem cells found in bone marrow). There are several types of stem cells in bone marrow, and the different types differ in degree of radiosensitivity. Of these, red blood cell precursors, or erythroblasts, are the most radiosensitive. White blood cell precursors, or myelocytes, follow. Platelet precursor cells, or megakaryocytes, are the least radiosensitive. Myocytes are mature muscle cells that are fairly radioresistant.
If a patient received 1,400 mrad during a 7-minute fluoroscopic examination, what was the dose rate?
0.2 rad/min
EXPLANATION: A measure 1,400 mrad is equal to 1.4 rad. If 1.4 rad were delivered in 7 minutes, then the dose rate would be 0.2 rad/min:
1.4rad/7min = xrad/1min
Which of the following statements regarding film badges is (are) correct?
- Film badges should be read quarterly.
- Film badges must not leave the workplace.
- Film badges measure quantity and quality of radiation exposure.
2 and 3 only
EXPLANATION: Film badges are supplied by a dosimetry service. They contain pieces of dental film held within a holder containing filters. When used properly, film badges measure the quantity and quality of radiation exposure. Film within the badges is usually changed monthly. The sensitive film emulsion is susceptible to deterioration and false readings if the badges are worn for longer periods, or if they are damaged by water, heat, light, and so on. To avoid the possibility of damage or exposure, film badges should not leave the workplace.
The annual dose limit for occupationally exposed individuals is valid for
beta, x-, and gamma radiations.
EXPLANATION: The occupational dose limit is valid for beta, x-, and gamma radiations. Because alpha radiation is so rapidly ionizing, traditional personal monitors will not record alpha radiation. Because alpha particles are capable of penetrating only a few centimeters of air, they are practically harmless as an external source
Radiographers use monitoring devices to record their monthly exposure to radiation. The types of devices suited for this purpose include the
- pocket dosimeter.
- TLD.
- OSL dosimeter.
2 and 3 only
EXPLANATION: The OSL is rapidly becoming the most commonly used personnel monitor today. Film badges and TLDs have been used successfully for years. A pocket dosimeter is used primarily when working with large amounts of radiation and when a daily reading is desired.
For radiographic examinations of the skull, it is generally preferred that the skull be examined in the
PA projection
EXPLANATION: Because the primary x-ray beam has a poly-energetic (heterogeneous) nature, the entrance or skin dose is significantly greater than the exit dose. This principle may be employed in radiation protection by placing particularly radiosensitive organs away from the primary beam. To place the gonads further from the primary beam and reduce gonadal dose, abdominal radiography should be performed in the posteroanterior (PA) position whenever possible. Dose to the lens is decreased significantly when skull radiographs are performed in the PA position.
Which of the following interactions between x-ray photons and matter involves a high-energy photon and the ejection of an outer shell electron?
Compton scatter
EXPLANATION: Compton scattering occurs when a relatively high-energy incident photon uses part of its energy to eject an outer shell electron, and in doing so changes its direction (is scattered). The energy retained by the scattered photon depends on the angle formed by the ejected electron and the scattered photon: The greater the angle of deflection, the less the retained energy. Compton scatter is very energetic scatter. It emerges from the patient and is responsible for scatter radiation reaching the image in the form of fog. In the photoelectric effect, a relatively low energy photon uses all its energy to eject an inner shell electron, leaving a vacancy. An electron from the shell above drops down to fill the vacancy, and in doing so gives up a characteristic ray. This type of interaction contributes most to patient dose, as all the photon energy is transferred to tissue. In coherent scatter, no energy is absorbed by the part; it all emerges as scattered photons. Pair production occurs only at very high energy levels, at least 1.02 MeV.
What percentage of x-ray attenuation does a 0.5-mm lead equivalent apron at 100 kVp provide?
75%
EXPLANATION: Lead aprons are worn by occupationally exposed individuals during fluoroscopic and mobile x-ray procedures. Lead aprons are available with various lead equivalents; 0.5 and 1.0 mm are the most common. The 1.0-mm lead equivalent apron will provide close to 100% protection at most kVp levels, but it is rarely used because it weighs anywhere from 12 to 24 lb! A 0.25-mm lead equivalent apron will attenuate about 97% of a 50-kVp x-ray beam, 66% of a 75-kVp beam, and 51% of a 100-kVp beam. A 0.5-mm lead equivalent apron will attenuate about 99.9% of a 50-kVp beam, 88% of a 75-kVp beam, and 75% of a 100-kVp beam.
