EXAM #2 (Ch. 36 Designing for Radiation Protection)

Question 1

Define: TLD

Answer 1

Thermoluminescent dosimeter.

Question 2

Define: Use factor

Answer 2

The percentage of time during which the x-ray tube is on and directed toward a particular protective barrier is called the use factor.

Question 3

Define: Diagnostic protective x-ray tube housing

Answer 3

Protective shielding in the x-ray tube housing designed to reduce leakage radiation to less than 1 mGya/hr, (100 mR/hr) at 1 m.

Question 4

Define: Glow curve

Answer 4

The graphically displayed response of a TLD as a function of temperature is the TLD glow curve.

Question 5

Define: Primary protective barrier

Answer 5

Barrier designed to shield against primary radiation. The image-intensifier assembly serves as a primary protective barrier. Secondary protective barriers are designed to shield areas from secondary radiation and are always less thick than primary barriers.

Question 6

Define: X-ray linearity

Answer 6

Radiation intensity is proportional to the mA station selected.

Question 7

Define: Secondary radiation

Answer 7

Leakage radiation; scatter radiation.

Question 8

Define: Occupancy factor

Answer 8

Level of occupancy of an area adjacent to x-ray rooms. The concept of the relative time that a protected area is occupied.

Question 9

Define: Geiger-Muller region

Answer 9

The voltage plateau over which the Geiger counter detector operates.

Question 10

Define: Resolving time

Answer 10

Applies to Geiger counter and scintillation detector. The minimum time between two interactions allowing both interactions to be detected.

Question 11

What do audible and visible signals indicate on the radiographic control console?

Answer 11

That the x-ray beam is energized and at what technique.

Question 12

List as many devices used for radiation protection on radiographic equipment as you can.

Answer 12

Protective drapes, Bucky slot cover, 5 minute reset timer, exposure switch that fastens to the console, dead-man switch, collimator, filter, tube housing, 6 ft cord on mobile unit.

Question 13

What is the result if the x-ray beam and the film are not properly aligned?

Answer 13

Misalignment causes unintended tissue to be irradiated, and intended tissue to be missed.

Question 14

What filtration is used for mammography equipment operated below 30 kVp?

Answer 14

30 m Mo or 50 m Rh.

Question 15

How are reproducibility and linearity different when the intensity of the x-ray beam is measured?

Answer 15

Reproducibility requires constant radiation intensity for multiple exposures at a fixed technique. Linearity implies proportional radiation exposure at adjacent mA stations.

Question 16

What characteristics of fluoroscopic equipment are designed for radiation protection?

Answer 16

The protective curtain on the image-intensifier tower, the Bucky slot cover, the 5 minute reset timer.

Question 17

How can filtration be measured if the amount of inherent and added filtration is unknown?

Answer 17

Experimentally determine the half-value layer and check to see that it exceeds the minimum HVL’s published by the NCRP.

Question 18

Name the three types of radiation exposure that are of concern when protective barriers are designed.

Answer 18

Primary radiation, leakage radiation, and scatter radiation.

Question 19

List five factors that are taken into consideration when a barrier for a radiographic room is designed.

Answer 19

Use factor, occupancy factor, distance, dose limit, kVp.

Question 20

What is the difference between a controlled area and an uncontrolled area?

Answer 20

Controlled areas are those primarily occupied by radiology personnel and patients. Normally, workers who enter a controlled area would be provided with an occupational radiation monitor. An uncontrolled area can be occupied by anyone; therefore, the maximum exposure rate allowed is based on the recommended dose limit for the public.

Question 21

What are the units of workload for an x-ray examination room?

Answer 21

The units of work load for an x-ray examination room are mA min/wk.

Question 22

Explain the use factor (U) as it relates to a protective barrier in an x-ray examination room.

Answer 22

The use factor of a room helps determine whether a given wall or the ceiling is a primary or secondary protective barrier and therefore what protective design is necessary.

Question 23

Why is the use factor for secondary barriers always 1?

Answer 23

Secondary radiation, leakage, and scatter are continuously present when an x-ray tube is energized, therefore the secondary barriers are always assigned a use factor of 1.

Question 24

Name the three gas-filled dosimeters.

Answer 24

Ionization chamber, proportional counter, Geiger-Mueller counter.

Question 25

Discuss the properties of TLD that make it suitable for personnel monitoring.

Answer 25

Sensitive, accurate, nonfading with time, unaffected by temperature and humidity, absorption characteristics equivalent to human tissue, reusable.

Question 26

Which modality of diagnostic imaging uses scintillation detection as a radiation detection process?

Answer 26

Nuclear medicine and CT.

Question 27

What are the two most widely used scintillation phosphors?

Answer 27

Sodium iodide (NaI), and cadmium tungstate (CdWO4).

Question 28

A photomultiplier has nine dynodes, each of which has a gain of 2.2. What is the overall tube gain?

Gain is synonymous with _____

Answer 28

Gain = (2.2)^9 = 1207.5.

Gain is synonymous with electrons

Question 29

Given the following conditions of operation, compute the weekly workload:

20 patients per day

3.2 films per patient

80 mAs per view on average

Answer 29

20pts x 3.2pics x 80mas x 5days/wk x (1/60)min/hr

203.2805(1/60) = 426.7

426.7 mA min/wk.

Question 30

The light field and alignment needs to be within +/- ?x% of the X-ray field

Answer 30

The light field and alignment needs to be within +/- 2% of the X-ray field

Question 31

What is the equation for photomultiplier tube gain?

Answer 31

PM = g^n

g = gain/electrons

n = dynodes

Question 32

The variation in X-ray intensity should not exceed ?x%

Answer 32

The variation in X-ray intensity should not exceed 5%

Question 33

The maximum acceptable variation in linearity is ?x% from one mA station to an adjacent mA station

Answer 33

The maximum acceptable variation in linearity is 10% from one mA station to an adjacent mA station

Question 34

Primary radiation is the _____ beam

Answer 34

Primary radiation is the useful beam

Question 35

What should the intensity of scatter radiation be 1m from the patient?

Answer 35

0.1% of the useful beam

Question 36

What is the annual recommended occupational dose limit? What unit?

Answer 36

50 mSv/yr

Question 37

The use factor for secondary barriers is always ?x

Answer 37

The use factor for secondary barriers is always 1

Question 38

The _____ of gas is the basis for gas-filled radiation detectors

Answer 38

The ionization of gas is the basis for gas-filled radiation detectors

Question 39

What is dynode gain?

Answer 39

Dynode gain = the ratio of secondary electrons to incident electrons

Question 40

What does high sensitivity mean in a device that detects radiation intensities?

Answer 40

The instrument can detect very low radiation intensities

Question 41

The size of the electron pulse is _____ to the energy absorbed by the crystal from the incident photon

Answer 41

The size of the electron pulse is proportional to the energy absorbed by the crystal from the incident photon

Question 42

How does a TLD work?

Answer 42

TLD is the emission of light by a thermally stimulated crystal following radiation

Question 43

What is a nearly tissue-equivalent radiation dosimeter material?

Answer 43

Lithium flouride