radreview equipment section 1.1 Principles of radiation physics (22 1/9/2017 Flashcards


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1.1 Principles of radiation physics (221/9/2017
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1

Which of the following is most likely to occur as a result of using a 30-in. SID with a 14 × 17 in. IR to radiograph a fairly homogeneous structure?

A Production of quantum mottle

B Density variation between opposite ends of the IR

C Production of scatter radiation fog

D Excessively short-scale contrast

The Correct Answer is: B
Since x-ray photons are produced at the tungsten target, they more readily diverge toward the cathode end of the x-ray tube. As they try to diverge toward the anode, they interact with and are absorbed by the anode “heel.” Consequently, there is a greater intensity of x-ray photons at the cathode end of the x-ray beam. This phenomenon is known as the anode heel effect. Because shorter SIDs and larger IR sizes require greater divergence of the x-ray beam to provide coverage, the anode heel effect will be accentuated. (Bushong, 8th ed., pp. 138–140)

2

Which of the following affect(s) both the quantity and quality of the primary beam?

Half-value layer (HVL)
Kilovoltage
Milliamperage

A
1 only
B
2 only
C
1 and 2 only
D
1, 2, and 3

he Correct Answer is: C
Kilovoltage (kVp) and half-value layer (HVL) change both the quantity and the quality of the primary beam. The principal qualitative factor of the primary beam is peak kilovoltage, but an increase in kilovoltage will also effect an increase in the number of photons produced at the target. The milliampere-seconds value is adjusted to regulate the number of x-ray photons produced at the target. X-ray beam quality is unaffected by changes in milliampere-seconds. (Bushong, 8th ed., p. 165

3

If exposure factors of 85 kVp, 400 mA, and 12 ms yield an output exposure of 150 mR, what is the milliroentgens per milliampere-seconds (mR/mAs)?

A 0.32

B 3.1

C 17.6

D 31

The Correct Answer is: D
Determining milliroentgens per milliampere-seconds output is often done to determine linearity among x-ray machines.

However, all the equipment being compared must be of the same type (e.g., all single-phase or all three-phase, six-pulse). If there is linearity among these machines, then identical technique charts can be used.

In the example given, 400 mA and 12 ms were used, equaling 4.8 mAs. If the output for 4.8 mAs was 150 mR, then 1 mAs is equal to 31.25 mR (150 mR ÷ 4.8 mAs = 31.25 mR/mAs). (Bushong, 8th ed., pp. 248–249)

4

To eject a K-shell electron from a tungsten atom, the incoming electron must have an energy of at least

A 60 keV.

B 70 keV.

C 80 keV.

D 90 keV.

The Correct Answer is: B
X-ray photons are produced in two ways as high-speed electrons interact with target tungsten atoms.

First, if the high-speed electron is attracted by the nucleus of a tungsten atom and changes its course, as the electron is “braked,” energy is given up in the form of an x-ray photon

This is called Bremsstrahlung (“braking”) radiation, and it is responsible for most of the x-ray photons produced at the conventional tungsten target.

Second, a high-speed electron having an energy of at least 70 keV may eject a tungsten K-shell electron, leaving a vacancy in the shell. An electron from the next energy level, the L shell, drops down to fill the vacancy, emitting the difference in energy as a K-characteristic ray. Characteristic radiation makes up only about 15% of the primary beam. (Bushong, 8th ed., p. 176)

5

If a high-voltage transformer has 100 primary turns and 35,000 secondary turns, and is supplied by 220 V and 75 A, what are the secondary voltage and current? A 200 A and 77 V B 200 mA and 77 kVp C 20 A and 77 V D 20 mA and 77 kVp

he high-voltage, or step-up, transformer functions to increase voltage to the necessary kilovoltage. It decreases the amperage to milliamperage. The amount of increase or decrease is dependent on the transformer ratio-the ratio of the number of turns in the primary coil to the number of turns in the secondary coil. The transformer law is as follows:

To determine secondary V,

To determine secondary I

Substituting known factors,

35,000/100 = x/220

100x = 7,700,000

x = 77,000 V (77kVp)

35,000/100 = 75/x

35,000x = 7,500

x = 0.214 Amps (214 mA)

(Selman, pp 84–85)

Vs/Vp=Ns/VP or ns/np=IP/Is

6

An increase in the kilovoltage applied to the x-ray tube increases the

  1. percentage of high-energy photons produced.
  2. beam intensity.
  3. patient absorption.

A 1 only B 1 and 2 only C 2 and 3 only D 1, 2, and 3

he Correct Answer is: B

As the kilovoltage is increased, a greater number of electrons are driven across to the anode with greater force.

Therefore, as energy conversion takes place at the anode, more high-energy photons are produced.

However, because they are higher-energy photons, there will be less patient absorption. (Fauber, 2nd ed., p. 58)

7

Which of the following will serve to increase the effective energy of the x-ray beam?

  1. Increase in added filtration
  2. Increase in kilovoltage
  3. Increase in milliamperage

A 1 only B 2 only C 1 and 2 only D 1, 2, and 3

he Correct Answer is: C

As filtration is added to the x-ray beam, the lower-energy photons are removed, and the overall energy or wavelength of the beam is greater

. As kilovoltage is increased, more high-energy photons are produced, and again, the overall, or average, energy of the beam is greater. An increase in milliamperage serves to increase the number of photons produced at the target but is unrelated to their energy. (Selman, 9th ed., p. 171)

8

How many half-value layers will it take to reduce an x-ray beam whose intensity is 78 R/min to an intensity of less than 10 R/min?

A 2

B 3

C 4

D 8

he Correct Answer is: B
HVL may be used to express the quality of an x-ray beam. The HVL of a particular beam is that thickness of an absorber that will decrease the intensity of the beam to one-half its original value. If the original intensity of the beam was 78 R/min, the first HVL will reduce the intensity to 39 R/min, the second HVL will reduce it to 19.5 R/min, and the third HVL will reduce it to 9.75 R/min. (Bushong, 8th ed., p. 52)

9

Characteristics of the metallic element tungsten include

  1. ready dissipation of heat
  2. high melting point
  3. high atomic number

A 1 only

B 1 and 2 only

C 2 and 3 only

D 1, 2, and 3

he Correct Answer is: D
The x-ray anode may be a molybdenum disk coated with a tungsten–rhenium alloy.

Because tungsten has a high atomic number (74), it produces high-energy x-rays more efficiently.

Since a great deal of heat is produced at the target, tungsten's high melting point (3,410°C) helps to avoid damage to the target surface.

Heat produced at the target should be dissipated readily, and tungsten's conductivity is similar to that of copper. Therefore, as heat is applied to the focus, it can be conducted throughout the disk to equalize the temperature and thus avoid pitting, or localized melting, of the focal track. (Selman, 9th ed., p. 138)

10

To be used more efficiently by the x-ray tube, alternating current is changed to unidirectional current by the

A filament transformer. B autotransformer. C high-voltage transformer. D rectifiers.

The Correct Answer is: D
Rectifiers (solid-state or the older valve tubes) permit the flow of current in only one direction.

They serve to change AC, which is needed in the low-voltage side of the x-ray circuit, to unidirectional current. Unidirectional current is necessary for the efficient operation of the x-ray tube.

The rectification system is located between the secondary coil of the high-voltage transformer and the x-ray tube. The filament transformer functions to adjust the voltage and current going to heat the x-ray tube filament.

The autotransformer varies the amount of voltage being sent to the primary coil of the high-voltage transformer so that the appropriate kVp can be obtained. The high-voltage transformer "steps up" the voltage to the required kilovoltage and steps down the amperage to milliamperage. (Carlton & Adler, p 78)

11

The source of electrons within the x-ray tube is

A electrolysis.

B thermionic emission.

C rectification.

D induction.

The Correct Answer is: B
The thoriated tungsten filament of the cathode is heated by its own filament circuit

. The x-ray tube filament is made of thoriated tungsten and is part of the cathode assembly. Its circuit provides current and voltage to heat it to incandescence, at which time it undergoes thermionic emission—the liberation of valence electrons from the filament atoms. Electrolysis describes the chemical ionization effects of an electric current. Rectification is the process of changing AC to unidirectional current. (Bushong, p 118)

12

With three-phase equipment, the voltage across the x-ray tube

  1. drops to zero every 180 degrees
  2. is 87% to 96% of the maximum value
  3. is at nearly constant potential

A 1 only

B 2 only

C 1 and 2 only

D 2 and 3 only

he Correct Answer is: D
With single-phase, full-wave-rectified equipment, the voltage is constantly changing from 0% to 100% of its maximum value. It drops to 0 every 180 degrees (of the AC waveform); that is, there is 100% voltage ripple. With three-phase equipment, the voltage ripple is significantly smaller. Three-phase, six-pulse equipment has a 13% voltage ripple, and three-phase, 12-pulse equipment has a 3.5% ripple. Therefore, the voltage never falls below 87% to 96.5% of its maximum value with three-phase equipment, and it closely approaches constant potential [direct current (DC)]. (Carlton and Adler, 4th ed., pp. 91–93)

13

The line-focus principle refers to the fact tha

t A the actual focal spot is larger than the effective focal spot

B the effective focal spot is larger than the actual focal spot

C x-rays travel in straight lines

D x-rays cannot be focused

The Correct Answer is: A
A distinction is made between the actual focal spot and the effective, or projected, focal spot.

The actual focal spot is the finite area on the tungsten target that is actually bombarded by electrons from the filament.

The effective focal spot is the foreshortened size of the focus as it is projected down toward the image receptor. This is called line focusing or the line-focus principle.

The quoted focal-spot size is the effective focal-spot size. (Carlton & Adler, 4th ed., pp. 112–114)

14

The electron cloud within the x-ray tube is the product of a process called

A electrolysis

B thermionic emission

C rectification

D induction

The Correct Answer is: B
The thoriated tungsten filament of the cathode is heated by its own filament circuit. The x-ray tube filament is made of thoriated tungsten and is part of the cathode assembly. Its circuit provides current and voltage to heat it to incandescence, at which time it undergoes thermionic emission—the liberation of valence electrons from the filament atoms. Electrolysis describes the chemical ionization effects of an electric current. Rectification is the process of changing alternating current to unidirectional current. (Bushong, 8th ed., p. 118)

15

Which of the following occurs during Bremsstrahlung (Brems) radiation production?

A An electron makes a transition from an outer to an inner electron shell.

B An electron approaching a positive nuclear charge changes direction and loses energy.

C A high-energy photon ejects an outer-shell electron.

D A low-energy photon ejects an inner-shell electron.

he Correct Answer is: B
Two types of interactions between high-speed incident electrons and the tungsten-target atoms account for the production of x-rays within the x-ray tube. (1) In the production of Brems (“braking”) radiation, a high-speed electron is attracted to the positive nuclear charge of a tungsten atom. In so doing, it is “braked” and gives up energy in the form of an x-ray photon. Most of the primary beam is made up of Brem

s radiation. (2) If the incident electron were to eject a K-shell electron, an L-shell electron would move in to fill the vacancy. It releases a photon (K-characteristic ray) whose energy equals the difference between the K- and L-shell energy levels. This is characteristic radiation; it is responsible for only a small portion of the primary beam. (Bushong, 8th ed., pp. 149–152)

16

Which of the following terms describes the amount of electric charge flowing per second?

A Voltage

B Current

C Resistance

D Capacitance

The Correct Answer is: B

Current is defined as the amount of electric charge flowing per second.

Voltage is the potential difference existing between two points.

Resistance is the property of a circuit that opposes current flow. Capacitance describes a quantity of stored electricity. (Selman, 9th ed., pp. 46–47)

17

What is the relationship between kV and HVL?

A As kV increases, the HVL increases.

B As kV increases, HVL decreases

C If the kV is doubled, the HVL doubles.

D If the kV is doubled, the HVL is squared.

he Correct Answer is: A

The HVL of a particular beam is defined as that thickness of a material that will reduce the exposure rate to one-half of its original value.

The more energetic the beam (the higher the kilovoltage), the greater is the HVL thickness required to cut its intensity in half. Therefore, it may be stated that kilovoltage and HVL have a direct relationship:

As kilovoltage increases, HVL increases. (Selman, 9th ed., pp. 122–123)

18

Characteristics of x-ray photons include

  1. a penetrating effect on all matter
  2. an ionizing effect on air
  3. travelling at the speed of sound

A 1 only B 1 and 2 only C 2 and 3 only D 1, 2, and 3

The Correct Answer is: B
X-rays are energetic enough to rearrange atoms in materials through which they pass, and they can, therefore, be hazardous to living tissue. X-rays are called ionizing radiation because they have the energetic potential to break apart electrically neutral atoms, resulting in the production of negative and/or positive ions. X-rays are infinitesimal bundles of energy called photons that deposit some of their energy into matter as they travel through it. This deposition of energy and subsequent ionization has the potential to cause chemical and biologic damage. Several of the outstanding properties of x-ray photons are: X-rays are not perceptible by the senses. X-rays travel in straight lines. X-rays travel at the speed of light. X-rays are electrically neutral. X-rays have a penetrating effect on all matter. X-rays have a physiological effect on living tissue. X-rays have an ionizing effect on air. X-rays have a photographic effect on film emulsion. X-rays produce fluorescence in certain phosphors. X-rays cannot be focused. X-rays have a spectrum of energies. X-rays are unaffected by a magnetic field.

19

The effective energy of an x-ray beam is increased by increasing the

  1. added filtration
  2. kilovoltage
  3. milliamperage

A 1 only

B 2 only

C 1 and 2 only

D 1, 2, and 3

The Correct Answer is: C
As filtration is added to the x-ray beam, the lower-energy photons are removed, and the overall energy or wavelength of the beam is greater.

As kilovoltage is increased, more high-energy photons are produced, and again, the overall or average energy of the beam is greater. An increase in milliamperage serves to increase the number of photons produced at the target but is unrelated to their energy. (Bushong, 8th ed., pp. 165, 166) Mark for Review Discussion Add to My Notes Performance Overall Time Spent 00:30:03 Performance in Last 10 Qs Hard Easy About this Question Question Difficulty Difficulty 71.77% (33691/46946) Students got it correct Study this topic • Pr