Ch. 5 The xray imaging system Flashcards Preview

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Flashcards in Ch. 5 The xray imaging system Deck (174):
1

What are the 3 main components of an xray system?

1. xray tube
2. operating console
3. high voltage generator

2

A high voltage generator contains...

the high voltage step up transformer and the rectification circuit

3

What are the voltages of an xray system?

25 - 150 kVp

4

What are the tube currents for an xray system?

100 - 1200 mA

5

Where is the fluoro tube usually located?

under the exam table.

6

To what degrees does a fluor table tilt? foot side and head side?

30 to the head and 90 degrees to the foot

7

Where would the generator be located within a xray room?

in an equipment cabinet against the wall

8

Radiation quantity is usually expressed in..

milligray (mGy) or mAs

9

How is radiation quality expressed?

kVp or HVL (half value layer)

10

All electric circuits that connect the meters and controls on the operating console are...

at low voltage to minimize the possibility of shock

11

Operating consoles are based on...

computer technology

12

Most xray systems are designed to operate on how much power?

220V..some are 110V or even 440V

13

Why would voltage provided to an xray machine vary by as much as 5%?

Because of variations in power distribution to the hospital and in power consumption by different sections of the hospital.

14

What does a line compensator do?

measure the voltage provided to the xray system and adjusts the voltage to 220V.

15

The power supplied to the xray machine is first delivered to the...

autotransformer

16

The autotransformer has a single winding and is designed to...

supply a precise voltage to the filament circuit and to the high voltage circuit of the xray imaging system.

17

What principle does the autotransformer work on?

the electromagnetic induction

18

How many windings and cores does the autotransformer have?

1 each

19

How much can the autotransformer step up?

it can step up voltage up to about twice the input voltage value

20

The voltage the autotransformer receives is called the ... and the voltage it provides is called the...

Primary voltage
secondary voltage

21

The primary and secondary voltage are directly related to...

the number of turns of the transformer enclosed by the respective connections

22

Some older consoles have adjustments for kVp called...

major kVp and minor kVp

23

What do the major and minor kVp controls do?

it allows the technologist to provide precisely the required kVp.

24

Exactly what does the minor kVp control do?

it fine tunes the selected technique

25

The minor and major kVp controls represent...

2 separate series of connections on the autotransformer.

26

kVp determines the ___ of the xray beam.

quality

27

What is a prereading kVp meater?

A meter that registers but no exposure is made and no circuit has current. It allows the voltage to be monitored before an exposure.

28

What does mA measure?

The xray tube current that crosses from cathode to anode

29

The number of electrons emitted by the filament is determined by...

the temperature of the filament

30

What controls the filament temperature?

the filament current

31

What is the filament current measure in?

amperes

32

As the filament current increases...

the filament becomes hotter, and more electrons are released by thermionic emission

33

What do filaments normally operate at?

3-6 amperes

34

Why is a correction circuit incorporated into a tube?

to counteract the space charge effect

35

Thermionic emission is the...

release of electrons from a heated filament.

36

What is the space charge effect?

As the kVp is raised, the anode becomes more attractive to the electrons that would not have enough energy to leave the filament area. These electrons also join the electron stream, which effectively increases the mA with kVp.

37

Xray tube current is controlled through the...

filament circuit

38

Voltage from the mA selector switch is delivered to the ...

filament transformer

39

What type of transformer is the filament transformer?

a step down.

40

What monitors the xray tube current?

with an mA meter that is in the tube circuit

41

Where is the mA meter connected?

at the center of the secondary winding of the high voltage step up transformer

42

What Hz does the secondary voltage alternate at?

60 Hz making the center of the winding always at 0 volts

43

What is the full title for the filament transformer?

filament heating isolation step down transformer

44

What does the filament transformer do?

It steps down the voltage to about 12 V and provides the current to heat the filament

45

Why are the secondary winding heavily insulated from the primary?

Because the secondary windings are connected to the high voltage supply for the xray tube

46

In a filament transformer, what are the primary windings made of?

thin copper

47

What current do the primary winding of the filament transformer carry?

a current of 0.5 to 1 A and about 150V

48

How are the secondary windings different from the primary windings?

They are thick and about 12V of electric potential and carry a current of 5-8 A (not mA)

49

The number of xrays that reach the IR is directly related to...

both the xray tube current and the time that the xray tube is energized.

50

What is a guard timer?

a timer that will terminate an exposure after a prescribed time

51

After how many minutes does the guard timer go off?

usually 6 seconds

52

What side is the guard timer usually located?

on the primary side of the high voltage transformer, where the voltage is lower

53

How many types of timer circuits are there?

4

54

Who controls the timer circuits?

3 are controlled by the radiologic technologist and the other is automatic.

55

In the US electric current is supplied at what frequency?

60 Hz

56

What is a synchronous motor?

an electric motor designed to drive a shaft at precisely 60 revolutions per second (rps)

57

Why do we care about synchronous motors in imaging systems?

because they are often used as timing mechanisms (60 rps)

58

How do we know if an imaging system is on a synchronous timer?

Because the minimum exposure time possible is 1/60 s (17ms) and intervals increase by multiples 1/30, 1/20.

59

Why can't serial exposures use synchronous timers?

because they need to be reset after each exposure

60

Electronic timers are the most...

sophisticated, most complicated, and most accurate of all the timers

61

How are electronic timers set up?

they have rather complex circuitry based on the time required to charge a capacitor through a variable resistance.

62

How accurate are electronic timers?

They allow a wide range of time intervals to be selected and are accurate to intervals as small as 1ms.

63

What type of procedures and why are electronic timers preferred?

interventional radiology because they can be used for rapid serial exposures

64

Most exposure timers are electronic and are controlled by...

a microprocessor

65

mAs determines the...

number of xrays emitted and therefore the exposure of the image receptor

66

What monitors the product of mA and exposure time and terminates exposure when the desired mAs value is attained?

the mAs timer

67

The mAs timer is designed to provide the ___ ___ tube current for the ____ ____ for any mAs selected.

highest safe; shortest exposure

68

Where is the mAs timer located and why?

it is on the secondary side of the high voltage transformer because the timer must monitor the actual tube current

69

What type of imaging systems are mAs timers used?

falling-load and capacitor discharge imaging systems

70

AEC measures...

the quantity of radiation that reaches the IR.

71

AEC automatically..

terminates the exposure when the IR has gotten the required radiation intensity.

72

What AEC system is used by most manufacturers?

one with a flat, parallel plate ionization chamber positioned between the patient and the IR

73

What is the ionization chamber of an AEC made of?

radiolucent material

74

What happens in the ionization chamber?

ionization in the chamber makes a charge and when the appropriate charge has been reached, the exposure is terminated

75

During a QA check on system made by the tech, what is happening?

the tech selects they type of exam, which sets the appropriate mA and kVp. At the same time, the exposure timer is set to the backup time. When the electric charge from the ionization chamber reaches a preset level, a signal is returned to the operating console where the exposure is terminated.

76

Why should AEC be used carefully during a mammo study?

because of varying tissue thickness and composition, the AEC may not respond properly to a low kVp

77

for a diagnostic xray, the electric backup timer should be set to what?

1.5 times the expected exposure time if the AEC fails

78

What are solid state radiation detectors used for ?

exposure timer checks

79

Why do solid state detectors work well as exposure timer checkers?

because they operate with a very accurate internal clock based on a quartz crystal oscillator. They can measure time as short as 1 ms, and when used with a oscilloscope, can display a radiation waveform.

80

What is the high voltage generator responsible for?

increasing the output voltage from the autotransformer to the kVp necessary for xray production.

81

A high voltage generator contains 3 primary parts:

1. the high voltage transformer
2. the filament transformer
3. rectifiers

82

What type of transformer is a high voltage transformer?

a step up transformer

83

What is meant when the term step up is used?

the secondary voltage is higher than the primary voltage because the number of secondary winding is greater than the number of primary windings.

84

Define turns ratio

the ratio of the number of secondary windings to the number of primary windings

85

voltage increase is proportional to...

the turns ratio, according to the transformer law...the current is also reduced proportionately.

86

What is the turns ratio for a high voltage transformer?

usually 500:1 and 1000:1

87

Why is the voltage waveform on both sides of a high voltage transformer sinusoidal?

because transformers operate only on AC

88

What is the difference between the primary and secondary waveforms?

their amplitude. the primary voltage is measured in V and the secondary voltage is measured in kV

89

The primary current is measured in ____ and the secondary current is measured in ___.

A; mA

90

Radiographers outside of the US and Japan may use what frequency?

50 Hz

91

What is the rectification process?

the process of converting AC to DC

92

What is the device that allows current flow in only one direction?

a rectifier

93

What type of current must a xray tube have?

DC

94

How are xrays produced?

by the acceleration of electrons from the cathode to the anode and cannot be produced by electrons flowing in the reverse direction.

95

What needs to be rectified?

the secondary voltage of the high voltage transformer (going from AC to DC)

96

Why must voltage be rectified?

to ensure that electrons flow from the tube cathode to the anode only

97

rectification is accomplished with...

diodes

98

What is a diode?

an electronic device that contains 2 electrodes.

99

Originally, all diode rectifiers were...

vacuum tubes called valve tubes

100

What have valve tubes been replaced with?

solid state rectifiers made of silicon

101

Why do semiconductors work?

tiny crystals in the semiconductors have some useful electrical properties and allow the semiconductors to serve as the basis for the solid sate microprocessors.

102

How many class types are there of the semiconductors? and what are they?

2. They are n-type and p-type

103

Describe n-type semiconductors

they have loosely bound electrons that are mostly free to moe

104

Describe p-type semiconductors

they have spaces called holes where there are no electrons. These holes are like the space between cars in traffic. holes are as mobile as electons

105

What would happen if a p-type and n-type were put together and formed a p-n junction with the potential on the p side?

If a higher potential is on the p side, then the electrons and holes will both migrate toward the junction and wander across it. This flow of electrons and holes constitutes an electric current.

106

What would happen if a p-type and n-type were put together and formed a p-n junction with the potential on the n side?

If a positive potential is put on the n side, both the electrons and the holes will be swept away from the junction and no electrons will be available at the junction surface to form a current.

107

solid state p-n junctions only conducts electricity...

in one direction and is called a solid state diode

108

Why are solid state diodes rectifiers?

because they only conduct electric current in one direction

109

Where are rectifiers located?

in the high voltage section

110

During a negative half of a cycle, current can flow only...

from anode to cathode, but this does not occur because the anode is not constructed to emit electrons (during a unrectified voltage)

111

What is half wave rectification?

a condition in which the voltage is not allowed to swing negatively during the negative half of its cycle

112

During the positive portion of the AC waveform, what is allowed to happen?

the rectifier allows electric current to pass through the xray tube

113

Since the rectifier does not conduct during a negative portion of the AC waveform and no electric current is allowed, what is the result?

the electric current is a series of positive pulses separated by gaps when the negative current is not conducted. What results is a rectified current because electrons flow in only one direction.

114

Why does half wave rectification have its name?

because only one half of the AC waveform appears in the output

115

What machines may have a vacuum tube rectifier?

some portable and dental xray systems

116

What are the systems called if they have a vacuum tube rectifier?

self-rectified

117

half wave rectified circuits contain how many diodes?

0, 1, or 2

118

How many xray pulses are produced by a half wave voltage generator?

60 xray pulses each second

119

What is a shortcoming of the half wave rectifier?

it wastes half the supply of power and requires twice the exposure time.

120

What circuit rectifies the entire AC waveform?

the full wave rectification

121

How many diodes in a full wave rectified xray imaging system?

4 in the high voltage circuit

122

How does the full wave rectifier work?

the negative half cycle corresponding tot he inverse voltage is reversed so the anode is always positive.

123

What is the advantage of full wave rectifier?

the exposure time is cut in half

124

What is the pulsed xray output of a full wave rectifier?

120 times each second

125

Half wave and full wave rectifiers are produced by...

single phase electric power

126

single phase power results in a

pulsating xray beam

127

Why does the xray beam pulsate with the single phase power ?

because of the alternate swing in voltage from 0 to maximum potential 120 times each second under full wave rectification.

128

Why do single phase voltage waveforms have little to no diagnostic value?

because of their low energy they can't penetrate well

129

How can single phase's little diagnostic value be overcome?

with 3 phase power that generates 3 simultaneous voltage waveforms that are out of step with one another.

130

What uses multiple voltage waveforms that are superimposed on one another, resulting in a wave form that maintains a nearly constant high voltage?

a three phase power

131

How many pulses are in a 3 phase power?

6 pulses per 1/60s

132

What application is applied when the voltage applied across the xray tube is nearly constant, never dropping to 0 during exposure?

a 3 phase power

133

What are the limitations to the speed of starting an exposure and ending an exposure?

the initiation time and the extinction time

134

What is one advantage to a high frequency generator?

its size-they are a lot smaller

135

High frequency generators produce...

nearly constant potential voltage waveforms,

136

What do nearly constant potential voltage waveforms do?

improve the image quality at lower patient dose

137

What systems now use high frequency circuits?

mammo and CT

138

What kind of circuits do high frequency generators use?

inverter circuits

139

What are inverter circuits?

high speed switches, or choppers, that convert DC into a series of square pulses (A square wave (pulse) is a non-sinusoidal periodic waveform (which can be represented as an infinite summation of sinusoidal waves), in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum.)

140

What does SCR stand for?

Silicon-controlled rectifiers

141

What do portables high voltage generators use in order to generate square waves?

storage batteries and silicon controlled rectifiers (SCRs)

142

What is the input that is generated by the portable generators to the step up transformer?

500 Hz

143

A high voltage step up transformer at 500 Hz is about the size of a 60 Hz transformer..it is pretty.....

heavy

144

High frequency generators are grouped by...

frequency

145

What type of rectification is used in almost all stationary xray imaging systems?

full wave rectification or high frequency voltage generation

146

How does the capacitor discharge generator work?

the generator charges a series of SCRs from the DC voltage of a nickel-cadmium battery (NiCd). By stacking the SCRs, the charge is stored at very high voltage. During exposure, the charge is released (discharged) to form the xray tube current needed to produce xrays.

147

During capacitor discharge, the voltage falls about ____ kV/mAs.

1 kV/mAs

148

Why is there a need for precise radiographic technique charts when using a capacitor discharge generator?

because the falling voltage limits the available xray tube current and causes kVp to fall during exposure.

149

What could happen after a given exposure time when using a capacitor discharge generator?

the capacitor bank continues to discharge which can cause continued xray emission.

150

How is extra xray emission stopped during a capacitor discharge?

by a grid controlled xray tube, an automatic lead beam stopper, or both

151

how does the grid controlled xray tube work?

It has a specially designed cathode to control xray tube current.

152

Why are a lot of systems used today have a falling load technique?

To ensure the shortest possible exposure time.

153

What is the theory of the falling load voltage?

the initial tube loading is higher and drops during exposure.

154

What is the drop rate of a falling load?

it follows the cooling characteristics of the xray tube anode

155

Where does falling load voltage work best?

in high capacity xray imaging systems such as interventional radiology where shorter the exposure time the better

156

How much voltage ripple does a single phase power system have?

100% voltage ripple

157

How much voltage ripple for a 3 phase, 6 pulse power system?

14% voltage ripple and voltage never falls below 86% of the maximum value.

158

How much voltage ripple for a 3 phase, 12 pulse power system?

4% voltage ripple and voltage never falls below 96% of the maximum value

159

How much voltage ripple for a high frequency generator?

1% voltage ripple-making it the best for xray quantity and quality

160

Less voltage ripple results in greater....

radiation quantity and quality

161

Why is radiation quantity best with less voltage ripple?

a greater number of xrays are produces when the electron energy is high than when it is low when a projectile electron is emitted by the xray tube filament

162

Why is radiation quality best with less voltage ripple?

because fewer low energy projectile electrons pass from cathode to anode to produce low energy xrays.

163

Fluoroscopy generally stays in ____ phase.

single

164

Fluoroscopic mA is ___ than xray mA.

lower

165

Why is fluor mA lower than xray mA?

Because the xray cables are long and have considerable capacitance, which results in a smoother voltage waveform.

166

Transformers and high voltage generators usually are identified by..

their power rating in kW

167

Electric power for any device is specified in...

Watts

168

Power = ...

power = current x potential

169

Watts = ....

watts = amperes x volts

170

How many kW is a high voltage generator for a basic unit?

30-50 kW

171

How many kW does an interventional radiology unit run?

up to 150 kW

172

High voltage generator power (kW) =

maximum xray tube current (mA) at 100 kVp and 100ms

173

Power is the product of...

amperes and volts

174

milliamperes x volts =

watts