1
Q
What are the 3 main components of the x-ray unit?
A
- operating console
- high voltage generator
- x-ray tube
2
Q
What are the 6 parts of the operating console?
A
- Line voltage source
- Line voltage compensator
- Autotransformer
- KV meter or indicator
- Exposure switches
- Primary coil of the main transformer
3
Q
What does the operating console provide control over?
A
mA, KV and exposure time and the ability to select other modalities
4
Q
What is the line voltage source?
A
The power that is coming into the x-ray unit from the wall which is controlled by a on/off switch
5
Q
What does a line voltage compensator do?
A
Automatically corrects the line voltage to the same value at all times, since the voltage provided is inconsistent
6
Q
What does a line voltage compensator help avoid?
A
Variations in radiographic density which is caused by voltage fluxuations
7
Q
What is the autotransformer?
A
It acts as the KV selector, and adjusts the voltage to match the preselected KV for the exam
8
Q
What does the autotransformer connect to?
A
Major and minor KV taps, the line compensator, the timer circuit and filament circuit
9
Q
What is the autotransformer made of?
A
Single coil of wire with an iron core
10
Q
What principle does the autotransformer operate by?
A
Self-induction
11
Q
What happens in self induction?
A
Single winding of wire incorporates both the primary and secondary coils of the transformer
12
Q
What is controlled by the autotransformer?
A
The KVP which can be manipulated by the operator
13
Q
What does the autotransformer supply?
A
It supplies the high voltage transformer and the filament circuit
14
Q
What does the high voltage transformer do?
A
Boosts the voltage to the KV selected on the console
15
Q
What is the autotransformer law?
A
Vs/Vp= Ns/Np
16
Q
An autotranfomer, connected to a 440 V supply contains 4000 turns. If 2300 turns are in the secondary coil, what is the secondary voltage?
A
Vs/Vp=Ns/Np
X/440=2300/4000
4000X=1012000
X=253 V
17
Q
What is another name for the KV meter or indicator?
A
Pre-reading meter
18
Q
What does the KV meter measure?
A
Volts, but is calibrated to show the KV after the transformation
19
Q
What is the “pre-reading” display?
A
The display that shows the KV on the control panel that will be produced by the transformer
20
Q
Where does the pre-reading voltmeter sit?
A
Between the autotransformer and high voltage transformer
21
Q
What is the exposure switch?
A
The switch that starts the exposure and is what the tech presses
22
Q
What are the two parts of the exposure switches?
A
A two part switch that has a “ready” function that boosts the filament temperature and energizes the induction motor to rotate the anode
23
Q
What does the timer circuit do?
A
Terminates the exposure
24
Q
What are manual timers?
A
Timers that allow us control of the exact exposure length
25
What are the four types of manual timers?
- Mechanical timers
- Synchronous timers
- electronic timers
- mAs timers
26
What are mechanical timers?
Timers operated by clockwork that the technologist needs to manually set
27
What is the minimum exposure time of a mechanical timer?
0.25 sec
28
What are synchronous timers?
Also mechanical timers that depend on an electric motor turning 60 revolutions per second
29
What are standard exposure times of synchronous timers?
1/60 second
30
What is the shortest exposure time of a synchronous timer?
1/120 sec
31
What are electronic timers?
Timers used in most modern x-ray equiptment that are highly accurate
32
What is the exposure time of electric timers?
As short as 1 ms
33
What are electronic timers best used for?
They can be used to provide rapid serial exposures because they reset very quickly
34
What is a mAs timer?
Monitors the product of mA and exposure time and terminates the exposure when the designate mAs is reached
35
What does a mAs timer provide?
The higherst safe tube current for the shortest exposure for any mas selected
36
Where is the mAs timer located?
On the secondary side of the high voltage transformer because it measure actual current
37
What are mAs timers mostly used with?
Failing load equiptment which provides the shortest possible exposure by starting at a high mA and reducing it during the exposureW
38
What do manaual timers cause?
The accumulation of charge in a capacitor
39
Which type of timer provides times which are multiples of 1/60 sec?
Synchronous timer
40
What is the most sophisticated, complicated and accurate x-ray timer?
Electronic timer
41
Which timer can be used for rapid serial exposures?
Electronic timer
42
The timer circuit is usually located in this part of the x-ray circuit. Which type of timer is an exception?
The timer is usually located on the primary side of the high voltage transformer, and the only exception is the milliampere seconds timer which is located on the secondary side of the high voltage transformer
43
Which obsolete timer is limited to exposures longer than 0.25 sec
The mechanical timer
44
What type of manual timer is the most common on modern x-ray machines?
Electronic timer
45
Which type of timer is able to provide the shortest possible safe exposure time?
Electronic timers
46
What does AEC stand for?
Automatic exposure control
47
What is AEC?
Device that measure the quantity of radiation reaching the IR and terminates the exposure when sufficient intensity is reached
48
What does AEC determine?
The length of the exposure based on the amount of radiation passing through the patient
49
What do AEC systems convert?
X-rays detected into electrical signal
50
What parameter of AEC are controlled by the tech?
All parameters are under control except the actual length of the exposure
51
When radiographs are taken in the AEC mode, what should the backup timer be set to?
2 times the expected exposure time as a backup
52
Why do we need a backup time when phototiming?
In case the AEC fails to terminate
53
What are the two types of AEC detectors?
Ionization chamber type
Photomultiplier type
54
What is the ionization chamber type?
The most common type that which relies on ion chambers in the bucky tray to detect the incoming radiation
55
How do ionization chamber types work?
The charge is accumulated in a capacitor, the Thyratron releases the charge to activate the Electromagnet, which pulls open the switch stopping the exposure
56
What is MRT?
The minimum response time or how long the exposure circuit takes to turn off based on radiation received by the AEC detectors
57
What is the typical MRT range?
0.002 to 0.02 seconds (newer to older units)
58
What happens if the exposure time is less than the MRT?
It will result in an overexposure because the system will use the MRT
59
What is the most common AEC error?
Choosing the incorrect bucky type (uprgiht vs table)
60
What happens if the wrong bucky is selected?
The exposure will go indefinitely until the backup timer kicks in
61
How is the intensity (threshold) changed using AEC?
It can be adjusted on the console, which adjusts the threshold at the capacitor
62
What does changing the threshold do?
Increases the threshold or decreases it which will turn the exposure off later or sooner
63
How should the part be positioned when using AEC?
Part needs to be centered over the photocell
64
What happens if the part is not centered correctly?
The exposure will be prematurely terminated, resulting in lower exposure
65
Where are the detector cells located?
In front of the grid and behind the patient
66
What is the photomultiplies type?
The older type that uses a two phase process
67
What are the steps of a photomultiplier type?
Photons that pass through the patient are converted to light by a phosphor, then that light is converted into electricity.
68
When should you never use AEC?
- If the antomy is too small or narrow to fit on the detector (fingers, toes, pediatrics)
- Anatomy that is peripheral to the edge of the body
- With large radiopaque prosthetics
69
What are general rules of AEC?
- Proper alignment and positioning should not be understated
- Collimation should be utilized
70
What are causes of AEC overexposure?
- Wrong bucky activated
- Needed Exposure time less than MRT
- Density control left on plus setting from previous patient
- Electronic malfunction of the AEC
- Incorrect detector cell configuration, such that activated cells lie under tissue thicker than the tissue of interest
- Presence of radiopaque artifacts in the Anatomy
- Presence of external radiopaque artifacts
71
What is the cause of AEC underexposure?
- Backup time shorter than Needed Exposure time
- Density control left on minus setting from previous patient
- Inadequate Collimation (excessive scatter radiation reaching sensors)
- Incorrect detector cell configuration, activated cells lies under tissue thinner than the tissue of interest
- detector cells not fully covered by the tissue of interest
72
How can a exposure be ended prematurely?
By removing your hand from the "dead-man" exposure switch
73
Where is the primary coil of the main transformer located?
In the low voltage portion of the main transformer
74
What are the 4 parts of the high voltage generator?
- secondary coil of the main transformer
- mA meter
- rectification system
- x-ray tube
75
What is the high-voltage step-up transformer?
A transformer that bumps the voltage up so the x-ray tube has a high voltage to make electrons that have enough energy for x-rays
76
What type of current does the high voltage transformer require?
AC (alternating current)
77
What type of transformer is the high voltage transformer?
Step-up transformer
78
What principle does the high voltage transformer operate by?
Mutual induction
79
What happens in mutual induction?
Magnetic field surrounding a wire with electricity flowing through it induces (causes) electricity to flow in a second wire placed within the force field
80
Where is the secondary coil of the main tranformer located?
In the high voltage portion of the main circuit
81
What is a turns ratio?
Number of turns of the wire in the primary coil compared with the number of turns of the wire in the secondary coil
82
What does a turns ratio determine?
How much the voltage is stepped up
83
What does a greater turns ratio indicate?
The higher the resulting KV
84
What is the turns ratio of the secondary coil?
Usually between 700 and 1000
85
What type of current does the X-ray tube require?
DC
86
What is the rectification system?
Converts AC coming from the step-up transformer to DC
87
What does the rectification system ensure?
That electrons flow from the cathode to the anode only
88
When does rectification occur in the tube?
After the voltage has been stepped up from volts to kilovolts
89
Where is the rectification system located?
Between the main (step-up) transformer and the x-ray tube
90
What is the electric symbol for the rectifier?
The general diode symbol
91
What are the types of recctifiers?
Vale tube rectifiers and solid state rectifiers
92
What are valve tube rectifiers?
Devices which conduct current only in one direction and blocks current flow in the other direction
93
What are the disadvantages of valve tube rectifiers?
Large size, heat production and frequent failures
94
What are solid state rectifiers?
Rectifiers that are based on the use of semiconducting materials
95
Which type of rectifier is used in current machines?
Solid state semiconductor diodes
96
What is the most commonly used element in solid state rectifiers?
Silicon which is combined with additives to produce two types
97
What are the type types of silicon additives?
- N type
- P type
98
What is the N type silicon?
Has extra free electrons
99
What is the P type silicon?
Has a deficiency of electrons
100
What are the two types combined to produce?
P-N junction which acts as a solid state diode
101
What are the 3 types of rectification in x-ray equiptment?
- Self rectification
- Half wave rectification
- Full wave rectification
102
What is self rectification?
Uses no rectifiers
103
How does self rectification work?
The x-ray tube acts as its own rectifier and suppresses the inverse phase of the
current
104
When is self rectification used?
Only in very low power tubes like dental equiptment
105
What does self rectification produce?
A half-wave waveform which is only one-half the electrical current used
106
How many pulses of radiation are produced in self rectification?
60 pulses of radiation per second
107
How many rectifiers does half-wave rectification use?
One or two rectifiers
108
What does half wave rectification produce?
Only half the voltage is used and the tube is protected by the rectifiers
109
How many rectifiers does full wave rectification use?
At least 4 rectifiers
110
How does full wave rectification work?
The rectifiers use both parts of the rectified alternating current and invert the inverse AC pulses so all the electricity is used
111
What are generators?
Converts mechanical energy to electrical energy
112
What does full wave rectification produce in a single phase generator?
Pulsating direct current
113
How many pulses of radiation are produced in full rectification in a single phase generator?
120 pulses of radiation per second
114
What is the ripple in full rectification in a single phase generator?
100% ripple
115
What is the ripple in full rectification in a 3 phase 6 pulse generator?
13% ripple
116
What is the ripple in full rectification in a 3 phase 12 pulse generator?
4 % ripple
117
What is the ripple in full rectification in a high frequency generator?
1% ripple
118
What does less voltage ripple result in?
A greater radiation quantity and quality
119
As the generator strength increases, how is ripple affected?
As generator strength increases, ripple decreases
120
Which type of generator results in lower patient dose?
High frequency generator
121
What is the main disadvantage of a single phase unit compared to 3-phase and high frequency units?
The 100% ripple and beam not being produced as efficiently
122
What are the disadvantages of three phase units?
Large size and cost
123
What is the mA meter?
Monitors the tube current in milliamperes
124
Where is the mA meter located?
Between the rectifier and x-ray tube
125
What are the parts of the filament circuit?
- voltage stabilize/space charge compensator
- mA selector
- filament selector
- filament transformer
- filament
126
What does the voltage stabilize/space charge compensator do?
Corrects the voltage if fluxuations occur and adjusts the filament current downward when KV is increased
127
What is the mA selector?
A bank of resistors that allows you to select the filament current
128
What is the mA selector calibrated in?
mA, but is actually a selection of the filament current which is measured in A
129
What is the mA contolled by?
The temperature of the filament
130
What does mA control regulate?
The number of electrons available at the filament to produce x-rays
131
How is filament amperage and mA related?
The higher the filament amperage the higher the mA
132
What does a hotter filament produce?
More electrons by thermionic emission, causing a higher mA
133
What is the filament selector?
Allows you to choose a small or large filament
134
Which filament is slected when higher mA stations are selected?
Large filament
135
What is the filament transformer?
A transformer that reduces the incoming voltage to 10 V and increases the amperage from mA to A
136
What type of transformer is the filament transformer?
Step down transformer
137
What does a step-down transformer dow?
Reduces voltage and increases current in response to the mA selected on the control pannel
138
What does stepping down the current do?
Compensates for the space charge effect and removes electrons that are not of sufficient power
139
What voltage does the filament circuit operate after being stepped down?
At 10 V but can range from 6-12 V
140
What is a typical turn ratio of a filament transfomer?
1:20 or 0.05
141
Which device regulates the voltage coming into into the x-ray machine?
Line voltage compensator
142
Which meter on the control console is a "pre-reading" meter?
KV meter
143
What is the incoming voltage for most fixed (installed) x-ray units?
220 V
144
Which device acts as the KV selector? Does it have a fixed turns ratio?
Autotransformer and it does not have a fixed turns ratio
145
If the incoming voltage is 220, what is the output voltage from an autotransformer if it has 5000 turns on the primary side and 4000 turns enclosed by the secondary taps?
Vs/Vp=Ns/Np
x/220=4000/5000
5000x=880000
x= 176 V
146
If the voltage from the problem above is sent to the tube through the main transformer which has a turns ratio of 600, what will be the kilovoltage through the tube?
Vp/Vs=Np/Ns
220/x=600/1
220=600x
0.37 kV
147
What is the relationship between filament temperatre and mA?
As the filament temperature increases the mA increase
148
Tube current (mA) is controlled by which circuit?
The filament circuit
149
What is the typical range for voltage and amperage in the filament circuit
3-6 A and 6-12 V
150
What portion of the x-ray circuit are the radiographer operated controls connected to?
The low voltage portion of the circuit
151
What is the reason its connected to the low voltage portion?
It prevents from a high voltage shock hazard
152
What is the main breaker?
Where the AC (alternating current) comes from to power the circuit
