Equipment Used in the Production of X-Rays
A. Autotransformer
1. Also known as a variable transformer
2. Provides for the variation of voltage flowing in the
x-ray circuit and applied to the x-ray tube
3. Single coil of wire with an iron core
4. Source for selecting kVp
5. Operates on the principle of self-induction
a. Single winding of wire incorporates both the primary
and the secondary coils of the transformer
6. Primary turns are fed 220 V from the radiology
department’s incoming line
7. Secondary turns (secondary taps) are selected by the
radiographer using the kVp select control
8. Voltage is stepped up or stepped down by only a small
amount and sent to the primary side of the highvoltage
step-up transformer
9. The high-voltage step-up transformer boosts the voltage
to the kVp that was selected
B. Prereading voltmeter
1. The voltmeter in the x-ray circuit indicates the voltage
that is selected
2. Called prereading because it indicates the kilovoltage
that will be flowing through the tube once the exposure
is made
3. Placed in the circuit between the autotransformer
and the high-voltage transformer
C. Timer
1. Used to regulate the duration of x-ray exposure
2. Wired in the circuit between the autotransformer and
the high-voltage transformer
3. mAs timer
a. Provides the safest tube current in the shortest
time possible
b. Measures total tube current
c. Located after the secondary coil of the highvoltage
transformer
d. Used with falling load generators
4. Electronic timer
a. Microprocessor controlled
b. Contained in most radiographic equipment
c. Allows exposure times of 1 ms (0.001 second)
5. Automatic exposure control (AEC)
a. Used to provide consistency of radiographic quality
b. Relies on excellent positioning skills and extensive
knowledge of surface and internal anatomy
because the part being imaged must be accurately
positioned over ionization sensors
c. Requires proper ionization chambers be selected
for part being imaged
d. Consists of a flat ionization chamber that is located
between the patient and the image receptor
e. Uses fixed kVp while machine controls mAs
f. As radiation passes through the ionization chamber,
it ionizes the gas contained inside
g. When a predetermined level of ionization is
reached (allowing time for a sufficient amount
of radiation to pass through to strike the image
receptor), the electronics terminate the exposure
h. Backup timer must be set to terminate the exposure
in the event of a malfunction
i. Backup timer protects the patient from overexposure
and the x-ray tube from overheating
j. Minimum response time: Shortest time possible
with an AEC because of the time it takes to
operate
k. Shortest time with an AEC is 1 ms (0.001 second)
l. Varying kVp when using AEC does not alter
receptor exposure, although contrast changes
m. Varying kVp serves to alter penetrating ability of the
beam, resulting in faster or shorter exposure time
n. Changing ± controls on AEC allows exposure to
be increased or decreased; each step normally represents
a change of 25% in receptor exposure
6. Falling load generator
a. Modern generator that takes advantage of extremely
short time capabilities and tube heat-loading
potential
b. Radiographer sets mAs and kVp
c. Falling load generator calculates the most efficient
method of obtaining the required mAs
d. X-ray tube current starts at highest level possible
for first portion of the exposure
e. When the maximum heat load of the tube has been
reached for that mA, the generator decreases the
mA to the next lower level that the tube can handle
f. The exposure continues at progressively lower levels
of mA for the shortest times possible until the
desired mAs is reached
g. Falling load generator always uses the shortest
times possible to obtain a given mAs
h. Disadvantages: Exams in which long exposure
times are used (e.g., breathing techniques for lateral
thoracic spine); rapid-sequence exposures in
which heat buildup in the tube may cause exposure
times to increase
D. Step-up transformer (high-voltage transformer)
1. Consists of primary coils and secondary coils
2. Requires alternating current to operate
3. Primary coil receives voltage from the autotransformer
a. Operates on principle of mutual induction
b. Magnetic field surrounding a wire with electricity
flowing through it induces (causes) electricity to
flow in a second wire placed within the force field
4. Voltage in the primary coil is boosted to the kilovoltage
level (thousands of volts) in the secondary coil
a. Number of turns of the wire in the primary coil
compared with the number of turns of the wire in
the secondary coil is called the turns ratio
b. Turns ratio determines how much the voltage is
stepped up
c. The greater the turns ratio, the higher the resulting
kilovoltage
d. Voltage is being induced in the secondary coil,
which has many more wire turns than the primary
coil
5. The turns ratio may be 500 to 1000, depending on
the machine
6. Voltage is varied at the autotransformer and sent to
the high-voltage transformer
7. Turns ratio in the step-up transformer is not varied
E. Rectifier
1. X-ray tube requires direct current to operate properly
2. Rectifier changes alternating current coming from
the step-up transformer to direct current
3. Rectifiers are solid-state semiconductor diodes
a. Consist of silicon-based n-type and p-type semiconductors
4. Located between the step-up transformer and the
x-ray tube
5. Unit with four diodes provides full-wave rectification
for single-phase generator
a. Full-wave rectification produces pulsating direct
current
b. Resultant waveform contains two pulses per cycle
(120 pulses per second)
c. Uses both portions of rectified alternating current
d. Results in 100% ripple, with voltage dropping to
zero 120 times per second
e. X-ray production ceases 120 times per second
6. Unit with 6 or 12 diodes provides full-wave rectification
for three-phase equipment
a. Because three-phase current is used, voltage never
drops to zero during the exposure
b. Voltage ripple for three-phase, 6-pulse is approximately
13%; the voltage actually used is about
87% of the kVp set
c. Voltage ripple for three-phase, 12-pulse is approximately
4%; the voltage actually used is about 96%
of the kVp set
d. Voltage ripple for high-frequency generators is
approximately 1%; the voltage actually used is
about 99% of the kVp set
e. High-frequency units result in lower patient dose
f. Three-phase, full-wave rectified waveforms produce
higher average photon energy (35% higher
for three-phase, 6-pulse; 41% higher for threephase,
12-pulse)
g. Three-phase and high-frequency units produce
12% to 16% more x-rays than single-phase units
h. kVp values used with single-phase equipment may
be decreased 12% to 16% when the same exam
is performed on three-phase or high-frequency
equipment
F. Milliammeter (mA meter)
1. Measures tube current in milliamperes
2. Wired between the rectifier and x-ray tube
G. mA control (filament circuit)
1. Regulates the number of electrons available at the
filament to produce x-rays
2. Voltage is provided by tapping windings of the autotransformer
and varying the voltage being sent to the
step-down transformer
a. Step-down transformer reduces voltage and increases
current in response to the use of variable resistors
(rheostats) manipulated by the radiographer
via the mA stations on the control panel
b. Resultant high current is sent on to heat the filament
c. A filament ammeter may also be connected
H. X-ray tube
1. Cathode assembly: Negative electrode in the x-ray tube
a. Contains two filaments—small and large
b. Filaments are made of tungsten (because of its
high melting point); a small amount of thorium is
added to reduce vaporization and prolong tube life
c. Filaments are heated slightly when the x-ray
machine is turned on; no electrons are ejected at
this low level of heating
d. During x-ray exposure, one filament is heated to
a level that causes electrons to be “boiled off” in
preparation for x-ray production (which is known
as thermionic emission)
e. Over time, filaments vaporize and coat the inner
surface of the x-ray tube with tungsten, leading to
tube failure
f. Cathode assembly also includes the focusing cup
g. The focusing cup surrounds the filaments on three
sides
h. The focusing cup has a negative charge applied,
which tends to concentrate electrons boiling off
the filaments into a narrower stream and repels
them toward the anode
i. Electron concentration keeps the electrons aimed
at a smaller area of the anode
j. Some x-ray tubes use the focusing cup as an electronic
grid that can turn the tube current on and
off rapidly, allowing very short and precise exposure
times, such as times needed for rapid serial
exposures (referred to as a grid-controlled tube)
2. Anode: Positive electrode in the x-ray tube
a. Consists of a metal target made of a tungsten–
rhenium alloy (because of its high melting point and
high atomic number) embedded in a disk (or base) of
molybdenum with a motor to rotate the target
b. Must be able to tolerate extremely high levels of
heat produced during x-ray production
c. Anode rotates from 3300 to 10,000 rpm, depending
on tube design
d. Rotation is achieved by the use of an induction
motor located outside the x-ray tube, which turns
a rotor located inside the x-ray tube; the target is
attached at the end of the rotor
e. Rotation of the target allows greater heat dissipation
f. Rotation of the target is stopped by a braking
action provided by the induction motor
g. X-ray machine should never be shut off immediately
after an exposure; machine should not be
shut off until the target has stopped rotating
h. Without the braking action, the target may spin
for 30 minutes, causing great strain on the bearings
i. Electrons strike the target on the focal track (sometimes
called the focal spot)
j. The focal track is beveled, producing the target angle
k. Target angle allows for a larger actual focal spot
(area bombarded by electrons), while producing a
smaller effective focal spot (the area seen by the
image receptor)
l. The larger the actual focal spot, the greater the
heat capacity; the smaller the effective focal spot,
the greater the radiographic image sharpness
m. This effect is called the line-focus principle
n. Target angle may be 7 to 20 degrees, depending on
tube design
o. Exposure switch should be activated in one continuous
motion, activating the rotor and then the
exposure button
p. The equipment allows the rotor to come up to
speed before making the exposure
q. Radiographer does not control this by activating
the rotor and waiting to press the exposure
button
r. Activating the rotor and allowing it to operate by
itself results only in unnecessary heating of the
filament and wear on the induction motor, shortening
tube life
3. Glass envelope with window
a. Cathode and target are inside the glass envelope;
some models use a partial metal envelope
b. Glass envelope also contains a vacuum so that
electrons from the filament do not collide with
atoms of gas
c. Tube window: Thinner section of glass envelope
that allows x-rays to escape
4. Tube housing: Encases the x-ray tube
a. Made of aluminum with lead lining
b. Supports and protects the tube, restricts leakage
radiation during exposure, and provides electrical
insulation
c. Also contains oil in which the x-ray tube is
immersed to assist with cooling and additional
electrical insulation
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